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onto the operations pad. See Figure 6 -Mine Operations Area for location of proposed holding ponds <br />identified as Pond #1 and #5. <br />Part IV - Site Soil Conditions <br />From the National Cooperative Soil Survey, as provided by the Natural Resources Conservation Service <br />of the USDA, the Hydrologic Soil Group for the mine site is classified or rated as a HSG C, described as <br />"Bodot -Silt -Rock outcrop complex, 5 to 25 percent slopes, very "bouldery". A portion of the site is <br />classified or rated as a HSG D soil group and described as "rock outcrop, 50 to 99 percent slopes ". The <br />area described with the 50 to 99 percent slopes is located to the east of the mine site and does not <br />represent the area influencing drainage for the mine site. The soils on the mine site, disturbed and <br />undisturbed areas, are made up primarily by the HSG C soil class /rating. HSG C is used for the drainage <br />analysis. The HSG C soils are characterized as having a layer that impedes the downward movement of <br />water or soils of moderately fine texture, with a slow rate of water transmission. Soils data information is <br />provided in Appendix 8 -Soil Survey Data. <br />PART V - Rainfall Data <br />Rainfall data for the area was obtained from the NOAA Atlas 14, Volume 8, Version 2 compiled by the <br />U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather <br />Service, office of Hydrology for the U.S. Department of Agricultural Soil Conservation Service, <br />Engineering Division. The Precipitation Frequency Estimates is presented in Appendix I - Rainfall Data <br />and IDF Curves. <br />PART VI - Time of Concentration <br />Time of Concentration (ToC) determination for nonurban areas, as prescribed in the Manual, was used. <br />The flow regimes of each basin were assumed, wherein the initial flow or overland flow path was <br />assumed for each basin, not to exceed 300 feet. The overland flow ToC used the equation prescribed in <br />the Manual as follows to determine the ToC: <br />t; = 1.8(1.1- K)Lo "' / Sl'3 <br />Where t; is ToC in the initial flow zone, K is Flow Resistance, L,, is length of the flow path in the basin, <br />and S is the slope of the flow path. Determination of K, Flow Resistance, is discussed below. The surface <br />slopes for each basin is also presented below in Part VII-Analysis Approach. <br />Time of Concentration for the channelized regime of the basins was determined by dividing the Velocity <br />of the flow by the length of the flow path. Where in the velocity of the flow was determined through the <br />equation, as presented in the Manual, of <br />V = C„ S`Vo.s <br />Where V is the velocity of the flow, C„ is the conveyance coefficient, and S is the slope of the ground <br />surface in the basin. Determination of C,, conveyance coefficient, is discussed below. <br />PART VII - Analysis Approach <br />Analysis for determining the storm water impacts to the developed mine site followed the prescribed <br />methods outlined in the Mesa County Stormwater Management Manual, henceforth the Manual. The <br />analysis was based on the Rational Method, one of the two allowed methods prescribed by the Manual, <br />for determining the peak runoff rates. The Simplified Peak Flow method was used for determining the <br />runoff volumes for sizing retention/detention ponds. <br />10 <br />