Characterisation of selected extreme flash floods in the Mediterranean region and implications for flood risk management

Zoccatelli D., Borga M., Marchi L. e Amponsah W., 2016, Characterisation of selected extreme flash floods in the Mediterranean region and implications for flood risk management, Giornate dell'Idrologia 2016, Trento, 27-29/06/2016,

The occurrence of flash flooding is of concern in hydrologic and natural hazards science due to the top ranking of such events among natural disasters in terms of both the number of people affected globally and the proportion of individual fatalities. On the average, these events cause more casualties worldwide than any other natural disaster, with an average rate of 5000 deaths per year (Douben, 2006). The potential for flash flood casualties and damages is also increasing in many regions due to the social and economic development bringing pressure on land use. Furthermore, evidence of increasing heavy precipitation at continental (Groisman et al., 2004) and global scales (Groisman et al., 2005) supports the view that the global hydrological cycle is intensifying as the planet warms. As a consequence, the flash flood hazard is expected to increase in frequency and severity in many areas, through the impacts of global change on climate, storm-weather systems and river discharge conditions. This work aims to provide a characterisation of flash flood events in the Mediterranean basin over various time and spatial scales, in order to characterise these events in terms of flood generating rainfall, peak discharges, runoff coefficient and response time, and to use the insight gained with this analysis to identify implications for flash flood risk management. High-resolution data enabling identification and analysis of the hydrometeorological causative processes of flash floods have been collected and analysed for 20 extreme flash floods occurred in Mediterranean countries. Criteria for flood selection were high intensity of triggering rainfall and flood response and availability of high-resolution reliable data. Hydrometeorological data collected and collated for each event were checked by using a hydrological model. The derivation and analysis of summarising variables based on the data archive has made it possible to outline specific characteristics of flash floods in the Mediterranean basin. Peak discharge data for more than 75% of the studied watersheds derive from post-flood surveys in ungauged streams. This stresses both the significance of post-flood surveys in building and extending flash flood data bases, and the need to continue developing methods for flash-flood hazard assessment able to take into account data from post-event analysis. Examination of data shows a peculiar seasonality effect on flash flood occurrence, revealing different climatic forcing. Consistently with this seasonality effect, spatial extent and duration are also varying. The runoff coefficients of the studied flash floods are usually rather low (less than 0.5 in most of the cases). Antecedent saturation conditions have a significant impact on event runoff coefficients, showing the influence of initial soil moisture status even on extreme flash flood events and stressing the importance of accounting soil moisture for operational flash flood forecasting. These results challenge the common wisdom that antecedent soil moisture is of little importance in determining the magnitude of extreme flash floods. Hence, accounting for antecedent soil moisture conditions is paramount for operational flash flood forecasting. The runoff response displays short lag times (mostly < 6 hours). The analysis of the response time provides information on the time resolution and the spatial density of the networks required for monitoring the storms that generate flash floods.

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