Sediment dynamics over sand ridges on a tideless mid-outer continental shelf

A sand ridge field with crests oriented NE–SW (52°) located between 55 and 85 m water depth on the Valencia continental shelf (Spain) was mapped with multibeam swath bathymetry and characterized with seismic profiling and sediment sampling. Boundary-layer hydrodynamic measurements conducted over a sand ridge at 66 m depth show evidence of wave and current sediment resuspension during a major storm event with a three-year recurrence period. From the measured intrawave sediment concentration profiles it is shown that the suspended sediment time evolution is related not only to mean currents but also to the wave field. The near-bottom shear stress generated during storm conditions reached around 1 N/m2 and was able to resuspend and transport sandy and muddy sediments for more than 32 h. The total suspended sediment transport in a layer of 1 m above the bottom was around 4000 kg per meter width and directed 160°. It is argued that these sand ridges were developed in a shallow environment and there is no evidence of present-day ridge migration or morphological degradation. It is hypothesized that sediment dynamics affecting sand ridges during storms favors the maintenance of their morphology.