Intrusions of sediment laden rivers into density stratified water columns could be an unrecognized source of mixing in many lakes and coastal oceans

Abstract

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.

Tracing Unconfined Nearfield Spreading of a River Plume Interflow in a Large Lake (Lake Geneva): Hydrodynamics, Suspended Particulate Matter, and Associated Fluxes

“…Actual settling velocities are likely to be larger than the Stokes settling velocity (Parsons et al, 2001). Therefore, faster settling may occur in the nearfield of the interflow, and processes such as sediment-driven convection-forming vertical finger plumes (Parsons et al, 2001;Davarpanah and Wells, 2016;Lu et al, 2022) can further accelerate the settling.…”

“…Following Lu et al (2022) who proposed determining sediment-laden interflow characteristics based on different temperature and sediment-derived densities of the river and the lake, the sediment-laden negatively-buoyant Rhône River plume is an interflow. By the same concept, i.e., considering the density difference between water and sediment, but not the particle size distribution, a mean settling velocity of ∼6 mm s −1 is obtained for 11 July and ∼5.5 mm s −1 for 18 September.…”

Sedimentary characteristics and depositional model of hyperpycnites in the gentle slope of a lacustrine rift basin: A case study from the third member of the Eocene Shahejie Formation, Bonan Sag, Bohai Bay Basin, Eastern China

“…Layered plant fragments are developed in the distal lobe deposits and levee deposits (Yang et al, 2019(Yang et al, , 2020. It is believed that the hyperpycnal flow developed in the fresh water lake basin is also common with floating processes (Lu et al, 2022) (Figure 3j). The floating stratified accumulation of plant fragments is caused by the reversal of buoyancy (Pritchard & Gladstone, 2009;Steel et al, 2016Steel et al, , 2017Stevenson & Peakall, 2010;Zavala, 2020;Zavala et al, 2012).…”

“…At the same time, the middle part of the bed is rich in plant fragments. The accumulation of those low-density materials may imply the reversed buoyancy (lofting) process of quasisteady low-density turbidity current in the last stage of flow evolution even in fresh water lacustrine basins (Lu et al, 2022;Pritchard & Gladstone, 2009;Steel et al, 2016Steel et al, , 2017Zavala & Arcuri, 2016). Therefore, Type D beds are interpreted as the result of flow transformation from quasisteady low-density turbidity current to muddy debris flow caused by flow reversed buoyancy.…”

“…(Figures 13 and 14). Therefore, the further study of formation mechanism and distribution patterns of lofting facies in fresh water basins is needed (Lu et al, 2022).…”