A discrete element model was used to highlight the influence of size, aspect ratio and roundness of blocks onto the propagation of granular flows in order to obtain a more realistic modelling of rock avalanches. The numerical model accounts for energy dissipations by shocks and friction within the granular mass or at the base of the avalanche. A parametric numerical study was performed, based on laboratory experiments involving an assembly of small bricks. A particular attention was paid to explain how the block and slope geometry influence the kinematics of the avalanche (sliding or rotation of particles), and how the dissipative modes are modified (shocks or friction). In particular the effects of elongation, size and roundness of blocks combined to slope undulation were investigated. It was shown that these geometric aspects may have some complex implications in the mechanism of propagation of the granular flow and need to be taken into account to predict properly the final position of the deposit and the area impacted by individual blocks.