E3S Web of Conferences 415, 03007 (2023)
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https://doi.org/10.1051/e3sconf/202341503007

Debris-flow detection for early warning purposes: recent
advances, open problems, and future challenges
Velio Coviello1*
1Research Institute for Geo-Hydrological Protection (IRPI), National Research Council (CNR), Padova, Italy

Abstract. 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 totheir flexibility and due to the new paradigm of smart sensor networks, proposed as a tool to monitor andgather 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 contributelimiting the adoption of EWSs in future and the possible benefits of communication and dissemination.

1 Introduction
Debris flows are among the most dangerous natural
hazards that threaten people and infrastructures in the
Alpine region due to their rapid motion and transportcapacity [1]. Starting from the 1960s, many efforts andinvestments have been devoted to build channel control
structures such as check-dams and retention basins allover the Alps. However, such structures are expensive
to build and to maintain because trapped sediments mustbe periodically removed. In addition, in narrow Alpine
valleys there is not always enough space forconstructing new channel control structures, which also
have a negative impact on mountain landscapes. The
current climatic change represents a further challenge
for the design of new mitigation measures. In the near
future, prolonged heat waves and extreme rainfall events
will likely increase both the intensity and the frequency
of debris flows.
Non-structural mitigation measures can considerably
decrease the impact of debris flows [2]. Long-term
datasets collected in catchments characterized byrecurrent debris-flow activity (i.e., at least 1-2 events peryear) are essential to improve hazard maps, to validate
the outcome of debris-flow models and to design early
warning systems (EWSs). A growing number ofscientists and local authorities are dealing with the
instrumental detection of debris flows with distributed
sensor networks. In particular, compact and low-costseismo-acoustic sensors represent a unique opportunity

to gather a continuous stream of data representative ofthe different processes occurring in the basin. In thiswork, I present an overview on the recent advances,
open problems, and future challenges in the field ofdetection of debris flows for early warning purposes.

2 EWSs for debris flows
Rainfall thresholds may have practical applications in
the determination of risk levels for shallow landslides
and debris flows at regional scale [3]. However, rainfall
thresholds are affected by significant uncertainty due tothe spatial variability of the rainfall field and because of
the heterogeneities in geology, climate, and land use thatcharacterizes mountain catchments [4]. Therefore,
EWSs based on rainfall thresholds may produce a large
number of false alarms and they still do not meet theperformance level required for effective applications at
the single catchment scale (Figure 1).
Operational EWSs are often based on the instrumental
detection of the debris flow upstream of a definedvulnerable site [2]. These EWSs typically consist insensors deployed within the channel (flow stage sensors
and video cameras) detecting the flow passage directly,
or on sensors deployed nearby the channel (seismic andinfrasound sensors) able to detect the flow remotely
through its elastic energy radiation in the ground and in
the atmosphere. They have a much smaller lead timethan systems based on rainfall thresholds but can be very

* Corresponding author: velio.coviello@cnr.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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effective in terms of detection rate (Figure 1). EWSs
making use of in-channel sensors are highly reliable but
typically require strong efforts for installation andmaintenance and in most of the cases result in a very
short lead time. EWSs based on seismo-acoustic sensors
permit to extend lead time by detecting the flow higher
up in the channel [5], [6]. Most of them adopt geophones
or the combination of a single geophone and an
infrasound microphone nearby the channel [7], [8].
Seismo-acoustic techniques can be very useful for risk
management, not only because they provide an early
detection of the debris flow but also allow estimating its
magnitude [8], [9]. Pulse doppler radars are employed tomeasure the flow velocity and detect intense rainfall but
their diffusion is limited as they require high computing
power and large storage capacity [2], [10].

acceptable [12]. However, the definitive closure of alinear infrastructure is always decided – at the best of
my knowledge – when the debris flow is detected by asensor network installed along the channel.
Table 1. Operational EWSs for debris flows in Italy,
France, Switzerland, and Austria. C contact sensors, R rain
gauges, H flow-stage sensors, G geophones, F infrasoundsensors, V videocameras, D pulse doppler radar.
Element at
risk
Road,
housesRoad

Sensors

Reference

C, R, H,
G, V
C, G

[12]

Road

C, R, V

Rochefort (AO), IT

Road,
housesRoad

R, H, S,V
G, V

Pasquini,
2018
[13]

Bouvaz (AO), IT

Road

C, V

[14]

Pont du Teu (AO),IT
Baudier (AO), IT

Road

C, V

[14]

Road

C, V

[14]

Regoud (AO), IT

Road

C, V

[14]

Berruard (AO), IT

Road

C, V

Bellet (AO), IT

Road

C, V

Claret, FR

C, V
C, V

[15]

C, V

[15]

Boscodon, FR

Road,
RailwayRoad,
RailwayRoad,
RailwayRoad

Segor,
per.com.
Segor,
per.com.
[15]

C

[15]

Merdaret, FR

Road

C

[15]

Arbonne, FR

C, H

[15]

C

[15]

C

[15]

C

[15]

C

[15]

Bondasca, CH

Road,
RailwayRoad,
RailwayRoad,housesRoad,housesRoad,housesRoad

C, R, V

[16]

Carrerabach, CH

Railway

H, V

[17]

Rotgraben, AT

Railway

G, F, V

[18]

Kühgraben, AT

Railway

G, F, V

[18]

Rosensteinergraben,AT
Masonbach, AT

Road

D, V

Railway

D, V

Kaunertal, AT

Road

D, V

Koschuch,per.com.
Koschuch,per.com.
Koschuch,per.com.

Torrent
Rovina di Cancia(BL), IT
Rotwandbach (BZ),
IT
Rabbia (BS), IT
Grissiano (BZ), IT

Fig. 1. Performances vs lead time of the most commonly
sensors employed in EWSs. I refer to the detection of a single
event, i.e. the main front. In case of secondary waves, theperformances of direct-contact sensors dramatically reduce.

Saint-Martin, FRSaint-Julien, FR

Despite the recent advancements and the growinginterest on non-structural mitigation measures among
partitioners and local authorities, very few successstories exist of EWSs operational for long periods andtrusted by local authorities. In most cases, contact
sensors such as trip wires and pendulum are employed(Table 1). This choice is consistent with the old rule“simpler is better” but it also descends from the need of
the non-specialist decision maker to see (and show) thatsomething solid is controlling the EWS [11]. In somecases, operational EWSs are based on both themonitoring of meteorological parameters, typicallyrainfall, and the detection of the debris flows after they
have started by means of geophones, flow stage sensors,and video cameras. However, in most cases the element
at risk to protect is a linear infrastructure such as a roador a railway crossing the torrent, thus the alarm is basedon the information provided by contact sensors andgeophones are employed (Table 1). This highlight how
most local authorities do not like the uncertainties thatare typical of critical rainfall thresholds and prefer tomanage a shorter lead time but a lower number of false
alarms (Figure 1). Progressive levels of alert –corresponding to different actions to take – have been
proposed to make the uncertainties of rainfall thresholds

La Ravoire, FR
Nant del’Armancette, FR
Abéous, FR
Riou Sec, FR

2

[11]

[14]

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4 Structural measures vs EWSs

A possible improvement of rainfall thresholds can beachieved by means of observations collected in thedebris-flow source areas with other instruments.
Recently, the analysis of geophone data and video
recordings show that different natural sources of groundvibration can be automatically classified and thehydrologic response to rainfall events in the initiation
area of the catchment can be used to improve thewarning performances of rainfall thresholds [19].

EWSs are often considered as an alternative – also less
impacting on landscape – to expensive structural
mitigation measures. Actually, the choice of the
mitigation measures to realize is often related to social
parameters such as composition of the local community
and their education to risk. As the extreme case of
Cancia teaches, the relocation of settlements is hardly
acceptable even if they fall into the debris-flow path[12]. On one hand, filter check dams can attenuate theseverity of debris flows by reducing the flow velocity
and retaining part of the volume before it reaches theareas at risk but, on the other, they alter the natural
sediment transfer and require regular maintenance. In
addition, the positive effects in terms of hazards
reduction could be not sufficient in case of extreme
rainfall events causing the debris flow to exceed themagnitude of the design event. This is particularly
relevant in Alpine valleys hosting multiple debris flow
channels that can be activated by the very same
meteorological event. Therefore, warning systems
would help manage the residual risk that results from thedecrease of effectiveness of structural measures. Even
when the performance of structural measures issatisfactory, EWSs can contribute to risk management
by providing real-time data on the evolution of theprocesses and on the structural response of criticalstructures like debris-flow breakers. EWSs could
suitably integrate structural mitigation measures inmany cases but in many cases the fragmentation ofcompetences and responsibilities among local andregional authorities do not contribute to their diffusion.

3 Seismo-acoustic detection
Passive, seismo-acoustic techniques are attractive
because they can provide monitoring data at a safe
distance from the active channel. In particular, sensors
such as geophones and microphones represent a unique
opportunity to monitor large areas with a dense and lowcost sensor network. On the other hand, instrument
installation, maintenance and data analysis require
efforts and skills in signal processing. Consequently, theemployment of seismo-acoustic sensors is experiencing
a growing interest among scientists but local authorities
still hesitate to adopt them in operational EWSs.
There are three approaches that have been proposed forthe seismo-acoustic detection of debris flows:
1. linear array of geophones,
2. network of seismometers,
3. array of infrasound sensors.
A linear array of geophones installed along a debrisflow channel was proposed as a possible tool for early
warning in the Moscardo basin [20]. Geophone data
demonstrated that the debris flow can be identified byanalysing the variability of signal amplitude. Recently,
this approach was further developed in the Gadria basin,
where an automatic algorithm based on the short-time
average over long-time average ratio (STA/LTA) of theseismic signal was used to early detect a debris flow ina continuous stream of seismic data and to filter out
different seismic sources [7]. In general, a linear arrayof geophones is effective in protecting specificvulnerable sites located on the alluvial fan. Greater the
distance array – vulnerable site, greater the lead time.
Seismometer commonly employed for tectonic andvolcano monitoring also detect signals produced bylahars [21], [22]. Arrival-time methods leveraging data
gathered at multiple stations have been used for tracking
seismic sources produced by debris flows at the basin
scale [6]. Recently, machine learning algorithms havebeen proposed to identify initial sediment mobilizationand extend warning times at Illgraben, Switzerland, but
their application in other locations is still limited due tothe need of training datasets collected by a dense
network of seismometers [23].
Low-frequency sound monitoring, typically in theinfrasound (<20 Hz) band, is an emerging technique fordebris-flow detection. Array of infrasound sensors andmicrophones can be installed at hundreds meters fromthe trajectories of debris flows [5]. Also, thecombination of a low-cost microphone with a geophone
installed at few meters from the active channel has been
proposed as a compact and single-station solution fordebris-flow detection [8].

5 The “information paradox”
The knowledge of debris-flow processes and thedevelopment of automatic detection systems are
sufficiently advanced to justify EWSs to be operational
in many real cases. Most limitations to the large-scale
adoption of such systems are due to the “responsibility
issue”. Who is responsible for issuing the alarm when apossible debris flow is detected? How many false alarms
are acceptable, and which is the minimum lead timerequired for a specific infrastructure? Which actions andcommunication strategies correspond to the different,
possible alert levels? From the (many) possible answers
to these questions descend what I call the “information
paradox”. The information gathered with sophisticateddetection systems (i.e., seismo-acoustic detection) might
be considered a problem instead of an opportunity bydecision makers if end-users are not informed about
debris-flow risk and they are not involved in theimplementation of warning protocols. This can also
produce negative feedbacks in terms of more demand
for structural protection measures from the local
communities and thus more constructions in future (i.e.,kind of levee effect), even though new constructions inhigh-risk areas are not recommended.
How to deal with that? More efforts in communication,dissemination, and promotion of the model of “shared
responsibility” from the scientific community would
surely help. This can also be considered a political3

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priority for the near future given the urgent need ofsaving space and resources in the Alpine region and the
expected growing debris-flow risk in areas consideredsafe so far due to the increasing impact of extreme
meteorological events produced by climatic changes.

[10]

Acknowledgements. I thank many colleagues for the fruitful
discussions and for providing precious information about
operational EWSs: Massimo Arattano, Matteo Berti, Marco
Cavalli, Francesco Comiti, Sandro Gius, Marcel Hürlimann,
Richard Koschuch, Pierpaolo Macconi, Lorenzo Marchi,
Guillaume Piton, Andreas Schimmel, Valerio Segor.

[11]

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