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' The pond size required to retain the volume from the <br />10-yr, 24-hr storm has been calculated using the SCS CN <br />method. The calculations are attached as Exhibit A and <br />indicate that a runoff .volume of 0.22 acre-feet (AF) is <br />required. In addition, a volume for sediment of 0.03 AF has <br />been allowed for. Sediment volume has been calculated using <br />the modified Universal Soil Lose Equation as shown in Exhibit <br />B. Total pond volume required is then 0.25 AF. <br />Peak discharges for the 10-yr,24-hr and 25-yr,24-hr <br />storms were calculated using SCS methods (see Exhibit A). The <br />peak discharges are 4.1 cfs for the 10-yr event and 5.7 cfs <br />for the 25-yr event. <br />The existing pond is not large enough to hold the <br />required volume of water and sediment and still meet the <br />requirement of one foot of freeboard between the maximum water <br />surface (25-yr flow over the emergency spillway) and the top <br />of the embankment. This is seen from the calculations below: <br />Emergency spillway: E1. 97.67 <br />Depth of flow: <br />Q = 5.7 cfs <br />Spillway width = 12 feet <br />Discharge coefficent = 2.5 (broad weir) <br />H = (5.7/(2.5 x 12))10.67 <br />H = 0.33 feet <br />Maximum water surface: <br />Spillway crest: E1. 97.67 <br />Depth of flow: 0.33 <br />Max. WS E1. E1. 9~-b0 <br />Freeboard required: 1 foot <br />Top of embankment required: E1. 99.00 <br />