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

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HISTORIC DRAINAGE ANALYSIS Design Storms: Runoff Quantities: Runoff Coefficients: Soil Conditions: Overland Flow Length: Time of Concentration: Rainfall Intensities: FINAL DRAINAGE REPORT IVERSON MINE WELD COUNTY, COLORADO Historic stormwater runoff was evaluated using two drainage basins for the site. The 5 -year storm event was used for minor runoff calculations and the 100 -year storm event was used for major runoff calculations. The drainage basins were less than 160 acres; therefore, the Rational Method was used for stormwater runoff calculations. Historic flows were calculated for 2, 5, 10 and 100 -year storms. Please refer to the Historic Drainage Plan located in the back pocket of this report for basin delineation. Minor Storm: 5 -year event Major Storm: 100 -year event The historic basin was less than 160 acres; therefore, design storm runoff rates were calculated using the Rational Method. The 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 I_ 28 '5 * P1 taken from the Urban Storm Drainage Criteria (10 + Tc) Manual. Historic flows are included in this section of Appendix D.