2011-07-11_PERMIT FILE - M2011001 (12)

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MINING DRAINAGE ANALYSIS The overall stormwater runoff for the mining development was evaluated using three drainage basins. Developed runoff rates for the 2 -year, 5 -year, 10 -year and 100 -year storm events were calculated using the Rational Method. Calculated flows were used in the onsite drainage facility evaluation. Please refer to the Mining Drainage Plan located in the back pocket of this report for basin delineation. Design Storms: Runoff Quantities: Runoff Coefficients: Soil Conditions: Overland Flow Length: Time of Concentration: Rainfall Intensities: FINAL DRAINAGE REPORT IVERSON MINE WELD COUNTY, COLORADO Minor Storm: 5 - year events Major Storm: 100 - year event Developed basins were less than 160 acres; therefore, design storm runoff rates were calculated using the Rational Method. Percent imperviousness was determined for the anticipated land use from Table RO -3 of the Urban Drainage and Flood Control District Drainage Criteria Manual. The runoff coefficients were then taken from the Urban Storm Drainage Criteria Manual. Gravelly substratum, Ascalon loam, Dacono clay loam, Nunn loam, and Otero sandy loam are the soil types on the site. The site soils belong to Hydraulic Soil Groups B, C and D. Flow length, L, was considered to be the furthest distance from the know discharge point to the upstream end of the basin. The time of concentration for the historic basin was calculated using equation RO -2. A maximum distance of 500 feet was used for the initial or overland flow time. Calculations and Urban Storm Drainage References are included in this section of the report. Rainfall intensity values were calculated using equation 28.5P1 1 _ (10 + Tc) °.386 Manual. taken from the Urban Storm Drainage Criteria Calculations of the mining runoff rates are included in this section of Appendix E.