Enhanced Sedimentation Beneath Sediment Laden Overflows and Interflows

Abstract eng:
Double-diffusive and settling-driven convection processes can enhance the sedimentation rate of particles beneath a sedimentladen plume. The two mechanisms can be active simultaneously, and previous experimental studies had difficulty distinguishing them visually. In experiments with a sediment-laden fluid overlying a saline layer, visual measurements of mean velocities can only be made in the optically clear lower layer. We measured the velocity and turbulence components of the flow field above and below the initial sediment/salt interface, using an Acoustic Doppler Velocimeter. The velocity of the sediment fingers in the lower layer were always larger than the Stokes settling velocity of the particles, leading to an asymmetry in the flow field of the two convective layers. Sediment fingers only dominated when there were marginal density differences between the two layers. We conclude that double diffusive sediment fingers control sedimentation beneath interflows in most lakes, whereas settling-driven convection is dominant in most oceanic overflows. When a sediment-laden river enters in to a stratified lake or the coastal ocean, it is of great interest to know how fast the sediment settles from the base of the resulting surface or subsurface flow (overflow or interflow). There is observational evidence of enhanced sedimentation near the mouth of the rivers, in the coastal ocean [6] and lakes [9], where the apparent particle settling rate is larger than that based on Stokes settling velocity of a single particle. However it is not clear whether such enhanced settling is due to flocculation [8], double-diffusive convection [4, 7] or settling-driven convection [5]. In the absence of flocculation, two related processes of enhanced particle sedimentation could occur beneath a sediment-laden intrusion; namely double-diffusive convection (illustrated in Figure 1a) and settling-driven convection (illustrated in Figure 1b). When a sediment-laden fluid overlies a denser saline layer, the accumulation of particles at the interface can develop a gravitationally unstable bulk density profile that produces Rayleigh-Taylor instability at the sediment front [5, 10, 11, 1, 2], shown in Figure 1b. In this case the settling-driven mechanism leads to convective sediment plumes below the interface, which drives vigorous convection. ... ... ... ... ... ... ... ... ... ... diffusion of salt being much faster than the Brownian diffusion of sediment. This process results in rapidly descending fingers of sediment beneath the intrusion as shown in Figure 1a.

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