Sedimentation from flocculated suspensions in the presence of settling‐driven gravitational interface instabilities

Abstract

We experimentally examine sedimentation from a freshwater suspension of clay flocs overlying saltwater in the presence of gravitational instabilities. The study seeks to determine: (1) if flocculation hampers or alters interface instability formation; (2) how the removal rates of sediment from the buoyant layer compare to those predicted by individual floc settling; and (3) whether or not it is possible to develop a model for effective settling velocity. The experiments were conducted in a tank at isothermal conditions. All experiments were initially stably stratified but later developed instabilities near the interface that grew into downward convecting plumes of fluid and sediment. Throughout, we measured sediment concentration in the upper and lower layers, floc size, and plume descent rates. The data showed that flocculation modifies the mixture settling velocity, and therefore shifts the mode of interface instability from double‐diffusive (what one would expect from unflocculated clay) to settling‐driven leaking and Rayleigh‐Taylor instability formation. Removal rates of sediment from the upper layer in the presence of these instabilities were on the same order of magnitude as those predicted by individual floc settling. However, removal rates were found to better correlate with the speed of the interface plumes. A simple force‐balance model was found to be capable of reasonably describing plume velocity based on concentration in the buoyant layer. This relation, coupled with a critical Grashof number and geometry relations, allowed us to develop a model for the effective settling velocity of the mixture based solely on integral values of the upper layer.

A settling-driven instability in two-component, stably stratified fluids

“…In the present work, we aim to understand how the traditional picture of double diffusion is modified if the unstably stratified scalar is a particulate phase with a Stokes settling velocity. A number of laboratory flow visualization experiments (Green 1987;Green & Diez 1995;Carey 1997;Chen 1997;Hoyal, Bursik & Atkinson 1999a,b;Maxworthy 1999;Parsons & García 2000;Parsons, Bush & Syvitski 2001;Manville & Wilson 2004;Carazzo & Jellinek 2013;Manzella et al 2015;Rouhnia & Strom 2015;Davarpanah & Wells 2016) demonstrate that thermal or compositional density gradients can dramatically alter the effective settling velocity of the particles by driving double-diffusive instabilities or settling-driven convection. This mechanism has also been hypothesized to be active in the field observations by Scheu et al (2015), and in the lake measurements of Sánchez & Roget (2007).…”

Enhanced sedimentation beneath particle‐laden flows in lakes and the ocean due to double‐diffusive convection

“…There is observational evidence of enhanced sedimentation near the mouth of the rivers, in both the coastal ocean [Nowacki et al, 2012] and lakes [Scheu et al, 2015], where the apparent particle settling rate is larger than that based on the Stokes settling velocity of a single particle. However, it is not clear whether such enhanced settling is due to flocculation [Rouhnia and Strom, 2015], double-diffusive convection [Hoyal et al, 1999a;Parsons et al, 2001], or settling-driven convection [Bradley, 1965;Hoyal et al, 1999b]. The sediment deposition rate is critical in determining the fate of particles that settle beneath an overflow (hypopycnal flow) or interflow (mesopycnal flow).…”

Sedimentation from buoyant muddy plumes in the presence of interface mixing: An experimental study

“…Because is not correlated with plume concentration (Figure b), we do not think that the increased sedimentation rate is due to shear‐induced density instabilities. If density instabilities were at play, we would have expected a positive correlation between and C [ Rouhnia and Strom , ]. However, if anything, is inversely related to C in our experiments.…”

“…The third potential mechanism for enhanced sediment removal rates are gravitational instabilities. Gravitational instabilities are classified by their initiation mechanism as double diffusive [ Green , ; Chen , ; Hoyal et al ., ; Parsons and Garcia , ] or settling [ Hoyal et al ., ; Blanchette and Bush , ; Rouhnia and Strom , ] driven. They appear in the form of positive irregularities in the density field near the stratification interface of two stagnant, or possibly laminar, layers of fluid.…”

“…They appear in the form of positive irregularities in the density field near the stratification interface of two stagnant, or possibly laminar, layers of fluid. As the density irregularities grow, they eventually evolve into downward advecting cells of fluid and sediment mixture in the form of small negatively buoyant plumes [ Hoyal et al ., ; Burns and Meiburg , ; Yu et al ., ; Burns and Meiburg , ; Rouhnia and Strom , ]. Instabilities at the stratification interface can also be driven by shear between the hypopycnal river discharge and the receiving water body.…”