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Upper Pit 6900 Access Road I Design Summary American Gypsum <br /> Tierra Group assumed the road falls into the"non-critical"category (no imminent danger to human <br /> life, minor environmental impact, and minor repair costs if slope fails (CO DRMS, 2016)), with <br /> strength measurements resulting from a single test per material type. Modeled FOS values <br /> exceeded the minimum recommended FOS values for this condition (1.3 for static and 1.15 for <br /> pseudo-static). <br /> 4.0 Surface Water and Sediment Controls <br /> Tierra Group designed surface water and sediment controls to ensure surface water within the <br /> new road area is conveyed under the road and reports to an adequately sized sediment control <br /> structure. Surface water from upland areas will be conveyed under the road via a single 18-inch <br /> corrugated steel culvert, where it will discharge into a natural stream channel and report to the <br /> existing M5O1 sediment control pond where it will evaporate, infiltrate, or report to the <br /> downstream drainage via the existing decant standpipe. Tierra Group developed a Sediment, <br /> Erosion, Discharge by Computer Aided Design (SEDCAD) model for pond M501 and determined <br /> it can sufficiently contain the additional sediment from the new road disturbance. <br /> 4.1 Design Criteria <br /> Design criteria for the surface water and sediment controls are shown in Table 4.1: <br /> TABLE 4.1: WATER MANAGEMENT DESIGN CRITERIA SUMMARY <br /> Parameter Value <br /> Design Storm—Culvert 100-year, 24-hour(BLM, 2015) <br /> Design Storm—Sediment Controls 10-year, 24-hour <br /> 4.2 Calculations and Modeling <br /> 4.2.1 Design Storms <br /> The 10- and 100-year 24-hour precipitation depths were obtained from the National Oceanic and <br /> Atmospheric Administration (NOAA) Atlas 14 (Perica et al., 2013) and were assumed to follow a <br /> Natural Resources Conservation Service (NRCS) Type II distribution. Precipitation depths for <br /> design storm events are shown in Table 4.2. <br /> TABLE 4.2: NOAA DESIGN STORM DEPTHS <br /> Storm Event Precipitation Depth <br /> (inches) <br /> 10-year 24-hour 1.47 <br /> 100-year 24-hour 2.21 <br /> 4.2.2 Drainage Basins and Soils Map <br /> Tierra Group delineated drainage basins for contributory areas using AutoCAD 2018 Civil3D <br /> (Autodesk, 2017) and determined land cover and hydrological soil groups for the watershed <br /> basins using the NRCS soil resource report (NRCS, 2016). Attachment D shows the drainage <br /> AG Upper Pit 6900 Access Rd 20181015 October 2018 Page 5 of 9 <br />