Characterization of extreme flash floods in Mediterranean Spain

Abstract

[eng] Flash floods are the most destructive natural hazard in the Spanish Mediterranean region, causing substantial fatalities and economic losses. However, characterizing these events has been challenging due to the lack of a comprehensive database. This study aims to fill this gap by collating and analyzing data using established methodologies. The analysis is divided into two parts. The first part examines extreme daily rainfall regimes in the Mediterranean and semi-arid climatic zones during the extended warm season. It then analyses primary data from 99 flash floods to identify spatial and temporal distributions and derive envelope curves. The spatial and temporal patterns of severe flash flooding closely mirror those of extreme daily precipitation amount. The envelopes curves are consistent with those from other regions. In semi-arid basins, flash floods exhibit higher magnitudes than those in Mediterranean watersheds, but the envelope curve declines more steeply with increasing drainage, reflecting differences in climatic and physiographic factors. The second part focuses on 13 major flash floods for which high-resolution hydrometeorological data are available. These events are characterized based on climate, morphology, precipitation, runoff ratio, lag time, and flashiness. The results align with previous findings regarding relief ratio, rainfall characteristics, and flashiness. However, event runoff coefficients in Mediterranean Spain are lower than in other European regions due to initially large soil storage capacities, which extend lag times in smaller drainage areas. At larger scales, flow hydraulics result in lag times that fall below the lower bound of the European envelope curve. These datasets and findings contribute to the development of a more comprehensive European flash flood database and offer valuable insights for improving flash flood risk management in Mediterranean Spain.