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.. <br />ATTACHMENT D <br />• <br />Hydrologic Protection <br />Design Specifications <br />The designs of the ditches and sedimentation pond were prepared <br />using the procedures developed in the Soil Conservation Service <br />publication "Urban Hydrology for Smal l Watersheds° Technical <br />Release No. 55(1975), hereafter referred to as TR-55. Design <br />rainfall magnitudes were obtained from. NOAA Atlas. 2 "Precipita- <br />tion Frequency Atlas of the United Statesa, Volume III, Colorado <br />(1973) for the location of the Mt .. Gunnison Mine facilitry. area. <br />The two design rainfall magnitudes utilized are: <br />10 yr., 24 hr. precipitation 1.8 inches <br />25 yr., 24 hr. precipitation s 2.2.inches <br />A71 ditches and diversions were designed to accomodate the peak <br />discharge from a 10 year, 24 hour .precipitation event with non- <br />. erosive velocities. Triangular channel designs will be used for <br />all ditches.' This design is proposed for ease of construction. <br />Overflow from the sediment pond through the top of the 24-inch <br />riser shown on Attachment C was designed to accamodate the peak :.. <br />discharge from a 25 year, 24 hour precipitation event. The <br />design procedures and specifications for the sediment control <br />structure follow: <br />A. Diversion of undisturbed runoff <br />1. A water bar across the U.S. Steel road and a diversion <br />ditch on a slight grade from the water bar to the road <br />ditch for Highway 133 will divert undisturbed runoff <br />• around the laydown area. The undisturbed area above this <br />D-1 <br />:~ _ <br />