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A new real-time debris flow and avalanches detection system
based on optical fiber sensing
Santina Aiassa1, Francesco Antolini1, Marco Barla2*, Alessandra Insana2, Roberto Gaudino3, Giuseppe Rizzelli
Martella3 and Saverio Pellegrini3
1Geosolving s.r.l., 10144 Corso Orbassano 336, Torino, Italy
2Politecnico di Torino, Dipartimento di Ingegneria Strutturale, Edile e Geotecnica, 10129 Corso Duca degli Abruzzi 24, Torino, Italy
3Politecnico di Torino, Dipartimento di Elettronica e Telecomunicazioni,10129 Corso Duca degli Abruzzi 24, Torino, Italy

Abstract. The real-time detection of potentially destructive water, earth and snow mixtures such as debris
flow and avalanches is a topic of growing interest to mitigate the risk in anthropized areas such as the Alpine
region. In view of this, a new cutting-edge debris flow and avalanche detection system, called OPTIALP,
was developed. The proposed system exploits the polarization variations induced on the fiber by mechanicalvibrations, for the automatic detection along their propagation path of potentially destructive snow and soilwater mixtures. One of the main values-add of the OPTIALP system is the “quasi distributed” and spatiallycontinuous detection along the whole fiber which improves the current monitoring technologies relying on"discrete" monitoring points and sensors. The OPTIALP system was designed and thoroughly tested in thelaboratory environment by means of a specific setup. Over 650 tests were carried out and a new signal
processing algorithm developed in Matlab environment capable to interpret the data acquired was created.
The results showed that the OPTIALP system is able to correctly identify the signals produced by lab-scalemass movements.

1 Introduction
In mountainous areas like the Alps debris-flow andsnow avalanche processes, despite occurring in different
seasons, often occupy common starting and runoutzones. Despite the different rheology of material
involved in the movement and the possible
transformation occurring during their pathways, acommon characteristic both of debris flow and snow
avalanches is that such mass movements are generated
by singular or multiple relatively shallow soil orsnow/ice failure collapses which during the movement
along a slope can combine with each other, channelize
and entrain further material. Debris flows require theavailability of cohesionless material and excess
moisture to saturate and mobilize the debris [1] whilesnow avalanches are a direct consequence of the
dynamic of the variable seasonal snow cover in steep
mountainous terrain. The prerequisite for the occurrenceof both snow avalanches and debris flows is a steep
slope.
These phenomena represent highly dangerous typesof mass movements, because of their high velocities,
large impact forces and long runout distances. Theirrepeated occurrence results in characteristic landforms,
such as cone-shaped debris accumulations at the mouthof gullies or torrent valleys where buildings andinfrastructures are very often located.
The protection of infrastructures in mountainenvironments against debris flow and snow avalanches

is of growing importance as an effect of global warming.
The frequency, the areal distribution, and the intensityof precipitations have slowly but continuously changed
while the occurrence of severe rainfalls has increased[2].
The consequent occurrence of debris flows and snow
avalanches has increased as well [3] despite the presenceof differentiated trends by basins not easily generalized
on a larger scale [4]. This frequency in the Alpine area
is sufficient to create high-risk conditions for people,settlements and infrastructures.
Consequently, there is a growing need for innovative
tools to increase the adaption to these phenomena. In
this sense, adaptation, i.e. the process of adjusting in
response to current or expected climate change andeffects, with the aim to reduce exposure andvulnerability to climate variability [5], is a fundamental
strategy for reducing and managing the risks of climatechange impacts on slopes and, more broadly, on all
geostructures.
The aim of the proposed new optical fiber system,
called OPTIALP, is therefore the detection and early
warning of high velocities and large impact force mass
movements like debris flows and snow avalanches. The
novelty of the proposed solution is that the sensing
system, capable to measure continuously and in realtime, can be used and extended over long distances (upto a few km) without any limitation so that the sensor
becomes the whole optical fiber. Another important
advantage is that optical fibers are completely buried in* Corresponding author: marco.barla@polito.it© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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the soil. Therefore, the visual impact of the system islimited thus allowing the installation in contexts
characterized by high environmental value (naturalparks and natural reserves). A patent application wasfiled for the OPTIALP system last 7th December 2022
(n. 102022000025158).
In the present paper the characteristics of theOPTIALP system will be first described. An
experimental setup, specially developed to test thetechnology at the laboratory scale, will then beillustrated along with the results obtained from the many
tests performed. Finally, conclusions and possible
further envisaged developments will be discussed.

stronger the intensity of the mechanical stress occurringon the fiber, the more sudden the change of the SOP.
This allows real-time detection if something
anomalous is occurring along the fiber. During theirpropagation, debris flows as well as snow avalanches
generate mechanical and vibrational stresses along theirpaths that can be monitored by observing the variationsof the SOP along the fibers in the domain of time.
Figure 1 also illustrates the installation scheme forthe optical fibers consisting of different loops placed
across the axis of the gully. As the optical fiber can be
subjected to large mechanical stress and impact forces
resulting in damages, it will be buried at a sufficientdepth (0.3-0.5 m) in the soil for protection.
A key point for the mitigation of the risk of debris
flows and rock avalanches occurrence is the prompt andcorrect detection of the masses propagating along aslope. Therefore, the adoption of a distributed sensing
system buried in the ground along the whole
propagation path represents an evident advantage withrespect to the use of discrete sensors.
At the same time, another requirement of an early
warning system for correct detection is to minimize thepossible occurrence of false alarms. Given this, it isessential to adopt a robust algorithm for signal
processing capable of correctly recognizing thevibrational characteristics of mass movements from anyother random noise affecting the fibers. Therefore, aspecific algorithm described in [6] and based on theselection of specific filtering and thresholding isadopted to process in real-time the SOP variation overtime.
From a technical point of view the advantages of theOPTIALP system compared to other monitoring
systems currently available on the market are:
- The spatially continuous detection of mass movements
over the entire length of the optical fiber that makes it
possible to use the system over long distances (up to 10km) and for very large areas (several km 2).
- The instantaneous response of the system and theextremely high sensitivity to mechanical vibrations.
- The higher wear resistance than traditional electromechanical sensors due to the burial in the ground.
- The relative ease of installation and the possibility ofdirect installation within structures such as rockfall netbarriers.

2 Characteristics of the system andprinciple of operation
A general layout of the proposed system to detect debris
flows and snow avalanches is shown in Figure 1.

Fig. 1. Layout of the OPTIALP system along a road thatcrosses a gully subjected to debris flows/snow avalanches.

The system is composed of the following devices:- Optical fibers;- Laser i.e. optical source;- Polarimeter;- Laptop or embedded microprocessor system to process
the acquired signals;- Radio node to send the alarms to the informativedevices;- Traffic lights or alternative informative devices toconvey specific messages to the elements at risk.
The laser source along with the polarimeter and thesystem used for signal processing, also called theinterrogator, represents the core of the OPTIALP
system.
The principle of operation of the system can besummarized as the measurement of the variation of thestate of polarization of the signal emitted by the laser
source along the optical fiber. The state of polarization(hereafter called SOP) can undergo changes over time
due to a well-known property of the materials calledbirefringence. Birefringence in the optical fiber iscaused by geometric asymmetries of its profile and ishighly susceptible to mechanical stresses such as
vibrations and distortions along the fiber path. The

3 Experimental validation
3.1 Description of the experimental setup
To assess the effectiveness of the system, a physicalscale model of a slope was built in the laboratory. The
model consists of a phenolic plywood base andplexiglass sides with a size of approximately 3 m inlength and 0.7 m in overall width (Figure 2).
For the experimentation carried out, a geometricconfiguration characterized by a uniform inclination
equal to about 30° with respect to the horizontal wasused. Above the wooden top, a layer of soil mixed with2

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Fig. 2. Cross section and plan view of the lab-scale model with an indication of the different optical fibers installation schemes (T, Uand Z).

sand of about 10-12 cm thickness was subsequently
created, lying within a layer of geotextile (Figure 2).
The latter has the function of protecting theunderlying wooden surface, facilitating the compactionof the mixture of soil and sand, and avoiding the removal
of material during the simulation of falling materials,
facilitating the execution of the tests. Inside the packageof soil that was made by depositing the material by
successive layers of a thickness of about 2-3 cm,different optical fibers of a length of about 10 m wereburied and arranged in three different geometricconfigurations (U – longitudinal, T - transversal and Z –by zones) and three different depths (1, 5 and 9 cm fromthe surface) of the optical fibers were installed, for a
total of 12 different configurations. Also shown inFigure 2 is a plan diagram of the physical model where
the different optical fibers configuration schemes
adopted are indicated. The green lines represent the Tlayout i.e. optical fibers installed in loops transversal to
the slope of the model, orange lines represent the U
layouts i.e. optical fibers installed parallel to the lateralboundaries both at the center of the physical model (Uc)
and along one side (Ul), while magenta lines represent
the Z fibers i.e. optical fibers installed in separate loops(by zones).
The fibers are connected to the optical source (i.e.
laser) and at the fiber output, light is coupled into apolarimeter to measure the polarization state with acertain sampling frequency fs. The samples are thenpassed to a PC where the signal processing is completed.
Three different types of experiments have been
carried out over the slope to emulate the mass
movements:- Cylinder (C): the downfall of a test cylinder made ofplastic material, 15 cm long and with a diameter of 5 cm.
- Single rock (SR): the downfall of a single rock with amean diameter of 8 cm.
- Debris (D): the downfall of 25 rounded rocks with amean diameter of about 5 cm.

The “background noise” on measurements has also
been acquired when no event was generated, to analyze
the rest conditions, and compare it to the effects of theevents in the model.
More than 1500 different tests were carried out anda total of about 5 Gb of data were collected.
3.2 Results and discussion of the tests
The typical results of the tests carried out can besummarized as the variation of the state of polarizationin the time domain (i.e. the duration of each test) as
shown by the blue line in Figure 3. The raw signals arecharacterized by complex patterns in frequencies andintensities both for the background noise measures aswell as for the C, SR and D simulations. Given thiscomplexity, the need emerged to adopt a specific realtime data processing that would allow distinguishing ina semi-automated way the patterns generated by themass movements from the background noise and anyother event (for instance a tree falling or the transit ofwild animals or people etc.) not related to thepropagation of debris flows and/or snow avalanches.
Consequently, a new algorithm for the processing of
the measured signals was created. The algorithm
consists of a digital time-moving average filter (Tmov)
that eliminates the noisy patterns from themeasurements and a SOP threshold value (ωth). These
two input parameters need to be defined in theprocessing scheme. Since they cannot be selected apriori, it was necessary to create specific matrices, calleddetection maps, able to synthesize the results in terms ofCorrect Detection (CD), Missed Detection (MD) andFalse Alarms (FA) for the different combination of thetwo parameters. These maps give a fundamental
indicator for the appropriate choice of Tmov and ωth andthey help the operators in minimizing the MDs and theFAs occurrence.

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In Figure 3 a summary of the laboratory test results
is shown for three different values of Tmov and ωth in thecase of the T and the Uc fibers layout. It is shown that asatisfactory combination of the two parameters can beidentified to correctly detect the mass movement.
Finally, a concept of a complete interrogator basedon the previous algorithm was created and tested. The
real-time interrogator performs the measurement of thestate of polarization along the optical fibers using a
specific sampling frequency and applies the algorithm
to identify the mass movement phenomena along theslope based on the selected values of Tmov and ωth. Whenthe threshold value is exceeded, the interrogator
generates, practically instantaneously (within less than
1 second), a specific warning sound and an emulated
traffic light is turned red.

A preliminary economic assessment for theapplication of this technique to a real case study has
indicated that, despite the initial additional costs for theexcavation and burial of optical fibers in the ground, theoverall costs are still comparable with those required forthe deployment of monitoring systems based on
traditional electromechanical sensors.
The advantage of the OPTIALP system with respect
to current detection systems based on a large number of"discrete" traditional sensors is evident in terms ofspatially distributed and continuous sensing, distances(up to 10 km) and areas (several km2), instantaneous
response and high sensitivity, higher wear resistance andrelative ease of installation.
The results obtained in the laboratory will beconfirmed in a relevant real-scale environment, i.e.
along a gully subject to debris flows and snow
avalanches. During the field activities, particularattention will be posed to study the potential scaleeffects between the signal generated by the mass
movements and the natural noise levels of the site.
The research was co-funded by Regione Piemonte under theP.O.R. FESR 2014/2020 Asse 1 Azione I.1b I.2.

Referencesa)

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3.
b)
Fig. 3. Summary of the results of the laboratory tests in termsof CD, MD and FA for three different values of Tmov and thand two different layouts of the optical fibers: T (a) and Uc(b).

4.
5.

4 Conclusions
A new cutting-edge debris flow and avalanche detection
system, called OPTIALP, was developed. The proposedsystem exploits the polarization variations induced on
the fiber by mechanical vibrations, for the automatic
detection along their propagation path of potentiallydestructive snow and soil-water mixtures.
The results of the experimental campaign at thelaboratory scale obtained so far are highly encouraging
and promising since the tests showed that the OPTIALP
system is able to correctly identify the signals producedby lab-scale mass movements. Moreover, the correct
identification of the mass movements seems to be
independent of the depth and the scheme of theinstallation of the optical fiber. This implies that on thefield it would be possible to adapt the path of the opticalfiber to the morphology and characteristics of thespecific site.

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