Highly resolved 3-D Large Eddy Simulation (LES) is used to study the interaction between a lock-exchange gravity current with a large volume of release and an array of bottom-mounted large-scale obstacles in the form of 2-D dunes or square ribs. The study of the interaction between a gravity current and an array of obstacles is important for many practical applications. For example, arrays of obstacles are often used as protective measures on the hilly terrains and on the skirts of the mountains to stop or slow down gravity currents in the form of powder-snow avalanches. Even if they do not arrest the flow, the retarding obstacles reduce the impact of the avalanche with the buildings situated downstream of the obstacles. The temporal variation of the impact forces on the obstacles is analyzed. This information is needed for the design of the retarding obstacles. Additionally, simulation results are used to understanding how this variation is related to the passage of the backward propagating hydraulic jumps and the different flow structures that develop within the flow. The loose bed surface over which the gravity current propagates in the environment is often not flat. Bed forms, typically in the form of ripples, dunes or anti-dunes are present at the seafloor or river bed. The presence of large-scale bedforms provides an additional mechanism for energy dissipation and can substantially modify the capacity of a compositional gravity current to entrain sediment with respect to the case of a flat bed. LES is used to understand how the shape and the relative size of the large-scale obstacles (roughness elements) affect the front velocity, the structure of the current, the energy balance, the bed shear distributions and sediment entrainment capacity of the current as it propagates over a loose bed. Finally, scale effects are investigated between Reynolds numbers at which most of the laboratory studies are conducted (Re~10⁴) and Reynolds numbers that are within the lower range of those encountered in the field (Re=10⁶).