Laboratory experiments on gravity currents moving on smooth and rough beds

The aim of this paper is the investigation of gravity currents moving on both smooth and rough beds by laboratory experiments. Gravity currents were produced in a Perspex tank of rectangular cross-section by lock exchange release experiments. The tank is divided into two parts by a sliding gate: one filled with tap water and the other one filled with salt water to the same height. Nine experiments were performed varying both the bed roughness and the initial density of the gravity current. All the experiments were recorded by a camera and an image analysis technique was applied to measure the spacetime evolution of the gravity current's profile. For the experiments with a smooth bed three different phases were observed in the gravity current’s dynamics: a first slumping or constant speed phase; a second self-similar phase, in which the front speed decreased as t-1/3, where t is the time measured from release; a third viscous phase, in which the front velocity decreased as t-4/5. In the experiments performed with a rough bed the measured front’s velocities were observed to be lower than the front’s velocity of gravity currents moving on a smooth bed.