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7000 C1D~rr Tu•enh'mi/r' Pond B Era(uahon 1 <br />TWENTYMILE COAL COMPANY <br />• POND B -HYDROLOGIC EVALUATION <br />1.0 INTRODUCTION <br />3 hydrologic evaluation of Pond B at the T~ventytnile Coal Company (TCC) mine has been completed. <br />The hydrologic evaluation was in response to comments from the Colorado Department of D4ining and <br />Geology (DD4G). The purpose of the evaluation was to predict, with modeling using SEDCAD4, that <br />existing Pond B will meet water quality standards during a 10-year, 24-hour storm event and show that <br />the existing spillway structure wtll safely pass the peak flow from a 25-year, 24-hour storm event. As- <br />budt information for the spillway and embankment structures was obtained from the Epp survey <br />(1996) and 1999 surve}' information obtained by TCC personnel. The hydrologic evaluation included a <br />review of the contributing watersheds for the pond. New subwatershed areas were delineated and new <br />hydrologic and sedrment yield parameters were assigned, as necessary. (Refer to the SEDCAD4 <br />printouts and Map 9B.) <br />The following is the methodology and assumptions that were used m the evaluation oFPond B. <br />The hydrologic, hydrauhc, and sediment calculations used to model the ponds were performed utilizing <br />the SEDCAD4 computer modeling program developed by Civil 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 />• Remm 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 />2.1 Precipitation Distribution <br />A precipitation distribution is input to model the tun-off hydrograph. SEDCAD4 allows dle user to <br />choose between the Natural Resources Conservation Service (NAGS -formally SCS) Type I, T}'pe LA, <br />`/ Type II, Type III, or user defined storm unit mass curves. The T}'pe II distribution was input as an <br />estimation of the TCC area storms. (Barfield et a/, 1981.) <br />2.2 Storm Duration <br />/~ A storm duration of 24 hours was used for the evaluation of Pond B. <br />• / 2.3 Return Period/Precipitation <br />A preupitanon amount is required for the appropnate return period. The following return periods and <br />precipitation events were used for the TCC D1ine. These values have been used in previous anal}'ses <br />bfont0omen' Watson Mining Group ' 165 5. Union Boulevard, Suite 410' LaAewood, Colorado 8022ft' (303) 763-5140 <br />