On the transport modes of fine sediment in the wave boundary layer due to resuspension/deposition: A turbulence‐resolving numerical investigation

Abstract

Previous field observations revealed that the wave boundary layer is one of the main conduits delivering fine sediments from the nearshore to continental shelves. Recently, a series of turbulenceresolving simulations further demonstrated the existence of a range of flow regimes due to different degrees of sediment-induced density stratification controlled by the sediment availability. In this study, we investigate the scenario in which sediment availability is governed by the resuspension/deposition from/to the bed. Specifically, we focus on how the critical shear stress of erosion and the settling velocity can determine the modes of transport. Simulations reveal that at a given wave intensity, which is associated with more energetic muddy shelves and a settling velocity of about 0.5 mm/s, three transport modes, ranging from the well-mixed transport (mode I), two-layer like transport with the formation of lutocline (mode II), and laminarized transport (mode III) are obtained as the critical shear stress of erosion reduces. Moreover, reductions in the settling velocity also yield similar transitions of transport modes. We also demonstrate that the onset of laminarization can be well explained by the reduction of wave-averaged bottom stress to about 0.39 Pa due to attenuated turbulence by sediments. A 2-D parametric map is proposed to characterize the transition from one transport mode to another as a function of the critical shear stress and the settling velocity at a fixed wave intensity.

A numerical investigation of fine sediment resuspension in the wave boundary layer—Uncertainties in particle inertia and hindered settling

“…Following previous studies (Ozdemir et al, 2010;Cheng et al, 2015), the wave condition with a free-stream velocity magnitude U 0.56 m/s 0 = ∼ and a wave period T 10 s = ∼ is considered to represent a relatively energetic flow condition when fine sediments form high concentration suspensions on continental shelves (e.g., Traykovski et al, 2000). The Stokes boundary layer thickness is calculated to be 2 / 1.78 mm Δ ν ω = = ∼ ∼ , in which ν is the fluid…”

“…By substituting Eq. (3) into the Eulerian two-phase equations for the fluid phase (Cantero et al, 2008), and adopting the Boussinesq approximation, which is appropriate for the relatively small volumetric concentration of suspended fine sediment on continental shelves (Traykovski et al, 2007), the resulting non-dimensional continuity and momentum equations of the fluid phase are written as (Cheng et al, 2015) …”

“…For the bottom resuspension, this term typically represents sediment-induced stable density stratification, which can attenuate the carrier flow turbulence (Cheng et al, 2015).…”

“…In the previous studies (Ozdemir et al, 2010;Cheng et al, 2015), the inertia terms associated with the particle Stokes number St in the particle velocity expression (Eq. (3)) are neglected by assuming that the Stokes number St is sufficiently small, and the particle velocity is calculated as…”

“…Comparing to the pseudospectral scheme, this hybrid scheme is more flexible to incorporate variable viscosity, and nonlinear boundary conditions. Cheng et al (2015) adopt this new scheme to study how a range of sediment/ bed properties associated with resuspension can control the sediment availability and the resulting transport. Simulation results reveal that the sediment availability is highly dependent on the resuspension/deposition mechanism from/to the bed.…”