M. Arattano

Ten years of debris-flow monitoring in the Moscardo Torrent (Italian Alps)

Abstract Field data on debris flows are of utmost importance for improving knowledge of these hazardous phenomena and for the development of attenuation measures. In alpine basins, debris flows generally occur with a frequency high enough to create serious risk to human settlements and transportation routes, but too low to justify monitoring activity capable of providing enough data in a sufficiently short time. The Moscardo Torrent, a small stream in the Eastern Italian Alps, is an exception to this general situation since it displays a high frequency of debris flows (commonly at least one event per year). In 1989, this torrent was instrumented and the results of the following 10 years (1989–1998) of debris-flow monitoring are presented, with an analysis of collected data. The equipment installed allowed measurement of rainfall, flow stage and ground vibrations caused by debris flows. Other important debris-flow variables, that is, mean front velocity, peak discharge and flowing volume were estimated from instrumental records. Video pictures have proved to be useful for the visual interpretation of debris-flow waves and have made it possible to estimate the surface velocity of debris flows. Recorded data are compared with other experimental data sets collected and documented worldwide. Advantages and shortcomings of different types of sensors are discussed with relevance both for research monitoring purposes and for possible use in debris-flow warning systems.

Monitoring the propagation of a debris flow along a torrent

Abstract The results are presented of a field experiment on debris flow monitoring that were obtained in the summer of 1996 in a small mountain torrent in the Italian Alps. A network of seismic detectors was placed about one kilometre upstream of some previously existing gauging stations equipped with ultrasonic devices. In June and July two debris flows occurred, which were recorded both by the seismic and the ultrasonic sensors allowing a comparison between the two results. For the first event, the plot of the mean velocity of ground vibration vs time revealed a strong similarity with the hydrograph recorded at the gauging sites: the graphs show the passage of a debris flow wave with a sharp peak followed by subsequent smaller waves. An estimation of the mean front velocity of these waves was performed at both sites.

Detecting torrential processes from a distance with a seismic monitoring network

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. Both networks were designed for debris flow monitoring purposes and for testing warning algorithms. In this paper, 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.

Processing the ground vibration signal produced by debris flows

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 impulses and the method of amplitude. 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 results of this work will provide means for decision to researchers and technicians who find themselves facing the task of designing a debris flow monitoring installation or a debris flow warning equipment based on the use of ground vibration detectors. We present the processing of the seismic signal produced by a debris flow.Two methods of seismic data processing, amplitude and impulses, are compared.We process the geophone signal of a debris flow to reveal its main features.The processing of the debris flow seismic signal reveals to be useful for warning.A geophone network allows an early detection of debris flow for warning purposes.

Monitoring Mud-Flows for Investigative and Warning Purposes: The Instrumented Catchment of Rio Marderello (North-Western Italy)

Mud-flows are natural phenomena that may occasionally occur in mountain torrents. They appear as waves with a steep front and a shape resembling that of debris-flows, but consisting of a muddy slurry that contains much less boulders and granular material than these latter. Nevertheless, mud-flows can cause severe damages to human settlements and infrastructures and may produce many casualties, as much as debris-flows do. The reduced boulder and debris content of mudflows may have an important influence on the results of the monitoring, which might be significantly different than those of debris-flows. This might be particularly true if seismic devices were employed as detecting tools, since the lack of huge boulders and large particles might impede the generation of strong ground vibrations. The ground vibration signals produced by mud-flows might also have different frequency ranges and different peak frequencies. These different behaviors should be investigated, not only for scientific purposes but also because they might lead to the choice of different parameters and algorithms in case seismic sensors were used for warning. For these reasons the Marderello basin, an alpine catchment prone to mud-flows located in North-western Italy, has been equipped with a micro-seismic network. The new monitoring system has been installed in the spring of 2013 and in July a mud-flow occurred that has allowed the collection of the first field data. These latter are presented in this paper together with a characterization of the catchment, a detailed description of the monitoring installations and an outline of future investigation plans.

Monitoring Rock Wall Temperatures and Microseismic Activity for Slope Stability Investigation at J.A. Carrel Hut, Matterhorn

Recent climate changes are increasing the frequency of rock-slope instabilities in the Alpine region. The formation of cracks leading to rockfalls causes a release of energy propagating in form of elastic waves. These latter can be detected by a suitable transducer array together with the vibrations generated by the impact of rockfalls. Geophones are among the most effective monitoring devices to investigate both these phenomena. A monitoring system composed by geophones and thermometers was installed at the J.A. Carrel hut (3829 m a.s.l., Matterhorn, NW Alps) in the framework of the Interreg Alcotra projects PERMAdataROC and MASSA by CNR IRPI and ARPA with the financial and logistic support of the Valle d’Aosta Region. The correlation between temperature trends and microseismic events is presented: cold periods characterized by a rapid temperature decrease present higher concentration of microseismic activity. However, not every drop in temperature is associated to microseismic activity, and the identification of the processes generating microseismic events in occasion of rapid temperature decrease is still uncertain. The objective of the ongoing research activity is to analyze in deep the statistical correlation between the number of microseismic records and the temperatures of air and rock in order to investigate the existence of recurrent patterns in the detected signals.

High Elevation Rock Falls and Their Climatic Control: a Case Study in the Conca di Cervinia (NW Italian Alps)

One of the impacts of climate warming in recent years is the evident increase of the number of rock fall occurrences at high elevations. With few exceptions, these events have small magnitudes and thus are rarely reported and documented, even less so in the past. Therefore it is difficult to use a statistical approach to analyze of the relationships between climate warming and rock slope instability. On the other hand, it is often difficult to carry out a time analysis of meteorological conditions responsible for rock fall triggering, considering that very few automatic weather stations (AWS) are located in the areas and in the altitudinal range that are affected by cryosphere degradation (i.e. above c.a. 3,000 m elevation in the Alps), and that climatic conditions in high elevation environments are spatially and temporally variable. The present study addresses the above-mentioned issues through analysis of a series of small rock falls that occurred in the last 10 years on the Matterhorn and surrounding rock slopes. A specific focus is temperature: we present a preliminary analysis of the spatial and seasonal variability of the vertical temperature gradient in the Conca di Cervinia, where the Matterhorn is located, to illustrate the uncertainty in estimates of the thermometric conditions at high elevation rock fall sites.

Interactive, 3D Simulation of Natural Instability Processes for Civil Protection Purposes

A computer simulation of a debris flow in motion, a landslide in evolution and a snow avalanche is here presented that was realized in the context of a series of risk education activities developed within the European project RISKNAT. The pc simulation allows to view in 3D and in their natural context of occurrence the three types of mass movements mentioned above. The peculiarity of the simulation is that the user is able to directly interact with the processes with the help of a special joystick that allows to vary the main parameters governing the process and also its sight. It is possible, for example, to choose an aerial view, or to get a view closer to the ground to observe in more detail the phenomenon in its progress. The computer simulation is based on real data and intends to spread among the local population, primarily exposed to risks, instructional and educational bases by offering a virtual experience of some recurrent natural processes occurring in alpine valleys. In fact it allows the user to reach a greater awareness of these latter, learning about their aspect, their mode of propagation, their velocity and their interaction with the natural and urbanized landscape. The simulation is therefore a tool to support the communication activities devoted to explain to the general public the civil protection basics, with the belief that the direct involvement, albeit virtual, in a situation of danger can be an important stimulus to better understand the forces of nature and to assimilate the basic behaviors needed for self defense.

3D Video Simulation of a Debris Flow

In 2011 a consortium of different institutions leaded by the Civil Protection Department of the Turin Province and including the CNR IRPI, the University of Turin and Thales Alenia, realized a 3D video simulation of a debris flow in motion. The 3D video, which is part of a larger installation that includes also the representation of a landslide and of a snow avalanche, allows a direct interaction with the process through a joystick. This latter, in fact, allows to change the viewpoint of the user, choosing an aerial view, a lateral one or a view that is closer to the ground permitting a more detailed observation of the phenomenon in its progress. The video has primarily instructional and educational purposes, enabling the user to acquire a personal reality of a debris flow and to become aware of its different aspects. It is also an example of the precious support that monitoring data may provide, since the virtual simulation was strictly based on real data recorded in field installations. The 3D video was created as a tool to support the communication activities devoted to explain to the general public the basic principles of civil protection through a direct involvement in a situation of danger. This involvement can be an important stimulus to better understand the natural phenomenon under investigation and to assimilate the basic behaviors needed for self defense. The video was realized with the funds of the European RiskNat project.