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REVISED <br />4/15/85 <br />SEDIMENTATION POND DESIGN <br />The sediment control facilities to be constructed and utilized at the <br />Fish Creek Tipple have been designed in accordance with the Regulations <br />of the Colorado Mined Land Reclamation Board for Coal Mining, MSHA Rule <br />30 CFR 77.216-2, and good engineering practices. These facilities <br />consist of an existing sedimentation pond, a detention basin, sumps, <br />diversion ditches, culverts, and other sediment control measures. The <br />existing facilities will be utilized to control runoff and sediment from <br />disturbed areas. All facilities are designed to convey, contain or <br />treat the runoff resulting from a 10-year, 24-hour precipitation event. <br />rnFTUnnrn nrv <br />Rainfall Parame <br />Rainfall information for the State of Colorado is published by the <br />• National Weather Service in a volume titled NOAA Atlas 2, <br />Precipitation-Frequency Atlas of the Western United States. The <br />publication contains maps giving total rainfall depths for the 2 through <br />100-year frequencies and the 6-hour and 24-hour durations. For the <br />24-hour duration, the 10-year rainfall total is 1.8 inches, the 25-year <br />total is 2.1 inches, and the 100-year total is 2.6 inches. <br />Runoff Parameters <br />Total runoff and peak flow rates are determined using the procedure <br />outlined in Chapter 10 of the National Engineering Handbook, Section 4, <br />Hydrology (SCS, 1972). The SCS method utilizes two parameters which are <br />characteristic of an individual watershed; the antecedent moisture <br />condition (AMC) and the curve number (CN). The AMC-II parameter is <br />considered applicable to average design conditions of soil moisture and <br />infiltration. The CN parameter was determined for each sub-watershed <br />using an area-weighted calculation. Values of CN were chosen using <br />• tables of Hydrologic Soil Group Classifications and Curve Numbers (SCS, <br />1977). <br />-2- <br />