GIS-Based analysis of sediment connectivity in small mountain catchments of Metauro River (northern Apennines, Italy)

Climate variations and escalating anthropogenic pressures stand out as two major factors driving the heightened occurrence and severity of highly destructive events. The increase in the intensity of rainfall phenomena leads to an imbalance in river systems, which are progressively prone to a rise in flood events, consequently elevating the likelihood of inundation, especially in small mountain catchments  characterised generally by high levels of (geological and hydrological) heterogeneity. This is the case in the Marche Region (central Italy), where in the last few years, high precipitation amounts were recorded in a very short time, leading to critical situations in the involved river basins. The most damaging occurred in September 2022 with flood events in the Candigliano, Cesano, Misa, and Sentino basins. The most significant damage due to sediment transport occurred in the municipality of Cantiano due to the flooding of the Burano River, the Candigliano River’s main tributary. Extraordinary rainfall was recorded close to the anticline of Mount Catria; the amount was about 30% of the annual precipitation, approximately 419 mm of rain. This event resulted in about 13 fatalities, 50 injuries and critical damages to infrastructure. A large amount of even coarse sediments with calcareous compositions have been mobilized causing the occlusion of the minor channels that have caused the leakage of water and accumulation in depression areas. Hence, the importance of studying these systems through a more detailed geomorphological approach trying to understand the evolutionary processes of the landscape. The proposed methodology consists of assessing the connection of sediments fusing the Connectivity Index (IC) toolbox defined by Cavalli et al. (2013) and its interpretation in light of the existing morphological context. The connection index represents the probability that particles near the river will end up there. By characterizing IC, it is possible to estimate the input of material into the riverbed, define the source areas of material, and study the transfer paths. In the mountain systems, the morphology is complex and constantly evolving; the spatial connectivity of the sediment is translated through the transfer of material on the slopes, in the valleys, and along the network of channels (e.g., catchment outlet, main channel network, a given cross-section along the channel). Using the toolbox, the geomorphological indices called weighted factors, i.e., rasters that are based on surface characteristics influencing the processes of runoff and inflow of sediment within a basin or on a slope (e.g., roughness, slope, lithology, curvature, land use, Italian Landslide Inventory (IFFI), etc.) were analysed. An integrated approach allowed to evaluate the connectivity of the sediments and to highlight the source areas, which is extremely useful for defining the general availability of sediments, the potential for achievement, and management priorities in ordinary conditions and emergency situations.