2009-283B - Laserfiche WebLink

Home Browse Search

Immediately before the first placement operation at Site IR- 14, the weir crest should be set to an initial weir crest elevation of +3 . 6 ft NGVD to provide the recommended mean operational ponding depth of 4 . 0 ft . Thus, the initial maximum ponded water elevation at the facility ' s first use lies at or below the existing mean elevation within the basin footprint . Given the initial 0 .2% bottom slope within the basin , a 4 . 0 - ft mean ponding depth corresponds to a 4 . 8- ft depth at the weirs . The initial weir setting prevents the release of effluent until the ponded water reaches its recommended operational depth . During this initial operational phase, the design dredge discharge (3 , 560 cy/hr) will increase the ponding depth at a rate of approximately 2 . 1 in ./hr and increase the ponded water surface elevation ( ponding depth plus deposition layer) at a rate of approximately 2 . 6 in ./hr. This relatively slow rise should allow for close continual monitoring of the entire dike perimeter for indications of slope instability . Inspection is most critical during periods when the ponded water surface elevation is allowed to rise above its previous maximum . Experience has shown that as the ponded water percolates into the interior dike slope, the coarser dike material filters the fine suspended sediment . This filtering reduces the dike permeability and thus decreases the dike ' s susceptibility to piping and saturation . As stated above , no effluent should be released until the surface of the ponded water approaches the weir crests ' initial setting of +3 . 6 ft NGVD . Notably, a flow control structure such as a weir cannot improve effluent quality beyond that of the surface water immediately upstream . Thus , the decision to release effluent over the weirs should be based on the analysis of water samples taken immediately upstream of the weir at the maximum depth of withdrawal . For Site IR- 14, recommended WES procedures (Section 2 . 3 . 2 ) determined this depth to be 2 . 0 ft, based on the design dredge discharge of 3 , 560 cy/hr and a design weir loading of 0 . 89 ft'/ft- sec . If testing shows that the turbidity of the interior surface waters remains unacceptably high, the release of effluent must be delayed by one of two methods : ( 1 ) raising the weir crests by adding flashboards or (2) shutting down the dredge plant. Additional alternative measures may include installing turbidity screens surrounding the weirs . _ Once the weir has begun to release effluent that meets established performance criteria ( Section 2 .3 ), the outflow over the weir must not exceed the design dredge discharge, or 0 . 89 ft'/ft- sec . As discussed below, static head over the weir then becomes the most practical criterion to ensure that the flow over the weir, and thereby the effluent quality, remains within the design limits . 31