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Appendix G-2 Hydrology Calculations <br /> Drainage basin numbers on the following analyses correspond to basins outlined on Map G-1. <br /> Basin Description <br /> 1-Baseline Main property west of ridgeline. <br /> 2-Baseline Property east of ridgeline. <br /> 1-Mining Affected area and uphill drainage during mining. <br /> 1 Reclaimed Affected area and uphill following reclamation_ <br /> All stormwater designs are based on the 10-YR & 100-YR 24-HR storm events for this area of <br /> Colorado. Calculations of runoff, both in terms of volume and flow, are according to the Rational <br /> Method. Runoff conditions are calculated in the mining condition only, as no stormwater control <br /> structures will remain in reclamation. <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 /> Map G-1 summarizes the drainage basins (A). Runoff coefficients are based on land conditions <br /> (C). NOAA data for rainfall intensity (i) is used. Runoff coefficients are typical values from <br /> ASCE' tables included in the hydrology software. The coefficients are based on either <br /> unimproved ground (0.20-0.25) or light industrial ground (0.5-0.6) for disturbed areas of the site. <br /> Assumptions: The following assumptions are made for the 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 /> 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 /> American Society of Civil Engineers <br /> Yampa Valley Gravel Pit <br /> August 2025 G-6 l.eti�id i i� nsso�:lames <br />