E3S Web of Conferences 415, 03018 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341503018

Monitoring debris flows in the Gadria catchment (eastern ItalianAlps): data and insights acquired from 2018 to 2020
Lorenzo Marchi1, Velio Coviello1, Marco Cavalli1, Francesco Comiti2, Stefano Crema1, and Pierpaolo Macconi3
1CNR-IRPI National Research Council, Research Institute for Geo-Hydrological Protection, corso Stati Uniti, 4 , 35127, Padova,Italy
2Free University of Bozen-Bolzano, Faculty of Science and Technology, piazza Università 5, 39100 Bolzano, Italy
3Civil Protection Agency, Autonomous Province of Bozen-Bolzano, via Cesare Battisti 23, 39100 Bolzano, Italy

Abstract. This work analyses seven debris flows recorded between 2018 and 2020 in the Gadria
instrumented catchment (South Tirol). We focus on three aspects not previously explored in this catchment:(i) the debris-flow transfer times between the headwaters and the outlet; (ii) the longitudinal variability ofdebris-flow velocity between the three downstream monitored cross-sections, and (iii) the characteristics ofthe secondary surges observed in three debris flows. In most cases, the mean velocity of the debris flow
estimated from the upper to the lower channel reaches (for travel distance of 2155 m) is rather low, ranging
between 1.9 and 3.9 m/s. This result could indicate a progressive slowing down, and possibly even temporary
stops of debris flows along the path. Some variability in flow velocity was observed between two channel
reaches in the lower part of the catchment (0.7 – 2.3 m/s in the upstream reach, and 1.4 – 4.7 in thedownstream one). Regarding the secondary surges, these have been noted to occur superimposed on slowmoving slurry-type phases. The mean velocity of the secondary surges varied between 3.5 and 8.9 m/s, withan average value close to 6 m/s for all three events. Their regular shape, duration, and depth suggest thatsuch surges were generated by flow instabilities, with no external forcing.

1 Introduction
Debris-flow monitoring stations provide invaluable data
- gained through remarkable resource investments - onsuch hazardous processes. Pioneering experiences indebris-flow monitoring in China and Japan date back to
the 1960s-1970s [1-2]. In Europe, the first permanent
installation was the Moscardo Torrent in NE Italy [3]; inthe last two decades, several additional sites wereinstrumented for debris-flow monitoring [cf. 4 for areview]. Among these instrumented catchments, theGadria Creek, equipped and managed by the Civil
Protection Agency of South Tirol, is now acting as a hub
for field research on debris flows in the Eastern Alps
involving different institutions from Italy and Austria[5-6].
This contribution presents and discusses data on
debris flows recorded between 2018 and 2020 (7 events,no debris flows occurred in 2021) in the Gadria
catchment, with a specific focus on: i) the occurrence
and phenomenology of debris flows in different sectorsof the channel (and related flow velocities); and ii) thesecondary waves observed in three debris flows
recorded in 2020.

2 Study catchment and instrumentation
2

The Gadria catchment (Fig. 1) drains an area of 6.3 km
with a range in elevation between 1,394 and 2,945 ma.s.l.; its lithology consists of metamorphic rocks(paragneiss and orthogneiss); the climate is alpine withmean annual precipitation around 800 mm. Additional
information can be found in previous works [7-8].

Fig. 1. Map of the Gadria catchment and details of theinstruments installed at the lower monitoring station.

This work analyzes the debris-flow hydrographs
recorded by three radar sensors (D2, D3, and D4)
installed at the lower station. D2 and D3 are located 80
m away from each other with a mean channel slope of16%, while D4 is installed 549 m upstream of D3. The© 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, 03018 (2023)
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https://doi.org/10.1051/e3sconf/202341503018

mean slope between D4 and D3 is 15.6%. The upperstation, which was set up in 2018, is locatedapproximately 500 m downstream of the debris-flow
initiation area and 2155 m upstream of D4 (meanchannel slope is 32%), and is equipped with seismic
sensors and a videocamera [9].

the two events recorded in 2019 featured similar
velocity in the two channel reaches, the other debris
flows had higher velocity in the reach D3-D2 than in theupstream reach D4-D3. Although the two channel
reaches have similar mean slope, the middle sector of
the reach D4-D3 is characterised by wider sections (Fig.
2) and a lower channel slope, in correspondence to theconfluence with two tributaries. These features could
explain the lower velocity observed in the D4-D3
channel reach.

3 Results
3.1 Debris flows at the upper and lower stations

Table 2. Mean flow velocity, peak discharge, and total
volume of debris flows main surges at the lower station. For
the 21 July 2018 debris flow, the flow stage measurement at
D3 is disturbed and does not allow a reliable computation ofpeak discharge and volume.
Velocity Velocity
Peak
VolumeDate
D4-D3
D3-D2 discharge
(m3)
(m/s)
(m/s)
(m3/s)
21 Jul 2018
1.5
5.3

During the 7 events recorded between 2018 and 2020 in
the lower part of the catchment (stations D2-D4),
significant surface runoff and sediment transport (alsofeaturing immature debris flows) were observed at theupper station, just downslope of the initiation areas.
The travel time between the upper and the lower
stations was computed as the time difference between
the onset of intense runoff with intense sedimenttransport at the upper station and the first rise of thedebris-flow hydrograph at the D4 section. The mean
velocity of debris flows between the upper and lower
stations was computed as the ratio between the travel
time and the distance between the two stations. For mostof debris flows analysed here, the resulting values(Table 1) are rather low, especially considering the steep
channel slope. We believe that such low mean travel
velocities may stem from important variations in flow
velocity taking place along the path, including
temporary stops of the flowing mass, probably
associated to erosion-deposition processes and abruptchanges in channel section geometry. However, actual
data to substantiate our hypothesis are not available.
Remarkably different from the other events is the 12
August 2020 debris flow. For this event, the large flow
depth of the sharp debris-flow front (see Fig. 5a insection 3 of this work), as well as the high flow velocityobserved at the lower station (Table 2), are consistent
with the short travel time between the upper and thelower stations. In this case, the debris flow was likely
not affected by temporary stops or significant slowing
down along the path.

11 Jun 2019

2.1

2.4

25

7,750

26 Jul 2019

1.3

1.4

166,700

2 Aug 2020

0.7

1.7

16

9,700

10 Aug 2020

1.3

3.0

31

15,35012 Aug 2020

2.3

4.7

108

16,700

Table 1. Travel times and mean flow velocity of thedebris flows between the upper station and D4.
DateTravel time(hh:mm:ss)

Mean velocity(m/s)

21 Jul 2018

00:09:22

3.8

10 Jun 201900:09:35

3.7

11 Jun 201900:07:37

4.7

26 Jul 201900:11:12

3.2

2 Aug 2020

00:16:06

2.2

10 Aug 2020

00:07:51

4.6

12 Aug 2020

00:02:40

13.5

Fig. 2. Aerial photo of the channel between D4 and D3; theblue arrow indicates a low-slope, partly unconfined sector ofthe channel.

Table 2 also presents the peak discharge and the debrisflow volume calculated, respectively, as the maximum
and integral of the product of the mean flow velocity ofthe main surges in the reach D3-D2 by the flow crosssectional area. The cross-sectional area was estimatedusing the flow stage measured at station D3 andassuming the cross-section to be clean from any deposit.

3.2 Debris flows at the lower station
Table 2 presents the mean velocity of debris-flow fronts
in the two channel reaches of the lower station. While2

E3S Web of Conferences 415, 03018 (2023)
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https://doi.org/10.1051/e3sconf/202341503018

A significant uncertainty in the volume estimation – up
to 50% – is possible due to the variability of both flow
velocity and cross sectional area during the debris flow[8]. However, calculated values are representative of themain features (e.g., duration, flow height) of theanalysed debris flows and are consistent with both field
estimations and the sediment yield calculated at thecatchment scale using topographic techniques.
The velocity of the June 10, 2019 debris flow could
not be reliably estimated in the monitored channel
reaches D4-D3 and D3-D2. As shown in Fig. 3, the firstpart of the hydrograph recorded at D4 shows the typicalfeatures of debris flows, with a sudden and fast-risinglimb, followed by the peak. This permitted computing
the travel time and mean flow velocity between theupper station and D4 (Table 1). However, after the peak,the stage values become almost stable, with values
approximately 1.5 m higher than the initial value of thechannel bed. The flow levels recorded at D3 and D2 arelow and do not permit recognizing a well-defineddebris-flow (or flood) wave. A possible interpretation no video recordings are available for this event - is thatthe debris flow deposited right downstream of D4 (andaffecting its stage measurements after the peak) and onlya small, fluid flow reached the downstream sensors D3and D2.

velocity close to 6 m/s was observed for all three events,and velocities of the individual surges varied between
3.5 and 8.9 m/s. Therefore, the secondary waves display
similar kinematic characteristics which are unrelated tothe debris-flow front velocity of their ”parental” event
reported in Table 2.
Table 3. Mean velocity between D3 and D2 for thesecondary surges observed in August 2020.
Std. dev. is reported in parenthesis.
No. ofVelocity
Datesurges(m/s)
6.0
2 Aug 202017(1.2)
6.9
10 Aug 2020
18(0.7)
6.4
12 Aug 202031(0.9)

A common feature of the August 2020 debris-flow
events is a phase of slow-moving slurry on which
secondary surges are superimposed, travelling at a muchhigher velocity. Together with their regular shape,duration, and depth, the presence of slurry in the channel(with a thickness of 0.3-0.8 m) suggests that the surges
were generated by intrinsic flow instabilities, without
external forcing (e.g., dam-break occurrence upstream).

Fig. 3. Hydrographs of the June 10, 2019 event recorded atthe lower station.

3.3 Secondary surges
Fast-traveling surges are common features in debris
flows [10-11] but, as observed by Huebl et al. [12], fielddata on their development, shape and velocity areseldom available.
While small, isolate secondary surges are often
observed in the recession phase of debris flows in theGadria Creek, the three events observed on 2, 10 and 12
August 2020 display long (5-15 minutes) and regularsequences of secondary waves at the lower station (Fig.
4). Fig. 5 shows secondary waves recorded at the threemonitored cross-sections of the lower station during the12 August 2020 debris flow. At D3 and D2 (Fig. 5b) theduration of the secondary surges varies from 10 to 20 s,with an average flow depth between 0.6 and 0.8 m.
Table 3 reports the mean values and the standard
deviations of flow velocity relative to the secondary
surges observed during the August 2020 events. The
surge velocity was computed as the ratio between theirtravel time (i.e., time difference between the peak instage values at the radar sensors D3 and D2) and thedistance between these two cross-sections. An averageFig. 4. Two secondary surges of the August 10, 2020 debrisflow.

3E3S Web of Conferences 415, 03018 (2023)
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in the upper Gadria. Nicola Marchi collaborated in the analysisof the secondary waves.

References
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D’Agostino, D. Penna, J. Theule, J., Nat. Haz, 73(3),1175-1198 (2014)
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Environ. Eng. Geosci., 27(1), 95-111 (2021)
9. E. Ioriatti, V. Coviello, M. Berti, F. Comiti, Study ofdebris-flow initiation through the analysis of seismic
signals, submitted to DFHM8 conference (2022)
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11. B. Zanuttigh, A Lamberti, Rev. Geophys., 45,RG3006 (2007)
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Fig. 5. Debris-flow hydrograph of August 12, 2020 (a) andsequence of secondary waves monitored at the sensors D3
and D2 for the same event (b).

4 Concluding remarks
Debris-flow monitoring in other catchments [e.g. 12-13]
has shown the importance of experimental observations
in different sectors of the channel: we expect that
extending monitoring to the upper part of the Gadriachannel could provide useful insights on debris-flowformation and evolution. Previous published studies in
the Gadria catchment [7-8] focused on data collectedclose to the catchment outlet, but since 2018 it has beenpossible to extend the observations further upstream, up
to the headwaters, enabling a more complete monitoringof debris-flow propagation.
Rather low mean velocities were observed between
the upper and the lower stations for most of the eventsoccurred from 2018 to 2020, probably due to local
deceleration and temporary halting of debris flows.
Some important differences in mean flow velocity
were observed also in the short debris-flow channel
monitored at the lower station. In fact, velocities in thechannel reach D4-D3 were usually lower than in the D3D2 reach, most likely for the local channel widening andslope reduction in the former reach which favored
deceleration - and partial deposition - of debris flows.
Finally, three debris flows that occurred in August2020 were characterized by sequences of very regular,
fast-traveling secondary surges superimposed on a slowmoving slurry. The origin of such surges – whose
kinematics were not related to debris flow front
velocities – seem to be associated to flow instabilities
generated well upstream in the channel network.
However, more events – monitored from the headwaters
to the outlet - featuring secondary waves are needed toformulate a robust hypothesis on their genesis.
The upper monitoring station was funded by the project
Sediplan-r (FESR/EFRE 2015-2020). We thank Ricardo
Carrillo, Elena Ioriatti, Giulia Marchetti, Shusuke Miyata, andAndreas Schimmel for their support in the research activities

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