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-2- <br />• 3. Sediment Pond No. 1 <br />From the design drawing presented, it is obvious that, with any flow'through <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 we 11 as proposed ponds. However, WF has assumed 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 = CLH3/2 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 spill- <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) Weir flow <br />Sharp Crested Weir: 3.3 LH 1 5 = Q <br />where H = head over crest of pipe and <br />. L = circumference of pipe <br />^ 2) Orifice Flow <br />Sharp Edged Orifice: 0.6A (ZgH)'S = Q <br />where H = 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 ( 2gH )0.5 = Q <br />1+Ke+Kb+XcL <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 />Kb = 0.5 <br />Kc = 5087 n2/d4/3 <br />= app. 0.22 from 24" CMP <br />• <br />