Overtopping is a frequent cause of failure of earth dams. This paper introduces a novel method to characterize the evolution of the breach and to quantify the breach hydrograph following a dam overtopping event from laboratorial tests data. These include a combination of Large-scale PIV (LSPIV), optical techniques and acoustic instrumentation to determine the flow field near the breach and its geometry. The breach hydrograph results from the direct integration of the velocity field over the breach. The results relative to two different earth dam laboratory models, led to failure by overtopping under hydraulic and geotechnical controlled conditions are shown. Hydraulic erosion and geotechnical failure events were registered. Key features of the breach hydrograph are discussed. It was found that hydraulic erosion was the predominant breaching process but the erosion by undercutting was also present in both tests contradicting the regressive erosion widely referred in the literature as the predominant erosion mode. The novel method to quantify the breach hydrograph showed a good agreement with the mass balance estimates commonly adopted in literature demonstrating a good potential in responding to immediate changes in the breach morphology such as those originated by sudden mass failure.