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C.2.2.3 Stilling Basin <br />A stilling basin will be required at the downstream end of Reach 2 due to the high <br />velocities that will be developed in this section of channel during extreme storm events. <br />This stilling basin will provide energy dissipation between Reaches 2 and 3, and prevent <br />erosion damage to the downstream portion of the spillway. For reclamation cost <br />estimating purposes, it was assumed that the stilling basin will be a concrete-lined basing <br />with an 8-foot bottom width, a depth of 5 feet, and a length of approximately 10 feet. <br />C.3.0 WEST PIT PONDS STORMWATER DESIGN <br />C.3.1 West Pit Watershed Analysis <br />Modifications to existing channels and new stormwater channels are required in the West <br />Pit Area to divert stormwater around the West Pit Water Management Ponds as shown on <br />Figure 3. In order to design the proposed modification to the facilities, it is necessary to <br />estimate the storm water response of the upgradient terrain and corresponding data. To <br />determine the storm water response, information pertaining to basin characteristics <br />presented in previous documents such as Technical Revision 29 (BMRI, 2000) was used. <br />Additionally, the subbasins of the watershed were delineated to reflect proposed changes <br />to the local topography, including construction of the holding ponds as shown in Figure <br />G.2-4. <br />HEC-HMS (U.S. Army Corps of Engineers, 2006) was used to model the storm water <br />response of the subbasins shown in Figure 3. The subbasin characteristics used in storm <br />water modeling are shown in Table 5. To determine lag time for subbasins, the travel <br />time for each flow segment was determined, combined and then multiplied by a factor of <br />0.6 (Haan, et al, 1994). Sheet flow was assumed for the first 300 feet, and then shallow <br />flow was assumed until intercepted by a diversion channel (Haan, et al, 1994). The <br />parameters used in these calculations are presented in Table 6. The coefficients used to <br />determine sheet and shallow flow velocities are presented in Table 3 (Haan, et al, 1994). <br />The 100 year, 24-hour storm was selected as the design storm event for the stormwater