MISCELLANEOUS PAPER 1-1-72-5

PRACTICAL GUIDANCE FOR ESTIMATING AND CONTROLLING EROSION AT

CULVERT OUTLETS by

B. P. Fletcher, J. L. Grace, Jr.

TE~HNICAL INFORMATION CENTER US ARMY ENGINEER WATERWAYS EXPERIMENT STAnON

VICKSBURG. MISSfSSJPPJ

May 1972

Published by U. S. Army Engineer Waterways Experiment: Station, Vicksburg, Mississippi

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED

MISCELLANEOUS PAPER ~-72-5

PRACTICAL GUIDANCE FOR ESTIMATING

AND CONTROLLING EROSION AT

CULVERT OUTLETS by

B. P. Fletcher, J. L. Grace, Jr.

10 I , ~ 101 10 IDo Do 101

May 1972

Published by U. S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi

A RMY·MRC VICKSBURG, MISS

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED

!;)? ~ )1/)' 1-1 -? ~ - s -~(' n

FOREWORD

This report is a paper prepared by Messrs. J. L. Grace, Jr ., Chief

of the Spillways and Channels Section, Structures Branch, HYdraulics Divi­

sion, U. S. Army Engineer Waterways Experiment Station, and B. P. Fletcher

for presentation at the Mississippi Water Resources Conference held in

Jackson, Miss., 11-12 April 1972.

Director of the Waterways Experiment Station during preparation and

publication of this paper was COL Ernest D. Peixotto, CE. Technical Direc­

tor was Mr. F. R. Brown .

•

iii

7562.

PRACTICAL GUIDANCE FOR ESTIMATING AND CONTROLLING EROSION AT CULVERT OUTLETS

By

B. P. Fletcher and J. L. Grace, Jr. Engineers, Hydraulics Division

U. S. Army Engineer Waterways Experiment Station

INTRODUCTION

This paper swmnarizes the results of research conducted at the

U. S. Army Engineer Waterways Experiment Station (WES) during the past

nine years to develop practical guidance for estimating and controlling

erosion downstream of culvert and storm-drain outlets. Initial efforts

were concerned with investigation and development of means of estimating

the extent of scour to be anticipated downstream of outlets. Subsequent

efforts have involved investigation and evaluation of various schemes of

protection for controlling erosion such as horizontal blankets of rock

riprap, preformed scour holes lined with rock riprap and channel ex-

pansions lined with natural and artificial revetments. In addition,

efforts have been made to determine the limiting discharges for various

energy dissipators including simple flared outlet transitions, stilling

wells, U. S. Bureau of Reclamation type VI basins and St. Anthony Falls

stilling basin~. Empirical equations and charts are presented for

l

estimating the extent of localized scour to be anticipated downstream of

culvert and storm- drain outlets, the size and extent of various natural

and artificial type revetments and the maximum recommended discharge for

each type of energy dissipator investigated . With these results, de ­

signers can estimate the extent of scour to be expected and select ap­

propriate and alternative schemes of protection for controlling erosion

downstream of culverts and storm- drain outlets .

SCOUR AT OUTLETS

In general, two types of channel instability can develop downstream

from a culvert and storm- drain outlet, i.e. either gully scour or

localized erosion termed a scour hole. Distinction between the two con­

ditions can be made by comparing the original or existing slope of the

channel or drainage basin downstream of the outlet relative to that

required for stability. Gully scour initiates at a control point down­

stream where the channel is stable and will progress upstream if suffi­

cient difference in elevation exists between the outlet and the upstream

section of stable channel. Erosion of this type may be of considerable

extent depending upon the vertical and horizontal distances existing

between the stable channel section and the outlet.

A scour hole is to be expected downstream of an outlet even if

the downstream channel is stable. The severity of damage to be antici ­

pated depends upon the conditions existing or created at the outlet and

in general will consist of embankment erosion and structural damage of

the apron, end wall, and culvert sections if sufficient localized scour

is experienced. Noteworthy surveys of conditions at culvert outlets

2

have been accomplished by Keeley1

in Oklahoma and Scheer2 in Montana.

The observations and empirical methods developed by Keeleyl,3,4

which provide specific guidance relative to the conditions that produce

gully scour or only a localized scour hole as well as those required

for stable channels in several Oklahoma soils merit consideration and

application in general . An example of a chart developed by Bohan5 for

design of trapezoidal channels with 1 on 2 side slopes in a soil which

would depos i t and erode with Froude numbers of flow less than 0.15 and

greater than 0 . 35, respectively, is shown in fig. 1. Bohan also re-

ported the results of research conducted at WES to determine the ex-

tent of localized scour that may be anticipated downstream of culvert

and storm- drain outlets . Empirical equations were developed for esti-

mating the extent of the anticipated scour hole based on knowledge of

the design discharge, the culvert diameter, and the duration and Froude

number of the design flow at the culvert outlet . However, the relation-

ship between the Froude number of flow at the culvert outlet and a dis ­

charge parameter, Q/D;/2 , can be calculated for any shape of outlet

and the discharge parameter is just as representative of flow conditions

as is the Froude number . The relations between the two parameters for

partial and full pipe flow in square culverts are shown in fig. 2.

Since the discharge parameter is easier to calculate and is suitable for

application purposes, the original data reported by Bohan were reanalyzed

to determine the following relations for estimating the extent of lo-•

calized scour to be anticipated downstream of culvert and storm- drain

outlets .

3

Maximum Depth of Scour :

D sm

D 0

D sm D

0

- 0 . 80

- 0 . 74

Maximum Width of Scour :

w Q sm

1 . 00 -D5/2 D

0 0

w Q sm

0 . 72 -D5/2 D

0 0

0 . 375 0 10 t .

0 . 915t0 .15

0.915t0 . 15

Maximum Length of Scour :

L sm

D 0

L sm D

0

- 2 . 40

- 4.10

Volume of Scour:

v s

0.73 -= D3

0

v s

0.62 -= D3

0

Q 2 t0 . 375

D5/2 0

Q 2 t0 . 375

D5/2 0

Tailwater < 0 . 5D 0

Tailwate r > 0 . 5D - 0

Tailwater < 0 . 5D 0

Tail water > 0 . 5D - 0

Tailwater < 0 . 5D 0

Tailwater > 0 . 5D - 0

Tailwater < 0 . 5D 0

Tailwater > 0 . 5D 0

The variables are defined 1n table 1 and compar1sons of predicted

and observed values are shown in figs . 3-6. Dimensionless profiles

4

along the center lines of the scour holes to be anticipated with tail-

waters less than 0 . 5D and equal to or greater than 0 . 5D are presented 0 0

in figs . 7 and 8, respectively . The maximum depth of scour occurred at

a distance 0. 4 of the maximum length of scour downstream of the culvert

outlet for all tailwater conditions . Dimensionless cross sections of

the scour hole at this location are also shown in figs . 7 and 8. If the

location of the outlet is such that a scour hole is not objectionable ,

i t may be pr actical to allow localized erosion since the scour hole acts

as an excellent energy dissipater ; however, a cutoff wall which extends

to a depth of at least 0 .7 of the maximum depth of scour expected (fig . 7)

should be provided to prevent undermining .

SCHEMES OF CONTROLLING EROSION AT OUTLETS

The average size of stone and configuration of a horizontal

blanket of riprap at outlet invert elevation required to control or

prevent localized scour downstream of an outlet can be estimated based

on the results reported by Bohan and subsequent unreported tests . For

a given design discharge , culvert diameter or width , and tailwater depth

relative to the outlet invert , the minimum average size of stone for a

stable horizontal blanket of protection can be estimated by the follow-

ing relation :

D 0

0.020 TW Q

D5/2 0

4/3

The length of stone protection required (fig. 9) can be estimated by the

equation,

5

L sp - 1 7 D . + 8 0

The variables are defined in table 1 and the recommended configura-

tion of a horizontal blanket of riprap for control of erosion at culvert

and storm-drain outlets is presented in fig. 10. Details of a scheme of

riprap protection termed "preformed scour hole lined with riprap" are

shown in fig. 11. The relative advantage of providing both vertical

and lateral expansion downstream of an outlet to permit dissipation of

excess kinetic energy in turbulence rather than direct attack of the

boundaries is shown in fig. 12 which indicates that the required size of

stone may be reduced considerably if a riprap-lined, preformed scour hole

is provided in lieu of a horizontal blanket at an elevation essentially

the same as the outlet invert.

LINED CHANNEL EXPANSIONS

A research project sponsored by the Louisiana Department of High-

ways is in progress at WES to investigate the feasibility of lining chan-

nel expansions downstream of culvert outlets with either sack revetment,

cellular blocks, or rock riprap. After observing flow conditions with

various sizes of model culverts and geometries of channel expansions,

the channel expansion geometry shown in fig. 13 was select.ed as a

practical configuration. The dimensions of the lined channel expansion

are related in terms of that of square box culverts.

Sack revetment with length, width, and thickness of 2 ft, 1.5 ft,

and 0.33 ft, respectively, and weighing 120 lb was simulated at a scale •

6

. •

of 1 : 8 as shown in fig . 14 . Cellular blocks roughly 0 .66 ft by 0.66 f~

and 0 . 33 ft thick whi ch weigh 14 lb were simulated at a scale of 1 : 4 as

shown in fig . 15 . Rock of 6 to 8 inch diameter weighing 17 lb was

simulated at a scale of 1: 4 as shown in fig . 16 . The results of tests

to determine the condit i ons of discharge and tailwater r equired to dis -

place or fail each of the r evetments are shown in fig . 17 and indicate

t hat the thickness of geometrically similar revetments can be calculated

by means of the following empirical equation :

d50 T TB D Q 4/3 s

0 . 016 0 or - or -= -D D D TW D5/2 0 • 0 0 0

The variables are defined • table 1 . The relative effectiveness ln

of the lined channel expansion relative to the other schemes of riprap

protection described previously is shown in fig . 12 . The relations

presented in fig . 12 are recommended for selection of either the size

of revetment for a given scheme of protection , dischar ge , tailwater

depth , and culvert dimension or fo~ the selection of the slze of culvert

with which a given revetment and scheme of protection will remain stable

under anticipated conditions of discharge and tailwater depth .

The maximum discharge parameters , Q/D5/ 2 , of various schemes of 0

protection can be calculated based on the results presented herein and

comparisons relative to the cost of each type of protection can be made

to determine the most practical design of providing effective drainage

and erosion control facilities for a given site . There will be condi-

tions where the design discharge and economical size of culvert or

storm- drain will result in a value of Q/D~/2 , the discharge parameter,

7

greater than the max1mum value permissible with feasible schemes of

protection discussed previously and some form of energy dissipater will

be required . In other cases, the value of Q/D5/2 may be less than 0

that of the aforementioned feasible schemes of protection and a

simpler more econom1cal form of protection may be indicated .

FLARED OUTLET TRANSITIONS

Tests were conducted to determine the maximum values of the

discharge parameter (table 2) that were considered satisfactory with

various conditions of tailwater and 3-, 5-, and 8- D - long simple flared 0

outlet transitions whose details are shown in fig . 18 . Results of the

tests of these simple outlet transitions with the apron at the same

elevation as the culvert invert are shown in fig . 19 which indicate

that the maximum discharge parameter for a given culvert , length of

transition , and tailwater can be calculated by the equation ,

TW L - 1 .6o 0 0

0. 4 D

0

TW

1/3

Similarly , the length of transition for a given situation can be calcu-

lated by the equation ,

L D

0

- 0 . 30 D

0

TW

1/3 2 2. 5

Variables are defined 1n table 1 and from fig . 19 it can be seen that

this type of protection is satisfactory only for low values of Q/D5/2 . 0

The arbitrary extent of scour depth equal to or less than 0 . 5D • 0

was used

to classify satisfactory conditions .

8

Attempts were made to investigate the effectiveness of recessing

the apron of these flared outlet transitions and providing an end sill

at the downstream end; however, fig. 20 indicates that this modification

did not significantly improve energy dissipation or increase the appli­

cable maximum value of the discharge parameter, Q/D5/2 . 0

COMMONLY USED ENERGY DISSIPATORS

Grace and PicKering6

have reported the results of model tests to

evaluate the maximum values of the discharge parameter, Q/D5/ 2 , appli­o

cable to various sizes of three commonly used energy dissipaters;

stilling wells, U. S. Bureau of Reclamation Type VI Basins and St. An-

thony Falls stilling basins.

The stilling well consists of a vertical section of circular pipe

affixed to the outlet end of a storm drain as shown in fig. 21. The

recommended depth of the well below the invert of the incoming pipe is

dependent on the slope and diameter of the incoming pipe and can be

determined from the plot shown in fig. 21. The recommended height of

stilling well above the invert of the incoming pipe is two times the.

diameter of the incoming pipe. The top of the well should be located

at the elevation of the invert of a stable channel or drainage basin.

The area adjacent to the well may be protected by riprap or paving;

however, if there is no adjacent erodible embankment within two well

diameters of the periphery of the stilling well, there is no need for

protection. Energy dissipation is accomplished without the necessity

of maintaining a specified tailwater depth in the vicinity of the outlet .

9

Details of the U. S . Bureau of Reclamation Type VI b asin and the

St . Anthony Falls stilling basin are presented in figs . 22 and 23 . Maxi ­

mum values of the discharge parameter , Q/D512 , considered satisfactory 0

for various sizes of each of the energy dissipators are presented in

table 3 . These data are satisfied by the following equations which

may be used to compute the diameter or width of each type of energy

dissipator relati·ve to that of the incoming pipe :

Dw Q 1 . 0

0 . 53 stilling well -D5/2 D

0 0

WSAF Q 1 . 0

0 . 30 St . Anthony Falls stilling basin -D5/2 D

0 0

0 . 55 - 1 . 30 U. S . Bureau of Reclamation Type VI basin

It is recommended that the s1ze of stone protection to be provided down-

stream of these energy dissipators be estimated by the fol l owing relations :

or

wm - 0 . 020 TW

- 1 . 0

Q w 5/2

ED

3

4/3

Guidance other than engineering judgment for estimating the length of

stone protection required downstream of an energy dissipator is not

available due to the lack of systematic investigations of this aspect

of the problem. •

10

ACKNOWLEDGMENTS

The results reported herein are based on research conducted at the

U. S. Army Engineer Waterways Experiment Station and sponsored by the

Directorate of Military Construction, Office, Chief of Engineers and

the U. S. Department of Transportation, Federal Highway Administration

and the Louisiana Department of Highways. Permission for the authors

to present this paper does not signifY that the contents necessarily

reflect the views and policy of either the U. S. Army Corps of Engineers,

the U. S. Department of Transportation, Federal Highway Administration

or the Louisiana Department of Highways, nor does mention of trade

names or commercial products constitute indorsement or recommendations

for use.

11

A

B

D

D 0

g

H

L

n

Q

s

T

TABLE 1

DEFINITION OF VARIABLES

Cross-sectional area of flow, ft2

Base width of channel, ft

Depth of flow in channel, ft

Diameter or width of culvert, ft

Depth of scour, ft

Maximum depth of scour, ft

Diameter of stilling well, ft

Depth of uniform flow in culvert, ft

Diameter of average size stone, ft

Froude number of flow at culvert outlet, F = Q/A J gd

Froude number of flow ln channel, Fch = Q/ ~/T Acceleration due to gravity, ft/sec 2

Depth of recessed apron and height of end sill, ft

Length of flared outlet transition, ft

Length of scour, ft

Maximum length of scour, ft

Length of stone protection, ft

Manning's roughness coefficient

Discharge, cfs

Slope of channel bottom and energy gradient

Top width of flow in channel, ft

Thickness of cellular block, ft

Thickness of sack revetment, ft

(Continued)

TW

t

v

vs

WED

WSAF

WVI

ws

WSM

TABLE l (Continued)

Tailwater depth above invert of culvert, ft

Duration of flow, minutes

Average velocity of flow in channel, ft/sec

Volume of scour, ft 3

Width of energy dissipater, ft

Width of St. Anthony Falls stilling basin, ft

Width of U. S. Bureau of Reclamation type VI basin, ft

One-half total width of scour, ft

One- half maximum width of scour, ft

( (

TABLE 2

MAXIMUM DISCHARGE RECOMMENDED FOR VARIOUS FLARED OUTLET TRANSITIONS

Limiting Values of Q/D5/2 0

L/D h/D TW/D Q/D5/2 0 0 0 0 --

3 0 0 0.88 3 0 0.50 1.78 3 0 l. 00 2 . 56 3 0.25 0.25 1.28 3 0 . 25 0.50 1.78 3 0 . 25 1.00 2 . 56 3 o.so 0 . 25 1.58 3 0.50 0.50 2.00 3 0.50 1.00 2.56 5 0 0.25 1.20 5 0 0.50 2.40 5 0 1.00 3.20 5 0.25 0 . 25 1.58 5 0 . 25 0.50 2.78 5 0.25 1.00 3 . 47 5 0 . 50 0.25 1.47 5 0.50 0 . 50 2.77 5 0 . 50 1. 00 3.46 8 0 0.25 1.68 8 0 0 . 50 2 . 40 8 0 1.00 3.75 8 0.25 0.25 2. 17 8 0.25 0.50 3.36 8 0.25 1.00 4.44 8 0 so 0.25 2.46 8 0.50 0.50 3.65 8 0.50 1. 00 4.55

•

TABLE 3

MAXIMUM DISCHARGE RECOMMENDED FOR VARIOUS TYPES AND SIZES OF ENERGY DISSIPATORS

Relative Width and Type Maximum Q/D5/2 of Energy Dissipater 0

Sti lling Well

1 D Diameter 2 .0 0

2 Do Diameter 3 . 5

3 D Diameter 5 . 0 0

5 D 0

Diameter 10.0

USBR Type VI Basin

1 Do Wide 0.6

2 Do Wide 2.2

3 Do Wide 4 . 5

4 Do Wide 7.6

5 Do Wide 11.5

7 Do Wide 21.0

SAF Stilling Basin

1 Do h'ide 3.5

) Do Wide 7 . 0

., .) Do Wide 9.5

TABLE 4

LITERATURE CITED

1. Keeley, J. W., "Soil Erosion Studies in Oklahoma; Part III, Culvert Outlet Conditions and Downstream Channel Stability," 1963, U. S. Bureau of Public Roads, Oklahoma Division, Oklahoma City, Okla.

2. Scheer, A. C., "Large Culvert Studies in Montana," 1968, Department of Civil Engineering and Engineering Mechanics, Montana State University, Missoula, Mont.

3. Keeley, J. W., "Soil Erosion Studies in Oklahoma; Part 1, Water Erosion in Narrow Ditches and Channels; Part II, Erosion Control Devices for Ditches and Channels," 1961, U. S. Bureau of Public Roads, Oklahoma Division, Oklahoma City, Okla.

4. Keeley, J. W., "Soil Sedimentation Studies in Oklahoma; Deposition in Culverts and Channels," 1967, Federal Highway Administration, Bureau of Public Roads, Oklahoma Division, Oklahoma City, Okla.

5. J. P. Bohan, "Erosion and Riprap Requirements at Culvert and Storm-Drain Outlets; Hydraulic Laboratory Investigation," Research Report H-70-2, Jan 1970, U. S. Army Engineer Waterways Experiment Station, CE, Vicksburg, Miss.

6. J. L. Grace and G. A. Pickering, "Evaluation of Three Energy Dissipaters for Storm-Drain Outlets; Hydraulic Laboratory Investigations," Research Report H-71-1, April 1971, U. S. Army Engineer Waterways Experiment Station, CE, Vicksburg, Miss.

•

Q

VgBS

SBI/3

h2 0 0 0

• • • - C\J ~ I. 0 _ __, _ _,.r"'TrT'"TT""'T-r-.r-tT"'"T7""::;-.., 0 !'

0.6

0.4

0.2

0. I

0.04

0.01

0.006

0.004

I

I

~ o':-0

' oro <I) 0 o ·

~ oro o·

0~ o·

o'V o·

Fig . 1 . Characteristics of a trapezoidal channel with 1- on- 2 side slopes as a function of Froude number

D 8

5~---+------------+---~------------+-

F= Q A ../gQ 0.5

0.05 0.1 0.5 Q

Dos;z

1.0

d Do

o'>

F = FROUDE NO. Q =DISCHARGE A =AREA Do= DEPTH OF CULVERT d = DEPTH OF FLOW S =SLOPE N = COEFFICIENT OF

ROUGHNESS

5 10

Fig. 2. Square culvert - Froude numb~r versus discharge

30

0 w ...... u -

9r---~-----.----.-----~--~~--~-----

MINIMUM TAILWATER ~: = 0.80 Q D 5/2 0

0.375 t 0.10

8~---

MAXIMUM TAILWATER D~: = 0. 74 Q Do5/2

0.375 t 0.10

Jr-----T-----+-----~----4-----~----~----~

Dsm ,Do IN FT t IN MIN.

6 1--- Q IN CFS

/

/ o FULL PIPE FLOW /

5 1--- • PARTIAL PIPE FLOW -+-----ir--- ---1----4----- ---1 0 UJ

g: /o 0 00 ~ 4~----+-----+-----4---~~~--~----~----~ E cl)

0

.. PREDICTED SCOUR DEPTH /.

I ....__ _ __,,..._.., t-----+-

VS OBSERVED SCOUR DEPTH

I 2 3 4 5 6 7 Dsm/00 OBSERVED

Fig . 3. Predicted scour depth versus observed scour depth

-0

45------~--~-----.-----.-----.-----.~-----, I I I I 0.915

MINIMUM TAILWATER sm = 1.00 Q t 0 •15

Do D 512 0

40 t---

MAXIMUM TAIL WATER WDsom = 0. 72 Q DoS/2

Wsm ,00 IN-FT t IN MIN.

30 1--- Q IN CFS

0.915 t 0.15

/ o FULL PIPE FLOW /

25 ~------- e PARTIAL PIPE FLOW -+-----+---_____,....--; :..____---+----t

~ /o ~ 20~----+-----+-----~----~~~--~----~-----4 ~ 0 0 6/

o <P% 15~----r----~~-~~0~n~~~,0~--~----1-----~----~

0 0 0 0 0 0 . 0

to~--4-~oo~~~~~v-~o~-~---+-------+-----~ (~t,'r:j~ 0 0

, AP <P o

5 0° ~· o PREDICTED

r---n--v~-, SCOUR WIDTH vs

/. v •• 00 5 10

OBSERVED SCOUR WIDTH

15 20 25 30 Wsm/D0 OBSERVED

Fig. 4. Preducted scour width versus observed scour width

-

35

-0 w 0:::: 0..

0

90r----r-,---r-,---.-,---.----~--~--~ L I l 0.71 I

MINIMUM TAIL WATER 05

0m = 2.40 Q t 0.125

D 512 0

MAXIMUM TAILWATER L;: = 4.10 Q Do512

0.71 t 0.125

BOt--

~ 40 E Cl)

__J

30~--~~---4----- r----+-----+-----~--~

/ ~b

c 0 "

2 0 t------+--------.. o. lB

~ •l>

~1>s9 PREDICTED SCOUR LENGTH I 0 ....,._____ . V 5 -

~ •"' OBSERVED SCOUR LEN.GTH v··

00 10 20 30 40 50 60 L5m/D0 OBSERVED

Fig. 5. Predicted scour length versus observed scour length

70

1400~---4-----+-----r----~----~----T----/~

D0 IN FT 0

/

t IN MIN. o V 1200 t-- Q IN CFS /

V5 IN FT 3

8 0 / ~ 1000~----+-----+-----+-----,_----/~~-----r-----1

~ <P /

Mo 800~----~----+-----+-~n~~~~--~~----~---, ~ / > cy

600~----~-----+-----~~~--~----~~----~----4 0 /

/ 0

.! 400~----~~®~u~ao~r----~----_,----~r-----~----1

0

200 no, ,.. PREDICTED SCOUR VOLUME t----:;:'f_=--/~..-r-v--i.._,7-t- V S -

jrf lo 0

OBSERVED SCOUR VOLUME I~ 0 IZ_-

00 200 400 600 800 1000 1200 1400 V5 /D0

3 OBSERVED

Fig. 6. Predicted scour volume versus observed scour volume

L o

Ds

Ds ..

0 2

04

oe

08

10

1.2

FLOW.,

l I

I

I I

06 00 66

·Q ~ 0 ! I

i 0 01

0

0 2

0 4

Ds o 6 o~.

0 8

10

1.2

I U a 16

I

I I

I

'

I

I •

' I i

I 0 0 8

LEGEND

0

0

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I I

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I

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T : I • I

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I

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~I\ ~' ,... 0 " '"'

.1'\

0 . 3 0"'

! ~

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I '

I oo j -I oo v o l

..... ~

0 . .5 LsfLsM

v 6

'

6

0 6

..,--OIUG INAL GROvNO LIN£ I I

··~ I o ; jo 0 I

·0 ~0 . ~ oy :Q- - -I

~ I I 0 0 ' '

v~ --r- -

I o·o I

./ ; v I 6 I

~ I I

I ' -+----- - -·--

0 I I

I 6 I I I I

I - --I I ' I

I . I i

0 7 0 8 09 10

DIMENSIONLESS CENTER- LINE PROFILE

CD I __,-ORIGINAL GROUND LIN£

0 a 0

0 n ..IY' Kl. I

'"-6... ---,6~ 1...--cuLVERT ~ v ~ ~

I 0 6 I c

• 00 I

• I I .., .

I

\ I '/ 1 - -

6 I

I

' I I I I I I

I ' I IJ

I ' I

•

~ I A/ I I

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0 I I I

-

~ / . r

i

I ' I i I

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I I I i I I •

I I I l i I

o e 0.4 0 2 0 0 2 0 4 oe o.e 1 0

Ws/Ws.,.

DIMENSIONLESS CROSS SECTION AT 0 .4 LsM

Fig. 7 . Dimensionless scour hole geometry for minimum tailwater

•

0 G _,-l R l IN.AL GROUND L IN£ o FLow ..

0. 2

~I Aa~V [\.. I I

0 4

~0.& Dsw

0. 8

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DIMENSIONLESS CENTER- LINE PROFILE

CD _,.,- ORIGINAL GROUND LIN£

I []

~/ ~ I -

I • •

~ I 1.--cu~vERr /o I I f

' : I

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0 8

1.0

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•

.

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0 0 33~T-01AM C U L VERT 6 0 22 4-FT-DIAM CULVERT

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I

I ~ I

~

0 2

I

I

0

Ws/Ws ..

I l o v I I j 1 1

I

i/ ! l-JYI ~ l I I I

' 0 2 04

DIMENSIONLESS CROSS SECTION AT 0 . 4 L 5M

I I

• I •

-+ !

! I

I

i :

06 0. 8

' ! I I I

I • I • : I •

I

I I

I

! I I

I

' I I l

Fig . 8 . Dimensionless scour hole geometry f or maximum tailwater

--

10

1.0

50r---~-----.----.-----.---~~--~-----00 00

4 5 t------lf----~

o ADEQUATE 20 ....._-----+~,__--+---+---- o ALMOST ADEQUATE

• • INADEQUATE --

15~--~~----~----~----~----~----~--~

0 •• -LENGTH OF I 0 1---1----+- ....___--+---+

• STONE PROTECTION HORIZONTAL BLANKET · .. -

10 15 20 25 30 35 Q

Fig . 9. Length of stone protection, horizontal blanket

\ I \ "" I \ ~

I ....: \ I ~ \ I

...... ~

I \ \....

It I \

..., ..., I \ I

I \ I \ 5

I "" \ ~ I \ ....:

--+----- ------r---- ~

~ I \ I \....

\ ' I ..,

\ ...,

I \ I \ I \ I \ I \ I \ I \ I \ I \ I

Wsp (MAX. T. W.)

Wsp (MIN. T.W.)

Fig. 10 . Recommended configuration of riprap blanket subject to maximum and minimum tailwaters

A l .. Oo

I ON .J

I ON3 c§ 3 Do .......-

l A I ON 3 __ j

(\J

J

I ON.J

PLAN

3 _{ARIES (0-lO ~o)~ ,, ~-

SECTION A-A

Fig . l l. Preformed scour hole

75u28

0.5~----~----~-------------+-----n-------------r----~

4

3

dso._TW_ 0.05~----~----~-----+~T-~~-----+-------------r----~ Do Do dso Do ( Q )4/J

Do = C TW Dos/2

c TYPE PROTECTION HORIZONTAL BLANKET

0.01 0.0 I 60 LINED CHANNEL EXPANSION

0.0 I 25 0.5- D0 -DEEP PREFORMED SCOUR HOLE

0.005 0.0082 I.O-D0 -DEEP PREFORMED SCOUR HOLE

0.5 1.0 10 50 100

Fig . 12 . Recommended size of protective stone

CURTAIN WALL

I ON 2

PLAN

2.5 0 0

0.500

0./500 ____._ 0.500

500 Do

SECTION A-A

Fig. 13. Culvert outlet erosion protection, lined channel expansions

/ - -'r'

... #"

\ I \\

I

\

. \ I I ' •

\\ I I f I • '

I 1 I I 1

I I \ I

/1 • J . -.

I ) \:

I ,

' • J

I ( ' \ ' 1 ' I ' I ' I I I

t \ • • • \ I •

\ ' " . \ I • I

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•

1-I

• , ., I \ , .. •

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I

• •

• •

, '

Fig . 14 . Channel expansions lined with sack revetment

•

•

Fig . 15 . Channel expansion lined with cellular blocks

-

Fig . 16 . Channel expansion lined with riprap

Ta TW -·-Do Do

OR

Ts TW -· Do Do

OR

d~0 TW •

Do Do

0. I

0.05 0 <D

6~ 06~8

0 .01

0 0 RIPRAP

6 BLOCK

0 SACK

0 .001 0 .1 0 .5 1.0 5 10

Q D Sfz

0

Fig . 17 . Maximum permissible discharge for lined channel expansions

20

8 ,,

PLAN

L~---. . . . . . . . H ... · .·-: · .. : .

~--------------------------------------~~ . . . . . . . . .

5Do • I

ELEVATION

Fig . 18 . Flared outlet transition

Q

0.5 ...,.._---?c__ ________ +-____ .,__ ___ ~

5; - 1.60 D D Do 2 o o

D 1'3

5 L Do

10

Fig . 19. Maximum permissible discharge for various lengths of flared outlet transition and tailwaters

20

Q

D 5/z 0

4--------~------~------~------.-------,

2

0

0

L =3

l ________ L_~D~o~---L~--~~~~::===±=======J

---L -=5 Do :J:~::-:·;:~--------l -----~ ---/~

0.2 0.4 TW Do

H/D0

0 -. 0 .25 --- 0 .50

0.6 0 .8

Fig . 20 . Relative effects of recessed apron and end sill on permissible discharge

1.0

•

0.8

.

-- - - - -

1- 0.6 ~--cr .J w .J -> w z ~ -~ t:> 0 z -.J .J w .J m -

1-.J (/)

.J u. 0. 4 w 0 ~ cr u. w 0 1-I w 1- ~ a. ~ -w 0 0

II 0.2 t-lo

~

- -- . ·- 1- ---- -- - - - - -·- ~

~ v / --

v v

------- 1--·--··

v 1- /

v - --- - ·--- -- - - -- 1--- --

0 0 0.2 0. 4 0.6 0.8 1.0

SLOPE, VERTICAL/HORIZONTAL

DATUM

VERT

HOR 20 0

T

L-~w _J Fig . 21. Stilling well

Unclassified Security_ Classification

DOCUMENT CONTROL DATA . R & D (S~curlly cla••lflcat lon ol title, body of abetrect end lndeltlnQ ennotet/on muat be entered when the overall report Ia cleae/f/ed}

I . ORIGINATING ACTIVITY (Corporate author) p U. RE ORT SECURITY CLASSIFICATION

U. S . Army Engineer Waterways Experiment Station Vicksburg, Mississippi

3 . REPORT T I TLE

Unclassified 2b. GROUP

PRACTICAL GUIDANCE FOR ESTIMATING AND CONTROLLING EROSION AT CULVERT OUTLETS

4 . DESCR IPTIVE NOTES (Type o f report end /nc/ue/ve datee) Final report

II - AU THORCSI (Fitet name, middle In/tiel, l aet name)

B. P. Fletcher J. L. Grace , Jr .

O. REPORT OATE

May 1972 Ia. CONTRACT OR GRAN T NO.

b. PROJECT NO.

c.

d.

10. DISTRIBUTION STATEMEN T

7a. TOTAL NO. OF PAGES 7b. NO. OF REFS

40 6 De. ORIGINATOR'S REPORT NUM"BER(S)

Miscellaneous Paper H- 72-5

lib. OTHER REPORT NO(Sl (Any other nwnbare that may be aee/,.ed thl• report)

Approved for public release; distribution unlimited.

II · SUPPLEMENTARY NOTES 12. SPONSORING MILITARY A CTIVITY

U . ABSTRACT

This paper summarizes the results of research conducted at the U. S. Army Engineer Waterways Experiment Stat·on during the past nine .vears to develop practical guidance for estimating and controlling erosion downstream of culvert and storm- drain outlets . Initial efforts were concerned with investigation and development of means of estimating the extent of scour to be anticipated downstream of outlets . Subsequent efforts have involved investigation and evaluation of various schemes of protection for controlling erosion such as horizontal blankets of rock riprap, preformed scour holes l ined with rock riprap and channel expansions lined with natural and artificial revetments . In ~ddition, efforts have been made to determine the limiting discharges for various energy dissipaters including simple flared outlet transitions, stilling wells, U. S . Bureau of Reclamation type VI basins and St . Anthony Falls stilling basins . Empirical equations and charts are presented for estimating the extent of localized scour to be anticipated downstream of culvert and storm- drain outlets, the size and extent of various natural and artificial type revetments and the maximum recommended discharge for each type of energy dissipater investigated . With these results, designers can estimate the extent of scour to be expected and select appro­priate and alternative schemes of protection for controlling erosion downstream of culverts and storm-drain outlets .

DO .':!' .. 1473 III& .. LAC:&• 00 .. OIIIW 1.71. I JAN ••• WHICH 1• oe.OL&T& .. 0111 AIIIWY u•a. Unclassified

Unclassified Security Classification

I • • LINK A LINK B LINK C KEY WORDS

ROLE WT ROLE WT ROLE WT

Culvert outlets

Erosion

Energy dissipators

Riprap

Scour

Storm- drain outlets

Stilling basins

.

I I

Unclassified Security Claaalflcation