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Drainage Analysis Methodology <br />The permit level drainage analyses perforn1ed to assess stormwater flows for Foothill Sanitary <br />Landfill PCP MP utilized procedures in the County of San Joaquin Hydrology Manual 1 <br />(Hydrology Manual). Each perimeter drainage ditch was sized to handle run-off from the <br />regulatory required 100-year 24-hour storn1 event. <br />The rational method from the Hydrology Manual was used to estimate stonnwater run-off. <br />Specifically: <br />Q= CIA, where: <br />Q is the flow in cubic feet per second (cfs), <br />C is the runoff coefficient (dimensionless), <br />I is the rainfall intensity in inches per hour (in/hr), <br />A is the drainage area in acres (Ac). <br />Basin storage volume was estimated by the equation: <br />V = CIA, where: <br />12 <br />Vis the stom1water volume in acre-feet (Ac-ft), <br />The runoff coefficient, C, for landfill areas is dependent on surface slopes, soil infiltration <br />potential, vegetative cover, and the potential for "storage'', or ponding. Caltrans· Highway <br />Design Manual provides a figure for addressing these parameters and selecting an appropriate C <br />value. A value of 0.4 was selected for the landfill calculations <br />The rainfall intensity, I, was obtained from the Intensity-Duration-Frequency tables in the <br />Hydrology Manual. As necessary, Inlet Time or time of concentration (tc) was assessed to <br />determine the applicable intensity, I, for each calculation. <br />Contributory drainage areas, A x, were delineated and measured on the landfill grading plan for <br />the drainage analyses. (See Drainage Map, Figure 1). <br />The calculated stonnwater flows for contributory areas to each perimeter drainage ditch, and the <br />100-year 24-hour stormwater volume contributory to the stonnwater basins are presented herein. <br />Each open perimeter drainage ditch is sized for capacity using Manning's equation. <br />The typical detention basins shown on the landfill grading plan have adequate capacity for <br />mitigating peak flows and sediment removal before either release to the adjacent offsite natural <br />water course(s), or continued flow within the perimeter landfill ditches for release into the larger <br />existing on-site southeast detention/retention area. <br />1 "Hydrology Manual for San .Joaquin County, Department of Public Works:· September 1997. <br />O:\San Joaquin County\136605 Foothi/1\Prelim CP\Orainage Analysis\FSL Appx A Drainage Analyses R 1.doo: July 28, 2010