A two-dimensional finite element model for the morphodynamic evolution of a cohesionless bed is presented and tested against experimental observations of the formation and evolution of free bars. The model is then employed to study the strongly nonlinear interactions governing bar development. The attention is focused on the influence of the initial perturbation introduced in the flow field for bar inception. It is found that the type of the disturbance plays a role of utmost importance as it can strongly affect both the developing process and the equilibrium conditions.