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3-8 3.6 Drainage and Erosion Control 3.6.1 Drainage Control System Design The primary function of the FL Stage 1B Partial Final Closure drainage control system is to collect and convey storm water in a controlled manner to minimize erosion and infiltration of storm water into the refuse prism. The following sections describe the site hydrology and proposed drainage control features. 3.6.1.1 Hydrology A hydrology study was prepared in August 2001 for the proposed conditions at the entire FL site. The study was conducted in accordance with 27 CCR, Section 20365. Updated hydrology calculations were prepared by SWT in support of the Stage 1B Partial Final Closure area. The objective of the hydrology study was to calculate storm water run-off for sizing and location information to integrate with the Stage 113's storm drain facilities at closure. The Rational Method hydrology utilizing guidelines set forth in the San Joaquin County hydrology manual was used to calculate peak discharge rates for a 24-hour, 1,000-year storm event. A computer program developed by Advanced Engineering Software was used to compute the run-off. The hydrology study map indicating drainage subareas and discharge points and calculations for onsite and off-site flows is presented on Figure 1 of Appendix F. A summary of the peak discharge rates are included in the hydrology study calculations included in Appendix F. To size the downdrains for the Stage 1B Partial Closure Area, the Cl factors (runoff coefficient/rainfall intensity) was determined from the hydrology study. The Stage 1B partial closure area was broken into smaller subareas to calculate the flow for each downdrain. The calculations for the downdrain sizing are included in Appendix F. In addition, 100-year flood conditions were also reevaluated to develop a maximum flood elevation along the north and west perimeters of the site directly adjacent to Little John's Creek. 3.6.1.2 Proposed Final Drainage Control System The proposed final drainage control system for the FL Stage 1B is shown on Figures 7, 8 and 9. North Slope Runoff is directed down slope to benches, to bench crossings, to downdrains that join the downdrains constructed with the Stage 1A Partial Final Closure. The westerly access road will serve to intercept a portion of future top deck run-off and upper westerly slope run-off. The trapezoidal channel on the east side of the westerly access road will convey tributary flow southwesterly along the access road to a road crossing, and down drain to the West Sedimentation Basin. Flow from the lower bench (closed per Stage 1A) will also be conveyed southwesterly via a modified bench crossing, down drain, to the northerly (perimeter) access road/berm, and be extended adjacent to Little John's Creek from the Stage 1A Partial Final Closure limits (perimeter berm) southerly to the south central inlet located on the east side of the West Sedimentation Basin. The placement of additional soils along the northerly perimeter road (perimeter berm) serves to increase the elevation of the road above historic and potential flood levels. The flow from the perimeter access road/berm is directed southerly to a road crossing and downdrain to the West Sedimentation Basin. Stage 1B drainage improvements details are presented on Figures 4 and 6. Forward Landfill Stage 113 Partial Final Closure/Post-Closure Maintenance Plan-June 2011 SWT Engineering z:\projects\allied waste\forward\11-1014 partial fnl clsr pin\partial final closure plan document\text\sec 3.doc