Hyperpycnal plume formation from riverine outflows with small sediment concentrations

Abstract

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.

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

“…This produces an unstable band of sediment-laden saltwater, i.e., the so-called interface layer, sitting between the muddy freshwater and the underlying clear saltwater. This phenomenon has been experimentally observed by Hoyal et al [1999b], Parsons et al [2001], and Blanchette and Bush [2005] and examined through linear stability analysis by Burns and Meiburg [2012] and Yu et al [2013] and Direct Numerical Simulation (DNS) by Yu et al [2014] and Burns and Meiburg [2015]. A few defining characteristics of the settlingdriven mechanism are the general creation of the interface layer below the initial stratification contact [Hoyal et al, 1999b], the onset of Rayleigh-Taylor generated plume like structures, asymmetry in the downward and upward interface motions, and the overall larger length scales of the instabilities and instabilities spacing (centimeter scale) compared to the double-diffusive generated fingering (millimeter scale) [Hoyal et al, 1999b;Blanchette and Bush, 2005;Burns and Meiburg, 2012;Yu et al, 2013;Burns and Meiburg, 2015;Yu et al, 2014].…”

“…The primary outcome of the study was a phase diagram depicting the dominate instability mode as a function of a thermal stabilizing ratio and sediment concentration. Parsons et al [2001] similarly concluded that thermal stratification and high sediment concentrations are likely needed for double diffusion to dominate. In addition, they identified an instability that is different than classic double-diffusive fingering.…”

“…The result of the two-way diffusion is an unstable band of locally more dense fluid near the interface that develops into a regular and symmetric pattern of downward and upward convecting fingers of alternating densities. Double-diffusive fingering can also occur in thermally stratified systems with warm suspensions of sediment overlying cooler clear water [Houk and Green, 1973;Green, 1987;Hoyal et al, 1999a;Parsons et al, 2001]. Key in the development of double-diffusive fingering for suspensions of sediment is that the sediment in the buoyant layer settles very slowly so that substance diffusion, and not particle settling, is primarily responsible for changes in the vertical density profiles [Burns and Meiburg, 2012;Yu et al, 2013;Burns and Meiburg, 2015;Yu et al, 2014].…”

“…10.1002/2015JC010750 Parsons et al [2001] further investigated the instability mechanism question for a warm, sediment-laden freshwater plume overlying cooler ambient saltwater. The primary outcome of the study was a phase diagram depicting the dominate instability mode as a function of a thermal stabilizing ratio and sediment concentration.…”

“…A distinction between these three mechanisms is whether or not there is turbulent mixing at the interface between the hypopycnal river discharge and the receiving water body. If there is, then shear or turbulence is likely the mechanism leading to gravitationally unstable packets of fluid and sediment [Maxworthy, 1999;Parsons et al, 2001;McCool and Parsons, 2004]. However, in cases where mixing is suppressed due to lack of mean plume advection, or very strong stratification, gravitationally unstable mixtures of fluid and sediment can develop due either to, or in combination to, the double-diffusive or settling-driven mechanisms.…”