2013-07-29_PERMIT FILE - C1982056 (4)

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September 2009 Foidel Creek Mine * Refuse Disposal Area Expansion — Surface Water Control Plan * Page 3 • Temporary ditches were designed to control runoff from the 10 -year, 24 -hour storm event. Because the postmine channels, including the bench ditches, will be permanent structures left in place at mine closure, the postmine channels were designed to control runoff from the 100 -year, 24 -hour storm event. The channel utility sub - routine contained within SEDCAD4 was used to design the temporary ditches and postmine channels. The design methodology used by SEDCAD4 is based on the limiting permissible velocity method. If the actual design is less than the permissible velocity, the channel is stable. Two design types were evaluated: • erodible channel • riprap channel Channel sections with shallow slopes or low flows are evaluated as erodible. Channel sections with steeper gradients or higher peak flows were stabilized with rock riprap. In accordance with Rule 4.05.3, all temporary ditches and postmine channels were designed with a minimum freeboard of 0.3 feet. Rock riprap size was determined using the PADER (Pennsylvania Department of Environmental Resources, Stover, 1990) method. Given the uncertainty in determining rock D50 for steeper slopes, an additional 3- inches were added to the modeling result recommended D 5o size. Channel designs are summarized in Table 2, Channel Design Summary for the Refuse Disposal Area 20MMYard Expansion. Hydrologic information in the table includes: • . the design event peak flow (cubic feet per second) • channel gradient (percent) • design velocity • flow depth Minimum channel design information includes: • channel shape (trapezoidal or triangular) • bottom width (trapezoidal only) • side slopes (14:1 V) • design depth • channel lining (bare soil or rock riprap) 4.0 Culvert Designs Table 3, 20MM Yard Expansion Culvert Designs, presents the minimum design information for culverts necessary to route the runoff from the 20 million cubic yard expansion. Culvert locations are shown on Drawing C -101, Final Grading Plan and Drawing C -102, Initial Sequence Grading Plan. Design calculations are presented in Attachment B. The analysis assumed that the culverts will generally be no larger than 48- inches. Several of the designs calculated minimum pipe diameters larger than 48- inches; the conversion table included in Attachment B was used to size multiples for design calculations that exceeded 48- inches. • Hydro - Environmental Solutions, Inc. 5710` Street, Suite B P.O. Box 772996 Steamboat Springs, CO 80477 Revised 09/09