2018-08-08_PERMIT FILE - C1982056 (17)

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REVISED 7/30/85 SEDIMENTATION POND DESIGN The sediment control facilities to be constructed and utilized at the Fish Creek Tipple have been designed in accordance with the Regulations of the Colorado Mined Land Reclamation Board for Coal Mining, MSHA Rule 30 CFR 77.216-2, and good engineering practices. These facilities consist of an existing sedimentation pond, a detention basin, sumps, diversion ditches, culverts, and other sediment control measures. The existing facilities will be utilized to control runoff and sediment from disturbed areas. All facilities are designed to convey, contain or treat the runoff resulting from a 10 -year, 24-hour precipitation event. MGTunnnl nr_v Rainfall Parameters Rainfall information for the State of Colorado is published by the National Weather Service in a volume titled NOAA Atlas 2, Precipitation -Frequency Atlas of the Western United States. The publication contains maps giving total rainfall depths for the 2 through 100 -year frequencies and the 6 -hour and 24-hour durations. For the 24-hour duration, the 10 -year rainfall total is 1.8 inches, the 25 -year total is 2.1 inches, and the 100 -year total is 2.6 inches. Runoff Parameters Total runoff and peak flow rates are determined using the procedure outlined in Chapter 10 of the National Engineerinq Handbook, Section 4, Hydrology (SCS, 1972). The SCS method utilizes two parameters which are characteristic of an individual watershed; the antecedent moisture condition (AMC) and the curve number (CN). The AMC -II parameter is considered applicable to average design conditions of soil moisture and infiltration. The CN parameter was determined for each sub -watershed using an area -weighted calculation. Values of CN were chosen using tables of Hydrologic Soil Group Classifications and Curve Numbers (SCS, 1977). -2-