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Drainage improvements are proposed for the site. These include grading and installation of <br />improved roadway ditches along portions of the vent access road, installation of a containment <br />berm along three sides of the site, installation of one drainage culvert, and construction of a <br />retention pond to capture runoff from the two onsite basins. These improvements are intended to <br />shed offsite runoff and collect and retain onsite stormwater. The attached drawings in the back <br />of this document illustrate the locations and details of these proposed improvements. <br />In addition, the attached drawings also illustrate: onsite basin topography, boundaries, and BMP <br />components; offsite basin topography and boundaries; and mine permit boundaries. <br />The National Resource Conservation Service (NRCS — formerly known as the Soil Conservation <br />Service, or SCS) dimensionless unit hydrograph procedure is one of the most well -known <br />methods for deriving synthetic unit hydrographs and determining runoff peaks and volumes. For <br />drainage basins where no runoff has been measured, the Curve Number Method can be used to <br />estimate the depth of direct runoff from the rainfall depth, given an index describing runoff <br />response characteristics. This method was originally developed by the SCS for storm conditions <br />prevalent in the United States and was the method used in this study. <br />7.2 Design Storms <br />DRMS requires analysis using 10 -year and 100 -year, 24 -hour storms. The National Oceanic and <br />Atmospheric Administration (NOAA) Atlas 2, Volume III isopluvials were used in conjunction <br />with site locations (latitude and longitude) to determine precipitation amounts for the given <br />storms. Copies of these charts are included in the attachments. <br />7.3 Runoff /Discharge Calculations <br />Peak flows were determined for the 100 -year storm in all basins to determine required capacities <br />for the diversion channels. SCS Unit Hydrograph Methods were applied utilizing "Haestad Pond <br />Pack" software. Worksheets and results produced by the software are included in the <br />attachments. Primary input requirements are: runoff curve number (CN), precipitation, and <br />times of concentration. <br />Curve numbers were taken from tables available from many sources based on surface soil types <br />and vegetation. Surface soils at the site are considered "Hydrologic Soils Group B" (soils having <br />a moderate infiltration rate when thoroughly wetted and consisting chiefly of moderately deep to <br />deep, moderately well to well drained soils with moderately fine to moderately coarse texture) <br />given the soils report provided by NRCS. Surface vegetation is considered "Pinyon/Juniper" <br />with either poor or fair stands or a weighted combination of both. The larger offsite basins have <br />areas with varying concentrations of pinyon/juniper stands and are undisturbed; therefore, they <br />received weighted combinations of CN values (varying from 58 to 75 — fair to poor stands, <br />respectively). The onsite basins are mainly disturbed areas void of vegetation and were therefore <br />assigned a CN of 75 for "poor" stands in Type "B" soils. <br />Times of concentration were computed for each basin utilizing widely accepted formulas for <br />computation of sheet -flow, shallow concentrated flow, and channel -flow from the headwaters to <br />the receiving storage area. <br />ESWMP -5 <br />