Sand and seagrass wrack modelling in Port Geographe, south-western Australia

Port Geographe, a marina and residential canal estate development, was developed in the early nineties. The design incorporated a sand bypass system to transfer sand across the harbour entrance from the west (upstream) to nourish the beaches in Wonnerup in the east; however, the artificial bypassing became a challenging issue because of the presence of seagrass wrack, which was more efficiently trapped by the sand trap than the sand. The wrack accumulation caused severe environmental problems (e.g. odour, reduced beach use) on the western side of the development and erosion of beaches to the east at Wonnerup. In this paper, the results from a numerical model study of sand and seagrass wrack transport that was undertaken to alleviate these problems are presented. Models developed by the Danish Hydraulic Institute (DHI MIKE 2D) were used to predict the waves, currents, and changes in morphology and were linked to a particle-tracking model developed as part of the study to simulate wrack transport. The model runs included the existing coastal structures and other configurations designed to bypass naturally both sand and seagrass wrack. An optimum configuration for the coastal structures to promote the natural bypassing of sand and seagrass wrack included an angled groyne on the western side of Port Geographe to replace the existing breakwater and the removal of groynes associated with the pocket beaches by a foreshore seawall.