Optimising the identification of river sediment supply areas and their connectivity in the mountainous river basins of central Italy

A B S T R A C TDue to climate change, the Mediterranean countries are increasingly affected by weather-related extreme events, particularly by damaging flash floods. In light of the increasing frequency and intensity of rainfall events, a comprehensive understanding of the processes that are activated and contribute to the delivery of material into catchment areas is of critical importance. Given the strong connectivity between upstream and downstream regions, the transport of coarse sediment through the river network can mobilise substantial volumes of material towards lower reaches. This often includes inhabited areas, where such sediment influx can exacerbate existing vulnerabilities by causing obstructions in bridges and underpasses, clogging drainage systems and manholes, and consequently leading to localised flooding, in addition to posing significant direct destructive hazards. In this paper, we structured a GIS-based method that integrates connectivity, stream power and stream length gradient indices with a morphological approach to assess the spatial distribution of sediment source and sink areas, also in view of a relatively rapid emergency planning. This approach is both time- and cost-efficient, offering reliable insights into sediment transfer along slopes and river channels, coupling them into a unique final product, while maintaining a simplified operational structure. Its applicability under extreme rainfall conditions was demonstrated through a case study in a 92 km2 catchment in central Italy, which experienced extensive landslides, hillslope erosion, and severe flooding following the extreme rainfall event of September 15, 2022 (a peak of 419 mm was recorded in 9 hours at the centre of the considered area). The testing operation was conducted with field survey and remote sensing analyses, since a detailed post-event mapping of landslides and a lot of geographical information on the disruption were available.