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When a sediment laden river flows into a stratified water body, the water mass can either intrude as an overflow, interflow or underflow, depending upon the density contrast. Different modes of sediment driven convection occur in each case. For the case of overflows, convective sedimentation occurs beneath the plume, whereby sediment rich plumes rapidly transport fine materials to depth. If underflow of dense sediment laden waters initially occurs, then after sediment has been deposited, the light interstitial material can subsequently loft and potentially mixes the entire water column. For an interflow, both lofting and sediment driven convection can occur above and below the pycnocline. All of these different regimes can be described in terms of two dimensionless parameters: namely RS = ΔρS/ΔρC and RA = ΔρA/ΔρC, where ΔρA is the density contrast between the upper layer and the river inflow (due to just salinity or temperature differences), ΔρC is the density contrast due to sediment between river and upper‐layer, and ΔρS is the density contrast between upper and lower layers (due to just salinity or temperature differences). Laboratory experiments were used to describe the vigour of convection in terms of these dimensionless parameters, which then allows behaviour of various inflows to be predicted. In most cases the convective velocities observed were an order of magnitude faster than Stokes settling velocities. These observations are also applied to predict how a turbidity current could lead to lofting and possible overturn of the stratification of Lake Kivu, a large meromictic lake between Rwanda and the Democratic Republic of the Congo. [ABSTRACT FROM AUTHOR]

Sediment-laden gravity currents, or turbidity currents, are density-driven flows that transport vast quantities of particulate material across the floor of lakes and oceans. Turbidity currents are generated by slope failure or initiated when a sediment-laden flow enters into a lake or ocean; here, lofting or convective sedimentation processes may control flow dynamics. Depending upon the internal turbulent mixing, which keeps particles in suspension, turbidity currents can travel for thousands of kilometers across the seafloor. However, despite several competing theories, the process for the ultralong runout of these flows remains enigmatic. Turbidity currents often generate large sinuous channel–levee systems, and the dynamics of how turbidity currents flow around channel bends are strongly influenced by internal density and velocity structure, with large-scale flows being modified by the Coriolis force. Therefore, understanding some of the largest sedimentary structures on the Earth's surface depends on understanding the turbulence processes within turbidity currents. [ABSTRACT FROM AUTHOR]

Abstract: Fine sediment transport produced by a subglacial freshwater discharge is simulated with a 2D non-hydrostatic model. The circulation pattern revealed a buoyant jet issuing from the aperture representing the subglacial tunnel, a vertically buoyant plume and a surface gravity current forming part of an estuarine circulation. Momentum-dominated experiments are more sensitive to the presence of suspended sediment in the discharge. At low concentrations, the sediment stays in the vertical plume and surface gravity current, and its concentration is progressively decreased by mixing but no settling is observed through the water column. At high concentrations, the sediment settles in the far field and is transported back to the near field by the landward estuarine current. Sediment settled from the surface layer through convective sedimentation, a process that was more effective than flocculation to transport sediment vertically, and showed vertical velocities faster than ms−1. Implications of these results are discussed. [Copyright &y& Elsevier]

Abstract: Clastic mud beds rich in continental organic matter are observed recurrently in the Nile deep-sea turbidite system. They formed during flooding periods of the river similar to those that induce sapropel formation and occurred during periods of increased density stratification of the eastern Mediterranean. The very fine-grained flood deposits are intercalated within pelagic sediments, sapropels and Bouma-type turbidites. These flood deposits form by the successive reconcentrations of surface (hypopycnal) plumes by convective sedimentation, which in turn generate a fine-grained low-energy hyperpycnal flow. Sea-level high stands seem also to favor hypopycnal plume formation and increase clastic mud bed formation. Consequently, these muddy clastic beds provide a direct link between deep-marine sedimentary records and continental climatic change through flood frequency and magnitude. [Copyright &y& Elsevier]

Laboratory experiments were performed to observe the sedimentation of natural sediment in buoyant turbulent suspensions. A series of experiments were conducted in a flume modified to investigate sedimentation from a steady, stratified shear layer. Ambient salinity stratification in the lower fluid allowed the upper layer to remain buoyant, while turbulence induced by free shear in the water column produced instabilities between the two fluids. These experiments showed that convective plumes dominated sedimentation. These convective plumes had vortex tips and vertical velocities of 1–2 cm/s. The velocities observed are two orders of magnitude larger than those predicted by Stokes settling of the constituent particles. Surface plume concentrations as low as 380 mg/l were documented to support robust mixing-induced convective sedimentation. At high concentrations (>6000 mg/l), the experiments produced a divergent plume with a considerable amount of the sediment-laden fluid being diverted along the bottom of the flume. Our observed vertical sediment fluxes demonstrated a 50% increase as compared to previous studies of double-diffusive sedimentation. A simple scaling analysis using the dissipation rate of turbulent energy and the surface plume sediment concentration was able to collapse the experimental data and indicated a positive correlation between turbulent energy and sedimentation. The empirical scaling relationship accurately predicted effective settling velocities measured on the Eel River margin in 1997–1998. The quality of the agreement between the laboratory and field measurements provides strong evidence that sedimentation by mixing-induced convection may be the dominant mode of sediment removal from energetic, highly concentrated, buoyant river plumes. [Copyright &y& Elsevier]

A series of laboratory experiments has been conducted in order to elucidate the sediment-induced mixing processes accompanying riverine outflows; specifically, the discharge of a warm, fresh, particle-laden fluid over a relatively dense, cool brine. In a parameter regime analogous to recently acquired field measurements, hypopycnal (surface) plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm, fresh, sediment-laden plume and particle settling within it. Convection was robust in the presence or absence of intense turbulence, at sediment concentrations as low as 1 kg m-3, and took the form of millimetre-scale, sediment-laden fingers descending from the base of the surface plume. A consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal (bottom-riding) flow. The experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient. The resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond. [ABSTRACT FROM AUTHOR]