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<br />2. Estimatinq Contraction Scour, <br /> <br />a, CASE 1. <br /> <br />CONTRACTION SCOUR, OVERBANK FLOW BEING FORCED <br />BACK INTO THE MAIN CHANNEL. (Live-bed scour) <br /> <br />For case 1a and 1b use Laursen's 1960 equation (8) for a long <br />contraction to predict the depth of scour in the contracted <br />section. This equation was given in Chapter 2. It assumes that <br />bed material is being transported in the main channel, but not in <br />the overbank zones. <br /> <br />Y2 = ( Ome2) ~ ( We1) K, (.:2) k, <br />Y1 Omel Wc2 n1 <br /> <br />(1) <br /> <br />ys = yz - y, (Average scour depth) <br /> <br />Where: <br /> <br />y, <br />yz <br />We' <br />WcZ <br />Qmc' <br /> <br />QmcZ <br /> <br />nz <br />n, <br /> <br />V. jw <br />e <br /> <br />= average depth in the main channel <br />= average depth in the contracted section <br />= bottom width of the main channel <br />= bottom width of the bridge section <br />= flow in the approach channel that is transporting <br />sediment <br />= flow in the contracted channel. Often this is <br />Qtotal but not always, <br />= Manning n for contracted section <br />= Manning n for main channel <br /> <br />K, & Kz = exponents determined below <br /> <br />K, <br /> <br />Mode of Bed Material Transport <br /> <br />KZ <br /> <br /><0.50 <br />0.50 <br />to <br />2.0 <br />>2.0 <br /> <br />V'e <br /> <br />0.59 0.066 mostly coitact bed material <br />discharge ., <br />0.64 0.21 some susp nded bed material <br />discharge <br />0.69 0.37 mostly su pended bed material <br />discharge <br />= (gy,S,) 0.5, shear veloci t <br /> <br />w = fall velocity of D50 of ed material. (See Figure <br />4.2) <br />g = gravity constant <br /> <br />S, = slope, energ~ grade li ~ of main channel <br /> <br />37 <br />