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• MOFffAT PERMANENT <br />This section contains an evaluation to determine if the Moffat sediment pond's <br />spillways are sized adequately to safely pass the design storm event specified <br />in the permanent impoundment performance standards. The computer model <br />SEDCAD+ Version 3.1 was used to model the Moffat watershed and Moffat <br />Permanent Impoundment and predict the hydrologic outputs. The model indicates <br />that the West Ridge spillways are adequately sized to safely pass the <br />permanent impoundment design storm event. <br />Deeiga storm <br />A 25-year, 24-hour storm event was utilized to demonstrate that the <br />impoundment's emergency spillway is adequately sized. This storm event is <br />recommended in Table 4 - Minimum Spillway Design Storm from the publication, <br />Ponds -- Planning, Design, Construction, Soil Conservation Service's <br />Agriculture Handbook Number 590. This publication recommends using a 25-year, <br />24-hour storm event for an impoundment which has: a drainage area greater than <br />20 acres, an effective dam height less than 20 feet, and less than 50 acre- <br />feet of storage capacity (such as the Moffat impoundment). The Routt County, <br />• Colorado 25-year, 24-hour precipitation event is 2.04 inches. The storm type <br />utilized for this design is SCS Type II. The Moffat impoundment is shown as <br />on the attached SEDCAD+ input/output sheets. <br />Impouadmeat Location <br />The Moffat Permanent Impoundment is located in the NW1/4, SW1/4, Section 31, <br />TSN, R85W, Routt County, Colorado. <br />Draiange Area <br />The total drainage area for this proposed permanent impoundment is <br />approximately 470 acres. The general location is shown on the Exhibit 4.6-1 <br />(as S(2,1,1) of J2). <br />Lead IIae <br />The postmining land uses for the drainage area of this impoundment are <br />rangeland and wildlife habitat. <br />Ruaoff and Dincharge Estimates <br />The peak discharge rate for the 25-year, 24-hour storm event in the Moffat <br />watershed is predicted by SEDCAD+ to be 157.84 cfs. The predicted peak rate <br />of discharge for the impoundment will be 16.13 cfs. The SEDCAD+ storm event <br />• <br />Technical Revision 32 4.6R-1 September 5, 1995 <br />