Overview on Index of Connectivity applications: towards a quantitative approach.

The growing interest of the Earth sciences community in sediment connectivity has led this property to become a key concept for modern geomorphology. At the same time, a great effort has been made to develop new methods to measure it. The increasing availability of high-resolution Digital Elevation Models (DEMs) for different spatial and temporal scales from various sources, such as LiDAR and photogrammetry, has laid the foundations for the development of approaches exploiting DEM derivatives as slope, flow accumulation, and surface roughness through geomorphometry for a (semi)quantitative assessment of sediment connectivity. Among the different methods, the index of sediment connectivity, which aims at depicting spatial connectivity patterns to support the assessment of the contribution of a given part of the catchment as a sediment source and define sediment transfer paths, was particularly successful thanks also to its intrinsic simplicity and the relative ease of computation through the dedicated open-source software application. The index saw many applications in different geographical and environmental contexts and for different study objectives, reflecting the increasing need for an objective characterization of the linkages between landscape units. Traditional applications of the index are based on qualitative and semi-quantitative analyses to investigate hillslope-channel coupling, sediment transfer dynamics, the impact of extreme natural events, and prioritization of sediment source inventories. This work aims at presenting and discussing the main applications of the sediment connectivity index. If the intrinsic limitations of the index are considered, it is demonstrated that the index can provide a spatial characterization of sediment dynamics, thus improving the understanding of geomorphic systems' behavior. A specific focus will be made on the most recent applications that are moving towards a more quantitative approach to identify areas in a catchment that are both susceptible to slope instability/mass movements and connected to a specific target. Although other fields of application can be envisaged, the applications so far implemented deal especially with debris flows, one of the most active processes in supplying sediment to the main river channels. We believe that this kind of approach paves the way for a new generation of susceptibility maps that, encompassing the sediment connectivity concept, can be adopted for a better hazard and risk assessment and, consequently, for improved countermeasures planning.