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<br /> <br />. <br /> <br />4S <br /> <br />3.6.1 <br /> <br />Flow Confined to the Channel <br /> <br />. <br /> <br />Consider a situation where a normal river channel is narrowed by a <br /> <br />contraction. Scour due to the contraction may be determined in the following <br /> <br />. <br /> <br />manner (Nordin, 1971). The approach flow depth Y 1 and average approach <br />flow velocity VI result in the sediment transport rate qsl (Equation <br />3-1). Total transport rate to the contraction is Wlqsl in which WI is <br />width of the approach channel. If the water flow rate Q = wlqsl in the <br />upstream channel is equal to the flow rate at the contracted section, then <br /> <br />. <br /> <br />by continuity, <br /> <br />W, <br /> <br />'----- <br /> <br />. <br /> <br />WI <br />"'2 = W ql <br />2 <br /> <br />-- <br /> <br />W"2- <br />-------- <br /> <br />(3-2) <br /> <br />Here, ql = YlVl and q2 = Y2V2, <br /> <br />and the subscript 2 refers to conditions <br />J <br /> <br />in the contracted section. The sediment transport rate at the contracted <br /> <br />. <br /> <br />section after equilibrium is estahlished (i.e., the sediment transport rate <br /> <br />at the contracted section is equal to the sediment supply from the approach <br /> <br />channel) must be <br /> <br />qs2 <br /> <br />WI <br />::; W qsl <br />2 <br /> <br />v <br />-w. v <br />~3-=-3) <br /> <br />. <br /> <br />Knowing q2 <br /> <br />and <br /> <br />qs2' <br /> <br />Y2 <br /> <br />and <br /> <br />V2 <br /> <br />can be determined using q2 = Y2V2 <br /> <br />. <br /> <br />and the sediment transport equation (Equation 3-l). Depth of scour due to <br /> <br />the contraction can be determined as <br /> <br />Y =Y -Y <br />s 2 1 <br /> <br />(3-4) <br /> <br />3.6.2 Overbank Flow with Flow in the Channel <br /> <br />Laursen (1960) developed an equation for scour at a contraction where, <br /> <br />in addition to channel flow, there is overbank flow concentrating the con- <br /> <br />tracted channel (designated by subscript 2). The equation to predict depth <br />