EXPERIMENTAL RESULTS ON SEDIMENT ENTRAINMENT BY GRAVITY CURRENTS

Abstract

Gravity currents are geophysical flows responsible of the distal transport of high volumes of sediments. In particular, turbidity currents, a form of gravity currents where sediments in suspension confer the buoyancy that ignites the flow, are the main mechanism for distal sediment transport within lakes and reservoirs. In maritime environment, submarine clay-rich gravity currents can impact and may endanger human made infrastructures such as submarine cables and platforms. It is thus important to understand the dynamics of sediment transport associated to gravity currents. In the present research, it is intended to experimentally investigate the mechanisms of entrainment, transport and deposition of fine sediments caused by the passage of a saline gravity current. Conservative saline currents, with varied initial density, are let to flow over an erodible bed sector where fine sediments, with three different grain sizes, are at rest. A detailed description of the gravity current dynamics is reported using 3D instantaneous velocities measurements over a certain profile. Video records obtained synoptically and laterally through a transparent wall, provide a visualization of the entrainment and resuspension processes which is further related to the flow hydrodynamics. The critical threshold conditions for initiating sediment motion is frequently related to the balance of boundary shear stress and the submerged weight of the particle. However boundary shear stress is just one of several impelling forces and the particle submerged weight is just one of several inertial forces. Here the attention is first focused on the complete description of the flow velocity, in term of instantaneous and mean flow. A deep analysis of the hydrodynamic of one gravity current reproduced in laboratory is here presented and its role in sediments’ entrainment discussed.