The dynamics of wave and current supported gravity flows: Evaluation of drag coefficient and bulk Richardson number

Abstract

Wave and current supported gravity flows (WCSGFs) carry substantial fine-grained sediment across gently sloping seabeds of coastal and shelf waters, representing a primary mechanism of sediment transport. An existing buoyancy-friction model, which considers the important parameters of drag coefficient (CD) and bulk Richardson number (Rib), has been widely used to analyze WCSGF dynamics. However, an improved validation of the two parameters on the basis of in situ measurements is still outstanding. Here, we analyze 23 short-lived WCSGF events that were recorded at the shallow sea floor of a muddy open coast to quantify the parameters. The CD values, as defined in the buoyancy-friction model, were found to be around 0.02 and 0.01 for early flood and ebb phases of the tide, respectively, which are significantly larger than the range of 0.001–0.006 reported elsewhere from in situ observations, numerical simulations and laboratory experiments. The calculations of Rib from our WCSGF observations were all close to the critical value of 0.25, which supports the use of such a value in WCSGF models. Large CD values of our observed WCSGFs imply that strong currents can dilute the high-concentration layer and slow down the movement of WCSGFs, which reveals a remarkable difference between WCSGFs and gravity flows supported by waves alone.