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Upper Pit 6900 Access Road I Design Summary American Gypsum <br /> basins and soils maps used in the culvert sizing (D1), SEDCAD modeling (D2), and post- <br /> reclamation surface water diversion channel sizing (D3). <br /> 4.2.3 Curve Number <br /> The NRCS Curve Number (CN) is used to quantify the ability of various land surfaces to absorb <br /> runoff. Taking into account soil type and vegetation type and condition, it is used to calculate the <br /> runoff volume from a given rainfall.Tierra Group determined land cover and hydrologic soil groups <br /> for the watershed basin using the NRCS soil resource report for the site (NRCS, 2016) and site <br /> photographs taken by Tierra Group. Hydrologic soil groups are classified by the minimum rate of <br /> infiltration achieved for bare soil after sustained wetting. Group A soils have the lowest runoff <br /> potential and highest infiltration rates, while Group D soils have the highest runoff potential and <br /> lowest infiltration rates. A hydrologic soil group is assigned to each soil map unit in the soil <br /> resource report (NRCS, 2016). Vegetation consisted mainly of sparse grasses and shrubs, with <br /> scattered coniferous trees. Land cover was classified as "fair", with an estimated 30% to 70% <br /> groundcover. <br /> 4.2.4 Travel Time and Time of Concentration <br /> The time of concentration (TC) and lag time (TL) are used to measure the response of a <br /> watershed to a rain event; essentially the time needed for runoff from the most remote point in a <br /> watershed to reach the watershed outlet. TC and TL for each watershed were calculated using <br /> the NRCS lag equation (NRCS, 2010) and the standard relationship TL=0.6*TC. Lengths and <br /> slopes were estimated using AutoCAD Civil 3D (Autodesk, 2017). TC paths are shown on the <br /> watershed maps included with Attachment D. <br /> 4.2.5 Runoff <br /> Tierra Group developed a SEDCAD (Civil Software Design, LLC, 2010) model to calculate peak <br /> flows for sediment control design. Tierra Group also calculated peak flows using HEC-HMS (U.S. <br /> Army Corps of Engineers (USACE), 2010) for the culvert and post-reclamation surface water <br /> diversion channel. <br /> 4.3 Culvert Design <br /> Tierra Group determined contributory flows to the culvert from the 100-year, 24-hour event using <br /> the drainage basin delineation (Attachment D1) and HEC-HMS (USACE, 2010). The culvert will <br /> be an 18-inch corrugated steel pipe, as determined by the Federal Highway Administration <br /> (FHWA) HY-8 computer software (FHWA, 2015). <br /> The culvert inlet will be protected by a 12-inch thick riprap collar consisting of 6-inch D50 riprap, <br /> placed to a height of 5.0 feet and extending horizontally 4.0 feet from the edge of the culvert pipe. <br /> The culvert outlet will include a riprap apron with 6-inch D50 riprap. Riprap was sized according to <br /> methods outlined by the FHWA(FHWA, 1989). Drawing 04 (Attachment A) shows culvert details. <br /> 4.4 Sediment Controls <br /> Existing sediment controls at the site were designed to contain sediment and runoff water from at <br /> least a 10-year, 24-hour storm event. Tierra Group reviewed the pond M501 topography from <br /> September 2018 (Gamba, 2018) and determined that the pond has a capacity of 0.51 acre-feet, <br /> with 1 foot of freeboard. Tierra Group developed a SEDCAD model for pond M501 and <br /> AG Upper Pit 6900 Access Rd 20181015 October 2018 Page 6 of 9 <br />