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/anunn~?000 C7pmt Tnrnhnu/e"PmidG Evn/unbar ~ <br />TWENTYMILE COAL COMPANY <br />• POND G -HYDROLOGIC EVALUATION <br />1.0 INTRODUCTION <br />A hydrologic evaluation of Pond G at the Twenq•rrule Coal Company (TCC) rune has been completed. <br />The hydrologic evaluation was in response to comments From the Colorado Department of blirting and <br />Geology (DMG). The purpose of the evaluation was to predict, with modeling using SEDCAD4, that <br />existmg Pond G will meet water quality standards during a ]0-year, 24-hour storm event and show that <br />the existing spillway stmcture will safely pass the peak flow from the 100-year, 24-hour storm event. <br />As-built information for the spillway and embankment structures was obtained from the Epp sun•ey <br />(1996) and 1999 sun'ey information provided by TCC personnel. The hydrologtc evaluation included a <br />review of the contributing watersheds for the pond. New subwatershed areas were delineated and new <br />hydrologic and sediment yield parameters were assigned, as necessary. (Refer to the SEDCAD4 <br />printouts and Map 9A) <br />The following is the methodology and assumptions that were used in the evaluation of Pond G. <br />The hydrologic, hydraulic, and sediment calculations used to model the ponds were performed utilizing <br />the SEDCAD4 computer modeling program developed by CivJ Software Design (Civil Software <br />Design, August 1998) and were analyzed according to the following methodologies. <br />2.0 HYDROLOGIC CALCULATIONS <br />SEDCAD4 is the next generation of hydrology and sedimentology modeling software that replaces <br />SEDCAD+. SEDCAD4 has been previously' discussed in submittals for Pond D. Inputs to the <br />• hydrology' component of the SEDCAD4 computer model include: <br />• Precipitation Distribution <br />• Storm Duration <br />• Return Period/Precipitation <br />• H}'drograph Response Shape <br />• Drainage Basm Area <br />• Time of Concentration <br />• Muskingum Routing Parameters <br />• Curoe Number <br />Input values are explained in the following sections. <br />2.1 Precipitation Distribution <br />J A precipita don distribution is input to model the run-off hydrograph. SEDCAD4 allows the user to <br />choose between the Natural Resources Consemarion Service (NAGS -formally' SCS) Type I, Typc U1, <br />U~' Tvpe II, Tvpe III, oc user defined storm unit mass att•es. The T}'pe II distribution was input as an <br />estimation of the TCC area storms. (Barfield e! a!, 1981.) <br />2.2 Storm Duration <br />Q`~l A storm duration of 24 hours was used for the evaluation of Pond G. <br />• 2.3 Return Period/Precipitation <br />A precipitation amount is required for the appropnate return period. The following return periods and <br />precipitation events were used for the TCC Mine. These values have been used m previous anal}'ses <br />D1ont6omen' Watson bluung Group • IGS S. Union Boulevard, Sui[e 410' Lakewood, Colorado 80228' (30}) 7G3-51-00 <br />