Characterisation of the wave field and associated risk of sediment resuspension in a coastal aquaculture zone

The interaction between extreme environmental events and the marine aquaculture industry has, in the past, led to substantial economic losses for the industry. In 1996, the aquaculture industry based near Port Lincoln in South Australia experienced a sediment resuspension event linked to the passage of a vigorous ex-tropical cyclone. A key process that drives the sediment dynamics in coastal environments is the physical forcing due to surface gravity waves. The numerical model Simulating WAves Nearshore (SWAN) has been applied to hindcast two years of wave fields for the central South Australian coastline and is validated against five datasets, using both in-situ observations and satellite derived wave heights. Using the calibrated model, we investigate the spatial variability in the relative contribution of locally generated wind waves versus incoming swell to the mean wave energy budget. The potential for sediment resuspension is then assessed using theoretical thresholds for the initiation of sediment resuspension which are derived using the Shields parameter and observed sediment grain size. Linear wave theory in conjunction with the model hindcast is used to identify the conditions that could lead to a sediment resuspension event. Incoming swell from the Southern Ocean poses the greatest risk of a sediment resuspension event, while locally generated wind waves are only able to resuspend sediments in the shallow near shore regions. We have developed a simple tool that will allow policy makers and farm managers to identify the likely combination of significant wave height and mean wave period for a particular depth, that is likely to result in the mobilisation of unconsolidated particulate matter on the seafloor.