2009-283B - Laserfiche WebLink

Home Browse Search

water and impounded stormwater. The clarified effluent will be routed to the perimeter ditch and drained off site . 4 .2 Grading the Deposition Material Following the completion of dewatering, the dried sediment must be graded to prepare for the next dredging operation . Grading — that is, distributing the mounded sand , shell , and gravel over the remainder of the containment area — serves a number of necessary functions . These include reestablishing the initial uniform 0 . 2 % slope from the inlet down to the weirs, restoring the effective plan area of the containment basin , and improving subsequent dewatering ofthe fine-grained material by separating successive deposition layers with a free-draining substrate . As discussed in the next section , grading also provides for stormwater runoff control . Finally, a series of post-grading topographic surveys will assess material consolidation and refine estimates of remaining storage capacity . 4. 2. 1 Control of Stormwater Runoff As stated , grading the dewatered deposition layer provides the additional benefit of allowing the control and release of stormwater that drains from the interior slopes of the containment dike as well as the dewatered sediment. A shallow, uniform slope (0 . 2%f, Section 2 . 1 . 2 ) toward the weirs ensures adequate drainage, eliminates ponding of runoff in irregular depressions, and minimizes flow velocities and the risk of channelization and erosion . In compliance with regulatory policy, a sump or retention area of adequate capacity should be constructed adjacent to the weirs (with the weir flashboards in place) to retain the runoff from the first 1 in . of rainfall . For the IR. 14 containment basin interior area of 12 . 87 acres (from the dike crest centerline inward), a circular basin with a radius of 86 ft and an average depth of 2 ft will provide a retention pond with the required minimum capacity- of approximately 46 ,700 W . A site operator would then be responsible for the gradual release of the ponded runoff at intervals determined by local weather conditions . Providing shallow trenches or swales from the center of the retention basin to one or more weir sections may also be necessary to facilitate the rapid removal of runoff. As discussed in Section 3 . 1 , the clarified runoff will be routed from the containment basin to the perimeter ditch via the weir discharge culvert . The perimeter ditch, in turn , will drain to the on-site mangrove 38