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Summary: We propose a new simplified vision of flood dynamics through temporal and spatial scales to enhance understanding of potential and limits of human responses to flash floods. Based on data from a set of extreme flash floods that occurred in Europe, we analyze both the runoff response time of flash floods and the asynchronous character of flood peak time generation. We introduce the concept of “timeliness” to quantify the balance in time between the hydrological response of catchments to heavy storms and the human reactions. Timeliness is investigated across the range of spatial scales of concern for flash floods and for various types of anticipation actions. Results are reported for four extreme flash flood events occurred since 2002 in France, Italy and Slovenia. These results indicate that human actions adapt their pace to the physical context and are in a kind of “hurrying” process as the flood danger approaches in time. They show the importance of self-organization and ‘unofficial’ warning in response to flash flooding by individuals and groups. The work draws on flood hazard research findings from hydrology, meteorology and social science and put forward a framework for interdisciplinary collaboration between hydrologists and social scientists. [ABSTRACT FROM AUTHOR]

Observations and simulations of rainfall events are usually compared by analyzing (i) the total rainfall depth produced by the event and (ii) the location of the rainfall maximum. A different approach is proposed here that compares the mesoscale simulated rainfall fields with the ground rainfall observations within the multiscale framework of maximum intensity diagrams and severity diagrams. While the first simply displays the maximum rainfall intensity of an event at a number of scales, the second gives the frequency of occurrence of the maximum rainfall intensities as a function of the spatial and temporal aggregation scales, highlighting the space-time scales of the event severity. For use in a region featuring complex relief, severity diagrams have been generalized to incorporate the regional behavior of heavy rainfall events. To assess simulation outputs from a meteorological mesoscale model, three major storms that have occurred in the last decade over a mountainous Mediterranean region of southern France are analyzed. The severity diagrams detect the critical space-time scales of the rainfall events for comparison with those predicted by the simulation. This validation approach is adapted to evaluate the ability of the mesoscale model to predict various types of storms with different regional climatologies. [ABSTRACT FROM AUTHOR]

Based on weather radar detection, orographic rainbands parallel to wind direction may persist for several hours over a Mediterranean mountainous region prone to stable wind and humidity conditions. A statistical analysis shows that orographic rainbands are more active and more stable over the mountains than over the lower hills. By the mean of the range–time indicator technique, the northward advection velocity of the rain cells is deduced (60 km h[sup -1] ) and is slightly lower than the wind velocity (85 km h[sup -1] ) measured at the high-altitude weather station (Mont Aigoual, 1565 m above mean sea level). The detailed analysis highlights that the positioning of individual orographic cells in relation to the relief is not random: they are triggered by relief shoulders on their southeast flank. Their regular spacing (typically 15 km) is responsible for the general organization of the rainbands. Rain accumulations vary from 20 to over 100 mm day[sup -1] from the outside to the center of the rainbands. [ABSTRACT FROM AUTHOR]