Western Port in Victoria has experienced extensive loss of seagrass coverage believed to be due to large sediment inputs to the bay from its river catchments. This study is an attempt to advance understanding of the efficiency of sediment delivery from upland erosion through a channelized floodplain to seagrass beds within a semi-enclosed bay, Western Port. The results of sediment inputs from river catchments are presented here and the impact analyses are presented in two other companion papers.
Sediment transport modelling was carried out using the HEC-RAS model. The model was calibrated using observed stage height and discharge data. The sediment transport component of the model was calibrated using the bed elevations and total sediment loads. A separate model was constructed for each stream (i.e. Cardinia, Bunyip, Lang Lang and Bass) in the WPB catchment. Finer proportions of sediments in the Bunyip River are relatively small compared to Cardinia, Lang Lang and Bass rivers. This indicates relatively less percentage of sediment deposition in the Bunyip River. Preliminary modelling results show that the Cardinia Creek, at least, experiences deposition of substantial amounts of silt and clay. Model results were analysed to identify the potential area of erosion and deposition and sediment connectivity to the bay. Subsequent hydrodynamic and seagrass population models will use this information as driver. This information will assist natural resources managers to quantify possible future conditions and to identify actions within the catchment and along the coastline that are most likely to benefit seagrass habitats.