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1.3 RETURN PERIOD/PRECIPITATION <br />A precipitation amount is required for the appropriate return period. The following return periods and <br />precipitation events are used for the Colowyo Mine: <br />10 -year, 24 -hour storm event 1.8 in <br />25 -year, 24 -hour storm event 2.3 in <br />100 -year, 24 -hour storm event 2.7 in <br />1.4 HYDROGRAPH RESPONSE SHAPE <br />A unit hydrograph is chosen for each drainage area or sub -area modeled to predict the run -off response. <br />The hydrograph responses available in the SEDCAD4 model are slow, medium, and fast. A slow response <br />corresponds to a forested area or an area with a number of obstructions. A fast response corresponds to an <br />un- vegetated or poorly protected area. Medium and fast hydrograph responses are considered as most <br />representative for mine areas. <br />The internal convolution increment is 0.05 hours, and values are saved at the user - specified interval of 0.1 <br />hours or greater. A convolution increment of 0.1 is specified. It should be noted that a time of <br />concentration less than 0.125 hours bypasses the unit hydrograph technique and instantaneous run -off is <br />assumed. As explained in the SEDCAD4 User's Manual (Schwab and Warner, 1998), the time of <br />concentration restriction of 0.125 hours is dictated by a combination of array size restrictions, minimum <br />internal convolution interval, and the user - specified time increment for saving convolution values. <br />1.5 DRAINAGE BASIN AREA <br />Drainage basin areas are determined for each pond from scaled topographic drawings. In many cases <br />determination of the "worst case" condition with respect to volume of runoff has to be determined by <br />examining several stages of the mining operation. The final post- mining topography would usually be <br />expected to produce the largest volume or runoff as the entire watershed is contributing to the runoff by <br />this stage, and is always evaluated but may not control the design. Watershed (or sub - watershed) areas are <br />delineated consistent with four general categories representing various stages of reclamation and <br />disturbance. The watershed or sub - watershed delineations reflect anticipated disturbance /reclamation <br />status based on Colowyo's current mining and reclamation plans. <br />1.6 TIME OF CONCENTRATION, Tc <br />The time of concentration is calculated using the SCS upland method (Soil Conservation Service, 1986) as <br />incorporated into the SEDCAD software. The hydraulic lengths, drainage heights and associated slopes <br />are determined from topographic drawings. The values calculated for each structure are shown on a sub - <br />watershed basis in the SEDCAD4 printouts. <br />1.7 MUSKINGUM ROUTING PARAMETERS, K, X <br />The Muskingum Routing Parameters are also calculated using the SCS upland method. The hydraulic <br />lengths, drainage heights and associated slopes are determined from topographic drawings. The values <br />calculated between each structure and /or sub - watershed are shown in the SEDCAD4 printouts. <br />1.8 CURVE NUMBER, CN <br />The runoff curve number is a factor relating the amount of rainfall to the amount of runoff for a given area. <br />Curve numbers were jointly developed and agreed upon by the Division of Minerals and Geology and <br />Colowyo. <br />The curve number for the undisturbed lands is selected as Hydrologic Soil Group (HSG) "C" based on the <br />pre- mining mapped soils in the county SCS soil maps. <br />Exhibit 7 Page 2 Revision Date: 3/18/13 <br />Revision No.: TR -95 <br />