Surveying flash floods: gauging the ungauged extremes

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 frequently can such a flood be expected to occur? Collectively, these studies contributed to the establishment of regional peak discharge envelope curves and to the development of a better understanding of regional behaviour of extreme floods. However, focus on peak discharges and point rainfall maxima alone provides limited insight into the hydrological controls of flash-flood response. Flash-flood monitoring requires rainfall estimates at small spatial scales (1 km or finer) and short-time scales (15-30 min, and even less in urban areas). These requirements are generally met by weather radar networks. This is shown schematically in Figure 1, which reports typical monitoring scales of weather radar systems and rain-gauge networks, together with the time and space scales of a number of flash-flood generating storms observed in Europe in the last 15 years (Borga, 2007). Rapidly increasing availability of good quality weather radar observations is greatly expanding our ability to measure and monitor rainfall distribution at the space and time scales which characterize the flash-flood events (Borga et al., 2007). These technical advances have the potential to enhance the information content of post-event surveys. Realizing this potential calls for the development of a methodology for flash-flood response survey, which goes beyond the collection of indirect peak discharge estimates by focussing on three concepts that are revised in this short commentary.