A LES study on gravity currents propagating over roughness elements

Predicting the evolution of turbulent gravity currents is of great interest in many areas of geophysics and engineering, in particular due to their impact on the environment. In most practical applications in river, coastal and ocean engineering, gravity currents propagate over loose surfaces containing large scale bedforms (e.g., dunes). In others, arrays of obstacles (e.g., ribs) are often used as protective measures on hilly terrains to stop or slow down gravity currents in the form of powder-snow avalanches. To predict the capacity of a turbulent gravity current propagating over a loose bed to entrain, carry, and deposit sediment requires a detailed understanding of its structure and the role played by the large-scale instabilities present in the flow.

The present study uses high-resolution Large Eddy Simulation to study the physics of high Reynolds number compositional Boussinesq gravity currents with large and small volume of release in lock-exchange configurations and their dynamic effects on various obstacles (e.g., bedforms, flow retarding obstacles, submerged dams that are used to control sediment deposition in reservoirs). The study shows that gravity currents propagating over large-scale roughness elements reach a turbulent drag-dominated regime in which the front velocity decays proportional to t^-1/2, similar to the case of gravity currents propagating within a porous medium. Though the establishment of a regime in which the flow evolution is mainly determined by the balance between the turbulent drag and the buoyancy force driving the flow was expected, the fact that the law of decay of the front velocity with time is identical for gravity currents propagating over roughness elements and in a porous medium of uniform porosity is not obvious.

The simulations provide detailed information on the temporal evolutions of the front velocity, energy balance, sediment entrainment capacity and the flow instabilities, and of the distributions of the density, velocity, local dissipation rate and bed shear stresses at different stages of the propagation of the gravity current. The study investigates of the effect of the shape and relative size of the obstacles, with respect to the current height, on the structure of the current and on the differences with the simpler, but much more widely studied case of a gravity current propagating over a flat smooth surface. For example, the simulation results are used to explain why gravity currents propagating over dunes have a much larger capacity to entrain sediment than gravity currents propagating over ribs of the same height and with similar spacing.

The accurate estimation of impact of gravity current on the structures over its path is very important from engineering point of view since many submerged cables over the ocean bottom or submerged dams in reservoirs are under the risk of such impacts. The simulations of gravity currents propagating past arrays of ribs or isolated dams are used to estimate the characteristic times and magnitudes of the hydrodynamic impact forces on these obstacles. This information is crucial for the proper design of these structures. The study shows the critical role played by flow disturbances (e.g., backward propagating hydraulic jumps) that form as a result of the interaction between the current and the large-scale obstacles. Finally, the study investigates scale effects between the Reynolds numbers at which most experimental investigations of gravity currents are conducted and Reynolds numbers at field scale.