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Cross Gold Mine <br />December 2024 G1-2 <br /> <br /> <br />Drainage basin numbers on the following analyses correspond to basins outlined on Map G-1. <br />Basin Description <br />1 Diversion: Idaho-Upper <br />2 Potosi Shaft Area <br />3 Diversion: Idaho-Lower <br />4 Diversion: Cross Mine Adit <br />5 Idaho Tunnel Operations Area <br />6 Cross Mine Operations Area <br />7 Comstock Area <br />8 Caribou 300 Diversion <br />All stormwater designs are based on the 10-YR & 100 -YR 24 -HR storm event for this area of Colorado. <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 ASCE1 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 /> <br />1 American Society of Civil Engineers