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FOIDEL CREEK MINE BUILDING REMOVAL <br />• STORM RUNOFF DESIGN FOR PAD AND DITCHES <br />Peabody Energy Twentymile Coal Company (TCC) proposes to remove the CCYC office <br />building. There are paved parking lots above (north side) and below (south side) the <br />building, with an existing 1,073 feet of access road to the building located within the <br />Twentymile Coal-Foidel Creek Mine located in Routt County, Colorado. TCC proposes <br />to tear down and remove the building, and leave the paved areas. The building pad area <br />will be regraded to transition the slope gradually between the upper and lower lots. The <br />regazded azea will be ripped, roughened, and re-seeded. The purpose of this analysis is to <br />model the building pad (including parking lots and regraded building footprint) and the <br />access road hydrology to determine sediment control measures; and the sizing of and <br />erosion protection for access road runoff drainage ditches. The hydrologic and hydraulic <br />modeling was conducted using SEDCADT"' 4 (Build Date 2000.09.07). Watershed soil <br />and cover information were taken from site photos and a soil investigation report (Habitat <br />Management, Inc. October, 2003). The access road, watershed, drainage ditch and <br />CCYC office facility locations are shown on Figure 1 (Foidel Creek Mine Building <br />Removal Site). Typical road drainage ditch designs for respective sections of the access <br />road are shown on Figures 2 and 3. Supporting SEDCAD*M 4 modeling results (including <br />input data and final reports) aze attached. <br />The 10-year, 24 hour precipitation event (1.8 inches of rainfall) was used to model the <br />runoff from the building removal site and from watersheds contributing runoff to the road <br />• drainage ditches. The area where the CYCC office building would be removed was first <br />modeled as regraded without vegetation to generate worst case erosion and sedimentation <br />conditions. The silt fence subroutine within SEDCAD'TM was used to both size and <br />demonstrate the effectiveness of silt fence or sediment logs to control erosion and <br />sedimentation from the regraded azea prior to the re-establishment of vegetation. Based <br />on modeling results, it is recommended that silt fence or 18-inch diameter sediment logs <br />be placed immediately downgradient from the regraded azea (Figure 1). The azea was <br />also modeled as revegetated without the silt fence/sediment log $MP to demonstrate that <br />the revegetated area will adequately control erosion and sedimentation. Following is the <br />summary of the modeling design parameters and results for the azea where the CYCC <br />office building would be removed and regraded: <br />REGRADED BUILDING PAD RUNOFF DESIGN PARAMETERS AND <br />RESULTS: <br />Regraded Building Site with Sediment Log BMP <br />Design Pazameters: <br />Precipitation: <br />- --Use 1-0-yeaz,-24-hour-storm-precipitation-depth = 1.8 in- - <br />Land Use: <br />Paved area S WS #1 = 0.22 ac with CN = 98 (paved parking lot) <br />• <br />