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|Title:||Design and management of dredged material containment areas to improve hydraulic performance|
|Authors:||Hydraulics Laboratory (U.S.)|
Dredging Operations Technical Support Program (U.S.)
Shields, F. Douglas.
Thackston, Edward L.
Schroeder, P. R. (Paul R.)
Bach, Donald P.
|Keywords:||Confined disposal facility design|
Dredged material disposal
Modeling dredged material containment area
Residence time distribution
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: Dredged material is often disposed into diked containment areas (DMCAs). Typically, the dredged material is pumped into a containment area as a slurry that is 10- to 20-percent solids by weight. Water and solids separate in the containment area because of gravity sedimentation, and the clarified water is discharged over a weir and through a culvert to a receiving water body. The suspended solids concentration of DMCA effluent is inversely related to the residence time of water in the DMCA. The longer the water stays in the DMCA before it is discharged, the lower the effluent turbidity and suspended solids concentration will be. Residence time is therefore an important consideration in design of DMCAs. Designers attempt to size DMCAs to provide adequate sediment storage and effluent quality at least cost. The currently recommended procedure for sizing DMCAs uses the estimated average residence time. Prior to this study, estimates of mean residence time were based on only a few observations of operating DMCAs. The mean residence time was simply estimated to be 0.44 times the ponded volume divided by the inflow rate. This study presents observed residence time distribution data from some 12 DMCAs. These data were collected by injecting Rhodamine WT dye at the inflow point and monitoring dye concentration in the outflow. The resultant time-concentration curve is identical to the residence time distribution during the dye test. In addition, similar data for chlorine contact chambers, physical models, and waste stabilization ponds found in the literature are also examined. A composite data set from all different sources showed a strong relationship between the ratio of mean residence time to theoretical residence time (theoretical residence time is equal to the pond volume divided by the average flow rate) and the pond length-width ratio. Baffles or spur dikes can be used to increase the effective length-to-width ratio of a given DMCA, thereby increasing the mean residence time without increasing the overall size. The relationship between mean residence time and length-width ratio was incorporated into the DMCA design procedure, and a process for selecting DMCA size, shape, and spur dike layout to achieve minimum cost resulted. The utility of state-of-the-art mathematical hydrodynamic models for simulating flow through DMCAs is examined in Appendix A. It was found that the mathematical models are capable of reproducing the observed residence time dtstributions fairly well if model coefficients are arbitrarily adjusted to achieve best fit. However, better field data will be required to develop reliable a priori modeling capability. Although additional research based on higher quality field data is needed to provide more definitive information on DMCA residence time distribution, at this point it is apparent that in many cases DMCA economics could be improved by the use of baffles or spur dikes.
|Appears in Collections:||Technical Report|