Suspended material distributions in the wake of estuarine channel dredging operations

Abstract

Field sampling of the suspended material field downstream of a large volume bucket dredge operating in the Lower Thames River estuary near New London, Connecticut was conducted in order to examine the magnitude and character of the dredge-induced resuspension and to evaluate typical operational efficiency. These data indicate that approximately 1.5 to 3% of the sediment volume in each bucket-load is introduced into the water column producing suspended material concentrations adjacent to the dredge of 200 mg l −1 to 400 mg l −1 . These values exceed background levels by two orders of magnitude. Analysis of particulate organic carbon and grain size characteristics indicates that resuspension also alters suspended load composition increasing the percentage of inorganic materials and median grain size. Proceeding downstream, material concentrations along the center-line of the dredge-induced plume decrease rapidly approaching background within approximately 700 m. Compositional variations display similar trends with the major perturbations confined to the area within 300 m of the dredge. The observed spatial distributions indicate the dredge-induced resuspension is primarily a near field phenomenon producing relatively minor variations as compared to those caused by naturally occurring storm events. Previous work ( Bohlen & Tramontano, 1977 ) has shown that these latter systems can produce estuary-wide variations in suspended material concentrations, increasing the mass of material in suspension by at least a factor of two. This increase in total suspended load is nearly an order of magnitude larger than that produced by the dredge. These field observations also show that there is a distinct physical similarity between dredge and storm induced resuspension and provide some useful indications of the probable response of the larger scale coastal suspended material field to a variety of natural, high energy disturbances.