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The recession of Alpine glaciers causes an increase in the extent of proglacial areas and leads to changes in the water discharge and sediment balance (morphodynamics and sediment transport). Although the processes occurring in proglacial areas are relevant not only from a scientific point of view but also for the purpose of climate change adaptation, there is a lack of work on the continuous monitoring and multitemporal characterization of these areas. This study offers a multidisciplinary approach that merges the contributions of different scientific disciplines, such as hydrology, geophysics, geomatics, and water engineering, to characterize the Rutor Glacier and its proglacial area. Since 2020, we have surveyed the glacier and its proglacial area using both uncrewed and crewed aerial surveys (https://doi.org/10.5281/zenodo.8089499 , ; https://doi.org/10.5281/zenodo.10100968 , ; https://doi.org/10.5281/zenodo.10074530 , ; https://doi.org/10.5281/zenodo.10101236 , ; https://doi.org/10.5281/zenodo.7713146 ,). We have determined the bathymetry of the most downstream proglacial lake and the thickness of the sediments deposited on its bottom (https://doi.org/10.5281/zenodo.7682072 ,). The water depth at four different locations within the hydrographic network of the proglacial area (https://doi.org/10.5281/zenodo.7697100 ,) and the bedload at the glacier snout (https://doi.org/10.5281/zenodo.7708800 ,) have also been continuously monitored. The synergy of our approach enables the characterization, monitoring, and understanding of a set of complex and interconnected processes occurring in a proglacial area. [ABSTRACT FROM AUTHOR]

The mitigation of risk caused by debris flows is increasingly pursued by means of non-structural measures, including early warning systems (EWSs). Nowadays, EWSs are becoming attractive thanks to their flexibility and due to the new paradigm of smart sensor networks, proposed as a tool to monitor and gather intelligence from the surrounding environment. Also, an increasing number of extreme meteorological events is expected due to climatic changes, resulting in a consequent growing risk in areas considered safe so far. Although the technological development of detection systems based on low-cost sensor networks has recently spurred a great deal of interest, very few success stories exist of EWSs operational for long periods and trusted by local authorities. In this work, I present an overview on the recent advances, open problems, and future challenges in the field of detection of debris flows for early warning purposes, with a special attention to the European Alps. I discuss (i) the uncertainties related to the use of rainfall thresholds and their possible improvement based on field observations in the source areas, (ii) the new opportunities that seismo-acoustic sensors open in terms of warning performances and lead time, (iii) the problematic interaction of EWSs with structural mitigation measures, and (iv) the old but still actual problem of responsibility in issuing an alarm. Finally, I debate the “information paradox” that can contribute limiting the adoption of EWSs in future and the possible benefits of communication and dissemination.

Since debris flows represent one of the most dangerous natural hazard in mountain areas, Early Warning Systems (EWSs) play a crucial role in reducing the risk of these hazardous processes. Robust event pre-alert usually relies on long time series of local rainfall measures. Oftentimes regional rain gauge networks present an insufficient spatial density to grasp the highly variable spatio-temporal dynamics of debris-flow triggering events and thus relying on such networks for developing rainfall thresholds might lead to relatively low rainfall estimates. The present paper reports the development of operational rainfall thresholds for the Cancia EWS, Dolomites (NE Italy). The instrumentation configuration led to the derivation and implementation of a set of rainfall thresholds that significantly enhanced pre-alarm reliability thanks to an optimal spatial distribution of multiple rain gauges within the catchment. Notwithstanding the small number of debris flows occurred during the calibration period, rainfall thresholds were derived considering the whole population of rainfall events showcasing the statistical properties of those events that led to debris-flow initiation. Finally, the validation period served as proof of work for the proposed thresholds with no raised false alarms and with the identification of few minor, but correctly detected, debris flows.

On August 5, 2022 in the Rochefort torrent (Val Ferret, Mont Blanc), a debris flow occurred that invaded the road connecting the valley with the village of Courmayeur. The debris flow interrupted the car traffic and damaged the bridge that crosses the torrent and the aqueduct that serves the municipality of Courmayeur. Due to the recurrence of similar events, in 2017 the Valle d’Aosta Region had decided to install a monitoring and warning system for debris flows, close to the bridge on the Rochefort torrent, to interrupt the traffic in both directions through a pair of traffic lights in case of debris flow. The system, named ALMOND-F (ALarm and MONitoring system for Debris-Flow), has been installed along the torrent, few tens of meters upstream of the bridge. ALMOND-F adopts a warning algorithm that is based on the variation of the seismic signal intensity produced by debris flows and that had been thoroughly tested in previous years in the instrumented area of the Gadria basin. On August 5, 2022 the warning system activated the traffic lights and stopped the traffic about three minutes before the debris flow invaded the road. It is the first time that the ALMOND-F system is utilized in a real risk situation to protect the population, after some years of controlled tests carried out in an instrumented area. Even though this represents an undoubted technological success, the installation of ALMOND-F requires several issues to be addressed to grant the highest level of safety. For instance, the presence of other active debris-flow channels and/or natural risks in the same valley may represent a limitation to the installation of a site-specific alarm system. The installation of the Rochefort torrent, opportunely optimized also on the basis of the feedbacks of the August 5, 2022 debris flow event, could become a useful case study and so provide indications and suggestions on the mitigation of the debris flow risk through the use of warning systems.

Monitoring data gathered in the headwaters of the Gadria catchment, eastern Italian Alps, have been analysed to study debris-flow initiation. The active channel, located at 2200 m a.s.l., was instrumented with a geophone, a time-lapse video camera and a rain gauge. The peak amplitude and duration of the seismic signals and their frequency content were analysed and compared with video images. Results showed that different seismic sources produced signals with different characteristics and that it is possible to discriminate the most intense runoff by analysing the combination of peak amplitude and duration of the seismic signal. The further development of this research would be to create an algorithm able to automatically classify the seismic sources and identify intense channel processes that can generate debris flows. In perspective, the combination of seismic detection in the initiation area with monitoring just above the infrastructures at risk could represent an effective solution to expand the lead time of an early warning system.

This study presents monitoring data of a debris flow event in the Central Italian Alps. The debris flow occurred on August 16, 2021 in the Blè basin (Val Camonica valley, Lombardia Region) and was recorded by a monitoring station installed just few weeks before. The monitoring system was deployed to document the hydrologic response of the catchment to rainfall, and was designed to be lightweight, relatively cheap, and easy to deploy in the field. To this purpose, we combined video cameras with geophysical sensors (geophones and infrasound) and optimized the power supply system. The data recorded during the event allowed to identify the triggering rainfall, document the flow behaviour, and estimate surface flow velocity and flow rate using Particle Image Velocimetry algorithms. Moreover, the seismic signal generated by the debris flow revealed a peculiar frequency spectrum compared to regular streamflow. These results show that even a relatively simple monitoring system may provide valuable data on real debris flow events.

This work analyses seven debris flows recorded between 2018 and 2020 in the Gadria instrumented catchment (South Tirol). We focus on three aspects not previously explored in this catchment: (i) the debris-flow transfer times between the headwaters and the outlet; (ii) the longitudinal variability of debris-flow velocity between the three downstream monitored cross-sections, and (iii) the characteristics of the secondary surges observed in three debris flows. In most cases, the mean velocity of the debris flow estimated from the upper to the lower channel reaches (for travel distance of 2155 m) is rather low, ranging between 1.9 and 3.9 m/s. This result could indicate a progressive slowing down, and possibly even temporary stops of debris flows along the path. Some variability in flow velocity was observed between two channel reaches in the lower part of the catchment (0.7 – 2.3 m/s in the upstream reach, and 1.4 – 4.7 in the downstream one). Regarding the secondary surges, these have been noted to occur superimposed on slow-moving slurry-type phases. The mean velocity of the secondary surges varied between 3.5 and 8.9 m/s, with an average value close to 6 m/s for all three events. Their regular shape, duration, and depth suggest that such surges were generated by flow instabilities, with no external forcing.

Channel incision and narrowing have occurred in the 20th century in most Alpine rivers. However, the causal links between sediment-related human engineering and exploitation and morphological changes in rivers are mostly unclear. This study presents an analysis of the evolutionary trajectories of the main active channels in the upper Etsch/Adige River basin (Eastern European Alps) coupled with their modifications in terms of coarse sediment transport. Channel planform variations were quantified in 15 rivers (total length of 630 km) using multi-temporal analysis of historical maps and orthophotos. Sediment volumes excavated from river channels or trapped by hydraulic structures (dams and retention basins) were retrieved from historical records, along with geospatial information regarding the presence of lateral and longitudinal consolidation works and land use variations. Results indicate that most rivers underwent slight narrowing and some of them experienced widening, from the mid-19th century to the 1950s. From the 1950s to the late 1990s, severe variations in terms of narrowing and morphological simplification took place in all rivers. The analysis of channel changes in relation to human activities shows that gravel mining carried out in the period 1970s-1990s appears to have been the main cause of sediment imbalance in the rivers which narrowed the most. Since the 2000s, when gravel mining was banned by law, channel adjustments have become negligible throughout study area. Nevertheless, the trapping of a large share of coarse sediment fluxes--at the river basin scale--by retention check dams and hydropower dams has impeded rivers from recovering to their original conditions. [ABSTRACT FROM AUTHOR]

The recession of Alpine glaciers causes an increase in the extent of proglacial areas that leads to changes in the water and sediment balance morphodynamics and sediment transport. Although the processes occurring in proglacial areas are relevant not only from a scientific point of view but also for the purpose of climate change adaptation, there is a lack of studies on the continuous monitoring and multitemporal characterization of these areas. This work offers a multidisciplinary approach that merges the contributions of different scientific disciplines such as hydrology, geophysics, geomatics and water engineering to characterise the Rutor glacier and its proglacial area. We surveyed the glacier and its proglacial area since 2020 with both uncrewed (drone) and crewed aerial photogrammetric flights; we determined the bathymetry of the most downstream proglacial lake and the thickness of the sediments deposited on its bottom. Water depth at four different locations within the hydrographic network of the proglacial area and the bedload at the glacier snout were continuously monitored. The synergy of our approach enables the characterisation, monitoring and understanding of a set of complex and interconnected processes occurring in a proglacial area. [ABSTRACT FROM AUTHOR]