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MORRISON-KNUDSEN COMPAN Y.IN C. <br />• ENGINEERING REPORT <br />POWDERHORN COAL CO. <br />ACCESS ROAD CULVERTS continued <br />Page 2 <br />the 10 year 24 hour event, developed by GEX can be factored by .46/.29 = <br />1.59 to determine the 20 year 24 hour peak runoff. The 20 year 24 hour <br />peak runoff was determined to be 862 cfs. <br />Two methods were used to calculate the maximum peak flow the exist- <br />ing culverts can handle. The first method using the "Handbook of Steel <br />Drainage and Highway Construction Products", yielded a total combined <br />capacity of 690 cfs. The second method utilized a modification of the <br />Bernoulli and Manning equations. This method yielded a combined flow <br />capacity of 763 cfs or about ten percent greater than the first method. <br />• The two existing culverts in the access road do not have adequate <br />capacity to handle a 20 year 24 hour precipitation event. The culverts <br />are undersized 13% utilizing the peak flow capacity calculated using <br />method two. Recognizing the emperical nature of the peak runoff cal- <br />culations the 13% design deficiency may not be considered significant. <br />The two culverts do have adequate capacity to handle a 10 year 24 hour <br />event. <br />Supporting calculations and charts are included for reference. <br />•~ :fit" ~~~~~ies E. Stover <br />~' U~ A',9tate of Colorado <br />*'~' 2923 <br />~ ~ p~fessional Engineer <br />•A ; ;~cense No. 19230 <br />• •~ <br />13-1-19 <br />