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The GSE Drainage Design Manual Chapter 4 — Design Methods And Concepts • Active Operation Stage I — Model leachate flow into the LCRS based on landfill at a representative point in time in the landfill's development phasing plan. The waste thickness might be on the order of half of the final thickness of the waste. The slope might be fairly flat, with an intermediate cover and fair vegetation. • Active Operation Stage II — Model leachate flow into the LCRS based on the landfill at final grades with an intermediate cover in place and fair vegetation. • Post -Closure Stage — Model leachate flow into the LCRS based on the final closure conditions. The landfill will be at final grades with a permanent cover in place. Often this condition is modeled in HELP as simply the amount of infiltration through the final cover system. Since this value is typically very small when compared to the value at other stages, a more complex analysis is not needed. Impingement rate, q;, should be obtained from HELP model for each of the assumed stages for the landfill. Required transmissivity can then be calculated for the each case as [Giroud et. al., 2000b]: 011q=q;.L sin,8 (4.13) where aeq = required transmissivity for geocomposites (m3/sec per in width); ql = liquid impingement rate (m/sec); L = horizontal length of slope (m); and J3= slope angle (degrees). It is generally convenient to prepare a table that presents impingement rate, stress, required transmissivity and allowable transmissivity for each of the stages. Typically, higher transmissivity values are required at low stress levels during initial stages of landfill filling process. As a landfill reaches its closure stage, significantly lower transmissivity is required for the drainage layer albeit at a higher normal stress. Example calculations are provided in Chapter 6 along with a typical table providing requirements for drainage layer at various landfill stages. In terms of thickness of the composite, or the maximum depth of leachate in the drainage layer, Equation 4.13 can be expressed as: k.tm - q; -L sin ,3 (4.14) where k = required permeability of the drainage layer (m/sec); and t,,. = maximum thickness of the drainage layer. 4.2.4 Leakage Detection System The main reason for including an LDS in a landfill is to capture and to laterally convey liquid entering due to leakage of a primary liner. Figure 4.7 presents a schematic of a leak detection system, indicating the primary source of leakage. There can be additional sources of leakage into the LDS including (i) construction and compression water already present in the LDS; (ii) consolidation water from the upper compacted clay liner (if a compacted clay liner is present); (iii) infiltration water from leaks in the lower geomembrane; and (iv) liquid flow from leakage of Page 4-9