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between the current tailings pool level and the embankment crest to contain the entire <br />volume of runoff from the PMP event on the tailings surface while maintaining adequate <br />freeboard. The TDA spillway design takes advantage of this attenuation capacity by <br />incorporating a weir to control the rate of outflow from the TDA surface. This weir <br />controls the peak flow rates entering the spillway, thereby reducing the potential for <br />erosion damage to downstream areas. Table 4 summarizes the storage volume, water <br />surface elevation, inflow and outflow of the tailings pond in response to the design storm. <br />The results on Table 4 show the estimated peak runon onto the TDA during the design <br />storm event of 7881 cfs. The peak flow through the TDA reclamation spillway is <br />reduced to approximately 427 cfs. The resulting water pond on the TDA surface reaches <br />a maximum elevation of 8,610.3 feet during the PMP. The pond level recedes to an <br />elevation of 8,605.7 feet within 24 hours, with less than 1 foot of water remaining on the <br />TDA surface after 2 days. Topographic mapping of the TDA shows the dam crest <br />elevation varying from 8618 to 8620, indicating 8 to 10 feet of freeboard being <br />maintained during the PMP event for the reclamation design. <br />C.2.2 Design/Hydraulic Analysis <br />Using the hydrograph output information from HEC-HMS, flow depths and velocities in <br />the three spillway reaches and associated downgradient structures were analyzed using <br />Manning's equation (U.S. Army Corps of Engineers, 1991, Chow, 1959). Figure 2 <br />shows the locations of these proposed structures, consisting of a spillway, stilling basin <br />and outlet weir for the tailings pond. The design and detailed analyses of these structures <br />is presented below. <br />C.2.2.1 Outlet Weir <br />The outlet weir has been designed to control the outflow of water from the TDA during <br />extreme precipitation events. This structure will act to reduce the rate at which water is <br />released from the tailings impoundment during extreme precipitation events, thereby <br />reducing the size of the spillway required, and potential for damage to downstream areas.