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• Pond 010 is the only pond in the amendment area which will route the 25 year storm through the <br />emergency spillway. This is because the terrain in this area involves significant cut to build the <br />storage volume needed. The stage storage relationship forthe pond shows that the pond does hold <br />the three year sediment volume and the inflow from the 10 year 24 hour event. <br />The emergency spillway will be trapezoidal with a 10 feet wide bottom, rip-rap lining of D50 of 12 <br />inches and a principal spillway pipe of 24 inch diameter. The following page shows the layout of <br />the SCS TR-20 program which will route the 25 year storm of 2.40 inches. In this layout, the <br />watershed is divided into 4 subwatersheds with the respective areas, times of concentration and <br />curve numbers shown. These areas are seen on Map 2.05.3-12. The curve numbers have been <br />selected to provide a worst case scenario over the life of the operation. The southernmost <br />watershed in the basin (145.7 acres) must be routed through a reach to get to the sediment pond. <br />TR-20 allows reaches where the length of the reach is input together with two factors "x" and "m" <br />which define the characteristics of the reach channel. <br />These factors are based on the formula for the reach: Q = x A'" where A =cross-sectional area <br />of the channel in sq. ft. and Q = flow in cfs. A number of different areas and flows are gathered for <br />the same cross-section and the resulting data is fit to an exponential curve to obtain m and x. In <br />the case of Pond 010, the reach involved is actually Collection Ditch C5 which is the trapezoidal <br />ditch of 10 feet bottom width. Using a slope of 1 % and a manning's n Of 0.028, a series of flows and <br />. areas were developed from Manning's equation to get: <br />m = 1.43 <br />X = 1.46 <br />The reach length is 633 feet. As is seen from the TR-20 printout, the maximum flow in the <br />emergency spillway is 6.09 cfs, which corresponds to an elevation of approximately 0.4 feet water <br />flow height. Since the top of dam is 5620.0 feet and the bottom of the emergency spillway is 5618.5 <br />feet, the amount of freeboard at peak flow is still 1.1 feet, which is more than adequate. The level <br />of water at the start of the 25 year storm is 5612.2 feet which is 0.3 feet below the principal spillway <br />inlet. In the case of the initial water level at 5612.5 (the level of the principal spillway inlet), the <br />output change is insignificant to the peak flow in the emergency spillway. <br />lJ <br />(Revised 12!00) Attachment 2.05.3(3) -10-40 <br />