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OCR for page 95
95 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).

OCR for page 95
96 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

OCR for page 95
97 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).

OCR for page 95
98 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