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Nell'ambito del progetto Interreg SedInOut è stata effettuata una campagna di rilievi nei mesi di maggio e giugno 2021 nel bacino del torrente Liera (Canale d'Agordo -BL), al fine di: (i) valutare in campo le mappature delle aree sorgenti di sedimento pre- e post-evento ottenute dall'interpretazione di ortofoto ad alta risoluzione e le risultanze delle analisi del confronto tra Modelli Digitali del Terreno (DTM) derivati da dati LiDAR aerei (risoluzione di 1 m) acquisiti nel 2015 e nel 2019; (ii) raccogliere dati mediante compilazione di schede tecnico-descrittive che permettono di caratterizzare in dettaglio le aree sorgenti in situ; (iii) valutare la dinamica di sedimento nell'area di studio considerando trasporto, disponibilità, connessione-disconnessione del sedimento. In questo report vengono riportati i principali risultati.

Sediment connectivity can be regarded as a descriptor of the internal linkages between different landscape components within a catchment. The recent focus of the scientific community on connectivity related topics, both concerning hydrological and sediment connectivity, stresses the importance of understanding the main active pathways for a better estimation of energy and matter transfer at catchment scale. This task can be addressed using topography-based indices that analyse the linkages between landscape units. This approach to characterize connectivity is known as structural connectivity. The main limitation of structural connectivity is that it does not account for the processes driving sediment and energy fluxes (i.e., functional connectivity). In this work the integration between structural and functional approaches is proposed for characterizing sediment connectivity in mountain catchments. The structural approach, based on a topography-based sediment connectivity index, was used for assessing hillslope-to-channel connectivity. Since field data on processes driving sediment transport along the channel network are available, a functional approach has been devised to estimate withinchannel connectivity. An index of unit stream power computed from the hydraulic properties of the channel (i.e., discharge, slope and channel width) has been compared with the critical unit stream power computed from incipient motion thresholds derived from field data to identify the cells of the Digital Terrain Model (DTM) in which sediment can be mobilized under near-bankfull conditions. The index expressing the within-channel connectivity is given by the length of the reaches consisting of contiguous cells that exceed the critical unit stream power. During high-magnitude floods, when unit stream power values exceed the threshold for incipient motion, channels experience an increase in both hydrological and sediment connectivity. The proposed index characterizes those sections of the channel network that are effectively connected under near-bankfull conditions. The model has been developed and tested in a small mountain catchment in the Italian Alps (Strimm catchment, 8.42 km2 ) in which a high-resolution DTM and monitored discharge and sediment transport data are available. The integration between structural and functional indices of sediment connectivity has permitted characterizing the spatial pattern of sediment connectivity on the hillslope and pointing out potential sites for sediment erosion, entrainment and deposition along the main channel network. This combined approach represents a new development of the index of connectivity considering sediment transport process along the channel network, with the only limitation related to field data availability. However, further testing is required to validate the index in order apply it to other catchments.

The degree of linkage between the sediments sources and downstream areas (i.e. sediment connectivity) is one of the most important properties controlling landscape evolution. Many factors have been found to affect sediment connectivity, especially at the catchment scale. In particular, the degree of linkage between different areas within a catchment depends largely on the morphological complexity of the catchment (relief, terrain roughness, stream network density and catchment shape) and the combined effects of vegetation, such as land use changes and land abandonment. Moreover, the analysis of the spatial distribution of sediment connectivity and its temporal evolution can be also useful for the characterization of sediment source areas. Specifically, these areas represent sites of instability and their connectivity influences the probability that a local on-site effect could propagate within a multiple-events feedback system. Within this framework, the aim of this study is to apply a geomorphometric approach to analyze the linkage between landscape complexity and the sediment connectivity at the catchment scale. Moreover, to assess sediment delivery, the index of connectivity (IC) proposed by Cavalli et al. (2013) was used to evaluate the potential connection of sediment source areas with the main channel network. To better understand the relationship between morphological complexity of the catchment's landscape and the sediment spatial distribution and mobilization, two catchments with different size and geomorphological and land use characteristics were analysed: the Rio Frate and Versa catchments (Oltrepo Pavese, Southern Lombardy, Italy). Several shallow landslides, which represents the main sediment source area type in the catchments, were triggered especially in the period from 2009 to 2013. Moreover, relevant modification of land use and drainage system during last decades, especially related to land abandonment, have conditioned the sediment connectivity of these two catchments. To analyse these different aspects, IC was applied by using different stream networks densities. This allowed to investigate the sediment connectivity changes according to different landscape settings. Moreover, a multi-temporal, land-use change analysis was carried out to evaluate its role on the sediment dynamics. Finally, a sediment delivery assessment was performed. Shallow landslides inventories, with the identification of extent, location and typology of landslides phenomena, were used to better characterize the sediment supply to the channel network. This analysis represents an important step toward the realization of a useful tool for the improvement of sediment management strategies.

Climate changes are predicted to increase precipitation intensities and occurrence of extreme rainfall events in the near future. Scandinavia has been identified as one of the most sensitive regions in Europe to such changes; therefore, an increase in the risk for flooding, landslides and soil erosion is to be expected also in Sweden. An increase in the occurrence of extreme weather events will impose greater strain on the built environment and major transport infrastructures such as roads and railways. This research aimed to identify the risk of flooding at the road-stream intersections, crucial locations where water and debris can accumulate and cause failures of the existing drainage facilities. Two regions in southwest of Sweden affected by an extreme rainfall event in August 2014, were used for calibrating and testing a statistical flood prediction model. A set of Physical Catchment Descriptors (PCDs) including road and catchment characteristics was identified for the modelling. Moreover, a GIS-based topographic Index of Sediment Connectivity (IC) was used as PCD. The novelty of this study relies on the adaptation of IC for describing sediment connectivity in lowland areas taking into account contribution of soil type, land use and different patterns of precipitation during the event. A weighting factor for IC was calculated by estimating runoff calculated with SCS Curve Number method, assuming a constant value of precipitation for a given time period, corresponding to the critical event. The Digital Elevation Model of the study site was reconditioned at the drainage facilities locations to consider the real flow path in the analysis. These modifications led to highlight the role of rainfall patterns and surface runoff for modelling sediment delivery in lowland areas. Moreover, it was observed that integrating IC into the statistic prediction model increased its accuracy and performance. After the calibration procedure in one of the study areas, the model was validated in the other study area, located in the central part of Sweden, since this experienced flooding in relation to the same triggering event.

Geomorphic coupling, i.e. linkages between geomorphic system components, and sediment connectivity, i.e. the degree of linkage that controls sediment fluxes throughout landscape, have important implications for the behavior of geomorphic systems and have become key issues in the study of sediment transfer processes. The detailed characterization of the topographic surface plays a fundamental role for studying sediment dynamics in a catchment. Digital Elevation Models (DEMs) can both improve geomorphological interpretation (e.g. individuation of sediment source areas) and enable the quantitative modeling of sediment fluxes and connectivity. In particular, the availability of LiDAR-derived high-resolution Digital Terrain Models (DTMs), exploited using geomorphometric analysis, extends the applicability and potentialities of topography-based modeling approaches. Indeed, geomorphometry allows to derive detailed characterization of drainage pattern and surface roughness, which are two of the most important parameters in the study of sediment delivery. Since the late 1990s, some GIS-based approaches mainly based on stream-power have been developed for modeling the topographic potential for erosion and deposition and evaluating the impedance to sediment conveyance. But it is in the more recent years that an increasing interest for the quantitative characterization of the linkage between landscape units can be observed in literature. The development of geomorphometric indices, such as the sediment connectivity index (IC) by Borselli et al. (2008) and the version of IC proposed by Cavalli et al. (2013), and related freeware applications, has certainly contributed to this increased interest. In this work, the state of the art on the use of DEMs for sediment connectivity assessment, with a specific focus on the sediment connectivity index and following applications, will be presented. Future perspectives will be also discussed. References Borselli L., Cassi P., Torri D., 2008. Prolegomena to sediment and flow connectivity in the landscape: a GIS and field numerical assessment. Catena, 75(3), 268-277. doi:10.1016/j.catena.2008.07.006 Cavalli M., Trevisani S., Comiti F., Marchi L., 2013. Geomorphometric assessment of spatial sediment connectivity in small Alpine catchments. Geomorphology 188,31-41. doi:10.1016/j.geomorph.2012.05.007