1988-04-18_PERMIT FILE - C1981008A (2)

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DESIGN ANALYSIS GENERAL Sedimentation Pond 002 was evaluated and redesigned to meet or exceed the design and performance requirements of the CMLRD while utilizing as much of the existing structure and topography as was possible. Pond 002 was designed to adhere to the following minimum specifications: 1. Provide storage for the runoff from the 10-year, 24-hour precipitation event without having any discharge through the emergency spillway (0.26 ac-ft.). 2. Provide an emergency spillway capable of passing runoff from the 25-year, 24-hour storm event (0.33 ac-ft. ). 3. Provide adequate storage for the sediment generated from the 10-year, 24-hour (MUSLE) storm event (2.0, 10-year, 24-hour MUSLE storm events or 0.049 ac-ft. ). 4. Provide a nonclogging dewatering device and principal spillway that will provide adequate detention time for the 10-year, 24-hour storm event and still allow a method to dewater the pond to the 100 percent sediment storage level in a reasonable amount of time. HYDROLOGY The hydrology of the watershed was analyzed by utilizing the SEDIMOT II computer modeling program. In performing this analysis, the characteristics of the minesite and reclamation procedures along with onsite sediment control procedures were considered. Addendum A, B, and C contain complete listings of the SEDIMOT II inputs and outputs for the 10-year, 25-year, and 100-year design storms, respectively. Table 1 is a summary of these results. Input consists of precipitation amount (inches) , watershed area (acres) , time of concentration (hours) , runoff curve number (CN) , representative particle-size distributions of soils in the area, sedimentology information, spillway information, etc. The drainage area for Sediment Pond 002 was determined from a 1" = 400' topographic map (see Exhibit 13-4) . 2