The influence of nearshore sandbars on coastal stability in Port Geographe, south-west Australia

The coastline of Geographe Bay, south-west Australia, is dominated by a system of shore attached oblique sand bars that are 1-3 km long, up to 1 km wide and extend from the shoreline to a depth of ~5 m. The Port Geographe coastal development, initially developed in the 1990s, included a residential canal estate and a marina. Subsequent to the construction of this system a series of coastal problems were encountered that included the accumulation of seagrass wrack upstream of the breakwaters and erosion of beaches downstream of the development at Wonnerup. In 2014, a reconfiguration of the marina breakwater layout was completed to enable natural bypassing of the seagrass wrack and sand. Hydrodynamic and morphological model output using the DHI MIKE21 numerical modelling suite together with repeated bathymetric surveys were analysed to determine the shoreline evolution subsequent to the completion of the new marina layout. Results indicated that the Port Geographe coastal environment is complex with a variety of features influencing sand transport in the region. This included sheltering from the large headland at Cape Naturaliste, presence of offshore sand bars with sand transport occurring predominantly during winter storms when relatively larger waves approach the coast from the northwest. Model predictions indicated that there were strong interactions between the incident waves and currents with the nearshore sandbars. The sandbars refracted the waves to create regions of high and low energy along the coastline and acted as an obstruction to the flow and thus currents were enhanced along the crests of the sandbars and in the nearshore. In combination, these features resulted in shoreline changes corresponding to the location of the sandbars and highlighted the importance of the sandbars in controlling nearshore sand transport.