Classification of the Australian continental shelf based on predicted sediment threshold exceedance from tidal currents and swell waves

Estimates of significant wave height and period, together with tidal current speed over a semi-lunar cycle, were used to predict the area on the Australian continental shelf over which unconsolidated sediment was mobilised (threshold exceedance). These sediment-entraining processes were examined independently to quantify their relative importance on the continental shelf. Using observed grain size data, mobilisation from swell waves occurred on ∼31% and tidal currents on ∼41% of the continental shelf. Swell wave energy is sufficient to mobilise fine sand (0.1 mm diameter) to a water depth of 142 m on the Otway Shelf near the western entrance to Bass Strait. Tidal currents in King Sound (northwest shelf) are capable of mobilising large areas of medium sand (0.35 mm diameter) 100% of the time. Superimposing the distribution of threshold exceedance by wave and tidal currents indicates that there are areas on the shelf where either wave-induced or tidal currents dominate, some areas where waves and tides are of relatively equal importance and still other areas where neither is significant. We define six shelf regions of relative wave and tidal energy: zero (no-mobility); waves-only, wave-dominated, mixed, tide-dominated and tides-only. Our results provide a predictive, process-based understanding of the shelf sedimentary system that has applications to marine engineering projects and to regional studies of pollution dispersal and accumulation where significant shelf sediment mobilisation is a factor.