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The GSE Drainage Design Manual Chapter 4 — Design Methods And Concepts uniform and less dependent on short-term (i.e., hourly) events, as compared to the final cover. A number of computer models and manual methods are available for performing water balance analysis, including: (i) the Simplified Manual Method [Koerner and Daniel, 1997]; (ii) the HELP Model [Shroeder, et. al., 1994a, 1994b]; (iii) the LEACHM Model [Hutson and Wagenet, 1992]; (iv) UNSAT-H [Fayer and Jones, 1990]; (v) SoilCover [SoilCover, 2000]; and (vi) HYDRUS-21) [Simunek et. al. 1999]. A detailed review of each of these water -balance analysis models is provided in the EPA Guidance Document titled "Technical Guidance for RCRA/CERCLA Final Covers" [2004], which was still in draft form at the time this manual was prepared. We recommend the use of HELP Model for calculating impingement rate within a leachate collection layer for the following reasons: (i) relative simplicity when running simulations; (ii) industry -wide familiarity with the software; (iii) availability of input parameters and internal database of information; and (iv) reasonably accurate and conservative results. Figure 4.6 shows a typical cross-section of a landfill leachate collection system. Landfill LCRS impingement rates depend on the operational stage of a landfill, which can be conveniently broken down as follows: (i) initial operation stage; (ii) active operation stage; and (iii) post -closure stage. Early in the landfill operation, surface water control may not be well established and may allow for a relatively large portion of surface water to infiltrate into the LCRS. As filling progresses, the use of protective soil and improved grading techniques reduce the amount of infiltration into the waste, thus decreasing the LCRS flow rate. In the post -closure period, the application of the final cover system greatly reduces the amount of infiltration into the waste, and thus greatly reduces the amount of leachate entering the LCRS. We recommend breaking the landfill into several operational stages (as few as three and as many as six) with varying geometry, waste thickness, cover slopes and cover materials. Separate HELP analyses should be performed for each operational stage modeled. An example of what the designer might consider when modeling a landfill broken into four stages is presented below: • Initial Operation Stage — Model leachate flow into the LCRS based on a "fluff' layer of waste being placed in the landfill cell. A typical waste thickness might be on the order of 10 feet. The slope might be fairly flat (-2%) with a six-inch daily cover layer. `past erj-�f- (�': .�Drainage Layer a .'"..� Primary Liner a.;: L Drain Figure 4.6 Typical landfill leachate collection and removal system configuration (LCRS). • Page 4-8