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landfilling and ancillary facilities which will drain to the proposed detention facility include <br /> approximately 270 acres. The area presently drains to Uttle Johns Creek which bisects the site <br /> in a northeast to southwest direction. Portions of the southwest comer of the site are located <br /> within the Little Johns Creek floodway. The current projected peak flow rate of Little Johns Creek <br /> is approximately 800 cis based on studies performed at the time that Farmingham Dam was <br /> constructed. <br /> Current Conditions Run-off Parameters <br /> Area: <br /> Approximately 270 acres, which is divided into four areas. See Figure A for the <br /> delineation of these areas. <br /> o Area A - Approximately 130 acres used for landfilling operation (either landfilling <br /> or borrow pits) <br /> o Area B - Approximately 108 acres undisturbed by landfill operations <br /> o Area C (1 &2) -Approximately 17 acres of borrow pit area excavated below grade <br /> which act as a closed depression and do not drain to Uttle Johns Creek <br /> o Area D - Approximately 15 acres in the FEMA flood plain <br /> Therefore, the total area contributing to Little Johns Creek is approximately 253 acres. <br /> Run-off Curve Number: <br /> It was assumed that all soils, including the existing cover over the garbage, are SCS l <br /> group D soils corresponding to highly plastic clays. Group D soils were chosen based upon a J <br /> review of the report by Twining Laboratories, October, 1991. The run-off curve numbers were <br /> obtained from Table 2-2a of the TR-55 manual; this table is included as Attachment B. <br /> o Area A - Assume 94 for newly graded areas <br /> o Area B-Assume 84 for open space with fair vegetative coverage (grass 50-75%) <br /> o Area D - Assume 100 for open water <br /> o Composite curve number 90 <br /> Time of Concentration: <br /> The time of concentration was estimated based upon TR-55 methods. The time of <br /> concentration was calculated from an approximate high point on the existing landfill area to the <br /> location where Uttle Johns Creek exits the landfill property. The method by which the time of <br /> concentration was calculated was obtained from the TR-55 manual and is also included in <br /> Attachment B. The calculation is found in Table A. <br /> r� <br />