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0 Introduction <br />Peabody Energy Twentymile Coal Company (TCC) has constructed a new intake shaft installation at <br />TCC's Foidel Creek Mine, located in Routt County, Colorado. The previous drainage and sediment <br />control system design considered the worst -case scenario (greatest runoff and sediment volume into the <br />sediment pond) that would occur during the construction and operation of the facility. Previous modeling <br />determined that the worst -case scenario occurred during the construction phase. Now that the facility is <br />constructed and operational, the hydrologic conditions at the intake shaft installation have subsequently <br />been improved with the addition of imported gravel surfacing, and reclamation of portions of the shaft <br />pad. The purpose of this analysis is to model as -built conditions for the hydrology and sedimentology of <br />site runoff from this installation, and to demonstrate that the existing perimeter drainage ditches, incised <br />sediment pond, spillway and spillway drainage channel are adequately designed to passively retain, treat <br />and discharge runoff from the intake shaft pad. The pond will completely contain three (3) years of <br />sediment storage (0.003 acre -feet) plus the 10 -year, 24 -hour runoff volume of 0.7822 acre -feet with 1.0 <br />feet of remaining freeboard. With the pond completely full at the start of the 10 -year, 24 -hour runoff <br />event, discharge from the sediment pond will still meet NPDES limits for allowable sediment <br />concentrations (as per CMLRB Rules and Regulations, Section 4.05.6(3)(a & b)). The single open - <br />channel spillway will pass the 25 -year, 24 -hour runoff peak flow with a minimum of 1.0 feet of remaining <br />freeboard and the downstream spillway channel will pass the same peak with a minimum of 0.3 feet of <br />remaining freeboard (as per CMLRB Rules and Regulations, Section 4.05.9(2)(a)(ii) and 4.05.9(2)(c)(ii)). <br />Since the up- gradient boundary of the facility is located on a watershed divide, no up- gradient drainage <br />ditches are required to collect or divert runoff from undisturbed areas. The location of the 6MN Shaft Pad <br />Facility relative to the surrounding area is shown in Exhibit 1. An expanded plan view of the 6MN Shaft <br />Pad Facility is shown in Exhibit 2. <br />The hydrologic, hydraulic and sediment modeling was conducted using SEDCAD. Watershed soil and <br />cover information was taken from site photos and a soil investigation report (Habitat Management, Inc. <br />October, 2003. Fish Creek Shaft & Access Road Soil Investigation, Routt County, Colorado, Twentymile <br />Coal Company Permit #82056). Composite soil samples of the as -built gravel pad surface were collected <br />by TCC in December 2008 and analyzed by North West Colorado Consultants, Inc. (North West <br />Colorado Consultants, Inc. Material Sort by Weight/Hydrometer and Sieve Analysis December 12, 2008). <br />The resulting base coarse particle -size distribution is presented in the SEDCAD reports as 3 /4 " B. Coarse. <br />Shaft Pad <br />A plan view of the intake shaft pad and pond is attached as Exhibit 1. The runoff from the 13.43 -acre <br />intake shaft facility area is collected in a small sediment pond both directly and via two perimeter <br />drainage ditches (Ditch 1 and Ditch 2, Exhibit 2). The up- gradient boundary of the facility is located on a <br />watershed divide. Therefore, no up- gradient drainage ditches are required to collect or divert runoff from <br />undisturbed areas. The disturbance can be further delineated by noting that of the 13.43 acres of <br />disturbance 5.25 acres have been seeded and reclaimed, 0.25 acres are facilities (generating negligible <br />sediment), 0.83 acres are the perimeter drainage ditches, 6.11 acres are the gravel pad, and 0.99 acres are <br />the sediment pond including the embankment strip around the pond. Reclamation was completed in fall <br />2009 on the cuttings pit, stockpiled topsoil, and a strip of land between the disturbed portion of the shaft <br />pad and the sediment pond. The intake shaft pad was constructed from the native Binco soil. Imported <br />pit run gravel was placed on top of the native soil. The intake shaft pad was surfaced with a layer of 3 /4 " <br />base - coarse gravel placed over the pit run. As -built gravel pad particle -size distribution data were <br />obtained from laboratory analysis of composite samples collected from the pad. <br />TCC 6MN Intake Shaft Facility 1 Water & Earth Technologies, Inc. <br />Drainage and Sediment Control System Design April 24, 2009 <br />