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MORR150 N-KNUOSEN COMPANY. INC. <br />• <br />ENGINEERING REPORT <br />POWDERHORN COAL CO. <br />ACCESS ROAD CULVERTS <br />Morrison-Knudsen Company, Inc. was requested to review the design <br />of two existing culverts in the access road near the waste area. The <br />culverts are steel pipe arches 81" x 59". According to current M.L.R.B. <br />Rule 4.03.1(4)(e)(i) these culverts must pass a 20 year 24 hour peak flow. <br />Substantial work had been done by GEX Colorado, Inc. in determining <br />the Peak Discharge of the Coal Creek Drainage, October 1980. It is M-K's <br />understanding that these calculations were accepted by the M.L.R.B. The <br />• work was done on the basis of a 10 year 24 hour event. The procedure <br /> used was that published by the Soil Conservation Service. "Procedures <br /> for Determining Peak Flows in Colorado". M-K used the GEX work as a <br />starting point for its analysis. M-K did review the GEX work and accept <br />the work as being correct. <br />The 20 year 24 hour precipitation was determined by graphically <br />interpolating the 5 year, 25 year, 50 year and 100 year data contained in <br />Figure S-7 of the Procedures for Determining Peak Flows in Colorado. This <br />interpolation resulted in a 1.7 inch figure for the 20 year event which <br />compares to a 1.4 inch figure for a 10 year event. By using curve 82 of <br />the same manual the runoff resulting from 1.7 inches of precipitation is <br />determined to be .46 inches. This compares to .29 inches taken from the <br />same table for 1.4 inches of precipitation. <br />• The ZO year 24 hour runoff is .46 inches compared to the 10 year <br />24 hour runoff of .29 inches. Therefore the hydrograph peak flows for <br />13-1-18 <br />