Flash floods and Urban flooding Spain
The defintion of a flash flood
Although there is no single definition, a flash flood is usually defined as a sudden flood in a small catchment area (usually less than 1000 km2), occurring within 6 hours or less of the causative event (heavy rain, dam break, levee failure, rapid snowmelt or glacier-outburst flood) and often within 2 hours of the start of high intensity rainfall. Flash floods are usually caused by heavy rainfall that can either be local, affecting only one or two catchments, or more extended, producing flash floods as part of the framework of a major flood event (16).
Flash floods caused on average 50 casualties per year in Europe, 70% of the total number of deaths due to floods (8). The fatalities mainly occur in countries surrounding the Mediterranean Sea, where a large population density exists at the coastal areas due to the important urbanization processes in this area during the last decades (14). Particularly in Spain, almost 90% of the victims due to floods are caused by flash floods (15).
The Mediterranean region is prone to flash floods, especially in the northwest (17), where littoral and pre-littoral mountain chains favour not only torrential rain concentrated in small catchments, but also heavy rainfall. The short concentration times and the extraordinary runoffs that develop can turn into catastrophic flash floods like those that occurred on 25 September 1962 in north-eastern Spain, when 815 people died in less than 5 hours (18,21). In the Mediterranean region, summer events are usually local and of short duration, while in the autumn, the warmer sea surface temperature, as well as the large number of cyclones and organised perturbations, can give rise to extended catastrophic flash flood events (17). Rainfall during the summer-autumn period is the most important cause of flash floods in the Mediterranean region (16,19). In Catalonia, for instance, a region in the northeast of Spain that is particularly vulnerable to flash floods, flash floods are concentrated between August and October, which is consistent with the distribution of convective precipitation (17).
The island of Madeira was struck by a flash flood on 20 February 2010, with 185 mm of rain falling in one day, leading to 47 deaths and widespread damage in the capital Funchal (13).
A total of 219 floods were recorded across the region of Catalonia in the period 1981 – 2010, 11% of them have led to serious damage or the destruction of hydraulic installations, infrastructure, paths and roadways, buildings, livestock, crops produced catastrophic damage, and so on (20). A dramatic example of a more recent flash flood is the Biescas disaster in Spain in 1996, where 160 mm of rain fell in 1 hour producing a flash flood that killed 87 people in a campsite (8).
Not all flash floods are equally destructive. A distinction has been made between 3 types of floods (16):
- ‘‘Ordinary or small floods” do not cause rivers to overflow their banks, cause some damage if activities are being carried out in or near the river at the time, and cause minor damage to hydraulic installations.
- ‘‘Extraordinary or intermediate floods” are those that cause the rivers to overflow their banks, inconveniences to the daily life of the local population, and damage to structures near the river or stream, with possible partial destruction.
- ‘‘Catastrophic or large floods” cause the rivers to overflow their banks and lead to serious damage or the destruction of hydraulic installations, infrastructure, paths and roadways, buildings, livestock, crops, and so on.
In Catalonia the number of extraordinary flash flood events shows an increasing trend for the 1900 - 2011 period. This was related to an increase of heavy, more concentrated rainfall events. Besides, the increase of exposition and vulnerability to flash floods in this area plays a role, as the population density in flash flood prone areas has increased considerably. This problem is aggravated in the summer, when some villages near the sea can go from a population of less than 1000 inhabitants to over 100,000 inhabitants. The latter affects the character of these flash floods, turning them from ‘‘ordinary or small floods” into ‘‘extraordinary or intermediate floods” (there is no trend in “catastrophic floods” in Catalonia). These results are coherent with the hypothesis formulated by the IPCC (2012, 2014) about the increase in torrential events and the role played by vulnerability and exposure changes (16).
An increase in intensive short-term precipitation in most of Europe is likely to lead to an increased risk of flash floods (1), particularly in the Mediterranean and eastern Europe (2). The flood risk from climate change could be magnified by an increasing impermeable surface due to urbanization (3) and modified by changes in vegetation cover (4) in small catchments.
The destruction of the vegetation cover and the alteration of the soil by fire have crucial consequences on the hydrological properties of the subsoil. Mediterranean regions are subject to violent flash floods that could be intensified in the future due to forest fire and/or climate change. These factors should be taken into account in future flood warning systems and flood policy. Research on the Llobregat River near Barcelona has shown that in the 21st century peak flood discharge may increase up to 40%. As a consequence, the frequency of flash floods is projected to change (9).
Results in the scientific literature are not univocal, however. One study reports a projected decrease of heavy winter and summer precipitation in the south of Europe in the 21st century with climate change (B2 and A2 emissions scenario) (5). Another projects an increase in precipitation extremes and variability over the Mediterranean region in winter, spring and autumn seasons, despite an overall decrease in mean precipitation (A2 emissions scenario) (6). Yet another study reports a projected decrease in summer precipitation and a dipolar change pattern in winter (increase to the north, decrease to the south), and an increase in both very dry and very wet seasons (A2, A1B, B2 and B1 scenarios) (7).
The Llobregat River basin in the eastern part of Catalonia (northeast of Spain) has all the specific characteristics to develop severe flash floods: Mediterranean climate, small watersheds with steep slopes and an orography that creates intense convective precipitation events (11). Besides, the consequences of flash floods in this area can be catastrophic due to the human developments that have been built around the rivers. There was an event in 1962, for example, with over 400 deaths (12). Future flash flood magnitude and frequency in two sub basins of the Llobregat River basin have been assessed for future projections over the period 2001–2100 compared with 1971–2000 (10). This has been done for the IPCC SRES scenarios A2 (severe) and B1 (moderate). The results of the analyses show different trends in the two sub basins: an increase of the occurrence of flash floods events in one of the sub basins, a decrease in the other.
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Spain.
- EEA (2004b), in: WHO (2007)
- Ludwig et al. (2003), in: WHO (2007)
- De Roo et al. (2003), in: WHO (2007)
- Robinson et al. (2003), in: WHO (2007)
- Copolla and Giorgi (2010), in: MET Office (2011)
- Goubanova and Li (2007), in: MET Office (2011)
- Copolla and Giorgi (2010), in: MET Office (2011)
- Barredo (2007)
- Versini et al. (2012)
- Velasco et al. (2013)
- Llasat et al. (2003, 2005), in: Velasco et al. (2013)
- López Bustos (1964), in: Velasco et al. (2013)
- Norwegian Meteorological Institute (2013)
- Llasat et al. (2010), in: Pino et al. (2016)
- Olcina and Ayala-Carcedo (2002), in: Pino et al. (2016)
- Llasat et al. (2016)
- Jansà et al. (2014), in: Llasat et al. (2016)
- Aulet et al. (2012); Gaume et al. (2009), both in: Llasat et al. (2016)
- Papagiannaki et al. (2013), in: Llasat et al. (2016)
- Llasat et al. (2014), in: Llasat et al. (2016)
- Martín-Vide and Llasat (2018)