Italy

Flash floods and Urban flooding Italy

Vulnerabilities

Mediterranean coastal regions in particular are regularly affected by localized heavy precipitation events, resulting in very dangerous flash floods, often of limited predictability (10). Due to its position, exposed to southerly moist flows from the Mediterranean Sea, and the steep orography near the coasts, one of the most affected areas is Liguria region in northwestern Italy (11). Extreme precipitation is usually observed between late summer and mid autumn, when heat and moisture fluxes from the Mediterranean Sea are the highest, thus suggesting a fundamental role of sea surface temperature in the generation and evolution of heavy precipitation. This precipitation results from the convergence between two different flows: a warm, moist southeasterly low-level jet on the eastern side, channeled between Corsica and Central Italy and impinging over the Ligurian Appennines, and a northerly shallow cold offshore flow, moving from the Po Valley through the lowest orographic gaps and affecting the western part of Liguria (9).

Especially the Genoa Metropolitan Area is known for its high number of flash floods. Rainfall intensities sometimes reach high values in this area due to the location of Genua with respect to the Mediterranean and the Alps. On 7/8 October 1970, for instance, 948 mm fell in 24 hours north of the city centre, one of the highest rainfall events ever recorded in Europe and the Mediterranean region. An increase in the number of flash floods has been observed in the Genoa Metropolitan Area. This is related to the urbanization (13) and the effects of climate change (most notably changes to the rainfall regime) (12). 

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.

A dramatic example of a flash flood is the event that killed 147 people in Sarno, southern Italy, in May 1998. That flood was generated by a sudden extreme rainfall event and consequent mudflows (8). In 2018, 10 people were killed by a flash flood in southern Italy (14).

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 21^st 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).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Italy.

1. EEA (2004b), in: WHO (2007)
2. Ludwig et al. (2003), in: WHO (2007)
3. De Roo et al. (2003), in: WHO (2007)
4. Robinson et al. (2003), in: WHO (2007)
5. Copolla and Giorgi (2010), in: MET Office (2011)
6. Goubanova and Li (2007), in: MET Office (2011)
7. Copolla and Giorgi (2010), in: MET Office (2011)
8. Barredo (2007)
9. Cassola et al. (2016)
10. Ricard et al. (2012), in: Cassola et al. (2016)
11. Silvestro et al. (2012); Rebora et al. (2013); Buzzi et al. (2014); Cassola et al. (2015), all in: Cassola et al. (2016)
12. Acquaotta et al. (2018)
13. Faccini et al. (2016), in: Acquaotta et al. (2018)
14. Avolio et al. (2019)