Insight on slope unit delineation for landslides susceptibility analysis at large scale

The study presented below is part of a research project which focuses on geo-hydrogeological susceptibility analysis at regional scale. Specifically, this phase of the research activities is addressed to the identification of optimal mapping units for delimiting areas prone to landslides. In literature, the units adopted for this analysis are grid cell [3], unique condition unit (UCU) [4] and slope unit (SU) [2; 3; 4]. Since the goal is the identification of mapping units that account for real morphology of the territory and physiographic environment within which landslides could occur, it was thought that the optimal mapping unit for this study could be the SUs. The widely used tool for slope unit delineation, the ‘r.slopeunits’ plugin [1; 2], has been implemented. The software, requiring only a Digital Elevation Model (DEM) as input data, implements an iterative and adaptive process, depending on both hydrological and morphological parameters settings. These parameters consist of: initial flow accumulation threshold, which defined the largest drainage basin size to be split into two ‘’half – basins’’ (left and right sides of basin); a reduction factor that controls the progressive decrease of the initial threshold and thus the dimension, in term of area, of the next basins that will be split; the number of the iteration during which the partitioning of basins is performed; the circular variance of the aspect and the minimum area, which refine the delineation of the SUs. The different combination of all the aforementioned parameters results into different slope units sets, in term of number of units, size, and local morphology delimitation. For this reason, the analysis has been focused on the different combinations of parameter values and tested on the area of the upper part of the Tronto River basin, located in the north-eastern part of the Lazio region and with an area of 251 km2. Other exploratory analysis has been conducted to evaluate: the effect of scale of analysis, delineating the SUs at regional scale and at basin scale, and the role of DEM resolution: 10-meter, 5- meter and 1-meter. For the latter, the analysis was conducted on a restricted portion of the study area (1 km2) [5] due to the computational time required, which is too costly and not manageable by the machine to work on the whole basin. Preliminary result, indicate that the flow accumulation threshold reduction and the number of iterations play a fundamental role in SUs partition and size. The circular variance of the aspect controls a more refined delimitation of slope units. Instead, the minimum area would not seem to affect the process much. Regarding the scale of analysis, apparently, setting the same sets of parameters at regional scale and basins scale, leads to identical geometry of mapping units for the study area . Finally, the test conducted on the variation of DEM resolution, led obtaining very small and unsuitable set of SUs using the DEM of 1 meter resolution , because the morphological variations are detected at a resolution which is lower than that at which the landslide occurs. These findings suggest that, despite the challenge in identifying the best combination of parameters, the delineation of mapping units controlled by hydrological and morphological parameters allow to obtain a natural subdivision of the territory differentiated in term of spatial extension and geometry. However, it seems that exists a limit in size and geometry below which these units could not be defined as slope units as do not respect the definition of ‘’terrain unit delimited by drainage lines and divide lines ’’. References [1] Alvioli, M., Guzzetti, F., & Marchesini, I. (2020). Parameter-free delineation of slope units and terrain subdivision of Italy. Geomorphology, 358, 107124. [2] Alvioli, M., Marchesini, I., Reichenbach, P., Rossi, M., Ardizzone, F., Fiorucci, F., & Guzzetti, F. (2016). Automatic delineation of geomorphological slope units with r. slopeunits v1. 0 and their optimization for landslide susceptibility modeling. Geoscientific Model Development, 9(11), 3975-3991. [3] Ba, Q., Chen, Y., Deng, S., Yang, J., & Li, H. (2018). A comparison of slope units and grid cells as mapping units for landslide susceptibility assessment. Earth Science Informatics, 11, 373-388. [4] Guzzetti, F., Carrara, A., Cardinali, M., & Reichenbach, P. (1999). Landslide hazard evaluation: a review of current techniques and their application in a multi-scale study, Central Italy. Geomorphology, 31(1-4), 181-216. [5] Ministero dell'Ambiente e della Sicurezza Energetica (MASE). (2025). Dataset LIDAR [Dataset]. Distribuito con licenza CC BY 4.0. Recuperato da https://gn.mase.gov.it/portale/distribuzione-dati-pst.