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<br /> Preci pitation and Storm Type. The precipitation amounts at the Yoast Mine <br /> area for the 10-year, 25-year and 100-year precipitation events were obtained <br /> from NOAA Pr ecipitation Frequency Atlas - Frequency Atlas of the Western <br /> United States - Volume II - Colorado (Miller, et al., 1973). A standard SCS <br /> Type II storm was utilized for each analysis. <br />• 10-year, 24-hour event: 1.7 inches <br />• 25-year, 24-hour event: 2.1 inches <br />• 100-year, 24-hour event: 2.5 inches <br />Sedimentology• To determine the sediment loading to the pond, the average <br />annual "R" method was used. The amount of sediment storage is determined for <br />a defined length of time and is based on the design storm R, design storm <br />sediment load, average annual R, and the number of storage years (Y) . R is <br />the rainfall erosivity factor. The equation for required sediment storage <br />is: <br />Required Storage = (Annual R)(# years)(Storm Sediment Load) <br />Storm R <br />The TR-37 SEDCAD4 demonstration used an annual R value of 10 versus the 2001 <br />as-built which used an R value of 30. This updated R value was obtained from <br />Figure 8A.1, Isolines of R factor for Western U. S., which was taken from <br />Design Hydrology and Sedimentology for Small Catchments (Haan, et. al., <br />1993). <br />A sensitivity analysis was conducted varying the number of storage years (Y) <br />to maximize sediment storage capacity and maintain compliance with the <br />applicable rules and regulations. The number of storage years (Y) was <br />increased in the Sedimentology sub-routine until the effluent settleable <br />solids concentration in Pond 014 discharge exceeded the water quality <br />criterion (0.5 milliliters per liter (ml/1]), or the model indicated there <br />was less than two (2) feet of storage room in the pond (the elevation <br />difference between calculated top of sediment storage and principal spillway <br />invert elevation), as recommended by Warner et al (1998). The SEDCAD4 model <br />results present the maximum sediment storage while retaining capacity to <br />store the design storm runoff and maintain compliance with applicable rules <br />and regulations. <br />Reclaimed Condition Storage Capacity. Results of the TR-37 SEDCAD4 <br />demonstration show the storm runoff volume for the entire watershed from the <br />10-year, 24-hour event to be 1.34 acre-feet (ac-ft). The following <br />summarizes Pond 013 storage capacity for the TR-37 10-year, 24-hour event <br />SEDCAD4 demonstration. <br />TR-37 13-17.1-3 Revised 02/08 <br />