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COLON/YO COAL COIvIPANY <br />bVORK AREA CRAI~IAGE PLAN <br />IN?RGCUCTiON <br />This report describes the drainage pion developed fora 137.2° ccre tract of land <br />located near Axial, Coloredo. The parcel includes the •.vork area used by the Colowyo <br />Coal Company in connection with its surface mining and reclamation operctions. <br />The essential features of the proposed drainage rystem are shown on Figure 1. <br />Pertinent design criterie end procedures are described in the "Summary" section of this <br />report; supporting calculations are included in the Appendix. <br />SUMMARY <br />A. General Design Criteria <br />~~_.~_~ ,:eguie;ions pertcining to Surfcce Coci i+lining and Reclamation Operations <br />require ther sedimentation ponds be of sufficient volume that the following criteria ere met: <br />a) sedimentation ponds must provide a theoretical detention time of not less <br />. ,_ <br />than twenty-.rou: ,curs for r,~z water inflow cr r~-.;== `-^~^ a 10-; ecr, <br />24-hour precipitation event. <br />b) such ponds must provide a minimum sediment storage volume equcl to the <br />accumulated sediment volume from the drainage area to the pond for e <br />minimum of 3 years as determined using the Universal Soil Loss.Equotion. <br />In addition, Federal Regulations require that "an appropriate combination of principal and <br />emergency spillways be provided to safely discharge the runoff from a 25-year, <br />24-hour precipitation event ." <br />Design rainfall figures for the project ores were obtained from Reinfal! Frequency <br />Maps contai.^.ed in the USDA Soil Conservation Service mcnual entitled Procedures for <br />Determining Peak Flaws in Coloredo. Rainfall amounts for two specific durations and <br />recurrence intervals were determined as follows: <br />10-year, 24-hour precipitation -- 1 .75 inches <br />25-year, 24-hour precipitation -- 2.00 inches <br />