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Non-Water Release Corrective Action Plan Corral Hollow Sanitary Landfill 4.1.6 Summary of Impacts from Seismic Hazards In summary, the potential for impacts to the CHSL from ground rupture, seiche, tsunami, liquefaction/ dynamic settlement, or refuse/cover system slope failure resulting from the design MCE is considered negligible and outside the definition of a reasonably foreseeable impact. 4.2 PRECIPITATION IMPACTS Storms causing damage to the cover, exposure of waste, clogging of the drainage system, erosion or failure of slopes, are all possible due to excessive amounts of water. Therefore, excessive precipitation is a foreseeable casual event that can affect a landfill. Landfills are required to maintain systems to control run-on and run-off due to precipitation throughout the active and post-closure period. The standard for Class III Landfills, such as the CHSL, is to design for a 100-year, 24-hour storm event, which is defined as a one percent chance of occurring in any one given year (1/100= 0.01 = 1%). CalRecycle's BMP's indicate that 1,000-year, 24-hour storm events have occurred in California, and solid waste landfills that are Class II landfills are now required to be designed to withstand the 1,000-year, 24-hour storm event. Although the CHSL is a Class III landfill, CalRecycle's proposed BMP for precipitation, as a causal event, is the 1,000-year, 24-hour storm event. The drainage system at the CHSL was designed to covey a minimum 100-year, 24-hour storm event. Previous stormwater analysis at the site have used the both the rational method and Soil Conservation Service (SCS) methods to determine stormwater run-off. This analysis utilized WinTR55, developed by the SCS to determine stormwater runoff. SCS parameters were adjusted as necessary in order to best match the Rational Method parameters used in the previous analysis to provide a more direct comparison in stormwater run-off amounts and volumes between methods. From the National Oceanic and Atmospheric Administration (NOAA) (see Appendix C), the 1,000-year, 24-hour storm event is 4.07 inches of rainfall; which is an additional 1.26 inches of rainfall over the 100-year, 24-hour storm event. This event would generate a peak flow of 62.9 cubic feet per second and a volume of 478,290 cubic feet over the entire site. It has been determined that 4 of the 5 V-notch channels (drainage benches and perimeter) would overtop due to the run-off generated from a 1,000-year, 24-hour storm (see Appendix • C). However, the overflow would be contained within the benches and/or adjacent areas. It is 12 Geo-Logic A S S O C I A T E S