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' REVISED <br /> 7/30/85 <br />' <br />r~ SEDIMENTATION POND DESIGN <br />' Th <br />di <br />l f <br />t <br />t <br />ili <br />i <br />b <br />t <br />d <br />d <br />li <br />d <br />h <br /> e se <br />ro <br />men <br />con <br />ac <br />es to <br />e construc <br />ze <br />at t <br />t <br />e <br />an <br />uti <br />e <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 /> <br />i <br />METHODOLOGY <br /> <br /> Rainfall Parameters <br />' <br /> Rainfall information for the State of Colorado is published by the <br /> <br />' `"', National Weather Service in a volume titled NOAA Atlas 2, <br />Th <br />F <br />A <br />l <br />f <br />h <br />U <br />d St <br />P <br />i <br />i <br />W <br /> estern <br />nite <br />ates. <br />rec <br />p <br />tation- <br />requency <br />t <br />as o <br />t <br />e <br />e <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 /> <br /> -2- <br /> <br />