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<br />46 <br /> <br />. <br /> <br />of flow at section 2 is <br /> Y2 _ Q W n <br /> ( --.!. ) 6/7 ( .-!. ) cl ( -2 )c2 <br /> Yl - Qc W2 nl <br />where, c1 = 6(2 + f) <br />7(3 + f) , <br /> 6f <br /> c2 7(3 + f) , <br /> Qc = the approach channel flow rate, <br /> <br />. <br /> <br />(3-5) <br /> <br />. <br /> <br />Qt = the contracted channel flow rate that is greater than the approach <br />channel flow rate by the amount of flow on the floodplain, <br /> <br />. <br /> <br />Y = the flow depth, <br />W = the channel width, <br />n = the Manning's roughness coefficient, <br />exponent f is given below <br /> U*/V f <br /> s <br /> < 0.5 0.25 <br /> 1.0 1.00 <br /> > 2.0 2.25 <br /> <br />. <br /> <br />. <br /> <br />Here, U* = the shear velocity (/T/P ) in the approach channel, <br /> <br />. <br /> <br />v = the fall velocity of the bed material. <br />s <br /> <br />3.6.3 Overbank Flow Only <br /> <br />For scour at bridges on a floodplain where there is no sediment trans- <br /> <br />. <br /> <br />port upstream, Laursen (1963) proposed that <br /> Y2 WI 6/7 V 2 )3/7 <br /> ( 1 <br /> ~- IV) ( Y l/3 2/3 <br /> 2 l20 1 DSO <br /> <br />(3-6) <br /> <br />. <br /> <br />where, Yl = depth of the approach channel, <br /> <br />VI = velocity of the approach channel, <br />WI width of the approach channel, <br /> <br />. <br /> <br />. <br />