Deposition of dredged sediment at open water sites

Abstract

Silt-clay dredge spoil released at the surface of near-shore waters is deposited on the sea floor within a few hundred meters of its impact point. Only a few percent of the spoil is lost into the water column in most disposal operations. Surveys of the deposits formed by the controlled release of dredged sediment show some to be compact (presenting minimum surface area to the ambient water) and others dispersed (extending over a large area as a thin layer). The principal factor controlling the degree of dispersion during placement is the cohesion of the spoil. Disaggregated spoil is deposited from a turbidity current in a thin annulus, aggregated or cohesive spoil, in a compact pile of discrete blocks or ‘clods’. Formation of a compact deposit of spoil requires that the clods survive impact with the bottom; their kinetic energy must be absorbed in plastic deformation without clod rupture. The impact speed and the kinetic energy density are calculated for free fall of clods in water. Laboratory measurements are made of the deformation-rate dependence of the mechanical work done to rupture clods of silt-clay marine sediment in impact. These show that plastic deformation will dissipate the impact energy for clods less than 0·8 m in diameter; larger clods will break up upon impact. Field observations confirm the presence of clods smaller than this in deposits of cohesive spoil. The disposal processes responsible for the formation of spoil deposits are insensitive to the water depth and current speed. A compact spoil deposit is most likely to result when cohesive sediment is dredged with a clam shell bucket and released in small quantities at low speed over a soft-bottomed disposal area.