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.

To provide a probable explanation on the field observed rapid sedimentation process near river mouths, we investigate the convective sedimentation in stably stratified saltwater using 3-D numerical simulations. Guided by the linear stability analysis, this study focuses on the nonlinear interactions of several mechanisms, which lead to various sediment finger patterns, and the effective settling velocity for sediment ranging from clay (single-particle settling velocity V 0 5 0.0036 and 0.0144 mm/s, or particle diameter d 5 2 and 4 lm) to silt (V 0 5 0.36 mm/s, or d 5 20 lm). For very fine sediment with V 0 5 0.0036 mm/s, the convective instability is dominated by double diffusion, characterized by millimeter-scale fingers. Gravitational settling slightly increases the growth rate; however, it has notable effect on the downward development of vertical mixing shortly after the sediment interface migrates below the salt interface. For sediment with V 0 5 0.0144 mm/s, Rayleigh-Taylor instabilities become dominant before double-diffusive modes grow sufficiently large. Centimeter-scale and highly asymmetric sediment fingers are obtained due to nonlinear interactions between different modes. For sediment with V 0 5 0.36 mm/s, Rayleigh-Taylor mechanism dominates and the resulting centimeter-scale sediment fingers show a plume-like structure. The flow pattern is similar to that without ambient salt stratification. Rapid sedimentation with effective settling velocity on the order of 1 cm/s is likely driven by convective sedimentation for sediment with V 0 greater than 0.1 mm/s at concentration greater than 10-20 g/L.

[1] Convective sedimentation in a stably stratified saltwater is studied using the linear stability analysis. Convective sedimentation is known to occur due to the double-diffusive mechanism and the settling-driven mechanism. In this study, a semi-empirical closure of sediment diffusivity based on the long-range hydrodynamics effect is adopted. The sediment phase can act as either the slow-or fast-diffusing agent in the double-diffusive system for the given salt diffusivity. Moreover, the settling-driven effect is proportional to the square of the sediment diameter via Stoke settling law. We consider sediment concentration (grain size) in the upper freshwater layer to be in the range of 0.1 to 39.4 g/l (2 to 60 mm), which is on top of a saltwater layer with salinity 35 ppt. Linear stability analysis allows us to identify the dominant mechanism that triggers the instability, the growth rate, and the resulting characteristic finger width. Model results suggest that for fine sediment with grain diameter smaller than 10 mm (settling velocity 0.09 mm/s), doublediffusive mechanism controls the instability and the resulting sediment finger size is of millimeter scale. For the given threshold of growth rate of O(0.01) s À1 , the minimum sediment concentration is about 8-15 g/l. For grain size greater than or around 10 mm, the settling-driven mechanism dominates and instabilities occur at sediment concentration as low as O(0.1) g/l with centimeter-scale fingers. Our findings may contribute to a better understanding on the observed rapid sediment removal in the plume of small mountainous rivers.

No abstract

[1] Sediment transport under nonlinear waves in a predominately sheet flow condition is investigated using a two-phase model. Specifically, we study the relative importance between the nonlinear waveshape and nonlinear boundary layer streaming on cross-shore sand transport. Terms in the governing equations because of the nonlinear boundary layer process are included in this one-dimensional vertical (1DV) model by simplifying the two-dimensional vertical (2DV) ensemble-averaged two-phase equations with the assumption that waves propagate without changing their form. The model is first driven by measured time series of near-bed flow velocity because of a wave group during the SISTEX99 large wave flume experiment and validated with the measured sand concentration in the sheet flow layer. Additional studies are then carried out by including and excluding the nonlinear boundary layer terms. It is found that for the grain diameter (0.24 mm) and high-velocity skewness wave condition considered here, nonlinear waveshape (e.g., skewness) is the dominant mechanism causing net onshore transport and nonlinear boundary layer streaming effect only causes an additional 36% onshore transport. However, for conditions of relatively low-wave skewness and a stronger offshore directed current, nonlinear boundary layer streaming plays a more critical role in determining the net transport. Numerical experiments further suggest that the nonlinear boundary layer streaming effect becomes increasingly important for finer grain. When the numerical model is driven by measured near-bed flow velocity in a more realistic surf zone setting, model results suggest nonlinear boundary layer processes may nearly double the onshore transport purely because of nonlinear waveshape.