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Drainage Calculations Peak discharges and average velocities were calculated to adequately size drainage ditches and drain pipes by means of the Rational Method. Subwatershed slopes, areas, and overland flow lengths were obtained directly from the preliminary drainage plan and existing topography. Drain pipes and ditch configurations were modeled with FLOWMASTER to obtain velocities used in travel time calculations. Rainfall intensities used in the Rational Method calculations were based on rainfall intensity curves developed by the County of San Joaquin, Department of Public Works. Runoff coefficients ranging from 0.20 to 0.95 were used based on specifications from the Runoff Coefficient Standard, San Joaquin Improvement Standards, September 1991. Rainfall intensity curves were obtained from Standard Drawings D-4 and D-5. Hydrology calculations are included in Table E-1. Drainage Pipes and Ditches All closed conduits and drainage ditches have been designed to prevent overtopping during a 10- year/24-hour frequency precipitation event. Diversion ditches around the Composting area have been designed to prevent overtopping during the 25-year/24-hour precipitation event of 2.25 inches in accordance with CCR Title 14. All drainage pipes will be manufactured from polyvinyl cloride (PVC) and range from 8 to 21 inches in diameter. Drainage pipes will be placed at a minimum grade of 0.5%. Drainage ditches will be graded at a minimum slope of I% and will be a minimum of I foot deep with 2:1 side slopes. Retention Basin The proposed retention basin will retain all surface runoff generated from the site. The only outlet from the basin is by means of evaporation and percolation. The retention basin was designed to accommodate twice the volume of runoff generated from a 10-year/24-hour precipitation event (0.06 inches/hour) in accordance with San Joaquin County Improvement Standards, September 1991. LAPROJECT\397MAIN.M �Dmbcr 1993 E-2