Numerical investigation of internal wave‐induced sediment motion: Resuspension versus entrainment

Abstract

We present numerical simulations of near‐bed instability induced by internal waves shoaling over topography using a model with an explicit representation of the sediment concentration. We find that not all separation bubble‐bursting events lead to resuspension, though all lead to significant transport out of the bottom boundary layer. This transport can significantly enhance chemical exchange across the bottom boundary layer. When resuspension occurs, we find that it is largely due to two‐dimensional evolution of the separation bubble during the bursting process. Three‐dimensionalization occurs once the resuspended sediment cloud is transported out of the bottom boundary layer, and hence, redeposition is strongly influenced by three‐dimensional effects. We derive a criterion for resuspension over a linearly sloping bottom in terms of two dimensionless parameters that encapsulate the sediment properties.

Experimental investigation of sediment resuspension beneath internal solitary waves of depression

“…Understanding this process is of great interest due to the importance of sediments on aquatic biogeochemistry and acoustics [Carr et al, 2008]. Field observations [e.g., Bogucki et al, 2005], laboratory experiments [Boegman and Ivey, 2009] and numerical simulations [e.g., Stastna and Lamb, 2002;Diamessis and Redekopp, 2006;Stastna and Lamb, 2008;Aghsaee et al, 2012;Olsthoorn and Stastna, 2014] suggest that global instability of the wave-induced boundary layer [Huerre, 2000], where coherent vortices are shed from the separation bubble in the adverse pressure gradient region, is the main resuspension mechanism beneath ISWs; although direct evidence of this remains lacking.…”

“…Moreover, all prior research has been limited to two-dimensions (simulations have been for a 2-D fluid [e.g., Stastna and Lamb, 2002;Diamessis and Redekopp, 2006;Aghsaee et al, 2012] and experiments have been planar 2-D observations within a 3-D flow field [Carr et al, 2008;Boegman and Ivey, 2009]). These have not directly observed/simulated sediment resuspension; the lone exception being the recent 3-D simulations by Olsthoorn and Stastna [2014], which investigate ISW-induced resuspension over a Gaussian ridge. There is a need for further research that (1) investigates the ability of ISWs to directly resuspend bed sediment and (2) predicts resuspension based on readily measured wave and sediment characteristics available to a field oceanographer.…”

Topographically generated internal waves and boundary layer instabilities

“…The vortex shedding and global instability produce strong bottom shear stresses which may enhance sediment resuspension. 6 This behaviour has also been examined numerically by Stastna and Lamb 7 who found that vortex shedding can occur when internal solitary waves of elevation travelled in the presence of a background current (as would be the case for resonantly generated waves) and that a trapped core was not necessary for vortex shedding to occur. Some observational and experimental studies suggest internal wave induced pumping of sediments high into the water column.…”

“…1,7,12,17 Recent numerical work has confirmed that sediment resuspension occurs in a coupled hydrodynamic-sediment model. 6 More broadly, a wide variety of field studies have pointed to a link between increased near-bed sediment concentrations and shoaling internal waves on a slope. [18][19][20][21][22] Quaresma et al 8 have offered clear evidence of nonlinear internal waves influencing sediment resuspension over the northern shelves of Portugal.…”

“…23 Some work has been completed on the three-dimensionalization of internal waves shoaling over isolated topography and the generation of near-bed instabilities. 6 This work also included a sediment resuspension model and found that three-dimensional effects can influence the deposition of suspended sediment. The three-dimensional characteristics of boundary layer instabilities that form due to internal solitary wave interaction with isolated topography has also been investigated 24 finding that separation-induced instabilities exhibit strong three-dimensionalization.…”