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RIVERCITY <br />744 HORIZON COURT SUITE 110 970 - 241 -4722 <br />GRAND JUNCTION, CO 81506 970 - 241 -8841 FAX <br />Email: info @rccwest.com <br />To: Susan Patton, Agapito Associates, Inc. <br />From: Marc Kenney, River City Consultants, Inc. <br />Date: December 29, 2010 <br />Re: Revised DWDA Stormwater Calculation Summary <br />The purpose of this letter is to present my analysis of the 100 year 24 hour storm for the <br />catchment that includes the Development Waste Disposal Area (DWDA). This analysis <br />included runoff modeling, ditch sizing, and culvert sizing. The purpose of this analysis is to <br />complete the Development Waste Pile application filed with the Colorado Division of <br />Reclamation, Mining, and Safety (CO DRMS). The CO DRMS issued a letter dated <br />September 16, 2010 stating the application was incomplete due to an incomplete surface <br />water control design. This analysis completes the requirements noted in the September 16, <br />2010 letter. Further, this analysis also sizes the clean water diversion ditch located just north <br />of the DWDA (north of the access road on the north side of the DWDA) and updates the <br />• analysis letter issued November 2, 2010. <br />The catchments of interest are located at the New Elk Coal Mine, owned by New Elk Coal <br />Company, which is a subsidiary of Cline Mining. The New Elk Coal Mine property is <br />located south of Highway 12 approximately 20 miles west of Trinidad, Colorado. The <br />DWDA is located south of Highway 12, just east of the main entrance/ exit gate for the <br />mine. The DWDA catchment is 4.09 acres and is isolated from other catchments by <br />diversion ditches and grade breaks. This analysis sizes the diversion ditches for the 100 year <br />24 hour storm. <br />Vegetative cover within the DWDA catchment is limited and thus these calculations assume <br />bare soil. Vegetative cover within the clean water diversion catchments range from <br />impervious to pasture and juniper forest. Soil types were all assumed to be D. This is <br />consistent with natural soils in the vicinity of the DWDA. The hydrologic analysis was done <br />using US EPA SWMM, SCS curve number, Manning's equation, and kinematic wave within <br />Autodesk's Storm and Sanitary Analysis 2011 (formerly BOSS International's StormNET). <br />NOAA Atlas 2 Volume III - Colorado was used to determine the 100 year 24 hour total <br />precipitation (4.2 inches). A SCS Type II storm was used to model the 100 year 24 hour <br />storm. <br />. SAPROJECTS \1134 Agapito - New Elk Mine\Desi n \StormNET\Revised DWDA Cale Summa .doc <br />9 N <br />TR -53 Page E 39 -2 Revised 1/3/11 <br />