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III. Environmental Setting, Impacts, and Mitigations <br /> K Hydrology, Geohydrology, and Water Quality <br /> TABLE III.K.1: EXISTING 100-YEAR PEAK RUNOFF <br /> Approximate <br /> Hydrologic Area Area(acres) Current Land Use Runoff Curve Number/a/ <br /> A 130 Active Landfill 94(newly graded) <br /> B 108 Undisturbed; Agriculture 84(open space; 50-75% <br /> grass cover) <br /> C 17 Borrow pit 0(does not drain to the <br /> creek) <br /> D 15 Floodplain 100(open water) <br /> TOTAL* 270 <br /> Peak Flow at the downstream property 134 cfs <br /> boundary <br /> * This estimate does not include flows from the parcels north of the existing landfill(126.4+ <br /> 6.5 acres), as it is assumed that they do not contribute to peak flow in Little Johns Creek at the <br /> site downstream boundary,under existing conditions. [Beck: Shouldn't total equal <br /> 405.5 acres rather than 402.9?] <br /> /a/ The Runoff Curve Number represents the average percent impervious surface and includes <br /> consideration for the hydrologic properties of the soil. <br /> Assumptions:Time of concentration was calculated from high point on existing landfill to the <br /> location where Little Johns Creek exits the property(1.4 hours). SCS Type 1 <br /> rainfall distribution; Average 2 antecedent moisture conditions; 100-year 24-hour <br /> storm of 3.2 inches. <br /> SOURCE: R.W. Beck and Associates,March 1992. <br /> has relatively impermeable materials both above and below it. The water level in a confined <br /> aquifer is described as the height that the groundwater would reach in a well tapping the aquifer <br /> (since water in confined aquifers may be under greater-than-atmospheric pressure,the water may <br /> rise above the top of the aquifer when tapped by well). <br /> III.KS <br />