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42 TABLE 5.7 Calculations for isolating the attack angle effect in pier scour depth B Zmax Zmax(0) L/B B/B H/B Kw Ksp Ka Ka () (mm) (mm) (mm) 15 4:1 61.00 1.00 72.99 72.99 6.15 1.00 1.00 1.00 1.00 30 4:1 122.07 2.00 103.09 72.99 3.07 1.00 1.00 1.41 1.41 45 4:1 174.83 2.87 117.65 72.99 2.14 1.00 1.02 1.61 1.59 60 4:1 215.67 3.54 151.50 72.99 1.74 1.00 1.05 2.08 1.98 90 4:1 241.81 3.96 196.08 72.99 1.55 0.99 1.08 2.69 2.51 45 1:1 86.27 4.00 208.77 72.99 1.54 0.98 1.09 2.86 2.67 45 2:1 129.40 1.41 147.06 73.53 4.35 1.00 1.00 2.00 2.00 45 6:1 301.93 2.12 161.29 73.20 2.90 1.00 1.00 2.20 2.20 tion width and Equation 5.5 in both the transverse and the 5.19 ATTACK ANGLE EFFECT ON INITIAL longitudinal direction. The difference between Ka and Ka is SCOUR RATE linked to the influence of the water depth and pier spacing effect that occur when the angle of attack becomes more The initial scour rates for the attack angle flume tests are severe. In order to examine the pure attack angle effect, the plotted in Figure 5.26 for both the transverse and vertical shallow water effect and pier spacing effect need to be elim- direction. In this case, the scour rates show a large scatter inated. The proposed equations for shallow water depth and pattern. This is because several opposing factors are involved pier spacing are used to isolate the angle of attack effect. The in the initial shear stress or the initial scour rate under a results of the calculations are shown in Table 5.7. The cor- given attack angle condition. Based on the observations in rection factor, Ka, for pure attack angle effect on scour depth previous sections, the scour rate will decrease with an increase is obtained based on the following relationship: in pier width and flow depth, but increase with an increase in pier contraction and the sharpness of pier corners. There- K a = K w K sp K a (5.6) fore, the relative magnitude of the scour rate under a given attack angle depends on the balance between these influ- Figure 5.25 shows that when the attack angle is less than encing factors. 30 degrees, the correction is not significant and that for angles of attack larger than 30 degrees, the correction factor 5.20 ATTACK ANGLE EFFECT ON SCOUR would be overestimated if the correction was not done. Fig- HOLE SHAPE ure 5.25 also shows that using the projection width with the SRICOS exponent of 0.635 leads to a reasonable and often The attack angle also strongly affects the shape of the conservative prediction of the correction factor Ka. There- scour hole. If the four corners of the rectangular pier are fore, this approach is adopted for the proposed method. numbered as in Figure 5.24, test observations indicate that 3.0 3.0 2.5 2.5 Initial Rate (mm/hr) Initial Rate (mm/hr) 2.0 2.0 1.5 1.5 1.0 1.0 0 30 60 90 0 2 4 6 Attack Angle (Degree) L/B (A: Transverse Direction, L/B=4) (B: Vertical Direction, Attack Angle =45) Figure 5.26. Initial scour rates for the attack angle flume tests.