Wave generation of gravity-driven sediment flows on a predominantly sandy seabed

Abstract

Wave-supported gravity flows (WSGF) generate rates of sediment flux far exceeding other cross-shelf transport processes, contributing disproportionately to shelf morphology and net cross-shelf fluxes of sediment in many regions worldwide. However, the conditions deemed necessary for the formation of WSGF limit them to a narrow set of shelf conditions; they have been observed exclusively in regions where the seabed consists of very fine-grained sediment and typically co-occur with nearby river flood events. Here we document the occurrence of a WSGF event on a predominantly sandy seabed and in the absence of a preceding river flood. Our measurements confirm that the dynamics are governed by the friction-buoyancy balance observed in other WSGF, but also reveal how grain size influences the structure and transport rate of WSGF. We observe that, although sufficient concentrations of fine sediment are required for their formation, WSGF can form in mixed grain-size environments and can transport high concentrations of sand. The occurrence of WSGF on a sandy seabed suggests that they may occur under a much wider range of conditions and, given the global prevalence of sandy shelves, they may be a more frequent and more ubiquitous feature of shelf dynamics than previously thought.