The problem of the spreading of a granular mass released at the top of a rough inclined plane was investigated. The evolution of the avalanche was measured experimentally from the initiation up to the deposit using a Moiré image-processing technique. The results are quantitatively compared with the prediction of an hydrodynamic model based on depth-averaged equations. In the model, the interaction between the flowing layer and the rough bottom is described by a non-trivial friction force whose expression is derived from measurements on steady uniform flows. We show that the spreading of the mass is quantitatively predicted by the model when the mass is released on a plane free of particles. When an avalanche is triggered on an initially static layer, the model fails in quantitatively predicting the propagation but qualitatively captures the evolution.