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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).

OCR for page 103
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