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EDNA MINE CENTER RIDGE PERMANENT IMPOUNDMENT This section contains an evaluation to determine if the Center Ridge sediment pond's spillways are sized adequately to safely pass the design storm event specified in the Permanent Impoundment Performance Standazds. The computer model SEDCAD+ Version 3.1 was used to model Ccnter Ridge watershed and Center Ridge Permanent Impoundment, and predict the hydrologic outputs. The model indicates that the Center Ridge spillways are adequately siud to safely pass the permanent impoundment storm event Design Storm A 25-year, 24-hour storm event was utilized to demonstrate that the impoundment's emergency spillway is adequately sized. This storm event is recommended in Table 4 -Minimum Spillway Design Storm from the publication, Pondr -- Planning, Design, Construction, Soil Conservation Service's Agriculture Handbook Number 590. This publication recommends using a 25-yeaz, 24 hour storm event far an impoundment which has: a drainage area greater than 20 acres, an effective dam height less than 20 feet, and less than 50 acre-feet of storage capacity (such as the Center Ridge impoundment). The Routt County, Colorado 25-yeaz, 24-hotu precipitation even[ is 2.04 inches. The storm type utilized for this design is SCS Type II. The Center Ridge impoundment is shown as Structure 1 of Branch 1 of Junction 1 on the attached SEDCAD+ input/output sheets. • Impoundment Location The Center Ridge Permanent Impoundment is located in the SW 1/4 of the SE t/a of Section 12, TSN, R86W, Routt County, Colorado. Drainage Area The total drainage area for this proposed permanent impoundment is approximately 1,016 acres. The general location is shown on the attached 1" =400' watershed boundary map. Land Use The postmining land uses for the drainage azea of this impoundment arc rangeland and wildlife habitat. Runoff and Discharge Estimates The peak discharge rate for the 25-year, 24-hour storm event in the Center Ridge watershed is predicted by SEDCAD+ to be 163.25 cfs. The predicted peak rate of discharge for the impoundment will be 100.05 cfs. The SEDCAD+ storm event summary predicts that the peak stage will be 27.5 ft. From the SEDCAD+ Elevation-Area-Capacity-Dischazge table, this peak stage equates to a depth of flow through the emergency spillway of 0.9 feet. Additional assumptions and routing results aze shown on the attached SEDCAD+ computer printout. App. 4.6-0-1 September 7, 1993