Polydispersed Gravity Currents Along a V-Shaped Valley: Experiments and Box Model.

Abstract:

Turbidity currents, which occur at the continental margins and transport sediments along submarines canyons are particulate gravity currents made of poorly sorted particles. In such currents, the flow is to a large extent controlled by the grain size distribution of the particles at the source. Here we present the combined results of a box model and lock-exchange experiments of particulate gravity currents at small volumetric concentrations of particles (< 4%) flowing along a V-shaped valley, for which we looked at the grain size distributions along the deposit. The currents were made of 1) Silicon Carbide (SiC), 2) Glass Beads (GBs), 3) combined poorly sorted SiC/GBs, and 4) moderately sorted combined SiC/GBs. Experimental particles sizes were in the range 3-100 microns. We used 20 particle size classes to characterize the distributions. First we show that a box model that solves for the deposition of all the particle size classes, remarkably describes the speed of the currents, the mass deposited by the currents, and the material density of the deposit for those currents initially made of SiC and GBs. Last but not least, the box model also reconstructs the mass based size distributions along the deposits characterized by various fining. Yet, the reconstructed distributions differ with the shape of the particles. In the presence of only round GBs particles, the mass based size distributions given by the box compare extremely well with the experimental counterparts. In the presence of SiC, there is a systematic shift towards the smaller grain sizes of the mass based size distributions given by the box compared to the experimental ones. Several hypotheses to explain this shift are currently being investigated. The present work was supported by the Fundaçao para a Ciência e a Tecnologia (FCT, Portugal) under Project Pest-OE/CTE /LA0019/2013-2014.