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' o o Transit Mix Concrete Co. <br /> involves subdividing the drainage area into sub-watersheds with relatively uniform surface <br /> characteristics. SEDCAD utilities were used to determine surface areas, storage capacities, <br /> routing parameters, and storm runoff for the proposed ditches and sediment basins. The curve <br /> number method flow calculations are also included in Attachment G-1. <br /> a. Site Climate <br /> Precipitation data for the Colorado Springs area are available from NOAA Atlas 14, point <br /> precipitation values for site area. Table G-3 shows the 24-hour storm event data. <br /> Table G-3 <br /> Storm Event Estimates <br /> Return 24-hour Rainfall <br /> Period (in) <br /> 2 Years 1.93 <br /> 10 Years 2.99 <br /> 100 Years 5.38 <br /> The data in DCM Volume 1 are based on NOAA Atlas 2, which has been superseded by NOAA <br /> Atlas 14; however, the data referenced in the DCM Volume were utilized for these calculations <br /> following El Paso County rules and regulations, due to the more conservative nature of the data. <br /> A Natural Resource Conservation Service (NRCS, formerly the Soil Conservation Service)Type <br /> II rainfall distribution was applied to the 24-hour storm. The Type II distribution applies to mid- <br /> continent locations where there are intense localized storms. <br /> The 100-year, 24-hour storm event was modeled for the design of culverts and ditches, and for <br /> the design of all sediment basins in accordance with state and county regulations. The SEDCAD <br /> result model reports are included in Appendix G-1 for reference. <br /> b. Site Curve Numbers <br /> The modeled runoff volume is a function of the curve number (CN), watershed area, and storm <br /> size, with the soil curve numbers used to define the runoff from soils in the model. Runoff curve <br /> numbers are typically determined from soil survey hydrologic soils group classifications and <br /> vegetation cover and condition. NRCS classifies soil types into four hydrologic soil group(HSG) <br /> ratings: A, B, C or D, with A-type soils having very high infiltration rates and D-type soils having <br /> very low infiltration rates. The area, curve number (CN), and the time of concentration were <br /> determined for each of the subwatersheds and entered into the SEDCAD model. A curve <br /> number of 98 was assigned to the open pit areas based on the impervious nature of the bare <br /> rock. A CN of 91 was used to represent the runoff response from overburden, fines and topsoil <br /> Hitch Rack Ranch Quarry Permit Application <br /> October 3, 2017 G-18 <br />