2007-12-19_REVISION - M1988112 (16)

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controls structures associated with the West Pit Ponds. The 100 year, 24-hour storm event for the area was determined to be 2.76 inches from the NOAA Atlas for Colorado (Miller, et al, 1973). This rainfall was distributed over time consistent with an SCS Type 2a storm. Figure 3 illustrates the final contributing up gradient watershed and subbasin boundaries as well as the longest overland flow path for each subbasin. C.3.2 Design/Hydraulic Analysis The kinematic wave method within HEC-HMS was used to analyze storm water routing over upland areas and through proposed down gradient structures. Using the hydrograph output information from HEC-HMS, flow characteristics of proposed down gradient structures were then analyzed using Manning's equation (U.S. Army Corps of Engineers, 1991). Figure 4 shows the locations of these proposed and existing structures, consisting of diversion channels and two detention ponds in the west pit area. The design and detailed analyses of these structures is presented below. C.3.2.1 Diversion Channel Design A network of diversion channels shall be constructed to route storm water flow from up gradient areas around the West Pit Ponds. Design flows based on the 100-year 24-hour SCS Type 2a storm, as determined using the HEC-HMS software, are presented in Table 7. The channels have been sized using Manning's and velocity head equations. Figure 4 shows the locations and configuration of the proposed diversion channels. C.3.2.2 West Pond The West Pond will be constructed by excavating into the existing pit backfill, and placement of a compacted earth fill perimeter berm as shown on Figure G.2-5. Approximately 12,000 cubic yards of excavated material will be used in construction of the perimeter berm, 10 feet high at its maximum section, resulting in a non jurisdictional impoundment according to SEO criteria. The pond interior will lined with a synthetic