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°` c STATE OF COLORADO <br /> c p <br /> Roy Romer, Governor <br /> +1876+ ` <br /> DEPARTMENT OF NATURAL RESOURCES <br /> MINED LAND RECLAMATION DIVISION <br /> FRED R. BANTA, Director <br /> June 1 , 1989 \ <br /> Mr. Lew Thompson <br /> Environmental Coordinator <br /> Mid-Continent Resources, Inc. <br /> P.O. Box 158 <br /> Carbondale, Colorado 91623 <br /> RE: Re-Design of Coal Refuse Area Drainage Ditches , Coal Basin Mine (C-81-017 ) <br /> Dear Mr. Thompson: <br /> The Division has reviewed the material submitted by Scott Jones , P.E., <br /> concerning the drainage ditches at the Sutey and the Tunnel Waste refuse <br /> disposal areas. The ditches draining the refuse piles , and also clean-water <br /> diversions above the piles, must now be designed to convey a 100-year/24-hour <br /> precipitation event. <br /> At the Rock Tunnel disposal area, the ditches in question are the south and <br /> west side clean-water diversion, and the north side ditch which drains the <br /> pile. All appear, as designed, to have sufficient capacity. The rip-rapped <br /> section of the south side diversion ditch , however, is designed at a 50% <br /> grade, and at the design flow of 23.8 cfs would experience a calculated <br /> velocity of approximately 15 ft./sec. The Division requires that <br /> Mid-Continent Resources submit design details of this section, including <br /> rip-rap size and any other stabilization methods used. Additionally, the 24- <br /> inch CMP culvert under the sediment pond access road will carry the design <br /> flow of 23.8 cfs only at a headwater depth of about three feet (HW/Diam.=1 .5, <br /> from Chart 5, Hydraulic Charts for the Selection of Highway Culverts) . It is <br /> likely that damage to the road and the upper sediment pond would occur under <br /> these conditions. Therefore, the Division recommends replacement with a <br /> 36-inch culvert, which should pass the flow with very little headwater <br /> occurance . <br /> The East and West drainage ditches at the Sutey pile were upgraded to 100-year <br /> capacity as partial abatement of NOV C-88-022, and no further design work is <br /> needed on them. Material submitted for the ditch labeled A-A' (to be built in <br /> 1989) on Map II-B-App. 5-10, Vol . 2-A of the Permit Application shows that it <br /> will handle the 100-year runoff of 105 .2 cfs. Due to its 10% slope, however, <br /> the water would run at a highly erosive velocity of about 13 ft./second. The <br /> Division requires that you submit a plan for stabilizing this ditch , possibly <br /> using check structures to lower the velocity and rip-rap to protect the <br /> channel bed. <br /> Sincerely, <br /> William B . Crick <br /> Reclamation Specialist <br /> 215 Centennial Building, 1313 Sherman Street Denver, Colorado 80203-2273 Tel.(303)866-3567 <br /> WRr'/vih <br />