ESTUftRINE RESPONSE TO DREDGING IN THE TWEED RIVER, AUSTRALIA

Abstract

Between mid 1974 and mid 1975,760,000 m3 of sand was dredged from the bed of the Tweed River for the purpose of nourishing cyclone damaged beaches of the Gold Coast (Queensland). A comprehensive field data programme was established in 1976 to record the changes in the hydraulic processes of the Tweed River brought about by the dredging. The field measurements demonstrated that the dredged area was being infilled with sediments of both marine and estuarine origin. The dredging increased tidal ranges throughout the lower estuary, the effect being more pronounced at low water. Sediment bedload rates were estimated from detailed measurements of bedforms and used to calibrate a sediment transport formula. The formula was used in conjunction with a 1 Dim. numerical model of tidal hydraulics to simulate estuarine shoal dynamics by means of a simple sediment routing technique. The results showed that the dredging had altered the tidal hydrodynamics so as to enhance the ebb transport of sediment towards the dredged hole. In the long term it was found that the sediment transport switched to a weak net upstream movement of sediment. The detailed hydraulic mechanisms involved are discussed. The study demonstrates that the impact of dredging can be minimised by location upstream of the entrance plug of marine sand.