Increased flash flooding and climate change in the Genoa Metropolitan Area (Italy)

The Genoa Metropolitan Area (GMA) is historically affected by flash floods for two main reasons: the meteorological conditions due to the Genoa Low and the city’s geomorphologic arrangement: a narrow coast belt bounded by mountains. In the last years in GMA there is also an increase in flash floods due to man-made landforms and changes in rainfall regime. In 2014 several floods took place: in some areas of the hinterland the cumulate year rainfall has exceeded 4000 mm, while 2000 mm fell on the coast. The research analyses the thermo-pluviometric statistics collected over more than 100 years, registered by Genoa University (since 1833) and Chiavari (since 1877) stations, which represent the central and the eastern zone of the GMA. An analysis was based on mean annual air temperatures, rainfall, rainy days and rainfall rate, which means an annual rainfall and rainy days ratio. The air temperatures show a positive trend, statistically significant, rising from an average of 15 °C in the 19th Century to a current average of 16° C. The annual rainfall does not show any trend, remaining at an average of 1270 mm for Genoa and 1180 mm for Chiavari [5]. The number of rainy days shows a negative trend, statistically significant, from more than 110 days in the 19th Century in Genoa (90 in Chiavari) to 90 nowadays (80 days in Chiavari). As a consequence, the rate of daily rainfall shows a positive trend: we can say that rainfall is more intense. The scenario above is confirmed by the analysis of maximum precipitation recorded by the two pluviographs for 1,3,6,12 and 24 hours in 1945-2014 period; although there is no statistical relevance, all the analyzed values indicate an increasement over time, showing the intensification of flood events in recent decades.