Rainfall intensity in the Genoa Metropolitan Area (Northern Mediterranean): secular variations and consequences

According to historical papers, the Genoa Metropolitan Area (GMA) has been affected by floods since the Middle Ages. An increase in flash floods has been recorded over the last few years, and in 2014 at least four floods were observed, three of which occurred during autumn. Following these observations, the secular pluviometric variations for the region have been analysed. Pluviometric data recorded at the Genoa University station (since 1833) and Chiavari station (since 1877), which represent the central and the eastern zones of the GMA along the coast, have been analysed. Weather data recorded at the Isoverde station (since 1921) and Giacopiane Dam station (since 1925) have allowed the analysis to be completed in the western valley and eastern mountain regions of the GMA hinterland, respectively. This analysis was based on the mean annual, seasonal and monthly rainfall amounts, the number of rainy days and the rainfall rate (the ratio between the former and the latter) recorded at the four stations. Furthermore, the annual maximum hourly rainfall data have been analysed for the Pontecarrega station (at the Bisagno stream, the most affected valley in Genoa) and the Giacopiane Dam station. No trend was found for annual rainfall, though significant decreases were observed for the numbers of rainy days. As a result, an increase in the rainfall rate for the coastal and mountain stations was found. Moreover, an increase has also been found in the 3- and 6-hourly rainfall values. These findings could be related to the intensification of flash flood events that has been observed over the last decade. Interventions to reduce flood risks in the GMA should therefore no longer be postponed.