Gravity currents moving on smooth and rough beds

In this work gravity currents moving on both smooth and rough beds were studied. Full-depth lock exchange experiments were performed and the instantaneous velocity field was investigated by particle image velocimetry (PIV). A Perspex tank (Figure 1) with rectangular cross-section was divided into two parts by a vertical sliding gate placed at a distance x0 from the left end wall of the tank, as shown in figure 2. The left portion was filled with salty water with density ρ01 and the right part with tap water with density ρ2 (ρ01>ρ2) at the same height h0. All the experiments were acquired by a CCD camera and an image analysis technique was applied to measure the space-time evolution of the gravity currents’ profiles. The experiments were performed changing the bed’s roughness and keeping constant ρ01, ρ2, x0 and h0. In agreement with previous study, three different phases in the gravity current’s dynamics were observed: a first slumping or constant speed phase; a second self-similar non-viscous phase, in which the front speed decreased as t-1/3; a third self-similar viscous phase, in which the front velocity decreased as t-4/5. Results show that as the bed’s roughness increases the front speed decreases.