ENTRAINMENT IN TURBULENT GRAVITY CURRENTS

LABORATORY gravity currents are frequently used to model a range of en vironmental and industrial flows1. The manner in which these flows bec ome diluted with distance by the surrounding fluid has important impli cations for turbidity currents2, pyroclastic flows3,4, avalanches5, ac cidental dense gas releases6, fire propagation7 and emission of indust rial pollutants. Here we present an experimental technique for quantif ying the entrainment of ambient fluid into the head of a gravity curre nt propagating along a horizontal surface. The technique relies on the neutralization of an alkaline current by entrainment of acidic ambien t fluid, and is visualized by using a pH indicator. Dimensional analys is indicates that the proportion of ambient fluid entrained into a gra vity current head depends only on the initial volume of the current an d distance from the release point, and is independent of the initial v alue of the density difference. This result is confirmed by the experi mental data, which also show that little dilution of the head takes pl ace during the slumping phase8,9. Thereafter the dilution increases wi th the downstream distance, in quantitative agreement with the predict ions of a theoretical model which evaluates the volume of entrained fl uid. We apply the results to show that sediment slumps of initially hi gh sediment concentrations will become dilute turbidity currents owing to entrainment of sea water before they have propagated extensively o ver the floors of ocean basins.