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EDNA MINE <br />WEST RIDGE PERMANENT IMPOUNDMENT <br />This section contains an evaluation to determine if the West Ridge sediment pond's spillways are <br />sized adequately to safely pass the design storm event specified in the permanent impoundment <br />performance standards. The computer model SEDCAD+ Version 3.1 was used to model the West <br />Ridge watershed and West Ridge Pemrattent Impoundment and predict the hydrologic outputs. <br />The model indicates that the West Ridge spillways are adequately sized to safely pass the <br />permanent impoundment design storm event. <br />Design Storm <br />A 25-year, 24-hour storm event was utilized to demonstrate that the impoundment's emergency <br />spillway is adequately sized. This storm event is recommended in Table 4 -Minimum Spillway <br />Design Storm from the publication, Pondt -- Planning, Design, Construction, Soil Conservation <br />Service's Agriculture Handbook Number 590. This publication recommends using a 25-year, 24 <br />hour storm event for an impoundment which has: a drainage area greater than 20 acres, an effective <br />dam height less than 20 feet, and less than 50 acre-feet of storage capacity (such as the West Ridge <br />impoundment). The Routt County, Colorado 25-year, 24hour precipitation event is 2.04 inches. <br />The storm type utilized for this design is SCS Type II. The West Ridge impoundment is shown <br />• as Structure 1 of Branch 1 of Junction 1 on the attached SEDCAD+ input/output sheets. <br />Impoundment Location <br />The West Ridge Permanent Impoundment is located in the NEt/4 of the NWtla of Section l3, <br />T4N, R85-86W, Routt County, Colorado. <br />Drainage Area <br />The total drainage area for this proposed permanent impoundment is approximately 919 acres. The <br />general location is shown on the attached 1" =400' watershed boundary map. <br />Land Use <br />The postarining land uses for the drainage area of this impoundment are rangeland and wildlife <br />habitat. <br />Runoff and Discharge Estimates <br />The peak dischazge rate for the 25-year, 24-hour storm event in the West Ridge watershed is <br />predicted by SEDCAD+ to be 163.66 cfs. The predicted peak rate of dischazge for the <br />impoundment will be 68.38 cfs. The SEDCAD+ storm event summary predicts that the peak stage <br />. will be 12.6 ft. From the SEDCAD+ Elevation-Area-Capacity-Discharge table, this peak stage <br />equates to a depth of flow through the emergency spillway of 0.6 fat Additional assumptions <br />- and routing results ate shown on the attached SEDCAD+ computer printout <br />App. 4.6-N-1 <br />_September 7, 1993 <br />