Slope stability time evolution of a shallow landslide restored by Soil and Water Bioengineering (SWBE) techniques: A case study in Northwest Tuscany (Italy)

The well-known extreme flood event occurred on 19 June 1996 in Versilia, north-western Tuscany, Italy, causing an impressive number of landslides and debris flows, dramatic floods and 15 fatalities. The Tuscany Region financed Soil and Water Bioengineering (SWBE) interventions to restore slope stability and mitigate soil erosion. The Pomezzana landslide was the greatest one restored by such kind of Nature-Based Solutions (NBS) in 1998, applied in Tuscany for the first time. The study presents the 2023 landslide monitoring surveys, concerning the vegetation evolution state and the slope stability conditions of two different situations: AdS1-SWBE restored landslide and AdS2-unrestored landslide. The root systems of the main tree species present (chestnut, alder, and hornbeam) have been characterised and used to evaluate the effects of vegetation on slope stability according to the historical monitoring 1998–2013-2023. The Root Area Ratio (RAR) values obtained in the field, and therefore soil cohesion values also considering the deterioration of the wooden crib wall, drove us to calculate the Factor of Safety (FS) through SSAP 2010™ software. Various scenarios of slope stability were evaluated with the infiltration depth obtained with the landslide back analysis with the event rainfall obtaining the time evolution of the slope stability. The study confirms the technical effectiveness of the slope stabilisation of the SWBE interventions, as in the restoration of superficial landslides triggered by rainfall the research also shows the positive effects of vegetation evolution over slope stability and ecological succession, triggering natural native species succession with increasing soil reinforcement. The results show the SWBE restoration project hastened the slope stabilisation process through the combination of vegetation development and SWBE techniques. Slope stability temporal evolution in a shallow landslide restored by Soil and Water Bioengineering (SWBE) techniques: a study case in Northwest Tuscany (Italy). • Promote Soil and Water Bio-Engineering (SWBE) such as Nature-Based Solutions (NBS) for natural hazard mitigation. • Long-term monitoring evaluating technical effectiveness of a SWBE intervention realised for a shallow landslide restoration. • FS increased over time confirming how vegetation evolution contributes positively to soil cohesion and slope stability. • SWBE restoration accelerates ecological succession and slope stability, compared to naturally evolved site. • Native species succession, triggered by SWBE techniques, is evolving toward a stable form of vegetation.