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<br />ys = Yz - Y, Average scour depth <br /> <br />Coefficients Laursens equation <br />Bed material is sand with D50 =0.30 mm. = 0.00098 ft <br />Fall velocity (w) = 0.13 ft/s <br />Average Shear stress = 62,4 X 8,64 X 0.00076 = 0.41 Ib/ftZ <br />V,c = (0.41/1.94)0.5 = 0.46 ft/s <br />V./w = 0.41/0.13 = 3.50 The mode of bed material transport <br />ii mostly by suspension . Therefore, <br />K, = 0.69 and Kz = 0,37 <br /> <br />Yz = (43.600)6/7 (398)0.69 (0,024)0.37 = 1.59 <br />9 25,890 389 0,024 <br /> <br />Y = 14.3 - 9 = 5,3 ft <br />s <br /> <br />Comments <br /> <br />This amount of contraction scour may be unacceptable because: <br />1. This amount of contraction scour plus the local scour <br />could place the foundations (footings or pile caps and piles) <br />too deep. <br /> <br />2. The bed material that would be scoured out will <br />deposit downstream, either in the channel or on the floodplain. <br />If deposited in the channel, it could increase flooding. <br /> <br />Solutions would be to set the abutments back from the channel. <br />Another possibility would be a relief bridge. <br /> <br />A relief bridge to decrease the flow through the bridge would <br />decrease the ~ontraction scour further. However a relief bridge <br />would be very costly. <br /> <br />will accept this amount of contraction scour. In problem 2 will <br />calculate the discharge needed through the bridge opening to reduce <br />the contraction scour to 2 ft. <br /> <br />65 <br />