revised final closure plan - Salinas Valley Solid Waste Authority

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Revised Final Closure Plan July 2010 Crazy Horse Sanitary Landfill -21- 103-97133 conditions in the upstream phases could potentially overwhelm the existing benches and overside drains on the south slope and the subsurface HDPE pipe currently in the main channel alignment flowing from the entrance area to the southeast discharge at the end of the Module 1 perimeter road. Storm Water Sedimentation Basins and Retention Capacity After stormwater run-off and run-on has been conveyed away from the closed landfill, it enters a system of proposed energy dispersion and existing retention structures. Stormwater flow from Sedimentation Basin A, Subbasin C, and Subbasin B will discharge to a rip-rap energy dispersion apron at the location shown on Figure 17. The peak discharge to this apron will be approximately 162 cfs (Appendix E). The apron is sized with a riprap D 50 diameter of 12.5 inches. This classifies as a Light riprap (Caltrans) to be placed 2 feet deep over a 15-foot by 20-foot area. Flow exiting the riprap apron will enter an approximately 500-foot long natural drainage course as shown on Figure 17. This vegetated drainage will remove most sediment before flows enter Sedimentation Basin B. This drainage course discharges into the headwater pool of a culvert emptying into Sedimentation Basin B. Discharge from Sedimentation Basin B leaves the landfill property and flows into Pesante Creek. The peak discharge of Sedimentation Basin B was compared to estimates of pre-landfill discharge in accordance with Low Impact Development (LID) practices. A detention analysis of the two basins is detailed in Appendix E and summarized below. Sedimentation Basin A receives stormwater discharge from Subbasin A on the west side of the topdeck area. The southern low point of Sedimentation Basin A discharges into an existing 36-inch CSP which discharges to the rip-rap apron dicussed above. Ultimately discharge from Sedimentation Basin A contributes to peak flow into Sedimentation Basin B further downgradient (the lowest point of the site’s property). As discussed in Appendix E, Sedimentation Basin A has an adequately sized discharge structure (36” culvert) for the 100-year design storm. Hydraulic analysis of the culvert outlet flow capacity to the peak stormwater flow revealed no improvements to this basin should be required according to the analysis. To satisfy LID restrictions on peak discharge, Sedimentation Basin B will require some minor improvements to control peak discharge: 1. Installation of a sluice gate on the existing outlet to restrict peak discharge. 2. Raising the southern berm to provide required freeboard. 3. Installation of a diversion channel to accommodate 100-year peak flow into the basin. The existing Sedimentation Basin B outlet structure consists of a concrete-lined trapezoidal weir approximately 8 feet wide and 2.5 feet deep with an inlet elevation of 312-ft MSL. The basin is surrounded by a mixture of tall vegetation and trees, thus protected from wind induced wave action. The sideslopes of the weir are approximately 2H:1V. Flow calculations indicate the weir can adequately manage peak discharge off of the proposed artificial turf final cover system from the 100- yr storm event. However, that peak flow would exceed the estimated peak flow from pre-landfill conditions. A flow restriction device is required to reduce peak discharge from the basin to that estimated for the pre-landfill condition. As discussed in Appendix E, a sluice gate installed to a height of 1.3 feet above the bottom of the existing 8-foot wide weir will adequately restrict peak flow as required. The sluice gate will be fastened to the existing concrete lining of the weir and supported if required to resist any hydrostatic forces that may develop on the upstream side. Golder Associates
Revised Final Closure Plan July 2010 Crazy Horse Sanitary Landfill -22- 103-97133 Capacity estimates were performed in Appendix E with the sluice gate in place. The calculations indicate the surface water elevation in Sedimentation Basin B will rise to an elevation of approximately 314.5-ft MSL at the time of peak flow into the basin. This is the current elevation along portions of the existing berm along the southwestern side of the basin. Therefore, the southwestern berm of the basin will be raised to an elevation of 315.5-ft MSL to provide a nominal 1- foot of freeboard from the top of the estimated maximum water surface elevation in the basin to the top of the berm. Retention time and particle settling velocity calculations (Appendix E) indicate any eroded sand ballast component of the artificial turf final cover system should be entirely retained in the basin at peak flow conditions (100-yr, 24-hr flows) during the postclosure period. Figure 17 indicates a diversion channel is to be installed to accommodate peak inlet flows to Sedimentation Basin B. The existing 36-inch culvert inlet to the basin is inadequate to manage that peak flow condition. To avoid uncontrolled flooding in the area surrounding the headwater pool, installation of the diversion channel is necessary. The channel will be 4 feet wide at its base with 2H:1V sideslopes. It will be graded at a slope of 3.6-percent from the headwater pool to the basin. Subsurface Drainage. Drainage media in the structured geomembrane will provide adequate subsurface drainage of the artificial turf on the steeper slopes (2.5H:1V, and those approaching 2H:1V) present within the manufacturers recommended factor of safety. However, the length of the 3H:1V slopes on the west side of the landfill will require mid-slope outlets of the drainage media to avoid saturated conditions within the sand ballast material. 2.15 Slope Protection and Erosion Control 27 CCR 21145, 21150, and 20365(c) Closure will incorporate the “Drainage and Erosion Control Program” to minimize sand ballast erosion. This program addresses drainage facilities, design slope length, and vegetation. The major program elements are summarized below: • A diversion berm graded to a flowline slope of 1-percent will be placed to intercept runoff on the topdeck and to direct this runoff to overside drain inlets. The purpose is to reduce the erosion associated with the high velocities of runoff flowing down steep gradients. Flow calculations included in Appendix E indicate flow velocities (100-yr storm flows) of less than 5 feet per second and subcritical flow conditions as recommended for vegetated soil. That design parameter will be used to minimize hydraulic stress on the sand ballast where possible. • The Final Drainage Plan will use corrugated HDPE pipe for overside drains on the sideslopes to contain and control high velocity flows. • Monthly inspection and following high intensity rainfall periods to identify and schedule repairs of eroded areas with particular attention paid to the existing vegetative layer in Module 1. • Temporary repairs, including the use of geosynthetic tarps, when the ground is too saturated for equipment to make repairs without causing additional damage (Module 1). • Annual maintenance of the sedimentation basins, if necessary. Golder Associates
Page 1 and 2: REVISED FINAL CLOSURE PLAN (2010) C
Page 3 and 4: Revised Final Closure Plan July 201
Page 27: Revised Final Closure Plan July 201
Page 41 and 42: TABLES Table 1 - CalRecycles Final
Page 43 and 44: Initial Closure Final Cover Phase I
Page 45 and 46: This figure was originally produced
Page 47 and 48: CRAZY HORSE CANYON ROAD CRAZY HORSE
Page 49 and 50: A - 1 - - TYPICAL ARRAY SECTION LEG
Page 51 and 52: ID Task Name Duration Start Finish
Page 53 and 54: LEGEND NOTES REVISED FINAL CLOSURE
Page 55 and 56: LEGEND A-55 SW-1 NOTES REVISED FINA
Page 57 and 58: LEGEND E A-15 NOTES REVISED FINAL C
Page 59 and 60: NE SW A 4 12 LANDFILL CROSS SECTION
Page 61 and 62: LEGEND NOTES J 14 14 OVERSIDE DRAIN
Page 63 and 64: 1 16 16 2B 16 16 2A 16 16 U 4 16 MA
Page 65: V 18 18 NOTES V 18 18 MODULE 1 FINA

revised final closure plan - Salinas Valley Solid Waste Authority

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