2010-05-07_PERMIT FILE - C2009087A (39)

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3.0 Design Analysis • This sump system was designed to meet or exceed the applicable design and performance requirements of the Regulations of the Colorado Mined Land Reclamation Board for Coal Mining. Specifically, the designs adhered to the following requirements. • Provide for an adequate sediment storage volume and periodic sediment removal. • Provide a non - clogging dewatering device or conduit spillway to remove water storage resulting from inflow in accordance with Rule 4.05.6(3)(b). • Provide a spillway system for either a combination of principal and emergency spillway, or a single spillway configured as required by Rule 4.05.9(2)(a), capable of passing runoff from the 25 -year, 24 -hour storm event in accordance with Rule 4.05.9(2)(c)(ii). • If a combination of principal and emergency spillway are designed and constructed then there shall be no out -flow through the emergency spillway during passage of runoff from the 10 -year, 24 -hour storm event in accordance with Rule 4.05.9(2)(b). Coal Stockpile Sumps are upstream of Pond 002 (Wadge Impoundment). The sump system serves as pre - settling basins to pre -treat the sediment loading from the South Surface Facilities Area, with the exception of the Portal Area. The sumps will not be permitted under the Colorado Water Quality Control Divisions' (WQCD) Colorado Discharge Permit System (CDPS) and, consequently, there • are no effluent limitations (e.g., settleable solids concentrations) for the discharge from LCC. Pond 002 is the permitted CDPS structure for the drainage network designed for the South Surface Facilities Area. Since there will be no effluent limitations that apply to discharges from LCSS, Rules 4.05.2(7) and Rule 4.05.6(3)(b) are not applicable. 4.0 SEDCAD Demonstration The hydrology and sedimentology of the watershed were analyzed using the SEDCAD4 computer program (Schwab, 1998). Model methodology and assumptions are presented in Exhibit 2.05.3 -E2, Drainage and Sediment Control Plan. Addendum 2.05.3- E2.E -1 and 2.05.3- E2.E -2 of this appendix present the Coal Stockpile Sump System SEDCAD modeling demonstrations for the 10 -year and 25- year, 24 -hour design storm events, respectively. Inputs include precipitation (inches), watershed area (acres), time of concentration (hours), runoff curve number, representative particle size distribution of soils in the area, sedimentology information, spillway dimensions, etc. The drainage area for the watershed, as well as, slope lengths and distances for time of concentration estimates were measured from a permit topographic map (see Map 2.05.3 M4). • The UCSS design assumed that is a constant inflow of surge water from the Portal Area. The design assumed an inflow rate of 80 gallons per minute (0.35 acre -feet [ac -ft] per day). This results in a 2.05.3 -E2.E2 12/09