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The monitoring of flash-flood events gives us the unique opportunity to observe how catchments respond when most of the surface and subsurface hydrologic flow paths are active. These events often reveal aspects of hydrological behaviour that either were unexpected on the basis of weaker responses or highlight anticipated but previously unobserved behaviour (Delrieu et al., 2005; Archer et al., 2007). Characterizing the response of a catchment during flash-flood events, thus, may provide new and valuable insight into the rate-limiting processes for extreme flood response and their dependency on catchment properties and flood severity.

Flash-flood events, however, are difficult to monitor because they develop at space and time scales that conventional measurement networks of rain and river discharges are not able to sample effectively (Creutin and Borga, 2003). As these events are locally rare, they are also difficult to capture during classical field-based experimentation, designed to last a few months over a given region, or on experimental catchments with drainage areas of a few square kilometres. This explains why the investigation of flash-flood events is by necessity event-based and opportunistic as opposed to driven by observations from carefully designed field campaigns. Post-event surveys therefore play a critical role in gathering essential observations concerning flash floods. Traditionally, indirect peak discharge estimates and collection of rainfall maxima have been used to document these events, as well as to provide an answer to the questions that are invariably asked after a major flood: Why did such a major flood occur? How …