Medium-term geomorphological changes along rivers in urban areas: the lower Bisagno and Polcevera Valleys in Genova city (Italy)

Numerous urban centres developed along rivers because of the resources that they provide. Consequently, many of the world’s rivers are impacted by urbanization. Thus, complex and two-way interactions have established between rivers and cities, resulting in potential benefits or damage in both directions. Focusing on fluvial geomorphological processes, the consistent presence of vulnerable elements close to the riverbed generally results in severe geo-hydrological risk conditions.

Based on multi-temporal comparison of historical maps and present-day aerial images, geomorphological survey, boreholes data analysis, high-resolution topographic data investigation, and archival document collection and review, this contribute aims to illustrate in detail the geomorphological changes associated with urban sprawl that has affected the lower Bisagno and Polcevera rivers since the first half of the 19th century.

From the 10th century to 1815 the urban development of Genova was limited to the hills surrounding the so-called Porto Antico Bay, which correspond to the present-day historical centre. From the 19th century onwards, Genova has developed over the Bisagno and Polcevera valleys, Est and West of the Porto Antico Bay, respectively. The cultivated narrow and elongated alluvial-coastal plains close to the seaside were progressively turned into industrial and residential areas. Meanwhile, the Polcevera and Bisagno lower reaches experienced relevant narrowing, simplification of landforms and complete channelization. The downstream-most section of the Bisagno River was culverted in the 1930s; at the mouth of the Polcevera, a progradation of the coastline to the sea occurred, completely due to the filling of the seaside to realize flat surfaces for industrial activities. Nowadays, the surface behind bank protections overall corresponds to made ground or landscaped ground and no pristine landforms are recognizable. Minor tributaries were squeezed, diverted, channelized, and culverted. In conclusion, the anthropogenic interventions completely reshaped the early-nineteenth-century landscape leading to the current geomorphological setting.

This research is framed in a larger project concerning (i) the quantitative analysis of urban river morphological dynamics at medium- and short-term temporal scales and (ii) the geomorphological evolution of the waterfront of Genova, which aims to support proper management of urban areas and geo-hydrological risk mitigation within a city of historical and cultural value that is very prone to flash floods.