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• a 24-hour, 1,000-year storm event and to protect the landfill from erosion. The preliminary design criteria for the stormwater sedimentation basins are listed below: • North Area surface water sedimentation basin volume 38 acre-feet • South Area surface water sedimentation basin volume 12 acre-feet • West Area surface water sedimentation basin volume 30 acre-feet • Maximum basin side slope 3:1 (H:V) • Minimum basin base slope 3 percent The sedimentation basins have adequate storage volumes to hold potential silt generated from the landfill based on the USLE analysis (see Appendix 1). 5.3.7 LANDFILL GAS GENERATION AND AIR EMISSIONS CALCULATIONS In accordance with the S)VAPCD regulations, a landfill gas control/recovery system has been designed for the future Forward Landfill. Calculations regarding • estimated gas generation and air emissions were conducted to properly design the gas control system. These calculations were made to provide for the construction of an efficient LFGES. The proposed LFGES will be expanded accordingly as the landfill develops. Estimates of the landfill gas generated in each of the current (year 2000) in-place refuse prisms were added to the estimate of the landfill gas generation from the future combined fill areas. The landfill gas generation rate estimates were determined using the US EPA "Landfill Gas Emissions Model Version 2.0" (LandGEM) and EPA "Compilation of Air Pollution Emission Factors (AP-42)". In accordance with AP-42 ("Compilation of Air Pollutant Emission Factors", 5'h Ed., EPA, 1997), a default k of 0.02/yr (the methane generation rate constant) is approved for use in the LandGEM program for arid areas like the Central Valley Region, which receive less than 25 inches of rain per year. A Lo value of 170 and 112.5 cubic meters of methane per megagram of refuse was also used in the LandGEM program for the in-place refuse at the existing Forward and Austin Road Landfills. From the compositional analysis of the material received at the Forward Landfill, a conservative estimate is that 31 percent of the accepted material is decomposable, whether it degrades easily or slowly. The landfill gas • generation rates are presented in Appendix H. Forward Landfill JTD 5-25 L\AI lied\2000.193\R eports\Jtd:Sec-5.0:05/24/02 BRYAN A.STIRRAT&ASSOCIATES