E3S Web of Conferences 415, 03005 (2023)
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https://doi.org/10.1051/e3sconf/202341503005

A model-based early warning system for runoff-generated
debris-flow occurrence: preliminary results
Federico Cazorzi1, Matteo Barbini2, Alberto Beinat1, Martino Bernard2, Mauro Boreggio2, Matteo Cesca3, Sara
Cucchiaro1, Roberta Dainese3, Alberto De Luca1, Christian Demmler4, Carlo Gregoretti2, Karl Hagen4, Veronika
Lechner4, Eleonora Maset1, Michael Neuhauser4, Paolo Nicolosi3, and Christoph Zingerle5
1University of Udine, Department of Agricultural, Food, Environmental Sciences, 33100 Udine (UD), Italy
2University of Padova, Department of Land, Environment, Agriculture and Forestry, 35020 Legnaro (PD), Italy
3ARPAV, Regional Agency for Environmental Protection and Prevention of Veneto, Regional Department for Land Safety, 32100

Belluno, Italy
4Austrian Research Center for Forests, Department of Natural Hazards, Seckendorff-Gudent-Weg 8, Vienna, Austria
5Zentralanstalt für Meteorologie und Geodynamik (ZAMG), Division: Customer Service Tyrol and Vorarlberg, Fürstenweg 180,6020 Innsbruck, Austria

Abstract. Early warning systems for debris flows are low cost measures for mitigating this kind of hazard.
The early warning systems provide a timely alert for upcoming events in order to take protective measures,such as closing railways-roads, evacuating people from the threatened areas, and put rescue forces into
readiness. These systems usually are sensor-based, and the alert time is the interval between the timing of
the first detachment of debris flow by a sensor and its arrival into the threatened area. At the purpose ofincreasing the alert time, we propose an early warning system based on a model-cascade: nowcasting,hydrological- and triggering models. Nowcasting anticipates rainfall pattern that is transformed into runoff
by the hydrological model. The triggering model estimates the volume of sediments that the runoff canentrain, and compares it with a critical threshold. If this is exceeded the alert is launched. The proposed early
warning system is tested against the available data of the Rovina di Cancia (Northeast Italy) site.

1 Introduction
Early Warning Systems (EWS) are basically integrated
systems of forecasting, hazard monitoring andcommunication, enabling organisations to take timely
actions to reduce disaster risks as minimizing loss of lifeand damages in case of hazardous events. In essence, theEWS provides an alert on a future or on ongoing event[1]. The increasing occurrence of extreme precipitation
causing debris flows, induced by climate change effects,
points to a wider and wider use of EWSs as mitigation
measure for the risk management [2]. This system has
no marked negative effects on landscape or ecology, and
it is more flexible than structural countermeasurescharacterized by high costs, that for this reason can be
implemented only with limitations [3,4]. An EWS for
debris-flow occurrence is usually sensor-based, and it iscomposed by a monitoring system linked to acommunication system. When the monitoring system
detects an ongoing event, the communication systemprovides the alert to the managing authority and/or
directly to the population by means of sirens and traffic
lights on the vulnerable roads. The monitoring system
usually consists of one or two types of sensors: i) sensors
recording the rainfall that triggers the debris flow, suchas rain gauge and weather radars, and ii) sensors
detecting the transit of the debris flow: trip wires,geophone, and flow level sensors (radar, laser, acoustic,pressure transducers). The first type of sensors canspread the alarm about 10 to minutes on average before
the debris-flow trigger (the time for runoff generation
depends on terrain characteristics and morphology as
well as by moisture conditions and moisture) bycomparing the measured precipitation with an intensity
thresholds. Conversely, sensors of the latter type canprovide the alert between 1 to 20 minutes, on average,
before the debris flow impacts the threatened area. As
the alert time (i.e. the interval between the debris-flow
detection and its reaching areas the threatened area) is
crucial to take adequate civil protection measures, it
should be extended as much as possible. The sensorbased EWS has the limit of the alert time that is imposedby lag time between rainfall occurrence or debris-flowtransit detection, and debris-flow arrival into the area toprotect. In order to increase the alert time, [5]successfully used a 1h-nowcasting system based on
portable weather-radar for the identification andtracking of storm cells. When the rainfall intensity of theidentified cells exceededs the threshold value for debrisflow occurrence, they were able to launch the alert at
least one hour in advance. The solution adopted by [5]can be used if the identification and tracking of cellstorms is possible. In order to increase the alert time ineach situation, the Interreg Italy-Austria program
funded the project INADEF (Innovative eArly warning
system for Debris Flows, 2019-2022) to develop a EWS
based on a cascade of models that provides the rainfall© 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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pattern, the generated runoff volume andcorresponding sediments volume of debris flow.

https://doi.org/10.1051/e3sconf/202341503005

thechannel length from the initiation area to the threatenedarea). The EWS operates at two levels: it forecasts thedebris-flow occurrence and estimates the event
magnitude. The first level corresponds to thecomparison of runoff discharge with the criticalthreshold, while the second level derives from thecomparison of the computed volume of sediments witha threshold. This threshold is the value of the sedimentvolumes of the debris flow that stops just upstream of
the threatened area and it is site-dependent. When eachthreshold is exceeded, the corresponding alert islaunched (Figure 1 vii.).
Since the model cascade needs to be executed for each
nowcasting timestep, an online server was selected as
the best solution to make the EWS operational.
Therefore, a Web-Gis Application (WGA), wasdesigned and implemented, with a back end in charge oforchestrating the model’s cascade, and a front end todisseminate the simulation results (Figures 2 and 3).
The system operates as follows (Figure 1): i) INCA
uploads updated 6-hours rainfall nowcasting every 15
minutes on the INADEF server. ii) When a new
nowcasting is uploaded, the WGA extracts theinformation it needs from the nowcasting grids and iii)
feeds it (along with other information about the studysites) to the hydrological model ZEMOKOST). iv) The
results of the hydrological model are used as inputs forthe triggering model (v), and vi) the results of thetriggering models are analysed. When a debris flow is
forecasted for a site vii) the web app displays a warning,
on a map and notifies selected recipients via e-mail.

2 The proposed model-based
EWS
The proposed EWS for debris-flow occurrence iscomposed by a cascade of models integrated into a web
application (Figure 1). The cascade of models simulate
the rainfall pattern six hours in advance, nowcasting therunoff generation and the triggering of debris flow withan estimate of the potential sediment volumes. The
nowcasting is provided by INCA (Figure 1 i.) [6], amulti-parameter analysis and nowcasting system formeteorological parameters, developed by ZAMG.
Based on the spatial and temporal pattern of
precipitation (Figure 1 ii.) the runoff is calculated withthe conceptual precipitation-runoff model ZEMOKOST(Figure 1 iii.) [7]. According to the channel network,
sub-catchments are defined and information onroughness, runoff coefficient, and terrain specifications(area, inclination, etc.) are surveyed.
For each sub-catchment, the calculated hydrograph isforwarded to the sub-catchment area below until the last
node in the spatial description is reached (Figure 1 iv.).
The model provides the input for the debris-flow
triggering model (Figure 1 v.), discharges in m³/s every
minute, and for each sub-catchment.

Fig. 2. The web app front end showing the INCA rainfall
nowcasting maps and the INADEF study sites

Fig. 1. The INADEF architecture.

The triggering model estimates by means of the massconservation equation the potential entrainablesediments volume associated to the runoff volumecontributing to debris flow (Figure 1 vi.), that is therunoff volume corresponding to a discharge larger thana critical threshold for debris-flow initiation [8]. Theparameter controlling the volume of entrainablesediments is the sediment concentration c. This value
depends both on rheology and site morphology (i.e., theFig. 3. A close-up of the web app front end: on the map thestudy sites where a debris flow is forecasted, are highlighted.

3 The site of Rovina di Cancia
2

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The EWS was tested on five test sites, three in Northeast
Italy (Moscardo, Rovina di Cancia and Rudan), and twoin Austria/ Tyrol (Bettelwurfmure and Gröbentalbach,Figure 3). In the present work, we only show the test bedsite of Rovina di Cancia because it is the site with thelarger number of events, and therefore, it can provide
more reliable results.
The first known debris-flow event
in Rovina diCancia [9] is dated
1868, when a catastrophic debrisflow of about 100,000 m3 caused 21 victims. Debrisflow occurrence increased in the last ten years. The
Rovina di Cancia channel originates on the scree at thefeet of Salvella Fork (2500 m a.sl.). The upper part ofthe channel is covered by giant boulders and water
surges delivered by rocky cliffs cannot entrain large
quantities of sediments. This occurs around anddownstream the giant rock at a quote of 1666 m a.s.l.,where some tens of thousands of sediments can beentrained. A flat area at a quote of 1330 m a.s.l. stops
debris flows transporting a sediments volume up to10,000 m3 and reduces the solid content of events
transporting larger volumes. Downstream of the flat area
there is an open check dam that dampens the erosion of
the upstream reach, and, just downstream of it there isthe confluence of the Bus de Diau Creek that drains thebasin identified by the yellow border in Figure 4 (thebasin contributing to the initiation area is identified by
the red borders). The channel ends at two retention
basins that protect the downstream inhabited area. Mostof the sediment entrainment occurs between theinitiation area and the flat area. Downstream of the open
check dam, debris flows alternate phenomena ofentrainment and deposition. A monitoring station (n.1 ofFigure 4) and rain gauges were installed in 2014, while
another two monitoring stations were installed in 2019.
The monitoring stations were upgraded in 2020 with
infrared cameras and flow level sensors (radar and lasermeter) during INADEF project. During the project
period three debris flows occurred: on 1 July, 11 and 29
August 2020. The corresponding mobilized sediments
volumes were estimated by differencing the pre andpost-event Digital Elevation Models (DEMs) obtained
fromUnmanned
AerialVehicle(UAV)photogrammetric surveys. At this site a sensor-based
EWS [10,11], that is not displayed in Figure 4, isoperating as well.

volume. Using c = 0.5, the model-cascade hydrologicaltriggering model is runned for all the ten occurred eventsof debris flows.
The overall comparison provides satisfactory results. At
the first level, all events are identified. In addition, thedifference between the estimated and predictedoccurrence times ranges in the interval [-8 - +8] minuteswith a mean value of 4.5 minutes. The comparison
between the volumes of sediments predicted by themodel cascade and those estimated is shown in Figure
5.

Fig. 4. The site of Rovina di Cancia with the rain gauges(triangles, dark of ARPAV and white of INADEF) and thenumbered monitoring stations (circles) used for testing theWGA. The red and yellow lines identified the basins drainedby the initiation area and the mouth of Bus de Diau Creek,
respectively.

4 Testing of the EWS
The EWS was tested for the period 2009-2021, were ten
debris flows occurred. Before running the nowcasting,the single couple hydrological-triggering model is tested
using the rainfall data of the rain gauges, the estimatedvolumes of entrained sediments, and the timings ofdebris-flow occurrence. The value of the sedimentconcentration for computing the potential entrainablesediment volume is 0.5. This is the value of the main
parameter of the triggering model for computing thedebris-flow hydrograph. Such a value is the maximum
one obtained by the comparison between the estimatedsediment volume and that computed through therelationship that provides the potential entrainable

At the second level, the volume of an unique occurred
event is underestimated and such an event results to bebelow the threshold almost certainly because themissing of recorded rainfall pattern on the basin
contributing to the triggering area; two values of thepredicted volumes of sediments are overestimated andover the threshold. This means that there is one missed
alarm and two false alarms on ten events in twelve years.

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5 Conclusions
A model-based EWS for debris flow is proposed andtested in a monitored basin in a period of twelve years,
in which ten events of debris flows were observed. The
model cascade using the observed rainfall is tested
satisfactorily. Conversely, the model cascade using thenowcasting has a lower performance in terms ofpredicted sediments volume but a satisfactorily one interms of predicted events. These are initial results. A
substantial improvement of the event prediction quality
in the single catchments can be achieved by thecalibration of the model by (extensive optical) event
monitoring. Moreover, the permanent ongoingdevelopment of the precipitation-forecast as important
input parameter should rise the hitrate as well.

Fig. 5. Comparison between the estimated and predicted
sediment volumes for the occurred events using the observed
rainfall pattern.

This work was supported by the Interreg program Italy-Austriaof the European Union.

The EWS is then tested using the whole model cascade,using therefore, the rainfall pattern provided by
nowcasting for the period 2009-2021.

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Fig. 6. Comparison between the estimated and predicted
sediments volumes in the period 2009-2021.

Figure 6 shows the comparison between the sedimentvolumes of debris flows predicted by EWS with thoseof the estimated volumes of occurred debris flows fortwelve years. The green circles correspond to thesediments volumes both predicted and estimated. Thiscase represents the relative majority of the events. It isfollowed by the case of false events (yellow circles),missed events (red circles), and runoff events (bluecircles). About the first level, i.e. the debris-flow
forecasting, the performance of the EWS is quasi
satisfactory because in a period of 12 years there are fivefalse alarms (runoff events are assimilated to those ofdebris flows) and three missed events: about one each
two and four years respectively. The ROC analysis, infact, provides the following values for true positive, true
negative, false positive, and false negative rates: 0.77,0.88, 0.12, and 0.3 respectively (for the ROC analysis
the runoff events are treated as false events). About thesecond level, i.e the prediction of the sediments volume,the performance of the EWS is not satisfactory because
it captures only a high magnitude and a low magnitude
event. However, most of the false and missed events arebelow the threshold for alarm.

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