INFLUENCE OF THE INITIAL VOLUME OF LOCK EXCHANGE GRAVITY CURRENTS ON THE SEDIMENT ENTRAINMENT CAPACITY

In the lock-exchange technique used to produce gravity currents, two fluids of different densities are initially at rest in a tank and separated by a vertical barrier, the lock gate. When this gate is suddenly removed, the two fluids come into contact and, due to differences in the hydrostatic pressure, the denser fluid flows along the bottom boundary of the tank, while the lighter fluid flows in the opposite direction above the gravity current. The initial volume of the denser mixture was found to highly influence the hydrodynamics behavior of the gravity current. The artifacts produced by the geometry of a limited lock exchange release are partly suppressed by increasing the volume of denser fluid and making it, at least, equal to the ambient fluid volume. In the present study the influence of the initial volume release is investigated focusing the attention on the potential capacity of the current to entrain sediments from an erodible bed. In fact, the erosive capacity has found to be a crucial characteristic of these type of flows responsible for the displacement of important sediment volumes. These flows are thus an important mechanisms of distal transport of sediments in the subaqueous environment. In these experiments, the lock gate was placed at three different locations to have three initial volumes of release. Two initial gravity current’s densities were tested to have a total of nine experiences to compare. Using 3D instantaneous velocity measurements, the hydrodynamics of the gravity currents are analyzed. The bottom shear stress is estimated from the hydrodynamics measurements. Through the investigation of the instantaneous flow and turbulent quantities, the differences in the potential erosion capacities are discussed as function of the initial conditions of the gravity current.