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maximizes the retention of suspended solids. Specific expected performance characteristics of the weir system are discussed later in this section . As mentioned above, the width of the weir crest affects withdrawal depth . Weirs typically employed in dredged material containment facilities are described as sharp-crested or narrow-crested based on their crest width relative to the static head over the weir. A weir is described as sharp- crested if the thickness ( T) of the weir crest is significantly less than the static head (H) over the weir, typically HIT > 1 . 5 . Under specific conditions, sharp- crested weirs may result in a shallower withdrawal depth than weirs with a broader crest, that is, for weirs with a value HIT :g 1 . 5 (Walski and Schroeder, 1978 ). To withstand hydrostatic pressure and reduce deformation and seepage the proposed weir design specifies the flashboards (discussed below) to be nominal 6 in . X 6 in . timbers . The timbers ' finished dimension of 5 . 5 in . , combined with the design static head overthe weir of 4. 9 in . , yields H/T = 0 . 89, a ratio within the range of a narrow-, rather than a sharp -crested weir system . However, in the present application, the use of a narrow-crested weir should be adequate (Gallagher and Company, 1978) . The weir parameter that most directly influences withdrawal depth and effluent quality is weir crest length . The Selective Withdrawal Model (Walski and Schroeder, 1978) developed by the U . S . Army Engineer Waterways Experiment Station ( WES) under the Dredged Material Research Program (DMRP) relates weir crest length to withdrawal depth through the parameter ofweir loading. Weir loading is defined as the ratio of the liquid discharge of the dredge (Q) to the effective weir crest length (B ) . Project planning guidelines used by the Jacksonville District Corps of Engineers indicate that an 18 - in . O . D . dredge will likely be used for future channel maintenance in Reach III of Indian River County . Given typical design output specifications for a 244n . dredge (discharge velocity of 16 ft/sec, a volumetric discharge of 3 , 560 cy/hr, and a 20/80 solids/liquid slurry mix), the Selective Withdrawal Model indicates that a weir crest length of 24 ft should produce a 2 . 0-11 withdrawal depth, based on a design weir loading (QB) of 0 . 89 ft'/ft• sec . As discussed in the next section, this depth falls below the recommended minimum ponding depth at the weir (2 . 8 ft) and thus should not result in the release of effluent with a high suspended sediment concentration . Moreover, DMRP research indicates that under field conditions, the actual depth of withdrawal may fall significantly below that predicted by the WES Selective Withdrawal Model . Therefore, the use of the WES Selective Withdrawal Model provides a conservative containment basin design . 14