Journal of Conference Abstracts

The Effects of External Flows on Gravity Currents

Andrew Hogg (a.j.hogg@bristol.ac.uk)¹, Herbert Huppert (heh1@esc.cam.ac.uk)² & Mark Hallworth (hallwort@esc.cam.ac.uk)²

¹Centre for Environmental and Geophysical Flows, School of Mathematics, University of Bristol, BS8 1TW, U.K.

²Institute of Theoretical Geophysics, D.A.M.T.P., University of Cambridge, CB3 9EW, U.K.

The propagation of a high Reynolds number, two-dimensional gravity current at the base of a uniform flow is considered. Experiments are reported for which relatively heavy fluid is introduced midway along a channel in which there is a uniform flow of water. The density of the released fluid is rendered in excess of the ambient fluid by either dissolving salt or suspending particles in water. Fluid is introduced into the channel either at a steady rate or by releasing a known volume over a relatively short duration. The upstream and downstream propagation of the current was measured for different initial concentrations of salt, input rates and particle sizes and concentrations. For the particle-driven currents, the distribution of the deposit was also determined.

These flows are mathematically modelled using both simple 'box' models and the appropriate shallow-water equations. Both of these models balance the inertia of the current with the buoyancy forces that drive the flows. In the case of the particle-driven currents, they also account for the sedimentation of particles through the current to the underlying boundary. Box models permit the construction of analytical expressions for the motion and the deposit from the current, whereas the shallow-water equations must be integrated numerically. Both sets of solution are in very good agreement with the experimental data.

These studies may be applied to natural phenomena that occur when a buoyancy-driven motion interacts with a uniform flow. For example, winds and tides influence the buoyancy-driven motion in the atmosphere and oceans, respectively. A novel application is that of water-injection dredging. A water jet is directed towards the bed to mobilise the sedimentary particles. Once suspended they form a particle-laden cloud with a density in excess of the surrounding estuarine water. Hence, a gravity current arises. However, this flow is strongly influenced by tidal currents. Prediction of the fate of the dredged material, which is of considerable environmental importance, can be tackled using the analysis of this study.