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d® <br />~~. <br />RECENED <br />ocT 1 s 1996 <br />T e r r a M a t r i x <br />October 17. 1996 <br />gwiaidn nt Minerals ~ Geoi09Y <br />Ms. Erica Crosby <br />Division of Minerals and Geology <br />1313 Sherman Street, Room 215 <br />Denver. Colorado 80203 <br />Re: Comparison of Design Alternatives for the Colowyo Coal Company - Gulch A Pond <br />Dear Ms. Crosby: <br />IIIIIIIIIIIII Ill II <br />Slmunarized herein is a comparison of the design alternatives for the Colowyo Coal Comparry - Gulch A Pond. <br />Included in the summary is a description ofeach of the hydrologic, hydraulic, and sediment input parameters used <br />to analyze the Gulch A Pond. <br />The Gulch A Pond was evaluated according to existing conditions where the majority of the disttubed flow is <br />contained in the open pit. The pond was then evaluated for the post-mining condition prior to seeding the fmal <br />reclaimed pit area contributing to the pond. Figures showing the watershed boundaries and surface conditions <br />are presented in Attachment A. <br />1.0 HYDROLOGIC AND HYDRAULIC CALCULATIONS <br />The hydrologic and hydraulic calculations used to mode! the Colowyo mine site were performed utilizing the <br />SEDCAD+ computer modeling program developed by Civil Sofhvare Design. The results of the SEDCAD+ <br />analyses are presented in Attachments B and C. <br />SEDCAD+ is a comprehensive, computer based model used to evaluate storm water, erosion, and sediment <br />control management techniques. The SEDCAD+ model calculates run-off and peak flow via a numerical <br />modeling technique based on user inputs of a design storm event (i.e., rainfall amount and duration, selection <br />of rainfall distribution, and convolution increment). Hydrographs are developed on asub-watershed basis with <br />the input of area, time of concentration, Soil Conservation Service (SCS) curve ntunber, and the selection of ane <br />of three dimensionless double triangle unit hydrograph shapes. Routing of hydrographs is accomplished by <br />Muskingum's Method. <br />Inputs to the hydrology component of the SEDCAD+ Computer Model include: <br />• Precipitation Distribution <br />• Storm Duration <br />• Return Period/Precipitation <br />• Hydrograph Response Shape <br />• Drainage Basin Area <br />• Time of Concentration <br />• Muskingum Routing Parameters <br />• Curve Number <br />Fnglneermg G Envlrvnmenral Services <br />i65 South Union Boulevard. Suite boo <br />Izkewood. Colorado BozzB <br />Phone 7o776).5yo raz 7077679007 <br />Sream6oal Springs Denver Fort Collins SeaNe gantlago <br />