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999 <br />ENGINEERING dE SURVEYING COMPANY <br />BOX 674 • 443 DODGE DELTA., COLORADO, x1416 303-874-7593 <br />I! <br />2780 <br />I'. <br />', June 30, 1981 <br />State of Colorado <br />Mined Land Reclamation <br />423 Centennial Building <br />1313 Sherman Street <br />Denver, Colorado 80203 <br />ATTN: Rich Domingue <br />RECEIVED. <br />/ . <br />JUL2 1931 <br />MItJED LAND RECLA/RATION ~ CORE <br />COLO. DE7T, OF. NATURAE AE50~0.CE5 <br />Re: Section 2.05.3(4) Ponds, Impoundments and Diversions - Grand <br />Mesa Coal Company. <br />i <br />I' This letter is to verify the work done to date on responding to the <br />Mined Land Reclamation Adequacy Letter, dated May 8, 1981 regarding <br />the above referenced section. <br />I Enclosed you will find the revised tables and figures for the ditch <br />' and culvert sizing and peak flow calculations. Please review them <br />i and let us know of your approval so we can rewrite the accompanying <br />text in the Permit Application. <br />~ We have also enclosed calculations used to determine the size of <br />the pond and spillways. According to our calculations, the pond <br />size using the triangular hydrograph method is not significantly <br />h ~ smaller than the originally sized pond. <br />1 The principal spillway was sized using the peak flows of a 25-year, <br />24-hour precipitation event. The 12-inch corrugated metal pipe <br />spillway can discharge a volume of 0.30 acre-feet at an average flow <br />.,, of 2.66 cfs in 81 minutes. <br />Since the volume from a 25-year - 24-hour event can be stored with <br />0.30 acre-feet discharged through the principal spillway during the <br />storage period, the water level Drill not reach the emergency spillway. <br />Under these circu tances, what flow do we use to size the emergency <br />spillway. <br />~ r <br />i <br />E. LeRoy Black <br />CSK/ELB/ab <br />enclosures <br />cc: Grand Diesa Coal Company <br />