SCOUR DEVELOPMENT DURING MULTIPLE FLOODS AT · PDF fileSCOUR DEVELOPMENT DURING MULTIPLE...

UNIVERSITA DI PISA, DIPARTIMENTO DI INGEGNERIA CIVILE,

CORSO DI DOTTORATO IN “SCIENZE E TECNICHE DELL’INGEGNERIA CIVILE”

SCOUR DEVELOPMENT DURING MULTIPLE

FLOODS AT ENGINEERING STRUCTURES IN

RIVERS

BORISS GJUNSBURGS, ROMANS NEILANDS,

ROBERTS NEILANDS, and ZANNA BUDJKO

Riga Technical University, Civil Engineering Faculty,

Water Engineering and Technology Department, 16/20 Azenes Street,

Riga, LV 1048, Latvia



� During the past few decades, the equilibrium and temporal scour depth at piers, abutments, and spur dikes was studied.

� In empirical formulae suggested in the literature, the authors calculated river flow parameters at the peak of the flood with unrestricted duration for equilibrium or restricted one for temporal depth of scour (Melville & Coleman, 2000).

� The duration of the tests in flumes - the time necessary to reach the equilibrium stage of scour under steady flow conditions was up to 6 weeks (Radice et al., 2002): this question is the matter of continuous discussions (Grimaldi et al., 2006).

� The equilibrium scour depth at bridge piers, abutments, and spur dikes was studied and fundamental books were published by Breuser & Raudkivi (1991), Hoffmans & Verheij (1997), Melville & Coleman (2000), and May et al. (2002).



� However, in nature, the action of flow loads on engineering structures has the form of hydrograph, and multiple floods form scour holes at piers, abutments, spur dikes, and guide banks.

� Because of the difference between laboratory tests and physical process of scour in nature, the existing formulae overestimate the scour depth value (Kwak et al., 2004) and considerably increase the costs of engineering structures.

Table 1. Comparison of pier scour depths

Table from - Kwak, K. et al. (2004), Scour and erosion characteristics at Choji bridge, in Proceedings of Int. Conf. on Scour and Erosion, vol.1., pp. 292-300, Stallion Press, Singapore.

� The scour development during multiple floods at abutments and guide banks was investigated by Gjunsburgs et al. (2005, 2006).

19.2Neill Equation

13.9Laursen Method

19.7Froehlich Method

16.5CSU Method3.9

6.7SRICOS Method

Measured scour depth (m)Predicted scour depth (m)Method



� The prediction of scour depth during the multiple floods is of great importance for the stability and safety of bridge constructions.

� The bridge failure during the floods as a result of local scour at piers, abutments, and guide banks, or of contraction scour can lead to considerable environmental damages and losses.



Study tasks ............

� to study the influence of contraction of the flow, hydraulics, river bed parameters, multiple floods probability, duration and sequence on the scour at engineering structures;

� To work out the method at local scour computing at the abutments and guide banks during the flood at plain rivers;

� To make theoretical analysis at the method to study the physics of scour process during the floods;

� To make comparison of computed data with tests.



Answers........

� When scour starts and stops?

� How depth of scour developing during the flood?

� How depth of scour developing during the several floods?

� How long the depth of scour is developing?



Experimental setup

� The tests with a rigid bed were performed for different flow contractions and Froude numbers in order to investigate the velocity and the changes in water level near the embankment, along it, and near a model abutment.

� The tests with a sand bed were carried out for studying the scour processes, the changes in velocity with time, as well as, the influence of hydraulic parameters, contraction rate of the flow, grain size of the bed material, and time, on the scour.

� The openings of the bridge model were 0.5, 0.8, 1.2, and 2.0 m in the first flume and 0.445, 0.575, 0.775, and 0.975 m in the second one.



Experimental setup

L

L

L

L

b1

b2

b3

b4

h =12cmch =7cmf

floodplain

channel

300 50 80 120 200 350

Figure 1. Laboratory flume cross section with bridge openings 50-200 cm



Experimental setup

� The contraction rate of the flow Q/Qb (where Q = a discharge of the flow and Qb = a discharge of the flow in a bridge opening in the un-contracted conditions) varied from 1.25 to 5.69 at a depth of floodplain of 0.05, 0.07, and 0.13 m; the Froude numbers varied from 0.078 to 0.151. The slope in the first and second flumes was 0.0012 and 0.0015, respectively.

� The tests with a sand bed were carried out in the conditions of clear water. The sand was placed 1 m up and down the contraction of the flumes. The mean grain size was 0.00024 and 0.00067 m in the first flume and 0.0005 and 0.001 m in the second one with a standard deviation.



� The condition that FrR = Frf was fulfilled, where FrRand Frf were the Froude numbers for the plain river and for the flumes, respectively.

� The test duration in the flumes was 7 hours, the vertical scale was 50, and the time scale was 7. The test duration, as referred to the real conditions, was equal to 2 days.

� That was the mean duration of time steps, into which the flood hydrograph was divided according to the method presented. The tests were intended to investigate the scour process in time intervals within one step of 7 hours and two steps of the hydrograph of 7 hours each with different flow parameters.

Experimental setup



5000120900.15107.3010.585134.5S3

4450104000.13606.509.525134.5S2

300071100.08904.246.305134.5S1

14300143000.087647.109.9013350L9

11395160100.075641.388.7413350L8

9740137000.066535.487.5113350L7

7550/7840138000.133928.1311.17350L6

6140/6410112800.109423.489.077350L5

5590/5660102700.098420.818.167350L4

7190122800.124323.6010.37350L3

6060100100.010322.708.587350L2

439075000.078016.606.477350L1

RefRecFrQ

(l/s)

V

(cm/s)

hf

(cm)

L

(cm)

Test

Table 2. Experimental data for open flow conditions in flumes.



Fig.1 Velocity distribution in approach to abutment. Test SS1.

� At the upper head of structures are streamline concentration, a local increase in velocity, vortex structures, and development of scour hole were observed.



.

According to laboratory tests, w = 1/6 πm2 hs3 = a volume of

the scour hole, t = a time, and Qs = a sediment discharge out of the scour hole.The left-hand part of Eq. 1 can be written as:

,where hs = a depth of scour, and m = a steepness of the scour hole

sQdt

dw=

[2]

[1]

The differential equation of equilibrium of the bed sediment movement in the conditions of clear water has the form:

dt

dhah

dt

dhhm

dt

dw s

s

s

s

222

2

1== π

Method



,where B = mhs = a width of scour hole, Vl = a local velocity at the abutment, and A = a parameter (Levi, 1969).The sediment discharge in the process of scour development is:

,

where b = AmVl4 , k = a coefficient of changes in the

discharge caused by the scour, hf = a water depth in floodplain.

4

ls VABQ ⋅=

The sediment discharge for the plain bed is (Levi, 1969) :

.

Method

[3]

4

4

21

+

=⋅=

f

s

s

lss

h

hk

hbVmhAQ

[4]



The hydraulic characteristics, the backwater value ∆h, the contraction rate of the flow Q/Qb, the velocities V0 and Vl, the grain size di in different bed layers H, the sediment discharge Qs, as well as the depth hs and width of the scour hole varied B during the flood.

.

Method



Taking into account Eq. 2 and 4, Eq.1 can be presented as:

.

After integration, we have:.

Method

[6] .sf

s

x

x

si dhh

hhDt

4

21

2

1

+= ∫

4

2

21

+

=

f

s

sss

h

hk

hb

dt

dhah [5]



After integration with new variables, x = 1+ hs/2hf, hs = 2hf(x-1) and dhs = 2hfdx, we get :

,

where Ni = 1/6xi6 – 1/5xi

5 and ti = a time interval.Scour depth can be found:

[7]

Method

124−+= i

fi

ii N

hD

tN

,

where km = a coefficient depending on the side-wall slope of the abutment (Yaroslavtsev, 1956), ks = a coefficient depending on the shape of the abutment, and kα = a coefficient depending on the angle of flow crossing (Richardson and Davis, 1995).

( ) αkkkxhh smfs ⋅⋅⋅−= 12 [8]



0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 450

hs (

m)

Test SL 19

Test SL 4

t (min)

Calculated data Experimental data

350 250 150 50

Figure 2. Scour development in time under steady flow conditions. Tests SL4 and SL19.

0,00000,0200

0,0400

0,06000,0800

0,10000,12000,1400

0,1600

0 100 200 300 400 500 600 700 800 900t (min)

Calculated dataExperimental data

hs

(m)

0,00000,0200

0,0400

0,06000,0800

0,10000,12000,1400

0,1600

0 100 200 300 400 500 600 700 800 900t (min)

Calculated dataExperimental data

hs

(m)

Figure 3. Scour development in time for unsteady flow conditions. Test TL1.



According to our method, the flood hydrograph was divided into time steps and each step was in turn divided into small time intervals, as shown in Fig. 4.

t 1 t 2 t 1 i t n

Time steps

Floodplain

Time intervals

t t

Q

flooded

Fig. 4. Hydrograph divided into time steps and time intervals.



,

� To determine the scour depth during the flood, we divided the hydrograph into time steps with duration of 1 or 2 day, and each time step – into time intervals up to several hours. In laboratory tests, the time steps were divided into 20 time intervals.

� For each time step, the following parameters must be determined: hf – depth of water in the floodplain, Q/Qb –contraction rate of the flow, Δh – maximum backwater, d – grain size, H – thickness of the bed layer with d, and γ – specific weight of the bed material. As a result, we have Vl, V0, A, D, Ni, Ni-1, and hs at the end of time intervals and finally at the end of the time step.

� For the next time step, the flow and bed parameters were changed because of the flood and the scour developed in the previous time step.



� Considerable influence on depth of scour had flow parameter changes from step to step of hydrograph.

� For example: In test SL4 with Q = 16.6 l/s, hf = 7cm, Q/Qb = 3.66, d = 0.24 mm and Δh = 1.19cm, depth of scour hs was 10.1 cm in 7 hours. In test SL 19 with Q = 35.48 l/s, hf = 13 cm, Q/Qb = 4.05, d= 0.24 mm and Δh = 1.42 cm depth of scour was 13.6 cm in 7 hours.

� In test TL1 we were modeling two steps of hydrograph, where test SL4 was as first step and test SL19 as a second. The depth of scour hs in 14 hours (test TL1) was 14.3 cm, but not equal to sum of the two separate tests – 23.7 cm.



According to our method, the flood hydrograph was divided into time steps and each step was in turn divided into small time intervals, as shown in Fig. 5.

t 1 t 2 t 1 i t n

Time steps

Floodplain

Time intervals

t t

Q

flooded

Figure 5. Hydrograph divided into time steps and time intervals



Figure 6. Hydrograph of multiple floods divided into time stepsand time intervals



In calculating the scour, the flood hydrograph was divided into six steps and each step into 20 intervals. The scour stopped on the forth step at the peak of the flood with a depth of 6.69 m. The second flood proceeded with the same probability as the first one, and the scour started on the ninth step and stopped on the tenth step at the peak of the second flood. The scour depth in the second flood increased by 0.79 m, as shown in Fig. 7.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 1000000 2000000 3000000

t (sek)

hs (

m)

First

flood

Second

flood

Figure 7. Development of scour depth in time during the two equal floods at the abutments.

t 1 t 2 t 1 i t n

Time steps

Floodplain

Time intervals

t t

Q

flooded

t 1 t 2 t 1 i t n

Time steps

Floodplain

Time intervals

t t

Q

flooded



Example

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time, t (days)

Dep

th o

f sc

ou

r h

ole

, h

s (m

)

I flood II flood III flood

0 6 12 18 0 6 12 180 6 12 18

Lo

w w

ate

r

Lo

w w

ate

r

Figure 8. Scour depth development for three floods of the same probability at an elliptical guide bank



Example

0.0

1.0

2.0

3.0

4.0

5.0

Time, t (days)

Wid

th o

f sc

ou

r h

ole

, B

(m

)


0 6 12 18 0 6 12 180 6 12 18

Lo

w w

ate

r

Lo

w w

ate

r

Figure 9. Scour width development for three floods of the same probability at an elliptical guide bank



Example

Figure 10. Scour hole volume development for three floods of the same probability at an elliptical guide bank

0.0

5.0

10.0

15.0

20.0

25.0

30.0

Time, t (days)


0 6 12 18 0 6 12 180 6 12 18

Lo

w w

ate

r

Lo

w w

ate

r

Vo

lum

e o

f sc

ou

r h

ole

, W

(m

3 )



� Scour usually stops at the peak of the flood

� Time of scour is less than flood duration

� Resistance capacity of the flow with scour development is increasing

� Existing methods and formulas tends to overestimate abutment scour

Remarks



Conclusions

� The differential equation of equilibrium for the bed sediment movement in clear water was used and a new methods for calculating the scour development with time at the abutments and guide banks during the multiple floods was elaborated.

� Scour depth at the guide bank and at the abutment is different.

� In calculations by the method suggested, the depth of scour can be determined during/after multiple floods.

� The scour development usually stops at the peak of the flood or just after it.



Conclusions

� In the second flood of the same probability, the scour depth continues to develop, but starts closer to the peak of the flood, with less intensity and less duration.

� During the forthcoming flood of higher probability, the scour depth remains on the level of the previous high flood. The time of scour is always less than the time of flood.



� The relative depth of scour depends on the contraction of the flow, relative grain size of the river bed, stratified riverbed conditions, kinetic parameter of the flow, relative depth and velocity of the flow, Froude number in relation to the slope, unsteadiness of the flow, multiple floods probability, duration and sequence (Gjunsburgs and Neilands, 2001, 2004, 2005, 2006).

� As found in the earlier investigations, the scour depth depends also on the slope of the side walls (Yaroslavtsev, 1956), as well as on the shape of construction and the angle of flow crossing (Richardson and Davis, 1995).

Conclusions



Thank you

December 2005, Riga

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