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5. Miscellaneous Paper

Title:	Tampa Bay dredged material disposal site analysis
Authors:	United States. Army. Corps of Engineers. Jacksonville District Williams, David T.
Keywords:	Dredging Dredged material disposal Waste disposal sites Dredging spoil Ocean waste disposal Tampa Bay (Fla.) Tampa Bay Channel Deepening Project
Publisher:	Hydraulics Laboratory (U.S.) Engineer Research and Development Center (U.S.)
Series/Report no.:	Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; HL-83-8.
Description:	Miscellaneous Paper Abstract: The Jacksonville District is considering an open-ocean disposal site, designated as Al ter native Site 4, for placing dredged material from the Tampa Bay Channel Deepening Project and from the annual maintenance of that project. Initial dredging would remove 3.6 million cu yd. The annual maintenance dredging to be placed at the site is predicted to be 0.4 million cu yd. The proposed disposal site is a square, 2 n.m. on each side, located in the Gulf of Mexico 17 n.m. offshore in water 72 ft deep. The material to be disposed is predicted to be 35 percent silt and clay, 43 percent sand, and 22 percent gravel. Currents in the area are produced by wind stress, the Gulf loop current, tides, and tropical storms. Of these mechanisms, the first two dominate in time but hurricane currents dominate in magnitude. WES collected current data during a 2-month period, 9 March through 12 May 1983, but no other field data are available for currents. General guidance published in the literature cite normal Gulf currents of up to 1 knot, and hurricane-generated currents of up to 4 knots can be expected in this vicinity. Wave data for 29 years are available at Fort Myers. Using these data bases, probability density function curves of wave conditions and current speed were developed, from which the expected annual value of sediment transport was calculated using a bed shear stress approach. This study was undertaken to predict both the short-term and the long-term fate of sediment placed in Alternative Site 4. The short-term fate refers to the fate of the turbidity plume which forms as the water column entrains sediment during disposal. In this case, the short-term fate was analyzed using a numerical model. The plume was tracked for 15,000 ft (4-1/2 km) from the dump, and by that distance the average concentration had reduced to 2 mg/l, which is the background concentration for ocean water at Site 4. At the point where the plume crossed the boundary of Site 4, the average concentration was 4 mg/l. The amount of sediment entrained in the plume is predicted to be 35 to 40 cu yd per dump or about 2-1/2 percent of the load. Long-term fate refers to the resuspension, transportation, and redeposition of sediment from the primary deposition mound. The critical shear stress approach was used to predict when particles of a given size would become mobile. Both waves and currents were included in the computation of bed shear for resuspension using a Bijker-type equation, but only currents under resuspension conditions were considered in the transportation analysis. Five size classes were identified ranging from clay/silts (0.03-mm particle diameters) to gravels (32-mm diameter). Because the larger particles move more slowly and less frequently than the smaller ones, sorting and armoring processes were included in the analysis. The predicted depth of scour is 0.1 ft (30 mm) per year. In the early years of disposal, this amounts to about 3 percent of the annual volume disposed. By 40 to 45 years, the predicted scour (resuspension) rate will increase to about 20 percent of the annual disposal volume. By about the year 70, the edge of the disposal mound will have reached the boundary of the disposal site and the depth at the center will be 18ft (5-1/2m), indicating a useful life for Site 4 of about 70 years. Procedures have been developed to predict the growth of the disposal mound and annual loss rate should the annual maintenance rate change at any point in time.
Rights:	Approved for public release; distribution is unlimited.
URI:	http://hdl.handle.net/11681/10651
Appears in Collections:	Miscellaneous Paper