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water and impounded stormwater. The clarified effluent will be routed to the perimeter ditch and drained <br /> off site . <br /> 4.2 Grading the Deposition Material <br /> Following the completion of dewatering, the dried sediment must be graded to prepare for the next <br /> dredging operation . Grading — that is, distributing the mounded sand , shell , and gravel over the remainder <br /> of the containment area — serves a number of necessary functions . These include reestablishing the initial <br /> uniform 0 .2% slope from the inlet down to the weirs, restoring the effective plan area of the containment <br /> basin, and improving subsequent dewatering ofthe fine-grained material by separating successive deposition <br /> layers with a free-draining substrate. As discussed in the next section, grading also provides for sormwater <br /> runoff control . Finally, a series of post-grading topographic surveys will assess material consolidation and <br /> refine estimates of remaining storage capacity. <br /> 4. 2. 1 Control of Stormwater Runoff <br /> As stated , grading the dewatered deposition layer provides the additional benefit of allowing the <br /> control and release of sormwater that drains from the interior slopes of the containment dike as well as the <br /> dewatered sediment. A shallow, uniform slope (0 .20/of, Section 2 . 1 .2) toward the weirs ensures adequate <br /> drainage, eliminates ponding of runoff in irregular depressions, and minimizes flow velocities and the risk <br /> of channelization and erosion . In compliance with regulatory policy, a sump or retention area of adequate <br /> capacity should be constructed adjacent to the weirs (with the weir flashboards in place) to retain the runoff <br /> from the first 1 in. of rainfall . For the IR- 14 containment basin interior area of 12. 87 acres (from the dike <br /> crest centerline inward), a circular basin with a radius of 86 ft and an average depth of 2 ft will provide a <br /> retention pond with the required minimum capacity- of approximately 46 ,700 W . A site operator would then <br /> be responsible for the gradual release of the ponded runoff at intervals determined by local weather <br /> conditions. Providing shallow trenches or swales from the center of the retention basin to one or more weir <br /> sections may also be necessary to facilitate the rapid removal of runoff. <br /> As discussed in Section 3 . 1 , the clarified runoff will be routed from the containment basin to the <br /> perimeter ditch via the weir discharge culvert. The perimeter ditch, in turn, will drain to the on-site mangrove <br /> 38 <br />