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this reason as well as to facilitate revegetation work, that the bench width of 15 feet is maintained. The bench configuration has ample capacity to carry accumulated runoff at low controlled velocities. Figure G-5 graphically illustrates the planned bench construction and provides basic design details for the drainage collection system. While this drawing shows the drainage system for the final stages of the project, the collection systems indi- cated will be installed at the start of disposal operations in the active areas, and will be expanded over the life of the project. The location and size of the various components will be changed periodically as the operation progresses, but the early phases of the system will be laid out in conformance with the indicated plan. Interceptor channels The graded bench terraces are to be sloped from drainage breaks • to collection points. At the collection point on each terrace, a grated inlet of sufficient capacity to drain all runoff concen- trated at that bench level will be installed. The inlets will be interconnected with a corrugated steel pipe having banded watertight joints, running transverse to the benches down the slope. This pipe will be on a steep gradient, but because of its short duration, infrequent usage, material scour of the metal should not be a problem. The flexibility of the steel pipe ,is desirable over more rigid pipe materials; moreover, the longer lengths and easy installation will facilitate the progressive construction sequence necessary. That sequence of construction dictates that after a 45-foot lift is completed, a trench up the slope from the previous bench level be dug and the pipe from that bench be extended to the newly completed bench where an inlet catch basin is constructed and attached. A temporary plug covers the end of the pipe at each level and a locating marker erected. The trench must be backfilled and compactd to prevent the creation G-40