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EFA Result (Layer 2)
8
7
Scour rate Shear stress
(mm/hr) (N/m2) 6
Scour Rate (mm/hr)
0 3 5
0.1 4
1 6 4
2 9
4 18.5 3
5 27
2
6 40
6.9 60 1
0
0 10 20 30 40 50 60 70
2
Shear Stress (N/m )
Figure 12.2. EFA Results for Soil Layer 2 (Example 1).
Channel geometry: Channel upstream width B1 = 50 m Use SRICOS-EFA program Option 1: Complex Pier Scour.
Flow parameters: Angle of attack: 20 degrees
70 years predicted hydrograph Results:
EFA result: Layer 1: Thickness 10 m; critical shear After a 70-year period of flood, the final pier scour is
stress 2 N/m2
Layer 2: Thickness 20 m; critical shear Z = 4.7 m
stress 4 N/m2
Time duration: 70 years Table 12.2 lists data input for this example. Figure 12.5 illus-
Determine: The magnitude of maximum pier trates the scour depth development with time. Figures 12.6
scour depth through 12.8 provide further information.
12.3 EXAMPLE 3: GROUP RECTANGULAR
PIERS WITH ATTACK ANGLE
12.2.1 SRICOS-EFA Method: AND APPROACHING CONSTANT VELOCITY
Computer Calculation
Given:
Since the hydrograph is used in this case as hydrologic Pier geometry: Pier width B = 1.22 m, pier length
data input, the relationship between discharge and velocity Lpier = 18 m, rectangular pier, number
and the relationship between discharge and water depth need of piers, N = 3, spacing, S = 18 m
to be defined. The HEC-RAS program can be a good tool to Channel geometry: Channel upstream width B1 = 150 m
define these relationships. The following charts present the Flow parameters: Water depth H = 3.12 m,
results obtained from HEC-RAS for this case. Angle of attack: 20 degrees
Scour Depth vs. Time (Example 1)
4500
Flow 4000
Pier Scour Depth (mm)
3500
3000
2500
2000
1500
1000
500
0
B 0 200 400 600 800
Time (Day)
Figure 12.3. Plan view of single circular pier scour case
(Example 1). Figure 12.4. Scour depth versus time (Example 1).

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TABLE 12.2 Summary of data input (Example 2)
Input Unit SI 1
Output Unit SI 1
First Date of Analysis 01-01-2003
Last Date of Analysis 01-01-2073
No. Of Input Data 25569
Upstream Channel Width 50
Type of Pier Rectangular Pier 2
Pier Width 1.22
Pier Length 18
Attack Angle 20
Number of Piers 1
Time Step Hours 24
Type of Hydrologic Input Discharge 1
Number of Regression
Points Discharge vs. Velocity 8
1.42, 0
14, 0.02
141, 0.16
566, 0.49
Values of Regression Discharge, Velocity 1415, 0.87
Points 5663, 1.75
Input 13592, 2.97
Hydrologic 19821, 3.56
Data Number of Regression
Points Discharge vs. Water Depth 8
1.42, 3.86
14, 4.18
141, 5.02
Discharge, Water Depth 566, 6.18
1415, 7.83
Values of Regression 5663, 11.33
Points 13592, 13.15
19821, 14.19
No. Of Layers 2
Properties of 1st Layer Thickness 10
Critical Shear Stress 2
Number of
Regression Points Shear Stress vs. Scour Rate 8
1, 0
2, 0.1
Estimate Initial 4,1
Scour Rate Value of Regression Shear Stress, Scour Rate 6,2
Points 9, 3
20, 6
40, 8
60, 8.9
Properties of 2nd Layer Thickness 20
Critical Shear Stress 4
Number of
Regression Points Shear Stress vs. Scour Rate 8
3, 0
4, 0.1
Estimate Initial 6,1
Scour Rate Value of Regression Shear Stress, Scour Rate 9,2
Points 18.5, 4
27, 5
40, 6
60, 6.9

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Future Hydrograph Layer 2: Thickness 20 m; critical shear
14000 stress 4 N/m2
Flood period: 2 days for hand calculation
12000
2 years for computer calculation
10000 Determine: The magnitude of maximum pier
Discharge (m /sec)
scour depth
3
8000
6000
12.3.1 SRICOS-EFA Method: Hand Calculation
4000
(1) Calculate the K factors for max and Zmax as follows:
2000
H 3.12
-4 -4 ×
0
kw = 1 + 16 e B = 1 + 16 e 7.3 = 3.9
0 10 20 30 40 50 60 70
Time (Year)
( ) (37.12
.3 )
0.34 0.34
K w = 0.85 H B = 0.85 × = 0.637
Figure 12.5. Seventy years future approaching hydrograph
(Example 2).
Approaching constant velocity V = Here, B is the projected width of pier.
3.36 m/sec
EFA result: Layer 1: Thickness 10 m; critical shear B = Lpier sin + W cos = 18 × sin 20° + 1.22 ×
stress 2 N/m2 cos 20° = 7.3 m
EFA Result (Layer 1)
10
9
8
Scour rate Shear stress
(mm/hr) (N/m2) 7
Scour Rate (mm/hr)
0 1 6
0.1 2 5
1 4
2 6 4
3 9 3
6 20 2
8 40
8.9 60 1
0
0 10 20 30 40 50 60 70
2
Shear Stress (N/m )
Figure 12.6. EFA results for Soil Layer 1 (Example 2).
EFA Result (Layer 2)
8
7
Scour rate Shear stress
(mm/hr) (N/m2) 6
Scour Rate (mm/hr)
0 3 5
0.1 4
1 6 4
2 9
4 18.5 3
5 27
2
6 40
6.9 60 1
0
0 10 20 30 40 50 60 70
2
Shear Stress (N/m )
Figure 12.7. EFA results for Soil Layer 2 (Example 2).

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Since in this case, the pier is a rectangular pier, so
18
-4
Flow 20° ksh = 1.15 + 7 × e 1.22 = 1.15
Since there are three piers in this case, the effect of a
Lpier group pier exists.
ksp = 1 + 5e
( -1.1D
S
) = 1 + 5e( -1.1718
.3) = 1.33
B
Figure 12.8. Plan view of single rectangular pier scour B1 150
Ksp = = = 1.17
case (Example 2). B1 - nB 150 - 3 * 7.3
There is attack angle of the flow, so
( )
( )
0.57 0.57
20
k = 1 + 1.5 = 1 + 1.5 = 1.636
90 90
TABLE 12.3 Summary of data input (Example 3)
Input Unit SI 1
Output Unit SI 1
First Date of Analysis 01-01-1998
Last Date of Analysis 01-01-2000
No. Of Input Data 730
Upstream Channel Width 150
Type of Pier Rectangular Pier 2
Pier Width 1.22
Pier Length 18
Attack angle 20
Number of piers 3
Pier spacing 18
Time Step Hours 24
Type of Hydrologic Input Velocity 2
Number of Regression Points Velocity vs. Water Depth 1
Values of Regression Points Velocity, Water Depth 3.36, 3.12
No. Of Layers 2
Properties of 1st Layer Thickness 10
Critical Shear Stress 2
Number of Regression
Points Shear Stress vs. Scour Rate 8
1, 0
4, 1
Estimate Initial 6,2
Scour Rate Value of Regression Shear Stress, Scour Rate 9,3
Points 6, 30
100, 10
200, 12.5
400, 16
Properties of 2nd Layer Thickness 20
Critical Shear Stress 4
Number of Regression
Points Shear Stress vs. Scour Rate 8
3, 0
4, 0.1
Estimate Initial 6,1
Scour Rate Value of Regression Shear Stress, Scour Rate 9,2
Points 18.5, 4
27, 5
40, 6
60, 6.9