Debris flow seismic monitoring and warning

Standardization of measurement procedures and performances are important goals in every field of science and are in general intensely pursued by scientists. Certain natural phenomena, however, present particularly difficult challenges in this regard and many efforts are still needed to actually reach standardization and systematic performance of measurements. Debris flows certainly belong to this latter category. Due to their low frequency of occurrence, their short duration and their sudden and abrupt nature they are extremely difficult to be measured. Only instrumented basins where debris flows occur with a sufficiently high frequency per year allow systematic monitoring activities. Even though during the last decades several such basin have been instrumented, field measurement data are still scanty and methods of measurement are not yet sufficiently standardized. One of the goals of the European Territorial Cooperation project SedAlp “Sediment management in Alpine basins: integrating sediment continuum, risk mitigation and hydropower” (Alpine Space Programme 2007-2013) is to make some advancement in this direction. One of the expected outputs of the SedAlp project is a protocol on debris flow monitoring. Ground vibration sensors have been increasingly used and tested, during the last few years, as devices to monitor debris flows and they have also been proposed as one of the more reliable devices for the design of debris flow warning systems. The need to process the output of ground vibration sensors, to diminish the amount of data to be recorded, is usually due to the reduced storing capabilities and the limited power supply, normally provided by solar panels, available in the high mountain environment. There are different methods that can be found in literature to process the ground vibration signal produced by debris flows. In this paper we will discuss the two most commonly employed: the method of amplitude and the method of impulses. These two methods of data processing are analyzed describing their origin and their use, presenting examples of applications and their main advantages and shortcomings. The two methods are then applied to process the ground vibration raw data produced by a debris flow occurred in the Rebaixader Torrent (Spanish Pyrenees) in 2012. The detection of debris flows through seismic devices occurs at a certain distance from the channel bed. Ground vibration detectors are installed outside of the flow path, usually along the banks of the torrent or on the surrounding valley slopes, in order to avoid damage or even complete destruction. Seismic networks, however, are also prone to detect other earth surface processes that can be confused with the passage of a debris flow. Recognizing these other processes is important, particularly when the seismic network is used for warning purposes and not only for monitoring. To this aim, two seismic networks were installed in two instrumented basins located in the Italian Alps: the Marderello (Western Italian Alps) and the Gadria (Eastern Italian Alps). Both networks were designed for debris flow monitoring purposes and for testing warning algorithms. The seismic recordings of torrential processes that occurred at different distance from the monitoring networks, within and outside the monitored channels, are presented and discussed. It was found that knowledge of the waveform that these different processes produce is critical to the successful design and implementation of seismic networks for debris flow warning. The output of the seismic devices commonly employed for the monitoring of debris flows, such as geophones and seismometers, is a voltage that is directly proportional to the ground vibration velocity. The output signal in analogical form is usually digitalized at a fixed sampling frequency to be opportunely processed. The processing is performed to both reduce the amount of data to be stored in a data-logger and to reveal the main features of the phenomenon that are not immediately detectable in the raw signal, such as its main front, eventual subsequent surges, the wave form and so on. The processing also allows a better and sounder development of algorithms, when seismic devices are employed for warning purposes. However, the processing of the raw signal alters in different ways the original raw data, depending on the processing method adopted. This may consequently limit or reduce the efficacy of the warning. To this aim, the methods of amplitude and impulses have been also applied to some seismic recordings obtained in the instrumented basin of Illgraben (Switzerland) and Chalk Cliffs (USA). Data from two years of seismic monitoring performed in the Gadria basin are presented and discussed, together with the warning algorithm integrated in the system since 2014. In summer 2014 the alarm was correctly triggered by a debris flow and a red light was activated 3 minutes before the passage of the main front through the cross-section where this experimental semaphore is installed. The alarm lasted for the whole duration of the flow, correctly switching off after 20 minutes. Testing the algorithm on the whole seismic dataset recorded during both summers 2013 and 2014, the two debris flows event occurred in this time interval were correctly detected and only 3 false alarm where produced. After numerical simulation, it was possible to state that to eliminate false alarms a non-simultaneousness criterion in the threshold triggering can be adopted. The analysis of the frequency content of the seismic traces recorded at Gadria also allowed to draw some preliminary conclusions on the spectral characterization of the debris flow phenomena through geophone data. Some future developments are also outlined: the next step in the definition of a more advanced and robust warning system would be possible integrating the spectral information in the warning algorithm

Debris-flow detection for early warning purposes: Recent advances, open problems, and future challenges

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

Performance of the debris flow alarm system ALMOND-F on the Rochefort Torrent (Val d’Aosta) on August 5, 2022

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

Study of debris-flow initiation through the analysis of seismic signals

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

Debris-flow monitoring and warning: review and examples

Debris flows represent one of the most dangerous types of mass movements, because of their high velocities, large impact forces and long runout distances. This review describes the available debris-flow monitoring techniques and proposes recommendations to inform the design of future monitoring and warning/alarm systems. The selection and application of these techniques is highly dependent on site and hazard characterization, which is illustrated through detailed descriptions of nine monitoring sites: five in Europe, three in Asia and one in the USA. Most of these monitored catchments cover less than ~10 km2 and are topographically rugged with Melton Indices greater than 0.5. Hourly rainfall intensities between 5 and 15 mm/h are sufficient to trigger debris flows at many of the sites, and observed debris-flow volumes range from a few hundred up to almost one million cubic meters. The sensors found in these monitoring systems can be separated into two classes: a class measuring the initiation mechanisms, and another class measuring the flow dynamics. The first class principally includes rain gauges, but also contains of soil moisture and pore-water pressure sensors. The second class involves a large variety of sensors focusing on flow stage or ground vibrations and commonly includes video cameras to validate and aid in the data interpretation. Given the sporadic nature of debris flows, an essential characteristic of the monitoring systems is the differentiation between a continuous mode that samples at low frequency (“non-event mode”) and another mode that records the measurements at high frequency (“event mode”). The event detection algorithm, used to switch into the “event mode” depends on a threshold that is typically based on rainfall or ground vibration. Identifying the correct definition of these thresholds is a fundamental task not only for monitoring purposes, but also for the implementation of warning and alarm systemsPeer ReviewedPostprint (author's final draft

Monitoring debris flows in the Gadria catchment (eastern Italian Alps): Data and insights acquired from 2018 to 2020

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

When instrument location makes the difference on rainfall thresholds definition: Lessons learned at Cancia, Dolomites

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