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Appendix G-1 Hydrology Calculations <br /> Drainage basins are shown on the Exhibit G map. Drainage basins 1 and 2 represent the areas <br /> that drain directly to the three pods before, during, and after mining. Drainage basin 1 <br /> encompasses the westernmost pod and western processing/operating area. Drainage basin 2 <br /> encompasses the center and easternmost pod and eastern processing/operating area, as well <br /> as some off-site areas that drain onto the site. <br /> Runoff conditions are calculated in four conditions: premine, mining, and reclaimed (two <br /> reclamation options). This is to show the baseline runoff condition of the site, how mine <br /> disturbance will change that, and what the post-mine runoff conditions will be. <br /> All stormwater designs are based on the 100-YR 24-HR storm event for this area of Colorado. <br /> Runoff modelling is conducted for both operating and reclaimed conditions. Calculations of <br /> runoff, both in terms of volume and flow, are according to the Rational Method. <br /> The Rational Method is a widely used technique in hydrology for estimating peak discharge from <br /> small drainage basins during storm events. It is based on the premise that peak discharge is <br /> proportional to rainfall intensity, catchment area, and a runoff coefficient that accounts for land <br /> use and soil type. The method uses the formula Q = CiA, where Q is the peak discharge (cubic <br /> feet per second or cubic meters per second), C is the runoff coefficient, i is the rainfall intensity <br /> (inches per hour or millimeters per hour), and A is the catchment area (acres or hectares). This <br /> method is particularly useful for catchments where the time of concentration is relatively short. <br /> The Exhibit C maps summarize the drainage basins (A). Runoff coefficients are based on land <br /> conditions (C). NOAA data for rainfall intensity (i) is used. Runoff coefficients are typical values <br /> from ASCE3 tables included in the hydrology software. The coefficients are calculated as a <br /> weighted average of the land areas and their runoff coefficient values. The coefficients area <br /> based on unimproved vegetated ground (0.20-0.25), disturbed light industrial ground (0.5), and <br /> impervious areas such as open water (1.0). <br /> Assumptions: The following assumptions are made for the Evans surface hydrology model. <br /> 1. NOAA rainfall intensities that are publicly available are accurate. <br /> 2. The computed maximum rate of runoff to the design point is a function of the average <br /> rainfall rate during the time of concentration to that point. <br /> 3. The hydrologic losses in the catchment are homogeneous and uniform. <br /> 4. The runoff coefficients represent the average soil antecedent moisture condition, <br /> imperviousness, and type of soil. <br /> 5. The depth of rainfall used is one that occurs from the start of the storm to at least the <br /> time of concentration, and the design rainfall depth during that time period is converted <br /> to the average rainfall intensity for that period. <br /> s American Society of Civil Engineers <br /> Evans Mining Resource <br /> February 2025 G-10 Lewicki&Associates <br />