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ensure that all three weir crests remain at equal elevations, and that flow over all three weirs (as measured by the depth of flow over the weir crests) remains essentially equal , within the limits of adjustment provided by the weir boards . 3. 3. 1 Operational Ponding Depths Before dredging begins, the contractor must set the initial elevation of the weir crest . This initial weir crest elevation corresponds to the minimum ponding depth at the weirs that must be reached before the contractor releases any water from the basin. The minimum ponding depth reflects considerations of the basin 's retention performance with respect to the settling characteristics of the dredged sediment. The retention analysis presented in the IR- 14 Management Plan (Taylor et al . , March 1999) recommended a minimum 4. 0-ft ponding depth based on a retention analysis that reflected available sediment data and the performance characteristics of an 18-in . dredge . The analysis determined that for the 10 . 18-acre interior plan area of the IR- 14 containment basin, a 2 .0-ft ponding depth provided a maximum retention time of 6 .2 hours. This time exceeds the 4 . 1 hours required for the finest fraction of sediment anticipated for placement in the IR- 14 basin to settle out of the 2 .0-ft withdrawal depth, based on the projected settling characteristics of the sediment to be dredged . However, consideration of actual field conditions suggests that additional retention time may be required to provide adequate settling and further reduce effluent turbidity levels . Given that retention time is directly related to' ponding depth, increasing ponding depth to 4 . 0 ft also increases the maximum retention time to 12.4 hours, or over three times the required settling time . Experience with other similar FIND dredged material containment facilities has shown that increasing the ponding depth to 4 .0 ft should not compromise dike stability. As a result, to ensure that the DU- 8 facility produces acceptable effluent under all foreseeable conditions, the recommended mean operational ponding depth is set at 4 .0 ft, with 5 .0 ft the recommended maximum ponding depth . Section 3 . 3 . 2 further discusses maximum ponding depth and related dike inspection requirements . Based on a recommended 4.0-ft mean operational ponding depth, the contractor should set the weir crest to an initial elevation of 4. 0 ft above the -0.4 ft NGVD mean elevation of the basin floor, or +3 . 6 ft NGVD . Given the slope of the basin floor, this initial weir crest elevation also corresponds to 4 . 8 ft above the - 1 . 2 ft NGVD elevation of the weir base or sill . Lacking a bottom slot and spline to tie it to the weir base plate, the bottom weir board should be set in a thick layer of roofing asphalt or other approved sealant to minimize the potential for water to leak below the bottom board . Because the water nearest the basin floor contains the highest concentration of suspended sediment, the contractor must take particular care to seal the bottom weir board to the weir base plate as well as to the flange extension. All - 15 -