The distributions and movements of sediments on the North West Shelf have a major influence on the ecosystem, primarily through their effect on primary production and benthic habitat distributions. They are also highly relevant to human activities on the NWS, particularly construction and maintenance of offshore structures, dredging, and other coastal developments involving habitat modification. This study provides a general characterisation of the distributions of suspended and bottom sediments based on available data. These data confirm the existence of a high turbidity zone extending from the coastline to around the 20 m isobath. Bottom sediments also show a marked cross shelf zonation, with gravel and sand dominating the inner shelf, and finer sand, silts and clays dominating the less energetic outer shelf. A three dimensional sediment transport model was developed to estimate major resuspension, deposition, and suspended transport patterns over the NWS. The simulations covered the three years 1996, 1998, and 1999. They successfully reproduced the winnowing of fine sediments (i.e. very fine sand, silts and clays) in the relatively high-energy environment of the mid shelf, and their subsequent export onto the continental slope. The model also predicted a persistent strip of elevated suspended sediment concentrations along the inner shelf. Near surface suspended sediment concentrations in this zone typically increased by an order of magnitude from the neap to spring tide. Long-termed average sediment fluxes were directed southwest along the shelf and off the shelf. While there were significant uncertainties associated with these fluxes, the results suggested that sediment losses from the shelf exceeded the mean annual river loads by at least a factor of three over the simulation period. The model also predicted major sediment transport events during tropical cyclones. Modelled off shelf loads during Tropical Cyclone Bobby far exceeded both annual river loads and annual loads during non-cyclone conditions. These simulations also demonstrated how cyclone induced ocean eddies could capture large quantities of suspended sediments and carry them offshore to be deposited within highly localised patches in the deeper ocean. While there were various limitations in the model formulation, its predictions were broadly consistent with the limited available observations. It is suggested that further progress in sediment modelling on the NWS should be based on extended data sets for sediment processes, such as grain size distributions, flocculation, consolidation, and river loads, as well as improved wind and wave products, and inclusion of bedload transport.
