E3S Web of Conferences 415, 03004 (2023)
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https://doi.org/10.1051/e3sconf/202341503004

Characterization of a debris flow event using an affordable
monitoring system
Matteo Berti1*, Andreas Schimmel2, Velio Coviello3, Mario Venturelli1, Luca Albertelli4, Luca Beretta5, Francesco
Brardinoni1, Massimo Ceriani5, Marco Pilotti6, Roberto Ranzi6, Marco Redaelli5, Riccardo Scotti7, Alessandro Simoni1,Laura Turconi8, and Fabio Luino8
1 University of Bologna, Department of Biological, Geological, and Environmental Sciences (Italy)
2 Andreas Schimmel - Alpine Monitoring Systems (Austria)
3 National Research Council, CNR-IRPI Padua (Italy)
4Land & Cogeo s.r.l. (Italy)

5 Civil Protection Agency, Lombardia Region (Italy)
6 University of Brescia, Department of Civil, Environmental, Architectural Engineering and Mathematics (Italy)
7 Servizio Glaciologico Lombardo, Lombardia Region (Italy)
8 National Research Council, CNR-IRPI Turin (Italy)

Abstract. 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 wasrecorded by a monitoring station installed just few weeks before. The monitoring system was deployed todocument 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 thedebris flow revealed a peculiar frequency spectrum compared to regular streamflow. These results show thateven a relatively simple monitoring system may provide valuable data on real debris flow events.

1 Introduction
Debris flows are complex processes characterized byunsteady, non-uniform surges of a multiphase material[1, 2]. Because of this complexity, making progress on
the understating of debris flows requires data on realevents [3]. Monitoring data are necessary to study debris
flow dynamics, develop and calibrate prediction models,design effective warning systems, and ultimately
mitigate the hazard [4]. Unfortunately, the cost andlogistic requirements of most monitoring systems isgenerally very high. High costs mostly depend on thepurchase of specialized equipment (rugged sensors,loggers, and communication devices designed to operate
in harsh environments), but also on installation costs [5,6]. Installation works include the realization of complex
support structures to hang sensors over the channel, as
well as the transportation of material in dangerous
places and the employment of highly skilled team. For
these reasons, debris flow monitoring is generally
undertaken within large research projects [7, 8], making
data from real events still limited.
In this work we present an affordable monitoring systemdesigned to collect data from real debris flow events.
The system is relatively simple, lightweight, andrelatively inexpensive, and was installed in July 2021 in

the Blè basin (Lombardia Region, Central Alps of Italy)
to document the basin response to rainfall. On August16, a large debris flow was triggered by an intenserainfall. The monitoring system detected the event andcollected valuable data on flow behaviour.

2 Study area
The Blè basin (Fig. 1) is located in the Val Camonica
valley, in the Central Italian Alps (Lombardia Region).
The elevation ranges from 2358 m at the ridge to 360 m
at the confluence with the Oglio River. The basin has adrainage area of 5.6 km2 and it is dominated by Triassic
carbonate rocks [9]. The bedrock forms massive rockcliffs in the upper part of the catchment, while themiddle and lower part the basin is blanket by Quaternary
talus deposits. Debris flows are initiated at the rock-talus
contact by surface runoff. Here, the overland flow
generated over the rock slopes impacts the loose debris
stored in the hollows or at the base of cliffs, causing theprogressive entrainment of the material.
The Blè Stream has a long history of debris flows as
evidenced by the volume of its alluvial fan, which has
slowly formed over the last few thousand years. In
recent decades, the events of August 1950 andSeptember 1960 are remembered.

* Corresponding author: matteo.berti@unibo.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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https://doi.org/10.1051/e3sconf/202341503004

Fig. 1. Schematic map of the Blè basin

Debris flows generally occur during the summer season(from June to September) after high-intensity shortduration rainfall. Apparently, the frequency of debris
flows in the Blè basin is increasing. Three events
happened in the last four years: August 25, 2018 (20.000m3), July 27, 2019 (20.000 m3), and August 6, 2019(100.000 m3), after a period of long dormancy.
Although these debris flows remained into the channel
causing limited damages, much larger events are
possible in case of channel barrage. This can happen if
the La Tavola landslide (Fig. 1) will collapse damming
the channel and forming a lake. At present, the landslide
moves slowly feeding material to the channel, but astrong increase of debris flow activity might destabilize
the slope by undercutting its toe.

geophone (Mark Products L-4 Seismometer); 1
DataCube seismic data recorder; 1 tipping-bucket rain
gauge (0.2 mm per tip); 1 Campbell CR200 datalogger;
1 time-lapse Brinno camera TLC-200. The unit runs
continuously at constant speed. Rainfall is measured
every 5 min by the CR200 datalogger, while groundvibrations are recorded by the DataCube at 200 samples
per second. The Brinno camera takes a picture of thechannel every 10 min.
The Mamodis unit (U2) consists of: 1 infrasound sensor(modified differential pressure sensor); 2 verticalgeophones 4.5 Hz; 1 Raspberry Pi Zero board with Pi
camera module; 1 microcontroller with a 50-MHz ARM
Cortex-M3 microprocessor; 1 Internet Stick. This unit
operates as debris flow detection system. The geophone
and infrasound data are recorded at 100 Hz and innormal operation mode the Pi camera takes a photo of
the channel every 15 min. These pictures, event data andhourly status messages of the system are sent to a webserver (http://mamodis.ddns.net/). When a debris flow is
detected by the internal detection algorithm [10], awarning message is sent via SMS and the camera isstarted to record a video at 10 fps for 1 hour.
The power to both units is supplied by two 12V 18Ahbatteries in parallel recharged by a 40 W solar panel. Alldata are stored on-site and periodically retrieved by
GPRS connection.
Basically, the monitoring system is designed to collecthigh-frequency ground-vibration data suitable tocharacterize the flow processes that occur in the channel.
The type of process (regular streamflow, streamflow
with bedload transport, or debris flows) can be inferredby observing the time-lapse pictures and the videos. The
videos also allow to derive the surface flow velocity andestimate the flow rate.

3 Monitoring system
The monitoring system was designed to fulfil several
specific requirements:- detect the passage of a debris flow
- collect data to estimate flow depth and velocity- have a low power consumption- be lightweight and quick to deploy in the field (without
the need of heavy support structures)- be easy to remove at the end of the summer (to avoid
possible damage during the winter season)- be relatively inexpensive.
To achieve these goals, we combined video cameras
with geophysical sensors and optimized the energy
consumption to minimize the power supply system.
The system is made of two main components: a groundvibration unit (U1) and a MAMODIS infrasound unit(U2) [10].
The ground-vibration unit (U1) consists of: 2 vertical
4.5 Hz geophones (Pasi SIS-911-050); 1 vertical 1 Hz

The monitoring system was installed in the lower reachof the Blè Torrent on 2 July 2021 (Fig. 1). The site waschosen to have a broad view of the channel, a full2

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sunlight exposure, and (apparently) a stable bank. All
the equipment was mounted on a single pole (2 m, 50mm diameter) with the base embedded in a 30x30 cm
cockpit filled with rocks and concrete (Fig. 2). The
geophones were installed in a straight line parallel to thechannel, at a distance of few meters from the bank.

datalogger, the DataCube, and the Raspberry Pi Camera
survived. The data retrieved from these loggers provideda detailed documentation of the event.
Rainfall at the monitoring spot started at 17.50. At thattime little streamflow was present in the channel. The
thunderstorm cell strokes the upper basin at 18.10(according to the weather radar); five minutes later thephoto taken by the monitoring system showed asignificant increase of surface runoff and water
turbidity. At 18.19.22, the MAMODIS system detecteda debris flow and started the video recording(https://www.youtube.com/watch?v=hW2TGFB3040).
In the meantime, the rainfall intensity at the monitoring
point increased reaching a peak of 13 mm in 5 minutes.
The debris flow passed in front of the station at 18.27.25,
8 minutes after the detection. The flow was fast,turbulent, with a steep bouldery front followed by amore dilute body. Three main surges occurred in the first
10 minutes, then the flow began to subside. The
lowering of the flow level revealed the strong erosion
caused by the debris flow along the bed and on the sides.
The undercutting destabilized the channel bank, causing
the collapse of the slope at 18.38.
Fig. 3 shows the data recorded by the 1 Hz geophone
installed at the base of the station and by the 4.5 Hz
geophone placed 30 m upslope. Both sensors show a
sharp increase of the vibration intensity caused by thepassage of the debris flow. The seismic signal starts torise when the debris flow passes in front of the camera,
apparently without any gradual increase before. A
background noise of about 0.05 mm/s due to waterrunoff is visible in both signals during the 30 min
preceding the debris flow.

Fig. 2. Picture of the monitoring station installed in the Blèbasin (location in Fig. 1).

The 1 Hz geophone and one 4.5 Hz geophone of theMAMODIS unit were placed close to the station, while
the other three 4.5 Hz geophones were placed
respectively 30 m downslope and upslope, in order tomeasure the average front velocity over about 60 m.
Installation took about 4 hours by three people and did
not require any heavy equipment.

4 The 16 August 2021 event
On August 16, 2021, a debris flow occurred in the BlèTorrent. The event was triggered by an intense rainfallof 40 mm in 1 hour caused by a localized storm cellmoving west to east. The debris flow mobilized avolume of about 60.000 m3 and travelled along thechannel for nearly 2.3 km. The front stopped at anelevation of 580 m (about 1 km downstream themonitoring spot) covering a local road with debris andlarge boulders.
The day after the event we went to the site to survey thedebris flow and download the data collected by themonitoring system. Once in place we realized that alarge bank failure wiped out the station dragging all theequipment into the flow. Bank stability is difficult topredict, but our mistake was not having tied the station
to a stable point (a tree or a rock) with a steel cable, toprevent collapse into the channel. Luckily, most of theinstrumentation was found by the Civil Protection
volunteers about 170 m downstream the monitoring
spot, within a debris flow lobe deposited into thechannel. The solar panel, the power box, and theprotective cases were almost destroyed, but the CR200

Fig. 3 Data recorded by the 4.5 Hz geophone (upper) and 1
Hz geophone (lower) during the event.

The sensors also detect the multiple surges observed inthe video. Preliminary analyses of the frequency
spectrum show that the seismic signal generated by thedebris flow is characterized by a low-frequency
component (10-30 Hz) almost absent in regularstreamflow. A detailed analysis is underway toinvestigate the change of spectral frequency during theflow and compare our findings with published results.

3

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https://doi.org/10.1051/e3sconf/202341503004(Brescia): field monitoring of the Val Rabbia and Blè debris
flow catchments.”.

A second preliminary analysis was done to estimate thevelocity and peak discharge of the debris flow based onvideo images. To this purpose we combined two freeMatlab tools: PIVlab, that computes the flow vectors
based on particle image velocimetry analysis [11], andRIveR, that processes the flow vectors to estimate theflow rate making simple assumptions on the verticalvelocity profile [12]. The analysis requires the crosssection channel geometry, which was obtained bycomparing two accurate Digital Elevation Models
created by drone photogrammetry before and after theevent. Despite the uncertainties, the analysis provides
realistic results. The computed peak velocity is 4.4 m/s
for the first surge (from 18.27.24 to 18.27.33; Fig. 4a)and 5.4 m/s for the second surge (from 18.29.44 to18.29.40). In both cases the surface flow vectors show
the parabolic profile expected for open channel flow
(Fig. 4b).

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Fig. 4. Results of the Particle Image Velocimetry analysiscarried out for the first debris flow surge. a) flow vectors; b)peak flow discharge.

The estimated peak discharge is 224 m3/s and 227 m3/s
respectively. These values roughly agree with thosepredicted by the empirical relationship between peakdischarge and total volume proposed by [13], that for a
60.000 m3 debris flow indicates a peak discharge
ranging from 100 to 700 m3/s depending on flow
composition.
This work was funded by the Lombardia Region under theproject “Dynamics of debris flows in Val Camonica valley4