at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood...

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Transcript of at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood...

Page 1: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 2: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 3: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Inthis

study,the

effectsof

Y/b,Fp

andF

onthe

scourdepth

havebeen

investigatedas

theem

piricalequationsconsidered

inthis

studyuse

theseparam

etersforpredicting

thescourdepth.

EflectofFroudenum

ber

Therelative

scourdepth,d/bhas

beenplotted

againsttheFroude

ntunberfordifferentZ/bratio

andd50

inFigure

1.Itisseen

fromthe

plotthattherelative

scourdepthincreases

with

theincrease

inFroude

number.Since

theFroude

ntunberisa

linearfunctionofapproach

velocity,V,itcan

besaid

thatforagiven

approachdepth,sourdepth

isdirectly

proportionaltothe

approachvelocity.The

positiverelationship

betweenthe

scourdepthand

theFroude

numberhas

beenused

invarious

empirical

equationsas

suggestedby

Shenet.

al.,Chitale

andBata

(Kabir,1984).

Moreover,the

scourdepthincreases

asthebed

materialbecom

esfiner.

Eflecz‘ofApproachD

epth

Thevariation

ofd/bw

ithrespectto

therelative

approachdepth,Y/b

fordifferentI/bratio

andd50

isshown

inFigure

2.Here,

therelative

scourdepth

showna

linearlyincreasing

relationshipw

iththe

relativeapproach

depth.Thescourdepth

changesw

iththe

approachdepth

atanaverage

rateof1.95,w

hichshows

onlym

inorvariation

fordifferentpier

shapesand

bedm

aterials.Laursen

hasestim

atedthis

rateas

2.0(G

ardeand

Raju,1985),

which

isin

closeagreem

entwith

therate

of1.95found

inthis

study.

EjfectofPierShape

Figure3

showsthe

scourpattemaround

piersoffourdifferentshapes.The

same

I/bratio

hasbeen

maintained

forthe

rectangular,round-nose

andsharp-nose

piers.Am

ongthese,

thesharp-nosed

pierhadthe

minim

umscourand

therectangularpierhad

them

aximum

scour.Thepattem

ofscouraroundthe

pierissim

ilarforthe

rectangularandround-nose

shapes.However,the

locationofm

aximum

scourdepthis

neartheupstream

edgefor

allthepiers.

Theeffectof

piershapehas

beenincorporated

inthe

scourdepthestim

ationin

asim

plifiedform

throughthe

width

ofthepier.However,

itis

generallyobserved

thatstreamlining

thenose

ofthepier

canreduce

thescourdepth.

3

Page 4: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 5: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

J1

15

I_

—-—

--—-—

—Sharp-n05e

-——

——

——

——

-Round

nosepier

l/b=3-00‘

D=0.;_om

m

?—

@i@

—-—

Rectangular'

‘Inu‘

I.

\.\.\= /'

s00-1.

.-o-s

~o-sI

0-1i

-——

-—c~

Y_/b

*0-—

o+

—C‘

lI

'=-

soIrcu

orP16!‘

‘libI00

ID

50:049m

ml

A'//

.

.v

///

-///

;/"///

I'

0.

,y/i

Z

pierI/b=3-0Q

QD50

=0-9I._mm

PIE!’I/b=3~00

1

/,4

,//

./

1//

.'/

U

/A/

._/

./

p

//

0-e0-9

1-o‘

i—

Q1

Fig.2Relative

scorn’depthversus

relativeapproach

depth

5

Page 6: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 7: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 8: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 9: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 10: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 11: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 12: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 13: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 14: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 15: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

fotmdation

arethe

safetyagainstthe

maxim

umscour,the

minim

umgrip

belowscourlevel

requiredto

resistover-turning,slidingofw

ellandthe

permissible

fotmdation

pressures.

InIndia,

thepractice

isto

choosea

preliminary

valueofthe

depthoffoundations

accordingto

theIndian

Roadscongress

codeofpractice

forBridges.G

enerally,one-third

ofthe

maxim

umanticipated

scourbelow

thehighest

floodlevel

(HFL)

isprovided

asthe

minim

umgrip

length.Thus,the

coderecom

mends

thatthedepth

ofthefoundation

be1.33

times

thedesigned

HFL.

Them

aximum

depthofscouris

takenastw

icethe

normaldepth

ofscour.Terzaghiand

Peck(1962)reportoffailure

oftwo

bridgepiers.

One

pierhadits

baseplaced

ina

bedcontaining

bouldersoflarge

size.In

anothercase,thebase

ofthepierwas

establisheddeep

intoa

gravelstratum.

Yet,duringseasons

ofhighwater,the

piersfailed

dueto

scour.Hence,itis

advisablethatthe

baseofthe

foundationbe

establishedata

depthbelow

low-w

aterchannelequaltonotless

thanfourtim

esthe

highestfloodlevel.

CO

NC

LUS

ION

Scourdepth

canbe

predictedto

areasonable

precisionw

iththe

helpofem

piricalor

theoreticalcorrelationsinvolving

parameters

likesilt

factor,bed

factor,depth

offlowetc.

which

affecttheintensity

ofscour.As

bridgescontinue

tofail

inspite

ofthetheoretical

precision,reliableforecasts

ofscourneedto

begiven

basedon

experience.A

largefactorof

safetyhelps

becauseofthe

uncertaintyinvolved

inthe

predictionof

scourdepth.

Itis

advisableto

followthe

localstandardcode

ofpractice.

RE

FER

EN

CE

S

1.G

radeR.J.,

1961,"LocalBed

VariationatBridge

Piersin

AlluvialChannels",Research

Journal,University

ofRooskee,Roorkee,India.2.

LaursenE.M

.,1963,

"AG

eneralizedM

odelStudy

ofScour

aroundBridge

Piersand

Abutments",Proceedings

ofMirm

esotaIntem

ationalHydraulics

convention,M

irmesota,

USA.

3.Standard

specificationsand

CodeofPractice

forBridges,Section

1:GeneralFeatures

ofDesign,Section

2:Loadsand

Stresses,IndianRoads

Congress,New

Delhi,India.

4.Straub

L.G.,

1942,"M

echanicsofR

ivers",Physics

oftheEarth,

PartIX

,H

ydrology,M

cGraw

-HillBook

Company,N

ewYork,U

SA.5.

TerzaghiK.,PeckR.B.,

1962,"SoilMechanics

inEngineering

Practice",AsiaPublishing

House,New

Delhi,India.

6.VenkatadriC.,Rao

G.M

.,HussianS.T.,Asthana

K.C.,

1956,"ScouraroundBridge

Piersand

Abustments",JournalofCentralBoard

ofIrrigationand

Power,Vol.22,No.1,N

ewD

elhi,India.

15

Page 16: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

TH

ES

RIC

OS

ME

TH

OD

:A

SU

MM

AR

Y

by

Jean-LouisBriaudl,

H.C.Chenz,KiseokKwak3

AB

ST

RA

CT

Inthe

USA,the

scourdeptharound

abridge

pieriscurrently

calculatedusing

theH

EC-

18equation.

Thisequation

wasdeveloped

forpiersfounded

insand

andthere

isa

sensethatin

claythe

depthofscouris

notaslarge.

Theptu-pose

ofthisstudy

wasto

developa

method

forclays,

silts,and

dirtysands.

TheSRICO

Sm

ethod(http://tti.tam

u.edu/geotech/scour)was

developedon

thebasis

offlume

tests,numericaltesting,and

erosiontesting

ofthesoil.

Anew

apparatuscalled

theEFA

(ErosionFunction

Apparatus)wasbuiltforengineers

totestthe

soilforerodibility

inthe

laboratory.The

outputofthe

simple

SRICOS

method

isa

scourdepth

aftera

giventim

e.If

ahydrograph

isused

asinput,the

extendedSRICO

Sm

ethodcan

beused

andresults

ina

scourdepth

versustim

ecurve.

1Spencer

J.BuchananProfessor,

Dept.ofC

ivilEngineering,

TexasA

&M

University,

CollegeStation,

Texas77843-3136,U

SA(briaud@

tarnu.edu)2Associate

professor,DeptofCivilEngineering,Texas

A&

MU

niversity,CollegeStation,Texas

77843-3136,USA.

3Ph.D.Student,DepartmentofC

ivilEngineering,TexasA

&M

University,College

Station,Texas77843-3136,

USA

is

Page 17: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Inthe

caseofcohesive

soilsthe

scourratecan

bethousands

oftimes

slowerthanin

thecase

ofcohesionless

soils.Cohesive

soilsinclude

siltsand

clays.According

tothe

unifiedsoil

classificationsystem

,siltsand

claysare

soilsw

hichhave

more

than50%

byw

eightofparticlespassing

the0.075m

msieve

opening.Siltsizeparticles

arebetween

0.075mm

and0.002m

mand

claysize

particlesare

smallerthan

0.002mrn.

Cohesivesoils

arenotclassified

bygrain

sizebut

insteadby

theirdegreeofplasticity

which

ism

easuredby

theAtterberg

limits.

Becausecohesive

soilsscour

som

uchslow

erthan

cohesionlesssoils,

itis

necessaryto

includethe

scourratein

thecalculations.

Indeed,while

oneflood

may

besufficientto

createthe

maxim

umscourdepth

z-maxin

cohesionlesssoils,the

finalscourdepthafterm

anyyears

offloodhistory

atabridge

incohesive

soilmay

onlybe

afraction

ofemax.

Thescour

rateeffect

incohesive

soilsis

measured

bythe

erosionrate

versusshear

stresscurve

which

canbe

usedto

calculatethe

reductionin

scourdepthin

thecase

ofcohesivesoils.

Theerosion

rate2':is

definedas

thedepth

ofsoilscouredperunitoftim

eand

isconveniently

quotedin

rrmr/hr.

Theshear

stress2'is

theshearstress

imposed

atthewatersoilinterface

andis

givenin

N/m

i.The

2'vs.

rcurve

isa

measure

oftheerodibility

ofthesoil(Figure

1).Typically

theerosion

rate2

iszero

untilthecriticalshear

stressrs

isreached

andthen

2increases

asr

increases.The

2':vs.

rctu"ve

canbe

measured

with

theEFA

(patentpending)(ErosionFunction

Apparatus)(http://tti.tam

u.edu/geotech/sour)(Briaud

etal.,1999(a)).

Inthe

EFA(Figure

1)a

soilsam

pleis

erodedby

waterflow

ingover

it.The

sample

iscollected

fromthe

sitein

astandard

thinw

allsteeltube,placedthrough

thebottom

ofarectangularcross

sectionpipe,and

alm

rnprotrusion

iseroded

overtime.

Oncethe

2':vs.

rcurve

isobtained

them

ethodto

predictthepier

scourdepth

asa

functionoftim

eproceeds

asfollow

s.Firstthe

maxim

umshearstress

rmaround

thebridge

pieris

calculated(Briaud

etal,l999)(a)):

rm,=0.094pl/2L

-l

(1)logRe

10where

pis

thedensity

ofwater,v

them

eanapproach

velocity,and

Rethe

pierReynolds

number.

Secondthe

initialscourrate2,.

correspondingto

rm,

isread

onthe

z'vs.

2"curve.

Thirdthe

maxim

umdepth

ofscourem

iscalculated

(Briaudetal.,l999)(a)):

em,(m

m)

=0.18

Re0'635(2)

whereRe

isthe

pierReynoldsnum

ber.Note

thatequation(2)

givesa

valueof

em,

which

isvery

closeto

theone

forcohesionless

soils.Indeed

itwas

fotmd

thatthem

aximum

depthof

scourinclays

andin

sandswhere

approximately

thesam

ein

flume

experiments.

Inthose

same

experiments

howeveritwasfound

thatthescourhole

inclay

developedto

theside

andin

theback

ofthepierand

notinthe

frontofthepier.

Thisindicates

thatforscourinclay

thefrontof

thepierm

aynotbe

thebestplace

toinstallm

onitoringequipm

ent.Fourth,the

equivalenttime

regis

calculated.The

time

teqis

defmed

asthe

time

overwhich

thedesign

velocityvdes

would

haveto

beapplied

forthedepth

ofscourz

tobe

equaltothe

depthofscourreached

afterthehydrograph

spanningthe

designlife

ofthe

bridger,,.,

hasbeen

applied.The

time

re,is

calculatedas

(Briaudetal.,1999)(b)):

I

1..(hrs)=73I10.Iyeerslim(palm/S))1'7°6(a(mm/hr))*“°(3)

Fifth,thescourdepth

eversus

time

tcurveis

givenby:

17

Page 18: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

‘A

.Erosion

=Rate

IZ

~

>I

iliinl,Shear

Stress(N/m

2)I

Fig.1-TheEFA:( __é-T

b_-TU

_-

__

Sample

“E

rosionFunction

ppI

EFA'—

"_'1i11r5.:¢_'.,..

.,

_.'----"

.1"

-.'.-.

_-1-_---..".'_--r».

'_.__-__-.-i_-;,.,_-*._-_'

:-I=.'-£1»;...r-.-.;;=.‘-..-,=_-',;..;;'.-,'.',r_=-‘."'

'"-

.'.3'1

-'.-3

"'_-_.-_'t.,"_"._-:-_j_

.'._.-_1

.2:1--_~':.-1-1_'€*_.='£11--I-If-53-'--..—...1:-.;

-_-3.'.‘_;.-,

'_,.

.._'.~_-':;._-'1_,.-’

-.:=‘.'--::-.--.--r-'.'0

I_'.I'.I!-'.-‘-‘J:-"_'~"'-‘.1‘:if-'.'-'-_'::i__'.‘:1}:-.

r-'.~..,;:''-':--.

--..-..;_-_.-._:-

Aaratus

a)Diagram

oftheApparatus,(b)Res

18

(b)

(E1)

ultofanEFA

tests

Page 19: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 20: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 21: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 22: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

AC

KN

OW

LED

GE

ME

NTS

TheSRICO

Sm

ethodwas

initiallydeveloped

undersponsorshipby

theTexas

Departmentof

Transportation.It

isbeing

furtherdeveloped

undersponsorship

bythe

National

CooperativeH

ighway

ResearchProgram

.

REFER

ENC

ES0

Briaud,J.-L.,

Ting,F.C

.K.,Chen,

H.C.,G

udavalli,R.,

Perugu,S.,

Wei,

G.,

1999(a),

“SRIC

OS:

Predictionof

ScourRate

inCohesive

Soilsat

BridgePiers”,

Journalof

Geoteclm

icalandG

eoenvironmentalEngineering,Vol.

125,No.4,April1999,pp.237-246,

American

SocietyofC

ivilEngineers,Reston,Virginia,USA.

0Briaud,J.-L.,Ting,F.C

.K.,Chen,H.C

.,Gudavalli,R.,Kw

ak,K.,Pl1ilogene,B.,Han,S.W.,

Perugu,S.,Wei,G

.,Nurtjahyo,P.,Cao,Y.,Li,

Y.,1999(b),“SR

ICO

S:Prediction

ofScourRate

atBridge

Piers”,Report

2937-Fto

theTexas

Department

ofTransportation,

TexasA

&M

University,C

ivilEngineering,CollegeStation,TX

77843-3136,USA.

22

Page 23: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 24: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 25: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 26: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 27: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 28: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 29: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 30: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

8

h:pore

waterpressurein

head(variation

fromhydrostatic

pressurerelative

tom

eanwaterlevel)

psg;w

eightofunitvolume

oftheindividualsand

grain/1,.:variation

ofwaterpressureacting

onthe

surfaceof

thebed

relativeto

theinitialwaterlevel

h’:excesspore

waterpressurez

:depthofthe

sandlayerm

easuredfrom

topofthe

sandsurface

asdatum

p_,.:densityofsand

densityofwater

gravitydue

toacceleration

:porosityofwaterpart

:porosityofthe

sandcolum

n(it=

/1,,+2.,,/1,,:porosity

ofairpart)2>-ix}->0Q‘Q

Substitutingequations

(2)and(3)into

equation(1),and

assuming

/1,,2/1

,

0",+pg/1'=

(p,-

p)gz(1--/1)=

constant(4)

Thus,theliquefaction

statecan

beexpressed

by:

(p._—p)sZ(1-1)

(/>.—r>)sYz'(1-1)

EX

PE

RIM

EN

TA

LD

ET

AILS

TheShields

diagramwas

usedto

determine

criticalshearvelocity,u.c,againstm

eangrain

size,0'50.

Therelation

betweencritical

shearvelocity

andm

eangrain

sizecan

beexpressed

as:

u.,=0.034;?(6)

where,24.‘,is

inm

/s,anddso

isin

mm

.C

riticalshearvelocitycan

beconverted

intocriticalm

eanflow

velocity(U6)by

usingthe

following

logarithmic

expressionofthe

velocityprofile:_

g:

5.75log[5.5I-ii](7)

u"'cdso

where,ho

=flow

depth.

30

Page 31: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 32: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 33: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 34: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 35: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 36: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 37: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 38: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

SC

OU

RO

FR

OC

KA

ND

OTH

ER

EA

RT

HM

AT

ER

IALS

AT

BR

IDG

EP

IER

FOU

ND

ATIO

NS

ByG

eorgeW

.Annandalel,StevenP.Sm

ithzand

Tamara

Butler3

AB

ST

RA

CT

Bridgefoundations

mustbe

designedto

withstand

theeffects

ofscourfromextrem

eflood

eventsthatcould

potentiallyoccur

duringthe

structure’slife.

Many

equationsare

availableto

assistinthe

predictionofscour

atbridgecrossings

butuntilrecently,few

accountedfor

theeffects

ofgradationand

nonefor

theeffects

ofcohesionand

consolidation.N

oquantitative

procedurefor

predictingbridge

pierscour

inrock

haspreviously

beenavailable

topracticing

engmeers.

Recognizing

thisneed

theU

nitedStates

TransportationResearch

BoardCom

mittee

onH

ydraulics,H

ydrologyand

Water

Quality

identifiedscour

inerodible

rockand

consolidatedm

aterialsas

oneof

itstop

threehydraulic

problems.

Oneof

them

ethodsthat

havebeen

developedto

addressthis

need(Sm

ithet

al.1997)

isbased

onthe

ErodibilityIndex

Method

(Annandale1995).

TheE

rodibilityIndex

Method

isa

semi-em

piricalmethod

thatcanbe

usedto

estimate

theerodibility

ofanyearth

material,including

cohesionlessgranular

soil(sand,graveland

cobbles),cohesivesoil,andjointed

andfractured

rock.

Thispaper

summ

arizesthe

ErodibilityIndex

Method

andexplains

howit

isused

tocalculate

thedepth

ofscouraroundbridge

piers.Application

ofthem

ethodis

demonstrated

bym

eansofa

casestudy.

INT

RO

DU

CT

ION

Thispaper

introducesconcepts

thatcan

beused

toexplain

thescour

resistanceof

complex

earthm

aterialssuch

asrock,

slickensidedand

cohesiveclays,

andalso

non-cohesivegranularm

aterial.A

semi-em

piricalapproachthatcan

beused

toquantify

therelative

abilityof

theseearth

materials

toresistscouris

presented,concomitantly

with

amethod

thatcanbe

usedto

calculatethe

depthofscouraround

bridgepiers.

Thefirstbridge

pierscouranalysisusing

theErodibility

IndexM

ethodwas

conductedfor

theN

orthumberland

StraitBridge

(Anglio

etal.

1996)(Figure

1).This

analysisentailed

verificationofthe

ErodibilityIndex

Method

byusing

materialproperties

andestim

atesofthe

erosivepow

erofwater

thatcausedscour

inrock

aroundone

ofthebridge

piers.Laboratory

1DirectorW

aterResourceEngineering,G

olderAssociatesInc.,44

Union

Blvd,Suite300,

Lakewood,Colorado80228.

2Hydraulic

DesignM

anager,URSG

reinerWoodw

ardClyde,4582

SouthU

lsterStreet,Denver,Colorado

80237.3Engineer,G

olderAssociatesInc.,44

Union

Blvd,Suite300,Lakewood,Colorado

80228.

38

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Page 41: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

occuralong

thefractures

andjoints

inthe

rock,and

alongthe

fissuresand

slickensidesin

theclay,before

failureofthe

blocksofrock

orclumps

ofclaythem

selvesoccur.

MaterialResistance

When

scourina

complex

earthm

aterialsuchas

rockis

considered,therelative

abilityof

suchearth

materials

toresisterosion

isdefined

bym

ultipleparam

eters.Materialproperties

thatdeterm

inescour

resistanceofrock

includeintact

material

strength,block

size,shear

strengthbetween

blocksofrock,and

therelative

shapeand

orientationofthe

rockblocks.

By

making

useofparam

etersthatrepresentthe

relativerole

ofeachofthese

propertiesto

resisterosionit

ispossible

todefine

ageo-m

echanicalindexthatquantifies

therelative

abilityofearth

materialto

resisterosion.

Research(Annandale

1995)has

shownthat

therelative

abilityofother

earthm

aterialsto

resisterosion,such

cohesionlesssilt,

sand,gravela_nd

cobbles,and

cohesiveearth

materials,

canalso

bequantified

with

thesam

eset

ofparam

etersas

usedfor

rock.The

Erodibility

Index,which

isidenticalto

Kirsten’sExcavatability

Index(Kirsten

1982)isdefined

bythe

equation

K=M,-K,-K,-J,

(1)W

hereMS

=intactm

aterialstrengthnm

nber;Kb=

blockorparticle

sizenum

ber;Kd=

discontinuityorinter-particle

bondshearstrength

nmnber;and

J8=

relativeshape

andorientation

number.

Tablesand

methods

toquantify

theconstituentparam

etersare

presentedin

Annandale(1995),Kirsten

(1982)andthe

NationalEngineering

Handbook

(NRCS1997).

ErosivePow

erofWater

TheE

rodibilityIndex

Method

usesstream

power,w

hichis

equivalentto

therate

ofenergy

dissipationin

flowing

water,torepresentthe

erosivepowerofwater.

(Theseterm

sare

usedinterchangeably

inthis

paper).By

making

useofthis

variableitis

possibleto

quantifythe

relativem

agnitudeofpressure

fluctuations,which

playan

importantrole

ininitiating

sediment

motion

andm

aintainingsedim

enttransport.In

ordertosupportthe

hypothesisthatthe

rateof

energydissipation

canbe

usedto

representthe

relativem

agnitudeofpressure

fluctuations,Annandale

(1995)analyzedobservations

byFiorotto

andR

inaldo(1992)who

measured

pressurefluctuations

underhydraulic

jumps.

Theresults

ofthe

analysisindicated

thatthe

standarddeviation

ofpressure

fluctuationsis

directlyproportional

tothe

rateof

energydissipation

(Figure3).This

findingsupports

theuse

ofstreampow

ertoquantify

therelative

magnitude

ofthe

erosivepow

erofwater.

Increasesin

streampow

erare

relatedto

increasesin

fluctuatingpressures,w

hichform

thebasis

oftheconceptualm

odeloftheerosion

processschem

atizedin

Figure2.

Am

ethodthatcan

beused

todeterm

inethe

magnitude

oftherate

ofenergydissipation

arotmd

bridgepiers

arepresented

furtheronin

thispaper.

41

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Page 46: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

JErosionThreshold

Line

Erosionpredicted

forpoints

plottedin

this/I’

region/I’

StreamPower

\/,

//”

No

Erosion//'

predictedfor

//'

pointsplotted

in/"

thisregion

Erodibility

Index

Fig.7D

etermination

oferodibilityofearth

materials.

Determ

inationofExtentofScour

Theextent

(depth)of

scouris

determined

bycom

paringthe

streampow

erthat

isavailable

tocause

scourwith

thestream

powerthatis

requiredto

scourtheearth

materialunder

consideration.The

availablestream

powerrepresentsthe

erosivepowerofthe

waterdischarging

overtheearth

material,whereas

therequired

streampoweris

thestream

powerthatisrequired

bythe

earthm

aterialforscourto

comm

ence.Ifthe

availablestream

poweris

exactlyequalto

therequired

streampower,

them

aterialis

atthe

thresholdof

erosion.In

caseswhere

theavailable

streampow

erexceedsthe

requiredstream

power,them

aterialwillscour.

Otherwise,it

willrem

ainintact.

Figure8

showshow

theavailable

andrequired

streampower,both

plottedas

afunction

ofelevationbeneath

theriverbed,

arecom

paredto

determine

theextentofscour.

Scourw

illoccurwhen

theavailable

streampow

erexceedsthe

requiredstream

power.O

ncethe

maxim

umscourelevation

isreached

theavailable

streampoweris

lessthan

therequired

streampower,and

scourceases.

Therequired

streampow

eris

determined

byfirstindexing

ageologic

coreor

boreholedata.

Thevalues

oftheE

rodibilityIndex

thusdeterm

inedw

illvaryas

afunction

ofelevation,dependenton

thevariation

inm

aterialproperties.O

ncethe

indexvalues

atvariouselevations

areknow

n,therequired

streampow

eris

determined

fromFigure

4or

5,asconceptually

shownin

Figure9.

Figure9

indicatesthatthe

streampowerrequired

toscoura

particularearthm

aterialisdeterm

inedby

enteringthe

erosionthreshold

graphon

theabscissa,w

iththe

Erodibility

Indexknow

n,moving

verticallyto

theerosion

thresholdline,and

readingthe

requiredstream

poweronthe

ordinate.Figure

10illustrates

thattheprocess

isrepeated

asa

functionofelevation

belowthe

riverbed.The

valuesofthe

requiredstream

poweristhen

plottedas

afunction

ofelevation.

46

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Page 49: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 50: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

0.65

-351=2.0-K,-K,-K,Fr,°"“3

(5)yl

Y1I

/E

II

~.-

-'

__—

'_/

ix-I

1\

--

-1'

-I_

.fl

‘R,

whereys

=scourdepth

(ft),y1=

flowdepth

directly/ups’tr/eamofthe

pier(ft),

K1=

correctionfactor

forpier

noseshape,K2

=correction

factor......f6rangleofattack

offlow,

K3=

correctionfactor

forbed

condition,a

=pier

width

(ft),L

'=length

ofpier(ft),

Frl=

FroudeN

umber

=V1/(gy1)1/2,and

V1=

mean

velocityofflow

directlyupstream

ofthepier(ft/s).

With

Yumassum

edto

bethe

maxim

umscour,

thescour

depthestim

atedw

iththe

ErodibilityIndex

Method

canneverexceed

thisvalue.

Therange

ofscourdepth

estimates

forthis

method

istherefore

0f

ysfymax.

CA

SE

STU

DY

:WO

OD

RO

WV

VILS

ON

BR

IDG

E

TheW

oodrowW

ilsonBridge

overthe

Potomac

River

isan

essentialpart

oflocal,

regionalandnationaltransportation

systems

(Figure12).

TheBridge

carriessix

lanesofCapital

Beltway

trafficbetween

Alexandria,V

irginiaand

Oxon

Hill,

Maryland

andis

thelast

rivercrossing

forapproxim

ately50

miles

downriver.

Congestionand

thefrequency

ofdrawbridge

openingsform

arinetraffic

causetraffic

delayatthe

bridge.TheW

oodrowW

ilsonBridge

isone

ofafew

onthe

Interstatehighw

aysystem

thatcontainsa

movable

span.Under

currentCoastG

uardregulations,the

50-foothighdrawbridge

opensapproxim

ately240

times

peryeartoallow

forthepassage

ofmarine

traffictraveling

thePotom

acRiver.

Thefive-m

ilesection

oftheBeltw

ayw

ithinthe

projectareaserves

asa

systematic

linkforlocaltraffic

onm

ajornorth-southroadways

feedinginto

interchangesatTelegraph

Road,USR

outel,I-295

andM

D210.

Furthennore,the

easternhalf

ofthe

Beltway,

includingthe

Woodrow

Wilson

Bridge,is

designatedas

I-95and

constitutesa

criticallinkin

theM

aineto

Floridainterstate

route.Becausethe

adjacentsectionofthe

hecticBeltw

ayis

eightlanesw

ide,the

currentsix-lane

Woodrow

Wilson

Bridgerepresents

asevere

bottleneckon

thehighw

aysystem

.Furthermore,

theexisting

Woodrow

Wilson

Bridgecannot

lastm

uchbeyond

2004w

ithoutmajor

structuralrehabilitation.Theinspections

andrepairactivities

attheBridge

would

resultin

extendedtraffic

delaysand

increasedcosts.

Replacingthe

Bridgebefore

2004w

illgreatly

reducetraffic

delaysin

thearea.

In1992,

aC

oordinationCom

mittee

ofaffectedjurisdictions

fromM

aryland,Virginia,

andthe

District

ofC

olumbia

andlocal,

regional,state

andfederal

govemm

entsbegan

investigatingsolutions

tothe

trafficproblem

sat

Woodrow

Wilson

Bridge.The

Comm

itteeapproved

a“Preferred

Altemative”in

1996,which

featureda

facilityw

ithside-by-side,m

ovablespan,tw

inbridges

with

a70-footnavigationalclearance.

Thenew

twin

bridgesw

illcarry

10lanes

oftrafficplus

two

futureH

ighO

ccupancyVehicle

(HO

V)lanes.

Thenew

Bridgedesign

willclearthe

riverby70

ft,which

willreduce

thenum

berofopeningsby

more

thantw

o-thirds,thus

decreasingtraffic

delays.

50

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Page 53: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

ForpierM10

column

(4)ofTable1

showsthe

valueofSu

(kPa)ortheSPT

blowcount,

whicheveris

applicableaccording

tothe

log,atvariousdepths

belowthe

riverbedsurface.

NotethatSu

valuesappearasdecim

alnumbers;blow

cotuitsappearasw

holenum

bers.Colurrm

(5)ofTable

1shows

theestim

atedvalues

ofMS.

BlockorParticle

SizeN

umber

Theborehole

logm

aterialdescriptions

wereused

todeterm

inethe

particle/blocksize

ntunber,Kb.Kb

wasassigned

avalue

ofoneforallm

aterialsexceptthe

hardclay.The

hardclay

ofthecretaceous

periodPotom

acgroup

wasassigned

avalue

of100.The

reasonforusing

Kb=

100forthe

cretaceousperiod

clayis

thattheclay

isso

hardthatit

canbe

viewed

assoftintact

rockaccording

totables

inAnnandale

(1995).Kb

determinations

forpier

M10

areshown

incolum

n(6)ofTable

l.

Discontinuity

orInter-particleBond

ShearStrengthN

umber

Theshear

strengthnum

ber,Kb,

wascalculated

usingthe

following

equation(Annandale

1995):

K.=en(¢).<8)

where(I)=

8.1°forthevery

softtosoftclay

material,but=

30°forallotherm

aterials.Kb

valuesw

ithdepth

forpierM10

areshown

inTable

1column

(7).

Relative

Shapeand

Orientation

Num

ber

Avalue

ofone

wasassigned

tothe

groundstructure

number,

J5,in

allcases

(Annandale1995).

J,isshown

incolum

n(8)ofTable

1.

Erodibility

Indexand

Required

Power

Erodibility

Index(EI),

theproduct

ofM

s,Kb,

Kd,and

Ks,is

shownin

Table1

column

(9).The

powerrequired

toscour

thePotom

acR

iver’sbed

material

wasdeterm

inedusing

Annandale’s(1995)erosion

threshold(W

ittler,etal.1998)forlow

strengthm

aterials(as

aconservative

approach):

pR=0.48-51°“

(9)where:

pR=

power

requiredto

scourgranular

material

(kW/m

2).Required

streampow

eris

calculatedin

Table1colum

n(10)forpierM

10.53

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Page 58: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

thresholdforallearth

materials,including

rockand

cohesiveand

non-cohesivegranularm

aterialand

hasbeen

appliedsuccessfully

topredict

theonset

ofscour

ofrock

aroundbridge

piers(Anglio,etal.1996).

Am

ethodthatcan

beused

topredictthe

extentofscourbycom

paringthe

streampow

erthatis

requiredto

scourearthm

aterialwith

thestream

powerthatis

availablearound

bridgepiers

isalso

presented.The

method

requiresindexing

oftheearth

materialunderlying

theriverbed,

anddetennination

oftherequired

streampower

bym

akinguse

oftheerosion

thresholdline

definedby

theErodibility

IndexM

ethod.In

additionitalso

requiresquantification

ofthestream

powerthat

isavailable

tocause

scour.This

isdeterm

inedby

making

useof

dimensionless

graphsthatdefine

thechange

instream

powerasafim

ctionofscourhole

development(Sm

ith,etal.1997).

Acase

studythatillustrates

applicationofthe

method

tocalculate

scouraroundbridge

piersis

presented.

RE

FER

EN

CE

S

1.Anglio,C.D.,N

aim,R

.B.,Comett,A.M

.,Dtmaszegi,L.,Tum

ham,J.and

Annandale,G.W

.,1996,

BridgePier

ScourAssessm

entfor

theN

orthmnberland

StraitCrossing,

CoastalEngineering

1996,Proceedingsofthe

Twenty-fifthInternationalConference

heldin

Orlando,

Florida,September2-6,1996,B

illyL.Edge,Editor.

2.Annandale,G.W

.,Wittler,R.J.,R

uff,J.F.andLew

is,T.M.,1998,Prototype

Validationof

ErodibilityIndex

forScourin

FracturedR

ockM

edia,Proceedings

oftheW

aterResources

Conference,American

SocietyofC

ivilEngineers,Mem

phis,Tennessee,August.

3.Annandale,

G.W

.,1995,

“Erodibility.”

JournalofHydraulic

Research,Vol.33,N

o.4,

pp.471-494.

4.Briaud,J-L,Ting,F.C

.K.,Chen,H.C

.,Gudavalli,R.Perugu,S.and

Wei,G.,1999,SRICO

S:Prediction

ofScour

Ratein

CohesiveSoils

atBridge

Piers,Jom

nalofG

eotechnicaland

Geoenvirom

nentalEngineering,American

SocietyofC

ivilEngineers,Vol.125,N

o.4.

5.Cohen,E.and

VonThun,I.L.,

1994,DamSafety

Assessmentofthe

ErosionPotentialofthe

ServiceSpillw

ayatBartlettDam

,Proc.InternationalCom

mission

onLarge

Dams,Durban,

SouthA

fiica,pp.1365-1378.

6.Federal

Highw

ayAdm

inistration,1995,

EvaluatingScour

atBridges,

ThirdEdition,

Hydraulic

EngineeringC

ircular18

(HEC

-18),Publication

No.

FHW

A-IP-90-017,US

DepartmentofTransportation,400

SeventhStreet,W

ashingtonD

.C.20590.

7.H

julstrom,

F.,1935,

TheM

orphologicalA

ctivityof

Rivers,Bulletin

ofthe

Geological

Institute,Uppsala,Vol.25,chapter3.

8.Kirsten,H

.A.D.,

1982,Aclassification

systemforexcavation

innaturalm

aterials,TheC

ivilEngineerin

SouthAfrica,July,pp.292-308.58

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Shields,A.,1936,Application

ofSimilarity

Principlesand

TurbulenceResearch

toBed-Load

Movem

ent(in

Gennan),

Mitteiltm

gender

Preuss.Versuchsanstalt

furW

asserbauund

Schiffbau,Berlin,No.26.

Smith,

S.P.,Annandale,G.W.,Johnson,P.A.,Jones,J.S.and

Um

brell,E.R.,1997,“Pier

ScourinResistantM

aterial:CurrentResearch

onErosive

Power”,Proceedings

ofManaging

Water:

Copingw

ithScarcity

andAbundance,27“

Congressofthe

IntemationalAssociation

ofHydraulic

Research,SanFrancisco,C

alifornia,pp.160-165.

U.S.NaturalResources

ConservationService

(NRCS),1997,Fieldprocedures

guidefor

theheadcut

erodibilityindex,

Chapter52,

in"Part

628:Dam

s"ofthe

National

EngineeringHandbook.W

ashingtonD

.C.

:U.S.DepartmentofAgriculture.

Wittler,R.J.,Annandale,G

.W.,Abt,S.R.,R

uff,J.F.,1998,“New

TechnologyforEstim

atingPlunge

Poolor

Spillway

Scour.”Proceedings

ofthe

J998Annual

Conferenceof

theAssociation

ofStateDam

SafetyO

flfcials.October11-14,Las

Vegas,NV.

Yang,C.

T.,1973,

IncipientM

otionand

Sediment

Transport,Journal

ofthe

Hydraulics

Division,ASCE,Vol.99,N

o.HY10,Proceedings

Paper10067,pp.1679-1704.

59

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Page 62: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

0.3472

d_

iii}?r50

_iii-_

-(S.—

)8An

alternativeequation

wasprovided

byFarraday

andCharlton

(1983)w

hoconsidered

thesizing

ofriprapforthe

generalwaterenvirom

nentwith

anadditionalcoefficientto

accountforflow

changesin

certainsituations.

Theyproduced

theequation

@=c"1-"rt

(4)y.

whereC

*is

acoefficientdeterm

inedfrom

laboratoryand

field-testing,yo

isthe

averageflow

depth,andFris

theflow

Froudem

unber.Atbridge

piers,theysuggested

usinga

safetyfactorof

2and

acorresponding

C*

valueof0.28.

Theform

ofequation(4),a

Froudenm

nbermultiplied

bya

coefficient,is

ausefulway

ofexpressingm

anyriprap

equations.This

method

hasan

advantageoverthe

stabilitym

unberformulae

asthe

riprapsize

canbe

calculateddirectly

forthegiven

flowconditions.

Altematively,

small-scale

experimentalresults

havealso

beenused

todevelop

stonesize

criteriaspecific

tobridge-pierriprap.

Lauchlan(1999)

providesa

summ

aryofa

largenm

nberofpier

riprapsize

predictionequations.

Herew

ew

illdiscuss

anm

nberofthe

equationsand

compare

theirresults.

Breusersetal.(1977)provide

riprap-sizingcriteria

basedon

previouspierscourexperim

entsby

Carstens(1966),

Hancu(1971)

andN

icolletand

Ramette

(1971).It

wasdeterm

inedthatfor

givensedim

ent,scouring

beginsathalfthe

criticalvelocity,irrespective

ofthepier

diameter.

Breusersetal.(1977)suggestthatthe

riprapshould

thereforebe

sizedso

thatthecriticalvelocity

ofthestones

istw

icethe

maxim

mn

mean

velocityofthe

flow.The

resultingequation

isprovided

below,whereUmax

isthe

maxim

umm

eanflow

velocity.Theform

ulais

basedon

Isbash(1935)

usingE

=0.85.

2

drso=

2.83—

(]l"“‘——

(5)(5.-08

TheIsbash

(1935)equation

wasalso

rearrangedby

Richardsonand

Davis

(1995)to

giveequation

(6),whereU

=m

eanflow

velocity.The

‘K’factorisintroduced

toaccountforvelocity

changesassociated

with

differentpiershapes(Table

1).d

_0.692(11<r/)2(6)

r50(-51--

lieQ

uaziand

Peterson(1974)

conductedan

earlyexperim

entalriprapstudy.

Theyundertook

aseries

ofsmall-scale

experiments

with

ariprap

layerplacedaround

around-nosed

pier,andlying

flushw

iththe

bed.They

developedthe

following

relation.d

-0.20

NS,=1.14[-is-°-J(7)

Alsobased

onexperim

entalresultsis

theequation

ofCroad(1997),w

hichis

inthe

same

formas

(7),butwith

theaddition

ofapiershapefactor,A.ll6

NS,=2.1A[-ii](8)

dr5

0

62

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Page 67: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

0.035,while

Breusersand

Raudkivi(1991)

give0.009.

Inorderto

chosethe

mostappropriate

equation,thereasons

forthesedifferences

mustbe

assessed.

Breusersetal.

(1977)produces

asignificantly

largerriprap

sizethan

therem

ainingequations.

Forthisequation

thenum

ericalvalueofthe

coefficientisgreaterthan

2.The

otherequations

havecoefficients

lessthan

1.

Conversely,equationsbased

onthreshold

ofmotion

typecriteria

tendto

leadto

ratherlow

d,5Ovalues.

Forexample,Breusers

andR

audkivi(1991),Croad(1997),and

Chiew

(1995)forb/dr50

=4,produce

verysm

allriprapsizes.

Thereason

forthis

may

bethatthe

thresholdofm

otionapproach

toriprap

stabilityis

notonlyaffected

bythe

choiceofcriticalcondition,w

hichvaries

substantiallyw

iththe

exposureof

stones,but

alsoby

theshape

ofthe

stones.The

dragcoefficientvaries

with

theshape

androughness

ofthestones

andthe

asymm

etricshape

ofrocksalso

introducesan

unknown

liftforceeffect,R

audkivi(1990).Therefore

anyequation

basedon

thiscriterion

shouldalso

beable

toadaptto

changingem

bedmentlevels

andstone

shapefactors.

Given

thelack

ofconsistencyam

ongstthem

ethods,itisprudentto

selectam

ethodthatleads

toconservatively

largeriprap

relativeto

theotherrem

ainingm

ethods.O

nthis

basis,them

ethodsofRichardson

andD

avis(1995)

andLauchlan

(1999)are

recomm

endedfor

selectingsuitable

riprapfor

bridgepier

protection,M

elvillea.nd

Coleman

(2000).These

methods

wereused

toassess

riprapsize

requirements

forthe

HuttEstuary

Bridge(M

elvilleand

Lauchlan,1998)

andprovided

goodagreem

entwith

modelstudy

results(Lauchlan,M

elvilleand

Coleman,2000).

Inorderto

improve

confidencein

theuse

ofriprapsize

predictionform

ulaeit

isnecessary

tocom

parethese

equationsto

fieldresults.

Also,additional

laboratoryw

orkcould

includeassessing

ripraplayerperform

anceunderunsteady

flowconditions.

CO

NC

LUSIO

NS

1.Riprap

failurem

echanisms

areaffected

byriprap

size.Shearfailure

canbe

eliminated

bycorrectly

sizedstones.

2.There

isa

lackof

consistencyam

ongstexisting

riprapsize

predictionequations,

anda

conservativeapproach

following

them

ethodof

Richardsonand

Davis

(1995)or

thatof

Lauchlan(1999)is

recomm

endedas

appropriateunderm

ostsituations.3.

Furtherlaboratory

andfield

studiesare

requiredto

improve

confidencein

theprediction

methods.

L1X

.1‘la

(>

7!

{Will

tr/I!I

KI13‘.

1'{,3

K.,

r3:1

f

REFERENCES"

1

,11.

Breusers,H.

N.

C.,and

Nicollet,

G.,and

Shen,H.

W.

(1977).“Local

ScourAround

CylindricalBridge

Piers.”JournalofHydraulic

Research,15(3),211-250.2.

Breusers,H.

N.

C.,andR

audkivi,A.

J.(1991).

Scouring,A.

A.Balkem

a,Rotterdam

;Brookfield.

3.Carstens,M

.R.(1966).“Sim

ilarityLaws

forLocalisedScour.”Journalofthe

Hydraulics

Division,92(H

Y3),13-36.

67

Page 68: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Chiew,Y.

M.

(1995).“M

echanicsof

RiprapFailure

atBridge

Piers.”Journal

ofH

ydraulicEngineering,121(9),635-643.

Croad,R.

N.

(1997).“Protection

fromScour

ofBridgePiers

Using

Riprap.”

No.PR3-007],W

orksConsultancy

ServicesLtd,CentralLaboratories,Low

erHutt.Ettem

a,R.(1980).“ScouratBridgePiers,”PhD,The

University

ofAuckland,Auckland,N

ewZealand.

Farraday,R.

V.,and

Charlton,F.

G.(1983).

“BankProtection

andR

iverTraining.”

Hydraulic

Factorsin

BridgeDesign,H

RW

allingford,Wallingford,55-65.

Hancu,S.

(1971)“Sur

lecalculdes

affouillements

locauxdam

sla

zonedes

pilesdes

ponts.”14thInternationalAssociation

ofHydraulic

ResearchCongress,Paris,France.

Isbash,S.

V.(1935).

“Construction

ofDam

sby

Dum

pingStones

inFlow

ingW

ater(Translated

byA.D

orijkow).”,U.S

Arm

yEngineer,Eastport.

Lauchlan,C.S(1999).“PierScourCounterm

easures,”PhD,TheU

niversityofAuckland,

Auckland.Lauchlan,

C.S.,M

elville,B.W

.,and

Coleman,

S.E.(2000).

"Protectionof

theH

uttEstuary

BridgeAgainstLocalScour."InternationalSym

posirunon

ScourofFoundations,M

elbourne,Australia,November,2000

Melville,B.W

.,andColem

an,S.E.(2000).BridgeScour,W

aterResourcesPublications,

Colorado,USA:550pp

Melville,B.W

.,andLauchlan,C.S.

(1998)"H

uttEstuaryBridge

PhysicalModelStudy

ofScouratBridgePiers."Auckland

UniservicesLim

ited,AucklandN

icollet,G

.,and

Ramette,

M.

(1971)“Affouillem

entsau

voisinagede

pilesdes

pontcylindriques

oirculaires.”14th

InternationalAssociation

ofH

ydraulicResearch

Congress,Paris,France.Parola,A.

C.(1995).

“BoundaryStress

andStability

ofRiprapatBridge

Piers.”R

iver,Coastaland

ShorelineProtection:

ErosionC

ontrolusingR

iprapand

Armourstone,C.R.

Thorne,S.

R.Abt,

F.B.

J.Barends,

S.T.

Maynord,

andK.

W.

Pilarczyk,eds.,

JohnW

iley&

Sons,149-159.Q

uazi,M.

E.,andPeterson,A.

W.

(1974).“A

Method

forBridge

PierRiprapD

esign.”FirstCanadian

Hydraulics

Conference,Edmonton,Canad,96-106.

Raudkivi,A.J.(1990).Loose

BoundaryH

ydraulics,Pergamon

Press,Oxford.

Richardson,E.V.,andDavis,S.R.(1995).“Evaluating

ScouratBridges.”FHW

A-IP-90-0]7,Fairbank

TurnerHighw

ayResearch

Centre,McLean,Virginia.

68

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Page 71: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

channelthalweg,bend

scour,bed

fonns,and

confluencescour.

Theresulting

flowdepth

forgeneraland

contractionscourys

iscom

binedw

iththe

localscourdepthata

foundationds

togive

thetotalscourdepth

atthefotuidation

(Figurel),nam

ely

J*s)r0z‘aZ=

J/s+ds

(1)

Asum

mary

ofexpressionsforthe

determination

ofthevarious

scour-depthcom

ponentsatbridge

abutments

isgiven

below,wheresym

boldefinitionsare

givenatthe

endofthe

paper.Additional

expressionsfor

localpierscour

andlateralerosion

aregiven

inM

elvillea11d

Coleman

(2000).Use

ofanyscour

formulae

must6I1SUI6

thattheexpressions

arerelevantto

thecharacteristics

(flows,cham

ielparameters,

andsedim

ents)ofthe

siteunder

investigation.The

limits

ofuse,assum

ptions,andinadequacies

oftheform

ulaeshould

alsobe

establishedbefore

theform

ulaeare

applied.Exam

plesofapplication

ofthem

ethodologyofFigure

lare

givenin

Coleman

andM

elville(2000),Colem

anetal.(2000),and

Melville

andColem

an(2000).

GE

NE

RA

LE

QU

AT

ION

S

Generalequations

are:A

=W

yforarectangularchannel

(2)R

=W

y/[W+2y]

forarectangularCl18.I1I16l(3)

V=

Q/A

(4)

q=Q/W(5)

u*c=

[¢9c(SS-l)gd5g]°'5where

66can

beobtained

fromShields

diagram(6)

V,=5.75u.c

log

[5.5

3forfully

turbulentflowand

abedroughness

ofk=

2d50(7)

50

Thevariable

yin

theabove

equationsis

appropriateto

thesituation

beingconsidered,

thatis(Figure

1),forcalculationofym

Sadopty

Eyu;for(ym

S)cadopty

Eymsg

foryrsorybs

adoptyE

(ymslc;foryes

adoptJ’Eym

s;andfords

adoptJ’Eys-

GE

NE

RA

LD

EG

RA

DA

TIO

N

Useofa

rangeofthe

following

fourmethods,com

binedw

ithfield

andsubsurface

observations,togetherw

ithengineering

judgement,typically

providesthe

bestapproachto

initialquantitativeevaluation

ofmean

scouredflow

depthym

sresulting

fromdegradation

atasite.

Lacey(1930)

1/3yms

=0.47[-gj

wheref=

l.76dm0.5

(8)

Thisapproach

isindicated

tobe

tooconservative

forlargesedim

ent.The

relationforf

appliesfordm

<1.3

mm

.

71

Page 72: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Blench(1969)2/3

yms111201

forsandsof0.06

<djg

(mm

)52.0

(9)50

:2/3

yms=1.231

forgravelsofSS

w2.65

andd50

>2.0

nun(10)

_50

Maza

Alvarezand

Echavarria

Alfaro

(1973)0.784

Qym,=0.365[-__-—

(11)W

0.784d26l57

Thism

ethodis

validonly

forsedim

entsofd75

<6

mm

,principally

sandsand

gravels,w

ithpredictions

beingnoted

todiffer

toobservations

forfiner

materials.

Fora

narrowriver,

thechannelhydraulic

mean

radiusis

adoptedin

lieuofchannelw

idthW

.W

atson(1990)

reportsextensive

useofthis

method

forgravel-bedrivers

inN

ewZealand.

Holm

es(1974)

Theauthorindicates

totalscourtobe

thesum

ofysand

localscour,where

,,V

Kys

1sthegreaterof

ys=

yuor

ys=

—l—

‘——

(12)x/(A

/W1

2/3

with

K-

W0959

3l,

V1=C[—

‘Q-1—J)“——]

andC

=1.2

whereconverging

flows

are4.83Q

'A

AlW

encountered,suchas

inbraided

streams,and

1.0in

othercases.

Thism

ethod,whereys

incorporatesdegradation

andcontraction

scoureffects(and

alsopossibly

thalweg,bendscour,confluence

scourand

b6(1-fOI'I11effects),

isbased

onfield

datacovering

aw

iderange

ofsedimentsizes

collectedin

New

Zealandforscourfailures

atanum

berofrailway

bridges.The

method

incorporatesno

safetyfactorow

ingto

theuse

ofconservativedesign

flows

inanalyses.

Watson

(1990)reports

onconservative

predictionsof

scourfor

deepincised

channelsin

gravel-bedrivers,

especiallywhen

additionallyincorporating

asafety

factorw

ithinthe

analyses.

AV

ER

AG

EC

ON

TR

AC

TIO

NS

CO

UR

Forlive-bed

conditions(V/Vc

21)

inthe

(degraded)approach

channelofym

s,the

averagescoured

flowdepth

fora

contractedsection

(ymS)c

canbe

estimated

basedon

Richardsonand

Davis(1995)(m

odifiedfrom

Laursen,1960),namely

72

Page 73: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

6/11:,

=(1,,

ym

sQ

lmW

2

Thelong

rectangularcontraction

basisofthis

approachm

ayresultin

conservativepredictions.

Valuesofthe

exponentk]aregiven

inTable

1.

Table1.

ValuesofContraction

ScourCoefficientk]

<0.501

0.59\

Mostly

contactbedm

aterial8

0.50-2.030.64

lSom

esuspended

bedm

aterialdischarge

Forclear-waterconditions(V/Vc

<1)in

theapproach

channel,(ymS)c

canbe

estimated

basedon

competentvelocity

beingachieved

throughthe

bridgesite,nam

ely

O)m

S)c

=W

he

reV

:V

c:

Eachof(13)

and(14)

assumes

arectangular

channelsection.Analyses

forthalweg

andbend

effectscan

besubsequently

adoptedto

incorporateallowances

forvariations

inflow

depthsacross

thebridge

section.

BEND

SCO

UR

Them

aximtun

scouredflow

depthin

abendybscan

beevaluated

using(M

aynord,1996)

y,__,=1~i;(y,,,,),{1.s-0.051(r,/W)+0.00s4[W

/(y,,),]}(15)

wherea

conservativesafety

factorofFS

=1.19

isadopted

herein.This

method

isvalid

forW

/(ymS)c

<125

andrc/W

<10,rc/W

=1.5

beingadopted

forrc/W

<1.5,and

W/(ym

S)c=

20being

adoptedfor

W/(ym

S)c<

20.The

expressionofThom

e(1988)can

alsobe

adopted,namely

J»...=6...).{Z-07-0-191nl(r./W)—all<16)

wherethis

method

isvalid

forrc/W>

2.Equations

(15)and(16)are

recomm

endedto

belim

itedto

flows

ofoverbankdepths

upstreamofthe

bendofless

than20%

ofthem

ainchanneldepth.

Inlieu

ofadopting(15)

or(16),itcanbe

assumed

thatthescoured

areabelow

theflood

level,of

averagedepth

(ymS)c,

canbe

redistributedin

asim

pletriangular

form(N

eill,1973)

togive

apeak

flowdepth

inthe

bendof

ybs=2(J’ms)c

(17)

73

Page 74: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Alternatively,thescoured

areabelow

theoriginal(upstream

,unscoured)bed

level,ofaverage

depth[(ym

S)c-yu],can

besim

ilarlyredistributed

togive

ybs=

J/u+

210/ms)c

'J/ul(18)

Itisrecom

mended

thattheabove

methods

forassessmentofbend

scourthatareappropriate

toa

givenbridge

sitebe

usedtogetherto

determine

anappropriate

valueofyg,Sforthe

site.

THA

LWE

GEFFEC

TS

Theflow

depthin

thethalweg

yrscan

beestim

atedfora

straightchannelasthem

aximtun

of

y,,=1.27(y,,,),(Lacey,1930)

ory,=(ym),+(h/2)

(19)

wheream

plitudeh

isthe

maxim

umofthe

thalwegam

plitudeorthe

heightofaltemate-bars

inthe

channel.Expressions

enablingcalculation

ofthalwegam

plitudeand

altemate-bar

heightare

givenin

Melville

andColem

an(2000).

Alternativegraphicalm

ethodologiesfor

redistributingaverage

scouracross

across-section

toallow

forthalweg

effectsare

asused

forbend

scourestim

ation,nam

ely(17)

and(18)

above.In

addition,y;S

canbe

estimated

byscaling

thecalculated

averagescoured

flowdepth

(ymS)Cby

theratio

ofmaxim

umto

mean

flowdepths

forthe

unscouredchannel(M

azaAlvarez

andEchavarria

Alfaro,1973).

CO

NFLU

EN

CE

SC

OU

R

Them

aximum

flowdepth

ina

confluenceyc-S

canbe

calculatedfor

differentsedim

entclassesusing

theexpressions

(Ashmore

andParker,1983;and

Klaassenand

Vermeer,1988)

¢J_/9-=2.24

+0.031a

fornoncohesivesands

andgravels

anda

=30°to

90°(20)

ym

s

%5“—=

1.01+

0.0300:forcohesive

material

(21)yfllS

¥-=1.29+0.03701for0.6<Q,/Q;<1anduniform

sandof0.1s<d50(n1rn)<0.25(22)

yms

where37,",is

theaverage

flowdepth

inthe

degradedanabranches

approachingthe

confluence.In

regardto

(20),poorly-sortedbed

materialis

notedto

havelesserconfluence

scourdepthsthan

well-sortedm

aterialofthesam

em

eansize.

74

Page 75: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

BE

D-F

OR

ME

FFEC

TS

Thepeak

scouredflow

depthow

ingto

them

igrationofbed

forms

throughthe

bridgesite

canbe

estimated

as

y.=JG.+(/*1/2)or

1/.=yr.+(/1/2)(23)

whereh

isthe

maxim

umbed-form

heightfortheexpected

typesofbed

fonnsoverthe

rangeof

flows

occurringatthe

bridgesite.

Thisapproach

ism

oreapplicable

tothe

migration

ofC1L1I‘l6S,bars

orantidunes,

ripplesbeing

typicallysufficiently

small

asto

beinsignificant

inaffecting

scourmagnitudes.

Means

ofdetermining

thetypes

ofbedform

soccurring

forarangeofflow

sin

ariver,

andm

eansofevaluating

theheights

ofthesebed

forms,

arediscussed

inM

elvilleand

Coleman

(2000).

LOC

AL

AB

UT

ME

NT

SC

OU

R

Localabutrnent-scour

depthds

belowthe

surroundingbed

levelis

calculatedbased

onthe

analysesofM

elville(1997)and

Melville

andColem

an(2000),nam

ely

as=1<,,K,K,,1<_;K;1<GK,(24)

wherethe

factorsof(24)

aredefined

inTable

2and

Figure2.

Forthecalculations

ofTable2,

d50aE

0150andVa

EVcforLm

iformsedim

ents.

Table2.

FactorsInfluencing

LocalAbutment-scourDepth

Factor,1

KM

ethodofestim

ation‘

L‘

KyL

=2L

——<1

Flowdepth-

itys

8.l)L1lII'1'l6I1'[.

1,K

-YL

:‘2,

]_<

—gI—

<

sizeJ/S

Ky,=10)»,§>25

\lFor.Luiifonn

sediments:d50a

Edjg

land’VaE-VC'

I-

JFornonuniformsedim

ents:Flow

_I

am,=am,/1.8s»a34/1.8=O'gd50/1.8;and

““"’“S“Y‘K1

-,l’_qf..Q-§Yc.¢1z_‘i‘f1?E1j?_Yea.i§E?l9}!l?F?§f9F.f1§Qa.H§lP.%_@.?;‘}[email protected]_V-0’.—V.)

VG

7K,=1.0for[V-(Va-V6-)]/Vc21for[V-(Va-Vc)]/Vc<1

75

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Page 78: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

NO

TA

TIO

N

AAcCdmdmax

dnds450d50aFsF

rf8hKKdKGK1K

S»

KS

*

KrK},-LK

0,Ke*kklLL

*

72n*

QQ1QsQ]m

Q2QRFcSe5'0SSI14*

in

-1

-

1*

—

flowarea;

criticalflowarea

forsedimententraim

nent;convergence

coefficientforHohnes(1974);

effectivem

eandiam

eterofbedm

aterial;m

aximum

particlesize;

sedimentsize

forwhich

12%ofthe

sedimentis

fmer;

localscourdepthbelow

thesurrorm

dingbed

level;m

ediansize

ofbedm

aterial(byw

eight);m

edianparticle

sizeofarm

ourlayer;safety

factor;Froude

Ntunber;

Laceysiltfactor;

accelerationofgravity;

amplitude

(cresttotrough),bed-fonn

height;coefficient,factor;sedim

ent-sizefactor;

approach-channel-geometry

factor;flow

-intensityfactor;

foundation-shapefactor;

time

factor;flow

depth-abutrnentsizefactor;

fotmdation-aligm

nentfactor;bed

roughness;contraction-scourcoefficient;abutm

entlength(including

bridgeapproach)m

easuredperpendicularto

thechannelcentreline

(Figure2);

width

offloodplain

(Figure2);

Marm

ingroughness

coefficientforthem

ainchannelofa

compound

channel;M

anningroughness

coefficientfortheflood

plainofa

compound

channel;flow

rate,mean

discharge,designdischarge;

largeranabranchflow

rate;sedim

enttransportrate,smalleranabranch

flow;

flowrate

inthe

approachm

ainchannel(notflood

plains)transportingsedim

ent;totalflow

ratethrough

thebridge

(contracted)section;flow

rateperunitchannelw

idth,q=

Q/W

;channelhydraulic

radius;centreline

radiusofbend

curvature;energy

slope,streamslope;

channelslope;specific

gravityofsedim

entparticles,SS=

ps/p;flood

peakduration;

bedshearvelocity;

78

Page 79: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 80: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

TH

EE

FA,E

RO

SIO

NFU

NC

TION

AP

PA

RA

TUS

:A

NO

VE

RV

IEW

BYJean-Louis

Briaudl,H.-C.Chenz,FrancisTing?’

AB

STR

AC

T

Anew

apparatusis

describedto

measure

theerosion

functionofa

soil.The

apparatusis

calledthe

EPA(patent

pending)or

ErosionFunction

Apparatus(http://tti.tam

u.edu/geotech/scour).The

erosionfunction

isthe

relationshipbetween

thehydraulic

shearstressapplied

atthesoil-w

aterinterfaceby

thewaterflow

ingoverthe

soiland

theerosion

rateofthe

soil.This

erosionfunction

canthen

beused

topredictscourof

soilbywater.

1Spencer

J.BuchananProfessor,

Dept.ofC

ivilEngineering,

TexasA

&M

University,

CollegeStation,

Texas77843-3136,U

SA,([email protected])

2AssociateProfessor,Dept.ofC

ivilEngineering,TexasA

&M

University,College

Station,Texas77843-

3136,USA.

3AssociateProfessor,Dept.ofC

ivil&Envirom

nentalEngineering,SouthDakota

StateU

niversity,Brookings,South

Dakota57007,U

SA.

80

Page 81: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 82: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Water

FlV1

’Owit

1mm

0~

-III

'----.o'.:',".°

».-I‘

,'

-.'-“.1

,'.n

-".I

I.0.-I,-0-_.c'.

.=

'uIa|-".

‘.0.’

u-:¢

..._'-::--

¢'u‘

-'.

0'

.I.I'..

I‘-

n'

Ic-'._

".

-.-.

.1‘I-1

'0.

‘.0’.-0

'-T-

Q...

..

I_.q

.-0

.'

,1»

,":0

--.-:.'.T.

ll-_

'-..-Q

.--

I.

soH-.'n

:.'_',_

¢_-

'-n

'Q

'-I

1‘

PistonPushing

A.

attheR

ate=

Z

2(m

m/hr)

TC1:(N/m2)

Fig.1

-Schematic

Diagram

andResultofthe

EFA(Erosion

FunctionApparatus)

82

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Page 88: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 89: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
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Page 91: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

I1....-

-*

!_.

-.,

lV.

LILC

:é.-\,-._,

rlf»:'-._

{I:.»Ir

[J'.l-

-iF.

r,.-\"/__(_

amountofsam

plem

aterialandbe

abletd

utilizerock

coresthatare

routinelycollected

aspartofbridge

pierdesignand

construction./

0The

laboratory-testingdevice

mustbe

ableto

createflow

ingwater

overthe

rocksam

ple.Specifically,

thedevice

mustbe

ableto

applya

hydraulicshear

stressto

therock

sample

surface.

0Along

thesam

elines

asdescribed

above,the

laboratory-testingdevice

must

beable

tom

easurethe

shearstressthatis

appliedto

thesam

plebeing

tested.

0The

testingdevice

mustbe

ableto

generateshearstresses

atlevelsexpected

indesign

stormflow

conditions.Therefore,

thelaboratory-testing

devicem

ustbeable

tooperate

atshearstresses

thatrangefrom

ambientto

beyonddesign

conditions.

IBased

oninfonnation

obtainedfrom

theliterature

review,

rockcan

behighly

resistanttoerosion.

Sincethe

erosionrates

arevery

smallas

compared

with

cohesionlessand

cohesivesedim

ents,thelaboratory-testing

devicem

ustbeable

toaccurately

measure

smallam

ountsof

lostmaterialw

hilecontinuously

operatingfordays.

Basedon

theabove-described

criteria,the

rotatingcylinder

erosiontesting

apparatuswas

selected.This

typeofdevice

hasbeen

usedby

severalresearchersto

determine

criticalstressesand

ratesoferosion

ofcohesivesedim

ents.A

descriptionofthis

device,which

issim

ilartothe

Couetteviscom

eter,isgiven

below.

PR

EV

IOU

SU

SE

OF

RO

TA

TIN

GC

YLIN

DE

RA

PP

AR

ATU

S

Moore

andM

asch(1962)

appliedthe

rotatingcylinderprinciple

usedin

viscometers

tom

easurethe

scourresistanceofcohesive

soils.The

devicewas

calledthe

rotatingcylinder

erosiontest

apparatus.A

cylindricalsam

pleofcohesive

sedimentwas

suspendedinside

alarger

circularcylinder.

Theouter

cylinderis

freeto

rotateabout

itsaxis.

Theannular

gapbetween

thecylinder

a.rrdsam

plewas

filledw

ithfluid.

Asthe

outercylinder

isrotated,

mom

entumis

imparted

tothe

fluidand

thefluid

moves,im

partinga

shearstressto

theface

ofthesam

ple.Thecohesive

soilsam

pleis

stationarybut

mounted

onflexure

pivotsso

thatthe

shearstress

transmitted

tothe

sample

surfaceresulted

ina

slightrotation

ofthe

supportingtube.

Theresulting

rotationwas

calibratedto

measure

thetorque

onthe

sample

fromw

hichthe

shearstresscould

becom

puted(M

ooreand

Masch,1962,p.

1444).

Asa

shearstresswas

appliedto

thesam

ple,materialwas

erodedfrom

theface

ofthesam

ple.The

amountofm

aterialerodedwas

measured

andthe

durationthatthe

shearstresswas

appliedwas

alsorecorded.

Fromthis

information,the

averagerate

oferosioncould

becom

putedfor

agiven

appliedshearstress.

Severalresearchersincluding

Rektorik

etal.(1964),Arulanandanetal.(1973),Sargunarn

etal.(1973),

Alizadeh(1974)

andChapius

andG

atien(1986)

haveused

similar

devicesw

ithim

provements

andenhancem

ents.A

kkyand

Shen(1973)

usedthe

rotatingcylinder

apparatus

91

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Page 93: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

0a

smallsam

pleofrock

canbe

usedin

thistype

ofdeviceasthe

outercylindercanbe

sizedto

accomm

odatethe

sizeofstandard

rockcores,

0aflow

ingwatergenerated

shearstresscan

beapplied

tothe

sample,

0the

averageshearstress

onthe

sample

canbe

measured

bym

easruingthe

torquethatis

beingapplied

tothe

sample,

0sm

allquantitiesofm

aterialbeingeroded

canbe

measured

usingprecision

balances,and

0the

apparatuscan

beoperated

forlongperiods

oftime

asthe

outercylindercanbe

drivenby

acontinuousduty

motor.

Itis

alsoim

portant,however,

todiscuss

thelim

itationsof

thistype

oftesting

deviceto

understandwhere

uncertaintyand

biasm

aybe

presentin

theresults.

Theshear

stressis

computed

bym

easuringthe

torqueon

thesam

ple.H

owever,the

torquebeing

measured

isthe

torquebeing

appliedto

theentire

sample.

Therefore,thecalculation

oftheshearstress

resultsin

theaverage

shearstressoverthe

entiresam

plesurface.

Theresults

fromthe

experiments

asstunethatthe

shearstressis

turifonnacrossthe

entiresurface

ofthesam

ple.In

actuality,thesurface

ofrock

samples

canbe

pittedand

uneven.Therefore,there

may

bevariations

inthe

shearstress

distributionoverthe

faceand

thuslocalshearstresses

arelikely

tobe

greaterthattheaveraged

valuecom

putedfrom

them

omenton

thesam

ple.

Insum

mary

theshearstress

computed

fromthe

measured

torquem

aybe

biasedin

thedirection

ofunderestimating

theshearstress

actingon

thesam

ple.Inaddition

tothe

variationsin

theshear

stressover

thesam

plesurface

therem

ayalso

becom

ponentsofthe

flowacting

indirections

otherthanthe

directionin

which

thetorque

isbeing

measured.

Thus,therem

aybe

acom

ponentofshear

stressthatis

erodingthe

surfaceofthe

sample

thatisnotbeing

accountedfor

inthe

measurem

ents.In

theapplication

oftheseresults,

therm

derestimation

ofshear

stressw

ouldprovide

conservativeestim

atesofthe

ratesoferosion

versusshear

stress.That

is,theresults

would

showgreater

erosionrates

fora

givenshear

stress.The

conservativenature

oftheseresults

would

beappropriate

fordesignapplications.

RO

TA

TIN

GC

YLIN

DE

RA

PP

AR

ATU

S

Therotating

cylindertestingapparatus

usedin

thisstudy

wassim

ilartothe

devicespreviously

used;however,some

modifications

havebeen

made.

Figures1

and2

areschem

aticdrawings

ofthe

rotatingdevice

andFigure

3is

aphotograph

oftheactualdevice.

EnglishLmits

areshown

forequipm

entdim

ensionsas

theywere

usedby

manufacturers

tospecify

equipment

sizes.The

metric

equivalentswere

alsoprovided.

Them

ajorcom

ponentsofthe

apparatusconsistofthe

following:

0Bodine

l/8-hpFram

e42A

motor

(2500R

PMat

50in-oz

[353m

m-N

]of

torque)w

ithcontroller,

93

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Page 97: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

cylinder.Enough

wateris

addedin

therotating

cylinderat

eachR

PMtested

tocover

theunderside

ofthebottom

plateonly.

Thetorque

ateachRPM

testedwas

recordedand

aplotwas

developed.As

will

bediscussed

inthe

experimentalprocedures

section,the

torqueon

thebottom

platefora

givenR

PMcan

beobtained

fromthe

plot.This

torqueis

subtractedfrom

thetotaltorque

reading.Itshould

benoted

thatduringa

particularerosiontest,only

enoughw

aterisadded

tothe

cylinderarm

ulusto

wetthe

sidesand

nottheend

plateon

thetop

ofthesam

ple.Therefore,only

theend

effectsfrom

thebottom

plateneeded

tobe

considered.

EX

PE

RIM

EN

TA

LS

AM

PLE

PR

EP

AR

AT

ION

Samples

thatweretested

inthe

apparatuswere

collectedfrom

rockcores

obtainedby

theFD

OT.

Thesam

plewas

fonnedby

drillinga

horizontalsolid

cylinderthrough

avertical

core.The

rationaleforcollecting

asam

plefrom

theside

ofacore

wasbased

onthe

resultsofa

preliminary

experimentperform

edatthe

University

ofFlorida.A

sample

oflimestone

wascollected

froma

FDO

Tcore

andthen

cutintoa

cube.To

obtainqualitative

information

abouttheanisotropy

ofthese

samples,a

pressurewasherwas

directedateach

faceofthe

sample.

While

thisdoes

notsim

ulatefield

conditions(tangentialflow

overabed),itdidprovide

some

insightintothe

erosionproperties

ofthesam

ple.It

wasdiscovered

thattherewere

differencesin

therates

atwhich

variousfaces

eroded.These

differencesin

erosioncan

beattributed

tothe

non-homogeneity

andanisotropy

ofrocksam

ples.Itwas

concludedthatin

ordertom

ostaccuratelysim

ulatethe

fieldcondition,the

sample

facebeing

erodedshould

bein

thesam

eorientation

asin

thefield.

By

cuttinga

horizontalsolidcylinderfrom

thecore,the

erodingsurface

willbe

closertothe

fieldsituation.

Thesam

plesfor

erosiontesting

weretaken

from4-in

(10.16-cm)

nominal

diameter

corescollected

bythe

FDO

T.The

samples

werecored

fromthe

sidesusing

aconcrete

wetcorerw

itha

2-in(5.08-cm

)diam

etercore

bit.This

produceda

sample

of1.75-in

(4.45-cm)

indiam

eter.The

endsofthe

sample

wereleveled

with

aconcrete

wetsaw.

Thislefta

sample

with

alength

ofapproxim

ately3-in

(7.62-cm).

Ahole

mustbe

drilledin

thecenterofthe

rockm

aterialtoconnectthe

endplates

asw

ellasto

allowthe

sample

tobe

cormected

tothe

torquecell.

Inpreparing

thesam

ples,itwasdiscovered

thatduringcoring,the

samples

couldeasily

fracture.Tom

inimize

thefracturing,a

3/16-in(0.48-

cm)diam

eterholewas

drilledthrough

thecenter.

Thism

inimized

thedisturbance

tothe

sample

andkeptthe

sample

intact.

EX

PE

RIM

EN

TAL

PRO

CED

UR

E

Thefollow

ingisthe

procedureused

toconductatypicalerosion

test.

Sample

Preparation

l.Prepare

thesam

plefor

erosiontesting

asdescribed

inabove

byusing

aconcrete

wetcorer

andm

asomy

drillbit.2.

Recordthe

mass

ofthesam

plew

iththe

mass

balance.

97

Page 98: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

3.Place

thesam

plein

thedrying

ovenfor

atleast16hours

todry.

Afterthattime,record

them

assofthe

sample.

Thesam

pleis

considereddry

whenthe

mass

changeis

lessthan

0.1%in

aperiod

greaterthan1hour.

Recordthe

sample

drym

ass.4.

Measrue

thediam

eterofthesam

plew

itha

PiTapeata

minim

tunofthree

locationsw

iththe

calipersand

recordthe

averagediam

eterofthesam

ple.5.

Measure

thelength

ofthesam

plew

iththe

calipers.6.

Measure

thevolum

eofthe

sample

bygently

submerging

thesam

plein

agraduated

cylinderand

measure

thevolum

eofwaterdisplaced.

7.Com

putethe

sample

drydensity

fromthe

abovem

easurements

ing/cm

3.8.

Collectthe

waterand

loosem

aterialina

dryingdish.

Placethe

dryingdish

inthe

dryingoven

torem

ovethe

water.Record

them

assofrem

ainingm

aterial.9.

Com

pletelyim

merse

thesam

plein

waterfor

atleast16

hoursto

hydrate.The

sample

ishydrated

tosim

ulatea

saturatedrock

formation

asm

aybe

fotmd

ina

waterw

aybed.

After

thattime,record

them

assofthe

sample.The

sample

isconsidered

hydratedwhen

them

asschange

isless

than0.1%

inaperiod

greaterthan1hour.

TestingProcedure

1.Secure

sample

onthe

threadedrod

with

theplatens

andplace

thesam

plein

therotating

cylindererosion-testingdevice.

2.F

illtherotating

cylinderannulusw

ithwaterto

theproperlevel.

Itisim

portanttonote

thatwaterfrom

theactualfield

sitewhere

thesam

plewas

collectedshould

beused.

3.Place

therubberstopperon

theacrylic

cylinderandthen

attachsam

pleto

torquecell.

4.Setthe

offsetofthetorque

cellwith

thetare

switch

to0.000

mrn-N.

5.Turn

onthe

motorand

increasethe

RPM

(asm

easuredby

thetachom

eter)untilthe

desiredtorque

isachieved.

6.A

llowthe

testtorun

fora

minim

umof72

hours.Record

theduration

oftheexperim

entinm

inw

iththe

stopwatch.Periodically

adjustthem

otorspeedto

keepa

constanttorqueon

thesam

ple.Record

thetorque

inm

m-N

appliedto

thesam

ple.Trun

offthem

otorandallow

thewaterw

ithinthe

armulus

tocease

motion.

Remove

thesam

plefrom

thetorque

cellandcylinder.

Empty

thewateroutofthe

cylinderandclean

outtheeroded

particlesin

thecylinder.

10.Placethe

sample

inthe

dryingoven

foratleast16

hoursto

dry.Afterthattim

e,recordthe

mass

ofthesam

ple.The

sample

isconsidered

drywhen

them

asschange

isless

than0.1%

inaperiod

greaterthan1hour.

Recordthe

sample

drym

ass.

.‘°9°.\'

Thereare

afew

importantitem

sto

notew

ithregards

tothe

experimentalprocedures.First,prior

tobegim

ringthe

actualerosionexperim

ents,apreparation

rimis

required.The

preparation11.111is

requiredto

remove

loosem

aterialfromthe

surfaceofthe

rocksam

pleprior

tom

easuringthe

erosion.The

coringprocess

disturbsthe

surfaceofthe

sample

andthis

may

causean

excessiveam

ountofm

aterialtoerode

thatm

aynot

haveeroded

otherwise.The

preparation11.111

wasconducted

afterthesam

pledim

ensionswere

recordedbutpriorto

thefirstexperim

ent.

Also,at

times,

aslight

amount

ofm

aterialw

ouldbe

removed

fi'omthe

sample

diningthe

saturationprocess.

Thism

aterialwascollected

andweighed

(dryw

eight).This

valuewas

then

98

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Page 100: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 101: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

CanadianH

ydraulicsCenter,forproviding

information

ontheirrock

erosionexperim

ents.Judy

Mrtraniand

KenKerrassisted

with

theexperim

entalwork

andsam

plecollection.

REFER

ENC

ES

Akky,M

.R.,and

Shen,C.K.,1973.

“Erodibility

ofaCem

ent-StabilizedSandy

Soil.”Soil

Erosion:Causesand

Mechanism

s;Preventionand

Control,ConferenceW

orkshopon

SoilErosion,Highw

ayResearch

BoardSpecialReport135,W

ashington,DC

,pp.30-41.

Alizadeh,A.,1974.

“Amountand

TypeofC

layand

PoreFluid

Influenceson

theC

riticalShearStress

andSw

ellingofCohesive

Soils.”Ph.D.dissertation,U

niversityofC

alifornia,D

avis.

Armandale,G

.W.,

1995.“E

rodibility.”JournalofH

ydraulicResearch,Vol.33,N

o.4,pp.471-493.

Annandale,G

.W.,

Smith,

S.P.,Naim

,R.,

andJones,

J.S.,1996.

“ScourPow

er.”C

ivilEngineering,Am

ericanSociety

ofEngineers,July,pp.58-60.

Arulanarrdan,K.,Sargunam

,A.,Loganathan,

P.,andKrone,

R.B.,1973.

“Applicationof

Chemicaland

ElectricalParameters

toPrediction

ofErodibility.”

SoilErosion:Causesand

Mechanism

s;Prevention

andControl,

ConferenceW

orkshopon

SoilErosion,

Highw

ayResearch

BoardSpecialReport135,W

ashington,DC.

Chapius,R.,

andG

atien,T.,

1986.“A

nIm

provedRotating

Cylinder

Techniquefor

Quantitative

Measurem

entsof

theScour

Resistanceof

Clays.”

CanadianJournal

ofG

eotechnicalEngineering,Vol.23,pp.83-87.

Com

ett,A.,

Sigouin,N

.,and

Davies,M

.,1994.

“ErosiveResponse

ofNorthurnberlandStraitTilland

Sedimentary

Rockto

FluidFlow

.”N

ationalResearchC

ouncilofCanada,Institute

forMarine

Dynamics,TR-1994-22,Septem

ber,Ottawa,Canada,pp.1-15,26-27.

Gordon,

S.,1991.

“ScourabilityofR

ockForm

ations.”FederalH

ighway

Administration

Mem

orandum,July

19,Washington,DC.

Jurnikis,A.R.,

1983.R

ockM

echanics.2nd

ed.,TRAN

STEC

HPublications,

Clausthal-Zellerfeld,FederalR

epublicofG

ermany,pp.37-45,51-53.

Merrington,A.C

.,1949.Viscom

etly.Edward

Amold,London,England,p

30.

Moore,W

.L.and

Masch.F.D.,

1962.“Experim

entson

theScourResistance

ofCohesiveSedim

ents.”JournalofG

eophysicalResearch,Vol.67,No.4,April,pp.

1437-1446.

PSI—ProfessionalServices

IndustriesInc.,

1996.Prelim

inaryG

eotechnicalEngineeringStudy,U.S.441

BridgeO

verSantaFe

River,W

PINo.2110486,October10,pp.2-10.

101

Page 102: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Rektorik,R.J.,and

Smerdon,E.T.,

1964.“C

riticalShearStress

inCohesive

Soilsfrom

aR

otatingShear

StressApparatus.”

PaperNo.64-216,

American

SocietyofAgricultural

Engineers,June.

Richardson,E.V.

andDavis,

S.R.,1995.

“EvaluatingScour

atBridges.”

3rded.,

Hydraulic

EngineeringC

ircularN

o.18,

PublicationN

o.FH

WA-IP-90-017,

Office

ofTechnology

Applications,HTA-22,W

ashingtonDC.

Rohan,K.

andLefebvre,

G.,

1991.“H

ydrodynamic

Aspectsin

theRotating

Cylinder

ErosivityTest.”

GeotechnicalTesting

Jorunal,Vol.14,N

o.2,June,pp.166-170.

Sargunam,

A.,R

iley,P.,

Arulanandan,K.,

andKrone,

R.B.,1973.

“Effectof

Physicochemical

Factorson

theErosion

ofCohesive

Soils.”Journal

oftheH

ydraulicD

ivisions,Proceedings

oftheAm

ericanSociety

ofCivil

Engineers,Vol.

99,No.

HY3,

March,pp.555-558.

Smith,S.P.,1994.

“Preliminary

Procedureto

PredictBridgeScourin

Bedrock.”Colorado

DepartmentofTransportation,ReportNo.C

DO

T-R-SD

-94-14,Denver,CO,Decem

ber.

VanR

ijn,L.C.,

1993.Principles

ofSedimentTransportin

Rivers,Estuaries,andCoastal

Seas.Aqua

Publications,Amsterdam

,TheNetherlands,p.4.1.

102

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Page 104: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

resultsare

interpretedto

prototypesituations.U

nfortunatelylittle

isknow

naboutthese

scaleeffects

andnone

oftheseeffects

havebeen

studiedin

asystem

aticm

anner(Stunera.nd

Fredsoe,1999).In

contrasttothe

scaled-downlaboratory

tests,num

ericalm

odelsof

localscour

arotmd

pipelinesdo

notsuffer

fromscale

effects.O

ncethe

numerical

model

isdeveloped,itcan

beapplied

todifferentoperationalconditions

includingthose

cannot

beachieved

underlaboratory

conditions.M

anyissues

thatcould

notbeinvestigated

thoroughlyby

modeltests

canbe

examined

numerically.

Atypicalexam

pleofthis

isthe

scaleeffects.

Sinceit

isvery

difficultto

carryout

experiments

with

largem

odelpipes

underlaboratoryconditions,the

understandingto

thescale

effectsis

stilllimited.

Howeverthescale

effectscan

beeasily

investigatedusing

apropernum

ericalmodel.

Num

ericaltestson

thesam

escourprocess

canbe

rtmtm

derbothm

odelandprototype

conditions.Theindividualfactors

thatmay

affectthescourprocess

canbe

isolatedand

controlledeasily

byntunerical

model.

Inthat

sense,a

goodnum

ericalm

odelcan

certainlybe

complem

entaryto

modeltests

andcan

assistdesignengineers

toidentify

them

ostcrucialcasesforw

hichm

odeltestsm

aybe

run.Theultim

ategoalofnum

ericalm

odelsw

illbe

replacing(at

leastpartially)

thecostly

model

testsand

tobe

useddirectly

inthe

designofpipelines.

Developmentofnum

ericalmodels

forlocalscour

aroundpipelines

hasbeen

slow,

despiteoftheirrelative

significance.There

mainly

two

kindsofntunericalm

odelson

localscour

belowa

pipelinehave

beendeveloped:

simple

mathem

aticalmodels

andintegrated

mathem

aticalm

odels(Sum

erand

Fredsoe,1999).

Thesim

plem

odelconcem

sthe

scouraround

afixed

pipew

hilethe

integratedm

odelconsidersdynam

icinteractions

betweena

flexiblepipeline

andthe

resultingscour

process.M

ostofthe

models

reportedin

literatureso

fararesim

plem

odels.Theidea

oftheintegrated

model

howeveristo

predicttheentire

scourprocesssuch

asthe

occurrenceand

disappearanceof

scouralong

apipeline,

scouringand

backfillingbelow

thepipeline

dueto

thesagging

ofpipeline.It

isobvious

thatsuch

am

odelism

uchm

orecom

plexthan

thesim

plescour

modeland

needsto

bebased

onthe

developmentofthe

simple

model.

Dueto

thecom

plexityofthe

problemand

thelim

itedcom

puterresources

thatwere

available,currentknowledgeon

thesim

plem

odelsprevents

acom

prehensiveintegrated

model

frombeing

developed.Therefore

thefocus

will

begiven

tothe

simple

mathem

aticalmodelin

thispaper.

Overthe

lasttwo

decades,mainly

two

kindsofnum

ericalmodelforscourprediction

havebeen

developed.Oneis

basedon

thepotentialflow

theory,suchas

Hansenetal.

(1986)and

LiandCheng

(l999a),and

theother

isbased

onthe

k-sm

odels,such

asLeeuwenstein

andW

ind(1984),

Brors(1999)

andvan

Beekand

Wind

(1990).It

hasbeen

demonstrated

thatthepotential

flowm

odelsare

ableto

predictthe

maxim

umdepth

andthe

upstreampart

ofscourhole

correctly.However,

noneofthe

potentialflow

models

canexplain

thegentle

slopeofthe

scourholefonned

downstreamthe

pipe(Liand

Cheng,l999a).Thisis

mainly

dueto

thefactthatthe

potentialflowm

odelcannotsim

ulatethe

vortexshedding

processassociate

with

theflow

aroundthe

pipeline.Ithas

beennunderstood

(Sumeretal.,1988)thatthe

gentleslope

ofthescourhole

fonneddownstream

thepipeline

ism

ainlydue

tothe

vortexshedding

aroundthe

pipeline.

104

Page 105: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Earlynum

ericalmodels

basedon

k-sturbulence

models

seemed

tohave

difficultiesto

predicttheshape

ofscourhole.

Leeuwesteinetal.

(1985)developed

anum

ericalm

odelbasedupon

k-aturbulence

modeland

asedim

enttransportequation.Anum

ericalpackage

named

OD

YSSEEwas

usedto

calculatethe

turbulentflowfield.

Asforthe

computation

ofthe

sediment

transportand

thevariation

inseabed

topographythey

reporteda

failurein

obtaininga

realscourholeshape

byusing

anem

piricalbed-loadform

ula.This

wasascribed

tothe

ignoranceofthe

suspended-loadcontribution

inthe

model.

Inthe

numericalpartofthe

investigationby

Stuneretal.(1988),the

so-calledC

loudin

Cell(CIC

)m

ethodwas

employed

tosim

ulatethe

flow.

Itwasreported

thatthe

CIC

method

generallygives

goodprediction

onthe

grosscharacteristics

ofthe

organizedwake

behindthe

pipeline.H

owever,

therewas

noevidence

inthe

papershow

ingthat

anum

ericalmodelwas

employed

tocalculate

theseabed

deformation.

Instead,bycom

paringthe

effectiveShields

parameterw

ithits

time

averagevalue,

anim

portantconclusionwas

drawnby

Sumeretal.(1988)thatthe

organisedwake

behindthe

pipelinehas

strongeffects

onthe

profileofscourhole

downstreamofthe

pipeline.The

time-averaged

bedshearstress

isnota

suitableparam

etertouse

inpredicting

thelee-wake

scouringbehind

apipeline.

Some

improvem

entson

thek-a

basedm

odelshave

beenachieved

recently.Van

Beekand

Wind

(1990)developed

anum

ericalmodelbased

onk-2

turbulencem

odeland

atransportequation

forsuspendedsedim

ent.Theapplication

ofthem

odeltoscour

predictionbelow

apipeline

with

andw

ithoutan

attachedspoiler

showedfairly

agreements

with

them

easuredequilibrium

scourholes,

althougha

certaindegree

ofunderestim

ationof

downstreamscour

holewas

quiteevident

inthe

report.The

predictedrate

oferosionwas

aboutthreetim

esas

fastasin

thephysicalm

odel.Brors

(1999)presenteda

modelthatincludes

thedescription

offluidflow

bythe

standardk-s

turbulenceand

thesuspended

andbed-load

sedimenttransports.

Density

effectswere

consideredin

theverticalm

omentum

equationand

inthe

turbulenceequations.

Flowaround

asurface

mounted

cylinderwas

predictedin

goodagreem

entw

iththe

experiments.

However,in

thescour

calculationthe

model

didnot

predictperiodic

vortexshedding,

evenduring

thelater

stagesof

scourdevelopm

ent.The

authorsuggested

thatafine

mesh

(5000nodes)is

neededto

predictthephenom

enaofvortex

shedding.For

thescour

calculations,the

predictionofa

clearwater

scourhole

(0=0.048,where

0is

theShields

parameter)

agreedw

ellwith

Mao’s

(1986)experim

entalm

easurements.N

oattem

ptswere

made

forcasesoflive

bedscour.

Recently,LiandCheng

(1999b)developeda

numericalm

odelforlocalscouraroundpipelines

employing

aslightly

differentapproach.

Theflow

arormd

thepipeline

issolved

usinga

LargeEddy

Simulation

(LES)m

odelthat

resultsin

more

accurateprediction

ofseabed

shearstress

thantraditional

k-2turbulence

models.

Theequilibrium

scourhole

isdetennined

byiterations,

basedon

theassum

ptionthatthe

shearstress

onthe

seabedis

equalorless

thanthe

farfield

shearstress

forlive

bedscour

(orthe

criticalshear

stressfor

clear-waterscour)

everywhere

whenthe

equilibriumscourhole

isestablished.

Thepredicted

equilibriumscourhole

compared

veryw

ellwith

theexperim

entalresultsby

Mao

(1986)forbothclearwaterand

live-bedconditions

(LiandCheng,1999b;2000a).The

advantageofthe

modelis

thatitdoesnot

employ

anyem

piricalsedim

enttransportform

ula.However,

thedisadvantage

ofthe

105

Page 106: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

modelis

thatitcannotdescribe

thetim

edevelopm

entofthescourhole

dueto

useof

theequilibrium

assumption

inthe

model.

Insm

nmary,it

seemsthatnone

ofthecurrentm

odelsare

ableto

simulate

thetim

edevelopm

entof

two-dim

ensionalscour

holeaccurately

evenunder

steadycurrent

conditions.Thekey

elements

indeveloping

acom

prehensivem

odeloftime-dependent

scourliein

twofolds:

1)anaccurate

flowm

odelthatcanresultin

accurateprediction

ofvortex

sheddingbehind

thepipeline

and,2)apropersedimenttransportm

odel.

Theobjective

ofthepresentpaperis

todevelop

anum

ericalmodelthatis

capableof

predictingthe

time

development

oflocal

scourbelow

apipeline.

Them

odelw

illem

ploythe

LESflow

modeldeveloped

byLiand

Cheng(2000a).The

morphological

changeofthe

seabedw

illbe

calculatedin

thesam

efashion

asthat

usedby

Brlzirs(1999).The

rateofbed-levelchange

willbe

determined

fromthe

depositionrate

Dand

theerosion

rateE.

Thedeposition

rateD

willbe

setequaltothe

differenceofsetting

velocityand

upwardturbulentvelocity

times

thenear-bed

concentration.The

erosionrate

Ew

illbedeterm

inedfrom

thenear-bed

turbulenceintensity

andthe

concentrationgradients.

Theconcentration

ofthesuspended-load

willbe

calculatedby

solvingthe

scalartransportequationofsuspended-load

concentration.Theboundary

conditionfor

thenear-bed

concentrationof

suspended-loadw

illbe

specifiedusing

anem

piricalform

uladerived

fromexperim

entalm

easurements

(Zysennanand

Fredsrae,1990).

Detailsofthe

modelim

plementation

willbe

givenin

thefollow

ingtw

osections.

MA

TH

EM

AT

ICA

LM

OD

EL

Flowm

odelIt

hasbeen

demonstrated

bothexperim

entally(Sum

eret

al.,1988)

andnum

erically(Li

andCheng,

Inlet0

Outlet

1999b)that

thelocal

scourbelow

a

FreeSurface

/E[la

/,pipeline

dependsstrongly

onthe

Divevortex

sheddingflow

aroundthe

pipeline.Li

andCheng

(l999b)Seabed

demonstrated

thatthe

fluctuatingFig.

1Definition

Sketch:calculationdom

ainSeabed

Shea-TStress

Plays311

important

rolein

theso-called

lee-wake

scourprocess.Thereforeaccurate

predictionofthe

fluctuatingseabed

shearstressis

verycrucialto

theprediction

oflocalscour

belowa

pipeline.Pastexperiences

ofauthors’

andsom

eothers

(Li|<—

D—>\

andCheng,

1999b;Beaudan

_v

andM

oin,1994)

indicatedtt-10-.

Q.......................--

thatthe

LttgtEddy

(u,v)=(1;)_|_‘l,,v,)Sim

ulation(LES)

model

issuitable

forthe

vortex|

-

Y>4

Fig.2D

efinitionSketch:a

pipeneara

wall106

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Page 108: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 109: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

co»)=tr

<11)yr

h“

J/’

whereh

iswaterdepth,b

isRouse

number,ford50

=0.36m

rnsedim

entB=

2.8,andcb

isreference

concentrationatthe

levelofybabove

seabedgiven

byEq.(9).

Num

ericalmethod

Thegoverning

equations(1)to

(4)togetherwith

theboundary

conditionsare

solvedusing

finitedifference

method

ina

curvilinearcoordinatesystem

.Theconvection

terms

inequations

(2)to(4)

arediscretized

usinga

third-orderupwind

scheme

andthe

otherterm

sare

discretizedusing

centraldifference.Asecond-orderschem

eis

usedforallthe

time

dependentterms.

Fordetailsofnum

ericalimplem

entation,readers

arerefered

toLeietal.(1999).

Morphologicalm

odelThe

presenceofpipeline

breaksthe

localsedimentbalance

andcauses

thevariations

inflow

field.The

locationat

which

depositionor

erosiontakes

placedepends

onwhether

theam

ountof

sediment

settling,D,

islarger

orless

thanthe

amount

ofsedim

ententraimnent,E.

Thenetcross

boundaryflux

ofsedimentis

zeroonly

underequilibrium

conditions.Ingeneral,there

isa

residualflux,which

isnorm

allythe

causeofm

orphologicalchangeofseabed.

Fortw

o-dimensional

suspended-loaddom

inantapplications,

thegeneral

sediment

continuityequation

canbe

written

as

<1-0%=rt».-tat.+

<12)where

nis

theporosity

ofbed;ysis

bedlevel.

Thefirsttenn

onthe

righthandofequation

(12)isthe

rateofdeposition

ofentrainedm

aterial,expressed

asvolum

eofsedim

entgrains

settlingfrom

suspensiononto

unitarea

ofbedperunittim

e.Thesecond

termis

theactualrate

ofentraimnentofsedim

entm

assfrom

thebed,

expressedas

volume

ofsediment

grainseroded

intosuspension

fromunitarea

ofbedperunittim

e.Equation(12)indicates

thatthebed

morphological

changeys

isnotonly

aresult

ofupwarddiffusive

fluxand

downw

ardsettlem

entbutalso

thecontribution

ofconvectivetransportvcb.

Itshould

benoted

thatthebedload

sedimenttransportis

notincludedin

them

orphologicalmodelgiven

inEq.

(12).This

implies

theuse

oftheassum

ptionthat

thegradient

ofbedloadtransport

inthe

flowdirection

isnegligible.

Thisis

mainly

basedon

theexperim

entalfindings

thatthe

bedloadsedim

enttransportisonly

confinedw

ithina

verythin

layerofathickness

ofafew

sedimentdiam

eters(Zyserm

anand

Fredsoe,1990).

Solutionprocess

Thesolution

processoflocalscour

belowa

pipelinew

illstartby

solvingthe

flowfield

andsuspended-load

concentrationfield

aroundthe

pipelinew

itha

specified

109

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Page 112: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

same

inthe

numerical

tests.For

allthe

numerical

tests,a

rectangulardom

ainof

3000x350m

mw

itha

pipeofdiam

eterof100m

mbeing

placedat1000

mm

fromthe

inletofthedom

ain.A

153x63m

eshw

ithgrid

pointsbeing

concentratedtowards

thepipe

surfaceand

theseabed

isem

ployedfor

allthe

casesafter

acareful

mesh-

dependencestudy.The

numericalresults

onthe

time

developmentofthe

scourholeas

wellas

them

aximtun

scourdepthare

compared

with

theexperim

entalresultsofM

ao(1986)

Tablel

Flowand

sedimentconditions

forthenum

ericaltests

CaseH

PipeT

TSandsize

=Flow

TInitialgap

Shieldsdiam

eterd5@

(mm

)velocity

ratioe/D

0param

eter(m

m)

3U0

(cm/s)

l9

1‘0

1000.36

35.0

0l

0.040*

2100

0.3650.0

A0

0.0903

1000.36

50.03

0.5l

0.098

Fig.7

andFig.

8show

thescourdevelopm

entbelowthe

pipelinein

time

forcase

land

case2,

respectively.In

thosetw

ocases,

thepipe

wasoriginally

placedon

theseabed.

Thecasel

isa

caseofclearwater

scourand

thecase

2is

acase

oflivebed

scour.M

easurements

ofscour

profilesare

availableeven

atvery

earlystage

ofthescourdevelopm

ent.Thisis

extremely

valuableto

validatethe

presentnumericalm

odel.

112

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Page 115: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 116: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

RE

FER

EN

CE

S

BEAUD

AN,P.A

ND

MO

IN,P.(1994)“N

umericalexperim

entson

theflow

pasta

circularcylinderatsub-criticalReynoldsnum

ber,”ReportNo.TF-62,StanfordU

niversity.

BRQ

RS,B.(1999).“N

tunericalmodeling

offlowand

scouratpipelines”J.Hydr.

Engrg.,ASCE,l25(5),511-523

CELIK,

I.A

ND

RO

DI,W

.(1988).

”modelling

suspendedsedim

enttransportin

nonequilibriurnsituations”J.Hydr.Engrg.,ASCE,ll4(l0),

1157-1191.

HAN

SEN,E.A.,FREDSO

E,J.AN

DM

AO,Y.(1986).“Tw

o-dimensionalscour

belowpipelines,”

Proc.Fifth

Int.Sym

p.on

Offshore

Mech.

andArctic

Engrg.,Am

ericanSociety

ofMechanicalEngineers,3,670--678.

JENSENB.L.(1987).“Large-scale

vorticesin

thewake

ofacylinderplaced

neara

wall.”

Proc.2"“

internationalconferenceon

Laseranem

ometry-advances

andapplications,Strathclyde,U

K,153-163.LEI,C.,C

HEN

G,L.A

ND

KA

VA

NA

GH

,K.(1999)“Afinite

differencesolution

oftheshear

flowovera

circularcylinder,”

Ocean

Engineering,V

ol.27,N

o.3,

pp.271-290.LEEU

WESTEIN

,W

.,BIJKER,E.A.,PEER

BOLTE,E.B.

AND

WIN

D,

H.G.

(1985).“The

naturalselfburialof

submarine

pipelines,”Proc

4”‘Int.

Conf.on

BehaviourofOffshore

Structure(BO

SS),EleevierSciencePublishers,717-728.

LEEUW

ESTEIN,W

.,AN

DW

IND

,H.G.(1984).“Thecom

putationofbed

shearin

anum

ericalmodel,”Proc

19”‘Int.Conf.oncoastalEngineering,Houston,TX,

Vol.2,

1685-1702.LI,

FAN

DC

HEN

G,

L.(1999a).

“Anm

nericalm

odelfor

localscour

underoffshore

pipelines”J.Hydr.Engrg.,ASC

E,125(4),400-406.LI,F.

AND

CH

ENG

,L.(l999b).

“Num

ericalsimulation

ofpipelinelocalscour

with

Lee-wakeeffects,”

The9”‘Intem

ationalConferenceon

Offshore

andPolar

Engineering,ISOPE

99,Brest,Vol,II,212-216.

LI,F.AN

DC

HEN

G,L.(2000a).‘Prediction

oflee-wakescouring

ofpipelinesin

currents”J.ofWaterway,Port,Coastal,and

Ocean

Engineering(subm

itted).M

AO,

Y.(1986).

“Theinteraction

betweena

pipelineand

anerodible

bed,”Series

Paper39,Tech.University

ofDenmark.

RIC

HAR

DSO

N,

J.F.

AND

ZAK

I,W

.N

.(1954)

”Sedimentation

andFluidisation,PartI,”Trans.Inst.Chem

.Engrs,Vol.32,No.1,pp35-53.

ROUSE,

H.(1939)

”Experiments

onthe

mechanics

ofsedim

entsuspension,”

Proc.5“Int.Congr.Appl.Mech.,Cam

bridge,Mass.,550-554.

SMAG

OR

INSKY,J.(1963).“G

ereralcirculationexperim

entsw

iththe

primitive

equations,I.The

basicexperim

ent”,Monthly.

Weather

Review,Vol.

91(3),99-164.

116

Page 117: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

SUM

ER,B.M

.,JENSEN,H.R.AND

FREDSQE,J.(1988).

"Effectoflee-wakeon

scourbelow

pipelinesin

current,"J.

ofWaterway,Port,

Coastal,and

Ocean

Engineering,Vol.114,No.5,599--614.

SUM

ER,

B.M

.A

ND

FREDSQE,

J.(1999).

"Wave

scouraround

structures,"Advances

inCoastaland

Ocean

Engineering,Editedby

PhilipL.-F.

Liu,Vol.

4,191--249.V

AN

BEEK.,F.AA

ND

W1ND.,H.G.(1990).“N

umericalm

odellingoferosion

andsedim

entationaround

pipelines.CoastalEngineering,14

107-128.V

AN

RIJN

,L.C.(1984).“Sedimenttransport,PartII:

suspendedload

transport,”J.H

ydr.Engrg.,ASCE,110(10),1613--1641.W

HITEH

OU

SE,R.

(1998).“Scour

atm

arinestructures,”

Thomas

TelfordPublications.

ZYSERM

AN,

J.A.

AND

FREDSQE,

J.(1990).

“Data

analysisof

bedconcentration

ofsuspendedsedim

ent,”J.Hydraulic

Engineering,Vol.120,No.9,

1021-1042.

117

Page 118: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

EXPERIMENTALSTUDYorTHESCOUR0NTHESEABED

UNDERAPIPELINEINOSCILLATING

FLow*

ByPu

Qunl,LiKunz

AB

STR

AC

T

An

experimental

studyof

thescour

ofthe

seabedtm

dera

marine

pipelineis

presentedin

thispaper.

Thetests

arecarried

outin

aU-shaped

oscillatorywatertunnel

with

abox

imbedded

inthe

bottomofthe

testsection.By

useofthe

standardsand,clay

andplastic

grainas

theseabed

materialthe

influenceofthe

bedm

aterialonthe

scouris

studied.The

relationshipbetween

thecriticalinitialgap-to-diam

eterratioabove

which

noscour

occursand

theparam

etersof

theoscillating

flowis

obtained.The

self-burialphenom

enonoccurred

forthepipeline

notfixedon

two

sidewallsofthe

testsectionand

isnot

observedfor

fixedpipeline.

Theeffect

ofthepipe

onthe

sandwave

formation

isdiscussed.

Them

aximum

equilibriumscour

depthsfor

differentinitial

gap-to-diameter

ratio,diflerentKcnum

beranddifierentbed

sandare

obtained.

INT

RO

DU

CT

ION

Thesour

arounda

pipelinem

ayinfluence

thein-place

stabilityof

them

arinepipeline,

soit

isim

portantforthe

safetyand

economy

ofsubmarine

pipelinedesign

[L2].The

scourphenom

enonaround

apipeline

isvery

complex

becausethe

scourcan

beinfluenced

bym

anyenvirom

nentalelements

suchas

theflow

,thetopography

andthe

soil.This

phenomenon

issubstantially

aresult

ofcoupling

actionsbetween

fluid,solid

andseabed.

Thescour

belowa

pipelineexposed

towave

isrelated

tooscillating

separatedvortex

flow.

Becauseofthe

seabederosion

andthatthe

seabedboundary

isin

adynam

iccondition,the

boundaryw

illchangeand

theseabed

materialw

illenterthewater,w

hichw

illcause

thedifference

betweenthe

separatedvortex

flowaround

apipeline

abovea

erodiblebed

andthat

abovea

planebed

B.O

nthe

otherhand,

startand

transportationofthe

sedimentin

aunsteady

flowis

afrontierproblem

insedim

entresearch[4].

Inaddition,there

iscom

plicatedinteraction

betweenm

ultipleseparated

vorticesduring

theprocess

ofthe

*Theprojectis

supportedby

theN

NSF

ofChina(19772065)and

theKey

Project(KZ951-A1-405)of“N

inthFive-yearPlan”ofCAS.

IProf.,InstituteofM

echanics,ChineseAcadem

yofSciences,Beijing

100080,P.R.China2SeniorEngineer,Institute

ofMechanics,Chinese

Academy

ofSciences,Beijing100080,

P.R.China

118

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Page 120: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Ah

\

C\

“T

J0

IllI3

0

I.

.g‘n

;dzn

n:rn

n-a

ll!!!

t_______._~2.12m

--------=

Fig.1Schem

aticdrawing

oftheU-shaped

watertunnel1.Testcylinder

2.Working

section3.A

differentialpressuretransducer

4.Waterlevel

5.Abutterfly

valve6.W

indtunnelconnected

with

anairblow

er

2.Pipeline

model

Thepipe

models

ofdiameters

D=28.9

and19.1m

mare

made

ofplexiglassand

arefixed

ontw

osidewalls

ofthetestsection.

Forthepipelines

directlyinstalled

onthe

seabed,the

pipem

odelsofouter

diameterD

=l4mm

andinner

diameter

D,=12m

mare

made

ofalum

iniumw

ithlength

of190mm

.D

ifferentmodelpipes

with

differentsubmerged

weight

arein

differentinitialburialdepth.

3.The

preparationofthe

soilsample

Foursoilsamples

areused

inthe

testsforfixed

pipelinem

odels:thestandard

sand,the

clayand

two

kindsofplastic

grainw

ithdifferentm

eandiam

eter.The

standardsand

isw

ithcharacteristics

ofd5g=0.20II]II1,

specificgravity

y=2.59and

saturatedunit

weight

}{,,,,=10.24kN/m3.Theclayiswithcharacteristicsofd50=0.0047mrn,1;=,/47,/d25=2.86and

with

percentfinegrains

72%.

Theplastic

grainis

with

d50=0.47,0.68mm

andspecific

gravity;=1.42.

Theclay

wasexcavated

at5-7mdepth

ofportareaD

ongyingofChina.

Theclay

sample

inthe

bedbox

usedin

thepresenttests

isobtained

fromthe

clayw

iththe

same

initialunitweightand

afterfourdaysofsedim

entationin

atank.

Thenits

unitweight

ism

easured.For

observationofthe

scourphenom

enonaround

apipeline

inoscillating

flowand

investigationofthe

effectofthesand

diameteron

bedscourthe

plasticgrain

isused

asthesoilsam

ple.The

standardsand

withd50=0.38mm,D,=0.37,}{,a,=19.0kN/m3

isusedasbed

materialin

thetests

ofunfixedpipe

model.

120

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Page 122: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Where

vis

kinematic

viscositycoefficientofwater.

TheShields

numbercorresponding

tothe

incoming

oscillatingflow

isdefined

by

_f-U3.

0_

(7—1)'8'dso

(4)

Where

gis

gravityacceleration,f

isthe

frictioncoefficientforthe

wave-boundarylayer,see

ref.[8].

EX

PE

RE

ME

NT

AL

RE

SU

LTA

ND

AN

ALY

SIS

1.Scourprocess

arotmd

thepipeline

Thescourphenom

enonforgap-to-diam

eterratioe/D=0

isdissim

ilartothatfore/D

notequaltozero.

AsKc

increasethe

flowseparation

andthe

vortexshedding

occur.For

e/D=0,theflow

arotmd

apipeis

theforw

ardseparated

flowatthe

cornerbetweenthe

lower

pipesurface

andthe

bedsurface

andis

theseparated

andreattached

flowbehind

thepipe.

Thisflow

undergoesan

acceleratedand

deceleratedprocess.

Vlfhateverthebed

materialis,

thetests

showthatthe

scouroccursatthe

forward

comerofthe

pipeatfirst.

Thesedim

entm

ovesbackwards

awayfrom

thepipe

andreaches

thevicinity

oftheseparated

pointBas

shownin

Fig3(a).

Forstandardsand

bedthe

reattachedflow

behindthe

pipeis

observed.The

sedimentm

ovestowards

two

oppositedirections

awayfrom

thereattached

pointC,asshown

inFig

3(a).The

locationofC

fromthe

pipeis

fartherthan

B.W

iththe

scourdevelopm

entthesedim

entstacksup

nearthepointB.

Thegap

betweenthe

pipeand

thebed

occursaftera

scourprocess,theflow

pattemchanges.

Fore/D

notequaltozero

theflow

pattemis

differentfromthatfore/D=0.

When

e/Dis

small,the

sedimentreciprocates

nearthepointA

andthen

stacksup

alittle

(seeFig

3(b)),whatever

thebed

ism

adeof

standardor

plasticsand.

When

e/Dis

largerthan

acertain

value,for

example

e/D=0.45~1.0for

standardsand

bed,thereciprocations

ofsediments

areobserved

atpointA,D

andE,shown

inFig.3(b).

Thelargerthe

e/D,thelargerthe

distancebetween

Aand

DorE.

Thisis

dependentonthe

actingofthe

sheddingvortex

ofthewake

onthe

sandbed.

With

thevelocity

increasedthe

transfigurationdevelopm

entofthebed

surfaceis

observedforstandard

andplastic

sandbed.

When

thevelocity

isa

littlelargerthan

thecriticalvalue,

thereis

onlyone

main

sandvalley

formed

undemeath

thepipe.

Asthe

velocityincreases,

thesecond

andthird

sandwaves

occursuccessively

onboth

sideofthe

main

sandvalley.

Thelargerthe

distanceofthe

sandwave

frompipe,the

lowerthe

peakofthe

sandwave

is.The

sandbed

farfrom

thepipe

isundisturbed.

Itlooks

likethere

isa

propagationand

consumption

ofthewave

energyin

theliquefied

sandbed.

Thescourholes

ofthestandard

sandbed

aresteeperthan

thatofplasticsand

bed.Itm

aybe

interpretedby

theirdifferent

restangle.

122

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Page 125: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

bedw

itha

pipeis

alittle

smallerthan

thatwithouta

pipe.Itis

thesam

efor

standardor

plasticsand

bed.In

presentexperimentitis

shownthatthe

criticaloscillatingam

plitudefor

thesam

ebed

soilanddifferente/D

andD

isalm

ostthesam

eand

thelength

ofthesand

waveis

alsothe

same.

Despiteofwhetherthe

pipeis

presentornot,aseries

ofstreamwise

thingroove

distributedalong

thepipe

axisdirection

isseen

onthe

sandwave

valleysurface.4.

Self-burialphenomenon

Thepipe

modelused

inself-burialtestis

notfixedon

bothside

ofthetestsection.

Thesubm

ergedw

eightofthepipe

modelis

0.68,0.94and

1.19N/mand

theinitialburial

depthis

0,3.6%and

7.1%respectively.

Thelightpipe

modelrolls

awayfrom

theoriginal

positionat

U,,,=0.09m/s.

Thereis

notanytrace

onthe

bedsurface.

Butforthe

heaviestpipe

modelas

thevelocity

increasegradually,the

scourofthebed

atbothsides

ofthepipe

takesplace

atcertainvelocity

andthe

sedimentstacks

upnearthe

separatedpointB.

Itissim

ilartothatforthe

fixedpipe.

Butitisdifferentfrom

thefixed

pipethatdue

tothe

sagofthe

pipeinto

thescour

valleythe

gapbetween

pipeand

bedcarm

otform.

When

theam

plitudeincreases

slowly,

ascour

holefonns

graduallynearpointP

thatisoutside

theseparated

region.The

bedsand

grainsm

ovealong

thedash

lineas

shownin

Fig.5.Follow

ingthe

flow,the

sandgrains

sedimentated

atthetop

ofthepipe

shakeleftand

right.As

aresultofthe

aboveprocess

thepipe

isself-buried

gradually.Thestability

oftheself-

burialpipeincreases.

Them

iddlew

eightpipem

odelshakes

alittle

inscour

holeat

acertain

velocity,but

doesnot

goout

oftheoriginal

place.W

henthe

flowam

plitudeincreases

drasticallyatthis

state,thepipe

would

rollup.It

isseen

inexperim

entthatthefonnation

oftheself-burialphenom

enonofthe

pipeis

relatedto

theaccelerating

processof

theflow

.A

slowacceleration

offlowleads

tothe

self-burial,while

asudden

accelerationofflow

would

bringaboutthe

instabilityofthe

pipe,especiallyaftera

littleshaking

ofthepipe.

Theself-burialphenom

enonis

alsorelated

tothe

pipew

eightor

theinitial

burialdepth.

Forthesam

eaccelerating

processthe

heavierofthepipe

weightorthe

deeperoftheburialdepth

them

oreeasy

theself-burialtakes

place.Itis

alsoobserved

inthe

experiment

thatthecharacteristics

ofthesedim

enttransportationnear

theseparated

pointare

major

factorsfor

theself-burial

fonnation.The

self-burialphenomenon

doesnot

occurin

thetests

forfixedpipe

model.

'*l'~4%

-.

_

_:.;_-.B.=-_-,;-_-1-._'-:.'.-...

Fig.5Schem

aticdrawing

ofself-burialprocess

5.M

aximum

equilibriumscourdepth

Theexperim

entalresultsofthe

maxim

umequilibrium

scourdepth

areobtained

ate/D=0~1.0

forthe

fixedpipe

with

D=2

and3cm

inthe

presentpaper.The

rangesofthe

experimentalparam

etersare

listedin

Tab.l.

125

Page 126: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Table1-Ranges

ofExperimentalParam

etersforD

ifferentBedM

aterials

-_

U,,,Tl

_U,,,D

l_

f-U3,Bed

material

qjKC

___D

qRe_.7

9_

i---(y_1)_g_d50

Fineplasticgrain"4.30~22.20

0.9><103~4.0><103I

0.052~0.273Coarseplasticgrain

M3.91~31.90

1.3><103~6.5><1030.049~0.39s

Standardsand7

6.07~32.57I

1.3><103~4.9><1030.030~0.245‘

Foreverybed

materialthe

testresultscan

berelated

bya

seriesofoblique

linesin

thelogarithm

coordinatesofS/D

andKc,where

Sis

them

aximum

equilibrirunscourdepth.

Fordifferente/Dthe

interceptsofthe

obliquelines

aredifferent,buttheirslopes

onlydiffer

alittle.

Takingthe

averageofthese

slopesthe

nonnalizedtestresults

aregiven

inFig.6,

Fig.7,andFig.8

inlogarithm

coordinatesofS/(DA)and

Kc,forfme,coarse

plasticsand

andfor

standardsand

bedrespectively.

HereA

representsthe

interceptsofthe

obliquelines

with

averageslope

andis

afunction

ofe/D

andD,

asseen

inFig.6(b),

Fig.7(b)and

Fig.8(b).Itm

ustbepointed

thatmostresults

forplasticsand

areobtained

atlive-bedcase,

butmostresults

forstandardsand

aretaken

fromthe

case,wheresedim

entfarfromthe

pipedoes

notmove.

Achanges

with

e/Dnotvery

much

inFig.6(b)

andFig.7(b)butitchanges

obviouslyin

Fig.8(b).From

Fig.6,Fig.7,andFig.8

thefollow

ingrelationship

betweenS/D

andKc

isgiven

by

S/D

=A

-KCm

(6)W

herem

isa

constantofthebed

material.

126

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Page 131: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

SCO

UR

PO

TEN

TLAL

EV

ALU

ATE

DIN

TER

MS

OF

EXCESS

POR

EPR

ESSUR

EIN

DU

CE

DB

YO

CE

AN

WAVES

By

KoukiZEN1andKiyonobu

KASAMA2

AB

STR

AC

T

Thedriving

forcescausingthescourareclassifiedinto

twotypesofexternalforces;one

istheshearforce

onthe

surfaceofsoillayercreated

bythe

waterflow.Atypicalexam

pleisthe

scouraroundthe

pierofbridgeacrosstheriver.The

otheristhewaveforcegeneratedbyoceanwaves.A

largescaleofscourin

thecoastalzonem

aybe

mainly

attributedto

theexcesspore

pressurefluctuation

producedby

waveaction

onthe

seabed.Thispaperpresentsthem

echanismofthescourcausedbythelatterforcein

theoceanenviromnent

When

oceanwavespropagate,theoscillatorypore

waterpressureiscreatedinthepenneableseabed.Dueto

thespatialdiiference

ofoscillatorywaterpressure,the

excesspore

pressure,namely

theexcess

hydraulicpressure,is

generatedand

thedistribution

ofexcesspore

pressureproduces

theseepage

flowin

theseabed

When

theupward

seepageforce

towardthe

seabedsurface

becomeslargerthan

theeffective

overburdenpressurein

theseabed,theliquefaction

orquicksandoccurs.Oncetheliquefactionoccursatthe

seabedsurface,the

sandparticles

areeasilytransported

bywaterflow,because

theshearresistanceofseabed

surfacebecomes

nearlyequaltozero.

Thewave-induced

oscillatorypore

waterpressurein

theseabed

canbe

calculatedon

thebasis

oftheconsolidation

theoryby

applyingthe

appropriateinitialand

boundaryconditions

andinputdata

Then,theexcessporepressureisevaluatedasthediiferencebetweenthewave-associatedwaterpressureontheseabed

'Professor,DepartmentofC

ivilEngineering,KyushuUniversity,6-10-1

Hakozaki,Higashiku,Fukuoka,812-8581,JAPAN2Research

Associate,Departm

entofC

ivilEngineering,

KyushuUniversity,

6-10-1Hakozaki,

Higashiku,Fukuoka,812-8581,JA

PA

N

131

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Page 134: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Consequently,theeifective

verticalstressexpressed

byEq.

(2)varies

periodicallyin

accordancew

iththe

chaiseOftheM0.0-P(Z.19};

¢T’t;(-'40=

O-,v(Z’0)+{Iv(0.0—P(Zi9}

(2)

Ifthecr’,(z,2)attains

zeroorless

atcertaindepths,the

soilskeletonw

illbecome

aliquefied

state.Thus,thecriterion

forthewave-induced

liquefactioncan

bederived

fromEq.(2)by

settingthe

verticalefiectivestress,

<r’,(z,1),equaltozero

orless;"

<I’..(Z,0)-5-£061I)-P(Z.»0}=H.(Z.0

(3)

where,p(0,19cmdp(z,1):the

wave-inducedoscillatorywaterpressure

onthe

seabedsurface

andin

theseabed

respectively,0",(z,0):theinitialverticaleffective

stressatarbitrarydepth,z,ofthedeposit,

o",,(z,Q:thevertical

effectivestress

atarbitrarydepth,z,ofthe

depositandtim

e,2‘,Ao",(z,I):the

wave-associatedeffective

stresschange,u,(z,r):theexcessporepressureatarbitrarydepth,z,ofthedepositandtim

e,t.The

solidcurvesinFig.2(b)show

theverticaleifectivestressdistribution

drawnbyreplacingthe0",(z,0),

with7/’z,

where)7

isthe

submergedunit

weightofdeposit.

Thelines

numbered

(Dand@

inFig.2(b)

correspondto

onesnumbered

@and

@in

Fig.2(a),respectively.InFig.2(b),the

liquefiedzone

shownbythe

slantlineswheretheverticalstressbecomeszeroorlessappearsnearthe

seabedsurface,underthewavetrough.As

theexcesspore

pressureispositive

inthis

situation,thetransientupward

seepageflow

isgeneratedtoward

theseabedsurface.

1

lo)

lb)W

aveCrest

lh%S_tillW

aterLevel__¥__

4W

oveTrou

h,

IP(°Tn_\‘_P(o,:1g

PressurgpO

EffectiveStress

av

TosauoranbP8

”Oscil|otory

,\

'ExcessPore

\I

ZPressure:Ue

Z0

(z,o)=Yz\

II

""11,"°7’Z*{P1c,n'P1z,n)

._,_

-

Fig.2Conceptofwave-inducedliquefactionanddensification:(a)oscillatoryexcessporepressure,(b)effectiveverticalstress

134

Page 135: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Iftheo"’,(z,0),

p(0,t)andp(z,2)are

given,theliquefaction

potentialcanbe

evaluatedby

usingEq.(3).

Asthe

o",6z,0)

iscalculated

fromthe

submergedunitweightofdepositand

thep(0,

t)is

convenientlyestimatedbythelinearwavetheory,theonlyfactorto

beknownisthewave-associatedporepressure,p(z,I),the

evaluationofwhichispresentedelsewhere(Zenand

Yanrazak,1990a,1990b).W

hereas,iftheverticaleffective

stresschange,

Ao",(z,2),reaches

positivevalues,say

itexceedsthe

initialverticaleffectivestress,o",(z,0),asshownbythe

linenum

bered®

inFig.2(b),the

wave-inducedstress

exertsaforceonthe

soilskeletontopossiblydensiiythe

seabed.The

spatialdiiferencesofthe

oscillatoryexcesspore

pressurein

theseabedw

illexertseepageforces

onthesoilskeleton.Asthe

incrementoftheverticaleffective

stressisrepresentedby;

5’<T’v(Z,Ui”

ft?’

(4)0"z

Theverticaleffective

stressisderivedbyintegratingEq.(4);

aura12=fre+1/Z0)

where,j;theseepageforce

andy’;thesubmergedunitweightofdepositEq.(5)isequivalenttoEq.(2)whenthe

0’,(z,0)isidenticalwith

)/z.Theseepage

force,j,isderived

fiomEq.(2)bytaking

o"o",(z,0)/o"’z=1/’and

0"p(0,r)/82

=0into

account;

5P(%1)

1--—

—-

(0o"z

Thehydraulicgradient,27,andtheflowvelocity,v,arerespectivelygiven

bythefollowing

equations;

J.

i=-m

(7)Va/9.

v=-

—-—

-(<9)

11.W

here,1/Wistmitweightofwater,k

isthecoefficientofpenneability._

Fig.3showsthe

seepageforce,j,calculatedbyusingthefielddataonthewave-inducedoscillatorywater

135

Page 136: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

i—

l.5-I

-0.5.O

tit-—

*-rm

lll<N/in’)

vs’rm

/m=t

-15-to

50

5"

I0|_

_..I'l_

,_:_

_'“l"'“

'-"r"-T

"I-15|

\~\

\Liquefied

ZoneI

.\\

Obtained

from:

\01Seepaqe

force‘.Excess

pore1

|Series

No.2\\%

'pressure

|\\

Wave:

No.7\

,t=

345s\

_(.|J

t=8T

.5°

(,

II'D

\/‘S

-1ll

Q—Q--U-—O--O-—O"-O-—O'-~Q—*.' -N.0 U1

,T)tf>°"°\.

to‘Q5,\\

-|.qo_4-1.0-0.5

0'2X

V(m

/s)

Fig.3Liquefaction

duetoseepageflow

pressure,p(z,2)(Zen

andYam

azaki,1991,Zen

etal.,1998).The

jis

approximately

calculatedusing

theequation,j

=Ap(z,I)/Z12.Then,the

z’andv

arecalculated

usingEqs.(7)and

(8),respectively.InFig.3,the

seepageforce

andhydraulic

gradientindicatedby

solidcircles

become

remarkably

largerneartheseabed

surfacethanthoseatdeeperseabed.Especially,thehydraulicgradientattainsmorethan

1.0attheseabedsurface.

Thisdifference

isconsidered

togenerate

theupward

seepageflow

directingto

theseabed

surface.Theopen

circlesandsolidline

aretheverticalelfective

stressobtainedfiom

Eq.(2)andEq.(5),respectively.Accordingto

theliquefaction

criterionrepresented

byEq.(3),the

liquefactionis

sureto

occuratthesurface

ofseabed.Theliquefaction

createsalargepotentialforthe

transportationofsuspended

sandparticles.This

isareasonthatthescourm

aybecloselyrelatedto

thewave-inducedliquefaction..

RE

LATIO

NS

HIP

BETVVEENSC

OU

RA

ND

LIQU

EFA

CTIO

N

Thephenom

enacalled

scourandsucking

areunderstoodthatthe

sandparticlesconsisting

ofseabedare

carriedaway

bywaves

and/orcurrentwithoutsuficientsupplyofsand

inthe

trace.Then,them

agnitudeof

triggeringpotentialforthe

scourisnoticedtobegin

with.Generallyspeaking,the

characteristicsofsandsuch

asdensityandstrength

aredifferentbythesedim

entationconditions.

Also,inthe

fieldofgeotechnology,itis

thecom

mon

sensethatthecharacteristicsrem

arkablychange

dueto

theaction

ofextemalforces.

V1/henthesand

depositbecomessuspended

dueto

somereason,the

floatedparticles

may

beeasily

transportedeven

bysm

allintensityofextem

alforcessuch

asbottom

flowor

vortex.In

thism

eaning,the

above-mentioned

136

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Page 139: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 140: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

strength.

AP

PR

OA

CH

TOC

OU

NTER

IVIEASUR

ES

Atpresent,thedesign

procedureand

practicalmethod

forpreventingcom

pletelythe

scourdonotexist.

Asthesim

plestway,them

ethodseparatingthe

shearstressfromwaterflow

isappliedFrom

geotechnicalpointofview,however,itisnotalways

sufiicientinoceanenvirom

nent.Inorderto

preventtheseabedfrom

scour,itis

recomm

endedto

adaptamethod

atleasttakingaccountofthe

wave-inducedliquefaction

Thisis

achieved,forexam

ple,byinstallingthewave

dissipatingconcrete

blocksnotdirectlyonto

theseabedbutonto

therubble

mound.Large

rubbleisnotsuitablesothattheoverburdenpressurem

aynotbeequallytransferredtotheseabed.

Itisdesirableto

increaseasm

uchaspossiblethecontactpressurebetweenthewave

dissipatingconcreteblocks

andseabed.The

problemofliquefaction,however,stillremainsatthetoe

ofrubblem

ound,becauseonly

quitesm

alloverburdenpressure

isexpected

there.Howto

dealwiththis

toeproblem

isthe

keyfor

thescour

protection.Furthennore,thoughtheconventionalcountermeasuressuchaswire-cylinder,cloth,asphaltm

atandgravelm

atexpecttheeffectto

separatetheseabedfrom

thewaveaction

and!orcurrentTheyhaveaweakpoint

againsttheliquefaction

sincethepropagationoffluctuatingpressureintosanddepositcannotbefullyrestrained.

Isthereany

countermeasureresistanttothe

liquefaction?The

scourprotectionin

oceanenvirom

nentmaybe

foundoutbytakingthe

liquefactionmechanism

intoaccount.

CO

NC

LUD

ING

REM

ARKS

Thewave-induced

excesspore

waterpressure

inthe

seabedwas

analyzedon

thebasis

oftheconsolidation

theoryby

applyingthe

appropriateinitialand

boundaryconditions

andinputdata.Then,the

excessporepressurewasevaluatedasthedifference

betweenthewave-associatedwaterpressureonthe

seabedsurface

andthe

oscillatorypore

pressurein

theseabed.The

seepageforce

wascalculated

astheinclination

ofoscillatorywaterpressuredistributiontothedepth

Theupward

seepageforcein

thepenneableseabedwasanalyzedusingthe

fielddatato

evaluatewhether

ornottheliquefaction

occurred.Theresultofanalysis

showedthatthe

liquefactionevidentlyhappened.Once

theliquefaction

occursin

theseabed

surface,thesandparticles

areeasilytransported

bythewaterflow,asthe

shearresistanceofseabed

surfacebecomes

nearlyequalto

zero.Inthatsense,the

excessporepressure

andwave-induced

liquefactionin

theseabed

arevery

importantin

predictingthe

scourpotentialofpemreable

seabedinthe

coastalzone.The

post-liquefactionphenom

enonis

oneofthe

furthersignificanttopicsin

making

clearofthescour

potentialintheoceanenvironm

ent.

RE

FER

EN

CE

S

14O

Page 141: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

l.Nishida,

H.etal.,

1985,“Subsidence

ofdetachedbreakwaters

surveyedby

usingcross-hole

method”,

Proceedingsofthe32”‘ConferenceonCoastalEngineeringinJapan,JSCE,pp.365-369

(inJapanese).

2.Zen,K.andYam

azaki,H.,l990a,“Mechanism

ofwave-inducedliquefaction

anddensification

inseabed”,

SoilsandFoturdations,Vol.30,No.4,pp.90-104.'

3.Zen,K.andYam

azaki,H.,1990b,“Oscillatorypore

pressureand

liquefactionin

seabedinduced

byocean

waves”,SoilsandFoundations,Vol.30,No.4,pp.147-161.4.Zen,K.and

Yamazaki,H.,

199i,“Fieldobservation

andanalysis

ofwave-inducedliquefaction”,Soils

andFoundations,Vol.31,No.4,pp.161-179.5.Zen,K.and

Yamazaki,H.,1995,“Slope

instabilitydue

towave-induced

liquefactionin

theseabed”,River,

CoastalandShorelineProtection:Erosion

ControlUsingRiprap

andArrnourstone,EditedbyC.RThom

eetal.,

JohnWiley&

Sons,pp.38l-393.6.Zen,K.,

Jeng,D.S..,Hsu,JR

C.

andOhyam

a,T..,1998,“W

ave-inducedseabed

instability:difference

betweenliquefactionandshearfailure”,SoilsandFoundation,Vol.38,No.2,pp.37-47.

141

Page 142: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

SC

OU

RD

OW

NS

TRE

AM

OF

DA

MS

ByGeorgew.Annandalel,RodneyWittlerzandGregA.Scott?’

ABSTRACTThe

papersumm

arizesm

ethodsthatare

usedto

predictscourdownstreamofovertopping

dams

inthe

United

StatesofAm

erica.These

methods

canbe

subdividedinto

physicalhydraulicm

odelstudies,rigorousconstitutive

computerm

odelingand

empiricalm

ethods.Conventional

proceduresare

usedto

conductphysical

hydraulicm

odelstudies.

Rigorousconstitutive

modeling

isbased

onKeyblock

Theory(G

oodman

&Shi1985

andG

oodman

andH

atzor1991),

whereasconventionalem

piricalmethods

thatareused

includethe

Veronese(1937),Y

ildizand

Uzucek(1994)and

theM

asonand

Artunugam(1985)

equations.Keyblock

theoryis

directedat

solvingscour

problems

inhard

rockblocks,

whereasthe

conventionalempiricalm

ethodsare

intendedto

predictscourincohesionless

granularmaterial.

Theessence

oftheErodibility

IndexM

ethod(Arm

andale1995)

thatisused

topredict

scourinany

earthm

aterial,includingrock,and

cohesiveand

non-cohesivegranularearth

isalso

presentedin

thispaper.

Thepresented

casestudies

illustratethe

applicationofthis

empirical

method

topredictscourofrock

andgranularm

aterial.Com

parisonofobserved

andcalculated

scourin

rockand

granularm

aterialforfield

andnear-prototype

experimentalstudies

indicatessatisfactory

correlation.

INT

RO

DU

CT

ION

Scourdownstream

ofdam

s,induced

byeither

largespillw

ayflow

sor

overtopping,influences

thesafety

ofdams.

Thisis

am

atterofinterestto

United

StatesFederaland

StateAgencies

thatown

orregulatedam

s.There

arecurrently

more

than75,000

dams

inthe

United

StatesN

ationalInventory

ofDam

s(P.L.

99-662,P.L.

104-303).The

U.S.Arm

yCorps

ofEngineers

(USACE)

maintains

andperiodically

updatesthe

inventory.M

anyofthese

dams

couldpotentially

besubjectto

damfoundation

erosionresulting

fromhigh

flows.

Theissue

isregularly

consideredduring

damsafety

reviewsand

re-licensingofprojects.

Some

ofthelegal

requirements

pertainingto

thesafety

ofdams

arecontained

inthe

NationalDam

InspectionAct,

P.L.92-367.

Thereare

two

generalapproachesto

thehydrologic

andhydraulic

safetyofdam

sin

theU

nitedStates.

Some

agenciespreferto

usethe

Pl\/LFora

proportionthereofto

assessthe

impact

ofhydrologicloading

ona

project.O

theragenciesuse

riskbased

approaches.The

emphasis

of

1Associate

andD

irectorW

aterResource

Engineering,G

olderAssociates

Inc.,44

Union

Blvd.,Suite

300,Lakewood,Colorado.2Research

Hydraulic

Engineer,WaterResources

ResearchLaboratory,US

BureauofReclam

ation,Departmentof

Interior,Denver,Colorado.3

SeniorTechnical

Specialist,Structural

AnalysisG

roup,US

Bureauof

Reclamation,

Department

ofInterior,

Denver,Colorado.

142

Page 143: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

thispaperis

onm

ethodsused

inthe

United

Statesto

predictandassess

damfoundation

erosion,partofeitherapproach

towardsdam

safety.

Damfoundation

erosionassessm

entmethods

usedin

theU

nitedStates

includephysical

hydraulicm

odelstudies,

rigorousconstitutive

computer

modeling,

andem

piricalprocedures.

Theresults

fromphysicalm

odelstudiesare

qualitative,although

theyprovide

valuabledesign

anddarn

safetyinform

ation.Rigorous

constitutivem

odeling,based

onKeyblock

Theory(G

oodman

&Shi

1985,G

oodman

&H

atzor1991)

iscom

plex,although

analysesto

evaluatespecific

components

ofscourproblems

havebeen

completed.

Currentanalysisprocedures

usingthis

approachdo

notfullyaccountforthe

fluctuatingpressures

causedby

thehydraulic

loadingthatoften

dominates

thescourprocess.

Empiricalequations

forpredicting

scourdepth

includethe

Veroneseequation

(updatedby

Yildiz

1994)and

theM

ason&

Arumugam

(Mason

&Arurnugarn

1985)equation.

Theprinciple

concemw

iththese

equationsis

theirinabilityto

comprehensively

accountformaterial

properties.The

Veronese/

Yildiz

equationdoes

notcontain

anyallowance

form

aterialproperties.

Althoughthe

Mason

&Arum

ugamequation

containsan

allowancefor

particlediam

eter,a

largentunber

ofdam

foundationerosion

problems

dealsw

ithscour

ofrock.

Selectionofan

appropriateparticle

diameter

torepresentrock

propertiespresents

apractical

problem.Research

bythe

United

StatesBureau

ofReclam

ation,G

olderAssociates

Inc.and

ColoradoState

University

intoan

empirical

method

known

asthe

Erodibility

IndexM

ethod(Annandale

1995)shows

goodagreem

entbetween

observedand

calculatedscour

ofearth

materials

thatincluderock,and

cohesiveand

non-cohesivegranularm

aterial.This

method

usesa

geo-mechanicalindex

toquantify

therelative

abilityofearth

materialto

resisterosion.A

nem

piricalrelationshipbetween

thegeo-m

echanicalindexand

theerosive

powerofw

aterthat

defmes

anerosion

thresholdforany

earthm

aterialmakes

itpossibleto

estimate

erosionpotential

andcalculate

scourdepth.

Thispapersum

marizes

thedifferentapproaches

toassess

damsafety

issuespertaining

tohydrologic

loadingon

dams,and

presentsm

ethodsthatare

usedin

theU

nitedStates

toassess

damfoundation

erosion.Keyblock

Theory,conventionalempiricalequations

andthe

Erodibility

IndexM

ethodare

brieflydiscussed.

Thepaper

concludesw

ithcase

studiesthat

illustrateapplication

oftheE

rodibilityIndex

Method.

PM

FA

ND

RIS

KB

AS

ED

AP

PR

OA

CH

ES

Currentpolicyby

USAgencies

toassessthe

hydrologicsafety

ofdams

includesProbable

Maxim

umFlood

(PMF)and

Risk

Basedapproaches.

Traditionalstandards-basedapproaches

toevaluate

thepotential

forovertopping

relyon

routingthe

PMF

orsom

epercentage

thereofthrough

areservoirsystem

,todeterm

inethe

potentialdepthand

durationofdam

overtopping,orthe

potentialfordam

agingspillw

ayflow

s.Spillw

aysand

outletworks

areassum

edto

functionin

accordancew

ithoperating

criteriaduring

theroutings.

Thedam

mustbe

shownto

bestable

forthism

aximum

loadingin

ordertopass

thestandard.

Suchan

analysisincludes

assessmentof

theim

pactofformdation

scourondam

stability.The

magnitudes

ofPMF’s

arebased

onH

ydro-

143

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6.A

waterpressure

isassigned

tothe

potentialopen

facesof

thecritical

keyblocksand

thelim

itequilibritunanalysis

isrepeated.

Instability(factorof

safetyless

than1.0)forthis

caseindicates

theblock

would

likelybe

removed

bythe

flowing

water.

Althoughsom

em

odestresearchexam

inesthe

effectsofhydrodynam

icforces

actingon

thepotentially

openjointplanes,to

datethe

analysesofabutm

enterosionhave

onlyconsidered

hydrostaticforces

actingon

thesejoints.The

hydrodynamic

forcesproduced

byim

pingingwater

penetratingthe

jointplanesm

aybe

much

largerthanthe

hydrostaticforces

actingon

ajointfullofwater.

Hence,thisis

anim

portantconsiderationw

ithregard

toabutm

enterosion,requiringfurtherresearch.

ConventionalE

mpiricalM

ethods

Equationsused

inthe

pasttocalculate

plungepoolscourare

theVeronese,M

asonand

Artunugam,

andY

ildizand

Uzucekequations.

Of

theseequations

onlythe

Mason

andArtunugam

equationacknowledges

thatmaterialresistance

playsa

rolein

scour.Equation

(2)isthe

Veronese(1937)

equation.The

equationyields

anestim

ateoferosion

measured

fromthe

tailwatersurface

tothe

bottomofthe

scourhole.

Y8

=1

.90

H0

.22

5q

0.5

4

Y,=depth

oferosionbelow

tailwater(m

eters)H

=elevation

differencebetween

reservoirandtailw

ater(meters)

q=

unitdischarge(m

3/s/m)

Yildiz

andU

zucek(1994)

presentsa

modified

versionof

theVeronese

equation,including

theangle,0:,ofincidence

fromthe

vertical,ofthejet.

Y,=1.90H°~’”q°-54cosa:(3)

Equation4

isthe

Mason

&Arurnugarn

(1985)prototypeequation.

xy

w

r—Kqi?

(ag

d

h=

tailwaterdepth

aboveoriginalground

surface(m

eters)d

=m

ediangrain

sizeofform

dationm

aterial,d5@(m

eters)g

=acceleration

ofgravity(m

/s2)

K=6.42—3.1-H°'1°

x=0.6—fl—

v=0.3300

147

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y=O.l5+—

I-{—w

=0

.l5200

z=0.lO

d=0.25m

Unlike

theVeronese

andthe

Yildiz

andUzucek

equations,theM

asonand

Arumugarn

equationincludes

am

aterialfactor,d.

Althoughit

isan

attemptto

acknowledgethe

rolethat

materialproperties

playin

resistingscour,itis

unlikelythatthis

factoradequatelyrepresents

thevariety

ofmaterialproperties

foundin

foundationm

aterials.

Erodibility

IndexM

ethod

Annandale(1995)

developedthe

Erodibility

IndexM

ethodby

analyzingscour

eventsfrom

approximately

150field

observationsand

byanalyzing

publishedlaboratory

datapertaining

tothe

initiationofm

otionofsedim

entparticlessubjectto

flowing

water.A

nerosion

thresholdwas

establishedby

plottingthe

ErodibilityIndex

fordifferentrock

typesand

cohesiveand

non-cohesive

granularsoilsagainststream

power,andnoting

whetherscouroccurredornotfor

eacheventunderconsideration.

TheErodibility

Indexthatwas

usedto

quantifythe

relativeability

ofthe

earthm

aterialtoresisterosion

isidenticalto

Kirsten’sExcavatability

Index(Kirsten

1982).Kirsten’s

ExcavatabilityIndex

isused

tocharacterize

rockfor

determining

thepow

errequirem

entsof

earthm

ovingequipm

entthat

canrip

thesubject

material.

Theindex,

asform

ulatedin

theE

rodibilityIndex

Method,is

expressedas11116productoffourparam

eters,

K=

MsK

bKdJs

(5)

K=

ErodibilityIndex

M,=

intactrockstrength

parameter

Kb=

blocksize

parameter

K4=

shearstrengthparam

eterJ,=

relativeorientation

parameter.

Thevalues

oftheparam

etersare

detenninedby

making

useoftables

andequations

thatare

publishedin

Annandale(1995)

andKirsten

(1982).The

intactearth

material

strengthparam

eteris

equatedto

itsunconfined

compressive

strengthin

MPa

forstrengths

greaterthan10

MPa.

Theblock

sizeparam

eterisa

functionofRQ

Dand

ajointsetnum

berinthe

caseof

rock,and

afim

ctionofm

edianparticle

diameter

inthe

caseof

cohesionlessgranular

earthm

aterial.The

shearstrength

parameter

isa

functionofa

jointroughness

number

anda

jointalteration

number,orthe

tangentoftheresidualintem

alangleoffiiction

inthe

caseofgranular

soils.Relative

orientation,inthe

caseofrock,is

afrm

ctionofthe

relativeshape

oftherock

andits

dipand

dipdirection

relativeto

thedirection

offlow

.The

material

characteristicsare

generallyobtained

fromborehole

data,fieldtesting

(suchas

vanesheartesting)

andlaboratory

testing(to

obtainthe

unconfinedcom

pressivestrength).

Itis

alsopossible

toobtain

parameter

valuesby

making

useofgeologic

descriptionsofthe

material.

Therelative

magnitude

oftheerosive

powerofthe

waterisquantified

bystream

power,also

known

asrate

ofenergydissipation.

Thisparam

eterisused

becauseofits

closerelation

to

148

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turbulenceintensity

andpressure

fluctuations(Annandale

1995),the

hypothesizedprincipal

causeofscour.

Streampower,P,is:

P=2/-q-AE(6)

=stream

power(KW

/m2)

=tm

itweightofwater(=

9.82KN/m3)

=tm

itdischarge(m

3/s/m)

=energy

lossexpressed

interm

sofhead

perunitlengthofflow

(m/m

)E;*s=x'"e

Annandale(1995)used

equation6

toderive

anum

berofotherequationsthatcan

beused

tocalculate

streampowerfor

avariety

offlowconditions,including

headcuts,knickpointflow,

hydraulicjum

ps,etc.Sets

ofgraphsthatcan

beused

tocalculate

streampow

eratthe

baseof

bridgepiers

(Smith

etal.1997)havealso

beendeveloped.

Applicationofthe

method

requiresexpertise

inengineering

geology,and

geotechnicaland

hydraulicengineering.

Approachingscouranalysis

froman

interdisciplinarypointofview

,especially

onlarge

importantprojects,is

advisable.

Theerosion

thresholdthatrelates

theE

rodibilityIndex

tostream

power

ispresented

inFigure

2.The

solidm

arkersrepresent

eventswhere

scourwas

observed,whereas

theopen

markers

representeventswhere

scourdidnotoccur.

Thedotted

linerepresents

theapproxim

atelocation

oftheerosion

threshold.

Theextent(depth)ofscouris

determined

bycom

paringthe

streampow

erthatisavailable

tocause

scourto

thestream

power

thatis

requiredto

scourthe

earthm

aterialunder

consideration.

Figure3

showshow

theavailable

andrequired

streampower,both

plottedas

afunction

ofelevation

beneaththe

riverbed,arecom

paredto

detenninethe

extentofscour.Scourw

illoccurwhen

theavailable

streampowerexceeds

therequired

streampower.

Once

them

aximum

scourelevation

isreached

theavailable

streampoweris

lessthan

therequired

streampower,and

scourceases.

149

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dam.

Lateranalysis

indicatedthatthe

darnw

ouldhave

beenovertopped

bythe

1964flood

evenifallgates

werefully

openedas

earlyas

June1st.

Althoughthe

rockstrength

andjointing

appearedto

bequite

resistanttoerosion

duringthis

event,a3to

5foot(0.9

to1.5

m)thick

concreteoverlay

with

anchorboltswas

placedwhere

theovertopping

flowim

pingedon

therightabutm

entandfoundation

(Figure8)to

protectagainsteven

largerfloodsup

tothe

probablem

aximum

flood(PM

F).The

steeperleft

abutmentwas

treatedw

ithrock

boltsand

aconcrete

capin

major

surfacefracture

zones(Figure

9).This

modification

wasdeem

edprudent

giventhe

largedegree

ofuncertainty

associatedw

ithdeterm

iningerodibility

atthetim

ewhen

modifications

weredesigned.

Backcalculations:

Theem

piricalapproachto

detenniningerodibility

presentedin

thispaperwas

subsequentlyused

todetennine

ifitwould

accuratelypredictthe

observedresponse.

Thisapproach

isbased

onthe

streampowerofthe

impinging

jet,andthe

erodibilityindex

ofthem

aterialbeinghit.

Annandale’s(1995)

graphsuggests

thaterosion

ofrockis

possibleif

theiaggqigfirble

streampower

isgreater

thanthe

erodibilityindex

raisedto

the0.75

power(Pr

>=

TheErodibility

Indexis

computed

asK=(M

,)(Kg,)(Kd)(J_,).The

valueof

eachof

theparam

etersm

akingup

theE

rodibilityIndex

isobtained

fromtables

inAnnandale

(1995).M,is

anevaluation

ofthem

ass(intact)

strengthofthe

foundation.This

variesdepending

onwhether

thefoundation

isa

granularsoil,acohesive

soil,orrock.The

majority

ofthefotm

dationrock

atG

ibsonDarn

islim

estoneand

dolomite

(referredto

aslim

estonein

much

ofthedocum

entation).Forthe

evaluationatG

ibson,thevalue

ofM,is

equaltothe

unconfmed

compressive

strengthin

MPa.

Theaverage

valuefrom

laboratorytests

was22,900

lb/in2(158

MPa).

Some

weakerintensely

fracturedbeds

(about6to

l0feet(1.8

to3.0

m)

thick)are

present,particularlyon

theleftabutm

ent.The

rockin

thesebeds

would

havea

lowerstrength,perhaps

bya

factorof2to

4(40-80

MPa).

Laboratorytesting

perfonnedon

theconcrete

duringoriginalconstruction

ofthedarn

resultedin

anaverage

unconfinedcom

pressivestrength

ofabout2940lb/inz

(20M

Pa).

Kbis

anindex

relatedto

them

eanblock

size.It

canbe

estimated

asthe

rockquality

designation(R

QD

)divided

bythe

jointsetntunber(Jn).

Thedam

foundationlim

estonevaries

fromthin

bedsa

fewinches

thickto

massive

beds,8to

10feet(2.4

to3.0

m)thick.

Therocks

werefound

tobe

brokenby

severalfissures,which

followed

thebedding

surfacesvery

closely.Anotherprom

inentjointsetwas

mapped

oneach

abutment,and

therewere

otherminorjoints.

Thiscorresponds

toajointsetnm

nberof2.24.The

RQ

Dwas

notloggedforholes

drilledon

thedownstream

rightabutm

ent,but

ingeneralthe

rockwas

recoveredin

longsticks

with

afew

fracturedzones.

Basedon

corerecovery

numbers

andfield

observations,theaverage

RQ

Dis

probablyabout

90-95%,

with

isolatedareas

rangingdow

nto

about80%

.This

resultsin

Kg,values

betweenabout35.7

and42.4.

Theintensely

fracturedbeds

would

havean

RQ

Dofabout

17%based

onfield

measurem

ents.This

correspondsto

aKb

valueofabout7.6.

Theconcrete

wasplaced

in4-foot(l.2m

)lifts

usinglarge

blocksgenerally

encompassing

theentire

thicknessofthe

dam.

Thecontraction

jointsare

widely

spaced(35

to60

feet(10.7to

18.3m

))andkeyed.

Althoughsom

eliftlines

exhibitminorseepage

athighreservoirelevations,the

liftswere

cleanedw

ellandalso

keyed.The

valueofK1,forthe

concreteshould

behigh,say

about80orhigher.

156

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Page 159: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

erodibilityindex

arebased

largelyon

theconditions

remaining

afterthe1964

overtoppingevent.

Theintensely

fracturedzones

would

beexpected

toerode,

exceptperhaps

nearthe

crest.Although

theobserved

amountoferosion

inthese

zoneswas

notexcessive,thisis

believedto

beconsistentw

iththe

observedbehavior.

Theresults

ofthisstudy

supporttheconclusion

thatthereprobably

wassom

eerosion

ofw

eakrock

(intenselyfractured

beds)or

damaged

concretein

areaswhere

therewas

littledissipation

ofenergyfrom

thetailwater.

Therewas

some

surficialerosionand

scouringofloose

materialduring

theexperienced

overtopping,butnotmuch,asjudged

fromthe

conditionofthe

foundationafterthe

overtopping.The

concreteshould

notbevulnerable

toerosion

providedthat

theconcrete

remains

intactandthere

isn’tdegradationofthe

concreteby

cracking,freeze-thawaction,or

vandalism.

Thedecision

toprotectthe

intenselyfractured

bedsappears

tobe

soundunder

anyscenario.

Theareas

ofabutm

entrock

most

susceptibleto

erosionfor

higherovertopping

flows

havebeen

protected.

159

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Page 161: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

concentrationofthe

jetatimpact(A;),the

nozzleelevation,the

watersurfaceelevation,and

them

ediandiam

eteroftheroad-base

material(D50)in

meters.

Table1

-Experimentalparam

etersIt

i‘

Hi’

'*7

’_I

11

3L

ii

I—

’_'

Experiment

¢p

0:Q

v,,v,-

A;

Noz.El.

WS

El.Az

D501

15°11.9°*2.74‘10T82'5.7564.7%6184

_l3.353.49'1.00E-'62

2'15°12.1°

2.74,10.82I5;80'63.1%6.84

3.663.18

1.0013-02°'

‘T’’

I7°

6.84DJ

3.6873.76-1-.00E-02

J

15.11.7°

2.7410.82

5.7266._0fi>

15°1l2.3°

2.74110.82“5.86.61.5%

16.84

-IL"-3.93

2.911.001;-02‘S

'25°20.2°2.74'10.82

5.8561.7%6.89'

U13.94

2.95;l.00E

-02ON

525°l19.5°,2.74[10.82'5.7565.0%

6.893.34

13.55l.00E

-02\lI

25°'18.8°2.7410T82l5.6568.3%

6.892.56

4.33T1.00E-02A

'25°I19.1°2.74.‘10.82‘5.6967.0%

I6.89“

O02.90

‘3.99'1.00E'-T025

9I

35°.26.9°2.7410.82'5.7-465.3%6.96

3.353.61

Il.00E-02

10'

35°27.9°2.7410.825.841619%

6.963.97

2.98 1.00E-02111

""35°26.'5°2.74'10.82'5.70

66.7%6.96

3.033.93

1l.00E-02

L_12_

_'35°_25.8°_2.fi'1p.82‘_5.6568.4%6.96

2.5914.37‘l.00E

-02

0:=arctan[

v°Si1:¢j

(7)\/(120cos¢)

+2gAz

Erodibility

Index(K):

TheE

rodibilityIndex,

_K,is

usedto

quantifythe

granularm

aterial’srelative

abilityto

resisterosion.Itis

theproductofthe

Mass

StrengthNum

ber,Ms,

theBlock

SizeNtunber,

Kb,the

ShearStrength

Ntunber,K4,

andRelative

Ground

StructureNum

ber,JS.

K=M.-K.~K.-J.<8)

Mass

StrengthN

tunber(Mi)

FromTable

1ofAnnandale

(1995)the

Mass

StrengthNum

ber,M,-,

forgranular

soilbetween

looseand

medium

densityis

equalto0.07.

BlockSize

Num

ber(K9)

Theparticle

BlockSize

Num

ber,Kb,

forcohesionless

granularm

aterialscan

bedetennined

directlyby

(Annandale1995):K

b=1000D§0

(9)

Am

ediangrain

size,D50,of10

mm

yieldsa

blocksize

numberequalto

1.00E-03.

161

Page 162: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

ShearStrengthN

umber(Kg)

Forgranularmaterials,the

ShearStrengthNtunber,Kd,approxim

atestan((p),where

cpis

theequivalentresidual(m

inimum

)fiiction

angle.This

anglefor

theexperim

entalmaterialis

approximately

40°,yieldinga

shearstrengthnum

berequalto0.84.

Relative

Ground

StructureN

umber(@

)

TheRelative

Ground

StructureNum

ber,J_,,isequalto

1.0forgranularm

aterials.

Erodibility

IndexC

alculation

Theproductofthe

fournumbers

yieldsan

erodibilityindex

roughlyequalto

5.87E-05.

K,=(0.07)(1.00E-03)(0.84)(1.0);5.876-05(10)

Required

Power:

FromAnnandale

(1995),the

power

(kW/m

2)required,pR,

toerode

earthm

aterialinthe

lowerrange

ofErodibility

Indexnum

bersis

afunction

ofK:

480044

=--1

611

PR1000

()

RateofEnergy

Dissipation

AvailablePow

er:Thepow

er(kW/m

2)availablefrom

theplunging

jettoerode

theearth

material

within

theplunge

poolis

afunction

ofjet

hydraulics.From

Bohrerand

Abt

(Bohrer1996)the

velocityalong

thecenterline

ofajetina

plungepoolis

afunction

ofthejet

velocityatim

pact,theangle

ofimpact,the

airconcentrationofthe

jetatimpact(represented

bythe

ratioofairand

waterdensities)and

gravitationalacceleration.Equation

(12)describes

thisfunctional

relationship,follow

edby

thelim

itsof

application.Equation

(13)expresses

thedistance

alongthe

centerline.

-111(1)=-0.5812111[J-"-*-I1/+37’)+2.107(12)

V1P...

8L

-0.293lnH£’—][L/in52.6

P...813

L=_-Z1"Z1

(13)C

OSO

!

Therate

ofenergydissipation,

oravailable

power,isa

discretizedfunction

ofthetotal

headatvarious

elevationsalong

thecenterline

ofthesubm

ergedjet.

Equation(14)

showsa

162

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(Theinter-particle

fiictionangle

correspondsto

thearctangentvalue

ofKd,i.e.arctangent(0.75)

=36.9°).

RelativeG

roundStructureN

umber

FromTable

7in

Annandale(1995)

thevalue

ofJ,for

a45

degreedip

anglein

thedirection

offlow,and

arelative

shapeof15.5/2.5

=6

(i.e.,aratio

of1:6),is0.44.

Erodibility

IndexC

alculation

Theproductofthe

fournum

bersis

anestim

ateofthe

erodibilityindex

roughlyequal

to69.

K,=(12)(17)(0.75)(0.44)s69(23)

RateofEnergy

Dissipation

Itisassum

edthatallthe

energyis

dissipatedatthe

locationwhere

thejetim

pingeson

thesim

ulatedrock.

Thetotalrate

ofenergydissipation

isequalto

theproductofthe

unitweightof

water(7),thedischarge

(Q),and

thechange

inenergy,AE.

P=;/-Q-AE(24)

Therate

ofenergydissipation

perunitarea(p)is

equaltothe

rateofenergy

dissipationdivided

bythe

horizontalprojectionofthe

areaofthe

jetatimpact,A

,-.14

9192»1,733‘

J/

P=LT

17;.(25)

'J’

[L2

l,-‘Ir

Inthis

application,the

changeof

energy,AE,

isequal

tothe

totalavailable

energybetween

thenozzle

andthe

tailwatersurface.

Thissupposes

that100%

ofthetotalavailable

energyis

dissipatedin

theerosion

process.The

watercushionin

theexperim

entwaskeptto

am

inimum

inorderto

simplify

theestim

ateofthe

rateofenergy

dissipation.

Table2

containsthe

calculationsand

parameters

usedto

determine

therate

ofenergy

dissipation.The

totalhead,H,is

thestun

ofthenozzle

elevationand

thevelocity

headm

inusthe

tailwater

elevation.The

calculationsassum

ethat

thetotal

availablehead

isconverted

intokinetic

energyand

dissipatesat

thepoint

ofcontact

with

thefoundation.

Forthis

experimentw

ithajetw

idthatim

pactof3.0ft(approxim

ately1m

)arateofenergy

dissipationof

P=22.6kW/m2isindicated.

167

Page 168: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 169: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 170: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Kirsten,H.A.D

.,1982,“A

Classification

Systemfor

Excavationin

NaturalM

aterials”,The

CivilEngineerin

SouthAfrica.pp.292

-308,July.

Mason,

P.J.,Arurnugam

,K.,

1985,“Free

JetScour

belowDam

sand

FlipBuckets.”

ASCE

JournalofHydraulic

Engineering,Vol.lll,

No.2.

Smith,

S.P.,Annandale,G.W

.,Johnson,P.A.,

Jones,J.S.and

Um

brell,E.R.,1997,

“PierScourin

ResistantMaterial:

CurrentResearchon

ErosivePow

er”,Proceedingsof

Managing

Water:

Coping

with

Scarcityand

Abundance,27“

Congressof

theIntem

ationalAssociationofH

ydraulicResearch,San

Francisco,California,pp.160-165.

Veronese,A.,

1937,“Erosioni

deFondo

aValle

diuno

Scarico.”Annali

deiLavori

Publicci,Vol.75,No.9,pp.717-726,Italy.

Yildiz,D.,Uzucek,E.,

1994,“PredictionofScourD

epthFrom

FreeFalling

FlipBucket

Jets”,Intl.WaterPowerand

DamConstruction,Novem

ber.

170

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Page 172: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 173: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Ycar19417

"Lefi;

R1gmb

aflk

bank5

-O

ct.14

3

5-

4-.

3‘

O°""2'55121:1315

2-

3;?

StJ

5___.

--—epi-

,185m3!'s

1..

"\"‘S

ept

15'

\/.¢

¢Q

J

Scpt.90

-T-

'""-»

131113751

\3

‘--0}_

°_._

_°_’.-0

°_,¢I~mflnwmg

\~==»'\ \-1

1

-2-,

#___/\--_%

2a_

3_

Oct14

-4-

-5-1

010

2030

40so

-6—

me

ters

Fig.1-G

eneralscourandsedim

entationrecorded

atSanJuan

River,nearB

luff,Utah,USA,

onSept.-O

ct.1941

NIE

TH

OD

PR

OP

OS

ED

Theinitialm

ovementofbed

loadparticles

andtheirlifting

toputthem

insuspension

arerandom

processesgovem

edby

theinstantaneous

shearstressdeveloped

inthe

vicinityofeach

particle.A

stochasticor

probabilisticapproach

toquantify

thebottom

scourseem

sto

bean

adequatesolution.

However,in

thisw

orka

differentcriterion

will

beapplied

becauseofthe

following

reasons:thelack

ofsufficientreliabledata

onthe

conditionsunderw

hicha

particlebecom

essuspended

andrem

ainsin

suspension;the

relativeaccuracy

ofthe

friction-relatedfonnulas

thatareused

toobtain

thedepth

ofscour;andthe

complexity

ofthepotentialresulting

equationsofno

practicaluseto

designers.

Hypotheses

1.-The

firsthypothesis

establishesthatthe

bedload

materialis

liftedand

suspendedwhen

theverticalcom

ponent0'

oftheturbulence

islargerthan

thefree

fallvelocitya1

oftheparticles.

Sincethe

peakvalue

of0'

isrelated

tothe

shearvelocityU

.,an

expressioncan

bew

rittenas

U.

=aco

(1)

173

Page 174: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 175: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

equationsdeveloped

byM

aza-Alvarezand

Garcia-Flores

(1986)were

usedto

obtainthose

diameters.

However,there

wassom

euncertainty

aboutthedegree

ofarmoring

thatcould

bereached

atthescoured

bedas

aresultofa

highsedim

enttransportrate.Itisquite

probablethat

them

eanbed

diameteratthe

time

oftheoccurrence

ofthepeak

scourbecomes

higherthanthe

valuecorresponding

tothe

originalsam

ple.Therefore,

D84oftheoriginal

sample

ofthebed

materialwas

selectedas

therepresentative

diameterto

obtainthe

fi"eefallvelocity;equation

(1)can

bethen

written

asfollows:

U.

=a

(084(4)

Inthis

lastequationthe

coefficientacan

takeinto

account,atleastona

partialbasis,thefactofconsidering

D84instead

oftheactualdiam

eterlikelyto

befound

atthepartly

armored

riverbed.VVhenfield

measurem

entsare

made,ifthey

aretaken

fromthe

scouredbed,the

valuesof

D,

anda),

proposedby

eachofthe

authorsofthe

flowresistance

equationsreferred

toin

subsequentparagraphsshould

betaken

intoaccount.

3.-The

thirdhypothesis

establishesthat

duringthe

transitoftheflood,

andtherefore

duringthe

scouringprocess,

thew

idthofthe

riverbed

remains

constant.Furtherm

ore,it

isconsidered

thatduring

thecalculations

theflow

dischargealong

aunit

width

remains

theoreticallyconstantwhen

thebed

elevationsubsides.

PR

OP

OS

ED

EQ

UA

TIO

NS

Inorderto

quantifythe

generalscouritisrequired

touse

asstartingequation,one

relatedto

flowresistance.

Itis

possiblein

theseequations

todifferentiate

thefriction

inducedby

theparticles

frombed

Lmdulations,

asoccurs

inthose

proposedby

Engelund,Paris,

andAlam

,Lovera

andKennedy.

Other

equationstake

intoaccountthe

combined

effectofparticles

andundulations,

suchas

thosedeveloped

byG

ardeand

Ranga-Raju,Brow

nlie,C

ruickshankand

Maza,and

Karimand

Kennedy.Thispaperpresents

thededuction

oftheequations

toevaluate

thegeneralscourbased

onthe

equationofflow

resistanceproposed

byCruickshank

andM

aza;subsequently,only

thefinalequations

ofgeneralscourobtainfi'om

theequations

ofKarimand

Kennedy,andfrom

Manning's

arepresented.

FR

OM

TH

EE

QU

AT

ION

OF

CR

UIC

HS

HA

NK

AN

DM

AZ

A(1973)

Theseauthors

suggestedtw

oequations

toobtain

them

eanvelocity

U.O

neofthem

correspondsto

flowunder

lowerregim

eand

theotherto

upperregime;both

areapplicable

toriver

bedsw

ithparticle

sizesw

ithinthe

rangeofsands

togravel

(0.000075

D502

0.008m

).

Thegeneraldepth

ofscouroftheriverbed

willbe

thendeterm

inedwhen

aflood

occurs,basedon

theform

ulaforlow

erregime

andindicating

allstagesnecessary

todeductit.

Theequation

proposedto

evaluatethe

mean

velocityU

,forlow

erregime

conditionsis:

175

Page 176: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

0.634Q_456

U=7.580),,

(5)(5)

1),,A0.35

1h

E2s3.5E@

)(6)

which

isvalid

if:

Thevariables

notyetdefined

are:h,

flowdepth,

inm

;D84,particle

diameter

ofbedm

aterialinw

hich84%

aresm

allerthanthatsize,in

m;0150,

freefallvelocity

inclearwaterfor

particlesw

ithdiam

eterD50,in

m/s;

andS

isthe

hydraulicgradient.

Thegeneralscouris

calculatedfora

flowdischarge

associatedto

acertain

returnperiod.

Toperform

thecalculation

ofscourit

isassum

ed,atleasttheoretically,

thatthedesign

flowdischarge

moves

throughthe

areaform

edbetween

them

aximum

elevationofthe

watersurfaceand

theoriginalcross

sectionarea

surveyedpriorto

theoccurrence

oftheflood

orbefore

theflood

season.This

conditiondoes

nothappen

innature

becausewhen

them

aximum

waterelevation

occurs,them

aximum

scouroftheriverbed

alsotakes

place.In

otherwords,fromthe

veryfirst

mom

entofthe

flood,and

whenthe

flowis

alreadycapable

ofsuspending

more

particlesthan

thosebeing

sedimented,the

scouringprocess

actuallytakes

place.Therefore,thebed

subsidencecontinues

whereasthe

flowdischarge

keepson

increasingatthe

crosssection

understudy.

Theassum

ptionreferred

tobefore

isapplied

with

thepurpose

ofdefiningthe

distributionofthe

unitflowdischarges

existingacross

thecross

sectionduring

thetransitofthe

peakflow

discharge.The

unitflowdischarge

alongany

verticalofariver

crosssection

canbe

obtainedw

ithtw

odifferentprocedures.The

firstofthemis

asa

frmction

oftheinitialtheoreticalflow

depthho,

measured

betweenthe

maxim

umwater

elevationupon

passageofthe

designflow

dischargeQd

andthe

bedelevation

givenby

theoriginal

crosssection

surveyedwhen

flowdischarges

aresm

all(dryseason)

(seefigure

2).In

thiscase

thehigher

hothe

highertheunit

flowdischarges.

Thesecond

approachto

fmd

outtheunitflow

dischargesim

pliestheirdirect

measurem

entduring

thetransit

oftheflow

dischargeQd,

assuggested

bySchreider

eral.

(1999).

176

Page 177: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

——

——

---E¢

e--i-—

-----

...ii!

__

__

__

fir?~tr

vlm

t|alprofile

'"f-*1,

.1.5

-._,’-j1.$Scouredprofile

M15».1

:;:.:-,:fi_,||-—

i

'7f'l$°-‘>

=19..-F’9.?)

""599.--'4

Fig.2-G

eneralscourataverticaland

scouredcross

section

Takinginto

accounttheabove

assumptions

andusing

equation(5),the

flowdischarge

passingthrough

thetheoreticalcross

sectionalready

describedcan

beexpressed

asfollows:

Q7.5805,,B,h,},~°34{s)°"““

(7)d

D8q.6a4A

whereB,

isthe

effectivew

idthofthe

freewater

surface,in

m;

andQd

isthe

designflow

discharge,i.e.thepeak

flowofthe

floodforw

hichthe

generalscourisgoing

tobe

calculated,inm

3/s.Q0.

isassociated

toa

certainreturn

periodT,

tobe

selectedbeforehand

intenns

oftheproblem

tobe

solved.Forexam

ple,forcalculatingthe

totalscourinbridge

design,thevalue

ofT

rangesfrom

50to

100years.

On

theotherhand,

hmis

them

eandepth

ofthecross

section,inm

,defined

bythe

waterelevationupon

passingofthe

peakflow

dischargeofthe

floodand

bythe

initialprofileofsuch

crosssection,and

givenby

therelationship

hm=

A/Be,where

Ais

thehydraulic

area,inm

2.

When

passingthe

flowdischarge

Qd,the

tmitflow

qrthatm

ovesthrough

anyvertical

(unitwidth)w

itha

depthho

isequalto:7.5805,,h,‘~°“

s°"“5°

qr-

ID8g.634

IA

(8)

After

dividingequation

(7)by

equation(8),

i.e.Qd

/qr,and

solvingfor

qr,the

following

expressionis

obtained:

177

Page 178: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Qh

1.634

=_d

._Q.9

q.Be

()

Thisequation

definesthe

distributionof

theunit

flowdischarges

throughthe

crosssection,as

afunction

oftheinitialdepth

ho.

Mention

hasbeen

made

thatsuchunitflow

dischargehas

topass

duringthe

whole

riverbed

scouringprocess

thatwillstop

uponreaching

acertain

depthhs

inw

hichthe

equilibritunis

achievedam

onglifted

andsettled

particles.Therefore,forthe

equilibrituncondition,i.e.forthe

peakscour,equation

(8)canbe

written

as:

7.5

80)

121.634510.456

qe_

1(1)6345

0.456(10)

DA

84

Inorder

toexpress

theunit

flowdischarge

interm

softhe

shearvelocity

U..,

thefollow

ingtransfonnation

canbe

made:

758Q50

hsrrvshs0.456

S0456g0.456

qeT

D8q.6s4g0.456

A0456(11)

whereq,

isthe

unitflow

dischargeunder

which

particlesthatbecom

esuspended

andthose

alreadysettled

arein

equilibritunwhen

theflow

depthbecom

esequalto

hs.

Ifitistaken

intoaccountthat

U.=

(gh

S)0'5,equation(11)is

transformed

asfollows:

7'5

8$

50

hsl.l7

8U

‘0.9

l2

qeD8q.634

A0456g0.456

(12)

Asestablished

before,during

thefullscotuing

processthe

unitflowdischarge

ateachtm

itwidth

remains

constant;therefore:

qr=<15(13)

Ifequations(9)and

(12)arem

atched,then:

Qi

fig1.634

—7.58

C050hS1.17sU*0.912

(14)

Behm

D801634(g

A)0.456

178

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Page 180: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 181: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 182: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 183: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 184: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 185: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Thecoefficient

aof

equations(16),

(25)and

(29)is

boundto

becorrected

whensufficient

dataare

available,particularly

inthe

caseofrivers

with

nosandy

bedm

aterials,because

equations(l5a),

(24)and

(28)were

onlyvalidated

fora

particlesize

rangeof

()000145

D845

()_()()59m_

Forlargerparticlediam

etersthe

valuesobtained

with

them

ethodof

Lischtvanand

Levedievwere

assumed

tohold

valid.

RE

FER

EN

CE

S

Breusers,H.N

.C.,R

audkivi,A.J.,1991,"Scouring",A.A.Balkem

a,Rotterdam,Netherlands,

pp.37-41.Brow

nlie,W

.R.,1981,

"Com

pilationof

Alluvial

ChannelData:

Laboratoryand

Field",W

.M.

KeckLaboratory

ofHydraulics

andW

aterResources,

Division

ofEngineeringand

AppliedScience,C

aliforniaInstitute

ofTechnology,Pasadena,Cal.,USA,pp.183-191.

Cruickshank,V.,

Maza-Alvarez,

J.A.,1973,

"F1owResistance

inSandy

BedChannels",

Proc.IntemationalSym

posiumon

RiverM

echanics,IAI—IR,Vol.

1,pp.337-345,Bangkok,Thailand.G

raf,W.H

.,1993,"FluvialHydraulics",John

Wiley

andSons,N

ewYork,U

SA,pp.356.H

amill,L.,1999,"Bridge

Hydraulics",E

andFN

Spon,London,pp.259-260.H

offinans,G

.J.C.M.,

Verheij,H.J.,

1997,"Scour

Manual",

A.A.Balkem

a,Rotterdam

,Netherlands,pp.26-29.Karim

,M.F.,Kennedy,J.F.,1981,"Com

puter-BasedPredictors

forSedimentDischarge

andFriction

FactorofAlluvialStream

",Iowa

InstituteofH

ydraulicResearch,ReportN

o.242,U

niversityofIow

aC

ity,Iowa,USA.

Karim,

M.

F.,Kennedy,

J.F.,1990,

“Menu

ofCouple

Velocityand

Sediment-Discharge

Relationsfor

Rivers”,

JournalofH

ydraulicEngineering,

ASCE,Vol.

104,N

o.H

Y7,pp

1045-1059.Leopold,L.B.,W

olman,M

.G

.,Miller,J.P.,

1964,"FluvialProcessesin

Geom

orphology",W

.H.Freeman

andCo.,San

Francisco,USA,pp.227-241.

Maza-Alvarez,J.A.,1966,"TotalScotu

inBridge

Piers",Thesisto

obtainthe

MasterDegree

inH

ydraulics,NationalU

niversityofM

exico,pp.4-41.Mexico

City.In

Spanish.M

aza-Alvarez,J.A.,Rico-R

odriguez,A.,

1970,"C

ontributionto

theEvaluation

ofGeneral

ScourinN

aturalChannels",Proc.IVLatin

American

CongressofH

ydraulics,IAH

R,Vol.

2,pp.214-224.Oaxtepec,M

or,Mexico.In

SpanishM

aza-Alvarez,J.A.,

Garcia-Flores,

M.,

1986,"D

istributionofBed

Sediments

inN

ationalChannels",

Proc.X

IILatin

American

Congressof

Hydraulics,

Vol.3,

pp.104-109,

SaoPaulo,Brasil.In

Spanish.Przedwojski,B.,Blazejew

ski,R.,Pilarczyk,K.W.,

1995,"RiverTraining

Techniques",A.A.Balkem

a,Rotterdam,Netherlands,pp.379-383.

Schreider,M

.,Scachi,

G.,

Reynares,M

.,Franco,

F.,1999,

"Aplicationofthe

Lischtvan-Levediev

Method

toCalculate

theG

eneralScour

inSandy

Beds".Proc.

InternationalCongress

onH

ydraulicEngineering

(inprint),Havana,Cuba.In

Spanish

185

Page 186: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

FLO

WA

NG

LEO

FA

TT

AC

KA

TP

ER

PE

ND

ICU

LAR

BR

IDG

EC

RO

SS

ING

S

ByM

arcoFalcon

1andM

auroNalesso

2

AB

ST

RA

CT

Inthis

paperthe

fonnationofaltem

atebars,

centralbars,or

multiple

transversebars,forflow

ina

straightriverreachofconstantw

idth,isstudied

asan

instabilityproblem

ofthem

obilebed.

Oncea

specific,doublyperiodic

perturbationofthe

mean

bedlevelis

selected,andthe

goveming

principles,laws,andem

piricalfrictionand

sedimenttransport

relationsare

imposed,expressions

areobtained

forthebed

wavecelerity,rate

ofgrowth

ofthe

bedam

plitude,andadditionally,itis

possibleto

estimate

thelongitudinalwavelength

tow

hichthe

maxim

umrate

ofgrowth

corresponds,andalso

todeterm

inethe

dominanttype

ofbars:altemate,centralorm

ultiple.Instead

ofw

orkingw

itha

totallyvertically-integrated

versionof

thegoverning

equations,carehas

beentaken

tosolve

fora

three-dimensionalsolution

ofthetransverse

velocitycom

ponent,thusavoiding

theerror

ofhavingto

relatethe

transversebed-shear

with

them

eantransverse

velocitycom

ponent.Assum

ingthatthe

linearizedperturbation

solutionsare

acceptable,upto

valuesof

theratio

ofbedam

plitudeto

depthequalto

onetenth,itis

foundthatsignificantdeviation

anglesofthe

localvelocityvector(angles

ofattack)fromthe

longitudinaldirectionexist,at

certainspecific

locations,dependingupon

thetype

ofbeddefonnations

present.Thusitis

shownthatbridge

pilesand

abutrnentsadequately

alignedw

ithstraightparallelbanks,still

canbe

subjectedto

locallyapproaching

flows

atsignificantanglesofattack

owing

tothe

developmentofvarious

typesofsedim

entbars.

INT

RO

DU

CT

ION

Scourofrectangular-shaped

bridgepiles

issignificantly

sensitiveto

theangle

ofattack

oftheapproaching

flow:apile

isgenerally

setsuchthatits

majorlength

isparallelto

theapproach-flow

velocityvector

(ina

straight,constant-width

reach,the

major

lengthw

ouldbe

paralleltothe

banks),and

thusthe

magnitude

ofscourw

illdepend

uponthe

smaller

rectangularlength

(width

ofthepile),

asLaursen

andToch

(1956)and

Melville

(1999)havereported

(seefigure

1).How

evertheseauthors

showthatifforvarious

possiblereasons,the

angleofthe

approach-floww

ithrespectto

thepile

majoraxis

deviatesfrom

1Professor,

InstituteofFluid

Mechanics,

FacultyofEngineering,

CentralUniversity

ofVenezuela,Caracas,Venezuela.2

Graduate

Student,Institute

ofFluid

Mechanics,

Facultyof

Engineering,Central

University

ofVenezuela,Caracas,Venezuela.186

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Thepresentm

odelcanbe

usedto

testwhich

typeofbars

willbe

dominantforgiven

flows

conditions.As

thew

idth/depthratio

increasesit

correctlypredicts

atendency

fromalternate

barstowards

centralandm

ultiplebars.

Theresults

indicatethatifalternate

barsare

present,anglesofattack

atmid

width

(atthechannelcenter)w

illbegreatest;ifcentral

barsare

dominant,then

thelargestangles

ofattackw

ouldoccuratB/4

fromthe

banks.In

some

torrentialflows

alternateand

centralbarsoccur

simultaneously

(Furbish,1998).Field

researchregarding

thesuperposition

characteristicshould

beconsidered.

Inconclusion,

straight-approachflow

stowards

bridgestructures

arescarce

andit

would

appeartobe

worthw

hileto

takefield

dataupstream

ofthebridge

sitein

orderto

checknaturaldeviation

anglesofthe

velocityvectorfordesign

purposes.

RE

FER

EN

CE

S

1.Laursen

E.M

.,Toch

A.,1956,

“ScourAround

BridgePiers

AndAbutm

ents,”Iow

aH

ighway

ResearchBoard,

BulletinN°4,

Iowa

InstituteofH

ydraulicResearch,

StateU

niversityofIowa,Iowa,U

SA.2.

FurbishD.

J.,1998,

“IrregularBed

Forms

InSteep,

RoughChannels,”

Water

ResourcesResearch,Vol.34,No.

12,December,1998,pp

3635-3648.3.

Melville

B.W

.,1995,

“PierAnd

Abutment

Scour:Integrated

Approach,”Journalof

Hydraulic

Engineering,Vol.

123,No.

2,February,

1997,pp

125-136,ASCE,

New

York,N.Y.,U

SA.4.

Yalin

M.

S.,SilvaA.M

.F.,1991,“O

nThe

Formation

OfAltem

ateBars,”

Euromech

262,Balkema,Rotterdam

,1991.

5.G

onzalez,N

.,Nalesso,

M.,

1998,“Analisis

dela

Formacion

deIslas

FluvialesCentrales,”Degree

Dissertation,UniversidadM

etropolitana,Caracas,Venezuela.6.

Garcia,M

.,Nifio,Y.,

1994,“Dynam

icsofSedim

entBarsIn

StraightAndM

eanderingChannels:

Experiments

On

TheResonance

Phenomenon,”

Journalof

Hydraulic

Research,Vol.31,No.6,1993.

197

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Intotal239

resultswere

availableforpredicting

maxim

runscourdepth

and175

forpredictingthe

locationofthe

maxim

umscour

depth.The

datafrom

Ruffetal.

(1985)did

notinclude

information

onthe

locationofthe

maxim

umscour

depthand

thisinform

ationwas

alsonot

availablefor

8results

fromAbida

andTownsend

(1991)and

1resultfrom

Aderibigbeand

Rajaratnam(1998).

Table2

belowshows

thedivision

ofthedata

intotraining,testing

andvalidating

sets.Table2.D

ivisionofdata

intotraining,testing

andvalidating

sets.

Trainingdata

Testingdata

Validatingdata

Scourdepth186

2627

ILocation

ofscourdepth130

1827

RE

SU

LTO

FTR

AIN

ING

FOR

SCO

UR

DE

PTH

Fromthe

trainednetw

orkthe

densimetric

Froudem

unberis

consideredto

bethe

most

significantvariable

forpredicting

scourdepth.

Thisis

alsothe

conclusionof

severalexperim

entalstudies(Rajaratnarn

andBeny,

1977,BlaisdellandAnderson,1988,Lim

,1995).

Theeffectofrem

ovingdensim

etricFroude

number

fromthe

setofvariableshad

a4

times

largereffectthanthatofrem

ovingthe

nextmostsignificantvariable.O

utletshapewas

foundto

bethe

nextmostsignificantclosely

followed

byw

idthofthe

receivingcharm

elandw

idthofthe

culvertoutlet.Thesedim

entsize,sedim

entgradationand

tailwater

depthwere

formd

make

onlya

slightdifferenceto

theoverallprediction

ofscourdepthwhen

theywere

removed

fromthe

inputvariables.

Studiesthathave

exploredoutletshape

havereported

thatshapeis

asignificantfactorand

thatthe

scourdepthcan

varyby

upto

40%depending

onthe

shapeofthe

outlet(Abtetal.1984).

Theeffectoftailw

aterdepthhas

receivedsom

eattention

experimentally

howevertheeffectof

tailwaterdepth

isstilla

pointofdebate.Forlowtailw

aterdepthsitis

consideredthatthere

isan

effectonthe

depthofscour.The

tailwater

depthsin

thedatasetcover

alarge

rangeand

thereforethe

overallresultshowsthe

tailwaterdepth

tobe

insignificant.Adifferentresultm

aybe

obtainedifthe

inputdatawas

restrictedto

asm

allerrangeoftailw

aterdepths.

VA

LIDA

TIO

NO

FA

NN

Figure2

belowshows

acom

parisonofthe

measured

scourdepth

andthatpredicted

bythe

trainednetw

orkusing

previouslyunseen

dataand

showsgood

agreement.The

testdatasetin

thiscase

includessom

eof

theauthors’

experimental

dataand

datafrom

3previously

publishedstudies

byotherresearchers.

Itcan

beclearly

seenthatthe

artificialneuralnetwork

issuccessfully

predictingthe

scourdepth

tow

ith+/-

15%in

mostcases.

204

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Page 208: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Theapplication

ofartificial

neuralnetworks

toscouring

showsthe

potentialto

leadto

aflexible

toolforengineershoweverthe

lackofexperim

entaldataavailable

atthepresenttim

elim

itsthe

model.A

rtificialneuralnetworkshave

theadvantage

ofbeingapplicable

toa

wider

rangeofhydraulic

conditionsthan

traditionalempiricalm

odelsrem

ovingthe

requirementof

thedesigner

tochoose

theappropriate

equationfor

theanticipated

hydraulicconditions.

Furtherwork

isrequired

toprovide

acom

pletedata

settotrain

thenetw

orkand

validateit

usefulness.

RE

FER

EN

CE

S

1.Abida

H.and

TownsendR.D.

(1991).“Local

scourdownstream

ofbox-culvert

outlets.”JofIrrigation

andD

rainageEng.

ASCE.Vol

117.May/June

1991pp425-

440.2.

Abt

S.R.,Kloberdanz

R.L.

andM

endozaC.

(1984).“U

nifiedculvert

scourdeterm

ination.”JofH

ydrEng.ASCE.Vol110.O

ct1984ppl475-1479.

3.Ade

F.andRajaratnam

N.(1998).“G

eneralisedstudy

oferosionby

circularhorizontalturbulentjets.”J

ofHydrRes.IAI—

IR.V0136pt4.A

pril1995pp613-635.

4.Aderibigbe

O.andRajaratnam

N.(1998).“Effectofsedim

entgradationon

erosionby

planeturbulentw

alljets.”JofH

ydrEng.Vol124

pt10O

ct1998ppl034-1042.

5.A

liK.H.M

.andLim

S.Y.(1986).“Localscourcausedby

submerged

walljets.”Proc.

InstofCiv

Engrs.Vol81

pt2Dec

1986pp607-645.

6.Blaisdell

F.W.

andAnderson

C.L.(1988).

“Acom

prehensivegeneralised

studyof

scouratcantileveredpipe

outlets.”JofH

ydrRes.V0126pt4

April1988

pp357-376.7.

ChatterjeeS.S.,G

hoshS.N.and

ChatterjeeM

.(1994).“Localscourdue

tosubm

ergedhorizontaljet”.J

ofHydrEng.V

ol120pt8

Aug1994

pp973-992.8.

Chiew

Y.M.

andLim

S.Y.(1996).

“Localscour

bya

deeplysubm

ergedhorizontal

circularjet.”JofH

ydrEng.ASCE.V

ol122Sept1996

pp529-532.9.

LimS.Y.(1995).“Scourbelow

unsubmerged

fullflowing

culvertoutlets.”Proc.Inst

ofCivilEngrs

Water,M

aritime

andEnergy.V

ol112pt2.June

1995pp136-149.

10.Liriano

S.L.(1999)“The

influenceofnear-bed

turbulentburstingstructures

onscour

downstreamof

pipeculvert

outlets”.PhD

Thesis,U

niversityof

Hertfordshire,

Hatfield,U

K.11.

Opie

T.R.(1967).“Scouratculvertoutlets.”

MSc

Thesis,ColoradoState

Univ.,Fort

Collins,Colorado,extractonly.

12.Rajaratnam

N.

(1981).“Erosion

byplane

turbulentwalljets.”

JofH

ydrRes.IAH

RV

ol19pt4

April1981

pp339-358.13.

RajaratnamN

.and

BenyB.

(1977).“Erosion

bycircular

turbulentwalljets.”

Jof

HydrRes.IA

HR

Vol.15pt3

Mar1983.pp277-289.

14.Rajaratnam

N.and

MacDouga1l

R.K.

(1983).“Erosion

byplane

wall

jetsw

ithm

inimum

tailwater.”J

ofHydrEng.ASCE.Vol.

109no.7

July1983

ppl061-1064.15.

RaoS.G.(2000).“A

rtificialNeuralN

etworks

inH

ydrologyI:Prelim

inaryConcepts.”

JofH

ydrologicEng.ASCE.V

ol5N

o.2April2000

pp115-123.16.

Ruff

J.R.,Abt

S.R.,M

endozaC,

ShaikhA.

andKloberdanz

R.(1982)

“Scourat

culvertoutletsin

mixed

bedm

aterials.”Colorado

StateU

niv.Engineering

ResearchCentre

Reportno.FHW

A/RD

—82/011.FortC

ollins,Colorado.Sept1982.17.

TrentR.,

Gagarin

N.

andRhodes

J.(1993).

“Estimating

pierscour

with

artificialneuralnetw

orks.”H

ydraulicEngineering

‘93,San

Francisco,C

alifornia25-30

July1993.pp1043-1049.

208

Page 209: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

18.Trent

R.,M

olinasA.

andG

agarinN

.(1993).

“An

artificialneural

network

forcom

putingsedim

enttransport.”Hydraulic

Engineering‘93,San

Francisco,California

25-30July

19931049-1054.

209

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SO

ME

AS

PE

CTS

OF

THE

AR

GE

NT

INE

EX

PE

RIE

NC

EO

NLO

CA

LS

CO

UR

ByRaulAntonio

Lopardol

AB

STR

AC

T

Thispaper

dealson

threedifferent

subjects,as

examples

ofthecontribution

ofArgentineresearch

onlocal

scour:local

scourat

bridgepiers,

localscour

downstreamski

jump

structuresand

localscourdownstreamhydraulicjum

penergy

dissipators.Firtly,the

proposalofCottaand

Jensento

avoidthe

localscouratbridgepiers,developed

atthe

sixtiesin

LaPlata

NationalU

niversity.Thepracticalresults

werepublished

inthe

LatinAm

ericanCongress

oftheIA

HR

inSpanish.

Ithas

gooddiffusion

inSouth

America,

asit

wasincluded

inthe

BridgeDesign

ManualofVenezuela.

Secondly,the

INC

YTH

equationfor

localscour

downstreamflip

buckets,intentionally

oversimplified

inorder

tobe

ofuse

asan

initialestim

ateof

scour.It

requiresonly

theknowledge

oftheunitdischarge

andthe

fallheightfromthe

reservoirleveltotailw

aterlevel.Itshould

benoted

thatduringthe

designstage

many

parameters

areusually

unknown,suchas

thesize

ofblocksfonned

byfracture

oftherock

atdifferentdepthsnearthe

jetimpact.For

preliminary

calculations,thisequation

demonstrates

acceptableperform

ancefor

thedata

ofotherauthors,even

with

datapublished

laterthanits

fonnulation.Finally,the

phenomenon

oflocalerosiondownstream

ofhydraulicjtunp

energydissipators

inlarge

flatlanddam

s,when

rockssubm

ittedto

severepressure

fluctuationscom

posethe

riverbed,which

isw

elldifferentofthecase

customarily

studiedin

alluvialbedrivers.

Thisproblem

wasanalyzed

bym

eansof

resultsof

fluctuatingpressures

(statisticvalues

ofam

plitudesand

frequencies)in

thebase

ofa

hydraulicjum

pstilling

basin,downstream

acontinuous

endsill,

takinginto

accountthe

amplification

oftheprocess

andthe

trendto

induceim

portantdepressions.Vlfhengranularm

aterialscom

posethe

bed,thepresence

oftheend

sillcollaborates

with

acontrolled

scourdownstream

nearthe

structure.If

largerocks

compose

thebed,

prototypeexperim

entaldata

demonstrates

thatthe

endsill

hasnot

abeneficialaction.Itincreases

flowfluctuations

andconcentrates

theturbulentenergy

arotmd

adom

inantfrequency,removing

biggerblocks

andfavoring

localscour.Am

ethodologyis

proposedforthe

calculationofthe

weightofeventualprotecting

blocksorthe

anchoragebars

toavoid

therem

oval,takinginto

accounttheincidentFroude

Num

berofthe

jump

andthe

relationshipam

ongbroad

andlength

oftheblock.

INT

RO

DU

CT

ION

Asit

isusualin

othercountries,m

orethan

50%ofthe

bridgefailures

inArgentina

werecaused

byhydraulic

problems,m

orespecifically

becauseunderestim

atedscour.Itis

true

IScientificM

anager,NationalInstitute

ofWaterand

Environment(IN

A),C.C.46,(1082)Aeropuerto

Ezeiza,Argentina

210

Page 211: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

thatmostofthem

hadnotgood

hydrologicestim

ationoffloods

butscourestimation

wasalso

discussed.

Thechange

ofstatisticalaspectsoffloods

(theannualvolum

eofla

PlataR

iverbasinincreased

inthe

lasttw

entyyears)

requiresa

newevaluation

ofbridgedesign

andscour

recalculation.When

bigfloods

arepresentin

thedownstream

regionofthe

ParanaRiver,the

floodvalley

hasm

orethan

fortykilom

etresw

ideduring

largeperiods

(more

thanone

month).

Inthis

conditionsthe

scourinbridges

cannotbecalculated

byusualhydraulics

conditions.W

henscour

isdeveloping

thesection

isincreasing,

buttheupstream

waterlevelrem

ainspractically

constant.Forthistype

ofprocesses,thereare

some

localexperiences.

In1949

Dr.Schoklitsch

wasappointed

"ExtraordinaryProfessor"

atthe

HydraulicsInstitute

oftheExactSciences

andTechnology

SchoolattheU

niversityofTucum

an.In1953

Dr.Schoklitsch

beganhis

academic

andscientific

activitiesatthe

NationalUniversity

ofCuyo,in

SanJuan,atthe

footoftheAndes

Mountains.Itshould

bepointed

outthatDr.Schok1itsch's

researchdealtbasically

with

them

ovementofwater-transported

solids.Basedon

hisworks

toverify

Stemberg's

theoriesand

reviseD

uBoys's,he

formulated

newequations

onthe

movem

entofuniform

grain-sizesands,specific

fiiction,and

criticaldragforce.

On

theother

hand,his

known

equationto

allowthe

maxim

umdepth

calculationoflocal

scourdownstream

energydissipators

waspublished

previously,whenthe

authorwasprofessorin

Europe.Nevertheless,the

influenceof

Schoklitschequations

onthe

argentinedesign

ofstructures

underscour

conditionswas

detectedin

awiderange

ofengineers.

Threedifferent

subjectscan

exposethe

contributionofArgentine

researchon

localscour:localscouratbridge

piers,localscourdownstreamskijum

pstructures

andlocalscour

downstreamhydraulicjum

penergy

dissipators.

1-TheproposalofC

ottaand

Jensento

avoidthe

localscouratbridgepiers

Theprotection

againstlocalscouraround

bridgepiers

inalluvialrivers,

isan

localsystem

developedby

Cottaand

Jensen(2),aftera

longexperim

entalresearchfrom

laboratorytests

atthe"G

uillermo

Céspedes"Hydraulic

Laboratoryofthe

LaPlata

NationalU

niversity.

Theconstruction

systemconsists

ina

corbelslabem

ergingfrom

thepier

sectionjust

atthebed

level.Theslab

avoidsthe

horseshoevortex

actionon

them

ovablebed.

Thecom

prehensivedesign

oftheprotection

structurecan

beobserved

inthe

Figure1,

',

IF.

'~

——

-—¢»-

-i-i —

g,

‘_

"""""

"'

‘""'-

I4

._

=t-

.-~Q

-i

--,

_rtr

g.-

—5

;~

.>

_5

__H

D

i._

__

i...

.__.---

:Y

!A

-'

—.'

.I

'"

0--

-M

-_

__¢,

-r

._

-5_

...

i7

Jgm

“'7'

-..-

I.

__’_

Fig.1G

eneraldesignofthe

protectionstructure

211

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Page 215: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Asit

isobserved

inthe

Figure5,

theH

\lCYTH

equationis

alwaysa

goodm

eansolution

forthese

newprototype

datacom

ingfor

authors.Nevertheless,

dueto

thesafety

conditionsneeded

byengineering

design,itisbetterto

includea

safetycoefficientto

coverthe

99%of

existingprototype

data.It

isdem

onstratedthat

thiscoefficient

must

beapproxim

ately1.3.The

INC

YTH

equationw

iththis

safetycoefficientis

alsopresented

inthe

Figure5

asa"suggested

designcurve".

3-Localscouronrocky

bedsdow

nstreamstilling

basins

Energydissipators

oflargeflatlands

dams

usuallyconsist

ofhydraulicjum

pstilling

basins.Intemalflow

ofahydraulic

jump

isessentially

macroturbulentshowing

severerandom

pressureand

speedfluctuations.

Incidentkineticenergy

hasa

transformation

alongthe

jump

convertingto

potentialenergy.

Italso

generatesfluctuation

energyw

hichis

transportedby

differentscalesvortex.This

energyis

graduallydissipated

downstreamthejum

p.

Rapidly

variablepressure

fieldsin

spaceand

time

trendsto

enlargeexisting

beddiscontinuities

orcreate

themwhen

itis

composed

byerodable

rock.Fluctuating

pressurespropagation

insidethe

rockystructure

causesitto

breakinto

minorpieces.Altem

ativesforces

inducedby

pressurefluctuations

pullsome

blocksoutofthe

bed.Drainageestablished

among

theblocks

reducesfluctuations

amplitude.The

cavitycreated

previouslyenlarges

byascending

streams

generatedin

thezone

whenthejethits

againstthebed.

Water

pressureis

highlyvariable

alongthe

interfacerock-water

beinggreater

thanaverage

incertain

zonesand

lesserinthe

others.VI/henrocky

blocksdim

ensionsincrease,space-

time

correlationoffluctuating

pressurestrendsto

reduce.Astrong

ascendantinstantaneousforce

willbe

generatedifthere

isan

importantsim

ultaneityofactions.Blocks

shouldhave

reduceddim

ensions,turbulenceshould

beofa

largescale

orbothcauses

shouldpresenttogetherforthis

tohappen.There

areclearevidence

thatmacroturbulence

largelyexceeds

conventionalstillingbasins

length.Hepresented

some

practicalexamples

whereturbulence

intensitydecay

begunat

twice

thebasin

length(basinsdesigned

usingclassicaljtunp

length)

Macroturbulentnature

offlowinside

ahydraulic

jump

stillingbasin

isresponsible

forthe

existenceofstrong

pressurefluctuations.M

anypapers

hasbeen

written

givinga

warningsaboutits

highlydestructive

nature(5).Presence

ofstructuraldiscontinuitieshelps

toseverely

amplify

pressurefluctuations

amplitude.

Alsospectrurns

showa

tendencyof

energyconcentration

arounda

dominantfrequency.Forced

hydraulicjumps

canbe

producedinserting

achute

element

inthe

laboratorycanal.

Considerthat

thiselem

entconsist

ofa

despicablethickness

situatedata

distancex0

fromjum

pstart,ofheighthb

andReynolds

numberis

highenough

toadm

itviscous

forcesinfluence

isnegligible.

Theexpression

tocalculate

anon

dimensional

coefficientthat

groundupon

rootm

eansquare

fluctuatingpressure

amplitudes

(\li>'2)iS=,2

C711,0:/iii/2

(3)C'i1>=C'p(X/hi.X0/hi.hb/111,F1)

(4)where

Section1

isthe

upstreamsection

ofthejum

p(where

itbegins)and

p=

fluidspecific

mass,h1

=verticaldepth

atSection1,V1

=velocity

atSection1,and

F1=

FroudeNum

beratSection

1.Thisexpression

hasbeenobtained

throughdim

ensionalanalysis.

215

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Page 217: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

where"a"

and"b"

arethe

dimensions

ofthe

calculationslab.

Thoseexperiences

wheredeveloped

usingdepths

atSection1higherthan

3cm

andReynolds

nmnberover100,000.

Only

resultsobtained

forF1=

4and

x/h1=

26w

illbeused

forfurthercalculationsin

thispaper.

a/hl=3

C'1==0.021

a/hl=4

C'F=

0.026(7)

3./1'11=

5C

'F=

0.0

l7

Estiinationsaboutthe

dimensions

ofbasalticrock

blocksthatcould

berem

ovedby

theflow

downstreamSalto

Grandedam

'sstilling

basinhave

beenm

ade.This

damis

locatedon

Uruguayriver(between

Argentinaand

Uruguay).An

exigentconditionthatreally

occurredwas

takenabase

ofcomparison:q

=70.87

m2/s,F1=

4.17,l=

64m

(basin'slength),h1

=3.09

m,L,=

82m

(jump's

length),U1=

22.96m

/s,x/h1=

26.5(non

dimensionalposition).

Resultsobtained

areenough

tocalculate

C'pcoefficientthrough

rootmean

squareforce

F(m,,).C'1=isnota

goodindicatorofextrem

einstantaneous

forcesvalues.Using

dataregistered

forFroudenum

berequalto4

with

fixbarrierexperim

ents,arelationship

betweenupliftextrem

e0.1%

andm

eanvalues

hasbeen

found.Extreme

valuesare

3.07to

3.20tim

eshigherthan

rootm

eansquare

values.Theidea

ofallthiscalculations

isto

obtainan

estimation

ofminim

umrock

blocksthickness

toassure

stabilityits

ownweight.Forthis

working

conditionsflow

canlifteven

ablock

of1.70m

ofthicknessand

lineardimensions

ofseveralmeters.This

numberis

ingood

concordancew

ithvisualm

easuresrealisedduring

arestorationofthe

stillingbasin.

Localisederosion

ofimportance

shouldnotbe

admitted

downstreama

spillway

with

ahydraulic

jump

stillingbasin

which

dimensions

havebeen

designedusing

classicalcriteria.

Though,thisis

neveraccomplished.

Ingeneral,

erosiondownstream

astilling

basinw

hichlength

isequalto

thetheoreticaljuinp's

lengthis

neverzero.Theproblem

isthatjum

p'slengths

givenby

macroscopically

conditionsbeing

thelastsection

theone

wherethe

downstreamdepth

correspondingto

fieejum

pis

reachedand

averagespeed

iseasily

detemiined

usingcontinuity

andm

omentum

equations.

Velocityprofile

isusually

assumed

asunifonnin

thatsection(aschannelturbulentflow

profile).Infactthis

isnottrue

beingitm

oreand

more

differentwhile

FroudeN

umberatSection

1decreases.This

disturbanceim

pliesvery

lowsurface

speedsand

highspeeds

nearthebottom

,w

ithan

erosivecapacity

highlysuperior

tothe

onecalculated

usingaverage

speed.It

hashappened

thatsome

kindofbasaltclassified

asnon

erodablehave

beenfound

inplaces

wherelarge

flatlandsdam

shave

beenbuilt.Using

standardcriteria

stillingbasins

havebeen

designedw

itha

lengthof60%

ofthetheoretical.End

sillsofthose

basinsgenerates

conditionsofforced

hydraulicjump

iftheyare

ofenoughheight,favouring

macroturbulence

andlocalerosion

oftheriver

bed.A

warningagainst

consideringthat

arocky

bedsubm

ittedto

severepressure

fluctuationsbehave

asan

alluvialbed

isgiven.

Equilibriumis

dynamical

inthis

lastcase,

reachinga

fmal

conditionafter

scouring.O

nthe

contrary,when

thebed

iscom

posedof

meteorized

rockofseveraldim

ensionsblocks,the

remotion

ofoneofthis

blocksis

irreversible.Erosion

processon

rockybeds

isthen

completely

differentthan

theanalysed

forgranular

materialin

laboratory.

217

Page 218: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Thepresence

ofan

endsill

isbeneficial

whenriver

bedis

alluvial,separating

thatcontrolled

butinevitableerosion

fi"omthe

structure.Vlfhenriverbed

iscom

posedby

rockend

sillsdoes

nothelpbutfavourerosion

generatingan

increaseofspeed

fluctuations,concentratingenergy

aroundadom

inantfiequencythattrendsto

remove

largeblocks.

RE

FER

EN

CE

S

(1)Balloffet,A.(1987):"Erosi6n

ysedim

entaciénzrevista

dem

étodosde

analisis"(Scour

andsedim

entation:revueofm

ethodsforanalysis),FirstArgentine

Seminaron

LargeDam

s,Ituzaingé,Conientes,Vol.2,pp.151-189.(2)

Cotta,R.D.

andJensen,

P.D.(1966):

"Dispositivo

paraanular

laerosién

enlechos

moviles

provocadapor

pilaresinterpuestos"

(Dispositive

toavoid

thescour

onm

ovablebeds

dueto

pier'sinterference),IILatin

American

IAH

RCongress,Caracas,

Venezuela,Vol.1,pp

14-26.(3)

Cliividini,M

.F.etAl(1983):

"Evaluacionde

lasocavacion

maxim

aaguas

debajode

aliviaderosen

saltode

esqui"(Evaluation

ofmaxim

tunscour

downstramski-jum

pspillw

ays),Proceedingsofthe

XIW

aterNationalCongress,Cordoba,Argentina,Vol.

6,pp.187-210.

(4)Kem

ing,A.

etAl

(1987):"Free

jetscour

torock

riverbed",X

XII

IAH

RCongress,

Energydissipation

Seminar,Lausanne,Switzerland.

(5)Lopardo,R

.A.,De

Lio,J.C.andVem

et,G.F.(1987):"Therole

ofpressurefluctuations

inthe

designofhydraulic

structures"in

DesignofH

ydraulicStructures,pressed

byR.

Kiaand

M.L.Albertson,Colorado

StateUniversity,FortC

ollins,pp.161-175.(6)

Lopardo,R.A.

andLapetina,

M.

(1997):"Local

scouron

rockybeds

downstreamstilling

basins,inM

anagementofLandscapes

Disturbedby

ChannelIncision,editedby

S.S.Y.W

ang,E.J.

Langendoenand

F.D.Shields,

TheU

niversityof

Mississippi,

Oxford,U

SA,pp.288-295.(7)

Lopardo,R.AndSly,E.(1992):

"Constatacionde

laproftuididad

maxim

ade

erosionaguas

debajode

aliviaderosen

saltode

esqui"(Validation

ofthem

aximum

depthof

scourdownstreamski-jum

pspillw

ays),RevistaLatinoam

ericanade

Hidraulica

IAHR

,Sao

Paulo,Brasil,N°4,pp.7-23.

(8)M

ason,P.J.andArum

ugam,K.(1985):"Free

jetscourbelowdam

sand

flipbuckets",

ASCE,JournalofH

ydraulicEngineering,Vol.

11,N°2,pp.220-235.

(9)O

liveiraLem

os,F.

AndM

atiasRam

os,C.

(1984):"H

ydraulicm

odellingoffree

jetenergy

dissipation",Sym

posiumon

ScaleEffects

inM

odellingof

Hydraulic

Structures,Esslingenam

Neckar,Gennany,pp.7.6/1-7.6/5.

(10)Riedel,R.(1989):"Socavacion

aguasabajo

delsaltode

esquidelvertederode

lapresa

deC

olbtin"(Scourdownstream

theski-jum

pspillw

ayofC

olbtinD

am),IX

National

CongressofChilean

Hydraulic

EngineeringSociety,Santiago,Chile.

(11)SardiSocorro,V

.A.and

Martiribez

dela

Plaza,C.(1972):"Manualdc

procedimientos

parael

calculohidraulico

fluvialde

puentes"(Procedure

manual

forthe

fluvialhydraulic

bridgedesign),M

inisteriode

ObrasPfiblicas,Caracas,Venezuela,Vol.2.

218

Page 219: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

BR

IDG

ES

CO

UR

PR

OB

LEM

SIN

KO

RE

A

ByTae

Hoon

Yoonl,SooSam

Kim”,G

yeW

oonChoi3,Sangseom

Jeong4__1r--.¢=:___-_L—

.-:_—;-—

.1—_—

--:—

_.__

\

AB

STR

AC

T

Bridgescourproblem

shave

recentlybecom

eim

portantissuesfor

civilengineersin

Korea.Thereasons

are:first,thecatastrophic

collapseofthe

Sung-suG

randbridge

hasled

toa

reconsiderationofsafety

measures

andinspection

methods

forallbridges,

includingIm

derwaterstructural

integrityinspections

andsedim

entscour

measurem

ents.Second,

relativelyolder

small

andm

ediumsized

bridgeswere

designedand

builtw

ithoutconsiderations

ofscour

effects.A

recentevaluation

of100

bridgesin

Koreafor

scoursensitivity

showedthatabout85%

arescourcriticaland

thus,theyw

ouldfailifsubjected

tothe

designflood.In

theserespects,this

paperpresentsthe

briefsumm

aryofscourpractices,

thetechniques

ofscourmonitoring

systems

andscourprotection

techniquesavailable

forbridge

foundationsin

Korea.

INT

RO

DU

CT

ION

InSouth

Korea,many

bridgeconstructions

arein

progressin

riverandcostalareas.

Bridgescour

problems

haverecently

become

important

issuesw

ithinthe

Koreangeotechnicaland

waterresources

engineeringcom

miuiity.

Thereasons

arenum

erousbut

them

ostimportantfactors

canbe

stunmarized

asfollow

s.First,thecatastrophic

collapseof

theSung-Su

Grand

Bridgecrossing

theHan

riverhas

ledto

areconsideration

ofsafetym

easuresand

inspectionm

ethodsforallbridges.These

safetyconcem

shave

pushedlocal

govemm

entsto

includeunderwater

structuralintegrity

inspectionsand

sediment

scourm

easurements.Second,relatively

oldersmalland

medium

sizedbridges

weredesigned

andbuiltw

ithoutconsideringscoureffects,and

measurem

entsshow

thatthesebridges

dosuffer

fromsevere

scourproblems.

Inaddition

tobridge

transversingrivers,active

developmentofthe

coastalareashas

broughtincreasedpressure

tobuild

coastalbridgessuch

asthe

Seohae,theYoungjong

andthe

KwanganG

randBridges.

Interestinglyam

ongthem

,the

siteofthe

SeohaeG

randBridge

wasnota

favorablesite

onw

hichto

builda

bridgedue

tothe

relativelylarge

tidalm

otion.It

istrue

thatm

uchw

orkhas

beendone

inthe

fieldof

riverbridge

scoiuing(Breusers

andR

audkivi,1991)and

thereseem

stoo

many

empiricalform

ulasto

apply.Itis

1Professor,Dept.ofCivilEngineering,Hanyang

University,Seoul,Korea.

2Professor,Dept.ofCiviland

EnviromnentalEngineering,Chung

AngU

niversity,Seoul,Korea.

3AssociateProfessor,Dept.ofC

ivilEngineering,University

ofInchon,Inchon,Korea.4Associate

Professor,Dept.ofCivilEngineering,YonseiU

niversity,Seoul,Korea.

219

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Page 223: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Table3.N

umberofDam

agedBridges

dueto

scouring

TE

EE

EM

ME

8223;;M

EN

InKorea,

Before1990

justa

fewresearches

relatedto

thescour

wereconducted

basedon

experiments.Lee(1984)conducted

theresearch

onthe

localscouratbridgepiers

andconcluded

thepierReynolds

numberand

turbulentstrengthare

them

ainparam

etersfor

forecastingthe

maxim

umscour

depth.Kim

(1985a)conducted

theexperim

entsaboutthe

maxim

umscour

depthincluding

thevortex

mechanism

atcircular

pier.Kim

(1985b)com

paredthe

maxim

umscourdepths

calculatedusing

theexisting

scourequationsw

ithhis

experiment

dataw

hichwere

obtainedfrom

them

odeltests

usingthe

diatomite

asthe

channelbedm

aterial.Since1990,m

uchw

orkhas

beendone

inm

anyfields:the

estimation

ofscour

depthutilizing

numericalm

odels,the

detailedcharacteristics

oflocal

scotu"at

bridgepiers

incohesive

soil(Choi,

1998),sack

gabionas

scourcounterm

easures(Yoon,

1998),andautom

aticreal-tim

escourm

easurements

(Leeand

Yeo,1998).

SCO

UR

MO

NITO

RIN

GSYSTEM

S

Thereare

some

monitoring

devicesavailable

forbridge

scourm

easurements

inKorea.They

areG

PR(Ground

PenetratingRadar),M

agneticCollar,Fathom

eterandburied

rod.Here,anautom

aticrealtim

ebridge

ScourMonitoring

System(SM

S),designedby

Leeand

Yeo(1998)

andthen

usedto

detectthescouring

processofSeohae

Grand

Bridge,is

introduced.Seohae

Grand

Bridgeis

acoastalhighw

aybridge

crossingAsan

BayChannel.Fig.

1shows

thebridge

locationm

apenclosed

byKorean

peninsula.Theconstruction

startedin

1994and

willbe

completed

in2001.The

mostsignificantproblem

fromthe

perspectivesof

costalengineers

oftheconstruction

siteis

thew

orld’sfam

ousand

largesttidal

motion

measuring

upto

atidalleveldifference

of9.3mbringing

tidalcurrentspeedup

to0.8m

/s.Also,concurrentdevelopm

entplansin

thatarearesulted

inthe

currentspeedadding

upto

2.65m/s.Field

bridgeengineers

reportedexcessive

bridgescourdepth

more

than8m

fromthe

designbed

level.

223

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Page 228: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Thestability

ofriprapstones

aroundpiers

issensitive

tochanges

inelevation

ofthetop

ofthefoundation.W

henthe

topelevation

isthe

same

asthestream

bedorslightly

abovestrearnbed,itacts

asa

scourprotectivelayerby

interceptingthe

downflow

andprotecting

thestream

bed.It

implied

thatfor

theidentical

conditionssm

allerstones

canbe

usedto

attainthe

same

levelofprotectionifthere

isa

footingwhose

topelevation

isatstream

bedorslightly

aboveit.Ifthe

heightoffootingrises

ftnther,criticalvelocitydecreases

resultingin

reducedprotection.

Thereare

some

othermethods

usedto

protectbridge-pierscourin

Korea;Mattress

protectionand

deflectors.M

attressprotection

hasbeen

proposedas

anew

conceptusingartificialprotection

forlocalprotection

arounda

big,circular

pierin

abed

offme

sand.This

protectionconsists

ofntunerous

bundlesof

polyesterfilam

entsw

hichcan

besuspended

undera

frame

cantileveredfrom

thepier.

Thefirst

prototypetest

showedprom

isingresults

(Carstens1976).In

general,specialattentionhas

tobe

givento

providinga

tightcomiection

betweenthe

mattress

andthe

pier,becauseeven

thougha

smallgap,the

downflow

caninduce

severeerosion

thatextendsunder

them

attress.D

eflectorhas

beenused

forreducing

theintensity

ofthedow

nflownear

thepier

(Carstens1976;

Dargahi1987).

But,this

method

hasnot

beenused

widely

andfrequently,

becausethere

areno

designrules

areavailable

forthedeterm

inationofthe

width

andthe

heightofthedeflector,

which

isfixed

tothe

pier,anditdid

notnoticeablyreduce

theerosion

depth.

CO

NC

LUS

ION

S

Them

ainobjective

ofthisstudy

wasto

gatherthegeneralinform

ationon

thescour

problem,

scourpractice,and

scoursolutions

inKorea.

Before1990,m

ostoftheresearch

doneon

bridgescour

wasbased

onexperim

ents.The

testresultsfrom

suchexperim

entswere

notapplicableto

multiple

situationsbecause

mostofthe

experiments

werecarried

outatlocalized

conditions.Thus

hydraulicand/orbridge

designengineers

areoften

ataloss

overwhich

method

orequationis

applicableforthe

specificbridge

sites.

Since1990,m

uchw

orkhas

beendone

inm

anyfields

inKorea:

theestim

ationof

scourdepthutilizing

numericalm

odels,thedetailed

characteristicsoflocalscouratbridge

piersin

cohesivesoil,sack

gabionas

scourcountermeasures,and

scourmonitoring

systems

incoastalzones.Lately

therehave

alsobeen

advancedresearch

focusingon

theevaluation

ofsafetyofhydraulic

facilitiesand

researchconcerning

with

enviromnentalm

atters.

AC

KN

OV

VLE

DG

EM

EN

TS

Theauthors

wish

tothank

Dr.Lee

andProf.

Yeo,H

ydrauliclab.

atM

yongjiU

niversityfortheircontribution

tothis

study.

RE

FER

EN

CE

S

1.Breusers,H.,

Raudkivi,A.

1991.“Scouring

Hydraulic

StructuresDesign

Manual

2,”IAHR

,Balkema,Rotterdam

.

228

Page 229: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Carstens,T.,1976,“Seabed

ScourbyCurrents

nearPlatforms,”

3'“Conference

onport

andocean

engineeringunder

arcticconditions,

University

ofAlaska:991-

1006.Chiew,

Y.M

.,1995,

“Mechanics

ofRiprapFailure

atBridgePiers,”

JournalofH

ydraulicEngineering,ASC

E,121(9),pp.635-645.C

hoi,G.W

.,Ahn,

S.J.,K

im,

J.S.,Alm

,C.J.,

1995,“Piers

ScourPrediction

inPressure

Flow,”Korean

JournalofHydroscience,Vol.6,June,Seoul,Korea.C

hoi,G.W.,Ahn,

S.J.,Kang,K.W.,

1991“State-of-the-artofPierScourPrediction

ofDesign

Application,”Korean

JournalofHydroscience,

Vol.2,

June,Seoul,

Korea.C

hoi,G.W.,K

im,G

H.,Ahn,K.S.,

Seoh,B.H.,

1998“LocalScouratBridge

Piersin

theCohesive

Soilin

theYellow

Sea,”W

aterResource

Engineering,Vol.

2,pp1894-1899,Seoul,Korea.Dargahi,

B.,1987,

“FlowField

andLocal

Scouringaround

aC

ylinder,”Bulletin

TITRA

-VB

I-137,Departm

entof

Hydraulics

Engineering,R

oyalInstitute

ofTechnology,Stockholm

,Sweden.Escaram

eia,M.,

1998.“LaboratoryInvestigation

ofScouraroundLarge

Structuresin

TidalWaters,”Proceedings

ofICH

E"98,Ham

burg,Germ

any.Jarrett,

R.D

.,Boyle.

J.M

.,1986,

“Pilot

Studyfor

Collection

ofBridge-scour

Data,”U.S.G

eologicalSurvey,Water-Resources

InvestigationsReport86-4030.

Jung,D.W

.,1991,

“Experimental

Studyon

CharacteristicsofProtection

againstLocalScouraround

BridgePiers,”M

S.thesis,Han

YangU

niversity,Seoul,Korea.K

im,J.H.,

1985,“TheExperim

entalStudyforthe

Maxim

mn

ScourDeptharound

BridgePiers,”M

S.thesis,SeoulNationalU

niversity,Seoul,Korea.K

im,

H.S.,1985,

“Studyon

theScour

aroundBridge

Piers,”M

S.thesis,

SeoulN

ationalUniversity.Seoul,Korea.

Lee,J.,Yeo,

W.K.,

1998,“Developm

entofaField

Monitoring

Systemfor

Real-Tim

eCoastal

DataAcquisition,”

TechnicalReport

ofO

bservationN

etwork

EnhancementW

orkshop,KoreaOcean

Researchand

DevelopmentInstitute,

105-115.Parola,

AC.,

1993,“S

tabilityofRiprap

atBridge

Piers,”JournalofH

ydraulicEngineering,ASC

E,l19(10),pp.1080-1093.Richardson,

E.V.,

Harrison,

L.J.,

Richardson,J.

R.,and

Davis,

S.R.,

1993,“Evaluating

ScouratBridges,Hydraulic

EngineeringC

ircularNo.

18,”FHW

A-IP-90-017,FH

WA,February,pp.41-44.

Yoon,T.H.,

Park,K.D

.,Kim

,C.H.,

1998“Effects

ofFoundation

Geom

etryon

Riprap

ScourCounterm

easuresarotm

dBridge

Piers”Proceedings

of3rfKorea-Japan

BilateralSym

posiumpp240-242

Water

Resourcesand

Enviromrrental

Research,Seoul.Yoon,T.H.,Nam

gung,D.,

1998“

EfiectofNonuniform

ityofPierD

imensions

onthe

BridgePier

Scour,”Journalofthe

Hydraulic

Division,Proceeding

ofKorean

SocietyofC

ivilEngineers,Vol18,No.2-6,pp553-562,Seoul,Korea.

Yoon,T.H.,Yoon,S.B.,Oh,C.S.,1998,“Sack

Gabion

asScourCounterm

easures,”Journal

ofthe

Hydraulic

Division,

Proceedingof

KoreanSociety

ofC

ivilEngineers,Vol18,N

o.2-1,pp13-22,Seoul,Korea.

229

Page 230: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

BR

IDG

ES

CO

UR

EV

AL

UA

TIO

NIN

TH

EU

NIT

ED

ST

AT

ES

byE.V.Richardson‘,J.R.Richardsonz,and

P.F.Lagasse3

AB

ST

RA

CT

InSeptem

ber1988

TheU.S.FederalH

ighway

Administration

(FHW

A)issued

TechnicalAdvisory

T514023entitled

"ScouratBridges"toState

Highw

ayDepartm

entsorDepartm

entsof

Transportationw

hichrecom

mended

thateachState

evaluateevery

bridgeovera

streamas

toits

vulnerabilityto

scourofitsfoundations.

Accompanying

theadvisory

wasa

documententitled

"InterimProceduresforEvaluating

ScouratBridges."This

wasthefirstU.S.publicationwhich

gavespecific

recomm

endationsand

equationsfor

evaluatingscour.

Subsequently,FI—

IWA

issuedH

ydraulicEngineering

Circulars(I-IEC’s)18,20,and

23w

hichgave

state-of-practiceforevaluating

bridgescourforbridgesoverriverine

andtidalwaterways,streamstability

athighwaystructures

andbridge

scourand

streaminstability

countermeasures.

Thispaper

presentsthe

FHW

A’srecom

mendations

forbridgescourand

streaminstability

evaluation.

INTR

OD

UC

TION

InSeptem

ber1988,the

U.S.FederalHighw

ayAdm

inistration(FH

WA)

issuedTechnical

AdvisoryT514023

entitled"ScouratBridges"

toState

Highw

ayDepartm

entsorDepartm

entsof

Transportationw

hichrecom

mended

thateachState

evaluateevery

bridgeovera

streamas

toits

vulnerabilityto

scourofitsfoundations.

TheAdvisory

stated:"Mostwaterways

canbe

expectedto

experiencescour

overa

bridge'sservice

life(w

hichis

nowapproaching

100years).

Exceptionsm

ightincludewaterways

inm

assive,competentrock

formations

wherescourand

erosionoccuron

ascale

thatism

easuredin

centuries.....The

addedcostofm

akinga

bridgeless

vulnerableto

scouris

smallwhen

compared

tothe

totalcostofafailure

which

caneasily

betw

oorthree

times

theoriginalcostofthe

bridgeitself.

Moreover,the

needto

ensurepublic

safetyand

tom

inimize

theadverse

effectsstem

ming

frombridge

closuresrequires

ourbest

effortto

improve

thestate-of-

practiceofdesigning

andm

aintainingbridge

foundationsto

resisttheeffects

ofscour."

Accompanying

theadvisory

wasa

documententitled

"InterimProcedures

forEvaluatingScouratBridges".

Thiswas

thefirstU.S.publication

which

gavespecific

recomm

endationsand

equationsforevaluating

scour.Subsequently,in

1991,thisdocum

entwasrevised

andissued

asH

ydraulicE

ngineeringC

ircular18

(HE

C-18)entitled

"Evaluating

ScouratBridges."

Also,in

1991a

companion

document

(HEC

-20)entitled

"StreamStability

atHighw

ayStructures

wasissued."

‘SeniorAssociate,AyresAssociates

andEm

eritusProfessorofC

ivilEngineering,ColoradoState

University,FortC

ollins,CO2AssistantProfessor,D

epartmentofC

ivilEngineering,University

ofMissouri,Kansas

City,M

O3SeniorV

icePresident,Ayres

Associates,P.O.B

ox270460,FortC

ollins,C

O80527

230

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Page 233: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Floodstend

toscourthe

materialatabridge

crossingduring

therising

limb

oftheflood

andrefillthese

scourholesduring

therecession

limb.

Often,the

redepositedm

aterialinthe

scourholeis

more

easilyeroded

bysubsequentfloods.

Post-floodinspection

ofthebridge

crossingm

ayindicate

them

aterialaroundthe

foundationsare

adequate,whenin

fact,thebridge

isin

jeopardyof

failingduring

thenextflood.

Thisinfilling

also,makes

itdifficulttoobtain

fieldm

easurementof

scourdepthsbecause

them

easurements

haveto

bem

adeduring

aflood.

IMAXIMUM

cram-w

rrsascoua

5EQUILIBRIUM

scour:oeern

i 7 Vs

SCOURDEPTH,

LIVE

-BE

DSC

OU

R

17C

LEA

R-W

AT

ER

SCO

UR

PER

TIM

E

Figure1.

PierScourDepth

ina

Sand-BedStream

asaFunction

ofTime

(nottoscale)(Richardson

andDavis,

1995).

Them

agnitudeofthe

scourdepthdepends

onthe

flowvariables

ofthestream

(discharge,flow

velocityand

depth,angleofthe

flowto

thebridge,etc.),bed

andbank

materialcharacteristics

(bedrock,

alluvialornon-alluvial,

cohesiveornon-cohesive,

sizedistribution,

etc.)and

bridgecharacteristics

(sizeand

shapeofthe

pierandabutm

ents,width

ofopening,elevation

ofthedeck,

etc).'

DesignD

ischarge

Them

agnitudeofthe

flowvariable

dependson

theselection

ofadesign

discharge.The

selecteddesign

dischargefora

bridgeis

basedon

thedesign

lifeofthe

bridge,bridgeim

portance,consequence

offailure,etc.The

designdischarge

foradivided

highway

with

largeaverage

dailytraffic

(AD

T)(interstate

highway,autobahns,etc.)

would

belargerthan

fora

farmto

marketor

loggingroad.

Some

engineersadvocate

amaxim

umpossible

floodforim

portantbridges(Laursen,

1998)others

recomm

endrisk

analysis.Im

portantbridgesare

thosew

ithlarge

ADT,

Interstatehighways,schoolbus

andam

bulancerouts,and

etc.

TheFederalH

ighway

Administration

(FHW

A)

inH

EC-18

(Richardsonand

Davis,

1995)recom

mendsthatbridges

shouldbe

designedtoresistthe

floodevent(s)thatare

expectedto

producethe

mostsevere

scourconditions.HIEC-18

recomm

endsthe

100-yearfloodorthe

overtoppingflood

233

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Page 235: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 236: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 237: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

CO

NTR

AC

TION

SCO

UR

EQ

UA

TION

S

Contractionscourequations

arebasedon

theprinciple

ofconservationofsedim

enttransport.In

thecase

oflive-bedscour,this

simply

means

thatthefully

developedscourin

thebridge

cross-section

reachesequilibrium

whensedim

enttransportedinto

thecontracted

sectionequals

sediment

transportedoutand

theconditions

forsedimentcontinuity

arein

balance.Forclear-w

aterscour,thetransportinto

thecontracted

sectionis

essentiallyzero

andm

aximum

scouroccurswhen

theshear

stressreduces

tothe

criticalshearstressofthe

bedm

aterial.

Todeterm

ineifthe

contractionscouratabridge

isclear-w

aterorlive-beddetennine

ifthecriticalvelocity

(V0)orcriticalshearstress(re)ofthe

median

diameter(D50)ofthe

bedm

aterialinthe

channelupstreamfrom

thebridge

openingis

largerthanthe

averagevelocity

orshearstress

(clear-waterscour)orsmaller(live-bed

scour).O

rcalculatethe

contractionscourdepths

usingboth

equationsand

takethe

smallerscourdepth

(Richardson

andD

avis,1995).

Live-BedC

ontractionScourE

quation

RichardsonandDavis

(1995)inH

EC-18

recomm

endam

odifiedLaursen

(1960)equation

forlive-bed

contractionscour.

Itisbased

ona

simplified

transportfunction(Laursen,

1956)to

obtainequilibrium

sedimenttransportin

alongcontraction.

Inshortcontractions

suchasatabridge

itoverestim

atesthe

scourdepth(Richardson

andDavis,

1995).The

equationis:

9K1

&I

0,yr

Q1

VV2

ys=

y0-y0

=(Average

scourdepth,m)

where:y,=

averagedepth

inthe

upstreamm

ainchannel,m

yz=

averagedepth

inthe

contractedsection,m

y0=

averagedepth

inthe

contractedsection

beforecontraction

scour,mW

,=

bottomw

idthofthe

upstreamm

ainchannel,m

W0

=bottom

width

ofmain

channelinthe

contractedsection,m

Q,

=flow

inthe

upstreamchanneltransporting

sediment,m

3/s,cms

Q0=

flowin

thecontracted

channel,cms.

Often

thisisequalto

thetotaldischarge

unlessthe

totalfloodflow

isreduced

byreliefbridges

orwaterovertopping

theapproach

roadwaykl

:exponentsdeterm

inedbelow

dependingon

themode

ofbedmaterialtransport

V.

=(8Y1$1)”shearvelocity

inthe

upstreamsection,m

/sw

=m

edianfallvelocity

ofthebed

materialbased

onthe

D50(see

Figure2)

g=

accelerationofgravity

(9.81m

/s2)

237

Page 238: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 239: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 240: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

=the

unitweightofwater(9,800

N/m3)

=M

anningroughness

coefficient=

Shield’scoefficient

=specific

gravity(2.65

forquartz)=

densityofw

ater(999kg/m

3)pg

=density

ofsediment(quartz

2,647Kg/m

3)g

=acceleration

ofgravity(9.81

m/s2)

-om:/up-4:1-<

Equation7

isthe

basicequation

fortheclear-waterscoured

depth(y)in

alongcontraction.

Shield’scoefficient

forinitiate

ofmotion

rangesfrom

0.03to

0.1(Vanoni,

1975).Strickler’s

equationfor

ngiven

byLaursen,

inm

etricunits,

isn

=0.041

D”°.

Researchdiscussed

inRichardson

etal.(1990)recomm

endsthe

useofthe

effectivem

eanbed

materialsize

(Dm)inplace

oftheD50

size.The

useof

Dmw

ouldalso

bein

accordancew

iththe

work

ofFroehlich(1995).

Dmis

approximately

1.25D50_

Using

KSof0.039,

n=

0.041Dm

1'6andSS=

2.65in

Equation7

resultsin:

2y

=[ ]3

”(3)

Dm

2/3

W2

yS=y

—yo

(Averagescourdepth)

(9)

where:Dm=

diameterofthe

bedm

aterial(1.25D50)in

thecontracted

section,mys

=depth

ofscourinthe

contractedsection,m

yo=

originaldepthin

thecontracted

sectionbefore

scour,mO

thervariablesas

previouslydefined.

Clear-watercontractionscourequations

assume

homogeneous

bedm

aterials.However,

instratified

materials,the

clear-watercontractionscourequation

couldbe

usedsequentially.

Boththe

live-bedand

clear-watercontraction

scourequationsare

thebestthatareavailable

andshould

beregarded

asa

firstlevelofanalysis.If

am

oredetailed

analysisis

warranted,a

sedimenttransportm

odelsuchas

BRI-STAR

S(M

olinas,1993)orHEC-6

(U.S.Army

Corpsof

Engineers,1993)couldbe

used.

CR

ITIC

AL

VE

LOC

ITY

FO

RM

OV

EIV

IEN

TO

FB

ED

MA

TE

RIA

L

Thevelocity

anddepth

givenin

Equations6

areassociated

with

initiationofm

otionofthe

indicatedsize

(D).

RearrangingEquation

6to

givethe

criticalvelocityforbeginning

ofmotion

ofbed

materialofsize

Dresultin:.

240

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Page 242: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 243: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 244: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 245: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 246: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 247: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 248: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 249: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 250: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

2PATTERN:3x3x3x3x3

1.51

l@

1K0

1..

2

s=>OU1O

'5'8R:

5..°1:11:13 EIEIEI3EJEJEI EIEIEIlIll.'_lEll

Angleofattack

Figure8.

K0as

afunctionofangle

ofattack(afterSalim

andJones,

1996).

Scourdepthsforcom

plexpile

groups(notuniform

lyspaced

laterallyand

longitudinallyin

thestream

flow)can

notbedeterm

inedby

thesem

ethods.Scourforcom

plexpile

groupsw

ouldrequire

aphysicalmodelstudy.

PileCaps

Placedatthe

WaterSurface

orinthe

Flow

Forpilecaps

placedatornearthe

watersurfaceorin

theflow

,HEC

-18(Richardson

andD

avis,1995)

recomm

endedthatthe

scouranalysis

includecom

putationofscourcaused

bythe

exposedpile

group,computation

ofthepierscourcaused

bythe

pilecap

andpierscourcaused

bythe

pierifthepieris

partiallysubm

ergedin

theflow

.A

conservativeestim

ateoflocalscourw

illbethe

largestpierscourcomputed

fromthese

threescenarios

When

computing

thepierscourcaused

bythe

pilecap,aconservative

estimate

istoassum

ethatthe

pilecap

isresting

onthe

bedand

determine

V4and

yffrom

Equation19.

Use

Equation12

forpile

cap,pier

shaftand

exposedpile

groupsas

recomm

endedin

theprevious

discussions.Research

isunderway

todevelop

alessconservative

andm

orerealistic

equation(Salam

andJones,

1996and

Jones,1998)

Multiple

Colum

nsSkewed

tothe

Flow

Scourdepthform

ultiplecolum

ns(asillustrated

asagroupofcylinders

inFigure

6)skewedto

theflow

,depends

onthe

spacingbetween

thecolum

ns.The

correctionfactorforangle

ofattackw

ouldbe

smaller

thanfor

asolid

pier.H

owm

uchsm

alleris

notknow

n.R

audkivi(1986)

indiscussing

effectsofalignm

entstates"..the

useofcylindricalcolum

nsw

ouldproduce

ashallower

250

Page 251: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

scour;forexample,w

ithfive-diam

eterspacingbetween

columns

thelocalscourcan

belim

itedto

about1.2tim

esthe

localscouratasinglecylinder."Thus

formultiple

columns

spaced5

diameters

ormore

apartandatan

angleRichardson

andD

avis(1995)recom

mend

thatthelocalscourdepth

canbe

takenas

1.2tim

esthe

localscourdepthata

singlecolum

n.

Formultiple

columns

spacedless

than5

pierdiameters

apart,thepierw

idth’a’is

thetotal

projectedw

idthof

allthecolum

nsin

asingle

bent,nonrial

tothe

flowangle

ofattack.This

composite

pierwidth

would

beusedin

Equation12to

determine

depthofpierscour.

Thecorrection

factorK,would

be1.0

regardlessofcolum

nshape.

ThecoefficientK2

would

alsobe

equalto1.0

sincethe

effectofskeww

ouldbeaccounted

forbythe

projectedareaofthe

piersnonnalto

theflow

(Richardson

andD

avis,1995).

Thedepth

ofscourforamultiple

column

bentwillbe

analyzedin

thism

annerexceptwhenaddressing

theeffectofdebris

lodgedbetween

columns.

Ifdebrisis

evaluated,itwould

belogical

toconsiderthe

multiple

columns

anddebris

asa

solidelongated

pier.

PressureFlow

Scour

Pressureflow

,which

isalso

denotedas

orificeflow

,occurswhen

thewatersurface

attheupstream

faceofthe

bridgeis

greaterthanorequalto

thelow

chordofthe

bridgesuperstructure

andthe

waterisin

significantcontactwith

thebridge

deck.A

thigherapproachflow

depths,thebridge

canbeentirely

submerged

with

theresultingflow

beingacom

plexcom

binationoftheplunging

flowunderthe

bridge(orifice

flow)and

flowoverthe

bridge(w

eirflow).

Inm

anycases,when

abridge

issubm

erged,floww

illalsoovertop

adjacentapproachem

bankrnents.Hence,_foranyovertopping

situation,thetotalw

eirflowcan

besubdivided

intow

eirflowoverthe

bridgeand

weirflow

overthe

approach.

Abed(Abed,

1991,and

Abedet

al.,1991),

froma

limited

clear-waterflum

estudy

atColorado

StateU

niversity,statedthat

pressureflow

couldincrease

pierscourdepthsby

2.3to

10tim

es.Theseresults

wereobtained

bycomparison

ofscourdepthsforfree

surfaceandpressure

flowsim

ulationsw

ithsim

ilarhydrauliccharacteristics.

How

ever,som

etimes

whena

bridgebecom

essubm

erged,theaverage

velocityunderthe

bridgeis

reduceddue

toa

combination

ofadditionalbackwatercaused

bythe

bridgesuperstructure

impeding

theflow

,anda

reductionofthe

dischargewhich

passesunderthe

bridgedue

tow

eirflowoverthe

bridgeand

approachem

bankments.

Asa

consequencescourdepths

arereduced.

Jones(Jones

etal.,1993,

1996and

Richardsonand

Lagasse,1999,p.288)in

clear-waterpressure

flowstudies

atFI—IW

A’sTumer-Fairbank

ResearchCenter,found

that(1)localpierscourw

ithpressure

flowhas

twocom

ponents;(2)onecom

ponentisverticalcontraction

scourcausedby

thebridge

superstructureand

theotheris

localpierscourcausedbythe

pierobstructingthe

flow;(3)

them

agnitudeofthe

localpierscourwith

pressureflowis

approximately

thesam

easforfree

surfaceflow

;and(4)thatthe

twocom

ponentsare

additive.

251

Page 252: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Ameson

(1997,ArnesonandAbt,1998),in

acomprehensive

live-bedflum

estudyofpressure

flowscoursponsored

bythe

FHW

Averified

Jonesfindings.

Equation12

isused

todeterm

inethe

localpierscourcomponentcausedbythe

pierobstructingthe

flow.

Fortheverticalcontraction

pierscourcom

ponenthedeveloped

thefollow

ingequation.

E?’-=—5.08

+1.27[l’l)+

4.44[-Hi]+0.1962?-J(22)

yt

Hb

y1vc

where:ycps

=depth

ofverticalcontractiondeck

scour,m=

flowdepth

upstreamofbridge

deck,m=

distancefrom

bridgedeck

tochannelbed,m

=average

velocityofflow

throughbridge

opening,m/s

=criticalvelocity

ofthebed

material,m

/so<o_<GE

.‘..<

ScourDepths

With

Debris

Debrislodged

onapierusually

increaseslocalscouratapier.

Thedebris

may

increasepier

width,localvelocity

anddeflectthe

flowdownward.

Thisincreases

thetransportofsedim

entoutofthe

scourhole.

When

floatingdebris

islodged

onthe

pier,thescourdepth

isestim

atedby

assuming

thatthepierw

idthis

largerthanthe

actualwidth.

Theproblem

isin

determining

theincrease

inpierw

idthto

usein

thepierscourequation.

Furthermore,atlarge

depths,theeffectof

thedebris

onthe

scourdepthsshould

diminish.

Also,debrislodged

onpiers

andabutm

entscan

deflecttheflow

againstanotherpierofabutmentresulting

invery

largeangles

ofattack,andlarger

velocities.This

may

beworse

thanthe

scouratthepierorabutm

entwith

thedebris.

Asw

ithestim

atinglocalscourdepthsw

ithpressureflow

,onlylimited

researchhasbeendone

onlocalscourw

ithdebris.

Melville

andDongol(1992)have

conductedalim

itedquantitative

studyof

theeffect

ofdebrison

localpierscour

andhave

made

some

recomm

endations.However,

additionallaboratorystudiesw

illbenecessaryto

betterdefinethe

influenceofdebris

onlocalscour.

TO

PW

IDT

HO

FP

IER

SC

OU

RH

OLE

S

Thetopw

idthofascourhole

incohesionless

bedm

aterialfromone

sideofapierorfooting

canbeestim

atedfrom

thefollow

ingequation

(Richardsonand

Abed,1993),RichardsonandD

avis,1995),and

Richardsonand

Lagasse,1999,p.311):

W=

ys(K

+cot

9)(22)

252

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Page 255: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

DesignforScouratAbutm

ents

Thelackofadequateabutm

entscourequations(fundamentallywrong

andoverconservative)lead

theFederalHighw

ayAdm

inistrationto

recomm

endthatfoundation

depthsforabutm

entsbe

setatleast1.8

meters

belowthe

streambed,including

long-termdegradation

andcontraction

scourandprotectthe

foundationsw

ithrock

riprapand/orguide

banks(Richardson

andD

avis,1995).To

aidin

designofthe

foundationsFroehlich’s’s

(1989)live-bed

scourequationfor

I./y<

25and

am

odificationofH

IRE’s

(Richardsonetal.,

1990)equationforL/y

>25

weregiven.

TheH

IRE

equationwas

basedon

fielddata

ofscourattheend

ofspursin

theM

ississippiRiver(obtained

bythe

USCorps

ofEngineers.

Froehlich’sA

butmentScourE

quationforL/y

<25

L/

0.43§

=2.27

K1K5

Fr°'61+

1(24)

where:K1=

Coefficientforabutm

entshape(see

table4)

K2=

Coefficientforangle

ofembankm

enttoflow

K2=

(0/90)(seefigure

9fordefinition

of0)0<90°ifem

bankmentpoints

downstream0>90°ifem

bankmentpoints

upstreamL’

=

AcFrV45=

Q5=

YaY5=

Lengthofabutm

ent(embankm

ent)projectednorm

altoflow

,mFlow

areaofthe

approachcross

sectionobstructed

bythe

embankm

ent,m2Froude

Num

berofapproachflow

upstreamofthe

abutment,V4./(gy5)"*

Q4/A4,m/s

Flowobstructed

bythe

abutmentand

approachem

bankment,m3/s

Averagedepth

offlowon

thefloodplain,m

Scourdepth,m

The1

addedto

theequation

causeditto

envelope98

percentofthedata

inthe

developmentofthe

equationby

statisticalmethods.

HEC

-18E

quationfor

L/y>

25

ii=

4p;°-33i

y,0.55

where:

Y5=

Y1=

(25)

Scourdepth,mDepth

offlowatthe

abutmenton

theoverbank

orinthe

main

channel,m

255

Page 256: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Fr=

FroudeNum

berbasedon

thevelocity

anddepth

adjacenttoand

upstreamof

theabutm

entK4

:Abutm

entshapecoefficient(from

table4)

Tocorrectequation

25forabutm

entsskewed

tothe

stream,use

Figure9.

RecentAbutm

entScourDepth

Equations

Recentabutmentscourequations

haveappeared

inthe

literaturethatare

notbasedon

theabutm

entlengthbuton

theflow

interceptedbythe

abutmentand

approachem

bankment.

Theseare:

1.Chang

andDavis

equation(Richardson

andLagasse,

1999,p.

401)w

hichis

basedon

Laursen,livebed

contractionscourequation.

2.Strum

(1999)equation

forabutments

incom

poundw

ithvariable

setbacksfrom

them

ainchannel.

3.Richardson

andTrivino

(1999)equationbased

onm

omentum

exchange

4.Kouchakzadeh

andTownsend

(Richardsonand

Lagasse,1999,p.417)equation

basedon

mom

entumexchange.

Theseequations

havenotbeen

testedin

thefield

sothe

1995HEC-18

recomm

endationsare

stillvalidin

theU.S.

CO

NIP

UTE

RM

OD

ELS

Thehydraulic

bridgeroutinesofeitherthe

computerm

odelsWSPRO

(Shearrnan,J.O.,1987)orH

EC-R

AS(U.S.Corps

ofEngineers,1997,RichardsonandLagasse,1999,p.669)can

determine

theone-dim

ensionsflow

variableforuse

inthe

determination

ofscourdepthsata

bridge.These

modelsdeterm

ineaverageflow

depthsandvelocitiesovertheroadwayandbridge,asw

ellasaveragevelocities

anddepths

approachingand

underthebridge.

ST

RE

AM

INS

TA

BILIT

Y

Streams

aredynam

ic.Areas

offlow

concentrationcontinually

shiftbank

lines.In

meandering

streams

havingan

"S-shaped"planforrn,

thechannel

moves

bothlaterally

anddownstream

.A

braidedstream

hasnumerouschannels

which

arecontinuallychanging.

Inabraided

stream,the

deepestnaturalscouroccurswhen

two

channelscom

etogetherorwhen

theflow

comes

togetherdownstreamofan

islandorbar.

Thisscourdepth

hasbeen

observedto

be1to

2tim

esthe

averageflow

depth(NorthwestH

ydraulicConsultants

Ltd.,1973)(Richardsonand

Davis,1995).

Abridge

isstatic.

Itfixesthe

streamatone

placein

time

andspace.

Am

eanderingstream

whose

channelm

oveslaterally

anddow

nstreaminto

thebridge

reachcan

erodethe

approach

256

Page 257: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 258: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

waterways,theequations

usedto

determine

riverinescourare

applicableifthe

hydraulicforces

arecarefully

evaluated.

Bridgescourin

thecoastalregion

resultsfrom

theunsteady

diurnalandsem

i-diurnalflows

resultingfrom

astronomical

tides,large

flows

thatcan

resultfrom

stormsurges

(hurricanes,nor’easters,and

tsunamis),and

thecom

binationofriverine

andtidalflows.

Also,thesm

allsizeof

thebed

material(norm

allyfine

sand)as

well

assilts

andclays

with

cohesionand

littoraldrift

(transportofbeachsand

alongthe

coastresultingfrom

waveaction)affectthe

magnitude

ofbridgescour.In

addition,tidalfloware

subjectedto

mass

densitystratification

andwatersalinity

butthesehave

onlyam

inoreffectonbridge

scour.The

hydraulicvariables

(discharge,velocity,anddepths)

andbridge

scourinthe

coastalregioncan

bedeterm

inedw

ithasm

uchprecision

asriverineflow

s.These

detenninationsare

conservativeand

researchis

neededfor

bothcases

toim

provescour

determinations.

Determining

them

agnitudeofthe

combined

flows

canbe

accomplished

bysim

plyadding

riverineflood

flowto

them

aximum

tidalfloworrouting

thedesign

riverineflow

sto

thecrossing

andadding

themto

thestorm

surgeflow

s.

Some

ofthesim

ilaritiesand

differencesbetween

tidalandriverine

flows

are:

~Tidalflow

sare

unsteadyw

ithshortduration

peakflow

s.R

iverineflow

sare

alsounsteady

andm

anyhave

shortdurationpeakflow

s.Existing

scourequationspredictscourdepths

forthese

shortdurationpeakriverine

flows.

Also,waterwaysin

thecoastalzone

arecom

posedof

finesand

which

erodeeasily.

Therefore,riverinescourequations

willpredictscour

depthsin

shortdurationtidalflow

s.

~Astronom

icaltides,with

theirdailyortw

icedaily

inand

outflows,can

anddo

causelong-

termdegradation

ifthereis

nosource

ofsedimentexceptatthe

crossing.This

hasresulted

inlong-term

degradationofseveralfeetperyearw

ithno

indicationofstopping

(ButlerandLillycrop,

1993)(Vincentetal.,

1993).E

xistingscourequations

canpredictthe

magnitude

ofthisscour,butnotthe

time

history(Richardson

etal.,1993,RichardsonandDavis,1995).

~M

assdensity

stratification(saltw

aterwedges),w

hichcan

resultwhen

thedenserm

oresaline

oceanw

aterentersan

estuaryortidalinletw

ithsignificantfreshw

aterinflow,can

resultinlargervelocities

nearthebottom

thanthe

averagevelocity

inthe

vertical(Sheppard,1993).H

oweverw

ithcarefulevaluation,the

conectvelocitycan

bedetem

iinedforuse

inthe

scourequations.

With

stormsurges,

mass

densitystratification

willnotnorm

allyoccur.

Thedensity

differencebetween

saltandfreshwater,exceptasitcauses

saltwaterwedges,isnot

significantenoughtoaffectscourequations.D

ensityandviscositydifferencesbetween

freshand

sediment-laden

watercanbe

much

largerinriverine

flows

thanthe

differencesbetween

saltandfreshwater.

Salinitycan

affectthetransportofsilts

andclays

bycausing

themto

flocculateand

possiblydeposit,w

hichm

ayaffectstream

stabilityand

mustbe

evaluated.Salinity

may

affecttheerodibility

ofcohesivesedim

ents,butthisw

illonlyaffectthe

rateof

scour,notultimate

scour.

258

Page 259: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 260: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Managem

entAgency,NationalOceanic

andAtm

osphericAdm

inistration,U.S.GeologicalSurvey,

U.S.CoastGuard,U.S.Corps

ofEngineers,stateagencies,etc.are

used.

Thecrossing

isclassified

asaninlet,bay,estuary

orpassagebetween

islandsorislands

andthe

mainland.

Thecrossing

may

betidally

affectedortidally

controlled.Tidally

affectedcrossings

donothave

flowreversal,butthe

tidesactas

adownstream

control.Tidally

controlledcrossings

haveflow

reversal.The

limiting

caseforatidal-affected

crossingis

whenthe

magnitude

ofthetide

islarge

enoughto

reducethe

dischargethrough

thebridge

tozero.

Theobjectives

oftheprelim

inaryanalysis

areto

determine

them

agnitudeofthe

tidaleffectson

thecrossing,

theoveralllong-term

verticalandlateralstability

ofthewaterway

andbridge

crossing,andthe

potentialforwaterwayand

crossingto

change.

DE

TE

RM

INA

TIO

NO

FH

YD

RA

ULIC

VA

RIA

BLE

S

Thegeneralprocedure

isto

determine

(1)designflow

s(100-and

500-yearstormtides

andriverine

floods),(2)hydraulicvariables

ofdischarge,velocity,anddepths.

Thesevariables

arethenused

todeterm

inethe

scourcomponents

(depthsofdegradation,contraction

scour,pierscour,andabutm

entscour)usingtheequations

andmethodsgiven

previously.,and(3)evaluation

oftheresults.HEC-18

givesm

ethodand

equationsfordeterm

iningthe

hydraulicvariables

forunconstrictedtidal

affectedwaterway

andconstricted

waterways.Also,describes

1-and2-dim

ensionalcom

puterprogram

sfordeterm

iningstorm

surgehydrographs

andresulting

hydraulicvariables.

These1-and

2-dimensionalm

odelsare

alsodescribed

byAyres

Associates,1994;BinghamYoung

University,

1997;Burkau,1993;Froehlich,

1996;U.S.Corps

ofEngineers,1996;

andZevenbergen

etal.,

1997a,b.

RE

FER

EN

CE

S

1.Abed,L.M

.,1991,"LocalScourAround

BridgePiersin

PressureFlow,"Ph.D.Dissertation,

ColoradoState

University,FortC

ollins,CO.2.

Abed,L.M

.,R

ichardson,E

.V.,

andR

ichardson,J.R

.,1991,

"Bridgesand

Structures,"Transportation

ResearchRecord

1290,Vol.

2,Third

BridgeEngineering

Conference,Transportation

ResearchBoard,W

ashington,D.C.3.

Ahmad,M

.,1953,"Experim

entsOn

DesignAnd

BehaviorO

fSpurDikes,"

ProceedingsIAI-IR

,ASCE

JointMeeting,U

niv.ofMinn.

4.AASH

TO,

1992,"Hydraulic

Analysisforthe

Locationand

DesignofBridges,"

Vol.V

II,H

ighwayDrainage

Guidelines,Am

ericanAssociation

ofStateH

ighway

andTransportationO

fficials,Washington,D.C.

5.Arneson,L.A.,

1997“The

EffectsofPressure

Flowon

LocalScourinBridge

Openings,”

Ph.D.Dissertation,ColoradoState

Univ.,FortC

ollins,CO.

6.Am

eson,L.A.

andAbt,

S.R.,1998,

"VerticalContraction

ScouratBridges

with

Water

Flowin

gUnderPressure

Conditions,"presented

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eeting,Washington,D

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ydraulicCom

puterModels

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Page 263: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

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aloftheH

ydraulicsD

ivision,ASCE,Vol.

119,HY9,

1993,pp.1069-1071.

Richardson,J.R.

andRichardson,

E.V.,1993,

"TheFallacy

ofLocal

Abutment

ScourEquations,"

Hydraulic

Engineering,ASCE,Vol.

1,pp.749-755.

Richardson,J.R.,Richardson,E.V.,and

Edge,B.L.,1995,

"BridgeScourin

theCoastal

Region,"Proc.oftheFourth

IntemationalBridge

EngineeringConference,TR

B,NationalResearch

Council,W

ashington,D.C.Richardson,J.R.and

Trivino,1999,PersonalCom

munication.

Salim,M

.andJones,J.S.,1996,"ScourAround

ExposedPile

Foundations,"ASCE

NorthA

merican

Waterand

Environm

entCongress,Anaheim

,CA,

10p.

Schumm

,S.A.,1977,

"TheFluvialSystem

,"W

ileyand

Sons,338p.

263

Page 264: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Shearman,

J.O.,1987,

"BridgeW

aterwaysAnalysis

Model

forM

ainframe

andM

icrocomputers,"

WSPR

O/H

Y-7,FederalHighw

ayAdm

inistration,U.S.Departmentof

Transportation,Washington,D.C.

Sheppard,D.M

.,1993,

"BridgeScourin

TidalWaters,"

TransportationResearch

Record1420,W

ashington,D.C

.,pp.1-6.Strum

,T.W.,

1999,"Abutm

entScourStudiesfor

Compound

Channels,"FH

WA

Report

FHW

A-RD

-99-156,U.S.Dept.ofTransportation,McLean,V

A,

132p.

U.S.Army

CorpsofEngineers,

1981,"The

Streambank

ErosionControlEvaluation

andDem

onstrationAct

of1974,"

FinalReport

toCongress,

ExecutiveSum

mary

andConclusions,W

ashington,D.C.U.S.Corps

ofEngineers,1983,

"Streambank

ProtectionG

uidelinesForLandowners

andLocalG

ovemm

ents,"W

aterwaysExperim

entStation,Vicksburg,MS,60

p.U.S.Arm

yCorps

ofEngineers,1993,

"ScourandD

epositionIn

RiversAnd

Reservoirs,Com

puterProgramH

EC-6,"H

ydrologicEngineering

Center,Davis,CA.

U.S.Arm

yCorps

ofEngineers,1997"H

EC-R

ASR

iverAnalysis,"H

ydrologicEngineering

Center,D

avis,CA

.Vanoni,V.A.,Editor,

1975,"Sedimentation

Engineering."ASC

EM

anual54,NewYork,

NY.

Vincent,M.S.,Ross,M

.A.,and

Ross,B.E.,1993,

"TidalInletBridgeScourAssessm

entM

odel,"TransportResearch

RecordQ1420,Washington,D

.C.,pp

7-13.Zevenbergen,L.W

.,Richardson,E.V.,Edge,B.L.,Lagasse,P.F.,and

Fisher,J.S.,1997,

"Development

ofH

ydraulicCom

puterM

odelsto

AnalyzeTidal

andCoastal

StreamH

ydraulicConditions

atH

ighway

Structures,"Final

Report,Phase

H.South

Carolina

DepartmentofTransportation,Colum

bia,SC.Zevenbergen,L.W

.,Hunt,J.H.,Byars,M

.S.,Edge,B.L.,Richardson,E.V.,and

Lagasse,P.F.,1997,"TidalH

ydraulicM

odelingforBridges;UsersM

anual,"PooledFundStudySPR-

3(22),AyresAssociates,FortC

ollins,CO.

264

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Table1-SoilC

onditionsatM

ajorBridgesC

rossingR

iversin

Sudan.

Bridge

RR

iverC

onstruct-W

Sion

Date

,SoilD

escriptionS

FoundationP

erform

anceBlue

Nile

BlueI

1908Bridge

\Nile

;G

reysand

ofvaryingdegree

offineness

with

clayand

gravel.Sandstone

belowa

depthof

15m.

Spreadfooting

atthe

northabutm

entbearing

ata

depthof

loft(3m)

onhard

clay.12-inch

(0.3m)

diameter

pinepiles

atthesouth

abutment.

Piersof

diameters

ofllft(3.35m

)and

l6ft(4.88m

)bearingon

sandstoneabout

60ft(l8.3rn)below

riverbed.

NoSCOUT

Old

\Vhitel928

White:Nile

1‘

NileA

Bridgeg

‘

3to

Smof

Softclay

andsilt

underlainby

greyN

ubianSandstone

Caissonof

diameters

l6ft(4.88m)

sockettedan

averageof

5ft(1

.524m)

intothe

Nubian

Sandstone

NoScour

Shambat

AThe.

1968N

ileBridge

About4m

ofsiltyclay

atbanks.Sand

belowthe

siltyclay

atthebanks

andfrom

riverbedatthe

main

stream.

Sandis

looseto

medium

denseand

about4

tol5m

inthickness

.Sandstone

with

inclusionsof

mudstone

_‘prevailsbelow

thesand.

Caissonssocketted

intothe

sandstone.B

N0SCO

UT

EWhite'Jan.2000

‘Tid

el

New

White

Nile

Bridge»

Softhighly

plasticsiltabout5

to7m

deepunderlain

bya

2mlayer

ofm

ediumdense

clayeysand

overlyingN

ubianForm

ation.The

Nubian

Formation

consistsof

altemating

layersof

weatheredsandstone

andm

udstoneup

to25m

.Sandstoneprevails

belowthatdepth.

Largediam

eterbored

piers(dia.

1.2m)

ata

depthofl5m

.

AtbaraBridge

Atbara\Jan.2000

‘River

About7.5m

tollm

ofmedium

stiffsilty/clay

onthe

banks.Loose

tom

ediumdense

sandbelow

thesilty

clayatthe

banksand

fromthe

riverbedat

them

ainstream

.Basementcom

plexofchist

belowa

depthofl_5_m.

Drilled

piersbearing

atleast

2minto

thefresh

bedrock.

TutiBridge

Nile

constructedlBlue

Tobe

Medium

stiff,"medium

tohighly

plasticsilt

onthe

riverbanks.

Thesilt

iscom

pressible.Looseto

medium

densefine-grained

siltysand

underliesthe

siltat

riverbanksand

fromthe

riverbedat

them

ainflow

section.W

eatheredN

ubiansandstone

belowa

depthofl2

tolfim

._

Drilled

piersbearing

onslightly

weatheredN

ubiansandstone

belowa

depthof

15to

20mbelow

theriverbed.

Pierdiam

eterlto

1.2m

268

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Table2

SoilConditions

atHighw

ayBridges

Crossing

SeasonalStreams

inSudan.

Bridge

orS

treamN

ame

Highw

ayiC

onstruc-tion

Date

SoilDescription

Foundation1TXPB

_P

erfor-m

anceG

adambalha

Medani-

Gadarif

1976H

ighlyplastic

clayto

adepth

of7to

Sm.

Thetop

2.5mare

soft.Theclay

isstiffto

verystiff

belowthat

depth.W

eatheredbedrock

underliestheclay.

i

1‘Boxculvert

lwith

2mapron

underthe

riverbed.

Scourfaflure

hiI998.

Um

dllkaM

edaniSennar

1979H

ighlyplastic

clayto

adepth

of15iBox

culvertybearing

onthe

shighlyplastic

Tclay

Scourfailure

3tim

es

Aw

mm

KhartoumShendi

19953m

oflooseto

medium

sensesand

overliesvery

denseclayey

gravellysand.

lSpread

footingon

adepth

of2.5m

overalm

compacted

fill‘player.

Scourfailure

in1996.

ElkrbkanKhartoumShendi

1995About

2to9m

ofdense

tovery

denseclayey

sandfollow

edby

verydense

siltysand.

Spreadfootings

on

verydense

clayeysandata

depthof4m.

No

scour

KaboushlaShendi-Atbara

1997M

ediumdense

todense

clayeysand,silty

sandand

sandygravel

upto

adepth

of2.5m

followed

bystiffhighly

plasticclay.

Mudstone

belowa

depthof7.5m

,Spreadfootings

on2m

ofcom

pactedfill

layer.Bottom

offoundation

at4m

.

NoSCO11I'.

ShendiAtbara

1997Very

stiffto

hardsandy

siltyclay

toa

depthof

17m.

Theclay

containssubstantial

amounts

ofgravel

andstone

fragments

inthe

top2to

3m.The

clayis

highlyplastic

belowa

depthof3

to4m

.N

aturalm

oisturecontent

ofthe

highlyplastic

clayis

lessthan

plasticlim

it.

3Spread

footingbearings

at4m.

No

5scour.

Zalingi-Elgenina

1998Loose

tom

ediumfine-grained

siltysand

underlainby

alayer

ofstiffclayey

sandand

densesandy

clayfollow

edby

weatheredsand

stone.

ADriven

steelH

-;piles

(HP14

andEHP12)

about6lto

20min

lengthdriven

torefusal

ponweathered

Lsandstone.

Noscour.

Haram

andA

buKhedra

Rabak-R

enkTo

beI

constructeJ

d

Verystiff

highlyplastic

clay(aboutIto

3m)

inthickness

underlainby

verydense

clayeysand

(more

than15m

deep).

*Spread

footingsbelow

thehighly

plastic1clay

(belowa

.4depthof

;3m).Scour

is<2m

dueto

flat.topography

andilow

flowspeed.

Nyala-El

RiheedElB

iridi

Tobe

.constructe

Ad

l

Loosesilty

sand(thickness

about6to

9m)

with

thininclusions

ofclay

andsilt.

Medium

denseclayey

sandbelow

theloose

siltysand

upto

adepth

of12to

15m.

Verydense

clayeysand

prevailsbelow

thatdepth

-._

_--

Pilefoundations

bearingon

theAvery

dense3clayey

sandbelow

adepth

of12

to15m

.Azoorn

Zalingi-Elgenina

Under

‘C

onstrucTion

Veryloose

toloose

poorlygraded

finegrained

sand(variable

thicknessof2.5

tol8.5m

)follow

edby

medium

densepoorly

gradedfine

grainedsand

gradinginto

athin

layer(2to4m

)of

verydense

clayeysand

andsilty

sand.Bedrockbelow

adepth

of3.5-40.0m.

SteelH

piledriven

tothe

bedrock.

_I

_4»

269

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YT"1

.1il

'1

Remedial

measures

consistedof

replacement

ofthe

culvertbya

5span

deckgirder

concretebridge.

Thebridge

wassupported

ondriven

precastconcretepiles

thatweredriven

tothe

weatheredbedrock.

Suspendedpile

capwas

constructedabout0.3m

abovethe

streambed.The

optionofpiles

andsuspended

capswas

consideredbecause

ofthe

expansivenature

ofthehighly

plasticclay.

Thebridge

wasconstructed

inthe

summ

erof1999and

performed

wellduring

thefall1999.

UM

DILK

AB

RID

GE

This2-cell

boxculvert

wasconstructed

in1979

onSennar-M

edaniRoad

(NH

A,2000).The

actualwidth

ofthestream

is26m

,however;thew

idthofthe

culvertwas

only3m

.Theculvertwashed

out3tim

essince

itsconstruction.

Postfailure

investigationin

1999revealed

thatthescourdepth

rangedfrom

1to

3m.

Thegeotechnical

investigationafter

failure(BR

RI2000)revealed

asubsoil

conditionthat

consistsofhighly

plasticclay

extendingto

adepthof15m

.Thehighly

plasticclay

isunderlain

byclayey

sandextending

tothe

maxim

umdepth

explored(about20m

).Based

ongeotechnical

andhydraulic

investigationsitwas

decidedto

replacethe

culvertw

itha

onespan

bridge.The

proposednew

bridgew

illbesupported

onpiles

bearingbelow

adepth

8m.

Thebridge

willbe

constructedthis

summ

er.

SU

MM

AR

YAN

DC

ON

CLU

SIO

NS

5

Areview

ofthegeotechnicalaspects

ofscourasrelated

todesign

ofbridgesin

Sudanis

presented.Tw

odistinguished

trendsoffoundation

designforbridges

areidentified:

-One

designpractice

form

ajorbridges

crossingperm

anentriversand

anotherfor

highway

bridgescrossing

seasonalstream

s.The

former

bridgesare

supportedon

pilesorpiers

bearingon

bedrockw

ellbelowthe

scourdepth.Settlem

entand

bearingcapacity

considerationsratherthan

scourdepthcontrolthe

designofthese

bridges.N

ofailure

casesw

ererecorded

forthesebridges

althoughsom

eofthem

wereconstructed

more

than80

yearsago.

Highw

aybridges

crossingseasonalstream

sare

susceptibleto

scourproblems

becauseof

inadequatehydraulic

dataand

incompetent

geotechnicalpractice.Casehistories

haveshown

thatscour

causedfailure

ofbridges.

Thefollow

ingfactors

contributedto

scour-inducedfailure

ofbridgesin

Sudan:1)

Construction

offoundationsw

ithinthe

scourzone.2)

Placementofinadequate

compacted

fillbeneaththe

foundation.3)

Lackof

geotechnicalinvestigation

orim

properim

plementation

ofrecom

mendations

givenby

thegeotechnicalconsultant.

4)Excavation

forfoundation

andplacem

entand

compaction

offillandbackfillis

notsupervisedby

ageotechnicalengineer.

5)Practicalm

ethodsare

notusedto

predictthescourdepth.

6)H

ydraulicand

hydrologicdata

isnot

sufficientw

hichresults

ininadequate

hydraulicdesign.

7)O

bstructionof

bridgeopenings

bysoilheaps,construction

debrisand

treebranches.

8)M

ostof

theseasonal

streams

inSudan

arem

eanderingand

requiretraining.

9)Stone

pitchingis

notusedto

protectthestream

bedin

thevicinity

ofthebridge

andthe

streambanks.

271

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SE

TTING

TR

AIN

SU

SP

EN

SIO

NR

ULE

SO

NTH

EB

RID

GE

SP

RO

NE

TOS

CO

UR

INC

ON

SID

ER

ATIO

NW

ITH

RIV

ER

-B

ED

CH

AR

AC

TE

RIS

TIC

S

byJrm

ichiTanaka1,MasanoriM

ikamiz

AB

ST

RA

CT

EastJapanR

ailway

Company

(JREast)

hasover

threethousand

riverbridges.

Thesebridges

areoccasionally

damaged

byscour

aroundthe

piersdue

toheavy

rainfallsor

typhoons.Am

ongthose

eventsthere

havebeen

thecases

oftiltingor

collapseofbridge

piers,which

mightputrailw

aytransportin

unsafe.Operationalrules

fortrainsuspension

incase

ofbridgescourhazard

havebeen

establishedand

renovatedon

thebasis

ofthosecases.

Thispaper

presentssom

eim

portantbridge

scourcases

andoutlines

thecurrent

trainsuspension

rulesin

JREast

which

representtheem

piricalknowledgelearnt

fromthose

cases.

RIV

ER

SIN

JAP

AN

TerrainCharacteristics

inJapan

arethatm

orethan

80percentareas

arem

ountainous.Therefor,The

characteristicsofriverin

Japanare

shortriverlengthand

steepriverbed

slopecom

paredw

iththe

Mainland

Rivers(Fig.1).W

aterofl00kmlength

riverfallenw

ithin6

to8

hours.In

addition,C

limate

ofJapan

hasm

anyprecipitation.

Forexam

ple,Average

precipitationin

Tokyowas

1460mm

oneyear

(1951to

1980).Forthis

purpose,riversin

Japanhas

abiggercoefiicientofriverregim

e.Thatisto

say,riverflowofinundation,and

conveyanceofgravelare

large,because

ofmuch

precipitation.A

prominent

Dutch

civilengineerwho

gavem

anysuggestion

aboutharnessriver

ofJapanin

thelate

19thCentury

said“R

iverofJapanthatis

notariver,thatisatorrent”.

RA

ILWA

YB

RID

GE

SO

FJR

EA

ST

.

Sixpassengerrailw

aycom

paniesand

atraffic

Railw

ayCom

panyform

edon

division,and

privatizationofJapan

nationalRailw

aysince

1987.EastJapanR

ailway

Company

(JREast)is

thelargestrailw

aycom

panyin

Japan.JREasthas

anetw

orkof7,538km

oftracksin

Tokyoarea,and

carrysixteen

million

andeighteen

thousandpassengers

ayear.Alm

ostlinesofJR

Eastmaintains

overthreethousand

riverbridges.Thesebridges

wereusually

builtbeforethe

SecondW

orldW

ar.Bridgesatthis

time

werebuiltatthe

pointof

1AssistantM

anager,Equipm

entMaintenance

Dept.,

EastJapan

Railw

ayC

ompany

2Manager,

TakasakiOffice,E

astJapanR

ailway

Com

pany

273

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Page 277: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

'4-$

7

Fig.3-AnInclined

PierofRokumaizawa

bridge

277

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JREastlines.

Thissystem

includeswater

gagesunderbridge

piersand

datatransm

issiondevices

betweensites

andeach

regionaltrain

operationcontrol

center(TO

C).A

tthe

mom

entwhenscour

depththatpresum

edfrom

waterlevelapproachto

theend

ofbridgepier,O

rdersto

stoptrain

willbe

issuedfrom

TOC.

Therefore,Trainw

illnotpassthrough

thebridge

thatisin

unsafestatus

accordingto

thisrule.(Fig.7)

However,disasters

of1995

and1997

mentioned

belowproved

thatthere

areseveral

typesofscore

casesthatcannotbe

foreseenby

thisrule.To

analyzedisasters

bothin

1995and

1997,Followthings

werestudied.

1)Cause1

'G

radientofriverbedin

disasterof1995was

steep,which

isthe

rivuletthanbe

calledriver.

Thus,waterlevelahnostdoesnotincrease

with

riverflow

sincrease,reversely,

erosionforce

increasedw

ithincrem

entof

flowrate.

Suchas

theserivers,

itis

impossible

toprestune

scourdepthon

thebasis

ofwaterlevel.(An

example

foracalculation)

Generally

waterlevelapproxim

ately10cm

Basis100

yearsprobable

rainfall

approximately

60cm

2)Cause2

Bothdisasters

of1995and

1997,headarisen

indownstream

region.This

isbecom

ew

ithctunulation

ofriverbeddecline

agelong.

RIV

ER

CH

AR

AC

TE

EE

RIS

TIC

SO

FS

CO

UR

CA

SE

S

Analysisoftw

elvebridge

piersscours

casesin

JR-EAST

thatthereare

some

causesof

scourwhich

areinherentto

theirrivercharacteristics.

Asfigure

8shows,

causesofscour

arerelated

toparticularities

which

riverbeddecline,

largeaffection

ofdownstreamhead,

curvatureofrivercourse

andothers.However,the

inferenceofscourdepth

sofarhas

beenm

adesolely

fromthe

waterlevelvalueofa

bridgelocation;variation

inrivercharacteristics

arenotbeen

takeninto

accotmtforscours

depthestim

ation.W

einvestigated

riverbeddecline

thathad

intenselyaffection

forscour

in19

riverssystem

,and

112localities

ofJR

-EASTarea.

Investigatedresults,

welooked

outsw

ifttendency

ofriverbeddecline

under1960

to1970.The

periodofgathers

riversandand

builtdam

sjustintim

eis

consistentwith

onhighly

developmentperiod

ofJapan.Fluctuate

rateofriver

bedshows

infigure

9.W

ecom

paredthe

mean

riverbedlevel

valuesperw

atersystem,and

founda

declinetendency

inallof19

watersystem

s.O

nthe

otherhand,

riverbeddecline

rateshow

aconvergent

tendencyin

thelast

10years.

Presumably,

itcan

beconsidered

tothe

effectof

socalled

“theriver

sandgathering

restrictionlaw

”,which

wasenacted

inand

around1970’s.

However,headswere

formed

indownstream

ofbridgesbecause

ofaccumulation

ofriverbeddecline

untilnow.

281

Page 282: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 283: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

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cases

283

K.

Page 284: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 285: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 286: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

NE

WZ

EA

LAN

DR

EFLE

CTIO

NS

ON

BR

IDG

ES

CO

UR

ByStephen

E.Colemanl,Bruce

W.M

elvillez,Christine

S.Lauchlan3

AB

ST

RA

CT

New

Zealandis

acountry

with

numerous

riversand

streams

anda

highdensity

ofbridgedriver

crossings.O

naverage,

atleast

oneserious

bridgefailure

eachyear

inN

ewZealand

canbe

attributedto

scourofthebridge

foundations,bridgescourhaving

beena

high-priorityissue

fortransportation

authoritiesin

New

Zealandover

many

years.Selected

casesof

bridge-scourdam

agethathave

occurredw

ithinN

ewZealand

arepresented

hereinto

providean

overviewof

therange

ofscourprocessesoccurring

within

thecountry.

Ow

ingto

New

Zealand’srem

arkablydiverse

terrain,the

casespresented

coverranges

ofbed

materials,

floodm

agnitudes,bridge

foundationconfigurations,

andriver

morphologies.

Thepresented

casestudies

highlightim

portantbridge-scourdesignconsiderations:including

relevantaspectsofriverm

orphologyto

beconsidered

(includingvariability

inriver

course);that

bridgescan

sufferpotentially

significantscour

damage

infloods

smaller

thanthe

designfloods

traditionallyused

inscour

analyses;thatthe

effectsofhum

anintervention

intoa

river(in

theform

ofmining

andriver

trainingw

orksfor

example)

inthe

vicinityof

abridge

sitecan

significantlyim

pactbridge

stability;and

thatthe

expectedscour

depthat

agiven

bridgefoundation

canbe

severelyunderestim

atedif

thecom

binationof

thefull

rangeof

possiblescour

components

isnot

consideredforthe

foundation.

TYPESO

FSC

OU

R

Thetypes

ofscourthatcanoccurata

bridgecrossing

canbe

classifiedasfollow

s(Figure

1):0

Totalscouristhe

combination

ofindividualscourcomponents

atabridge

crossing.0

Generalscouroccurs

irrespectiveofthe

existenceofthe

bridgeand

canoccuras

eitherlong-term

orshort-termscour.

0Long-tenn

generalscourdevelopsovera

time

scalenonnally

oftheorderofseveralyears

orlonger,and

includesprogressive

degradationoraggradation

andlateralbank

erosiondue

tochannelw

ideningorm

eandermigration.

ISeniorLecturer,DepartmentofC

ivilandResource

Engineering,TheU

niversityofAuckland,

Auckland,New

Zealand.2

AssociateProfessor,

Department

ofC

iviland

ResourceEngineering,

TheU

niversityof

Auckland,Auckland,New

Zealand.3

PostdoctoralFellow,D

elftUniversity

ofTeclmology,Faculty

ofCivilEngineering

andEarth

Sciences,Delft,the

Netherlands.

286

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Page 288: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

0Progressive

degradation(aggradation)is

thegenerallow

ering(raising)ofthe

riverbedatthe

bridgesite.

IShort-terrn

generalscourdevelopsduring

asingle

orseveralclosely-spacedfloods.

Thistype

ofscourincludesscourarising

froma

lateralshiftofa

channelbend,achannelbraid

orthechannelthalweg;verticalscourin

abend;scourata

confluence;and

scourarisingfrom

bed-formm

igration.0

Localisedscour

isdirectly

attributableto

theexistence

ofthe

bridge,and

includescontraction

scourandlocalscour.

0C

ontractionscouroccurs

wherethe

bridgefoundations

(includingapproaches)constrictthe

flow.

0Localscourresults

fromthe

directinterferenceofthe

bridgefoundations

with

theflow

,andincludes

abutmentscourandpierscour.

Theeffects

ofdebrisrafting

atabridge

sitefrutherm

agnifyany

erosionaround

thefoundations

andalso

anylateraland

verticalforceson

thebridge

dueto

debrisand

sedimentloads.

Casesofbridge-scourdam

ageare

presentedin

therem

ainderofthispaperin

ordertoillustrate

therange

ofthesescourprocesses

occurringforN

ewZealand

conditions.

NE

WZ

EA

LAN

DC

AS

ES

OF

BR

IDG

E-S

CO

UR

DA

IVIA

GE

Aggradation:B

ullockC

reekR

oadBridge

The49-m

-longfour-span

single-laneBullock

CreekRoad

Bridgeon

StateH

ighway

6was

builtin1938.

In1972,a

major

floodsetoffseveralslips

inthe

catchment,w

hichis

onthe

lineofthe

Alpinefault.

Thelandslip-deposited

materialhas

subsequentlybeen

moved

downstreamby

freshesand

floods,thism

aterialrepeatedlycausing

bedaggradation

andbridge

closru"eatthe

BullockCreek

bridgesite.

Thesedim

entat

thebridge

siteis

predominantly

gravelsand

cobbles,w

itha

representativesize

of30-150m

m,

andw

ithlargerfractions

upto

theorderof1

min

size.The

braidedchannelupstream

ofthebridge

flows

betweenlarge

terracesfonned

bydeposited

landslipm

aterialthathasbeenpushed

tothe

edgesofthe

channel.

Aflood

inJanuary

1983caused

aggradationto

alevelm

orethan

1m

abovethe

decklevelofthe

originalbridge.Subsequentto

thisflood,the

existingbridge

wasrem

ovedand

them

ainchannel

wasexcavated.

Areplacem

entbridgewas

constructedabout

100m

downstreamofthe

originalbridge,w

iththe

soffitlevelofthenew

bridge2-3

mhigherthan

thatoftheprevious

bridge.

Sincethe

bridgereplacem

ent,the

riverhas

aggradedand

cutdow

nquite

regularly.W

iththe

aggradation,theriverhas

outflankedthe

bridgeateach

end,althoughthe

newbridge

hasnotbeen

buried.Bulldozers

haveoccasionally

beenused

topush

aggradedbed

material

acrossto

theriverbanks,the

riverthencutting

downagain

inthe

centralchannel.Itis

expectedthatonce

theslips

inthe

catchmentstabilise,possible

futuredegradation

atthebridge

sitem

ayrequire

theconstruction

ofarockweirto

protectthebridge

foundations.

288

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Page 290: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Overthis

reach,theriveris

freeto

meanderw

ithinthe

floodway.Riverbanks

within

thefloodw

ayare

lowand

usuallygrassed,and

thefloodplain

islow

.Spillovers

intothe

extensivelow

floodplainare

minim

isedby

extensivestopbanks

andw

illowprotection.

Debrisaccrunulation

canbe

aproblem

forthepiers

centralinthe

river.A

ttheroad

bridge,theriverincorporates

aflood

plainof

approximately

150m

width,w

ithflow

approachingthe

bridgeatapproxim

ately70°

tothe

bridgecentreline.

Meandering

hasresulted

inthe

riverapproaching

theBranxholm

eR

ailBridgeatan

angleofabout20°to

thebridge

centreline.

Gravel-rrrining-accelerated

degradationhas

occurredoverthe

livesofboth

ofthesebridges

onthe

Oreti

River.Degradation

attheBranxholm

eR

ailBridge

hasbeen

furtherexacerbated

bythe

upstreaminterception

ofbed-loadsedim

entsbyaw

eirusedto

facilitatewaterwithdrawal.

Floodingin

February1994

resultedin

severalpiles

ofPier

7of

theBranxhohne

Rail

Bridgebeing

undemrined.

Thisflood

constitutesthe

thirdlargestannualm

aximum

overtheperiod

1977-1996.

In1975,aw

eircomposed

ofverticaltimberpiles

wasnoted

tobe

acrossthe

entirem

aincharm

el66m

downstreamofthe

centrelineofthe

OretiR

iverRoadBridge.

Between1977

and1978,a

rockw

eirwasconstructed

with

theexisting

timberw

eirasits

upstreamface

anddefining

thew

eircrestlevel.

Arock

mattress

wasalso

constructedbeneath

thebridge

(Figure3).

Them

attressextends

acrossthew

idthofthe

main

channel,with

the16-m

-widecrestatalevelof1.7

mbelow

thelevelof

therm

dersideof

thepile

capsand

centredalong

thebridge

centreline.Despite

continueddegradation

oftheriver

upstream,

therock

weir"and

mattress

haveprevented

frnthersignificant

degradationofthe

streambed

atthebridge

site.Although

some

rockhas

beenreplaced

dueto

fietting,thestructures

haven’tneededto

betopped

upto

compensate

forgeneralscour

resultingfrom

them

ining.

Asa

resultof

the1994

scourdam

ageof

theBranxholrne

Rail

Bridge,Piers

7and

11were

underpinnedusing

steelH-pilesdriven

to18-21

mbelow

raillevel,andarock

weirwas

constructeddownstream

ofthebridge.

Thew

eircentrelinevaries

from6

mto

12m

downstreamofthe

bridgecentreline.

Thecrestofthe

weiris2.5

mw

ideand

isgenerally

9m

belowraillevel,w

ithelevated

sectionstowards

thebanks

tofacilitate

fishpassage

alongthe

river.The

rockw

eirwasexpected

torequire

maintenance

bytopping

up,asaresultofflood

damage

forexample.

Flood-levelwamings

havebeen

settoenable

therailway

lineto

beclosed

inextrem

eevents.

Degradation:

AshburtonR

iverRoad

BridgeThis

roadbridge

(onState

Highway1)

overthe

AshburtonR

iveris

a340-m

-long,two-lane,

reinforced-concretestructure

thatwas

builtin

1931.The

bridgecom

prises31

slab-typepiers

(Figure4),about25

ofwhich

liew

ithinthe

activeriverchannel.

Eachpieris

formded

onseven

400m

mreinforced-concrete

octagonalpiles.These

pileswere

drivento

arelatively

uniformdepth

ofbetween

6.5m

and6.7

mbelow

theunderside

ofthepile

caps.

TheAshburton

Riverin

thevicinity

ofthebridge

siteis

about280m

wide,isstraight,rm

iformin

slopeand

width,and

isbounded

bytreesand

straightstopbanks.The

channelisbraided

andthere

isevidence

ofactivebed

movem

ent.The

bedm

aterialiswell-graded

gravel.O

verthelife

ofthebridge,various

stopbanking,river-clearingworks

andgravel-extraction

workshave

takenplace

overextensive

lengthsofthe

river,upstreamand

downstreamofthe

bridge.The

extractionofgravel

fromthe

riverhasbeen,andcontinuesto

be,controlled.

290

Page 291: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Generalscourexacerbatedby

gravelextractionfiom

theriverhas

resultedin

agraduallow

eringof

thebed

levelatthebridge

site(Figure

4).Concem

forthe

vulnerabilityofthe

bridgeto

scourdam

agewas

accentuatedby

theshallowness

ofthefoundation

piles,andalso

debrisrafts

fomring

atthe

siteincreasing

thepotentialfor

localscourpierundenniningand

bridgedam

ageor

failurein

significantfloods.

Rockaprons

wereconstructed

in1979

arormd

eachofthe

piersw

ithinthe

activechannel(Figure

4).Each

apronm

easures5

mwide,15

mlong

and1.6

mthick.

Rockriprap

ofam

ediansize

d50of0.5m

wasused

toconstructthe

aprons,w

iththe

uppersurface

ofeachapron

locatedbeneath

theriverbed

surfaceand

approximately

2m

belowthe

baseofthe

pilecap.

Apronsactto

reducethe

scourpotentialatpiersboth

byarm

ouringthe

bedagainstlocalscourdue

tolocalhydraulic

vortices,and

alsoby

protectingagainst

generalscour

bydropping

attheapron

extremities

asthis

scourdevelops.

Apronsnevertheless

cannotprovidetotalassurance

againstscouring,particularly

forongoing

generaldegradation.Inspection

in1994

indicatedthe

rockapron

tobe

exposedatone

pieronly.

Bedlevels

inthe

riverchannelcontinueto

beregularly

monitored.

Degradation

andC

hannelWidening

(alsoBend

andLocalScours):

BlackmountR

oadBridge

The82-m

-longsingle-lane

BlackrnountRoadBridge

(Figure5)crossing

theM

araroaR

iverforms

partofWeirRoad

fromC

lifdento

Manapouriin

theSouth

IslandofN

ewZealand.

PierB(C)was

supportedbytw

ostaggered

rowsoffour(three)driven

vertical0.4m

x0.4

mconcrete

piles.

About1.5km

downstreamofthe

bridgesite,the

riverflows

intothe

Waiau

River.O

wing

towater

levelsbeing

lowerthan

anticipated,thewaterway

areaatthe

bridgesite

isin

excessofflratrequired

topass

the100-yearflood.

Thebridge

islocated

ina

mild

right-handbend

oftheriver(radius

ofcurvature

w500

m),the

riverapproaching

thebridge

atabout60°to

thebridge

centre-line.The

wall-type

pierswere

alignedperpendicularto

thebridge

centre-line,with

PierBtowards

theoutside

ofthebend

(Figure5).

Belowthe

undersideofthe

pilecap

forPierBwere

about2m

ofgravels(large)

andboulders

underlainby

about11

moftightgravels

with

some

sandsand

sandlenses.

Debrisacctunulation,principally

partsoftrees,can

occuratthebridge

piers.

During

thefailure

eventinAugust1980,the

channelscouredacross

thew

idthofthe

floodflows.

Theflood

thatcausedfailure

peakedatabout900

m3/s,the

largestrecordedflood

peakoverthe

period1963-1996,w

itha

peakflow

durationofabout6

hours.The

flowsatan

angleto

thew

all-type

pierresultedin

undermining

andrem

ovalofPierB.W

iththe

lossofthe

foundation,thebridge

superstructurebuckled

butremained

inplace.

Thedeflected

bridgedeck

profilehad

am

aximum

deflectionatthe

positionofthe

removed

pierofapproximately

3m

.A

maxim

runscourdepth

ofabout2.9

mwas

subsequentlymeasured

attheposition

ofthefailed

PierB,scourdepthsdecreasing

with

distanceaway

fromthis

pier.Slipping

ofthe

rmderm

inedem

bankment

aroundPier

C(degradation-associated

channelwidening)exposedthe

pilesforthis

pier.

Remedial

work

consistedofrestoring

theriverbed

tothe

originallevel

usingnatural

riverbedm

aterial,replacingPierB

(about3.5m

closertoPierC),reinstating

theslipped

materialaround

PierC,protecting

thepiers

andthe

embankm

entsusing

rockriprap,

andreplacing

thesuperstructure

with

similarsteeltrusses.

Thenew

PierBconsists

ofa1.5-m

-diarneterconcretecylinderdown

tothe

restoredbed

level,thena

1.9-m-diarneterconcrete

cylinderencasedin

asteelshellextending

291

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Page 294: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Outsidebank.

March1988'

.-i.

Outside

bank,I

W5

250m

March

19791I

i%

Outside

ballkrI

700112January

1973

1B

ridge*

-"=1

5>

»=

Fig.6Schem

aticPlan

View

oftheN

ear-fieldforthe

Waipaoa

RiverR

ailBridge.

-.

4,.‘1

":.

-.._

.:r

.'

;.";1.2’.

_..__..‘.

..eara‘Z.‘*aas¢..

Old‘°““g"a"“"m°“‘,

I1’

;‘

‘w1°“%§§

I ,.-‘IT

Oldbudgeapproach_____________._

ID

G1-;_-:7-_._'.:._'-IT

D\F~

~. ,,§

yir;

-ISpanD-Er\-rr-

0'X553: ";1;‘.O-1‘.-.1

IvGreg“bankrwm

fllmhah

*£8%é$i§%

E

Iemfidifllsvidfibflnks

Fig.7Schem

aticSite

Planofthe

Waitangitaona

RiverRoad

BridgeFailure

(afterColeman

andM

elville,1999).

294

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Page 297: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 298: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Them

ainobjective

ofthispaper

isshow

howthe

hydraulicaspectofbridge

designis

tackledin

Malaysia.

Theeffecton

foundationscouron

theintegrity

ofthebridge

ishighlighted.

Threedifferent

bridgesites,

thatisPukin

River

Bridge,Keratong

River

Bridgeand

PlentongR

iverBridge

werechosen

asexam

plesto

illustratehow

remedialactions

weretaken

toarrest

erosion,althoughsuccess

isnotalways

guaranteed.

HY

DR

AU

LICD

ES

IGN

PR

AC

TIC

ES

INT

HE

PU

BLIC

WO

RK

SD

EP

AR

TM

EN

T(P

WD

)M

ALA

YS

IA

Becausehydraulic

considerationsare

extremely

important

toensure

theintegrity

ofa

bridgesand

culverts,theM

alaysianauthority

hasplaced

highem

phasisin

thehydraulic

designof

bridges.In

thePublic

Works

Department,

Malaysia,

thedesign

philosophybuilds

uponthe

considerationsthatbridges

may

faildueto:

(a)inadequate

flowcapacity

leadingto

over-toppingof

thebridge

deckor

theapproach

embankm

ents;(b)

increasedloading

onthe

structurefrom

water,sedimentordebris;and

(c)failure

oftheform

dationsorsupports

asaresultofbridge

scouring.

Thesolution

tothe

firstprobleminvolves

thedetem

rinationofthe

designdischarge

andthe

flowcapacity,

andto

ensurethat

thefonner

isless

thanor

equaltothe

latter.The

designdischarge

canbe

calculatedusing

eitherthem

easuredstream

flowdata

orrainfallrecords.

InM

alaysia,guidelines

forthe

proceduresto

calculatethis

valueare

containedin

aseries

ofdocum

entspublished

bythe

Drainageand

InigationDepartm

ent(D

ID)

underthe

Ministry

ofAgriculture.

Some

ofthesepublications

areH

eiler(1973,1974),Heilerand

Chew(1974),Lew

isetal(1975)

andTaylor

andToh

(1980).The

PublicW

orksDepartm

ent(PW

D)

inM

alaysiautilizes

a100-yearstorm

forbridgedesign

anda

50-yearstonnforculvertdesign.

Tocounteractthe

secondproblem

,the

departmentproposes

theprovision

ofafreeboard,

which

isthe

verticaldistancebetween

thehighestwaterleveland

thesoffitlevelofthe

bridgedeck.

Avalue

rangingfrom

0.3m

to1.0

mis

used,with

thelow

ervalueforchannels

thatarenot

expectedto

havedebris

orfloatinglogs.

However,ifdebrisand

floatinglogs

areexpected

inthe

river,theforce

exertedby

theseobjects

onthe

piersm

ustalsobe

consideredin

thedesign

ofthepier.

Thestandard

practiceby

thePublic

Works

Department

(PublicW

orksDepartm

entM

alaysia,1982,1985)tocalculate

theseforces

isasfollow

s:

Fordebris:tr

0the

forceshallbe

computed

basedon

am

inimum

depthof1.22

m(4

feet)ofdebris;andQ

theforce

shallbe

computed

basedon

theassum

ptionthat

thelength

ofthe

debrisis

equivalenttohalfthe

stunofthe

adjacentspans.

Forfloatinglogs:0

theforce

shallbecom

putedbased

onthe

assumption

thatthelog

weighs2

tonnes,andtravels

atthenorm

alstreamvelocity;and

298

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Page 308: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

abutment.

Furthermore,m

ostofthesestudies

wereconducted

tmdera

clear-watercondition,andtheirvalidity

whenapplied

toa

live-bedcondition

remains

unproven.O

nlyrecently,the

studyby

Chiew

andLim

(2000)hasventured

intoriprap

protectionundera

live-bedcondition.

Evenso,riprap

protectionaround

anabutm

entcontinuesto

remain

inuncharted

territories.

Inaddition,

theexperience

gainedon

thesuccess

orfailure

ofa

particularscour

countermeasure

tmdera

givenflow

conditionoften

remains

the"property"

ofaparticularagency

orcompany.

Thisknowledge

isoften

notsharedalthough

blame

shouldnotbe

leviedso

quicklyon

thepractitioners.

Generally,

thereare

notm

anyplatform

son

which

suchexperience

andknow

ledgecan

bedissem

inated.It

ishoped

thatthe

information

onhow

bridgedesign

isconducted

inM

alaysia,andthe

threeexam

plescited

abovew

illencouragediscussion

formutual

benefitsam

ongstresearchersand

practitionersin

thisarea.

AC

KN

OV

VLE

DG

EM

EN

TS

Theauthors

would

liketo

thankM

r.LeowChoon

Heng,AssistantDirectorofBridge

Unit

forprovidingthe

datarelating

tothe

threecase

studiesoutlined

inthe

paper.Specialthanks

alsogo

toD

r.H

iewK

imLoi,

Deputy

DirectorofDrainage

andIrrigation

DepartmentM

alaysiafor

infonnationon

Malaysian

rivers.The

viewsexpressed

inthis

papermay

notbethatofthe

PublicW

orksDepartm

entofM

alaysia.

RE

FER

EN

CE

S

1.C

hiew,Y.M

.(1995)."M

echanicsofRiprap

FailureatBridge

Piers".JournalofH

ydraulicEngineering,ASCE,121(9),pp.635-643.

2.C

hiew,Y.

M.

andLim

,F.H.

(2000)."Failure

BehaviorofRiprap

LayeratBridge

PiersunderLive-Bed

Conditions".

JournalofHydraulic

Engineering,ASCE,126(1),pp.43-55.

3.H

eiler,T.D.

(1973)."Estim

ationofthe

DesignRainstorm

",DID

HydrologicalProcedure

No.

1,M

inistryofAgriculture

andFisheries

Malaysia

(Revisedand

updatedby

Mohd.

Fadhillahb.H

j.Mahm

ood,Salenabt.Salleh,Leong

TatMeng

andTeh

SiewKeatin

1982and

reprintedin

1990).4.

Heiler,

T.D

.(1974).

"Rational

Method

ofFlood

Estimation

forRural

Catchment

inPeninsular

Malaysia",

DID

Hydrological

ProcedureN

o.5,

Ministry

ofAgriculture

andFisheries

Malaysia

(Reprintedin

1995).5.

Heiler,T.D

.,andChew,H.H.(1974).

"Magnitude

andFrequency

ofFloodsin

PeninsularM

alaysia",D

IDH

ydrologicalProcedure

No.

4,M

inistryof

Agricultureand

FisheriesM

alaysia.6.

JapanIntem

ationalCooperationAgency

andPublic

Works

DepartmentM

alaysia(1992).

"TheStudy

onthe

Maintenance

andR

ehabilitationofBridges

inM

alaysia",FinalReport,Voltune

II,Main

Report,August.7.

JapanIntem

ationalCooperation

Agencyand

PublicW

orksDepartm

entMalaysia

(1996)."The

Studyon

theStandardization

ofbridgeDesign

inM

alaysia",FinalReport,VoltuneII,

Main

Report,August.8.

Lewis,K.V.,CassellP.A.,and

FrickeT.J.(1975).

"Urban

DrainageDesign

Standardsand

Proceduresfor

PeninsularM

alaysia",M

inistryof

Agricultureand

RuralDevelopm

entM

alaysia.

308

Page 309: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Lim,

S.Y.

(1997)."Equilibritun

Clear-Water

Scouraround

anAbutm

ent".Journal

ofH

ydraulicEngineering,ASCE,123(3),pp.237-243.

Lim,

S.Y.and

Cheng,N.S.(1998).

PredictionofLive-Bed

ScouratBridgeAbutm

ents".JournalofH

ydraulicEngineering,ASCE,124(6),pp.635-638.

Ng,S.K.and

RazakR.A.(1998).

"BridgeH

ydraulicProblem

sin

Malaysia".

Proceedingsofthe

4thIntem

ationalSeminaron

Bridgesand

Aqueducts2000,Feb.7-9,

1998,Mum

bai,India.Parker,G

.,Toro-Escobar,

C.,and

Voigt,Jr.,

R.L.(1998).

"Countermeasures

toProtect

BridgePiers

fromScour",FinalReport,N

ationalCooperativeH

ighway

ResearchProgram

,Transportation

ResearchBoard,N

ationalResearchC

ouncil,StAnthony

FallsLaboratory,

University

ofMinnesota,Decem

ber,360pp.

PublicW

orksDepartm

entMalaysia

(1982)."Bridge

Loading"Lecture

Notesfor

BridgeDesign

CourseO

rganizedby

BridgeU

nit,Designand

ResearchBranch.

PublicW

orksDepartm

entM

alaysia(1985).

"Chapter6:

Pier",Bridge

DesignG

uide,Bridge

Unit,Public

Works

Department,M

alaysia.Public

Works

DepartmentM

alaysia(1995).

"AnnualBridgeInspection

Manual".

March.

Smith,D.W

.(1976)."Bridges

Failures".Proceedings

oftheInstitution

ofCivilEngineers,

Vol.60,Part1,pp.367-382.

Taylor,M.A.W

.,andToh

Y.K.(1980)."Design

FloodH

ydrographEstim

ationforRural

Catchments

inPeninsular

Malaysia",

DID

Hydrological

ProcedureN

o.11,

Ministry

ofAgriculture

Malaysia.

Yusof,N.M.(1996).

"AnnualMandatory

Inspection:Resultsand

Findings".International

Seminaron

BridgeEngineering

andM

anagement,Jakarta,Indonesia,Septem

ber10-13.

309

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Page 311: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Condition

ratingareusedto

describethe

existing,in-placebridge

ascomparedto

theas-builtcondition.

Inspectorsare

toaccurately

recordthe

presentconditionofthe

bridgefoundations

andthe

stream,in

additionto

thecondition

ofthesuperstructure,approaches

andetc.

Theyareto

identifyconditions

thatareindicative

ofpotentialproblems

forfurtherreview

andevaluation

byothers.

Thescour

evaluationprogram

wasstarted

in1988

asthe

resultof

TechnicalAdvisory

T5140.20w

hichwas

supercededby

T5140.23in

1991.The

evaluationis

tobe

conductedby

aninterdisciplinary

teamofhydraulic,geotechnicaland

structuralengineerswho

canmakethenecessaryengineeringjudgm

entstodeterm

inethevulnerabilityofabridge

toscour.This

programresulted

fromthe

failureofthe

I-90bridge

overSchoharieCreek

inupstateN

ewYork

which

killed10

people(N

TSB,1988and

Richardson,etal,1987).There

are471,407

bridgesoverwaterin

theN

ationalbridgeinventory.

AsofNovem

ber,1999,

481,155have

beenscreenedasto

theirscourvulnerabilityand353,738have

beenevaluated.

Thestatistic

fiomthe

screeningare

asfollows:

QLow

Risk

345,033v

ScourSusceptible23.492

Unknow

nFoundations

87,093Tidal

1,055ScourC

ritical23,582

Theevaluation

programin

theU.S.is

onschedule

andscourcounterm

easureshave

beentaken

onbridges

thathavebeen

identifiedas

scoursusceptible

orscour

critical.Replacem

entbridges

arebeing

constructedas

rapidlyas

fundscan

beprovided.

An

importantscourcounterm

easureis

riprapprotection,scourm

onitoringbefore,during

andaftera

floodand

theinspection

program(Richardson

andDavis,1995,Lagasse

etal,1995,1997a

and1997b).

Inspectionforscouris

extremely

difficultbecauseofthe

many

factorsthatim

pactthescourvuhrerability

ofabridge.Som

eofthese

factorsare

streaminstability,

drainagearea

changes,changesin

floodm

agnitude,potentialchangesin

angleofattack,

streamchangesupstream

anddownstream

ofthebridge,long

termdegradation,changes

inland

use,urbanization,gravelmining,and

etc.

Thispaperdescribes

inm

oredetailscourinspection

anduse

threecase

historiesto

illustratethe

difficultiesin

inspectionforscourvulnerability.

FH

WA

"RE

CO

RD

ING

AN

DC

OD

ING

GU

IDE

"(1995)

During

thebridge

inspection,thecondition

ofthesubstructure,

bridgewaterway

opening,charm

elprotection,and

scourcounterm

easuresare

evaluated,along

with

thecondition

ofthestream

.FHW

A’s

"Recordingand

CodingG

uide"(FH

WA,

1995)givesguidance"

forratingthe

presentconditionofthe

bridge.

311

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Table1.

Item60

-Substructure(FH

WA,

1995)

Thisitem

describesthe

physicalconditionofpiers,abutm

ents,piles,fenders,footings,orothercom

ponents.Rate

andcodethe

conditionin

accordancew

iththe

previouslydescribed

generalconditionratings.

CodeN

forallculverts.

Allsubstructure

elementsshouldbeinspected

forvisiblesignsofdistressincluding

evidenceofcracking,section

loss,settlement,m

isalignment,scour,collision

damage,and

corrosion.The

ratinggiven

byItem

l13-ScourC

riticalBridges,mayhave

asignificanteffectonItem

60ifscourhas

substantiallyaffected

theoverallcondition

ofthesubstructure.

Thesubstructure

conditionrating

shallbemade

independentofthedeckand

superstructure.

Integral-abutrnentwingw

allsto

thefirstconstruction

orexpansionjointshallbeincluded

inthe

evaluation.Fornon-integralsuperstructure

andsubstructure

turits,thesubstructure

shallbeconsidered

astheportion

belowthe

bearings.Forstructures

wherethe

substructureand

superstructureare

integral,thesubstructure

shallbeconsidered

astheportion

belowthe

superstructure.

Thefollow

inggeneralcondition

ratingsshallbe

usedas

aguide

inevaluating

Items

60:

CodeD

escription

O\\]OO\Dz

NO

TAPPLIC

ABLEEXC

ELLENT

CO

ND

ITION

VERY

GO

OD

CO

ND

ITION

-noproblem

snoted.

GO

OD

CO

ND

ITION

-some

minorproblem

s.SATISFAC

TOR

YC

ON

DITIO

N-

structuralelem

entsshow

some

minor

deterioration.5

FAIR

CO

ND

ITION

-allprimary

structuralelements

aresound

butmay

havem

inorsection

loss,cracking,spallingorscour.

4PO

OR

CO

ND

ITION

-advancedsection

loss,deterioration,spallingorscour.

3SERIO

USC

ON

DITIO

N-

lossofsection,

deterioration,spalling

orscour

haveseriously

affectedprim

arystructural

components.

Localfailures

arepossible.

Fatiguecracks

insteelorshearcracks

inconcrete

may

bepresent.

2C

RITIC

ALC

ON

DITIO

N-advanced

deteriorationofprim

arystructuralelem

ents.Fatigue

cracksin

steelorshearcracksin

concretem

aybepresentorscourm

ayhaverem

ovedsubstructure

support.Unlesscloselymonitored

itmaybenecessaryto

closethe

bridgeuntilcorrective

actionis

taken.1

"IMM

INE

NT"FAILU

RE

CO

ND

ITION

-majordeterioration

orsectionlosspresentin

criticalstructural

components

orobvious

verticalor

horizontalm

ovement

affectingstructure

stability.Bridge

isclosed

totraffic

butcorrectiveaction

may

putbackin

lightservice.0

FAILEDC

ON

DITIO

N-outofservice

-beyondcorrective

action.

314

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Table2.

Item61

-ChannelandChannelProtection

(1995)

Thisitemdescribesthephysicalconditionsassociatedw

iththe

flowofwaterthrough

thebridge

suchasstream

stabilityandthe

conditionofthe

charmel,riprap,slope

protection,or

streamcontrol

devicesincluding

spurdikes.

Theinspector

shouldbe

particularlyconcem

edw

ithvisible

signsofexcessive

watervelocityw

hichm

ayaffecttm

dermining

ofslope

protection,erosion

ofbanks,andrealigm

nentofthestream

which

may

resultinim

mediate

orpotentialproblems.Accum

ulationofdriftanddebrisonthe

superstructureand

substructureshould

benotedonthe

inspectionform

butnotincludedin

thecondition

rating.

Rateand

codethe

conditionin

accordancew

iththe

previouslydescribed

generalconditionratings

andthe

following

descriptivecodes:

CodeD

escriptionN

Notapplicable.Use

whenbridge

isnotovera

waterway(charm

el).

9There

arenonoticeable

ornoteworthy

deficienciesw

hichaffectthe

conditionofthe

channel.

8Banks

areprotected

orwellvegetated.

Rivercontroldevices

suchasspurdikes

andem

bankmentprotection

arenotrequired

orarein

astable

condition.

7Bankprotection

isin

needofminorrepairs.

Rivercontroldevices

andem

bankment

protectionhave

alittle

minordam

age.Banks

and/orchannelhavem

inoramounts

ofdrift.

6Bankisbeginning

toslum

p.R

ivercontroldevicesandem

bankmentprotection

havewidespread

minordam

age.Thereis

minorstream

bedm

ovementevident.

Debrisis

restrictingthe

channelslightly.

5Bank

protectionis

beingeroded.

Rivercontroldevices

and/orembankm

enthavem

ajordamage.

Treesand

brushrestrictthe

channel.

Bankand

embankm

entprotectionis

severelyunderm

ined.R

ivercontroldeviceshave

severedam

age.Largedeposits

ofdebrisare

inthe

charmel.

3Bankprotection

hasfailed.R

ivercontroldeviceshave

beendestroyed.

Streambed

aggradation,degradation

orlateralm

ovementhas

changedthe

channeltonow

threatenthe

bridgeand/orapproach

roadway.

2The

charmelhas

changedto

theextentthe

bridgeis

nearastate

ofcollapse.

1Bridge

closedbecause

ofchamrelfailure.Corrective

actionm

ayputback

inlight

service.

0Bridge

closedbecause

ofchannelfailure.Replacementnecessary.

315

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Page 317: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Table4.

Item113

-ScourCriticalBridges

(FHW

A,1995)

Usea

single-digitcode

asindicated

belowto

identifythe

currentstatus

ofthebridge

regardingits

vulnerabilityto

scour.Scour

analysesshall

bem

adeby

hydraulic/geotechnical/structuralengineers.Details

onconducting

ascour

analysisare

includedin

theFH

WA

TechnicalAdvisory5140.23

titled,"EvaluatingScouratBridges."

Vlflreneveraratingfactorof4

orbelowis

determined

forthisitem

,therating

factorforItem60

-Substructurem

ayneed

tobe

revisedto

reflecttheseverity

ofactualscourandresultant

damagetothe

bridge.Ascourcriticalbridge

isonew

ithabutm

entorpierfoundationswhich

arerated

asunstabledue

to(1)observed

scouratthebridge

siteor(2)a

scourpotentialasdetennined

fioma

scourevaluationstudy.

CodeD

escription

NBridge

notoverwaterway.U

Bridgew

ith"unknow

n"foundationthathas

notbeenevaluated

forscour.Since

riskcannot

bedeterm

ined,flagform

onitoringduring

floodevents

and,ifappropriate,closure.T

Bridgeover"tidal"

watersthathas

notbeenevaluated

forscour,butconsideredlow

risk.Bridge

willbe

monitored

with

regularinspectioncycle

andw

ithappropriate

underwaterinspections.("U

nknown"

foundationsin

"tidal"watersshould

becoded

U).

9Bridge

foundations(including

piles)ondry

landw

ellaboveflood

waterelevations.8

Bridgefoundations

determined

tobe

stablefor

assessedor

calculatedscourconditions;

calculatedscouris

abovetop

offooting.7

Countermeasures

havebeen

installedto

correctapreviouslyexisting

problemw

ithscour.

Bridgeis

nolongerscourcritical.

6Scourcalculation]evaluation

hasnotbeenmade.(Use

onlytodescribe

casewherebridge

hasnotyetbeenevaluated

forscourpotential.)5

Bridgefoundations

determined

tobe

stableforcalculated

scourconditions;scourwithin

limits

offootingorpiles.

4Bridge

foundationsdetennined

tobe

stableforcalculated

scourconditions;field‘review

indicatesactionisrequired

toprotectexposed

foundationsfrom

effectsofadditionalerosionand

corrosion.3

Bridgeis

scourcritical;bridgefoundations

determined

tobe

unstableforcalculated

scourconditions:-Scourw

ithinlim

itsoffooting

orpiles.-Scourbelow

spread-footingbase

orpiletips.

2Bridge

isscourcritical;field

reviewindicates

thatextensivescourhas

occurredatbridge

foundations.Imm

ediateaction

isrequired

toprovide

scourcountermeasures.

1Bridge

isscourcritical;field

reviewindicates

thatfailureofpiers/abutm

entsis

imm

inent.Bridge

isclosed

totraffic.

0Bridge

isscourcritical.traffic.

Bridgehas

failedand

isclosed

totraffic.

317

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GeneralSite

Considerations

Inorderto

appreciatethe

relationshipbetween

thebridge

andthe

riveritiscrossing,notice

shouldbe

givento

theconditions

oftheriverup-

anddownstream

ofthebridge:

~Is

thereevidence

ofgeneraldegradationoraggradation

oftheriverchannelresulting

inunstable

bedand

banks?

~Is

thereevidence

ofon-goingdevelopm

ent(urbanization)

inthe

watershedand

particularlyin

theadjacent

floodplainthat

couldbe

contributingto

channelinstability?

~Are

thereactive

gravelorsandm

iningoperations

inthe

channelnearthebridge?

~Are

thereconfluencesw

ithotherstream

s?H

oww

illtheconfluence

affectfloodflow

andsedim

enttransportconditions?

~Is

thereevidence

atthebridge

orinthe

up-anddownstream

reachesthatthestream

carrieslarge

amounts

ofdebris?Is

thebridge

superstructureand

substructurestream

linedto

passdebris,oris

itlikelythatdebris

willhang

upon

thebridge

andcreate

adverseflow

pattems

with

resultingscour?

~The

bestway

ofevaluatingflow

conditionsthrough

thebridge

isto

lookatand

photographthe

bridgefrom

theup-and

downstreamcharm

el.Is

therea

significantangle

ofattackofthe

flowon

apierorabutment?

Assessingthe

SubstructureC

onditionItem

60,Substructure,isthekey

itemforrating

thebridge

foundationsforvulnerabilityto

scourdamage.

When

abridgeinspector

fmdsthata

scourproblemhasalreadyoccurred,itshould

beconsidered

inthe

ratingofItem

60.Both

existingand

potentialproblems

with

scourshouldbe

reportedso

thatascour

evaluationcan

bem

adeby

aninterdisciplinary

team.

Thescourevaluation

isreported

onItem

113(Table

4)inthe

"Recordingand

CodingG

uide."Ifthe

bridgeis

determined

tobe

scourcritical,

therating

ofItem60

shouldbe

evaluatedto

ensurethat

existingscour

problems

havebeen

considered.The

following

items

arerecorm

nendedforconsideration

ininspecting

thepresentcondition

ofbridgefoundations:

1.Evidence

ofmovem

entofpiersand

abutments;

~Rotationalm

ovement(check

with

plumb

line)~

Settlement(check

linesofsubstructure

andsuperstructure,bridge

rail,etc.,fordiscontinuities;check

forstructuralcrackingorspalling)

-Check

bridgeseats

forexcessivem

ovement

2.Dam

ageto

scourcountermeasures

protectingthe

foundations(riprap,guide

banks,sheetpiling,

sills,etc.),

Hasriprap

placedarotm

dpiers

and/orabutm

entsbeen

removed

orreplaced

with

riverrun

material.

Acom

mon

causeofdam

ageto

abutmentriprap

protectionis

rrmofffrom

theends

ofthebridge

which

flows

down

318

Page 319: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

tothe

riprapand

rmdem

rinesit.

Thiscondition

canbe

correctedby

installingbridge

enddrains.

3.Changes

instream

bedelevation

atfoundations(rm

denniningoffootings,exposure

ofpiles),and

4.Changes

instream

bedcross

sectionatthe

bridge,includinglocation

anddepth

ofscourholes.

~N

oteand

measure

anydepressions

aroundpiers

andabutm

ents~

Note

theapproach

flowconditions.

Isthere

anangle

ofattackofflood

flowon

piersorabutm

ents?

Inorderto

evaluatethe

conditionsofthe

foundations,theinspectorshould

takecross

sectionsofthestream

,notinglocation

andcondition

ofstrearnbanks.Carefulm

easurements

shouldbe

made

ofscourholesatpiers

andabutm

ents,probingsoftm

aterialinscourholes

todeterm

inethe

locationofa

firmbottom

.If

equipmentor

conditionsdo

notpermit

measurem

entofthestream

bottom,this

conditionshould

benoted

forfurtheraction.

AssessingScourPotentialatBridges

Theitem

slisted

inTable

5areprovided

forbridge

inspectors’considerationin

assessingthe

adequacyofthe

bridgeto

resistscour.In

making

thisassessm

ent,inspectorsneed

tounderstand

andrecognize

theinterrelationships

betweenItem

60(Substructure),Item

61(Channeland

ChannelProtection),andItem

71(W

aterwayAdequacy).

Asnoted

earlier,additionalfollow-up

byan

interdisciplinaryteam

shouldbe

made

utilizingItem

113(Scour

CriticalBridges)

whenthe

bridgeinspection

revealsa

potentialproblemw

ithscour.

Cross-Sectionsand

Underw

aterInspectionsPerhaps

thesingle

mostim

portantaspectofinspecting

thebridge

foractualorpotentialdamage

fromscouris

thetaking

andplotting

ofmeasurem

entsofstream

bottomelevations

inrelation

tothe

bridgefoundations.

Where

conditionsare

suchthatthe

streambottom

cannotbeaccurately

measured

byrods,

poles,soundinglines

orothermeans,otherarrangem

entsneed

tobe

made

todeterm

inethe

conditionofthe

foundations.O

therapproaches

todetennining

thecross

sectionofthe

streambed

atthebridge

include:

rb-UJl\.)|-~ .Use

ofdivers.

Useofelectronic

scourdetectionequipm

ent.

Whatare

theshapes

anddepths

ofscourholes?Is

thefoundation

footing,pilecap,orthe

pilingexposed

tothe

streamflow

;and

ifso,whatisthe

extentandprobable

consequencesofthis

condition?5.

Hasriprap

arounda

pierbeenm

ovedorrem

oved?

319

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Post-InspectionD

ocumentation

Following

completion

ofthebridge

inspection,the

newchannelcross

sectionshould

becom

paredw

iththe

crosssections

takenduring

previousinspections.

Theresults

ofthecom

parisonshould

beevaluated

anddocum

ented.M

anybridge

inspectorsnow

utilizelap

topcom

puterstofacilitate

thedocrunentation

oftheinspection

findings.Com

putersw

illalsofacilitate

plottingofsuccessive

channelcross-sectionsto

enablerapid

evaluationofthe

changes.A

bridgescourexpertsystem

,CAESAR

,(TR

B,1999)isavailable

toassistin

thisprocess.

Notification

ProceduresThe

Stateshave

establisheda

positiveprocedures

ofprom

ptlycom

mtm

icatinginspection

findingsto

properagencypersonnelfor

action.Theprocedure

providesforaction

foranycondition

thatabridgeinspectorconsiders

tobe

ofanem

ergencyorpotentially

hazardousnature.In

some

statesthe

inspectorcanclose

abridge

which

heconsiders

dangerous.W

hereas,inotherstates

henotifies

adesignated

authorityw

hotakesthe

necessaryaction.Conditionswhich

donotpose

animm

ediatehazard,butstill

warrantfurther

action,are

conveyedto

thoseresponsible

foraction.

Nom

rally,an

independentrevueauthority

isestablishedto

besure

thatcorrectionsare

made

toaproblem

thataninspection

hasidentified.

CASE

HIS

TOR

IES

OF

BR

IDG

EIN

SP

EC

TION

PR

OB

LEM

S

Introduction

Since1987

therehave

beenthree

bridgefailures

with

lossoflife

thatillustratethe

importance

ofbridgeinspections.

Intwo

ofthefailures

inspectorsfailedto

observechanged

conditionsthatifcorrected

may

havesaved

thebridge.

Inthe

thirdcase,the

inspectorsdocum

entedthe

changes,buttherewas

nofollow

-upaction

toevaluate

thechanges

andto

protectthebridge.

Inthe

following

sections,theinspection

problems

associatedw

iththese

bridgefailures

aredescribed

andissues

relatedto

inspectionare

highlighted.

SchoharieC

reekB

ridgeFailure

On

April

5,1987

theN

ewYork

StateThruw

ayAuthority

Bridge(I-90)

overSchoharie

Creekcollapsedkilling

10persons(Richardsonetal,1987

andNTSB,1988).

TheN

ationalTransportationSafety

Boardinvestigated

thecollapse

andgave

asthe

probablecause

as:"............tlrefailure

oftheNew

YorkState

Thruway

Authority(N

YSTA)tom

aintainadequate

riprap

aroundthe

bridgepiers,w

hichled

tosevere

erosionin

thesoil

beneaththe

spreadfootings.C

ontributingto

theaccidentwere

ambivalentplans

andspecifications

usedforconstruction

ofthebridge,an

inadequateN

YS

TAbridge

inspectionprogram

,andinadequate

oversightbytheN

ewY

orkState

Departmentof

TransportationandtheFederalHighw

ayAdministration.C

ontributingtotheseverity

oftheaccidentwas

thelack

ofstructuralredundancyin

thebridge."

323

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Lagasse,P.F.,J.D.Schall,andE.V.Richardson,1995,"StreamStability

atHighw

ayStructures,"H

ydraulicEngineering

CircularN

o.20,SecondEdition,FH

WA-IP-90-

014,FederalHighw

ayAdm

inistration,Washington,D.C.

Lagasse,P.F.,M.S.Byars,L.W

.Zevenbergen,andP.E.Clopper,

1997a,"BridgeScourand

StreamInstability

-Countenneasures-Experience,Selection,andDesign

Guidelines,H

ydraulicEngineering

CircularN

o.23,

FHW

AH

I-97-030,FederalH

ighway

Adrrrinistration,Washington,D.C.

Lagasse,P.F.,E.V.Richardson,J.D.Schall,andG

.R.Price,1997b,

"Instrumentation

forM

easuringScour

atBridgePiers

andAbutm

ents,"N

CH

RP

Report396,TransportationResearch

Board,NationalResearch

Council,N

ationalAcademy

Press,Washington,

NTSB,1988,"Collapse

oftheN

ewYork

Thruway

(I-90)Bridgeoverthe

SchoharieCreek,NearAm

sterdam,N

ewYork,A

pril5,1987,"

NTSB/I-IAR

-88/02,NTSB,

Washington,D.C.

NTSB,

1990,"Collapseofthe

Northbound

U.S.Route51

BridgeSpans

overtheHatchie

River

nearCovington,

Temressee,"

April

1,1989,

NTSB/H

AR-90/01,

NationalTransportation

SafetyBoard,W

ashington,D.C.

Richardson,E.V.,P.F.Lagasse,J.D.Schall,J.F.Ruff,T.E.Brisbane,andD

.M.Frick,

1987,"Hydraulic,Erosion

andChannelStability

Analysisofthe

SchoharieCreek

BridgeFailure,

New

York,"Resource

Consultants,Inc.

andColorado

StateU

niversity,FortCollins,CO

.

Richardson,E.V.andD

avis,S.R.,1995,"EvaluatingScouratBridges,"H

ydraulicEngineering

Circular

18,Third

Edition,FH

WA-H

I-96-031,Federal

Highw

ayAdm

inistration,Washington,D.C.

Richardson,E.V.,J.S.Jones,andJ.C.Blodgett,

1997,"Findingsofthe

I-5Bridge

Failure,"ASC

EH

ydraulicEngineering

Proceedingsof

Theme

A,27th

IAH

RCongress,San

Francisco,CA.

Richardson,E.V.andLagasse,P.F.,Editors,

1999,"StreamStability

andScourat

Highw

ayBridges,"

Compendirnn

ofASCE

WaterResources

PapersEngineering

Conferences1991

to1998,Reston,V

A.

TransportationResearchBoard,1999,"C

AESAR:An

ExpertSystemforEvaluation

ofScourandStream

Stability,"N

CH

RP

Report426,NationalResearch

Council,

NationalAcadem

yPress,W

ashington,D.C.

328

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Page 336: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

inclinationsofbridge

piersstay

insideofthe

smalllim

itedrange

andm

uchsm

allerthanthe

inclinationthathas

abad

influenceon

thetrack.

Abovescourm

onitoringdevice

islow

-cost,becauseonly

onescourm

onitoringdevice

isseton

bridgepierin

ordertoevaluate

stabilityofbridge

pier.Andfollow

ingeffects

areexpected

incase

touse

abovescourm

onitoringdevices.

(1)Therule

forregulationoftrain

operationduring

highwaterw

ithabove

scourm

onitoringdevices

become

more

reliable,becausem

onitoringobjects

ofthesedevices

havecloserrelationship

with

stabilityofbridge

foundation.(2)These

devicesare

alsoeffective

forthefltune

with

steepslope

thatstreambecom

essuper-criticalflow

andwaterlevelhardly

risesduring

aflood

CO

NC

LUSIO

NS

Table1shows

summ

aryofthe

fourscourmonitoring

devicesdeveloped

byJR

East.Scourm

onitoringdevices

exceptforaccelerometer-type

werem

adeforpracticaluse.A

tpresent,new

regulationoftrain

operationduring

highwaterincluding

with

installinga

scourmonitoring

isconsidered

byJR

East.

Table1

Summ

aryofScourM

onitoringDevices

Typeofscour.

.Characteristics

Usefor

mom

toring

FloatingSw

itchType

Thosecan

monitoran

Assessmentofthe

riskof

ElectrodeType

elevationofriverbed.

abutmentor

revetmentfailure

IAccelerometerType

Thosecan

monitorstability

Assessmentofthe

riskof

BridgeC

linometerType

ofbridgepier.

pierfailure

RE

FFER

EN

CE

1)N

ishimura,A.,Tanam

ura,S.,1989“A

Studyon

IntegrityAssessm

entofRailw

ayBridge

Foundation(in

Japanese)”,RTR

IReportVol.3,No.8

336

Page 337: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

RE

AL-T

IME

BR

IDG

ES

CO

UR

AS

SE

SS

ME

NT

AN

DW

AR

NIN

G

ByJeffrey

M.D

iStasilandCarlton

L.H02

AB

ST

RA

CT

Am

ethodto

assessbridge

scourpotential

ispresented.

Thepurpose

ofthem

ethoddevelopm

entisto

provideagencies

suchas

stateDepartm

entsofTransportation

(DO

Ts),state

police,andlocalpolice

am

eansofassessing

thehazard

posedby

bridgescour.

Thism

ethodis

basedupon

aspatialdecision

supportsystem(SDSS).

TheSDSS

isan

interoperablecode

thatw

illallowfor

flexibilityin

spatialcalculation.The

SDSSis

aJAVA

basedprogram

designedw

ithinterchangeable

modules

(datam

anagement,

algorithm,

andgraphical

userinterface).

Becausethe

codeis

JAV

Abased,

theprogram

canrim

onany

platform.

Itis

important

torecognize

thatthe

SDSSwas

originallyenvisioned

foruse

asa

means

toassess

seismically

inducedlandslide

hazard,butisflexible

enoughto

beused

foralltypesofspatialanalysis

(Miles

etal,1999).

Forthisapplication,the

SDSScalculates

bridgescourhazard

usingdifferenttypes

ofdatasets,analyticalalgorithm

s,andspatialanalysis.

Thedata

setsare

archival(foundationdesign,geom

etries,streambed

contours,location,etc.)and

temporal(clim

atalogical,hydrological,bridgescourm

onitors,etc.).Archivaldata

canbe

periodicallym

odifiedas

needed.Analytical

algorithms

arebased

uponrecom

mendations

fromthe

FederalHighw

aysAdm

inistrationH

ydraulicEngineering

Circulars

18and

20.O

theralgorithm

scould

besim

plyim

plemented

usingthe

modularnature

oftheSDSS.

An

importantaspectofevaluating

thehazard

potentialisan

assessmentoftypes

ofdatathatare

availableand

theform

atinw

hichthese

dataare

kept.A

surveyofthe

availabledata

ism

adeofthe

New

Englandregion

oftheU

nitedStates

ofAmerica.

Asurvey

wasm

adeoffederal

andregional

agenciesto

detenninethe

typesof

availabledata.

Inaddition,

existingscour

programs

ineach

New

Englandstate

arereviewed

toprovide

anunderstanding

oftheeach

state’sprogram

andits

direction.Recom

mendations

arem

adeforthe

developmentofadditionalm

eansofdata

acquisitionfor

betterreal-tim

eassessm

ent.U

ltimately,

theSDSS

couldbe

usedas

aweb-based

monitoring

scheme

forreal-tim

edissem

inationofbridge

scourhazard

warning

forsafety

andm

aintenancerelated

publicagencies.

INTR

OD

UC

TION

Thecollapse

ofthe

SchoharieCreek

Bridgein

New

Yorkin

1987was

thefirst

catastrophicevent

thatbroughtbridgescourinto

thepublic

spotlight.Since

then,theFederal

Highw

ayAdm

inistration(FH

WA

)has

focusedits

effortson

identifyingand

codingallbridges

regardingtheir

scoursusceptibility

throughqualitative

andquantitative

means.

In1989,

theFH

WA

mandated

thatallstatesevaluate

streamstability

andthe

potentialforscour

atbridgesover

water.Technical

advisoriesTA5140.20

“Scourat

Bridges”(U.S.

Department

of

1Grad.Asst.,Dept.ofCiv.and

Env.Engineering,Univ.ofM

ass.,Amherst,M

A01003

2Assoc.Prof.,Dept.ofCiv.andEnv.Engineering,U

niv.ofMass.,Am

herst,MA

01003

337

Page 338: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Transportation,1988)

andTA5140.23

“EvaluatingScour

atBridges”

(U.S.Departm

entof

Transportation,1991)were

issuedin

1988and

1991,respectively,toprovide

guidancefor

statesas

theydeveloped

andim

plemented

scourevaluation

programs

forexisting

bridgesand

newbridge

designs.In

1991,H

ydraulicEngineering

Circular

20(H

EC-20)

“StreamStability

atH

ighway

Structures”was

publishedby

theFH

WA

tohelp

provideguidelines

foridentifying

streaminstability

athighw

aystream

crossings(Lagasse

etal,

1991).In

1995,H

ydraulicEngineerC

ircular18

(HEC

-18)“EvaluatingScouratBridges

ThirdEdition”was

releasedby

theFH

WA,

which

presenteda

revisedm

ethodologyfor

afullscouranalysis,including

thedesign,

evaluation,and

inspectionofbridges

forscour

(Richardsonand

Davis,

1995).O

fparticularconcenrare

scourcriticalbridges,which

arebridges

thatcouldexperience

catastrophicfailure

orbecom

estructurally

unstableas

aresultofexcessive

scourcausedby

adestructive

floodevent.

Asingle

digitratingsystem

within

theN

ationalBridgeInspection

Standards(N

BIS)was

alsodeveloped

bythe

FHW

Ato

helpclassify

thevulnerability

ofbridgesto

scour.N

owthatm

ostofthebridges

havebeen

evaluated,inventoried,andcoded

with

regardto

scour,the

nextlogical

stepis

todevelop

asystem

aticm

eansof

classifyingand

prioritizingbridges

forrem

ediation.The

objectiveof

thisresearch

isto

developa

strategyfor

theorganization

ofastatewide

network

ofscourmonitoring

devicesto

assistinthe

allocationof

resourcesduring

potentiallydestructive

floodevents,

which

would

includeassessing

bridgescourin

real-time.

Thisw

ouldbe

accomplished

usinga

web-basedapproach,w

hichconsists

ofaplatform

independentcodethatutilizes

aSDSS.

Unfortunately,

thescour

equationsfound

inH

EC-18

may

notpredict

accuratescour

depths.This

may

bedue,

among

otherthings,to

theinability

toconducttests

onlarge-scale

laboratorym

odels.Therefore,new

scourequationsshould

alsobe

researchedthatcould

alsobe

usedalong

with

thecurrentequations.

BA

CK

GR

OU

ND

Throughthe

NBIS,

scourcritical

bridgesare

addressedin

theItem

I113code

inthe

Recording

andC

odingG

uideforthe

StructureInventory

andAppraisalofthe

Nation’s

BridgesN

ationalBridge(ReportN

o.FH

WA-PD

-96-001).A

scourcriticalbridge

isclassified

assuch

accordingto

oneofthe

following:

(1)observedscouratthe

bridgesite

or(2)scorn

potentialasdeterm

inedby

ascourevaluation

study.A

singledigitcode

isused

todescribe

thestability

ofthe

bridge’spier

andabutm

entformdations.

Scourcriticalbridgesare

identifiedby

acode

of3orless

(U.S.Departm

entofTransportation,1995).O

verthepastten

years,stateD

OTs

inN

ewEngland

havedevoted

alarge

amountoftim

eto

assigningI113

codesfor

eachbridge.

Many

bridgeswere

assignedcodes

afteran

initialscreening

wasperform

ed,withoutthe

needfor

afullscour

analysis.This

wasdone

througha

reviewof

existinginform

ationfor

thebridges,

including,but

notlim

itedto,

bridgeplans,

hydrologicfiles,

FEM

Aflood

studies,and

USGS

streamgage

data.For

alarge

ntunberof

bridges,however,theinitialscreening

wasnotsufficientand

afullcom

prehensivescouranalysis

wasrequired.

Thisusually

meanthundreds

ofbridgesforeach

state.O

fthosebridges

thatwereput

througha

scourevaluation

study,m

anystill

remain

eitherrurcoded

orwere

assigneda

temporary

codeuntila

more

thoroughanalysis

couldbe

performed.

One

reasonforthis

hasbeen

thatfor

many

ofthe

olderbridges,

noplans

wereavailable

andthus

thefoundations

wereunknow

n.In

othercases,hydraulicorFEM

Astudies

werenotavailable

orcom

plexhydraulic

conditionsnecessitated

am

oreintensive

analysis.W

hilem

anyofthese

stateshave

codedthe

338

Page 339: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

majority

oftheirbridges,nostate

hascom

pletelyfinished

thistask

duein

largepartto

unknown

foundations,although

temporary

codesm

ayhave

beenused

forthe

time

being.Forthe

most

part,allofthebridges

with

known

foundationshave

receivedan

I113

code.There

havebeen

afew

differentapproaches

toaddressing

bridgesw

ithim

known

foundations.O

neis

tocode

thebridge

asa

3(scourcritical).

Thiswas

doneto

savetim

eand

money

andwas

basedupon

resultsofothersim

ilarbridgesthatreceived

fullscoruevaluations.

Anotherapproach

isto

codethe

bridgeas

U(unknow

nfoundation)

fornow

untilsubstuface

investigationscan

beperform

edbased

uponprioritization

(Nardone,2000).The

investigationscan

beaccom

plishedthrough

boringsorgeophysicaltesting

methods,such

asground

penetratingradar.

Obviously,

thisapproach

ism

orecostly,

butit

pemiits

afull

scourevaluation

tobe

conductedand

insteadofassum

ingallofthese

bridgesare

scourcritical,some

may

beable

tobe

removed

fromthis

list.This

couldalso

savem

oneyin

thelong

termas

well,

which

will

bediscussed

laterinthe

paper.A

llstates

must

dealw

iththe

same

scourproblem

,yet

whenit

comes

toavailable

resourcessuch

asm

oneyand

persomrel,no

twostates

arealike.

Theim

plementation

ofaSDSS

willhelp

levelthefield

forallstates,perm

ittingallparties

toevaluate

andm

onitorscourusing

thesam

em

odels.

GE

NE

RA

LS

CO

UR

AN

ALY

SIS

Tosom

eextent,allofthe

statesin

New

Englandhave

developeda

flowchartthatoutlines

thegeneralprocedure

followed

fora

typicalbridgescouranalysis.

Eachstate’s

scourprogramwas

aresultofa

collectiveeffortbetween

geotechnical,hydraulic,andstructuralengineers.

Inm

anycases,

theservices

ofoutside

consultantswere

alsoneeded.

Ingeneral,

allbridges

underwentaninitialreview

ofexistinginform

ation,which

wasm

ainlyqualitative.

Bridgesthat

wereclearly

stableorunstable

werecoded

afterthisprocess.

Abridge

thatdisplayedextensive

scourduringa

fieldinspection

couldbe

codedjustas

easilyas

abridge

thatwasfounded

onbedrock.

Forthosebridges

determined

tobe

scoursusceptible(i.e.

theirvuhrerabilityto

scourwas

notasapparent),they

wereputthrough

afullLevel2

scouranalysis

orsom

eabbreviated

analysis(G

lemi,2000).

Engineeringjudgem

entwasalso

broughtinto

theanalysis

asflood

andbridge

historywere

consideredalong

with

existingdata,

records,and

reports.For

example,

considerationsw

ouldbe

made

inthe

instancewhere

abridge

wasdetennined

tobe

scourcriticalfora10-year

event,buthadw

ithstoodtw

oseparate

50-yearevents.Analysis

ofthebridge

fora10-yearevent

may

haveestim

atedscorn"

depthsthat

classifythe

bridgeas

scourcritical,

butclearly

thestructure

didnot

fail.Som

estates

addressedthis

issueby

addinga

secondcharacter

tothe

single-digitI113code,w

hichwas

basedupon

am

odifiedversion

oftheM

arylandState

Highw

ayAdm

inistration(M

SHA)

113rating

system,

tosupplem

entthe

I113code

inorder

tom

orerealistically

describethe

scoursusceptibilityofa

bridge.A

nexam

pleofthis

isdescribed

below:

“...asingle-span

bridgew

hichhas

beenstanding

for50

yearsm

aybe

scourcritical,

solelydue

tothe

calculatedabutm

entscour.Forsuch

abridge,we

mightselecta

MS

HA

ratingof

‘3A’

which

denotesscour

critical,but

with

am

ildscour

risk.”(W

hitman

&H

oward,Inc.,1996)

339

Page 340: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Thisadditional

characterin

thecode

canalso

beuseful

inthe

prioritizationof

bridgesfor

remediation.

One

approachto

conductingbridge

scourstudies

isdiscussed

in-depthin

thefollow

ingsection.

CO

MP

AR

ATIV

ESC

OU

RA

NA

LYS

IS

TheCorm

ecticutDepartmentofTransportation

(CO

NN

DO

T)initiallyconducted

itsscour

evaluationstudies

ingeneralaccordance

with

theaforem

entionedprocedures.

Aftera

periodof

time,

facedw

ithprohibitive

costsand

alarge

number

ofbridges

remaining

tobe

analyzed,C

ON

ND

OT,

alongw

iththe

FederalH

ighway

Administration

(FHW

A)and

consultants,developed

anew

method

thatwould

providean

I113code

withoutrequiring

afullLevel2

scouranalysis.

Thisnew

method,called

thecom

parativescouranalysis,utilized

theresults

ofpreviousLevel2

scouranalysesw

hilegenerating

time

andcostsavings

(CH

A,1998).Prior

tothe

introductionofthe

comparative

scouranalysis,

approximately

300bridges

receiveda

fullLevel2scouranalysis,including

airI113code.

Atthis

point,CO

NN

DO

Ttook

astep

backfrom

theirscour

studiesand

decidedto

takeanother

approachfor

therem

ainingbridges,w

hichresulted

inthe

implem

entationofthe

comparative

scouranalysis.Itwas

decidedthatthe

1350rem

ainingbridges

would

beplaced

inone

ofthreecategories:

(1)Low

Risk,(2)

Level2,or

(3)“advance

tothe

nextphase.”Exam

plesofLow

Risk

bridgesincluded

thosew

hichwere

abox

culvertorthosefoim

dedon

competentrock.

Level2bridges

included,butwere

notlimited

to,thosethatwere

notconsideredlow

risk,had

complex

hydraulics,or

wereconsidered

veryim

portant,basedupon

trafficvolum

e,replacementcost,structure

size,etc.The

purposeofthis

categorizationwas

toidentify

thosestructures

thatwereclearly

stableorunstable.

Anybridge

notplaced

inthe

firsttw

ocategories

fellinto

the“advance

tothe

nextphase”

category(C

HA,

1998).The

susceptibilityto

scourwasless

clearforthe

bridgesplaced

inthe

“advanceto

thenext

phase”category.

Thesebridges

werecandidates

foreither

aPhase

Illor

PhaseIV

evaluation.U

poncom

pletionofan

officereview

andsite

visit,a

bridgecould

eitherbecoded

usingthe

comparative

analysis,recormnended

forafullLevel2

analysis,oradvanceto

thenext

phaseto

obtainadditional

information

(i.e.structural

stabilityanalysis)

todeterm

inethe

recomm

endedrating

withoutneeding

afullLevel2

study.A

structuralstabilityanalysis

would

beperform

edon

bridgesfor

which

predictedscour

depthswere

calculatedto

bew

ithinthe

spreadfooting

orpile

foundationthat

wasexposed,

butnot

completely

undermined.

Theanalysis

would

addressconcem

sthatthe

bridgew

ouldbecom

eunstable

dueto

thecalculated

scourdepths

andstability

checksw

ouldbe

made

toensure

minim

umfactors

ofsafety

stillapplied

forbearing

pressure,overturning,

andsliding.

Thedifference

betweenPhase

IIIand

PhaseIV

isthatPhase

IIIrepresented

thevalidation

processofthe

comparative

analysisand

includesbridges

which

arerepresentative

ofthosefound

acrossthe

state.Once

finalapprovalhad

beengiven

tothe

comparative

process,Phase

IVwas

initiatedand

therem

ainingbridges

wereevaluated

following

thesam

eprocedure

(CH

A,1998).Those

bridgesthatwere

alreadyrated

usingthe

Level2analysis

servedas

thegroup

ofrated

bridgesw

ithw

hichthe

unratedbridges

would

becom

pared.Thecom

parativeanalysis

waspurely

qualitativein

nature,butitwasstillused

toprovide

recomm

endationsforthe

NBIS

Item113

rating(Antoniak

andLevesque,2000).

340

Page 341: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Inorder

tojustify

thecom

parisonof

two

bridges,one

ratedand

theother

Lurrated,prim

aryand

secondarycriteria

hadto

bem

et.Prim

arycriteria

wereconsidered

tobe

Singlevs.

Multiple

Spanand

StreamCharacter

Category.Secondary

criteriawere

listedas

Estimated

StreamVelocity,

FoundationType

(atAbutments

andPiers),

Ratioofthe

UpstreamCharm

elW

idthto

theW

idthof

theCharm

elBeneath

theBridge,

andAngle

ofAttack.

Avalid

comparison

mandated

that,at

thevery

least,allprim

arycriteria

werem

et.The

greaterthe

numberofsecondary

criteriathatwere

met,the

closerthesim

ilarityofthe

two

bridges(C

HA,

1998)

Application

ofCom

parativeAnalysis

Althoughthe

comparative

analysism

aynot

beapplied

exactlythe

same

wayshould

anotherstateadoptit,the

conceptcertainlyw

ouldstillbe

valid.As

previouslym

entioned,I113codes

havebeen

assignedto

mostofthe

bridgesw

ithknow

nfoturdations.

Theonly

onesthat

remain

arebridges

thatareuncoded

orreceiveda

temporary

codedue

tounknow

nfoundations.

Thecom

parativeanalysis

may

provideincentive

tostates

thatm

ayhave

beenreluctant

todeterm

inethe

foundationsofbridges

thatarecurrently

unknown,whetherthey

doso

throughborings,

geophysicalmethods,

orrecoveredplans.

While

itis

easyand

inexpensiveto

codea

bridgew

ithunknow

nfoundations

asscour

criticalw

ithoutdoing

airyanalysis,

initialsavings

couldbe

lostifthatbridgeis

monitored

atalaterdate.

Itwould

beam

isuseofresources

toplace

am

onitoron

ornear

abridge

thatwasnotscour

critical,sim

plybecause

itwas

toocostly

todetennine

theform

dationsinitially.

Theresources

shouldonly

beapplied

tothose

bridgesthat

aretruly

scourcritical.Perhaps

evenm

oreim

portantly,thecom

parativeanalysis

would

allowforinfonnation

tobe

sharedam

ongstates.

Forexam

ple,a

databaseor

web-basedsystem

containinginform

ationpertaining

torated

bridges(fullLevel2

analyses)in

Connecticutcouldbe

accessedto

finda

bridgew

hichcould

becom

paredw

ithan

similarunrated

bridgein

Vermont,assum

ingthe

propercriteria

wasm

et.Through

theuse

ofqueries,

severalrated

bridgescould

bepulled

upfor

possiblecom

parisonofthe

Lurratedbridge.

Basedupon

areview

oftheLevel2

scourreportsfor

therated

bridges,adecision

willbe

made

todetennine

ifarating

canbe

recomm

endedusing

thecom

parativeprocess.

Ifaratingcannotbe

made,then

thebridge

couldbe

forwardedto

aLevel2

orabbreviatedanalysis.

Once

them

ostappropriatebridge

isselected

forthecom

parison,usingengineering

judgment,the

reportfortheLevel2

bridge,andfield

reportsforthe

imrated

bridge,an

appropriateItem

I113rating

willbe

reconnnended.Itshould

benoted

thatthecom

parativeprocess

onlycom

paresLurrated

bridgesw

ithbridges

thathavebeen

througha

fullLevel2scour

analysis.A

bridgethatreceived

arating

throughcom

parativeanalysis

carmotbe

considereda

ratedbridge

tobe

compared

with

anotherrurratedone.

Thereare

some

conditionsthatshould

besatisfied

ifthisanalysis

isto

beadopted

andused

byother

states.First,

alarge

andrepresentative

groupofbridges

mustbe

availablefor

comparison;

otherwise,the

analysisw

ouldbe

limited

inits

scope.States

may

evenw

ishto

developtheirow

ndatabase

ofbridgesthatwere

ratedusing

aLevel2

analysisifthey

feelthatw

ouldbetterrepresentthe

bridgetypes

andconditions

intheirstate.

Inthe

eventthatbridgeor

streamconditions

drasticallychanged,

abridge

couldbe

reanalyzedusing

thecom

parativeanalysis.

However,ifthis

occurredto

abridge

thatinitiallyreceived

aLevel2

analysis,therew

ouldbe

two

options.It

couldbe

placedback

intothe

groupof

ratedbridges

usedfor

341

Page 342: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 343: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

satisfied.Through

theintegration

ofarchival

andreal-tim

edata,

andthe

selectionof

anapplicable

scourequation,predictedscourdepths

willbe

calculatedin

real-time

andthe

systemcan

alerttheuserofapotentialbridge

failuredue

toscourduring

anevent.

Thisapproach

will

alsoallow

scourdepths

calculatedfrom

differentequations

tobe

compared

andperhaps

giveinterested

partiesa

betterLurderstanding

ofwhich

scourequations

work

bestundercertain

conditions.If

actualscour

depthsfrom

previousstorm

eventswere

measured

inthe

fieldand

storedin

thebridge

attributesas

well,then

thesedepths

couldalso

becom

paredw

iththe

predictedresults.

TheSDSS

codew

illbe

written

inSun

Microsystem

sJava

1.2for

them

odel.Java

isplatform

independent,which

isdue

tothe

applicationofthe

conceptofavirtualm

achine.The

virtualmachine

allows

forlow-leveloperating

systemim

plementation

tobe

separatedfrom

high-levelcoding.

Javawas

selectedfor

theapproach

asa

resultofits

algorithmic

flexibility,its

abilityto

beaccessed

fromthe

webby

allparties,andthe

familiarity

oftheauthors

with

it(Miles

etal,1999).

GR

AP

HIC

AL

US

ER

INT

ER

FA

CE

CO

MP

ON

EN

TS

Thegraphicaluserinterface

(GU

I)com

ponentsserves

thepurpose

ofmaking

thesystem

more

user-friendly.Itdoes

thisby

masking

thecom

plexityofthe

usertasksand

thedifferences

betweenthe

nrunerousunderlying

components.

Throughdirect-m

anipulation,the

GU

Iw

illinclude

severalcomponents

foruse

inm

anagingspatialdata

requirements

(inputparameters),

modelconfiguration

(algorithms),and

analysisoutput(M

ilesetal,

1999).D

irect-manipulation

refersto

performing

computing

tasksthrough

physicalaction

insteadof

syntax.Several

advantagesof

direct-mairipulation

arecited

bySchneiderm

an(1988),

suchas

control-displaycom

patibility,reducederrorrates,fasterlearning,longerretention,and

more

userexplanation.The

GU

Iw

illbebased

upona

treem

odel,w

hichw

illbefam

iliarand

comfortable

form

ostusers.Its

structurew

illnot

onlyaid

inthe

tmderstanding

ofinformation

suchas

model

configurationand

inputparam

eters,but

alsoin

thestructure

ofthe

infonnationin

terms

oforganization

andrelation.

Theflexibility

ofthetree

willalso

pennitmodification

ofspatialdataparam

eters,suchas

inthe

instancewhere

am

odelcomponentis

addedorchanged

(Miles

etal,1999).

Thethree

components

oftheG

UIinclude

theSpatialData

Manager,the

ModelM

anager,and

theO

utputManager.

TheSpatialData

Managerw

illcontainallspatialdata

requirements,

which

willbe

representedby

branchesofthe

treein

theG

UI.

TheM

odelManagerw

illlistallofthe

models

(algorithms)

thatcanbe

selectedfor

analysis,pennitediting

ofmodelparam

eters,and

perfonnthe

analysisthrough

direct-manipulation.

TheO

utputManagerw

illhavethe

same

configurationas

theSpatialData

Manager,exceptthatitw

illorganizeand

presenttheresults

ofthe

analysis(M

ilesetal,

1999).A

flowchartis

shownin

Figure1

toprovide

avisualreference

ofthesystem

components:

343

Page 344: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

(ArchivalData)L

(fiomServer)

7oee

Ngorit

Manager

I

IutputM

anager

"0'

4?

as,

aes,

Databases

Fig.1.Proposed

systemarchitecture

forspatialdecisionsupportsystem

(SDSS)

AP

PLIC

ATIO

NS

OF

THE

STR

ATE

GY

EngineeringApplication

Some

stateshaveexpressed

theirsatisfactionw

iththe

scourcodesthey

assignedusing

theH

EC-18

scourequations.

Theyunderstand

thattheequations

canbe

overlyconservative,

butthey

feeltherequirem

entsthatwere

establishedby

theFH

WA

regardingscour

analyseshave

beenm

et(Nardone,

2000).O

therstates

haveindicated

theirinterest

inidentifying

newequations

thatcouldm

oreaccurately

predictmeasured

scourdepthsin

thefield.

Theapplication

ofnewequations

ca.nnotonly

providebetterestim

atesofscourdepth,butcould

alsoreduce

thenum

berofbridges

determined

tobe

scourcritical

(Antoniakand

Levesque,2000).

Revisingcodes

forbridges

thatwere

initiallycoded

asscour

criticalcould

significantlyim

pactthe

allocationof

newm

onitoringdevices,

therouting

oftraffic

duringan

evacuation,and

theprioritization

ofbridgesforrem

ediation.Connecticuthas

alreadytaken

thisinitiative

ofadoptingnew

equationsby

amending

thelocalabutm

entscourequationby

Froehlich,aspresented

inH

EC-18,Third

Edition.The

currentfonnula

isas

follows:

0.431

If,/Y,,=2.27KK5

Fr°"”+1(1)

12

Y

344

Page 345: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

where

K1=

coefficientforabutmentshape

(SeeTable

6,Section4.3.6

inHEC-18

ThirdEd.,

datedN

ov.95)K2

=coefficientforangle

ofembankm

enttoflow

(RefertoSection

4.3.6,Figure16

inH

EC-18,Third

Ed.,datedN

ov.95)L1

=the

lengthofabutm

entprojectednorm

altoflow

Ya=

averagedepth

offlowin

thefloodplain

=A

,/a’A

,=the

flowarea

oftheapproach

crosssection

obstructedby

theem

bankment

Fr=the

Froudenum

ber=V

J(g

ya)°'5V

e=

Qe/

Ac

Q,=

theflow

obstructedby

theabutm

entandapproach

embankinents

Y,=scourdepth

CO

NN

DO

Treported

thatthe+1

valuewas

initiallyintended

asa

factorofsafety,butwasnotin

Froehlich’soriginalpaper.

Thisvalue

increasesthe

predictedscour

depthby

thedepth

oftheoverbank

flow.

Basedupon

theconservative

natureofthe

equationbased

uponlaboratory

dataw

ithregard

toactualfield

data,CO

NN

DO

T,afterconsultingw

ithresearchers,chose

toreplace

the+1

valuew

itha

valueof+0.05

andreanalyze

theirbridgesusing

thisrevised

equation.The

predictedscour

depthis

nowreferred

toas

theam

endedscour

depth(C

ON

ND

OT,

1999).Therefore,the

amended

localabutmentscourequation

is:

I0.43

Y/Y=2.27KK

5Fr°"”+0.05

(2)s

aI

2Y

Inthe

eventthatanew

equationis

developedor

modified,

suchas

theone

presentedabove,

itcould

veryeasily

beadded

asa

model.

Forthis

equation,no

additionalspatialdatarequirem

entsw

ouldbe

necessary,but

anadditionalbranch

ofthetree

would

beadded

tothe

Model

Manager.

Aflow

chartrepresenting

thesystem

architecturefor

theengineering

applicationis

shownbelow

inFigure

2:.

345

Page 346: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

(K1:

KQ

aL1)

i(Y

39A

e$Q

J

(FrandV6

canbe

icalculated

usingreal

time

data)V

Mo

1eAm

ene

Manager

ScourEqn.

OutputY5/Ya=2.27i<,i<,(L‘/Y,)°~43Fr°~61+0.05

Manager

Fig.2:Flow

chartofsystemarchitecture

forengineeringapplication

Theoutputw

ouldcontain,

among

otherthings,the

calculatedvalue

ofY5.Instead

ofgoing

backthrough

allofthescour

criticalbridgefiles

inorderto

recalculatepredicted

scourdepths

usingthe

newm

odel,thescouranalysis

canim

mediately

beconducted

throughthe

GU

I.The

newresults

canthen

becom

paredw

ithscourdepth

resultsusing

existingm

odels.The

more

models

thatareavailable

foranalysis,the

more

informed

DO

Tofficials

willbe

regardingthe

scoursusceptibilityofabridge.

Comparative

Application

Algorithms

suchasthe

Comparative

SCOUIAnalysiscould

alsobe

introducedand

usedin

theweb-based

approach.The

spatialdata,which

containsthe

bridgeattributes,w

ouldhave

toinclude

theprim

aryand

secondarycriteria

establishedforthe

ratedbridges.

Queries

couldbe

usedto

identifyrated

bridgesthatare

similarto

unratedbridges

accordingto

matching

criteria.From

thecom

parison,approxim

atescorn‘depth

valuescan

beestim

atedfor

theturrated

bridgefrom

therated

bridge.Therefore,ifflow

satthe

structureevaluated

bythe

comparative

analysisapproach

magnitudes

similarto

criticalflows

fortherated

bridge,thesystem

canw

arnthe

userofthe

potentialhazard.A

flowchartrepresenting

thisapplication

isshown

inFigure

3:

346

Page 347: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

SpatiaData

orRate

SpatiaData

orI

Bridges(m

cl.Primary

UnratedBridges

(incl.&

SecondaryC

riteria)Prim

ary&

Secondaryl

Criteria)l

Mo

1eCom

parativeM

anagerScourAnalysis

IutputM

anager

I113Ratm

g,CntrcaFlows,C

riticalScourDepths,W

arnings

Fig.3:Flow

chartrepresentingsystem

architectureforcom

parativeapplication

Connecticutplanson

usingthe

revisedabutm

entequationto

reevaluatethe

300rated

(fullLevel2

scouranalysis)bridgesthatserved

asthe

sample

ofbridgesw

ithw

hichunrated

bridgeswere

compared.

Thestate’s

goalisto

reducethe

numberofscourcriticalnrunberofbridges

anditis

hopedthatthe

revisedequation

canpredictm

oreaccurate

airdless

conservativescourdepths

(CO

NN

DO

T,1999).8Changes

inthe

resultsofthe

Level2bridge

scouranalysesw

ouldim

pactthe

ratingsofthose

bridgesthatwere

ratedusing

theCom

parativeAnalysis.

Theversatility

ofthe

systemw

ouldallow

theuser

toim

mediately

implem

entthe

newequation

andbegin

calculatingrevised

real-time

scourdepths

forthe

Level2

bridges.By

havingcritical

flows

storedw

ithinthe

spatialdata,new

scourdepths

couldalso

becom

putedor

evenupdated

forcriticalfloods

(e.g.overtopping,100

year,500year).

Iftheresults

oftheanalysis

ofaLevel2

bridgewere

changed,thenthe

Comparative

Analysiscould

bererturto

ensurethatbridges

ratedusing

thisprocedure

werestillcom

paredw

ithsim

ilarratedbridges.

Itwould

beeasy

andquick

tom

akeany

modifications

within

thesystem

.

347

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Page 349: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Creation

ofCoverages

ofExistingM

onitoringSystem

sin

GIS

Once

thesources

ofhydrologicdata

aredocum

ented,thelocation

ofexistingm

onitoringsystem

sw

illbeidentified

(e.g.DEPrain

gages,USGS

streamgages,and

bridgescoru

monitors).

Ageographicalinform

ationsystem

(GIS)w

illthenbe

establishedto

catalogthe

attributesofthe

monitoring

systems.

Onelayer

willbe

createdthatcontains

thelocation

andattributes

oftheexisting

monitoring

systems.

Thislayerw

illbeoverlain

byanotherone

containingthe

locationofscourcriticalbridges

(thosew

ithan

I113code

of3or

less),asreported

bythe

stateDO

Ts.Gaps

inthe

coveragew

illbeidentified

whereexisting

monitoring

systems

donotoverlap

regionsin

which

scourcriticalbridgesare

located(N

ardone,2000).Installing

monitoring

deviceson

allof

thebridges

would

bepreferable,

butbudget

constraintsprevent

this.Instead,

abetter

turderstandingofthe

spatialorientationofourm

onitoringsystem

sw

illaidin

theallocation

ofadditionaldevices,whetheritm

aybe

streamgages,rain

gages,orbridge

scornm

onitors.This

furtherreinforcesthe

ideathatim

known

foundationsm

ustbedetennined.

Otherwise,m

onitoringdevices

may

beinstalled

inplaces

wherethey

arenotnecessarily

needed.M

oneyw

illhaveto

bespentup

fronttodetennine

unknownfoundations,

butsome

ofthatmoney

couldbe

recoupedthrough

abetterdistribution

ofnewgages

andnew

monitors.

Creation

ofBridgeScourAction

Plans

ABridge

ScourAction

Plan(BSAP)

wasoriginally

developedfor

Massachusetts

thatw

ouldhave

prioritizedthe

implem

entationofactions

outlinedin

theplan

basedupon

thecostof

abridge

replacementalong

with

thestructure’s

vulnerabilityto

scour.Each

bridgewas

tohave

hada

BSAPthatidentified

thoseofficials

responsiblefor

monitoring

orclosing

thebridge

asw

ellasinfonnation

regardingthe

locationand

elevationofthe

foundationsand

thecriticalscour

elevation.This

plan,however,

neverwas

putinto

effect.The

statecancelled

remediation

projectsdue

toa

lackofftuids

andw

hileit

hasbeen

discussed,the

Emergency

Managem

entService

(EMS)

hasno

formal

emergency

procedurefor

closingbridges

duringa

floodevent

(Nardone,2000).The

developmentofBridge

ScourActionPlans,sim

ilartothose

originallyproposed

forM

assachusetts,is

highlyrecom

mended

forall

scourcritical

bridges.According

tothe

NBIS

program,allbridges

ina

statem

ustbegiven

anI113

code.The

problemexists

inthatnotallof

thebridges

areowned

andm

aintainedby

thestate

DO

T.G

enerallyspeaking,forbridges

ownedby

localmunicipalities

thatwere

identifiedas

scourcritical,

aletter

issent

outto

notifythe

ownerofthe

bridgeas

tothe

conditionof

thebridge

andthe

scourcritical

determination.

Recomm

endationsfor

repairor

coiurtermeasures

may

alsobe

included(Nardone,

2000).However,

forsm

alltownswhere

thereis

noengineer

orthe

owner

doesnothave

atechnical

backgrormd,

thefindings

inthe

letterm

aynotreceive

properconsideration,

especiallyif

itis

onlydistributed

once.Five

ortenyears

down

theroad,the

initiallettermay

notberem

embered.

Anotherproblemthatcould

ariseis

therepair

ofthebridge

isthe

responsibilityofthe

owner.Localm

tuiicipalities,evenafterreceiving

thefindings

ofthescouranalysis

fortheirbridge,may

notdecideto

implem

entarem

ediationprogram

,whetheritisdue

tocostortheircontention

thatthe

predictedscourdepths

areextrem

elyconservative.

Itwould

beprudentto

move

allbridgesunderthe

jtuisdictionofthe

stateD

OT.

TheD

OT

isthe

agencyresponsible

forinspecting

andm

onitoringbridges

andit

seemsnaive

tohave

theagency

performallofthe

analyses,butnotallow

themto

installcorrective

measures

eventhough

theD

OT

ism

ostfam

iliarw

iththe

349

Page 350: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

problem.

Regardlessofwhose

jiuisdictionthe

repairofthe

bridgefell

under,by

havingan

actionplan

inplace,an

activeresponse

isprepared

andready

incase

ofaflood

event.

CO

NC

LUS

ION

S

Difficulties

with

currentscouranalysesare

time

consruningand

costly.One

problemis

thatinformation

neededforthe

analysesis

notalwayscentralized

oreasilyaccessible.

Insom

ecases,severaloutside

consultantswere

usedby

stateD

OTs

toconductLevel2

scouranalyses.This

makes

itdifficulttoconsolidate

andorganize

information.

Anotherproblemis

thatcurrentanalyses

areunable

torespond

inrrnediatelyto

changingbridge

orstreamconditions.

Inaddition,

neworrevised

scourequationsthatare

introducedin

theliterature

would

requirea

greatdealofeffort

forDO

Tsto

rerunthe

scouranalyses.

TheI-IEC-18

equationsare

known

fortheir

conservativeresults

insom

einstances.

Some

statesdo

nothave

theresources

toperform

additionalanalyses.O

thersm

aybe

contentwith

theircurrentItemI113

codessince

theHEC-18

equationsare

conservativeto

beginw

ithand

thereis

noincentive

forthemto

dothe

analysesagain.

Thedevelopm

entofthestrategy

ofareal-tim

escourm

onitoringsystem

would

providestate

andlocalofficials

with

analtem

ativem

ethodto

evaluatingbridge

scour.The

advantagesof

thisare

numerous,w

iththe

versatilityofthe

approachbeing

thebestadvantage.

Theapproach

providesa

mechanism

forestim

atingbridge

scourautom

aticallyin

real-time.

Thedata

iscentralized

andvery

accessibleaird

theinterface

isuser-friendly.

Analysescan

beperform

edquickly

andatthe

user’sdiscretion.

Aparticularm

odelmightnotbe

avery

goodone,butitis

theresponsibility

oftheuser(s)to

decidew

hichm

odelsare

suitablefortheirpurposes.

Multiple

algorithms,whetheritbe

models

oranalyseslike

theCom

parativeScourAnalysis,could

beused

foranalysis,instead

ofjustone

ortw

o.There

isa

greatdealofflexibilitywhen

addingnew

models

andthe

resultsofthese

models

canbe

compared

veryeasily.

DO

Tsthat

may

haveinitially

shiedaway

fromperform

ingadditional

ornew

scouranalyses

would

nowhave

theability

todo

soata

much

smallercost.

Them

orepro-active

DO

Tsare

whendealing

with

scour,the

greaterthe

benefitto

themselves

andsociety

asa

whole.In

them

eantime,

gapsin

theexisting

coveragewhere

scourcriticalbridges

arenotoverlain

bystream

gages,raingages,or

bridgem

onitorsshould

beidentified

sothatresources

canbe

allocatedm

oreappropriately.

Theim

plementation

ofthis

strategycould

ultimately

aidin

theim

provedidentification

ofand

responseto

scourcriticalbridgesjeopardizedby

apotentially

destructivestorm

event.

AC

KN

OW

LED

GE

ME

NT

S

Theauthors

would

liketo

acknowledgethe

New

EnglandTransportation

Consortitun

forproviding

fundsfor

theresearch

presentedin

thispaper.

Theyw

ouldalso

liketo

thankthe

following

peopleand

theirorganizationsforthe

valuablecontributions

theyhave

provided:

PaulNardone,Massachusetts

Highw

ayDepartm

ent;JeffGlenn,EarthTech,

Inc.;EdwardKent,

EarthTech,Inc.;

YolandaAntoniak,

ConnecticutD

epartment

ofTransportation;

DennisLevesque,

ConnecticutD

epartment

ofTransportation;

David

E.Powelson,

NewHam

pshireDepartm

entofTransportation;ErickBoehm

ler,United

StatesG

eologicalSurvey;GeneParker,

UnitedStates

GeologicalSurvey;

Gary

Hoar,M

aineD

epartment

ofTransportation;R

ichard

350

Page 351: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Tetreault,Verm

ontAgencyofTransportation;

D.M

axSheppard,

University

ofFlorida;Joe

Boardman,Rhode

IslandD

epartmentofTransportation

REFER

ENC

ES

Antoniak,Yolanda

andLevesque,

D61’lIfiS.2000.

Cormecticut

Department

ofTransportation.

Fromconversation

onM

arch17”“.

Clough,Harbour&Associates

LLP.1998.

“Com

parisonM

ethodology.”Preparedfor

theC

onnecticutDepartm

entofTransportation.

ConnecticutDepartmentofTransportation.

1999.“C

ON

ND

OT

Comparative

Analysis.”PreparedforEarthTech,Inc.

Glenn,Jeff.

2000.EarthTech,Inc.

Fromconversation

onM

arch16”‘.

Lagasse,P.F.,Schall,

J.D.,Johnson,

F.,Richardson,

E.V.,Richardson,

J.R.,Chang,F.

“StreamStability

atHighw

ayStructures,”

FederalH

ighway

Administration

Hydraulic

EngineeringC

ircularNo.20,PublicationNo.FH

WA-IP-90-014,Feb.1991.

Miles,S.B.,Keefer,D

.K.,andHo,C.L.(1999).

“Seismic

landslidehazard

analysis:fromhazard

map

todecision

supportsystem

,”U.S.

Conferenceon

LifelineEarthquake

Engineering:Settle,WA

(abstractaccepted).

Nardone,Paul.

2000.M

assachusettsH

ighway

Department.

Fromconversation

onM

arch15”‘.

Richardson,E.V.and

Davis,S.R.“Evaluating

ScouratBridgesThird

Edition,”Federal

Highw

ayAdm

inistrationH

ydraulicEngineering

Circular

No.

18,Publication

No.

FHW

A-IP-90-017,Nov.

1995.

Schneiderman,

B.,1998,

Designing

theuser-interface:

Strategiesfor

effectivehLunan-

computer-interaction,

Addison-Wesley

Longman:M

enloPark,C

A,639p.

U.S.Departm

entof

Transportation,FH

WA,

1988,Technical

AdvisoryTA5

140.20,“ScouratBridges,”O

fficeofEngineering,Bridge

Division,W

ashington,D.C.

U.S.Departm

entof

Transportation,FH

WA,

1991,Technical

AdvisoryTA5l40.23,

“EvaluatingScour

atBridges,”

Office

ofEngineering,Bridge

Division,

Washington,

D.C.

U.S.DepartmentofTransportation,FH

WA,

1995,“Recording

andCoding

Guide

fortheStructure

Inventoryand

Appraisalof

theN

ation’sBridges,”

Office

ofEngineering,

BridgeD

ivision,PublicationN

o.FHW

A-PD-96-001,W

ashington,D.C.

351

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Page 357: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

n

l

iFigure

3-TypicalLocalScourH

olearound

theUnprotected

Pier(foralignedflow

anda

generalscourlevelof-3.1

mbelow

MSL).

357

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Page 360: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

AC

KN

OW

LED

GE

ME

NT

S

Theauthors

would

liketo

thankJohn

Wood,consulting

engineer,forhisinputto

thisproject.

RE

FER

EN

CE

S

Blench,T.

(1969).“M

obile-bedFluviology.”

University

ofAlberta

Press,Edm

onton,Canada.Colem

an,S.E.

(1997)."U

ltrasonicm

easurement

ofsedim

entbed

profiles."Proc.,

27”‘Congress

ofthe

Intemational

Associationof

Hydraulic

Research,San

Francisco,C

alifomia,U

SA,August,B221-B226Fisher,T.and

Watson,A.(1996).“H

uttRiverEstuary

BridgeScourAssessm

ent.”Works

ConsultancyServices

Report.Lauchlan,C.S.(1999).“PierScourCounterm

easures”,PhD,U

niversityofAuckland,

Auckland.M

elville,B.W

.(1997).

“Review

ofScour

Predictionsfor

theH

uttEstuary

Bridge.”Auckland

UniServicesReport,Auckland

Melville,

B.W.

andR

audkivi,A.J.

(1996).“Effects

ofFormdation

Geom

etryon

BridgePierScour.”Journalo

fHydraulic

Engineering,ASCE,122(4),

203-209Richardson,

E.V.and

Davis,S.R.

(1995).“Evaluating

Scourat

Bridges.”H

ydraulicEngineering

Circular

No.18,

FHW

A-IP-90-017,Fairbank

TrunerH

ighway

ResearchCentre,M

cLean,Virginia

Robb,C.

(1992).“Phase

1-H

uttRiver

FloodC

ontrolScheme

Review.Topic

4:R

iverChannelC

apacity.”ReportfortheW

ellingtonRegionalC

ouncil,Wellington

Woodw

ard-Clyde.(1998)."Assessm

entofEnviromnentalEffects

-HuttEstuary

Bridge."W

oodward-C

lyde,Wellington,Septem

ber360

Page 361: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

HY

DR

AU

LICP

RO

PE

RTIE

SO

FC

ON

CR

ETE

BLO

CK

SFO

RB

ED

PR

OT

EC

TIO

N

BySung-Uk

Choil,JoongcheolPaikz,and

WoncheolC

hol

AB

ST

RA

CT

Recentlaboratoryexperim

entsreported

thatcable-tiedblocks

performexcellently

among

many

countermeasures

forlocal

scouraround

thebridge

piers.G

-blocksare

concreteblocks

beingplaced

aroturdthe

bridgepiers

inorderto

protectthechannelbed

fromlocal

scour.The

slnfaceof

G-blocks

ism

adeuneven

with

Lurifonnroughness

heightto

increasefriction.

ForG

-blocksto

performtheir

rolesuccessfully,

theroughness

shouldnotbe

significantlydifferentfrom

thatofnaturalchannels.Otherwise

theyw

illcause

seriousproblem

soflocalerosions.

Furthennore,theyshould

besafe

againsttheflow

force.Iftheyare

moved

bythe

flow,they

willnotonly

beinvalid

forbed

protectionbutalso

make

adverseeffects

onthe

channelconveyance.Inthis

paper,hydraulic

propertiesof

G-blocks

areinvestigated

throughlaboratory

experiments.

Flume

experiments

indicatethat

bothlogarithm

icand

power

lawscan

beapplied

tointerm

ediate-scaleroughness

byG

-blocks,andthatthe

roughnesscharacteristics

ofG-

blocksare

similarto

thoseofnaturalchannels.Itis

alsoshown

thattheresistance

byG

-blocks

changesdepending

uponthe

placementangle

oftheroughness

elements

tothe

flowdirection.Furtherm

ore,thecriticalw

eightofG-blocks

requiredto

withstandstrong

currentis

estimated

interm

sofm

eanvelocity

foran

individualblock

andm

at-typeblocks.

INT

RO

DU

CT

ION

Flows

tendto

acceleratewhen

theyhave

tocircum

ventalongerpath

compared

totheir

neighbors.This

resultsin

localscour

aroundthe

bridgepiers,

piles,and

otherhydraulic

structuresconstructed

inwater

course.M

anycounterm

easureshave

beendevised

fortherem

edyoflocalscorn:

Examples

areriprap,

gabion,cable-tied

blocks,sacrificialpile,

andcollar.

Theyare

eitherarm

oringor

flow-altering

countermeasure.

Among

these,riprapshave

beenthe

mostcom

mon

choicebecause

ofthelong

designexperience.

However,supplying

riprapsfor

protectionagainst

scourbecom

espessim

isticin

thefuture.

Bigstones

forguaranteed

useare

beingexhausted

andenvironm

entalregulations

make

theuse

ofriprapsm

oredifiicultthan

ever.Furthermore,itis

truethat

riprapshave

beenthe

most

comm

onchoice

butnot

thebest

choice.Engineers

have

IProfessor,DepartmentofC

ivilEngineering,YonseiUniversity,Seoul,120-749,Korea

2PostdoctoralResearcher,DepartmentofC

ivilEngineering,YonseiUniversity,Seoul,120-749,Korea

361

Page 362: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

believedthat

thereshould

bea

more

effectivecounterm

easurethan

ripraps.This

encouragesengineers

tosearch

altematives

toripraps.

Recentexperiments

byParkeretal.(1998)

showedthatcable-tied

blocksprovide

outstandingprotection

forthe

bridgepiers

fromlocal

scouram

ongvarious

countermeasures.

Theiradvantages

includeflexibility,

abilityto

withstand

strongcurrent,a

pre-attachedgeotextile,resistance

toice,and

costcompetitiveness

(Parkeretal.,

1998).Although

theuse

ofcable-tied

concreteblocks

isa

relativelyadvanced

technique,fewattem

ptswere

made

regardinghydraulic

propertiesofthose

blocks.

Experiments

byJones

etal.(1995)indicatethatthere

aretw

ofailure

modes

inthe

blockspaved

onthe

channelbottom.The

firstisby

overturningofblocks

locatedatthe

leadingedge

andthe

secondis

byuplifting

ofinner

blocks.In

thelight

oftheir

experimentalresults,the

followings

canbe

suggested:First,itisim

portanttom

aintainthe

roughnessofthe

block-pavedbed

similar

tothat

ofnaturalchannels

inorder

topreventsevere

localerosionatthe

leadingand

tailingedges.This

willreduce

thechance

offailureofthe

first-typem

ode.Secondly,

theblock

shouldbe

heavierenough

towithstand

thesevere

flowsafely.The

onlyresisting

forceofthe

block(eithersingle

orm

at-type)againsttheflow

forcescom

esfrom

its(ortheir)subm

ergedweight.

Inthe

presentpaper,

basiclaboratory

experiments

areintroduced

tostudy

hydraulicproperties

ofcable-tied

blocks.Flow

resistancerelationships

suchas

logarithmic

andpower

lawsare

appliedto

block-pavedopen-charm

elflows,

andtheir

validityis

investigated.Values

ofManning’s

roughnesscoefficient

areestim

ated,and

theyare

compared

with

theroughness

ofnaturalchannelandthe

roughnessby

ripraps.Finally,the

criticalweightofthe

blockis

obtainednotto

make

am

otionwhen

theblock

isexposed

tostrong

flows.

FLO

W-R

ES

IST

AN

CE

RE

LAT

ION

S

Forfully-developed

turbulentopen-charm

elflow

sw

ithinterm

ediate-scaleresistance,the

logarithmic

orpow

erlaw

equationis

known

tobe

appropriatefor

them

eanvelocity.The

logarithmic

equationdue

toKeulegan

(1938)hasthe

formof

U1

R—

—=—

1—

1U

.tr

Ogkshg

()

whereU

=m

eanvelocity,

U.

=shearvelocity,

R=

hydraulicradius,

ks=

roughnessheight,

7c=von

Karman

constant(=0.41),and

,6=

constant.Inthe

problemathand,

thecharacteristic

roughnessheight(ks)in

eq.(1)characterizesnotonly

theheightofthe

roughnesselem

entsbut

alsotheir

orientationto

theflow

,geom

etricaligm

nent,and

spacing.M

anyrelationships

forks

correspondingto

variousbed

materials

havebeen

proposed(G

rifiiths,1981;

Aguirre-PeAnd

Fuentes,1990;M

aynord,1991;

Ferroand

Giordano,1991).By

replacingks

with

theheightofroughness

elements

Dand

byusing

Darcy-Weisbach

formula,eq.(1)is

rewritten

inthe

formof

362

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Page 366: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Inall

ctu'vefittings,

negligibleabsolute

averagedeviations

areobtained,

indicatingthat

bothlogarithm

icand

powerlaws

arevalid

todescribe

resistanceof

block-pavedopen-channelflow

s.

With

thehelp

ofeq.(4),

valuesof

Manning’s

roughnesscoefficient

nare

convertedfrom

f,and

theyare

plottedagainstdischarge

perunitwidth

inFigure

4.Itappears

thatG3

blocksproduce

thehighest

andthe

lowest

roughnesswhen

theroughness

elements

areplaced

normaland

paralleltothe

flowdirection,

respectively.This

conforms

tothe

previousfigure.

Inthe

figure,values

ofM

anning’srt

rangebetween

0.011-0.022,w

hichcorresponds

tovalues

of0.016-0.031for

prototype.This

suggeststhatoverallroughness

byG

-Blocksis

lessthan

theroughness

byriprap

ranging0.032-0.036

(Maynord,

1991).Also,

roughnessprovided

byG

-blocksis

notsignificantly

differentfromthatofnaturalchannels.

Inthe

designpractice

ofblocks,theconceptofperm

issibleshear

stresscan

beused.

Thatis,

theshear

stressby

thedesign

flowshould

notexceed

theperm

issibleshearstress

oftheblocks.

Figure5

depictsthe

changeofbed

shearstress

with

mean

velocity.Inthe

figure,alinearrelationship

betweenbed

shearstressand

mean

velocityis

observedm

ainlydue

tonon-constantvalues

ofCf.

ForG3P,the

permissible

shear

stressis

estimated

tobe

l70—240

kg/m2by

assmning

Coulomb

frictioncoefficientof

,u=

0.8.However,the

permissible

shearstressofripraps

with

diameter

of15-30

cmrangesbetween9.76-19.53kg/m2

(ASCEandWEF,1992).ThisindicatesthatG-

blocksare

much

saferfromm

ovementbythe

flowforce

thanripraps.

InFigure

6,thecriticalw

eightoftheblock

versusm

eanflow

velocityis

plotted.Here,the

criticalweightis

defmed

bythe

weightrequired

toresistflow

forces.First,am

attypeinstallation,where

blocksare

shackedby

U-bolt,is

considered.Thetotalshear

forceis

calculatedby

usingeq.(5),and

avalue

ofCoulomb

frictioncoefficientof

,u=

0.8is

assumed.

Thesecond

caseconsidered

isfor

asingle

block.Values

ofdrag

coefficientof0.80and

liftcoefficientof0.25,fromthe

hexahedron(Naudascher,

1991)are

usedin

accountingforthe

mobility

ofasingle

block.Resultingcriticalweights

ofm

at-typeand

singleblock

aregiven

asa

ftmction

ofmean

velocityin

Figure7.In

thefigure,the

lastletterSand

Min

thelegend

denotesingle

andm

at-type,respectively.Itis

observedthatblocks

withstand

theflow

betterwhenthey

aretied

together,andthat

cleardifferencebetween

two

criticalweightsis

seen.

CO

NC

LUSIO

NS

Laboratoryexperim

entswere

carriedoutto

investigatehydraulic

propertiesofG

-blocks

forbedprotection

againstlocalscouraroundbridge

piers.Ithasbeen

shownthat

bothlogarithm

icand

powerlaws

areappropriate

forintennediate-scaleroughness

byG

-blocks.

Roughnessof

thechannel

bedpaved

byG

-blockdid

notshow

significantdifference

compared

with

thatof

naturalchannels.

Theperm

issibleshear

stressestim

atedfrom

theexperim

entsrevealed

thatG

-blocksare

much

saferfiom

thetransport

bythe

flowthan

ripraps.Finally,

thecritical

weight

neededto

resistflow

forceswere

calculatedfora

singleblock

andm

at-typeblocks.

366

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Page 368: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 369: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

TH

EP

RO

BLE

MO

FS

CO

UR

AN

DC

OU

NTE

RM

EA

SU

RE

S

ByDr.B.R.PhaniKum

ar1

AB

ST

RA

CT

Theproblem

ofscour

hascaused

many

bridgefailures

and,therefore,

needsto

beprevented

bysuitable

cormterm

easures.Several

methods

havebeen

suggestedfor

theprevention

ofscour.They

arebased

onm

odelstudiesforthe

protectionofem

bankrnentsand

piersfrom

undemiining

byscour.

Ripraparrests

theprogress

oftheerosive

cycle.W

allsof

riverbanksare

protected,insom

ecases,by

gabions.Sim

ilarly,spurdikesand

mattresses

ofbam

boosticks

arealso

successfullyused

forpreventing

scour.The

paperdiscusses

theproblem

ofscouranddifferentcounterm

easuresin

detail.

INT

RO

DU

CT

ION

Scourisa

seriousproblem

tocom

batwith

inbridge

engineering,as

ithas

beenthe

majorcause

offailureofm

ostofthebridges.

Scourisa

naturalphenomenon

inw

hichsoil

particlesare

removed

fromtheir

environmentby

moving

water.Because

ofthiscontinuous

removalofsoil,bridge

piersand

abutments

aresubjected

tounderm

ining(Terzaghi,

1936).

SC

OU

RC

HA

RA

CT

ER

IST

ICS

Threetypes

ofscourhavebeen

recognized.The

firstformofscour(Lane

etal,1954)takes

placeas

aresultofflooding.

Thiscauses

suspensionofm

aterialinthe

riverbed.The

velocityofwater

increases,therebyincreasing

theerosive

capacity.Solid

particlesw

illbelifted

andm

ovedand

suspendedwhen

theflow

returnsto

normal.

Thistype

ofscour,calledgeneralscour,is

particularlygreatwhere

thechannelis

narrow.

Bridgepiers

aregenerally

foundedbelow

thedepth

ofgeneralscour.

Thesecond

formof

scour,called

localscour,

iscaused

bythe

presenceof

some

obstructionto

thestream

suchas

apier.

Within

theproxim

ityofthe

pierthe

flowpattem

changes,becomes

turbulentanderodes

thesoil.

Thislocalized

scorn(Laursen

etal,1956)is

ofgreaterintensityand

acceleration,andis

afunction

ofvelocityofflow

,shapeofthe

pieretc.

Thethird

formofscour

isbank

scourw

hichoccurs

dueto

lateralbendingofthe

channel.W

aterstrikesthe

outersideofthe

bendand

erodesthe

material.

Theinside

ofthebank

isfilled

with

theeroded

sandsand

silts.The

foundationsofany

structureplaced

adjacenttothe

outsideofabend

mustbe

protected. ‘D

r.B.R.Phani

Kumar,

Department

ofC

ivilEngineering,

JNTU

Collegeof

Engineering,Kakinada-533

003,IND

IA

369

Page 370: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Thedifferentcharacteristics

ofscourare

afunction

oftherelationship

betweenthe

solidm

aterialandthe

capacityofthe

rivertorem

oveit.

Theerosive

capacitydepends

onthe

flowvelocity

which,

inturn,

dependson

thehydraulic

characteristicsof

theriver,

theintensity

oftheflood

andthe

characteristicsofthe

materialin

theriverbed.

Theresistance

ofthe

bedm

aterialto

erosionm

aynot

beoften

satisfactorilycharacterized

interm

sof

itsproperties

likedry

Lmitw

eightin

caseof

cohesivesoils

andaverage

diameter

incase

ofcohesionless

soils.Hence,

thereis

aneed

todepend

onthe

realobservations

and,accordingly,resortto

preventivem

easures.

CO

UN

TER

ME

AS

UR

ES

Variouscounterm

easuresto

scorn"have

beensuggested

andpracticed.

Theyhave

beenfound

toshow

satisfactoryperform

ancein

protectingem

bankments

andpiers.

Adiscussion

ofdifferentmeasures

forpreventingscourfollow

s:

Ripraporcoarse

rockfillis

oneofthe

mostw

idelyused

protectionm

easrues(Searcy,

1967).G

enerally,a

filteris

placedbetween

theriprap

andthe

naturalgroundin

orderto

preventthefinersoilfrom

cloggingthe

voidspace

oftheriprap.

Byusing

riprapthe

progressofthe

erosivecycle

isstopped

andthe

hydraulicarea

ofthechannelis

notreduced.

Insom

eplaces

wherecoarse

rockfor

riprapis

notavailable

gabionsw

illbe

used.G

abionsare

wire

basketsfilled

with

gravelorcoarse

aggregate.They

arestacked

toform

bankprotection

walls.Ifcoarse

aggregateis

notavailable,stacks

ofclothbags

filledw

ithsand

a.ndcem

entareused

forprotecting

thew

allsofbanks.

Gabions

arequite

effectivein

controllingscour.

Anotherpreventivem

easureforscouris

theuse

ofmattresses

forbankprotection.

Itis

theone

oftheoldest

practices.These

mattresses

consistofwoven

sticksofw

oodor

bamboo.

Sometim

es,concreteslabs

arealso

usedasm

attresses.

Thestructures

employed

fordiverting

thehigh

velocityflow

fromreaching

thebank

arecalled

spurdikesw

hichhave

beenfound

togive

excellentresults.They

areem

beddedin

theriver

bankand

extendinto

thechannel.

Theirdesign

isa

fimction

ofrivergeom

etry,length

ofthedikes

andspacing

betweenthe

dikes.Ifthe

spurdikesare

builtwith

theirtopssloping

downtowards

thecenterofthe

river,localscourarotm

dthe

endofthe

dikecan

bereduced.

CO

NC

LUS

ION

Many

bridgeshave

beeninvestigated

tofaildue

totm

dermining

byscour.

Localizedscour

inthe

proximity

ofanyobstruction

likea

pieris

ofquickprogress.

Scourcan

beprevented

bydifferentcounterm

easures.R

iprapw

itha

filterreducesthe

progressoferosive

cycle.G

abionsand

mattresses

made

ofwood

orconcreteslabs

protectwalls

ofriverbanks.Spurdikes

arrestlocalscour.

370

Page 371: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

RE

FER

EN

CE

S

1.Lane,E.W.,Borland,W

.M.,

1954,"RiverBed

ScourduringFloods",Transactions,ASC

E,Vol.119,p.1072.2.

Laursen,E.M

.,Toch,

A.,1956,

"Scouraround

BridgePiers

andAbutm

ents",Iow

aH

ighway

ResearchBoard,Bulletin

4.3.

Searcy,J.K.,1967,

"UseofR

iprapfor

BankProtection",

Hydraulic

EngineeringC

ircularNo.11,Bureau

ofPublicRoads.

4.Terzaghi,

K.,1936,

"FailureofBridge

Piersdue

toScour",

Proceedings,International

Conferenceon

SoilMechanics

andFoundation

Engineering,Cambridge,M

ass,Vol.2,p.26.

371

Page 372: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

SHEARSTRESS

CONCEPT

ING

RANULARFILTERS

By

GijsJ.C.M.Hoffmans‘,HenkdenAdel-2andHenkJ.Verheij3

AB

STR

AC

T

Scourisa

naturalphenomenon

causedby

theflow

ofwaterinrivers

andstream

sand

occursas

apartofthe

morphologicalchanges

causedby

riversor

asa

resultofman-m

adestructures.

Forthe

Dutch

Delta

Works

hydraulicstructures

wereoften

constructedon

finesw

ithloose

packing.Toguarantee

thegeoteclm

icalstabilityofthese

structuresthe

bedin

theirim

mediate

vicinityhad

tobe

protected.Though

severaltypes

ofbed

protectioncan

bedistinguished,

forexam

pleconcrete

blocks,asphalt,

andgranular

filters(w

ithor

without

geotextiles)the

scopeof

thispaper

islim

itedto

granularfilters.

Inthe

Netherlandsgeom

etricallysand-tight

filtersare

usuallyused.

Thestability

ofthese

filtersis

mainly

determined

bythe

geometricalproperties

ofthem

aterials.Consequently,these

classicalfiltersw

ithnturrerous

layersare

veryexpensive.

Inthis

studya

model

relationfor

sizinga

geometrically

opengranularfilteris

discussed.Ourgoalis

toprom

otediscussion

thanratherto

tryto

solvethe

many

problems

inthe

complex

fieldoffiltration

ingeotechnicalengineering.

INTR

OD

UC

TION

Non-geometricalgranularfilters

haveahydraulic

mode

ofoperation;i.e.thereduction

ofthe

hydraulicshear

stresseson

thebase

materialis

suchthaterosion

isprevented.

Availableknowledge

ofthehydrodynam

icforces,

liftand

drag,actingon

particlesin

granularfilters

ism

ainlybased

onexperience

andlaboratory

andfield

measurem

entsw

hichhas

proveninad-

equatefor

thepurpose

ofdevelopinga

highlyaccurate

designcriterion.

Thisis

dueto

thenum

erousfactors

thatinfluencethe

stability,andto

thedefinite

probabilisticnature

oftheacting

forcesw

hichm

ayattim

esbe

significantlyin

excessofm

eanvalues

andconsequently

causem

ovement.Verheijand

DenAdel(1998)calibrated

andvalidated

modelrelations

forgranular

filtersthatare

basedon

theN

avierStokesequation

forrmiform

flow,the

so-calledForchheim

errelation

andthe

hypothesisofBoussinesq.

Figure1

showsa

horizontalone-layer

filterw

itha

thicknessd

abovethe

basem

aterial.Considering

uniformflow

theshearstress

distributionin

theopen

flowis

linear.Usually

them

eanflow

velocityin

thedownstream

directioncan

beapproxim

atedby

alogarithm

icfunction.

Thevelocities

andshearstresses

inthe

filterlayerwillbe

brieflydiscussed

byapplying

thethree

aforementioned

equations.

‘Ministry

ofTransport,PublicW

orksand

WaterM

anagement,H

ydraulicEngineering

Division,D

elft(emailis

g.j.c.m.hoffm

ans@dw

w.rw

s.minvenw

.nl)2G

eodelft,Delft,The

Netherlands3D

elftHydraulics,D

elft,TheNetherlands

372

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Page 376: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 377: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Althoughcharacteristic

valuesforboth

loadingand

strengthare

included,theresulting

ratioof

Df/D

bis

independentofthefluctuations

inthe

loading.There

aretw

oreasons

forthis

ratherunexpected

result:First

itis

assumed

thatboth

filterand

basem

aterialw

illdisplay

initialm

ovementunder11116same

loadingconditions.

Second,fluctuationsin

theload

exertaload

onthe

filtermaterialsim

ilartothaton

thebase

material.The

effectsofnon-unifonn

flowhave

beentaken

intoaccorm

tbyapplying

equation(10)and

canbe

representedby

thestandard

deviationof

theinstantaneous

bedshearstress

(Hoffmans,1992,1996).

With

equation(14)the

influenceofparticle

gradationon

thestability

ofthebase

materialcan

beexplained

ina

qualitativeway.

Forexam

ple,whenthe

basem

aterialism

oregraded

thanthe

filtermaterial,ac-,1,is

greaterthanaqfConsequently,the

requiredratio

Df/D

bis

lesswhen

thisvalue

iscom

paredto

situationswhere

baseand

filtermaterials

dohave

thesam

egradation.If

onlythe

filterm

aterialisbroadly

graded,orqfis

greaterthanarc,1,,so

them

aximum

valueof

D];/Dbis

higherthan

forsim

ilarlygraded

materials.

Thesepredictions

correspondw

ithobservations

influm

eexperim

ents.Abroadly

gradedbase

materialhas

more

finesthan

am

oreuniform

material.The

materialin

thefilterlayerhas

topreventthe

erosionofthe

fines.Thiscan

onlybe

achievedby

reducingthe

filtervelocitiesorby

puttingm

orefines

intothe

filterlayers.Abroadly

gradedm

aterialinthe

filterlayer

hasrelatively

more

fmes,

which

reducethe

porevelocity

inthe

filterandso

alsothe

loadingon

thebase

material.Hence,the

broadlygraded

filterm

aterialisallowed

tohave

anaverage

grainsize

thatislargerthan

forunifonnm

aterial.

Usingthe

assumptions

ac’);=

aqfandm

ultiplyingboth

sidesby

D]§,,/D]f,;,,equation

(14)reduces

forhighvalues

of§dinto:

Df,1s

_D

f,15I

LPc,G,bAg,

(15)

Db,50D1150

77\Pc,G,t

At

Thevalue

of17hasbeen

calibratedby

usingexperim

entalresultsobtained

byVan

Huijstee

etal.

(1991).In

these9

flume

experiments

theinstability

ofthefilter

layerand

thebase

layerwas

simultaneously

observed.Them

eanvalue

of17isabout0.01

with

theboundaries

0.005<

17<0.025.Hoffrnans

(1996)also

founda

valueof0.01

onthe

basisofthe

Japanesetests

ofShim

izuetal.

(1990).Remark

thatthecalibration

andvalidation

of17wasbased

onunifonn-

flowexperim

ents.

Theresem

blanceto

traditionalrelations

derivedby

Terzaghiis

surprising.The

stabilitybetween

filterandbase

layerforgeometrically

sand-tightfiltersis:

Pia

V-l3fi°<10

(16)D

b,85D

b,50w

hichm

eans275

0.1.

Thedifferences

betweenequations

(15)and(16)can

beascribed

toa

safetyfactorthatvaries

from4

to20.

Thisanalysis

showsthatfor

uniformflow

therelations

forgeom

etricalsand-tightm

aterialsare

stronglyoversized.

When

theturbulence

intensitiesare

much

higher,for

example

downstreamofsills,

thevalue

of17(17

20.01)

mightbe

questionable.Underthese

conditionsthe

ratioD

f/Db

probablytends

tothe

geometricalvalue

ofabout1750.1.Itshould

berem

arkedthat

inthis

study,equation

(15)has

notbeen

validatedfor

non-uniformflow

conditions.377

Page 378: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Bakkeretal.

(1995)and

Stephenson(1979)

discussedfilter

modelrelations,

which

predictsim

ilarratios

betweenparticle

sizesof

filterand

basem

aterial.Although

theprediction

potentialoftheserelations

isreasonable

fortheexperim

entsinvestigated,they

dependon

theratio

R/D

f,which

isnotrealistic

forrmiform

flowconditions.

DA

IVIP

ING

PA

RA

ME

TE

R

Thedam

pingparam

eter(5)

isrelated

tom

aterialproperties

bothfor

laminar

andturbulent

flow.

Forlam

inarflow

5is

approximately

30/D155.Follow

ingIkeya

(1991)the

damping

parametervaries

from14/Djfso

to30/Dfso.In

bothcasesthe

lengthscale

ofthedam

pingis

much

smallerthan

theparticle

diameter

ofthegrains

inthe

filterlayer.

Consequently,theinfluence

oftheboundary

layerispractically

negligiblein

thecase

oflaminarflow

.

Forturbulentflow

conditionsVerheij

etal.(2000)

foundtf5

5,5/Dfw

whereasIkeya

(1991)arrived

atthefollow

ing:1/Dfso

<5

<6/Dfso.Ikeya

discusseda

suggestionm

adeby

Stephenson(1979)thatthe

turbulentboundarylayerin

thefilterlayeris

approximately,

1.5Df5;,which

waslater

independentlyconfirm

edby

them

easurements

ofSuzuki(1992).Sum

rnarisingequation

(15)isvalid

forbothlam

inarandturbulentflow

.

Thedifference

betweenresults

oftheDutch

andJapanese

researcherscan

beattributed

toa

differentway

ofmodelling

theeddy

viscosityand

todifferentvalues

forthecoefficients

inthe

so-calledForcheim

errelation.TheJapanese

assmned

aconstanteddy

viscosityin

thefilterlayer.

Inthis

studythe

eddyviscosity

isrelated

tothe

varyingfiltervelocity

(seeequation

4).Note

thatthe

eddyviscosityis

notaphysicalparameter,butaparam

eterthathelpsusto

relatevelocities

toshearstresses.

Sinceno

measurem

entsoffiltervelocities

inrelation

toloading

parameters

areavailable

noconclusions

canbe

drawnatpresent.

CO

NC

LUSIO

NS

Inthis

studym

odelrelationsfor

boththe

filtervelocity

andthe

shearstress

attheinterface

filter-basem

aterialarepresented.

Althoughthe

exactrelationbetween

thedam

pingparam

eterin

afilter

material

andits

material

propertiesis

disputable,the

typeof

relationbetween

characteristiclength

scaleand

particlesize

will

hold,in

spiteofthe

factthat

theasstunptions

foracontinuumapproach

areviolated.

Itshouldbe

notedthatthe

termshearstress

issom

ewhatmisleading.In

factthedistribution

ofthe

shearstressin

filterlayershas

tobe

consideredas

adistribution

ofaloading

parameter.A

modelrelation

hasbeen

discussedfor

geometrically

openfilters,

which

canbe

usedfor

bothuniform

andnon-uniform

flow.This

relationis

basedon

simultaneous

instabilityoffilter

andbase

material.

Theinfluence

ofthegrading

effectsofthe

filterand

basem

aterialshas

beenshown

qualitatively.This

relationcorresponds

closelyto

thetraditional

stabilityrelation

ofTerzaghifor

geometricalfilter

designand

representsthe

rangeofthe

magnitude

ofthesafety

factor.

Toincrease

theaccuracy

ofthem

odelpresentedhere

more

detailedinform

ationis

needed,inparticularthe

valueof27,w

hichm

aybe

foundby

carryingoutexperim

entsw

ithnon-Lm

ifonnflow

conditions.Itisnecessary

touse

sophisticatedequipm

enttom

easurefiltervelocities

andloading

parameters.

378

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Page 380: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 381: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

INTR

OD

UC

TION

Alluvial

sediments

formbed

andbanks

ofriverNem

tmas,

mainly

bysand

andsandy

loam.Lenses

andlayers

ofgravelandclay

arecom

mon

inthe

banksofthe

river.Bottomof

theriver

israther

flexible.Sand

wavesand

bars,reinforcedzones

ofgravellayeroutcrops,

toughclay

andboulders

varyfrom

placeto

place.Averagevelocities

intalw

egvary

within

thelim

itsfrom

1.0to

1.5m

/s.Inspring

floodperiod

atsome

zonesthe

velocitiesm

ayreach

magnitude

of1.8

m/s,

sometim

esin

drytim

eit

may

dropto

0.8m

/slevel.

Velocitysignificantly

variesin

time

dependingon

the

20

Pa

\

0-F’'-A

E/-

../

‘A

/'I

~,_Q

I/_

,4'

‘ii\/I’

1--1

I/\\

//

\\

2\_

/-

II

V*""

19L,

2

\

'-_‘\

Sr

\a

'6_J_ _l___L.

_1?

1314

Lkm

Fig.2Longitudinalprofiles

ofriverbeddownstream

railway

bridgein:

1-1963;2-1978;3-1981;4-1985.

regime

ofa

hydropower

plant,scour

andsedim

entationm

aygo

onat

thesam

espot

atdifferentperiods

oftime.

Dueto

theeffectofKaunas

hydropowerplanttransform

ationsofriverbed

goeson

inthe

zoneof30

kmlength

atthecentre

andsuburbs

ofKaunastown.The

bottomlevelatpresent

ison

theaverage

1.0m

belowthe

previouslevelof1959,when

theplantwas

constructed.At

some

placesthe

bedwas

loweredup

to2

m(see

Fig.2).Moorings

andem

bankments

ofthetow

,pipelines

andcables

areunder

theconstant

dangerofdam

agedue

toscour

ofbothstructures

toeand

bottomof

theriver.

Municipality

ofthe

town

realisedpossible

consequencesofthe

threat.Theyapplied

toscientists

ofKaunasU

niversityofTechnology

toinvestigate

thephenom

enonand

toform

ulatesom

eproposals

tostop

thedeform

ationsofthe

riverbed.

Fieldand

laboratoryinvestigations

wereperform

edand

some

suggestionswere

offered.

SC

OU

RA

ND

SE

LFLINTN

GO

FN

ON

-CO

HE

SIV

EH

ETE

RO

GE

NE

OU

SG

RO

UN

D

Scourstrengthofnon-cohesive

soilsdepends

mainly

onthe

diameterofsoilparticles

anddegree

ofheterogeneity.

Thereare

many

formulas

forcom

putationof

admissible

non-scouring

velocityfor

thetype

ofthe

soil(Zdankus,

1965).M

ostof

theseform

ulasare

designedfor

computation

ofm

aximal

admissable

non-scouringm

eanflow

velocity.W

econsiderthatm

aximalm

omentary

bottomvelocity

isa

more

suitableparam

eterforestimation

offlowscotuing

powerandsuggestthe

following

formula

umbadm=42J5+21+370

,CIT]/S(1)

whereumbadm

ism

aximalm

omentary

bottomnon-scouring

velocityat

thedistance

ofD/2

fromthe

bottomleveland

Dis

mean

diameterofa

soilparticlein

cm.

381

Page 382: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Accordingto

theresults

ofour

investigationsrelationship

betweenm

aximal

mom

entarybottom

umband

mean

velocityofflow

vmay

beexpressed

byem

piricalformula

kb=”i=1.32+A

1-

0.272lg[ll-].(2)

v1)(50-1.551))=2.33

1)Form

edby

heterogeneousgravel

riverbottom

roughnessthe

characterof

flowvelocity

distributionin

verticaleand

soilscourstrengthdepends

onheterogeneity

ofthesoil

(Sieben,1999).

Thedependency

may

beestim

atedm

ultiplyingadm

issablenon-scouring

velocityby

acorrection

coefficientcomputed

byourform

ula1)

k,=1-0.32zg[-5'1],(3)

HereDm

andD

,arem

aximal(corresponding

percentageof95%

)andrated

diameters

ofsoilparticles

respectively.Itis

evidentthatsmallerthan

D,soilparticles

willbe

washedoutfrom

thesurface

ofthe

soiluntilselfliningis

completed

andthe

surfaceis

reinforcedby

thelayerofsoilparticles

largerorequalto

rateddiam

eterD,(see

Fig.3).Thickness

ofareinforcem

entlayermay

bedeterm

inedby

ourformula

h,=

0.023P,D,2/Dm

.(4)

HereP,is

percentageofrated

diametertaken

froma

grain-sizedistribution

diagram.

——

—>

II

-:—>

——

—>

I’_

—-—

—>

catSO."

-o0.~.<5. 3

.0.1

96

“

Fig.3Surface

ofheterogeneousground

before(a)and

afterselflining

(b)

Scourdepth

hs(see

Fig.3)

containinglarge

particlesnecessary

forlining

quantitym

aybe

computed

fromourform

ulaPT

h‘h'100-P,'

(5)Lining

layermay

bebroken

byan

occasionalfloodwave

ofhighvelocities.Then

thelayeroflarge

particlesthen

isbeing

mixed

with

smallerones.W

hilenew

reinforcinglayeris

beingform

edscourand

lowering

ofbottomlevelproceeds

onceagain

untilnewreinforcing

layerhasbeen

formed.

SCO

UR

OF

CO

HE

SIV

ES

OIL

Clayey

soilscourprocessdiffers

greatlyfrom

thatofnon-cohesivesoil.

Particlesofthe

lastonerem

ainim

mobile

ifvelocityis

lessthan

scotuing.Cohesivesoilscourgoes

onatany,

evenatm

uchless

thanadm

issiblenon-scouring,

velocity.A

tsm

allvelocitiesintensity

ofcohesive

soilscourissm

allandthe

processis

similarto

melting,where

thesm

allestparticlesgradually

loosecontact

with

them

assof

asolid

bodyunder

theprocess

ofsw

elling.Increm

entofflowvelocity

leadsto

gradualincrementofscourintensity,therefore,itis

rather382

'

Page 383: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

difficulttodeterm

inethe

velocityw

hichm

aybe

consideredas

non-scotuing(M

irtshullavva,1962)

Our

device(Zdankus,

1968)for

investigationofcohesive

soilscotu

strengthm

akesit

possibleto

observethe

scourprocessand

estimate

itin

quantitativeparam

eters.Outflow

ingfrom

anozzle

ofthedevice

(seeFig.

4)w

aterjetsim

ulatesthe

riverflow

bottomlayer,

thereforejetvelocity

isconsidered

equaltom

aximalm

omentary

bottomvelocity.Itm

aybe

appliedto

calculatem

eanflow

velocityusing

formula

(2).

._lI

.;I

-I

3

-"'/I._

7k;,

|——_-—

5\_—

—@

i*-.-E_

Fig.4D

eviceforinvestigation

ofcohesivesoilscourstrength:

1-soilsample;2-nozzle;3-tank;4-scale;5-watersupply

line

W

INF

LUE

NC

EO

FS

EE

PA

GE

AN

DS

LOP

EIN

CLIN

AT

ION

ON

SC

OU

R

Ground

waterflowm

ayinfluence

thescourprocess

(Zdankus,1965).Itincreasesstability

ofsoilparticleson

thesurface

whenwaterinfiltrates

intothe

bedofthe

riverand

decreaseswhen

water

leavesthe

soil(Nian-Shen

Cheng,Yee-M

engC

hiew,

1999).The

seepagechanges

bothdistribution

ofriver

flowvelocities

andm

agnitudeofm

aximal

mom

entarybottom

velocity.Thevelocity

ishigherin

acase

ofupwardseepage

andlow

erfordownw

ardseepage.

Enteringthe

grotmd

water

pressessoil

grainsto

thesurface

andincreases

theirstability.Leaving

thesoilwaterlifts

thegrains

andreduces

theirstability(see

Fig.5).

AZA

—Z

——

>-

ll{

5b

‘u=f(z)

ma,

E**>

u=f(z)M

l__

I"bl

Fig.5Schem

eofupward

(a)anddow

nward

(b)seepageinfluence

onsoilparticles

andriverflow

Conditionsofsoilparticle

stabilityon

aninclined

slopeofa

riverbankare

worsethan

thoseon

ahorizontalbottom

ofariverbed.Perpendiculargravity

forceFp,w

hichis

equaltothe

gravityforce

Fgprojected

tothe

slopeplane,tends

tom

ovesoilparticle

downalong

the

383

Page 384: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

slope(see

Fig.6).Simultaneous

influenceofseepage

andsoilsurface

inclinationon

stabilityofsoilparticles

may

beexam

inedanalysing

allforcesacting

ona

singlesoilparticle.

Seepageforce

may

beexpressed

asF_.,=0.7s5c,pg1.n3,

(6)11

whereCS

isseepage

flowdrag

coefficient,3

pis

waterdensity,Ivis

verticalcomponent

orofa

seepagehydraulic

gradient.5

Gravity

forceis

directeddownward.

F5Fg

Itsm

agnitudedepends

onthe

mass

ofaparticle2

andm

aybe

expressedin

thefollow

ingw

ayF1,

1b

Fg=O.524(pS-p)gD3.(7)

Herepgis

densityofa

soilparticle.H

orizontallydirected

hydrodynamic

dragforce

isFh=0.393c,p

u2,,,1,.(8)

HereC,is

dragcoefficientforriverflow

.Fig_

6Schem

eofforces

actingSoil

S°i1Pa1'l5i°l95Onthe

519199inclinedby

particleon

slope:a-verticalcross0:angle

areacted

byperpendicularforce

Segtign;b-p1ane;1-5011pa11;i¢1e;2-F

p=Fgsina(9)

slopeline;3-direction

ofgroundtherefore,they

areunderless

favourablewaterflow

stabilityconditions

thanthose

onhorizontal

surface(Zdankus,

1973).Thus,

inclinationofsoil

surfacein

theslope

reducessoil

scourstrength.Tw

oequations

fora

criticalstateofthe

particlefor

shiftandoverturn

may

besetup

andsolved

with

respectsto

velocityumb.M

aximalm

omentary

bottomnon-scouring

velocityform

ulaw

ouldbe

obtained.The

fonnulaw

ouldexpress

dependencyof

thevelocity

ondiam

eterDto

1.5degree.

Unforttm

ately,thereare

nopossibilities

totake

intoaccountthe

influenceof

seepageon

thestructure

ofa

boundarylayer

ofthe

flowand

maxim

alm

omentary

bottomvelocity.

Therefore,solution

ofthe

equationshas

nosense,

andonly

empiricalform

ulasm

aybe

suggested.Hereis

aform

uladeveloped

byus

onthe

groundof

ourinvestigationresults

t2

kS=(l-0.lIv)cosa1-£9‘-.

(10)raw

Hereks

isnon-scotuing

velocitycorrection

coefficient;Ivis

verticalcomponentofground

waterflowhydraulic

gradient:positiveforupward

seepageand

negativefor

downw

ardone;

orisslope

inclinationangle;gois

soilinternalfiictionangle.

Seepagegradient

dependson

theground

waterlevel

ata

riverbank,

permeability

ofground

andwater

levelriseor

droprate.

Draw

down

curvefor

groundwater

flowm

ustbecom

putedto

determine

ahydraulic

gradient.H

ydrogeologicalconditions

andperm

eabilitycoefficients

shouldbe

known

forcom

putations.O

urdevice

totest

non-cohesivesoils

(Zdankus,1987)suitableforperm

eabilityinvestigations

goodenough.

Itshould

bem

entionedhere,that

influenceofseepage

toscour

isim

portantmerely

tonon-cohesive

soils.Therefore,

ourattention

waspaid

mainly

tothis

typeofsoils.

Ground

watermotion

incohesive

soilsis

soslow,that

seepagem

ayhave

noinfluence

onscourof

suchsoils.

384

Page 385: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 386: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

CO

NC

LUS

ION

S

Suddenfluctuation

ofriverflow

rateand

waterlevelare

them

ainreasons

ofbedscour.Intensity

ofthescouris

asgreatas

suddenand

frequentfluctuationsare.

Heterogeneityof

non-cohesivesoils

influencesscouring

strengthof

thesoils.

Formulas

(3)and(5)are

suggestedforcom

putingnon-scouring

velocityand

scourdepth.

RE

FER

EN

CE

S

1.M

irtshullavvaT.E.,1962,“Instructions

forDeterm

inationofAdrnissable

(Non-Scotuing)

FlowM

eanVelocity

ofCohesive

Grounds”.

USSRM

inistryof

Agriculture.Adm

inistrationofW

aterEconomy.M

oscow,USSR.—in

Russian.2.

Nian-Shen

Chengand

Yee-Meng

Chiew,l999,“IncipientSedimentM

otionw

ithU

pward

Seepage”.JournalofHydraulic

Research,Vol.37,N

o5,pp

665-681,IAHR

,Delft,

TheNetherlands.

3.Sieben

J.,1999,

“Atheoretical

Analysison

Armouring

ofR

iverBeds”.

Journalof

Hydraulic

Research,Vol.37,No3,pp

313-326,IAHR

,Delft,The

Netherlands.4.

ZdankusN

.,1965,“Investigation

ThroughM

omentary

BottomVelocities

ScourStrengthofN

on-Cohesive

Ground

inPrism

aticChannels”.

Doctor

thesis.Kaunas

PolytechnicInstitute,Lithuania.—

145p.—

inRussian.

5.Zdankus

N.,

1968,“D

evicefor

Investigationof

CohesiveG

round”.D

escriptionof

invention,cl.42o,15,to

certificateN

o213433.M

oscow,USSR.—in

Russian.6.

ZdankusN

.,l973,”Scouring

CapacityofFlow

inN

arrowO

penChannel”,Proceedings

ofInternationalSym

posiumon

RiverM

echanics,pp.831-839,Bangkok,Thailand.7.

ZdankusN

.and

other,1985,“Field

InvestigationsofFilling

Wave”.

Proceedingsofthe

29”‘Scientific-Technical

Conference,V.2,

pp185.

KaunasPolytechnic

Institute,Lithuania.—

inRussian.

8.Zdankus

N.,

1987,“Device

forDeterm

inationofPerm

eabilityC

oefficient”.D

escriptionofinvention,cl.G

01N

l5/08,tocertificate

No

1296907.Moscow,USSR.—

inRussian.

386

Page 387: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 388: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
Page 389: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

bridgepiers,

theseflow

sthen

negotiatinga

left-handchannel

bendbefore

passingbeneath

thecentralregion

ofthebridge

(Figure2).

Forthe1998

failureflood,itappears

thatsuchflow

sdid

notpass

beneaththe

centralregionofthe

bridgebutcontinued

alongthe

upstreamside

ofthebridge

(Figure2).

Theseflow

sthen

merged

with

flows

atthetrue-rightside

oftheflood

channelbeforeim

pactingon

thepiers

atthetrue-rightend

ofthebridge

(Figure3).

The1998-failure-eventflow

beneaththe

bridgewas

concentratedw

ithinthe

onebraid

channel.The

velocitiesfor

theconcentrated

flows

werenoted

tobe

highand

inexcess

ofthe2

m/s

velocityestim

atedforflow

spassing

thefailed

pierfourdaysafterthe

failureevent.

Thesurface

width

ofthecharm

elhasbeen

estimated

atabout

12m

to20

m.

At

thepeak

ofthefailure

flood,flow

isestim

atedto

haveextended

approximately

fiomAbutm

entA

toPier

H(94

m),

with

flowand

erosionconcentrated

atPiersC

and(in

particular)B

onthe

outsideofthe

left-handbend

inthe

channel(Figure3).

Thefixed

natureofthe

cliffforming

thetrue-rightlim

itoftheflood

channelm

ayhave

aidedin

forcingthe

flowpassing

acrossthe

upstreamface

ofthebridge

topass

beneaththe

bridgeatthe

positionofPierB.

Theflood

flowwas

estimated

tobe

lessthanthe

mean

annualfloodatthe

site.The

writershave

notobtained

quantitativeestim

atesof

flowm

agnitudesat

thebridge

site.W

ithinabout

15km

downstreamofthe

BealeyBridge,the

mean

annualfloodatthe

MtW

hiteRoad

Bridgeacross

theW

aimakaririR

iverhasbeen

estimated

at1200m

°/s(M

elvilleand

Coleman,2000).

Recognisingthe

relativem

agnitudesofflow

sinto

theW

aimakaririR

iverbetween

theBealey

Bridgeand

theM

tW

hiteBridge,and

thatthefailure

eventwasless

thanthe

mean

annualflood,afailure

flowof200

m3/s

isadopted

forthe

risk-assessment

scouranalyses

presentedbelow.

Thebridge

failureis

estimated

tohave

occurredataboutthe

time

oftheflood

peakatthe

site.From

photographstaken

within

hoursafterthe

failureevent(Figure

3),floodlevels

atthebridge

appeartohave

peakedat

aboutthelevelofthe

topofthe

0.9-m-deep

pilecaps.

Thebed

materialwas

notedto

beofa

widegrading

with

particlesranging

fromsands

to200-300

mm

cobbles.A

median

bedsedim

entsizeofd5@

=50-70

mm

wasestim

ated.Design

drawingsindicate

abedm

aterialofshingle.

I-HS

TOR

ICA

LS

CO

UR

In1948,the

upstreamedge

ofPierQdropped

byabout60

mm

,with

PierQrotating

ir1theplane

ofthe

pier.ForPierQ,the

pointsofPiles

1,2and

3were

foundto

bedriven

to7.6

m,6.1

m,and

7.6m

respectivelybelow

groundlevel.

Thepier

wasunderpinned

bythree

pilesateach

endofan

extendedpile

cap(located

beneaththe

existingone).

Eachnew

pileconsisted

ofthreerails

weldedtogetherand

wasofa

designlength

of7.0m

.From

1949to

1958,thebridge

wasm

onitoredin

tennsofthe

levelsofthe

upstreamand

downstreamwheelguards

onthe

superstructure,perhapsin

recognitionofongoing

bederosion

problems.

389

Page 390: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

OC

TOBER

1998SC

OU

RE

VE

NT

Thefailure

flows

of19-20O

ctober1998

erodedthe

bedaround

thepiles

ofPierBleaving

thepier

basicallyhanging,supported

bythe

superstructure.A

t2am

on20

October

1998,PierBsettled

about1.7m

asa

truckpassed

overthebridge

towardsAbutm

entV.The

drivercarriedon

overtherem

ainderofthe

bridgeto

complete

hisjourney

westward.In

failing,Pier

Brotated

aboutthejunction

ofthedeckw

ithPierC.

Crackingwas

evidentatthetops

ofPierCand

AbutmentA

owing

torotation

oftheadjoining

decksectionsasPierB

settled.

Thecause

offailurewas

ascribedto

localscourexacerbatedby

acom

binationofflow

concentrationata

skewedangle

tothe

pier,bendscourand

alsoconfluence

scour(Figure3).

Consistentwith

theeffects

ofbendscouron

thefailure,increasing

scourdepthswere

noted(afterthe

failureevent)for

PiersD,C

andB

respectively.Forbraided

riversthen,itm

aybe

thatthem

aximum

floww

ithina

channelbraidm

ayconstitute

aworse

scourscenariothan

alargerflood

spreadacross

thew

idthof

theflood

channel.Confluence

scourmay

havecontributed

tothe

failurew

ithtw

ochannelbraids

intersectingim

mediately

upstreamofthe

failedpierforthe

failureevent.

Thetrue-leftem

bankment

extendinginto

thechanneldid

notappeartohave

influencedthe

failureby

causingany

comm

otionofthe

failureflow

s(ofm

inormagnitude),butitdid

contributeto

concentrationofthe

flows

ataskewed

angleto

thefailed

pier.

AS

SE

SS

ME

NTS

OF

SC

OU

RD

EP

THS

Attempts

tom

easurescour

depthssubsequentto

failurewere

fiustratedby

highflow

velocities.M

easurements

within

fourdaysoffailure

indicateapeak

scourdepthofabout2.7

mbelow

thelevel

ofthebases

ofthepile

caps.Itis

recognisedhowever,thatscourholes

tendto

fillinas

aflood

recedes.

Thefailure

flowcan

beapproxim

atedas

asingle

channelflow

approximately

parallelto

theupstream

faceofthe

bridge,with

theflow

passingaround

abend

beneaththe

bridge(atPierB)and

subsequentlyproceeding

ina

downstreamdirection

(Figure3).

Forafailureflood

ofabout200m3/s

atthebridge

site,them

ethodofBlench

(1969)predictsa

flowdepth

forasingle

channelof20m

width

approachingthe

bridgeofyms=

4.1m

.The

method

ofMaza

Alvarezand

EchavarriaAlfaro

(1973)predictsan

equivalentflowdepth

ofym,=3.5

m.

Forym,=4.0

mand

anestim

atedbend

radiusofcurvature

oftheorderof60

m,am

aximum

scouredflow

depthin

thebend

ofybs=6.8-8.3

mis

predictedby

methods

detailedin

Melville

andColem

an(2000)and

Coleman

etal.(2000).

Altematively,ifthe

failureflow

isconsidered

toconstitute

two

channelbraids(ofequalflow

sand

eachof20

mw

idth)meeting

atthefailed

pier,them

ethodsofBlench

(1969)andM

azaAlvarez

andEchavarria

Alfaro(1973)predicta

braidflow

depthofyms

=2.6

mand

yms=

2.0m

respectively.For

yms=

2.5m

anda

confluenceangle

ofabout50°,

am

aximum

scouredflow

depthin

theconfluence

ofycs=

9.5m

ispredicted

bym

ethodsdetailed

inM

elvilleand

Coleman

(2000)and

Coleman

etal.(2000).

With

nodegradation

orcontractionscourevidentforthe

bridgesite,the

totalscouratPierBis

givenby

thecom

binationoflocalscourw

iththe

general(bendorconfluence)scouroccurring

atthepier.

390

Page 391: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate

Thelocalscourdepth

hasan

upperlimitofd,=

4.4m

forthepierw

ithaprojected

width

of3.25m

(fortheflow

atanangle

of84°tothe

slab-typepier).

Thetotalscoured

flowdepth

ytcalculatedin

thism

anner(yt=yb,+

dsoryr=ycs+

ds)isthe

depthbelow

thewatersurface

fortheflood

flow,the

watersurface

forthe1998

failureeventbeing

estimated

atabout0.9m

abovethe

levelofthebases

ofthepile

caps.

Thepresentscouranalyses

areforthe

purposeofassessm

entofrelativerisk

ofscouratthebridge

piers,and

notfor

thedesign

ofpileem

bedrnentdepths,

which

would

requirem

orerigorous

analyses.The

aboveanalyses

arelirnited

bythe

adoptionofan

approximate

failureflood

andapproxim

ateriver

geometries,

andthe

extrapolationofthe

analysesto

thelarge

sediment

sizesoccurring

atthebridge

site.Nevertheless,the

analyseshighlightthe

largescour

depthsthatcan

occuratindividualpiers

ofthebridge.

Suchscourm

aybe

more

criticaltobridge

stabilitythan

scourforlargerfloodsfor

which

theflood

flowextends

acrossthe

entirebridge

channelsection.Assessm

entofhistoricalaerialphotographrecords

would

revealwhetherthe1948

failureofPierQ

wasofthe

same

originas

the1998

failureofPierB

with

braidflow

focussingata

piercausing

Lmderm

iningofthe

pier.r

Itis

comm

onlyrealised

thatdramatic

channelshiftcanoccurin

thecourse

ofasingle

floodfor

braidedrivers.

Thehistoricalvariability

ofthechannelbraids

atthepresentbridge

siteis

evidencedin

Figures1

and2.

Basedon

thisvariability,itm

ustberecognised

thateachpier

oftheBealey

Bridgeis

subjecttothe

same

riskofpierscouring

thatoccurredatPierB

in1998.

RE

ME

DIA

LA

CT

ION

S

Afterthe

failureofthe

bridge,achannelwascutbeneaththe

bridgea

fewpiers

awayfrom

thefailed

pierinorderto

divertsome

flowaway

fromthe

failedpier.

Within

two

daysofthe

failure,asingle-

laneBailey

bridgewas

installedoverthe

droppedpierand

theadjoining

spansto

facilitatetraffic

flows

acrossthebridge.

Theplanned

remedialaction

consistedofjacking

PierBback

intoposition,underpinning

thepier,

andrepairing

thedam

agedspans.

Additionalpiers

may

alsobe

underpinneddepending

onassessm

entsofboth

scourvulnerabilityfor

therespective

piersand

alsothe

overallvalueofthe

transitlinkprovided

bythe

bridge.A

considerationagainstsignificantrepairs

beingcarried

outatthe

siteis

thatin62

yearsthe

bridgehas

onlyhad

two

problems

with

respecttoscour.

Ineach

case,the

bridgewas

readilyrepairable,especially

interm

sofquickly

restoringtraffic

flows

throughthe

useofaBailey

Bridge.

REFER

ENC

ES

1.Blench,

T.(1969).

“Mobile-bed

fluviology.”U

niversityofAlberta

Press,Edm

onton,Canada.

2.Colem

an,S.E.,Melville,B.W

.,andLauchlan,C.S.(2000).

“Totalscourdepthatbridge

abutments.”

Proc.,Intem

ationalSym

posiumon

Scourof

Foundations,M

elbourne,Australia,Novem

ber,2000.

391

Page 392: at - Start · scour depth z-max in cohesionless soils, the nal scour depth after many years of ood history at a bridge in cohesive soil may only be a fraction of emax. The scour rate
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