Continuously-fed gravity currents propagating over a finite porous substrate

Abstract

We present the results of laboratory investigations of continuously-fed density currents that propagate first over a smooth horizontal bed and then over a porous substrate of limited length. Inflow discharge, initial excess density, and substrate porosities are varied. Density measurements, acquired through an image analysis technique, are performed above the porous layer simultaneously with quasi-instantaneous vertical velocity profiles. After a first phase in which the current sinks into the substrate, freshwater entrainment from the bed begins and, gradually, a mixing layer forms at the interface between the surface flow and the porous bed. Shear-driven and Rayleigh-Taylor instabilities rule the dynamics of this mixing layer. The porous boundary effects are observed in the vertical distributions of both density and velocity, especially in the near-bed region. Here, larger flow velocities are recorded over porous substrates. We argue that these are due to the presence of a longitudinal pressure gradient, which in turn is a consequence of the current mass loss. Its presence over the porous substrate is proved by the current interface longitudinal slope. However, other effects of the presence of the porous substrate, such as the relaxation of the no-slip boundary condition and the bed-normal momentum exchange, also affect the velocity field. The turbulent structure changes significantly over the porous substrate: while streamwise turbulence decreases, shear and bed-normal Reynolds stresses increase in large part of the current depth. Buoyancy instabilities further enhance the bed-normal momentum flux and, in the near-bed region, contribute to turbulent kinetic energy generation together with shear.