Abstract
Gravity currents are the primary means by which sediments, solutes and heat are transported across the ocean-floor. Existing theory of gravity current flow employs a statistically-stable model of turbulent diffusion that has been extant since the 1960s. Here we present the first set of detailed spatial data from a gravity current over a rough seafloor that demonstrate that this existing paradigm is not universal. Specifically, in contrast to predictions from turbulent diffusion theory, self-sharpened velocity and concentration profiles and a stable barrier to mixing are observed. Our new observations are explained by statistically-unstable mixing and self-sharpening, by boundary-induced internal gravity waves; as predicted by recent advances in fluid dynamics. Self-sharpening helps explain phenomena such as ultra-long runout of gravity currents and restricted growth of bedforms, and highlights increased geohazard risk to marine infrastructure. These processes likely have broader application, for example to wave-turbulence interaction, and mixing processes in environmental flows.
Highlights
Gravity currents are the primary means by which sediments, solutes and heat are transported across the ocean-floor
The upper shear layer has been studied in detail; with laboratory studies suggesting that the flow velocity is well approximated as a free-jet, with an exponentially decreasing profile, with a Gaussian decay with distance from the velocity maximum, in subcritical[9,11,18] and supercritical flows[11]; albeit it has been argued that linear or exponential decay may occur in some supercritical currents[18]
Where models do not fully resolve turbulent fluid motion the frictional turbulent diffusion of momentum, and analogously diffusion of material transported by the flow, are parameterized by a positive[23] eddy diffusivity model
Summary
Gravity currents are the primary means by which sediments, solutes and heat are transported across the ocean-floor. Self-sharpening helps explain phenomena such as ultra-long runout of gravity currents and restricted growth of bedforms, and highlights increased geohazard risk to marine infrastructure These processes likely have broader application, for example to wave-turbulence interaction, and mixing processes in environmental flows. Gravity current dynamics depend on variations in the excess density of the flow, i.e. stratification, and shear and turbulent mixing resulting from the flow’s interactions with the seafloor and the ambient seawater. In the outer region the flow follows the concave up profile of the inner region; a result of short range (in comparison to the length scale over which velocity varies) isotropic turbulent fluctuations generating a down-velocity-gradient momentum flux[23,24] (i.e. towards the bed in the lower shear layer and towards the flow ambient fluid interface in the upper shear layer). Irreversible breaking of Rossby, or other dispersive waves, near a critical layer where the background flow speed tends to the wave phase speed[29] result in deposition of wave momentum, with concomitant changes in angular momentum distribution[26], and generation of mean flow[30,31]
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