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This work presents the results of the basin-scale analysis of sediment connectivity between hillslopes and the main channel during an extreme event in a mountain catchment. The Stolla basin (South Tyrol, Italy) was affected by a high-magnitude event in August 2017, during which the channel experienced widening and bed level changes and more than 600 debris flows, and landslides occurred in the basin. Hillslope and channel processes were mapped and characterized by using geomorphological analysis of orthophotos and pre- and post-event Digital Terrain Models (DTMs). Mass wasting process connectivity was derived by combining field survey evidence and GIS analysis. The coupling between the debris flows and landslides and the main channel was also evaluated by applying the Index of sediment connectivity (IC) by Cavalli et al. (2013). Binary logistic regression and the receiver operating characteristic curve was used to define thresholds that allow to discriminate connected from disconnected debris flows and landslides, based on IC values with respect to the main channel of the Stolla stream. First results indicate that the Stolla's post-event channel width was up to five times the pre-event width. Channel widening occurred mainly through bank erosions, removals of riparian vegetation and over bank depositions. Widening appeared to be accompanied by channel bed aggradation up to 1 m or incision up to -2 m. Depending on the morphological characteristics of the channel and of the valley, sediments were eroded from the banks, from the bed or were delivered by connected debris flows, landslides, and toe erosion processes. Statistical analyses indicate a high efficiency of the logistic regression model and the associated threshold to separate connected from disconnected debris flows and landslides using the IC as a predictor variable.

Debris flow processes are known to contribute substantial amounts of sediment to the fluvial system in mountainous areas, such as the Alps. In fact, debris flow release areas represent relevant sediment sources that should be taken into account when mitigating flood hazards for lower order streams. However, terrain that frequently produces debris flows is not necessarily connected to the channel network while structurally connected areas may often not produce debris flows. Therefore, the relevance of an area to contribute debris flow material to a channel mainly depends on the co-occurrence of two aspects: a high debris flow susceptibility which coincides with a high structural sediment connectivity. In this work, we present a novel data-driven approach that allows to identify areas that are both, susceptible to debris flow initiation and structurally connected to the main channel network. The methodology was developed for a debris flow prone basin located in the South Tyrolean Dolomites and further tested for two other catchments that exhibit different geomorphological settings. The methodical approach was based on the manual mapping of event-specific connected and disconnected debris flows areas that allowed to (i) calibrate a statistically based debris flow release susceptibility model and (ii) to derive quantitative thresholds for the previously derived connectivity index map (IC). The joined results reflect debris flow connectivity-susceptibility maps that were evaluated from numerous perspectives, including the evaluation of the spatial transferability of the approach. We present (i) quantitative IC index thresholds that allow to discriminate connected from disconnected debris flow release areas, (ii) well-performing debris flow release susceptibility models and (iii) joint debris flow connectivity-susceptibility maps that allow identifying zones that are differently relevant in terms of debris flow connectivity. Issues related to the geomorphic plausibility of the results and the spatial transferability of the approach are discussed. The proposed approach requires few basic input data sets and has therefore the potential to be applied over vast areas.

Several heavy precipitation events causing flash floods, debris flows, landslides, and morphological channel changes have occurred in Europe over the last years. In mountain environments, mass movements along the hillslopes are important sources of sediment supply to the rivers and may enhance the geomorphic effects of floods. The Stolla creek (catchment area: 40 km2) is a confined/partly confined channel of the Dolomites (Easter Italian Alps), that was affected by an extreme flood in August 2017, and by a moderate flood in August 2020. The geomorphic effects caused by the two storms were investigated with the aims: to compare the morphological changes induced by the two events in the channel and along the hillslopes; and to assess the impacts of the lateral sediment connectivity to the channel response. A multi-methodical approach was applied, including radar rainfall estimation, rainfall-runoff modeling, field surveys, remote sensing, geomorphological and statistical analysis. Hillslope and channel processes were mapped by comparing multitemporal orthophotos and changes in bed level elevation in the Stolla channel and sediment volume budget along the main channel and along the hillslope were accomplished using the difference of multi-temporal DTMs (DoD technique). Debris-flow connectivity to the main channel was derived by combining field evidence and geomorphometric analysis. The 2017 flood was caused by rainfall with a short duration (6 hrs) and a rain rate exceeding 45 mm h-1. More than 600 debris flows were triggered along the hillslopes. The Stolla channel experienced widening occurred through bank erosion, and overbank depositions and bed level aggradation up to 1.2 m or incision up to -2.2 m. Although 294,000 m3 of sediments eroded in the connected debris flows, limited volumes of sediments (< 1000 m3) were exported to the catchment outlet. The 2020 flood event was characterized by a lower rain rate (max 17 mm h-1) and a long duration (48 hrs) and did not trigger debris flows. The moderate magnitude of the flood peak did not lead to channel widening, but only bed incision (up to -1.4 m) in some reaches. The results show that different rainfall intensities and durations controlled the sediment transport effectiveness during the two floods and that the structural connectivity had an important role on the sediment cascade from the hillslopes to the main channel.

Sediment connectivity, defined as the degree to which a system facilitates the transfer of water and sediment through itself by means of coupling relationships between its components, has recently emerged as a paramount property of geomorphic systems. The growing interest of the earth sciences community in connectivity led this property becoming a key concept concerning sediment transfer processes analysis and one of the building blocks of modern geomorphology. The increasing availability of high-resolution Digital Elevation Models (DEMs) from different sources as LiDAR and Structure from Motion (SfM) paved the way to quantitative and semi-quantitative approaches for assessing sediment connectivity. A geomorphometric index of sediment connectivity (Borselli et al., 2008; Cavalli et al., 2013), based on DEM derivatives as drainage area, slope, flow length and surface roughness, has been developed along with related freeware software tool (SedInConnect). The index aims at depicting spatial connectivity patterns at the catchment scale to support the assessment of the contribution of a given part of the catchment as sediment source and define sediment transfer paths. The increasing interest in the quantitative characterization of the linkages between landscape units and the straightforward applicability of this index led to numerous applications in different contexts (Cavalli et al., 2020). This work presents and discusses the main applications of this sediment connectivity index. Being a topography-based index, it is focused on structural aspects of connectivity, and quality and resolution of DEMs may have significant impact on the results. Future development should consider process-based connectivity and incorporate temporal variability directly into the index. Moreover, this work demonstrate that, when carefully applied considering the intrinsic limitations of the mainly topographic-based approach, the index can rapidly provide a spatial characterization of sediment dynamics, thus improving the understanding of geomorphic system behavior and, consequently, hazard and risk assessment.

The assessment of flow velocity has a central role in quantitative analysis of debris flows, both for the characterization of the phenomenology of these processes and for the assessment of related hazards. Large-scale particle image velocimetry (LSPIV) can contribute to the assessment of surface velocity of debris flows, provided that the specific features of these processes (e.g. fast stage variations and particles up to boulder size on the flow surface) are taken into account. Three debris-flow events, each of them consisting of several surges featuring different sediment concentrations, flow stages, and velocities, have been analysed at the inlet of a sediment trap in a stream in the eastern Italian Alps (Gadria Creek). Free software has been employed for preliminary treatment (orthorectification and format conversion) of video-recorded images as well as for LSPIV application. Results show that LSPIV velocities are consistent with manual measurements of the orthorectified imagery and with front velocity measured from the hydrographs in a channel recorded approximately 70m upstream of the sediment trap. Horizontal turbulence, computed as the standard deviation of the flow directions at a given cross section for a given surge, proved to be correlated with surface velocity and with visually estimated sediment concentration. The study demonstrates the effectiveness of LSPIV in the assessment of surface velocity of debris flows and permit the most crucial aspects to be identified in order to improve the accuracy of debris-flow velocity measurements.

Debris flows are one of the most relevant natural hazards in mountain regions. A combination of structural (e.g. check-dams) and non structural (e.g. warning systems) measures is needed in most cases to reduce debris flow risks, and both benefit from experimental data on debris-flow characteristics. A new experimental site is being equipped in the Autonomous Province of Bolzano (Italy) for both monitoring purposes and testing warning systems. The study site (Gadria basin) is a small channel subjected to frequent debris flows. The overall system includes the following components: 4 rain gauges located at different elevations, 5 radar sensors and 5 geophones, and 3 video cameras and flash lights to record the propagation and the deposition of the debris flow. Transmission of data and alerts from the instruments will use radio technology because GSM coverage is not available in the basin.

In the last three decades generalized atmospheric temperature rise has produced rapid glacial retreat and permafrost degradation in high mountain environments of the European Alps. While it is widely recognized that these changes can impart general instability to mountain slopes (i.e. several projects have documented single case studies of high magnitude-low frequency, catastrophic slope failures), to our knowledge, there appears to be little empirical data available for investigating the spatial distribution of mass-wasting intensity through time in high mountain areas in relation to permafrost degradation and deglaciation patterns. This is a critical shortcoming. Understanding and quantifying the influence of similar transient conditions on mass-wasting processes holds critical long-term implications on the overall sediment flux reshaping mountain landscapes (e.g., post-LGM landscape evolution), as well as more practical short-term applications on water quality issues, damage control/prevention to infrastructure, and reservoir siltation, to mention some. In this contribution we present an integrated approach that aims to: (i) document the spatial distribution of mass-wasting activity and sediment production in the Saldur River basin (97 km^2); (ii) detect causal linkages between mass-wasting intensity, the potential spatial distribution of discontinuous permafrost, and patterns of glacier retreat; and (iii) identify source-to-sink colluvial sedimentary pathways as modulated by the spatial organization of glacial and periglacial landforms. To these ends, we map rock glaciers, protalus ramparts and moraines, and compile a field- and air photo-based multi-temporal (1959-2012) inventory of colluvial sediment sources. We then combine these data with two historical datasets of debris flow and landslide events (both implemented and maintained by the Autonomous Province of Bolzano) and analyse mass-wasting spatial distribution and intensity in relation to proximity of glacier fronts, intact and relict periglacial landforms, and a permafrost index map (implemented through PermaNET; http://www.permanet-alpinespace.eu/). This work is part of SedAlp (www.sedalp.eu), a project funded through the Alpine Space Programme.