E3S Web of Conferences 415, 06015 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506015

Spatial distribution of natural debris-flow impact
Georg Nag *, Roland Kaitna and Johannes Hüb
University of Natural Resources and Life Sciences, Vienna, Austria

Abstract. The high destructive potential of debris flows poses a challenge for the design of structural
mitigation measures to protect vulnerable areas. An essential part of designing such structures is to determine
the magnitude and spatial distribution of the impact forces. The impact is expected to be related to thecomposition of the flow, which can vary from clay to large boulders along a flow event and between events.
Experimental studies are prone to scaling bias and benchmark observations at the full scale are rare so far.
Here we present measurements of the temporal and spatial variations of the impact of a natural debris flowin the Gadria creek, IT, onto an instrumented barrier structure. The flow event was preceded by a precursory
surge, which was then followed by the debris flow with multiple surges. The flow height reached up to2.3 m. We found that the impact of boulders occurs primarily in the upper half of the flow profile, the highest
forces were measured in first part of the flow, and deposition has a strong influence on the lower part of thestructure by damping and/or redirection.

1 Introduction
Debris flows exhibit the ability to transport large
boulders of several meters, trapped in a matrix with ahigh concentration of solids that can shape steep fronts
with high velocities that pose severe potential for
destruction of settlements and infrastructure. Theinteraction of this mass wasting process with mitigation
measures is a fundamental problem for the design and a
key aspect for the functional reliability of mitigation
structures. Measuring events on a real scale is achallenge for science. Only some efforts were done in
the past to quantify these parameters in real scale [1–4].
Due to the low data availability of natural events, mostinvestigations to enhance our understanding of the
impact process are based on laboratory experiments[e.g. 5–7]. Field measurements offer a complementary
method with no scaling difficulties and provide
quantitative observations of flow dynamics andinteraction. Considering that debris flows show a great
heterogeneity in their temporal and spatial evolution [8–10], this is particularly evident for impact forces.

necessary to determine the magnitude, location andduration of the impact forces exerted onto the structure.

Fig. 1: Schematic illustration of impact dynamics on themonitoring structure

In this contribution, we will present in-situ
measurements of impact forces of a natural debris andwe show the temporal and spatial distribution of impact
forces of a natural debris-flow on a monitoring structure.

Impact forces of debris flows are often divided into two
different contributions depending on the duration ofimpact (see Fig. 1). The impact of individual boulders
exhibits a high contact force in a short period of time,
while the bulk pressure of the mixture exerts a more
uniform force on the obstacle over a longer period oftime [2]. For the planning of mitigation measures, it is* Corresponding author: georg.nagl@boku.ac.at© 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, 06015 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506015

cells, we found no measureable resonancefrequency.

2 Methods
2.1 Catchment area and monitoring structure

The impact force of debris flows can be divided
into two different forms of impact dependingon the duration of impact. In order to obtainbulk force we separate boulder impact forces
and bulk pressure by calculating a binned
maximum and median value over one second[14].

3 Results and interpretation
Stage hydrographs recorded directly in front of thebarrier (sensor 2) and next to the barrier (sensor 1)
illustrate not only the interesting flow dynamics ofindividual events, but also reflect the interaction of thebarrier with the flow and the complex erosiondeposition pattern during the events.
On August 2nd, 2020, the debris flows was triggeredby intensive rainstorm. After increasing discharge, asediment-laden precursory surge (debris flood) hit thebarrier at 19:29:00 UTC (307 sec. on Fig.4). The flow
developed with moderate velocity. A big boulder
deposited in front of the barrier at 19:30:02 UTC (413sec.), and blocked the flow height sensor two (Fig. 3a).
.

Fig. 2: Monitoring structure in the Gadria creek.

The full-scale test site is situated at the Gadria creek(Northern Italy), draining a catchment area of about
6.3 km² with an altitude difference from 2,950 m a.s.l.
to 1,394 m a.s.l. at the fan apex. The steep headwater
catchments, metamorphic rock and a high degree offragmentation and glacial deposits provide the basis forfrequent rainfall-triggered debris flows in the Gadria
catchement [11,12]. The channel has a trapezoidal form
with a fixed channel bed of a longitudinal slope of 6°
and a rock-riprap channel bank. The channel bed is 40 m
upstream and 10 m downstream from the barrier secured
with concrete riprap to prevent erosion during a debrisflow event. The monitoring structure represent a single
part of a structure used in alpine regions to mitigate mass
wasting processes, as debris flows.
The barrier structure is one element of the construction
system and consists of a concrete element of 1 m width
vertical orientated on the center of the channel (see Fig.
2) and 3 m above ground, connected to a foundation
below the channel bed. Fourteen load cells (HBM C6Afrom 1 to 2 MN capacity) are installed on the front of
the structure to measure the spatial distribution ofimpact. Each sensor is protected by a steel plate with acircular area of 0.2 m in diameter. Upstream of thebarrier, a traverse check dam unconnected to the wall isflush to the ground and builds up the second part of thesystem with two weighing systems, one infront if thebarrier and und aside.
2.2 Analysis
For the analysis of the impact force, the time series wereprocessed as follows:We processed the data with a wave-let analysis
by using a Daubechie function (db4) anddecomposed into 8 levelswith a soft thresholdto eliminate random noise [3,13]. For the load

Fig. 3: Video snapshot of the debris flow event. (a) at
19:46:09 UTC (1336 sec) with deposited boulder of 2 m indiameter; (b) at 19:51:18 UTC (1645 sec.) impacting surge.

2E3S Web of Conferences 415, 06015 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506015

After this surge, the flow height decreased
continuously. Some minor surges could not remove theboulder in front of the barrier and deposited more debris
in the area around the barrier. At 19:51:18 UTC, (1700sec.) a surge with 2.2 m flow height re-activated thedeposited material and removed all boulders in front thebarrier (see Fig. 3b). Afterwards several surges hit thebarrier. During this period, the velocity approached zeroin between surges and the material stopped at rest withflow heights of 1 to 0.8 m and was reactivated by thefollowing surges (see Fig 4).

We found the highest relative impact forces wereconcentrated in the upper regions of the flow. This may
be connected to a wedge of deposited material thatdamped the impact of single boulders. Additionally, thehighest values were found in the first phase of the debris
flow, which is consistent with the arrival of a boulder
front.
3.1 Conclusions
Our in-situ measurements demonstrate the temporal andspatial distribution of impact forces of natural debris
flows, which can subsequently be useful for appropriate
design.
The monitoring barrier was constructed in cooperation of theDepartment of Civil Protection of the Autonomous Provinceof Bozen-Bolzano in Italy and the University of Natural
Resources and Life Sciences, Vienna, Austria.

References
123
45Fig. 4: Flow depths of debris-flow event 2nd August 2020,
grey line represent the sensor in front of the barrier and theblack line is beside the barrier. (T0 = 19:23.53 UTC)

6To illustrate the temporal and spatial impact
behaviour, we calculated the relative impact force as
median binned value of force /maximum binned value
for each second and compared this with the relative
height defined as the height of each sensor normalized
by the flow height, in Fig.5. The symbol size represents
the absolute magnitude of the force and the colour (fromblue to yellow) represents the time. Hence, if the pointis close to 1, the maximum value and the mean value arealmost the same. This represents a more fluid like
impact. Instead, if the relative impact force tents to zero,
the maximum and mean value is far apart, which weinterpret as a bouldery impact.

7891011121314Fig. 5: Relative impact force of each sensor (binned meanvalue / binned maximum value) versus the relative height(sensor height/ flow height in front of barrier)

3

H. Suwa, Focusing Mechanism of Large Boulders
to a Debris-Flow Front, Kyoto University,(1988).
S. Zhang, Natural Hazards, 7, 1,(1993)
K. Hu, F. Wei, Y. Li, Earth Surf. Process.
Landforms, 36, 1268, (2011)
L. Bugnion, M. Schaefer, P. Bartelt, Granular
Matter, 15, 771, (2013)
C. Scheidl, M. Chiari, R. Kaitna, M. Müllegger,A. Krawtschuk, T. Zimmermann, D. Proske,Surveys in Geophysics, 34, 121, (2013)
D. Wang, Z. Chen, S. He, Y. Liu, H. Tang,Landslides, 15, 1331, (2018)
D. Song, X. Chen, G. G. D. Zhou, X. Lu, G.
Cheng, Q. Chen, Engineering Geology, 291,106211,(2021)
M. Berti, R. Genevois, A. Simoni, P. R. Tecca,Geomorphology, 29, 265,(1999)
B. W. McArdell, P. Bartelt, J. Kowalski, Geophys.
Res. Lett., 34, L07406, (2007)
S. W. McCoy, J. W. Kean, J. A. Coe, G. E.
Tucker, D. M. Staley, T. A. Wasklewicz, J.
Geophys. Res., 117, (2012)
M. Cavalli, S. Trevisani, F. Comiti, L. Marchi,Geomorphology, 188, 31,(2013)
F. Comiti, L. Marchi, P. Macconi, M. Arattano, G.
Bertoldi, M. Borga, F. Brardinoni, M. Cavalli, V.
D’Agostino, D. Penna, J. Theule, Nat Hazards,73, 1175,(2014)
P. Cui, C. Zeng, Y. Lei, Earth Surf. Process.
Landforms, 40, 1644, (2015)
D. Schneider, R. Kaitna, W. E. Dietrich, L. Hsu,C. Huggel, B. W. McArdell, Cold Regions Scienceand Technology, 69, 70, (2011)