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Pond 010 is the only pond in the amendment area which will route the 25 year storm through <br />the emergency spillway. This is because the terrain in this area involves significant cut to build <br />the storage volume needed. The stage storage relationship for the pond shows that the pond <br />does hold 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 <br />12 inches and a principal spillway pipe of 24 inch diameter. The following page shows the layout <br />of 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 <br />been 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 <br />pond. TR-20 allows reaches where the length of the reach is input together with two factors "x" <br />and "m" 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 <br />• for the same cross-section and the resulting data is fit to an exponential curve to obtain m and <br />x. In the case of Pond 010, the reach involved is actually Collection Ditch CS which is the <br />trapezoidal ditch of 10 feet bottom width. Using a slope of 1 % and a manning's n Of 0.028, a <br />series of flows and 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 <br />water flow height. Since the top of dam is 5620.0 feet and the bottom of the emergency spillway <br />is 5618.5 feet, the amount of freeboard at peak flow is still 1.1 feet, which is more than <br />adequate. The level of water at the start of the 25 year storm is 5612.2 feet which is 0.3 feet <br />below the principal spillway inlet. In the case of the initial water level at 5612.5 (the level of the <br />principal spillway inlet), the output change is insignificant to the peak flow in the emergency <br />spillway. <br />Attachment 2.05.3(3)6-3 <br />