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ATTACHMENT D <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 Sma11 Watersheds" Technical <br />Release Na. 55(1975), hereafter referred to as TR-55. Design <br />rainfall magnitudes were obtained from NOAH Atlas 2 "Precipita- <br />tion Frequency Atlas of the United States", Volume III, Colorado <br />(1973) for the location of the Mt. Gunnison Mine facility area. <br />The two design rainfall magnitudes utilized are: <br />10 yr., 24 hr. precipitation = 1.9 inches <br />• 25 yr., 24 hr. precipitation 2.2 inches <br />A11 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 accomodate 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 />