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5.2.6.1 ON-SITE DRAINAGE CONTROL Permanent storm water runoff and drainage control facilities on site were designed to carry the peak discharge resulting from a 1,000-year, 24-hour storm event, as required by 27 CCR, Section 20320 for a Class II waste management unit. A discussion of the design Calculations is presented in Section 5.3.5. The drainage network for the completed landfill is designed to carry storm water at velocities that will control run-off and minimize erosion. The Forward Landfill surface water control plan consists of an integrated system of bench ditches, perimeter channels, top deck perimeter berms, and storm water sedimentation basins (shown on Drawings 7 and 9). The final landfill grades are designed so that surface water will run off the landfill via sheet flow until intercepted by a deck berm or bench ditch. The bench ditches subsequently drain toward trapezoidal AC down drains or corrugated steel pipes which discharge to perimeter channels. Finally, the perimeter channels drain to a stormwater sedimentation basin. All landfill surface water is routed to one of three sedimentation basins see Drawing 9. The sedimentation basins for the existing Forward Landfill are located in the southwest corner of the site, on either side of the South Branch. The sedimentation basin for the existing Austin Road Landfill is located southwest of the existing Austin Road Landfill. Water collected in the sedimentation ponds is sampled in accordance with the site's Stormwater Pollution Prevention Plan (SWPPP) before it is discharged to the North or South Branches. The erosion control measures incorporated in the site design include the following: • Collection and control of runoff, diverting it away from highly erodible areas. • Construction of intermediate and final landfill slopes with drainage benches at intervals designed to control slope runoff velocities and volumes. • Hydroseeding with fast germinating drought-tolerant grass seed on intermediate surfaces that will be exposed for more than 180 days and all surfaces that are at final grade. Seeded surfaces will be watered until growth is well established. Proposed landfill slopes for the three major areas of the Forward Landfill were evaluated using the Universal Soil Loss Equation (LISLE) (see Drawing 11). The Forward Landfill}TD 5-10 L\Allied\2000.193\Reports\JTD-PC PCMP702:Sec-5-0-5/24/02 Revision 1:07/16/02 BRYAN A.STIRRAT 6 ASSOCIATES