Laserfiche WebLink
^ V V V V CENGINEERING <br />611 SKYLINE ROAD - LARAMIE, WY 82070 - (307) 742-0031 FAX (307) 721-2913 MEMO <br />TO: JERRY NETTLETON, MANAGER ENVIRONMENTAL AFFAIRS <br />TWENTYMILE COAL, LLC <br />FROM: JUSTIN TERFEHR, P.E. <br />SUBJECT: HYDROLOGIC 1YDRAULIC ANALYSIS FOR THICKENER UNDERFLOW <br />BOREHOLE PAD AND ACCESS ROAD (REVISED PER CO DRMS COMMENTS) <br />DATE: DECEMBER 22, 2016 <br />This Memo documents hydrologic and hydraulic evaluation of culverts and diversion <br />ditches for the Thickener Underflow Borehole Pad and Access Road to be constructed by <br />Peabody Energy, Twentymile Coal, LLC (Figure 1). The analysis modeled the <br />upgradient hydrology and sized the proposed culverts and diversion ditches utilizing <br />SEDCAD and NRCS guidance. <br />Twentymile Coal uses 24 -inch diameter CMP culverts as a minimum for maintenance <br />and cleaning. The 24 -inch culvert is larger than what is required to pass the runoff from a <br />10 -year, 24-hour precipitation event for the proposed culvert under the proposed access <br />road. The SEDCAD model was used to determine the diversion ditch depths with a <br />minimum freeboard of 0.3 feet. Grades for each diversion ditch were determined from <br />the existing site topography. Due to the existing steep terrain, Diversion Ditch 1 has a <br />slope of approximately 11 percent; the SEDCAD model indicated potential erosive flow <br />velocities, therefore, a ditch section that is larger than the typical 1 -foot deep v -ditch and <br />check dams are recommended to be installed (See Figures 2-3). The check dams were <br />sized and spaced using NRCS guidance (Attachment 1). Hydraulic calculations were also <br />completed to check the freeboard depth over the check dams in Diversion Ditch 1 <br />(Attachment 1). Diversion ditches 2 and 3 shall be constructed as v -shaped ditches to a <br />minimum depth of 1.0 foot with 2:1 (horizontal: vertical) side slopes (Figure 4). <br />Watershed soil and cover information used for the SEDCAD modeling were taken from <br />the Topsoil Stockpiles Along 18 Right Shaft Access Road project (WET, Inc. 2005). <br />WET obtained the data from site photos and a soil investigation report (Habitat <br />Management, Inc. October, 2003. Fish Creek Shaft & Access Road Soil Investigation, <br />Routt County, Colorado, Twentymile Coal Company Permit #82056). All culverts and <br />diversion ditches were modeled for the 10 -year, 24-hour storm event with a precipitation <br />depth of 1.8 inches. <br />All SEDCAD modeling completed for this analysis are provided in Attachment 2. <br />1/1 <br />