Role of rainfall intensity and urban sprawl in the 2014 flash flood in Genoa City, Bisagno catchment (Liguria, Italy)

Flash floods are typical for the Mediterranean region, however they have been recurring at increasing frequency over the past few decades especially over the Italian Peninsula. The region of Genoa has recently moved into the international spotlight due to frequent and disastrous flash floods of the Bisagno River and its tributaries as a result of intense autumn rainfall and dense floodplain urbanization. The last flood events even caused loss in human lives, with 6 casualties in 2011 and 1 in 2014. However, it has not yet been investigated whether the increase in these flash flood events is due to a change in rainfall regime or due to urban expansion in areas prone to flash floods. This study investigates the meteorological conditions and the ground effects of the Bisagno River flash flood of October 9th, 2014 in the city of Genoa. Application of a Standard Anomaly Index and a Cluster Analysis reveals a positive trend in intense and short-duration rainfall recorded in the Bisagno Valley for the period of 1945-2016. Urban sprawl in the catchment, assessed through a multi-temporal cartographic and photographic comparison, highlights the progressive and total consumption of land in the floodplain. In the river basin surrounding Genoa the risk of flash floods has increased, both due to a change in rainfall regime and associated increase in discharge but also due to increased vulnerability caused by progressive urban expansion and land use change, including in particular transformations of the Bisagno riverbed. The results obtained demonstrate the need for urgent action to mitigate flood risk by introducing, amongst other measures, a citizen observatory for early warning of flash floods.