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