Modelling intensity and frequency of seabed shear stress and sediment mobilization on the Canadian Pacific Shelf

Ocean surface waves and currents can interact to produce strong seabed shear stress and sediments mobilization affecting infrastructure safety and benthic habitat distribution. An accurate understanding of these processes is important to inform decisions taken to ensure sustainable development of the oceans. Modelled waves, tidal current and circulation current data for a 3-year period were used in a combined-flow sediment transport model to simulate the seabed shear stresses and the mobilization of observed sediments on the Pacific Shelf of Canada. The modelling results are presented and analyzed to update the framework of seabed disturbance and sediment mobility on the Pacific Shelf. The shelf is affected by strong waves and tidal currents. Maximum mean significant wave height over the modelled period can reach 3 m and mean near-bottom tidal currents reach up to 0.6 m s⁻¹. Our modelling results indicate that the mean wave and tidal current shear velocities reach the maximum values of 4 and 3 cm s⁻¹ respectively. Observed sediments on the Pacific Shelf can be mobilized by tidal currents at least once during the modelled 3-year period over 46 % of the shelf area while waves can mobilize sediments over 47 % of the shelf area suggesting nearly equal tidal and wave effects. Furthermore, waves and currents interact to cause enhanced combined wave-current shear velocities >5 cm s⁻¹ that are capable of mobilizing sediments over 80 % of the shelf area, nearly double that due to tides or waves alone. Regionalization of disturbance types based on the spatial variation of the relative importance of component processes in mobilizing sediments, however, demonstrates that tide-dominant disturbance type accounts for 43 % of the shelf area while wave-dominant disturbance accounts for 32 % of the shelf area suggesting tidal disturbance is more important than waves on the Pacific Shelf. Mixed disturbance only occurs over a small 11 % of the shelf area. Universal indices of Seabed Disturbance (SDI) and Sediment Mobility (SMI) were applied to better quantify the exposure of the seabed to oceanographic processes and sediment mobility, incorporating both the magnitude and frequency of these processes. Strong casual correlation between modelled shear stress and observed grain sizes for a range of values and over widely distributed areas on the shelf suggests that sediments are largely in equilibrium with the present-day hydrodynamics on the Pacific Shelf of Canada. The spatial variation of modelled bed shear stress, its correlation with the observed grain size, and patterns of modelled sediment mobilization and disturbance types from our study support and potentially improve the surficial geology, morphology, and sedimentary process knowledge for several regions of the Pacific Shelf.