Modelling seabed shear stresses and sediment mobilization on the Scotian Shelf, eastern Canada

Ocean surface waves and currents can interact to produce strong seabed shear stress and mobilization of sediments that can significantly impact the seabed stability and benthic habitats on continental shelves. Modelled waves, near-bottom tidal current and circulation current data for a 3-year period were used in a widely applied sediment transport module to simulate the seabed shear stresses and the mobilization of observed sediment grain size on the Scotian Shelf of eastern Canada.

The Scotian Shelf is affected by strong waves and tidal currents. These waves, currents and/or their interaction cause maximum mean bed shear velocities of 5 – 10 cm s⁻¹. Observed sediments on the Scotian Shelf can be mobilized by tidal currents at least once during the modelled 3 year period over 28% of the shelf area while waves can mobilize sediments over 60% of the shelf area suggesting much stronger sediment mobilization by waves. Interaction between waves and currents can produce enhanced combined wave-current shear velocity that is capable to mobilize sediments over 74% of the shelf area. The spatial variation of the relative importance of sediment mobilization frequency by component processes was used to classify the Scotian Shelf into six disturbance types. In comparison with previous studies using depth-averaged tidal currents, the present study based on near-bottom tidal currents has resulted in reduced sediment mobilization frequency by tidal currents, smaller extent of high mobility areas and significant changes of the spatial pattern of disturbance type distribution on the Scotian Shelf. The universal Seabed Disturbance Index and Sediment Mobility Index have also been applied to quantify the seabed exposure to physical processes and sediment mobilization on the Scotian Shelf by accounting for both the magnitude and frequency of these processes. The results of this modelling study are important for environmental assessments and for the spatial planning and management of the Scotian Shelf bioregion.