The Bisagno stream catchment (Genoa, Italy) and its major floods (1822, 1970 and 2014): geomorphic and land use variations in the last three centuries

The city of Genoa (Liguria, Italy) and the Bisagno Valley are affected by frequent floods, oftenwith loss of human lives. Historically characterised by high flood hazards, the Bisagno Valleywas recently affected by a flood event on 9 October 2014, less than three years after the tragic flood event of 4 November 2011 when six people died, including two children. In the last 50 years, four destructive floods occurred in the Bisagno Valley, in addition to some other events that caused significant damage and economic losses. This paper examines the three largest flood events in terms of intensity and ground effects which affected the Bisagno Valley in the last three centuries: the flood of 25 October 1822,well documented by contemporary sources, the flood of 8 October 1970, undoubtedly the most tragic on record, and the very recent event of 9 October 2014. For this purpose scientific and historical-geographicalmethodologies were adopted, the latter particularly useful for the reconstruction of the flood event of 1822 and the landscape history of the Bisagno Valley in the nineteenth century. This comparison shows that the Bisagno Valley is characterised by climatic and landform features that have been making the flood events historically common in the area. However, recent climate change and land-use variations, including some major modifications of the catchment basin, have progressively determined a decrease of the concentration time and an increase of runoff, solid transport, and flood hazard. Consequently, in recent decades a growth in the number of flood events occurred, to the extent that the Bisagno today is a famous case study on an international scale.