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Prymorys Environmental Consnl6ng, Inc. <br />"Dediiatin8raoiuw to tvmortow's mvrtmmrnl fa today's is~utry." <br />March 28, 2006 <br />Mr. Matt Carnahan <br />P.O. Box 195] <br />Bayfield, Colorado 81122 <br />Re: Rock Dog Storm Water Facility Design <br />Mr. Matt Carnahan, <br />Please find below and attached to this letter, plans and specifications for run-on storm water diversion <br />tacilities tecornmended for the Rock Dog Gravel Mine, as well as associated design calculations for 1 OYR- <br />24hrand 100YR-24hr storm events, in accordance with the memo you provided me from Rllen Sorenson of <br />the Division of Minerals and Geology (DMG) dated March 27, 2006, <br />Drainage Area Calculations <br />Two drainage azeas were identified for diversion azound the Rock Dog PHASE A area to effectively <br />protect the east and north overburden and/or top-soil berms.; (I) the north run-on area, and (2) the east run- <br />on area. Such delineations were detemtined from separation of flows at the highest elevation point of the <br />north-east comer of the PHASE A mining area. Peak discharges associated with a 100YR-24hr storm <br />event, for each drainage azea, aze provided in the attached storm calculations document. <br />Run-On Diversion Ditches <br />Asper the above Memo, the storm water control infrastructure for PHASE A has implemented diversion <br />ditches to convey up-gradient and off-site storm flows around the PHASE A mining area. Design <br />dimensions were made congruent with the affected area detention pond ditches (7 ft crest & ] ft. trough <br />widths at approx. 1.5 R. deep) and were modeled for the 100YR-24hr storm event peak flows from the <br />identified drainage areas (Attached storm calculations document). <br />East Run-On Area Access Entry Culvert Design <br />An 18 inch diameter, standard corrugated metal pipe (CMP) culvert was chosen to convey diversion ditch <br />flows under and through the mining access entry (DMG memo; #2b). At a ditch design slope of 0.0054, <br />and design dischazge at [he peak flow of a I OOYR-24hr event (4.98 cfs), an estimated 35 ft. non-submerged <br />CMP will capacitate the peak flow with a headwater depth of 17.5 inches. Culvert design assumes <br />projecting inlet and outlets into the diversion channel and outlet control. <br />Design engineering providing compacted fills above the CMP of at least y, its height is recommended [o <br />prevent crushing or pinching of the installed CMP. <br />Erosion Protection <br />Three (3) to 4 inch diameter (or greater) cobble rock "check dams" are recommended at 75 ft intervals <br />across and through the north diversion ditch and at 125 ft. intervals across and through the eastern diversion <br />ditch to prevent sedimeatation and erosion of the ditch-ways. The check dams should be "keyed" into the <br />-----P.O. Box 4470-- <br />Durango, Colorado 81302 <br />