Gravity currents interacting with bottom large-scale roughness

Gravity currents, driven by density variations caused by gradients in temperature, salinity, or sediment concentration, arise due to hydrostatic imbalances between adjacent fluids. These flows play a pivotal role in a wide range of geophysical and engineering applications, shaping atmospheric, terrestrial, and subaqueous environments. In natural settings, the propagation of gravity currents often encounters uneven topographies, where the dynamics of the dense flow are significantly influenced by topographic features. Recent research has increasingly focused on understanding gravity currents moving through channels obstructed by finite-size patches of obstacles, which adds complexity to their behavior and mixing processes. This experimental study investigates the interaction mechanisms between gravity currents and such obstructions, providing insights into their dynamics and mixing implications through a non-intrusive image analysis technique based on light reflection to evaluate instantaneous density fields.

Laboratory experiments were conducted in a Perspex tank with dimensions of 3 m in length, 0.3 m in height, and 0.2 m in width. An array of rigid plastic cylinders, each with a diameter of 2.5 cm, was placed at a predetermined location spanning the entire width of the channel. The gravity current was reproduced using the lock-release technique with a density difference ∆ρ=6 kg/m³. A total of 12 full-depth lock-exchange experiments were performed to analyze the submergence ratio, i.e. the ratio between the initial current depth and the obstacle height, and the gap-spacing ratio, i.e. the ratio between the spacing of the bottom obstacles and the obstacle height.

The analysis of instantaneous density fields provides valuable insights into the complex dynamics of gravity currents. During the initial slumping phase, the front of the dense current advances at a constant velocity. However, upon reaching the obstacles, the gravity current slows down, leading to the emergence of distinct flow regimes. The evaluation of the density fields enabled a more detailed description of the flow evolution. It was observed that the front of the dense current detaches from the upstream face of each obstacle. In particular, the temporal and spatial evolution of the dimensionless depth-averaged density obtained by integrating the instantaneous density fields below a threshold of 2% excess density revealed significant phenomena near the current front. This region exhibited increased mixing and dilution as the ratio between the initial current depth and the obstacle height increased. Conversely, the influence of the spacing between the bottom obstacles appeared to be less significant.