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Runoff calculations contained in this appendix are based on a set of standardized basin sizes and <br /> conditions. All waste pile basins will be assigned to a standard basin size during mining, and the <br /> appropriate basin size will dictate the size of diversion structure conducted. <br /> Calculations of runoff, both in terms of volume and flow, are according to the Rational Method. Runoff <br /> conditions are calculated in the mining condition only, as no stormwater control structures will remain in <br /> reclamation. <br /> The Rational Method is a widely used technique in hydrology for estimating peak discharge from small <br /> drainage basins during storm events. It is based on the premise that peak discharge is proportional to <br /> rainfall intensity, catchment area, and a runoff coefficient that accounts for land use and soil type. The <br /> method uses the formula Q = CiA, where Q is the peak discharge (cubic feet per second or cubic <br /> meters per second), C is the runoff coefficient, i is the rainfall intensity (inches per hour or millimeters <br /> per hour), and A is the catchment area (acres or hectares). This method is particularly useful for <br /> catchments where the time of concentration is relatively short. <br /> Map G-1 summarizes the drainage basins (A). Runoff coefficients are based on land conditions (C). <br /> NOAA data for rainfall intensity (i) is used. Runoff coefficients are typical values from ASCE2 tables <br /> included in the hydrology software. The coefficients are based on either unimproved ground (0.20-0.25) <br /> or light industrial ground (0.5-0.6) for disturbed areas of the site. <br /> Assumptions: The following assumptions are made for the Lockhart 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 rainfall <br /> 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 time of <br /> concentration, and the design rainfall depth during that time period is converted to the average <br /> rainfall intensity for that period. <br /> 6. The maximum runoff rate occurs when the entire area is contributing flow. <br /> Hydrographs and storm outputs (runoff, runoff peak flow, etc.) are included in the Appendix. <br /> Designs for surface water control structures such as diversion ditches and sediment ponds are <br /> attached to this Appendix after the hydrographs. <br /> 2 American Society of Civil Engineers <br /> London Mountain Clean Up <br /> G1-2 Lewicki&Associates <br /> April 2025 <br />