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s _~_ • <br />3. Sediment Fond No. 1 <br />From the design drawing presented, it is obvious that, with any flow throuyh <br />the emergency spillway, the freeboard requirements of 4.05.6(8)(d), one foot <br />of freeboard under passage of the design event, are not met. This is required <br />of all existing as well as proposed ponds. However; WF has assunnd that the <br />peak outflow will be equal to the peak inflow, neglecting any flood attenuation <br />due to storage in the pond. This results in an extremely conservative design. <br />It was also noted that the spillways of this and other ponds were designed <br />using Manning's equation. Spillways more approximate weir flow and the weir <br />equation Q = CLH3/z is more appropriate for design and provides the advantage <br />of quickly determining depth of~flow. <br />Several options are available to WF to show that the existing pond will pass <br />the 25-year event. They are: <br />A. Route the flood through the pond assuming that the gate on the <br />principal spillway is always open. Flow through the principal spi11- <br />way will then experience changing control from weir to orifice to, <br />finally, pipe flow. The equations for these flows are given below: <br />1) [Jeir flow <br />Sharp Crested Weir: 3.3 LH 1.5 _ ~ <br />where H = head over crest of pipe and <br />L = ci rcumference of pipe <br />2) Orifice Flow <br />Sharp Edged Orifice: 0.GA (Zg!!)'s = Q <br />where [[ = head over crest of pipe, and <br />A = cross-.section area of pipe, and <br />g = gravitational constant 32 fps2 <br />3) Pipe Flow <br />A ( 2ylf )0.5 = g <br />1+Ke+Kb+KcL <br />where Ke = coefficient of entrance loss <br />Kb = coefficient of bend loss <br />KC = coefficient of friction Loss <br />in general: <br />Ke 0.9 <br />xb = o.s <br />Kc = 5087 n2/d4/3 = app. 0.22 from 29" CMP <br />