Factors Governing the Distribution of Dredge — Resuspended Sediments

Field observations are analyzed to determine the primary factors governing the distribution of sediments suspended by clam-shell bucket dredge operations. These data show the plumes produced under typical estuarine conditions to be relatively small scale features having maximum longstream dimensions of approximately 700 m. Plumes can be considered to consist of three contiguous zones: an initial mixing zone, a secondary zone and a final mixing zone. The initial mixing zone immediately adjacent to the dredge has dimensions governed by bucket induced mixing and suspended material concentrations determined by dredge efficiency. Observations indicate that dimensions can be reasonably estimated using wake theory and that efficiencies result in the introduction of 2-4% of the sediment mass contained in each bucket load. Resultant concentrations range between 200 and 400 mg/l. Progressing downstream into the secondary mixing zone concentrations decay rapidly due primarily to gravitational settling. The observed decay rates indicate an average settling velocity of 4.7cm/sec well in excess of values based on the grain size characteristics of the dredged sediment. The behavior suggests significant particle agglomeration within this area. At the downstream limit of this zone distributions become essentially exponential in character and remain so through the final mixing zone. In this area concentrations progressively approach the upstream background levels and variations are governed primarily by diffusion. In each of these zones the observed distributions appear amenable to relatively straightforward modeling.