RECONSTRUCTING THE SURFACE MOVEMENTS OF THE CAROBBIO LANDSLIDE AT TWO SITES ALONG THE PARMA TORRENT AT THE MULTI-DECADAL SCALE

Purpose: Slope failure along the Parma Torrent are recurrent landslides that result from the interaction of river dynamics, mainly removing material from the slope toes, and gravitational slope processes. To better understand the events occurring along the torrent and their frequencies through time, a time perspective is mandatory, and multi-decadal reconstruction of landslide surface movements can be performed on forested slopes, by means of tree-ring techniques. Methods: The integrated use of dendrogeomorphological and GIS techniques has led to successful reconstructions of landslide surface movements at the multi-decadal scale in the Italian Alps (Gattinoni et al., 2019; Leonelli et al., 2020). The same approach based on the analysis of the tree-ring eccentricity, was applied also at the Carobbio landslide, a complex (roto-translational) slide involving marlylimestone flysch formation, at two sites along the Parma Torrent, right river bank. At one site a landslide event occurred, likely in 2010- 2011, causing the displacement of 17-20 m of an area of approximately 36’000 m2; a second site was selected south of to the first one, on a slope not yet interested by landslide events in the recent years. Results: At the northern site, we could date slope movements up to the 1960s, however major signals where recorded since 1971 and 1972 in the higher portion of the slope (with eccentricity values at approximately 20%), whereas in the lower portion of the slope the major signal were recorded only since 1997 (with eccentricity values between 20 and 25%). At the southern site although no events occurred in the recent past, most of trees present decreasing eccentricity values (up to 20% in recent decades) indicating an ongoing recovery since higher disturbances recorded in the 1970s. Here, however, a group of trees shows an increasing trend of eccentricity, with values as high as 30% already in the 1970s and up to 40-50% in recent decades. Conclusions: The integrated use of dendrogeomorphological techniques on forested slopes along the Parma Torrent together with GIS techniques has allowed the reconstruction of landslide surface movements at the multi-decadal scale at two sites. As we found, disturbance signals have been recorded by trees on the slope already since 40 years (upper portion) and 15 years (lower portion) before the major event observed at the northern site. The slope at the southern site is mostly in equilibrium conditions, although it presents an area in the SE portion that is recording strong signals of surface movements, indicating an ongoing gravitational stress. References 1. Gattinoni, P., Consonni, M., Francani, V., Leonelli, G., Lorenzo, C., 2019. Tunnelling in landslide areas connected to deep seated gravitational deformations: An example in Central Alps (northern Italy). Tunnelling and Underground Space Technology 93, 103100. https://doi.org/10.1016/j.tust.2019.103100 2. Leonelli, G., Chelli, A., Consonni, M., Lorenzo, C., Gattinoni, P., 2020. Multi-decadal dating of surface slope movements in forested DSGSD areas of the European Alps: detecting precipitation triggering factors. Geografiska Annaler: Series A, Physical Geography 1–25. https://doi.org/10.1080/04353676.20 20.1813983