Multibeam bathymetric investigations of the morphology of sand ridges and associated bedforms and their relation to storm processes, Sable Island Bank, Scotian Shelf

Storm-formed sand ridges are important morphologic features occurring on many continental shelves of the world. Our understanding of the morphology, migration, and the controlling processes over this type of sand ridges under different environmental settings is still limited. In the present study, an integrated approach of combining multibeam bathymetry survey, seabed sampling, sidescan/seismic surveys and seabed instrumentation measurements was used to characterize the surficial geology and morphology of offshore sand ridges on Sable Island Bank, Scotian Shelf, and to evaluate the relationship between sand ridge morphology, migration and the storm processes on this storm-dominated outer shelf bank. Multibeam data obtained at selected sites demonstrated the series of NE–SW oriented sand ridges with an average height of 4.3 m and wavelength of 1.5 km. Sand ridges attain maximum spacing, height and steepness in the intermediate depths. Sand ridge size and steepness also decrease from west to east and this decrease is correlated with decreasing dynamic condition, sediment grain size, and depth. Sand ridge profiles show weak easterly asymmetry. This morphological asymmetry augments the sediment texture and grain size asymmetry to suggest eastward long-term sand ridge migration and net sediment transport in the study area. Sand ridge asymmetry also shows strong local variation and the number of sand ridges asymmetric to the west is nearly equal to that asymmetric to the east. The likely cause is the nearly equal number of major storms passing to the south versus to the north of the Island so that the westerly and easterly peak storm-driven currents associated with these storm path scenarios affect equally the sand ridges on Sable Island Bank. A suite of smaller bedforms were found superimposed on the sand ridges: sand waves widely superimposed on both flanks of sand ridges, mixed 2D and 3D megaripples commonly occurring in the sand ridge troughs and on the lower western flanks, and large-wave ripples ubiquitously found in the troughs of all megaripples. Surficial sediment is coarsest (coarse sand with shell fragments) in the sand ridge trough and becomes progressively finer eastward toward the sand ridge crest and down the eastern flank, while best sorting occurs near the sand ridge crest. Box cores collected across sand ridges reveal characteristic storm event bed structure that reflects the development and migration of bedforms with the evolution of storms.