Weir Design to Maintain Effluent Quality from Dredged Material Containment Areas
Walski, Thomas M. and Paul R. Schroeder
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The suspended solids concentration in the effluent water from an upland containment area being filled with fine-grained material can be significantly influenced by the length of the weir and the depth of the ponded water. This study develops a procedure for designing and operating the weir to maintain good effluent quality, given a flow and dredged material type. Stratified-flow and sediment transport models were investigated to describe the depth of withdrawal, velocity profile, and effluent suspended solids concentration, given a concentration profile and flow. Field data on these parameters were collected at three sites - Yazoo River, MS, Fowl River, AL, and Oyster Bay, AL. The Waterways Experiment Station's selective withdrawal model developed by Bohan and Grace, modified to fit observed data, was selected as the basis of the design procedure for silt and saltwater clays and for freshwater clays. The nomogram relates the flow, weir length, ponding depth, and effluent suspended solids concentration. The designer manipulates these four variables until he reaches a satisfactory balance between weir length and ponding depth, based on his design flow and effluent goal. Sharp-crested rectangular or shaft-type weirs are recommended. Proper operation of the weir can ameliorate the effects of short-circuiting or an undersized basin. In general, the weir crest should be maintained at as high an elevation as feasible during dredging operations. Guidance for operation of the weir for special applications is also presented.