Resuspension and clearing of dredge spoils after dredging, Cleveland Bay, Townsville, north Queensland

Field studies of the effects of dredging the 8km long Townsville navigation channel (Platypus Channel) were carried out in May 1989. The suction dredge was followed by a research vessel and the spoil plume, which formed in the lee of the dredge, was marked using a near-bottom sail drogue. Vertical profiles of temperature, salinity and suspended sediment concentration (SSC) were obtained next to the drogue. Cross sections of the suspended sediment density stratification in the water column next to the drogue and along transects perpendicular to the track of the dredge during dredging, were recorded using an acoustic sounder.In the laboratory, the settling characteristics of a composite sample of muddy silt from four areas of the harbour, that are dredged were investigated. Sediment was added to a column and subjected to turbulence. Floc sizes were measured and vertical profiles of SSC were obtained.
The purpose of this research was to identify optimum conditions for dredging, that would result in the least damage to environmentally sensitive areas. Research focused on the fate of sediments, which were resuspended due to mechanical disturbance of the bottom during the process of dredging, the dredge's turbulence and the sediment released from the dredge's surface overflow.
This research was undertaken to address concerns relating to the impacts of dredging and the dumping of dredge spoil in Cleveland Bay. Issues of concern included the possible effects of siltation on nearby coral reefs, seagrass beds and beaches and the potential effect of toxic metals carried in mud from Townsville Harbour.