E3S Web of Conferences 415, 03008 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341503008

When instrument location makes the difference on rainfall
thresholds definition: lessons learned at Cancia, Dolomites
Stefano Crema1*, Velio Coviello1, Matteo Cesca2, Roberta Dainese2, Lorenzo Marchi1, Alessandro Pasuto1, and Marco
Cavalli1
1CNR-IRPI National Research Council, Research Institute for Geo-Hydrological Protection, Corso Stati Uniti, 4 , 35127, Padova,Italy
2ARPAV, Regional Agency for Environmental Protection and Prevention of Veneto, Regional Department for Land Safety, via

Tomea, 7, 32100 Belluno, Italy

Abstract. 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 ofdebris-flow triggering events and thus relying on such networks for developing rainfall thresholds mightlead 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 thederivation 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 thesmall 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.

1 Introduction
When dealing with risk management and governance inmountain environments, debris-flow Early Warning
Systems (EWSs) can be regarded as one of the mosteffective non-structural risk mitigation measuresavailable to the authorities [1, 2].
Amongst the most dangerous natural hazards over
the Alpine region, debris flows undoubtedly get a primerole due to their destructive power and their impact onanthropic environment. When debris-flow EWSs arelocated in densely populated or touristic areas, theirreliability must be carefully validated through the years
also considering potential variations of the boundary
conditions such as triggering rainfall input variability,
sediment availability and channel morphology.
In the Dolomites (NE Italy) the most common
debris-flow initiation process is channel bed
failure/erosion due to excess runoff related to intense
summer convective events [3]. Oftentimes in such areas,
rainfall spatial variability manifests severe and localized
gradients that can’t be grabbed by the density of atypical regional rain gauge network [4, 5] and this in turn
indicates the need for accurate and distributed rainfall
measures within the EWS-instrumented catchment. In

the present work, we summarize the EWS upgrade
carried out at Cancia (Dolomites) as a scientificcooperation between regional authorities and research
institutions with a peculiar focus on the rainfall
threshold derivation approach for an improved alarm
reliability thanks to an optimal spatial distribution ofrain gauges within the catchment.

2 Study area, data and requirements
The “Rovina di Cancia” debris-flow catchment (totaldrainage area 2.04 km2, area at the debris-flowformation site: 0.65 km2, see Fig. 1) is located in a
typical dolomitic context where debris flows are
triggered by intense rainfall at the interface between
vertical cliffs and scree slopes or on steep channels that
entrenches the scree.
The study area is known for its frequent debris flows
and the related hazard is enhanced by the missingconnection of the Cancia channel to the receiving Boite
Torrent. The debris-flow channel in fact ends with aretention basin just upstream the Cancia village. In casethe retention capacity of the system is exceeded, theevent can seriously damage a vast area putting in danger
tens of lives [6]. Civil protection measures inconjunction with a monitoring and Early Warning* Corresponding author: stefano.crema@irpi.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/).

E3S Web of Conferences 415, 03008 (2023)
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https://doi.org/10.1051/e3sconf/202341503008

system (EWS) have been installed by provincial
authorities to mitigate debris-flow impact.
The EWS operational since 2014 was structured asfollows (Fig. 1):
 station 1, 2267 m a.s.l. (close to the debris-flow
triggering area), 2 rain gauges
 station 2, 1690 m, 2 level sensors, 2 geophones,
1 rain gauge
 station 3, 1525 m, 2 geophones, 2 trip wires station 4, 1280 m, 2 geophones, 2 level sensors,
2 trip wires station 5, 1080 m , 2 trip wires, 4 level sensors,traffic light on road/bridge and sirens
 station 6, in the village, traffic lights and sirens.
Data recorded by a rain gauge belonging to theregional hydrometeorological network located in thelower part of the catchment (Rovina Bassa, 1335 m a.s.l,Fig. 1) were also analyzed.

rainfall thresholds performance while focusing on the
pre-event warning phase.

Fig. 2. Example of event rainfall representativeness forstations inside (S1 and S2) and close by the catchment(distance in km is reported within the graph). Debris-flow
timing as retrieved by the trip wire activation is also indicated.

In particular, the rainfall thresholds derived in thepresent work take advantage of the availability of threerain gauge stations inside the catchment. The spatial
variability of convective rainfall events can indeed leadto ineffective thresholds when relying on rainfall
estimates computed just outside the catchment or far
from the debris-flow triggering location (Fig. 2) as
already observed in previous studies investigating awider dolomitic area [3].

3 Eventscharacterizationthresholds definition

andDuring the period 2014-2019, four major debris flows
were available for the development of rainfall thresholds(Tab.1).
Table 1. Date and volume estimation of the four major
considered events for the calibration period. Volume
estimation is derived by [7]Fig. 1. Study area and location of monitoring and early
warning stations.

Date

Volume (m3)

23.07.2015
04.08.2015
01.08.201829.10.2018

25000
20000
4500
11000

The last 2018 event represents the well-known Vaia
storm [8] that hit north-eastern Italy with prolonged
intense rainfall and whose temporal pattern is quite
different from the typical very intense and rapid summer
storms that usually trigger debris flows in the study area.
In addition, two minor events were also partially
recorded by the system or by local witnesses, but they
did not reach the retention basin. Major events wereindeed considered with particular attention since they
represent the conditions capable of stressing the wholesystem even if with different degrees of severity.
In the present work, we are not trying to correlate theevent magnitude with the rainfall volume, rather we’reseeking an event threshold that might trigger asignificant response at the catchment scale thus standing

All the stations are radio-linked to the main logic
system that is available to the authorities. Leveraging
the availability of monitoring instrumentation andthanks to the occurrence of several debris flows from the2014 system installation up to 2019, the comprehensive
aim of the EWS was to produce robust thresholds on
rainfall, seismic and flow-level data. Such thresholds
were intended to become operational as an update of theEWS working procedures that were previously relying
on the regional rain gauge network instruments.
In the present paper, seismic and flow-level analyses
remain in the background in respect to the main focus of
the work that wants to tackle rainfall variability and2

E3S Web of Conferences 415, 03008 (2023)
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https://doi.org/10.1051/e3sconf/202341503008

on the safe side of the estimate and at the same timerelying on similar recorded events that stressed thewhole system.
To rely on a large testing dataset, it was decided notto consider only the few rainfall events that triggereddebris flows, but rather to leverage the present rain
gauge configuration in a different way. All rainfall
events were thus automatically detected using aminimum accumulation threshold of 1 mm and a hiatusof 6 hours for separating two consecutive events. Suchvalues are considered appropriate for the test site (andcoherent with similar alpine catchment applications,generating a quite stable number of events [9, 10 andreferences therein] and bearing in mind also that thedebris-flow formation catchment is vastly covered bythick and highly permeable loose debris with no evident
permanent runoff.
Doing so, we obtained more than 800 events perstation (within the working period of the EWS thatusually covers the range between late spring and earlyautumn), and we tried to seek helpful information insidesuch a numerous dataset.

While we could not use debris-flow occurrence time
information (available only for a handful of events) we
could however investigate rainfall properties in therange 5-min/48-h (5, 10, 15, 30, 60 min and 1, 3, 6, 12,24, 48 hours) for each event, showcasing the statisticalproperties of those events that have actually triggered adebris flow. This permits the characterization of rainfall
properties independently from event duration or total
accumulation metrics that may suffer from the choice ofdifferent hiatuses or minimum rainfall depth for event
definition.
For all the analyzed events, debris-flow related
rainfall popped up as an extreme value at least for one
rain gauge location considering the percentile
distribution of the analyzed intervals in respect to theaverage population severity, and this was true from the5-min interval up to the 1-h and 3-h durations for typicalsummer events (Fig. 3). This satisfactory detection
capability can be ascribed to the optimal spatial
arrangement of the rain gauges. The upper twin raingauges (station 1), close to the initiation areas, record
rainfall intensities higher than the other rain gauges
during severe summer convective events (Fig. 3a),whereas the lower rain gauge (Rovina Bassa) better
detects the most severe conditions related to frontal
precipitation events (Fig. 3b). The middle rain gauge(station 2) exhibits an intermediate behavior with gooddetection capabilities of both types of events. Such acombination of “event-sensitivity” has thus the intrinsic
potential of detecting almost the whole spectrum ofconditions that can lead to debris-flow initiation inCancia.
More in general, gauge location is always one of thekey points for reliable thresholds development sincehaving at least one rain gauge close to the catchment
centroid is crucial for a proper spatial rainfall calibrationeven when using distributed measures such as radarderived rainfall maps [11].
In the case of Cancia, after having verified that all
the debris-flow events featured at least one rainfall
intensity interval very close or above the 99 percentileof the dataset, the 95, 98 and 99 percentiles of eachgauge location and each considered time interval wereretrieved from the analyzed populations and integrated
within the operational EWS associated with anincreasing level of pre-alert severity. It is worth
mentioning here that also some previous studies in the
dolomitic area [3] identified rainfall intensity duration
curves that fall roughly in the 98-99 percentile range
thus demonstrating the mutual robustness of the
proposed approaches.
Regarding the seismic dataset, the travel time of themajor debris-flow events was calculated, and a firstorder volume estimation was proposed based on theintegrated seismic energy detected by a single geophone[12]. An amplitude-based event threshold was identified
but, due to hardware limitations though, a dynamic
threshold based on moving windows computation was
not workable thus removing a degree of reliability to the
proposed thresholds. Anyway, the integration within theEWS of seismic data thresholds has the advantage ofproviding an alert in advance and detecting also minorFig. 3. Example of event characterization on a population ofmax intensity per 5-min (a) and 10-min (b) interval per eachautomatically identified rainfall event, data are related tostation 1 (a) and Rovina Bassa (b) gauges. Looking at the 2018
event, it quickly emerges as a very severe contribution from10 minutes up to all the considered durations especially in thelower Rovina Bassa station notwithstanding the frontal long
lasting precipitation type.

3

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events or short travelling ones thus increasing theoverall robustness of the system.

4 Approachperspectives

implementation

2.

S. Segoni, L. Piciullo, S. L. Gariano, Nat. Hazard
Earth Sys., 18 (2018)
3. C. Gregoretti, G. Dalla Fontana, Rainfall
threshold for the initiation of debris flows bychannel-bedfailure in the Dolomites, inProceedings of the Fourth International
Conference on Debris-Flow Hazards Mitigation:
Mechanics, Prediction, and Assessment, DFHM,
10-13 September 2007, Chengdu, China (2007)
4. E. I. Nikolopoulos, S. Crema, L. Marchi, F. Marra,
F. Guzzetti, M. Borga, Geomorphology 221, 286(2014)
5. M. Cavalli, S. Crema, V. Coviello, L. Marchi, C.
Gregoretti, A. Schimmel, F. Comiti, A. Pasuto,
ARPAV tecnical report, pp. 101 (2021)
6. M. Bacchini, A. Zannoni, Nat. Hazard Earth Sys.
3, 71 (1999)
7. A. Simoni, M. Bernard, M. Berti, M. Boreggio, S.
Lanzoni, L.M. Stancanelli, C. Gregoretti, Earth
Surf. Proc. Land. 45, 3556 (2020)
8. S. Davolio, S.D. Fera, S. Laviola, M.M. Miglietta,
V. Levizzani, Mon. Weather Rev. 148, 3571
(2020)
9. L. Marchi, F. Cazorzi, M. Arattano, S. Cucchiaro,
M. Cavalli, S. Crema, Nat. Hazard Earth Sys. 21,
87 (2021)
10. J. Hirschberg, A. Badoux, B. W. McArdell, E.
Leonarduzzi, P. Molnar, Nat. Hazard Earth Sys.
21, 2773 (2021)
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Earth Sys. 22, 1955 (2022)

andThe proposed thresholds together with related
operational measures (SMS with increasing alert level)
were tested during the 2020 season leading to a positive
evaluation of their performance. Rainfall severity wasclassified by percentile rank with different levels of preevent warning related to the 95, 98 and 99 percentiles.
Operational measures took also into consideration a
pre-event condition and a post-event one (trip wiresinterrupted) allowing the seismic and level values todirectly issue a warning in the post-event scenario. In
particular, during the 2020 season only a few minor
events occurred in the catchment (July, 1st, August 20thand 29th). Such events were characterized during postevent field surveys and monitoring cameras image
differencing since they did not have a magnitude
capable of stressing the whole system and deliveringsignificant amounts of debris to the channel outlet.
Notwithstanding this, the developed thresholds
revealed their effectiveness also in the detection of suchminor events proving their reliability without raising, forthe 2020 testing season, any false alarm. As a future
perspective, the continuously increasing rainfall events
population could be further analyzed so as to verify
which statistical property (or other paradigms such as
peak-over-threshold approaches) might better represent
debris-flow triggering rainfall and to attain even morerefined initiation threshold.
Finally, the collaboration among authorities,
decision-makers and researchers on such issues can beregarded as the way forward, helping also in thetransition from a catchment-scale EWS perspective
towards a more comprehensive valley-scale risk
management. This is true especially in light of theongoing investments in the area for the first European
smart road (Cortina 2021 and current developments,details at: https://www.anaspercortina2021.it/smartroad-cortina-2021) that will pave the way for a newgeneration of resilient infrastructures capable of sensing
the environment and broadcasting warnings at the onsetof an hazardous event.
Acknowledgements. The present research was carriedout in the framework of a research agreement among
ARPAV, CNR IRPI and the Universities of Padova andBolzano for the assessment of warning thresholds for themonitoring and warning system of Cancia debris flowand financed by Veneto Region.

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