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(Section 2 .3 . 6) will produce an additional 7,200 cy of material. Should some material excavated from the basin interior be unsuitable for dike construction, the ditch material can make up the deficit. Alternatively, the material excavated from the ditch can contribute to the dike requirement to reduce the excavation depth in the basin interior. The final excavation depth and distribution of material , determined in the final design phase, will reflect the results of detailed subsurface investigation. The interior of the containment basin must also be graded following excavation. Construction efficiency may initially dictate taking dike material from a perimeter trench inside the containment dike. However, before dredging operations begin, this trench must be eliminated and the site interior regraded to avoid flow channelization and unacceptable effluent quality. If left ungraded , the irregular topography within the basin will produce nonuniform flow and deposition patterns which, in turn, will result in isolated surface water ponding. Ponding will inhibit drying of the deposition layer and make initial attempts at surface trenching more difficult . For these reasons, a uniform grade with an adequate slope (about 0 .2%) must be provided from inlet to weir as part of initial facility construction . Thus, although the basin interior will maintain a mean excavated grade of -0.4 ft NGVD, the floor of the basin will slope uniformly downward from approximately +0 .4 ft NGVD near the inlet to approximately - 1 .2 ft NGVD at the weir. Once dredging operations begin , differential settling of varying grain size fractions ( i .e., rapid precipitation of the coarser fractions nearer the inlet with increasingly finer sediments deposited nearer the outlet) will maintain a rough downward slope from inlet to weir. 2.3 Additional Design Features 2. 3. 1 Inlet The number and locations of the dredge slurry outfalls, or pipeline inlets, govern the pattern of deposition within the containment basin . A single, moveable inlet offers several advantages over single or multiple fixed designs. A single, fixed inlet would produce a mound of coarse material at the fixed inlet point. If not mechanically redistributed, the mound would effectively reduce the basin ponding area. A multiple inlet manifold could overcome this disadvantage. However, the infrequent maintenance projected for this portion of the ICWW, once every 5 to 10 years, cannot justify the cost of a fixed, multiple inlet manifold system for the IR- 14 containment basin . More cost effective, a single inlet, periodically 12