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dlap 7999 Clpnrz TmrnAmdr'PondD Eva/nalian• I <br />TWENTYMILE COAL COMPANY <br />POND D -HYDROLOGIC EVALUATION <br />1.0 INTRODUCTION <br />A hydrologic evaluation of Pond D at the Twenty-mile Coal Company (TCC) mine has been completed. <br />The purpose of the evaluation was to show that the existing Pond D will meet, or has met water quahry <br />regulations during a 10-year, 24-hour storm event and assure that the existing spillway structures would <br />safely pass the peak Flow from a 25-year, 24-hour storm event. As-built information for the spilhvay <br />and embankment strucmres was obtained from the Epps survey (199G) and confirtned by TCC <br />personnel. The hydrologic evaluation included a review of the Pond D watershed. New subwatershed <br />areas were delineated and new hydrologic and sediment yield parameters were assigned, as necessary. <br />(Refer to the SEDCAD 4 printouts and Map 9A.) <br />Pond D is located adjacent to the TCC processing facility and currently controls surface water runoff <br />from a 890-acre watershed. In addition to controlling storm water run-off, the Pond collects water <br />from continuous flowing springs located in the watershed and is fiequentl}' used to store water pumped <br />From the mine workings. <br />Since water is continuously entering Pond D from mine pumping and springs, a permanent pond water <br />level was set to account for water inflow in the model. <br />A site visit was conducted on April 8, 1999 to look at the watersheds contributing to Pond D, and a <br />meeting was held on May 4, 1999 with DD$G personnel to discuss evaluation requirements. <br />The following is the methodology and assumptions that were used in the evaluation of Pond D. <br />• The hydrologic, h}'draulic, and sediment calculations used to model Pond D were performed utilizing <br />the SEDCAD 3.0 computer modeling program developed by Civil Software Design (Civil Software <br />Design, August 1998) and were anah~zed according to the following methodologies. <br />2.0 HYDROLOGIC CALCULATIONS <br />SEDCAD ~ is' the next generation of hydrology and sedimentologv modeling softvare that replaces <br />SEDCAD+. It is a comprehensive, computer-based model used to evaluate storm water, erosion, and <br />sediment control management techniques. The SEDCAD ~ model calculates run-oft and peak flow <br />via a numerical modeling technique based on user inputs of a design storm event (i.e., rainfall volume <br />and duration, selection of rainfall distribution, and convolution utcrement). Hydrographs are <br />developed on a subwatershed basis with the input of area, tune of concentration, SCS Cun•e Number, <br />and [he selection of 1 of 3 dimensionless double triangle unit hydrograph shapes. Calculated runoff is- <br />routed to the various strucmres input into the model. Muskingum routing parameters are used. <br />Inputs ro the hydrology component of the SEDCAD 4 computer model mdude: <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 />Input values are explained in the Following sections. <br />t`lumgomen~ \Paaon Dlining Group ' 1G5 S. Onion Buulecard, Sui[e 4ll1 ' Lakcwoud, f_ulurado 80228' 1303) 7G3-514[1 <br />D-5 <br />