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OCR for page 103

103
(7) The flood lasts 2 days (48 hours), therefore Maximum Contraction Scour Depth vs. Time
(Example 4)
Z(Cont) = 562 mm or 4% of Zmax(Cont) 14000
Z(Unif) = 553 mm or 5.6% of Zmax(Cont) 12000
Pier Scour Depth (mm)
10000
12.4.2 SRICOS-EFA Method:
Computer Calculation 8000
6000
Use SRICOS-EFA program Option 2: Contraction Scour.
4000
Results: 2000
After a 2-year period of flood having 3.36 m/sec velocity, the
final contraction scours are 0
0 200 400 600 800
Z(Cont) = 13.14 m Time (Day)
Z(Unif) = 9.39 m Figure 12.18. Maximum contraction scour depth versus
time (Example 4).
Table 12.4 and Figures 12.15 through 12.19 provide a sum-
mary of input data and illustrate the results. Flood period: 70 years
Determine: The magnitude of maximum contrac-
12.5 EXAMPLE 5: CONTRACTED CHANNEL tion scour depth
WITH 60-DEGREE TRANSITION ANGLE
AND APPROACHING HYDROGRAPH
12.5.1 SRICOS-EFA Method:
Given: Computer Calculation
Channel geometry: Upstream uncontracted channel width
B1 = 150, contracted channel width due Since the hydrograph is used in this case as hydrologic
to bridge abutment B2 = 50 m, contrac- data input, the relationship between the discharge and veloc-
tion length of channel L: = 30 m ity and the relationship between discharge and water depth
Abutment need to be defined. The HEC-RAS program can be a good
transition angle: 60 degrees tool to define these relationships. The following charts pre-
Flow parameters: 70 years predicted hygrograph sent the results obtained from HEC-RAS for this case.
Manning Use SRICOS-EFA program Option 2: Contraction Scour.
Coefficient: 0.02
Hydraulic Radius: 2.72 m Results:
EFA result: Layer 1: Thickness 10 m; critical shear After a 70-year period of flood, the final contraction scours are
stress 2 N/m2
Z(Cont) = 8.8 m
Layer 2: Thickness 20 m; critical shear
stress 4 N/m2 Z(Unif) = 6.6 m
V1 Uniform Contraction Scour Depth vs. Time
River Bank Flow River Bank (Example 4)
10000
9000
8000
Pier Scour Depth (mm)
B1
7000
6000
5000
90°
4000
B2
3000
L
2000
Bridge Abutment Bridge Abutment
1000
0
0 200 400 600 800
Time (Day)
Figure 12.17. Plan view of contracted channel scour case Figure 12.19. Uniform contraction scour depth versus
(Example 4). time (Example 4).

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104
TABLE 12.5 Summary of data input (Example 5)
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 Uncontracted Channel Width 150
Contracted Channel Width 50
Contraction Length of Channel 30
Transition Angle of Channel 60
Manning's Coefficient 0.02
Average Hydraulic Radius 2.77
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
Input 566, 0.49
Hydrologic Values of Regression Discharge, Velocity 1415, 0.87
Data Points 5663, 1.75
13592, 2.97
19821, 3.56
Number of Regression Points Discharge vs. Water Depth 8
1.42, 3.86
14, 4.18
141, 5.02
Discharge, Water Depth
Values of Regression 566, 6.18
Points 1415, 7.83
5663, 11.33
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
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