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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.

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.

· I fenomeni di piena eccezionale in Italia sono quasi sempre accompagnati da notevole trasporto di sedimento grossolano e di materiale legnoso, che modificano la geometria degli alvei e creano occlusioni. · Una pianificazione territoriale basata unicamente sui risultati di modellazioni idrauliche è nella maggior parte dei casi non adeguata ai fenomeni che realmente accadono durante eventi estremi. · Le valutazioni geomorfologiche permettono la definizione di scenari di evento più aderenti alla realtà e che possono essere analizzati successivamente tramite modelli numerici.

Channel widening taking place during large flood events can be substantial in mountain rivers, with consequent great potential damages to infrastructures and buildings. The purpose of this work is twofold: i) to provide a quantitative assessment of geomorphic effects of an extreme flood event (recurrence interval > 100 years); ii) to test on this study case a new hydromorphological methodological framework (IDRAIM) developed to guide river corridor planning and management. As to the first objective, field surveys were integrated with remote sensing, GIS and statistical analyses for a flood event occurred in 2011 in Northwestern Italy. Channel widening ratios (width after / width before the flood) were calculated and then correlated with different controlling factors, and envelope relationships were then obtained. The tool of the IDRAIM framework used for the second objective was the Event Dynamics Classification (EDC) applied to selected study reaches, whose widening ratios turned out to correspond well with the EDC classes. Based on the results obtained, a practical procedure for predicting the expected widening is finally proposed.

Gravel-bed rivers featuring a near dynamic equilibrium state usually display a good correspondence between geomorphological and riparian community gradients, whereas long-term human alterations may disrupt these patterns. The aim of this study was to investigate the distribution of woody riparian vegetation on three geomorphic units - floodplains, bars, and islands along the Piave River, a gravel-bed river located in Northern Italy suffering from various degrees of human pressure and disturbances. A total of 214 plots (4×4 m) along three cross sections in each of two sub-reaches of the river were surveyed in order to identify the different morphological units, dendrological characteristics of standing plants, and species composition. Three riparian woody communities, with decreasing woody species richness and tree size, were identified by cluster analysis: Alnion incanae (n=58), Salicion eleagni (n=52), and Rhamno-Prunetea (n=27). The first was significantly associated with floodplains and the others with bars. No community showed a relatively stronger association with islands. Linear mixed models showed that the three geomorphic units significantly differed in mean elevation, fine sediment depth, and geomorphic persistence, but not all Tukey contrasts were significant, and absolute differences in mean elevation above talweg were quite small (

A high-magnitude flash flood, which took place on 25 October 2011 in the Magra River catchment (1717km2), central-northern Italy, is used to illustrate some aspects of the geomorphic response to the flood. An overall methodological framework is described for using interlinked observations and analyses of the geomorphic impacts of an extreme event. The following methods and analyses were carried out: (i) hydrological and hydraulic analysis of the event; (ii) sediment delivery by event landslide mapping; (iii) identification and estimation of wood recruitment, deposition, and budgeting; (iv) interpretation of morphological processes by analysing fluvial deposits; (v) remote sensing and geographic information system (GIS) analysis of channel width changes. In response to the high-magnitude hydrological event, a large number of landslides occurred, consisting of earth flows, soil slips, and translational slides, and a large quantity of wood was recruited, in most part deriving from floodplain erosion caused by bank retreat and channel widening. The most important impact of the flood event within the valley floor was an impressive widening of the overall channel bed and the reactivation of wide portions of the pre-event floodplain. Along the investigated (unconfined or partly confined) streams (total investigated length of 93.5 km), the channel width after the flood was up to about 20 times the channel width before the event. The study has shown that a synergic use of different methods and types of evidence provides fundamental information for characterizing and understanding the geomorphic effects of intense flood events. The prediction of geomorphic response to a flood event is still challenging and many limitations exist; however a robust geomorphological analysis can contribute to the identification of the most critical reaches.

This work addresses the temporal dynamics of riparian vegetation in large braided rivers, exploring the relationship between vegetation erosion and flood magnitude. In particular, it investigates the existence of a threshold discharge, or a range of discharges, above which erosion of vegetated patches within the channel occurs. The research was conducted on a 14 km long reach of the Tagliamento River, a braided river in north-eastern Italy. Ten sets of aerial photographs were used to investigate vegetation dynamics in the period 1954-2011. By using different geographic information system (GIS) procedures, three aspects of geomorphic-vegetation dynamics and interactions were addressed: (i) long-term (1954-2011) channel evolution and vegetation dynamics; (ii) the relationship between vegetation erosion/establishment and flow regime; (iii) vegetation turnover, in the period 1986-2011. Results show that vegetation turnover is remarkably rapid in the study reach with 50% of in-channel vegetation persisting for less than 5-6 years and only 10% of vegetation persisting for more than 18-19 years. The analysis shows that significant vegetation erosion is determined by relatively frequent floods, i.e. floods with a recurrence interval of c. 1-2.5 years, although some differences exist between sub-reaches with different densities of vegetation cover. These findings suggest that the erosion of riparian vegetation in braided rivers may not be controlled solely by very large floods, as is the case for lower energy gravel-bed rivers. Besides flow regime, other factors seem to play a significant role for in-channel vegetation cover over long time spans. In particular, erosion of marginal vegetation, which supplies large wood elements to the channel, increased notably over the study period and was an important factor for in-channel vegetation trends.

The SedAlp project contributes to an integrated management of sediment transport in Alpine basins. It is directed to an effective reduction of sediment-related risk while promoting the enhancement of riverine ecosystems and reducing the impacts of hydropower plants (balancing the implementation of EU Directives and national law e.g. RES, WFD, NGP etc.). SedAlp includes pilot actions in various representative Alpine river basins of five countries and contributes in particular to monitor sediment and wood transfer in a large set of Alpine catchments. A major goal of the project was to understand spatial and temporal variability of processes and to provide planning, warning and predictive tools but also sediment and wood management recommendations for protecting people. Furthermore the project includes strategy policy development and implementation actions for the improvement of sediment continuity in Alpine river basins. SedAlp regards the geological and climatic variability across the Alps that generate complex patterns of sediment transfer, whereas management conflicts are similar. To reach the ambitious goals of the project, it is essential to understand the key processes of sediment and wood transport. Workpackage 5 with the title "sediment transport monitoring" was focused to enhance the knowledge about these transport processes by conducting the first standardized transnational monitoring of sediment and wood transport in Alpine basins. WP5 concentrated on monitoring and assessing fluxes of sediment and wood at the local scale. Measured fluxes are strongly depending on the supply from the catchment and on the connectivity to source areas, analysed in WP4. The output of WP5 feeds WP6 which studied the interferences between structures and sediment/wood fluxes and forms the basis for WP7 which linked the catchment with channel processes. Finally, the results of WP5 contribute to policy recommendations in WP8. Within WP5, monitoring activities on 28 different pilot sites in Austria, Germany, France, Italy and Slovenia have been conducted between 2012 and 2015. The monitored transport processes covered woody debris, debris flow, bedload and suspended load. The different pilot sites showed a great diversity in catchment size, channel slope and hydraulic parameters and covered the whole bandwidth from small, high alpine catchments (e.g. Strimm: 5 km² catchment size, 2427 m elevation a.s.l) to large catchments in the alpine valleys (e.g. Drau/Dellach: 2131 km² catchment size, 600 m elevation a.s.l). With the results of the monitoring activities, a database using different parameters regarding the transport process has been established and reflects the big diversity of the different pilot sites. The coordinated sediment transport monitoring outlined the links between the various processes responsible for sediment delivery at catchment scale and stresses out the need for a closer integration between the monitoring of various sediment transport processes in Alpine headwaters. Due to the wide range of different measurement methods used within the project, one focus of this work package was to standardize the measurement procedures and harmonize data analysis. This led to the 1st milestone "Protocol on standardized data collection methods in sediment transport monitoring for transboundary exchange". These protocols were intended to describe the used monitoring techniques and data processing methods for debris flows, wood transport and bedload transport. Furthermore, the protocols work also as guidelines to assist in choosing the appropriate monitoring method for supporting prospective monitoring efforts. 4 An important aspect of the project was to understand spatial and temporal variability of the monitored processes. The observed appearance of clockwise and counter clockwise hysteresis effects between sediment and discharge, showed the significant role of the location of the active sediment source in the temporal variability of sediment transport. Analysis of the spatial distribution showed an evolvement of the bedload transport width with increasing discharge which provides vital information for an improved planning of river related measures. To determine these process related variability, the use of appropriate monitoring methods as well as their right application is of great importance. Indirect monitoring methods (e.g. geophone devices) pointed out their potential in the automatic and continuous detection of these transport processes. The presented data reflect a high spatial and temporal variability in the occurrence of bedload, suspended load debris flow and wood transport and reveal significant consequences for the practical usage of monitoring methods, data and application. Many river engineering tasks require detailed information about the extent of sediment transport and wood mobility, which are provided by sediment and wood transport relations and equations. The 3rd milestone "First set of practically applicable bedload/wood transport relations and models" gives an overview about the most common transport relations and formulas. Furthermore it presents the difficulties and challenges in the application of these relations and shows the last developments in improving transport equations. The comparison between measured and calculated specific bedload transport rates showed substantial differences in the derived results. For the practical use of these relations it is strongly recommended to select, calibrate and validate the sediment and wood transport equations using monitored field data. The evaluation of river restoration projects showed the need of an improved process understanding between sediment transport an engineering measures. The functionality and sustainability of river restoration measures are, beside the hydrologic and hydraulic conditions, mainly depending on the superior sediment regime and thus the sediment input into the reach. By increase or decrease of the mean sediment input, the hydraulic and thus morphological conditions of the reach need to rearrange to the given input by e.g. lateral and vertical adjustments of the river bed or by changing the river type.