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• an infrequent event offers an upper parameter on which to base the design. A recurrence interval less rigorous, however, should be used. Although representing a recurrence interval between 50 and 100 years, the "Assumption B" storm and its consequence definition are a reasonable and practical basis of design. On this basis and the data developed in the CHZM Hill report, a runoff value of 0.7 cfs per acre is derived. Benched construction The primary element of the runoff control plan is the construc- tion of the embankment. The pile will be constructed in nearly horizontal lifts (6"-18") with outer faces having a maximum slope of 3:1 (3 feet horizontal to 1 foot vertical rise). After a depth of 45 feet has been constructed over the entire pile surface (or phased surface area), the embankment slope will be stepped in 15 feet forming a bench or terrace on the outer surface. Thus • every 45 feet of embankment height will have a 15-foot wide bench. The bench will be in-sloped, forming a water catchment for any surface water runoff falling on the slope between benches. The benches will be sloped on a 1 percent gradient from drainage breaks into a collection point. Spacing of drainage breaks and collection points are dependent on the area to be drained and the design storm. The selection of a 1 percent slope for the bench gradient is based on the use of a slope that is construc- tible with ease of control, and will carry the surface runoff accumulation to the collection points at velocities less than the material scour velocity. For this purpose, design velocities were kept to approximately 3 feet per second or less. Because accumulation of runoff along the length of the bench varies from near zero to a maximum just before the collection point, the velocities will be very low near the beginning of the bench runs. The low velocities may result in the deposition of material in these areas requiring maintenance of the benches. It is for • G-39