Wave Generation of Gravity‐Driven Sediment Flows on a Predominantly Sandy Seabed

Abstract

Wave‐supported gravity flows (WSGFs) 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 and that WSGF can form in mixed grain‐size environments and transport high concentrations of sand. The occurrence of WSGF on a predominantly 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.

A Turbulence‐Resolving Numerical Investigation of Wave‐Supported Gravity Flows

“…As summarized in Table , the present simulations give an empirical critical Richardson number around 0.01, regardless of wave direction. This value is significantly smaller than 0.25, but it is consistent with recent field observations of Flores et al (), who reported and laboratory experiments of Lamb and Parsons () showing where sediment concentration is directly measured. Although there exist larger uncertainties in earlier field measurements, many WSGF events are observed in the field to occur at much lower than 0.25 (Traykovski et al, ).…”

“…However, the shelf slope in these later two field sites is milder ( ) than that reported by Traykovski et al (). A recent measurement of the downslope current speed of WSGF by Flores et al () in a mixed sediment site also suggested a speed of 5 cm/s. A more careful comparison of our model results with these field data indicates that the main reason that the present simulations predict lower downslope gravity current speed is because the computed near‐bed sediment mass concentration is only about 26 kg/ (g/L).…”

The Formation of Turbidity Maximum Zones by Minor Axis Tidal Straining in Regions of Freshwater Influence

“…An example of the semidiurnal variations in stratification, current structure, and near-bed sediment transport induced by tidal straining is shown in Fig. 1 for a shallow site in the midfield region of Rhine ROFI Horner-Devine et al 2017). Although other processes, such as the propagation of tidal plume fronts (Horner-Devine et al 2017;Rijnsburger et al 2018), are also relevant in the midfield region, the main features of tidal straining are evident in the stratification and current time series (Figs.…”

“…The interplay between tides, density gradients, and bottom friction favors convergence processes that may lead to the formation of Negative fluxes are directed onshore, and positive fluxes are directed offshore. These measurements were part of the Stratification Impacts on Nearshore Sediment (STRAINS) field campaign in 2014 and were taken 10 km north of the mouth of the Rhine River, in 18 m of water, approximately 5 km off the coast Rijnsburger et al 2018). These measurements were taken during neap tide and low wind conditions.…”

“…Suspended sediment concentrations were measured using optical backscatter (OBS). The reader is referred to Flores et al (2017) for further details about the instrumentation and deployment. turbidity maximum zones in ROFIs (e.g., Souza and Lane 2013;van der Hout et al 2015;Brown et al 2015;van der Hout et al 2017).…”