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5-15 As a conservative measure, the HELP modeling that was completed for the project anticipated that the subgrade (liner) gradient beneath all development areas would be 2%, and that yearly thickness of refuse would no greater than 10 feet. While wastes that are initially placed in landfills are typically dry, with a moisture content that is well below their field capacity (moisture holding capacity), for the Forward Landfill leachate generation analyses, as a conservative measure it was assumed that the upper 10 -foot refuse section each yearly simulation would exist at its field capacity. The moisture content for underlying refuse layers was manually specified for each yearly simulation based on the results that were calculated for the previous year. As additional conservative measures, runoff was assumed to be impeded by a "fair stand of grass" though, for simulating transpiration, a bare ground (daily cover) condition was assumed. Current estimates are that the Forward Landfill may operate through approximately 2037 (GLA, 2012) (with the permitted closure year remaining at 2021 as discussed in Section 4.7). During this period, the landfill is expected to be developed within four major areas that have differing footprint areas, subgrade gradients, and anticipated refuse thickness as shown in Appendix H. The HELP simulations were run for a period of 30 years; 20 years of active refuse disposal, and 10 years of the post -closure period with final cover. The simulations conservatively ignored placement of intermediate cover soils, and assumed that a one -foot thick LCRS drainage layer would be placed at a 2% gradient on top of a geomembrane layer at the base of refuse. LCRS collection pipe was assumed to exist on 200 -foot centers. Model Results As shown below, the peak daily leachate that is calculated to be generated in the HELP analyses is approximately 0.027 inches. As shown in the graphs included in Appendix H, this occurs within five years of refuse placement with periodic spikes that follow and which may be related to variable annual rainfall. Within a one -acre landfill area, the peak daily leachate generation amounts to approximately 4,073 gallons or approximately 0.0045 cfs. The peak daily head on the liner system was calculated to be approximately 1.6 inches Accordingly, the 200 -foot spacing of the LCRS appears more than adequate to keep less than one -foot of leachate head on the liner. LCRS Pipe Capacity Since the largest tributary area within the liner system is approximately 23 acres, the peak daily flow through leachate collector pipes to the leachate sumps will be approximately 0.14 cfs. Recognizing that 27 CCR regulations require that the LCRS pipes be designed to accommodate double the peak daily leachate generation rate, the LCRS pipe at the facility should be designed to accommodate approximately 0.28 cfs. The maximum capacity of a 6 - inch diameter mainline LCRS pipe at a 2% slope is 1.03 cfs, therefore the factor of safety is Forward Landfill SWT Engineering Joint Technical Document - April 2014 z:\projects\allied waste\forward\five year permit rvw 2013\jtd-5 yr pr 2013\text\sec 5 final.doc