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<br />3.3. Top of Dam. <br /> <br />3.3.1. Level Crest. For a dam with a level crest, the <br />discharge over the top of the dam is computed by the weir flow equation: <br /> <br />Q = CLHE <br /> <br />(22) <br /> <br />where: <br /> <br />Q = discharge in cfs <br />C = weir coefficient, typically 2.6 to 3.1 <br />L = effective length of dam crest in feet <br />H = head or depth of water over dam crest in feet <br />E = exponent, typically 1.5 <br /> <br />3.3.2. Non-Level Crest. Critical flow over a non-level dam crest can be <br />computed from crest length and elevation data. The crest is divided into rectangular <br />and trapezoidal sections with the flow computed through each section and composited <br />for the entire crest. For a rectangular section, critical depth, d , is: <br />o <br /> <br />d = % * Hm <br />o <br /> <br />(23) <br /> <br />where: <br /> <br />Hm is the available specific energy which is the depth of <br />water above the bottom of the section. <br /> <br />For a trapezoidal section, the critical depth is: <br /> <br />d 0 = % (Hm + Y. * l:1.y) <br /> <br />(24) <br /> <br />where, l:1.y is the change in elevation across the section. Flow area, A, <br />is computed as T * d 0 for rectangular sections and as V. T (2d 0 - l:1.y) for trapezoidal <br />sections, where T is the top width. The flow through the section is computed as: <br /> <br />Q = (A 3 * oj .5 <br />T <br /> <br />(25) <br /> <br />where, g is the acceleration due to gravity. The total flow over the top <br />of dam is then the sum of flows through each section. <br /> <br />7-83 <br />