Episodes of extreme intensity of precipitation in Barcelona. Flash Flood Risk

Abstract

One of the main features of urban areas is the prevalence of impervious surfaces (sealed soils). Depending on its density and extent, soil sealing restricts natural soil functions, since it reduces water infiltration capacity and run-off regulation, thus increasing flash flooding risk. In the case of heavy rainfalls in urban areas with high imperviousness ratio, the sewage system could no longer be able to cope with the excess of rainwater and then pluvial floods develop. In many European zones, urban spread and densification have increased the impervious surface ratio (European Environment Agency, 2016). In Barcelona conurbation, most areas have high fractions of sealed soils (60–80%) and in the city, this fraction exceeds 80% (source: European Environment Agency, GMES/Copernicus). Heavy rainfall episodes are relatively frequent in this area, especially in autumn, and some of them have resulted in flash floods. In this study, rainfall intensity records from an urban network of 23 stations are analysed. The database consists of series of rain amounts collected at 5-minute intervals, which provides a measure of rainfall intensity (mm/5 minutes). The recording period encompasses 16 years (1994-2009). For all the records with durations greater than 60 minutes and total amounts greater than 25 mm (67 episodes, 499 records), the total rain amount, average intensity and maximum intensity are determined. We have selected the records exceeding, simultaneously, the 95th percentile of these three variables, thus obtaining 29 extreme records, which could lead to pluvial flash flood events. These extreme records correspond to seven different episodes, occurred from the end of July to October. Most of these extreme episodes are generated by convective processes at local scale as well as by eastern advections. The seven extreme episodes caused important affectations in road communications, breakdowns at traffic lights and telephone lines, as well as interruption of the service of some suburban lines, the airport and the railway network. Because of climate change and global warming, the frequency of episodes of very heavy precipitation is likely to increase. The development of policies aimed at limiting or reducing the proportion of sealed soils in urban areas would be a key element to mitigate, as far as possible, this risk of flash flooding.