Intensity and frequency of seabed shear stress and sediment mobilization on the Canadian Atlantic Shelf — A modelling study

Abstract

Ocean surface waves and currents can interact to produce strong seabed shear stress and sediments mobilization affecting infrastructure safety, benthic habitat distribution and decision for 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 Canadian Atlantic Shelf. The modelling results are presented and analyzed to update the framework of seabed disturbance and sediment mobility on the Atlantic Shelf. The Atlantic Shelf is affected by strong waves and tidal currents. Maximum mean significant wave height over the modelled period can reach 3.5 m and mean tidal currents reach up to 1.2 m s⁻¹. Our modeling results indicate that the mean wave and tidal current shear velocities both reach the maximum values of ∼4 cm s⁻¹. Observed sediments on the Atlantic Shelf can be mobilized by tidal currents at least once during the modelled 3-year period over 30 % of the shelf area while storms can mobilize sediments over 35 % of the shelf area suggesting a slightly stronger sediment mobilization by storms. Furthermore, waves and currents interact to cause enhanced combined wave-current shear velocities >5 cm s⁻¹ that is capable of mobilizing sediments over 63 % of the shelf area, double that due to tides or waves alone. The spatial variation of the relative importance of waves, tidal current and circulation current in mobilizing sediments was used to classify the Atlantic Shelf into six disturbance types. Wave-dominant and tide-dominant disturbance types are equally important and both occupy ∼25 % of the shelf area. Mixed disturbance is insignificant and accounts for only 3 % of the shelf area. Universal Seabed Disturbance Index (SDI) and Sediment Mobility Index (SMI) were applied to better quantify the exposure of the seabed to oceanographic processes and sediment mobilization, incorporating both the magnitude and frequency of these processes. The values of SDI and SMI on the Canadian Atlantic Shelf are found to be comparable to those on the Australian shelves and the Irish Sea shelf. These indices, together with the seabed disturbance type classification scheme, potentially can be used as standard parameters to quantify seabed disturbance and sediment mobility on other shelves of the world.