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The GSE Drainage Design Manual Chapter 4—Design Methods And Concepts • Active Operation Stage I — Model Ieachate 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 Sta e — 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 stages,a more complex analysis is nthe amount of infiltration through the final cover system. Since this value is typically very small when compared to the value at other ot 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): _ qf .L ®K9 sin,6 (4.13) where ®req = required transmissivity for geocornposites (in3/see per m width impingement rate(m/sec);L=horizontal length of slope(m);and/3=slope angle(degrees).liquid It is generally convenient to prepare a table that presents impingement transmissivity and allowable transtnissivity foa each of the staes rate, stress, required transmissivity values are required at low stress levels duringg Typically, higher ing Process. As a landfill reaches its closure stage, significantly lowerltransmisse v is requireds of landfill lfor 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.t = q, .L mH" sin,8 (4.14) where k=required permeability of the drainage layer(m/sec); and tar=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 Ieakage 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