Active Landslide Portions Contribute to Surface Water Concentration: Insights from GIS Analysis and Field Data in the Northern Apennines

The distribution of small water bodies associated to landslides in a part of the Northern Apennines (Italy) has been explored, for the first time, using GIS analysis, field surveys and geophysical imaging. The analysis on the whole investigated area was performed using the Topographic Wetness Index (TWI), a proxy for surface soil moisture patterns based on topographic characteristics. The case studies correspond to two large landslides deep-rooted in the bedrock. The Sauna landslide in the Parma torrent basin and the Berceto landslide in the Taro river basin, have been investigated through field work, geophysics, boreholes and radiocarbon dating for the time constraints of the water bodies. The TWI analysis carried out both at the regional and the case-study scale has shown that low values of this index (drier areas) are more associated with inactive landslides portions, whereas higher values (wetter areas) are more associated with active portions. The analyses on the case studies highlighted that the condition characterized by wet soil and/or the presence of small water bodies are spatially persistent across time in correspondence of the same portion of the landslide that preserves landforms able to maintain these waters. As highlighted by geophysics, these landforms are in connection with deep shapes of the sliding/rupture surface of the landslide that mimic those at the surface.