Vegetation height assessment through high resolution ALS data analysis: the role of monitoring in the H2020 RECONECT project - NBS measure for geo-hydrological risk reduction in the Portofino Park (Italy).

Man-made terraces and their role in slope stability issues are gaining more and more attention because of their involvement in mass wasting. Either the widely diffusion of terraces in hilly-mountainous regions or the increase of high intensity rains that characterize many areas, particularly in the Mediterranean climate change hot spot, contribute to this growing interest. Terraces, which have been largely used for the subsistence agriculture in the past occupying in many cases event more than 50% of a small catchment surface, are now mostly in abandon. Then, their slope stabilization and erosion reduction role have turned into a possible source of shallow landslides and mud-flows. Terraces abandonment has led to the progressive change in land cover, passing from cultivated crop to wild vegetation. In case of shallow landslide triggering along an abandoned terraced slope the wild vegetation may be involved. In Northern Italy Liguria region many events happened in the last 20 years: in 2011 in the Cinque Terre and Vara Valley areas, in 2014 in the Genoa Metropolitan area, in 2019 and 2021 in the western part of the region, always causing high damage and several casualties. In all the circumstance the landslides mass included vegetation, which often comprises trees that increase the damaging potential of the mass movement itself. In fact, considering the hilly-mountainous morphology of the region, the lower altitude areas along the sea are highly urbanized or in general intensively anthropogenically modified: soil sealing in the small flat areas and culverts along the streams. Then, the floating transportation that is generated in case of shallow landslides triggered along terraced slopes may contribute substantially in reducing the culverts transport capacity, often generating a dam like effect that increases the flooding in flat areas. In this framework, the vegetation height assessment, in particular along the abandoned terraced slopes, may contribute to the hazard evaluation in areas prone to rapid landslides triggering. The high-resolution ALS (Airborne Laser Scanner) data analysis may be successfully used to quickly and precisely estimate the vegetation height in large areas, using the DSM and the DTM. Then, this analysis may be related to shallow landslides susceptibility assessment and to the spatial analysis of the possible exposed elements: culverts, infrastructure and buildings. The research has been carried out within the framework of the Horizon 2020 RECONECT - Regenerating ECOsystems with Nature-Based Solutions for hydro-meteorological risk rEduCTion; the Italian RECONECT demonstrator is set in the Portofino Park. The monitoring activities conducted in the project include a high-resolution ALS survey that allowed to obtain a precise DSM and DTM. The results of this research contribute to improve the assessment of the risk associated to terraces collapse in case of intense rain event, both in terms of the potential source areas and in terms of possible impacted elements.