Owing to their destructive power, debris flow are the subjects of extensive investigations aiming at charaterizing and modeling their inner nature. Laboratory experiments on simulated dam breaks are one important way to trigger and study debris-flow waves. High-speed recordings of granular flows arising from a dam-break-like event can be processed to extract useful information about the flow dynamics. Although the techniques for measuring of velocities in liquid end gas flows are well established, they cannot be used directly on the flows arising from a dam break: this paper presents two required additions that were made to compute such flows. All the resulting quantities are being used to tune a mathematical model describing the observed flows.