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1.0 INTRODUCTION <br />Seneca Coal Company (SCC) has prepared a remediation plan for the Old WoIE Creek mine pit area at <br />the Seneca II Ivline The Old Wolf Creek area is apre-Surface Mining Control and Reclamation Act of <br />1977 (SMCRA) or pre-law area located in portions of Section 35,Township 6 North, Range 87 West. <br />The area was reclaimed prior [o August 1977 and, consequently, the permanent program rules and <br />regulations of the Colorado Mined Land Reclamation Board for Coal Mining which were promulgated <br />in response to SMCRA axe not applicable. <br />The Colorado Division of Reclamation, Mining and Safety (CDRMS) has requested that SCC improve <br />surface water drainage in the Old Wolf Creek area to, among concerns, decrease erosional instability, <br />SCC has developed the remediation plan, and specifically, this channel design in response to CDRMS's <br />request. <br />SCC has designed a drainage channel, designated as PL-1, to route runoff from a topographic feature <br />that pools water to a natural drainage in an undisturbed part of the Old Wolf Creek area. The channel <br />location is shown on Drawing 1, Old Wolf Creek Remediation Plan. The channel was designed using <br />the SEDCAD4 (Schwab, 1998), a hydrologic computer program. <br />2.0 HYDROLOGIC CALCULATIONS <br />The SEDCAD4 model calculates runoff and peak flows using a numerical modeling technique based on <br />user inputs of a design storm event (i.e., rainfall amount and duration, rainfall distribution, etc.). Runoff <br />hydrographs are developed on a subwatershed basis with the inputs of area, time of concentration, <br />curve number, and selection of a dimensionless double triangle unit hydrograph shape. Routing of <br />hydrogxaphs is accomplished using the Muskingum Method. <br />2.1 Precipitation Distribution <br />SCC used a Type II storm distribution as an estimation of the area storm distributions. This <br />distribution has been used for other Seneca II Mine hydrologic modeling. <br />2.2 Storm Duration <br />A storm duration of 24-hours was used fox the SEDCAD modeling. <br />2.3 Return Period/Precipitation Amount <br />Site-specific rainfall data collected by SCC indicates that the 10-year, 34-hour storm event precipitation <br />amount is 1.7 inches. This rainfall amount has been used for other Seneca II Mine hydrologic <br />modeling. <br />2.4 Hydrograph Response Shape <br />A unit hydrograph shape predicts the runoff response for each drainage area ox sub-area. The <br />hydrograph response shapes in SEDCdD4 are slow, medium and fast. A slow response corresponds to <br />Old Wolf Creek area Remedianon Plan 1 Revised 06/07 <br />