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

Abstract

Wave‐supported gravity flows (WSGFs) have been identified as a key process driving the offshore delivery of fine sediment across continental shelves. However, our understanding on the various factors controlling the maximum sediment load and the resulting gravity current speed remains incomplete. We adopt a new turbulence‐resolving numerical model for fine sediment transport to investigate the formation, evolution, and termination of WSGFs. We consider the simplest scenario in which fine sediments are supported by the wave‐induced fluid turbulence at a low critical shear stress of erosion over a flat sloping bed. Under the energetic wave condition reported on the Northern California Coast with a shelf slope of 0.005, simulation results show that WSGFs are transitionally turbulent and that the sediment concentration cannot exceed 30 kg/m 3 (g/L) due to the attenuation of turbulence by the sediment‐induced stable density stratification. Wave direction is found to be important in the resulting gravity current intensity. When waves are in cross‐shelf direction, the downslope current has a maximum velocity of 1.2 cm/s, which increases to 2.1 cm/s when waves propagate in the along‐shelf direction. Further analysis on the wave‐averaged momentum balance confirms that when waves are parallel to the slope (cross‐shelf) direction, the more intense wave‐current interaction results in larger wave‐averaged Reynolds shear stress and thus in a smaller current speed. Findings from this study suggest that the more intense cross‐shelf gravity current observed in the field may be caused by additional processes, which may enhance the sediment‐carrying capacity of flow, such as the ambient current or bedforms.

Direct Numerical Simulations of Miniature Along‐Shelf Current‐Supported Turbidity Currents: Conceptual Investigation of Velocity Structure and Drag Coefficient

“…Similar conclusions were drawn in Yue et al. (2020) for shore‐normal waves, and was observed to be as low as when waves are shore‐parallel. The only DNS study on ACSTCs was conducted by Ozdemir and Yu (2018), which investigated the fluid dynamical details of the bi‐directional nature of ACSTCs, particularly focusing on the normal and shear Reynolds stress balance equations.…”

“…when waves are normal to the shore. Similar conclusions were drawn in Yue et al (2020) for shore-normal waves, and d C was observed to be as low as  0.002…”

“…Therefore, we selected the critical bed shear stress as   0.01 Pa c , which is based on the study by Curran et al (2007), where the bottom shear stress as low as 0.01 Pa is sufficient to suspend loose fine sediments. Similar values for the critical shear stress were also adopted in wave-only supported turbidity currents to generate SSC similar to those observed in the field (see Yue et al, 2020). As mentioned, the relation between the erosion coefficient and the ratio between the bed shear stress and critical shear stress of erosion determines the erosion flux from the bed, that is,   , 0.11, and 0.25, where o S is defined as…”

“…For wave‐only supported sediment gravity flows, DNS were conducted by Ozdemir (2016) and recently by Yue et al. (2020). The results in Ozdemir (2016) suggest that the drag coefficient is dependent on the Reynolds number and can reach up to when waves are normal to the shore.…”

“…The same study reports almost a constant drag coefficient when rheological stresses dominate the wave boundary layer; whereas a power relation for the drag coefficient was conjectured for cases when rheology is absent. For wave-only supported sediment gravity flows, DNS were conducted by Ozdemir (2016) and recently by Yue et al (2020). The results in Ozdemir (2016) suggest that the drag coefficient is dependent on the Reynolds number and can reach up to  0.006…”