Gravity-driven sediment flows on the shallow sea floor of a muddy open coast

Abstract

Wave and/or current supported gravity-driven sediment flows (GDSFs) cause substantial sediment movement across the continental shelf, contributing to morphological evolution in many regions worldwide. However, they appear to occur episodically and ephemerally and, therefore, it remains a challenge to document them in detail with in situ measurements. Here we present solid evidence of frequent generation of such flows over the shallow sea floor of a muddy open coast. They were triggered by wave resuspension and/or sediment settling from the overlying water column, maintained by wave- and/or current-induced bed stress, and terminated due to upward spreading of bottom sediment within the high concentration layer. Randomly selected GDSF events were analyzed to realize parameterization with a buoyancy-friction model; the resultant bed drag coefficient for two of the cases is higher than the value of 0.003, which is attributed to the additional drag at the interface between the overlying flow and the moving GDSFs. Although short-lived, the observed GDSF events in the three different seasons indicate that they occur more frequently than previously thought.