Pier and Contraction Scour in Cohesive Soils (2004) / Chapter Skim
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Pages 56-67
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From page 56... ...
This profile identifies two separate scour depths: the maximum contraction scour depth zmax, which occurs xmax after the beginning of the start of the contracted channel, and the uniform scour depth zunif, which occurs after that. As described in this chapter, a series of flume tests were performed to develop equations to predict the values of zmax, xmax, and zunif in cohesive soils.
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From page 57... ...
Among them, the contraction ratio was varied in Tests 1, 2, and 3; the water depth was varied in Tests 4, 5, and 6; and the velocity was varied in Tests 2, 6, and 7. There were two groups of secondary tests: Tests 2, 9, 10, and 11 were for the transition angle effect on contraction scour, and Tests 2, 12, 13, and 14 were for the contraction length effect on contraction scour.
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From page 58... ...
. As a result, the contraction scour depth can be simply calculated by subtracting the upstream water depth H1 from the total water depth in the contraction section H2.
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From page 59... ...
Figure 7.5 shows a sketch of the contraction distribution in plan view. The measurement emphasis was placed on obtaining four parameters at the equilibrium contraction scour: the maximum contraction scour depth Zmax, the uniform contraction scour depth Zunif, the location of the maximum contraction scour Xmax, and the contraction profile along the channel centerline.
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From page 60... ...
Both Zmax and Zunif were normalized with respect to H1, the upstream water depth. Figure 7.12 shows the attempt to correlate the contraction scour depths to the Reynolds Number defined as V1B2/υ.
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From page 61... ...
Plain is the location of maximum contraction Figure 7.11. Overlapping of scour for contraction scour and abutment scour.
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From page 62... ...
The final form of the equation sought was The factors α and β were obtained by optimizing the R2 value in the regression on Figures 7.13 and 7.14. The proposed equations are where Zmax is the maximum depth of contraction scour; H1 is the upstream water depth after scour has occurred; V1 is the mean depth upstream velocity after the contraction scour has occurred; B1 is the upstream channel width; B2 is the contracted channel width; g is the acceleration due to gravity; τc is the critical shear stress of the soil (obtained from an EFA test)
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From page 63... ...
Indeed, the bridge is usually a fairly narrow contraction and the maximum contraction scour depth in this case can be downstream from the bridge site. Based on the flume test observations, the maximum contraction scour generally Z Z V gBunif Fr where Frmax .
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From page 64... ...
Transition angle effect on maximum contraction scour depth. Figure 7.20.
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From page 65... ...
Transition angle effect on the location of the maximum scour depth. Figure 7.22.
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From page 66... ...
; Line CD is the transition from the maximum contraction scour depth to the uniform contraction scour depth.
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From page 67... ...
Cont L Hec ( ) = × − ≥ θ τ ρ 1 90 1 49 0 7 20 1 0 5 1 1 3 1 67 depth of scour along the centerline of the contracted channel, Xmax is the distance from the beginning of the fully contracted section to the location of Zmax, V1 is the mean velocity in the approach channel, VHec is the velocity in the contracted channel given by HEC-RAS, B1 is the width of the approach channel, B2 is the width of the contracted channel, τc is the critical shear stress as given by the EFA, ρ is the mass density of water, n is Manning's Coefficient, H1 is the water depth in the approach channel, Kθ is the correction factor for the influence of the transition angle as given by Equation 7.24 below, and KL is the correction factor for the influence of the contraction length as given by Equation 7.25 below.
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