A new approach to mapping littoral cells within a tide-dominated embayment

Western Port is a large tide-dominated embayment located 75 km east of Melbourne, Victoria that is flanked by a rapidly growing population, significant industrial infrastructure, grazing lands and protected RAMSAR habitats. Remarkably little is known about the sediment dynamics of Western Port despite concerning rates of erosion of the swamp and floodplain deposits including cohesive clay cliffs that fringe the sunkland. As in other tide-dominated systems, sediment dynamics in Western Port are complicated by strong tidal currents, multiple sediment sources and complex circulation. We applied the coastal sediment cell framework to Western Port, in conjunction with high resolution satellite derived rates of coastal change to elucidate sediment sharing among the sub-embayment beaches. Using end member modelling of 20 sediment assemblages based on elemental and mineralogical composition we robustly identified seven end members that correspond to different point and diffuse provenances. Hierarchical clustering of the 20 beaches dispersed across the bay yielded nine coastal cells, delimited by headlands, deep water channels, estuaries or disruptive infrastructure. Analysis of shoreline change within the cells suggests most cells are neutrally or positively balanced but a northward movement of sediments is leading to rotation. The deep channels provide a pathway for Bass Strait sediments to bypass the embayment headlands and tidal currents enable cross shore exchange with the channels beyond wave derived closure depth in some locations. Understanding the sediment cell boundaries and direction of sediment transport can assist with the management of erosion hotspots; the novel approach applied here can resolve complex connected coastal systems such as estuaries and embayments.