Turbidity currents-underwater avalanches- constitute a special class of gravity currents which can occur in environments such as lakes, submarine channels, and continental slopes. They represent a large-scale geophysical flow phenomenon that plays an important role within the global sediment cycle as they can transport O(100) km3 amount of sediment into deep-sea regions. In the present investigation, direct numerical simulations of turbidity currents interacting with complex seafloor topographies are carried out to study relevant properties such as front velocity, runout length, deposit profiles, mixing and unmixing of ambient and clear fluid. Towards this aim, first, a highly parallel three-dimensional code called TURBINS is developed and validated against several numerical and experimental results available in the literature.

Subsequently, to investigate the influence of the seafloor topography on turbidity currents, bi-disperse currents produced by the so-called "lock-exchange" configuration are conducted. The results from three bottom topographies, viz. flow over a tall Gaussian bump (B2), over a shallow bump (B1), and over a flat bottom (FL), demonstrate the strong influence of the bumps on current properties. In all three cases, turbidity currents, initially, traveled with the same velocity. During the later stages, however, we observed a nonmonotonic influence of the bump height on flow properties such as front location and mixing dynamics, i.e. the currents in cases B1 and B2 travel, respectively, faster and slower than the current produced in case FL. Spatio-temporal information such as energy budgets and concentration profiles are employed to explain this nonmonotonic behavior.

On one hand, the lateral deflection of the current by the bump leads to an effective increase in the current height and its front velocity in the region away from the bump. At the same time, taller bumps result in a more vigorous three-dimensional evolution of the current, accompanied by increased dissipation, which slows the current down. For small bumps, the former mechanism dominates, so that on average the current front propagates faster than its flat bottom counterpart. For currents interacting with larger bumps, however, the increased dissipation becomes dominant, so that they exhibit a reduced front velocity as compared to currents propagating over flat surfaces.