Nearshore turbidity due to suspended fine sediment is a natural phenomenon around much of the western coast of southern Africa. However, there is concern about the potential impact on biotic communities of an increase in turbidity as a result of mining by means of marine dredgers. This paper discusses field measurements and the validation and application of mathematical models in investigations of the structure, composition, and decay of fine sediment plumes induced by dredging on a wave-dominated coast. In addition, the resuspension and mobility of the fine sediment deposited as a result of dredging are assessed. The expected trends of horizontal decay of concentrations with distance from the dredger and higher concentrations at greater depths were confirmed. The extent of plumes (i.e., where concentrations return to background levels) was predicted to be some 4 km at most, while a plume life span well in excess of 3 h was indicated. This extent and longevity of suspended sediment plumes were attributed to the high mobility of fine sediment in a wave-dominated environment resulting from elevated bed shear stress induced by swell wave action. Predictions indicate that these highly mobile fine sediments are transported tens of kilometres within weeks after discharge.
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Vol. 2008 • No. 241