The influence of diffusive convection on sedimentation from buoyant plumes

Diffusive convection driven by the differential diffusion of density alteri ng fluid properties may enhance the scavenging of particles from natural bu oyant plumes. For single-phase (fluid-solute-heat) systems this phenomenon has been extensively studied because salt fingering generated at the oceani c thermocline is a major mechanism of salt transport in the oceans. However , the influence of this process on particle laden plumes, for example, fluv ial plumes in lakes and estuaries volcanic clouds and seafloor hydrothermal plumes is largely unknown. In this paper, we present direct experimental m easurements of the interfacial particle flux at the plume base which can be applied to predict particle scavenging from natural buoyant plumes, Partic le flux is measured using a light attenuation technique employing a chain o f photodiodes which average concentration over a large number of fingers. T he results are in good general agreement with earlier studies based on fing er velocity. Flux measurements cover a wide range of conditions from those where diffusive convection dominates to those where settling and diffusive convection are of a similar magnitude. For very small particles double diff usive (salt finger) theory is applicable to two component particulate syste ms as suggested by earlier studies [Green, T., 1987. The importance of doub le diffusion to the settling of suspended material. Sedimentology 34, 319-3 31]. Two component diffusive convection theory is extended to three compone nts in order to predict particle scavenging from marine fluvial plumes whic h involve the diffusion of sediment, salt and heat. For larger particles wh ich settle significantly the flux can be approximated by adding the double- diffusive and settling fluxes. A theory to predict particle transport throu gh the lower layer and sedimentation at the bed Is developed based on the o bservation of strong convection below the plume. Application of our theory and experimental results indicates that while double diffusion may signific antly influence the longitudinal distribution and vertical sorting of depos its from lacustrine plumes, the diffusive convection process is generally i nsignificant in marine plumes. Observational evidence for lacustrine double diffusion based on water column measurements is presented. (C) 1999 Elsevi er Science B.V. All rights reserved.