Assessment of heavy rainfall triggered flash floods and landslides in the Sturla stream basin (Ligurian Apennines, northwestern Italy)

On 21 October 2013 a short-term heavy rainfall hit the Sturla Valley in the Ligurian Appenines (northwestern Italy). The Sturla Stream joins the Lavagna Stream in Carasco forming the Entella River. The Entella flows for eight kilometers towards the Ligurian sea between the towns of Chiavari and Lavagna in the Eastern Ligurian Riviera. The maximum elevation of the valley is Mount Aiona (1695 m asl in the natural reserve of Aveto Regional Natural Park). The basin shows short watercourses and steep slopes typical of the Ligurian mountain. During the event the rain gauge of Borzone (386 m asl) recorded more than 180 mm in 2 hours. The hydrological event was a flash flood in the lower stretch of the Sturla Stream which caused the collapse of a bridge in Carasco, with two fatalities, while many houses, infrastructures and business activities were damaged. In addition, the rainfall event triggered almost 100 shallow landslides. The objective of this study is to analyse the rainfall event and highlight the causal factors of floods and landslides risk and their recent increase. A rainfall and hydrometric analysis of the event was conducted. In addition, a climate analysis of the annual and maximum rainfall for 1, 3, 6, 12, 24 hours at the historical rain gauge of Giacopane Dam was carried out. The series were analysed with a Mann- Kendall test. There was a statistically significant increase in the rainfall rate and the presence of significant trends showing an increase in the hazard. A multitemporal comparison of different maps since the 19th century and the comparison of land use variations in the 20th and 21st century has shown the progressive narrowing of the Sturla riverbed at the confluence with the Lavagna Stream, the urbanization of the floodplain, the abandonment of the terraces and the development of a scarcely efficient woodland for the floods and landslides protection. The Curve Number analysis method shows the variations occurred in the last 80 years. The basin drainage conditions are becoming less efficient and the general hydrological conditions worsened. Furthermore, the building of secondary or minor roads triggered shallow landslides which affected thin debris covers; these were largely remodelled in the past to build terrace cultivations which are now almost totally abandoned. This paper demonstrates that both land use variations and climate change have contributed to increase the hazard while the urbanization increased the vulnerability of the area.