The 25 October 2011 rainstorm in Liguria and Tuscany (Italy): peculiarities and consequences on slope stability

On 25 October 2011 heavy rainstorms caused hundreds of landslides, debris torrents and floods in Eastern Liguria and North-western Tuscany. These phenomena produced severe damage to villages, infrastructures and road network and 12 fatalities in the Magra and Vara river valleys and in the Cinque Terre area (Massa-Carrara and La Spezia provinces). The Magra River reached very high values of water level (up to 9.40 m at the Calamazza-Aulla hydrometer - Centro Funzionale Regione Toscana, 2011), breaking embankments and causing floods at Aulla and Fornola. On 25 October 2011 the Brugnato (Vara R. Valley), Monterosso and Levanto (Cinque Terre area) rain gauges recorded 511.0, 365.4 and 310.8 mm in 12 hours (from 3.00 to 15.00), respectively. The highest hourly rainfall intensity was recorded from 13.00 to 14.00 at Brugnato (143.4 mm/h), from 9.00 to 10.00 at Monterosso (82.6 mm/h) and from 10.00 to 11.00 at Levanto - San Gottardo rain gauge (100.6 mm/h). The highest cumulative rainfall was also extremely high. For example, the Brugnato rain gauge recorded a cumulative rainfall of 303.4 mm (from 12.00 to 15.00) and 468.8 mm (from 9.00 to 15.00). A similar condition occurred in Lunigiana, where the S. Giustina-Pontremoli, Pontremoli, Parana (Mulazzo) and Patigno (Zeri) rain gauges recorded 323.0, 319.8, 265.6 and 240.2 mm in 12 hours (from 6.00 to 18.00), respectively. The highest hourly rainfall intensity was recorded at the S. Giustina rain gauge (72.8 mm/h from 16.00 to 17.00). The 25 October rainstorm triggered hundreds of landslides. Most of them were first-time shallow landslides such as debris slide-debris flows. These phenomena were usually superficial (0.3-3 m thick), linear (width/length ratio 0.03-0.5). They mostly involved soil and sometimes portions of fractured and semi-permeable or impermeable bedrock. They mainly started in hollows at the top of the zero-order basins. There, the concave morphology favours runoff concentration and infiltration, while the concave soil-bedrock interface induces concentration of subsurface flow, saturation and build-up of pore pressures. The mass movements especially affected the road network. Many roads were blocked, isolating villages for days. Some landslides involved houses, causing serious damage and three victims at Cassana (Borghetto di Vara). Tens of people had to leave various villages of the La Spezia and Massa-Carrara provinces