Dynamics of the head of gravity currents

Abstract

Gravity currents, from which turbidity currents are an example, occur in reservoirs and lakes with potential hazards on what concerns sedimentation and water quality issues. The dynamics of gravity currents produced by full-depth lock-release of saline water into a fresh water tank are herein investigated. More specifically, the present results concern to the head of gravity currents where main interaction with the ambient fluid is observed. The experiments were conducted in a 3.0 m long Perspex flume of horizontal bed and rectangular cross section of 0.20 x 0.30 m2 and recorded with a 25 Hz CCD video camera. An image analysis technique is used to evaluate time-space varying density distribution of unsteady density currents. The identification of the current head, based on the local vertically-averaged mass of the current, allows the characterization of its length and mass, both varying in time and space. Temporal evolution of these parameters show repeated cycles of stretching and breaking, where mass detachment from the head towards upstream, within the current body, is observed. This feature is related with instabilities signatures observed in the boundary region between current and ambient fluid and help to understand the entrainment phenomenon at the current head.