E3S Web of Conferences 415, 06004 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506004

Application of physical models to improve the mitigation
structures of debris flow in a case study
Davood Farshi*, and Dany Suter
Department of Civil Engineering, Eastern Switzerland University of Applied Sciences, Rapperswil, Switzerland

Abstract. Debris flows are a phenomenon in mountains areas, where a large amount of mobile rockmaterial, large amounts of water and steep slopes meet together. They are characterized by high capacity ofsediment transport, high concentration of sediment with wide range of grain sizes and high front-velocity.
To improve the protection measures against debris flow, it is necessary to know the processes of debris
flows and its interaction with structures of the measures. By investigation of the mitigation structures inexperimental scaled physical models it is possible to understand the functionality of the structures underdifferent scenarios as well as optimization of the structures. For the protection of the counties Agarn und
Leuk in Valais in Switzerland a multi-level debris flow basin system has been designed. By means thephysical model in scale of 1:55 the functionality and the dimensions as well as the details of the structures
could be optimized.

1 Introduction
Debris flow is a flow of sediment and water mixture ina manner as if it was a flow of continuous fluid driven
by gravity, and it attains large mobility from theenlarged void space saturated with water or slurry [1].
Debris flows are mainly subjected to the inertial andgravitational forces. Therefore, Froude's law ofsimilarity can be applied, for physical models of debris
flow [2].
Physical models of debris flow have of course
limitations and simplifications. For example, the terrain
surface in most of the models is non-erodible. The
sediment entrainment from torrent bed can’t beinvestigated in such models. The choice of the model
scale depends not only on the influence of scale effects
but also, among other things, on the space conditions in
the laboratory and handling of material logistics for therequired debris volumes. Based on our experiences, theappropriate choice of the model scale is usually between
1:40 and 1:60. This is accompanied by the fact that thefine components can no longer be reproduced in models.
The natural mixture of debris flow is strongly variable
and can’t be reproduced 100% in the laboratory.
However, in order to obtain meaningful results, thedebris flow mixtures are calibrated to target values. Thetarget values include the volume of surge 𝑉, themaximum discharge 𝑄𝑚𝑎𝑥 , and the maximum front
velocity 𝑣𝐹𝑚𝑎𝑥 of the debris flow. In this way the flow
behaviour of debris flow can be reproduced, although
the model mixture is not exactly the same as the mixture
in the nature. However, the large grain sizes are
considered in the model mixture, which are very

important for clogging of the outlet structures ofsediment traps.

2 The multi-level debris flow basin
system Meretschi
In the catchment area Meretschibach, between Agarn
and Leuk in Canton Valais, smaller debris flows
occurred repeatedly in the past years. Based on the newhazard assessment map, a large debris flow potential up
to 130'000 m3 with a probability of 0.003 in a year canoccur. The residential settlements and traffic routes
located in the valley are at considerable risk. In order tomitigate the risk a multi-level debris flow basin system
has been designed on the slope above the valley (Fig. 1).

Fig. 1. The multi-level debris flow basin system.

The five deposition areas have different tasks. For
example, the sediment trap 1 (Geschiebesammler 1) isintended to ensure the deposition of very frequent events(probability over 0.1 in a year) with a volume of up to
20'000 m3. When the spillway is activated(Überlastfall 1), all subsequent surges have to be safely
transferredtothesedimenttrap2* Corresponding author: davood.farshi@ost.ch© 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, 06004 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506004(Geschiebesammler 2). The sediment trap 2 provides adeposition volume of about 70'000 m3 and transfers thesubsequent debris flow surges in case of a full filling via
the spillway (Überlastfall 2A) further towards thesediment trap 3 (Geschiebesammler 3) with a capacity
is about 20'000 m3. The excessive debris flow load willbe transferred over the spillway 2B and 3 (Überlastfall2B and 3) to the corridors 2 and 3 (Überlastkorridor 2and 3) These complete the deposition capacity up to
130'000 m3 [3].

The tests are divided into three phases: preliminary
tests, main tests for the spillways structures (phase 1)
and main tests on the entire system (phase 2). The
preliminary tests have been used to calibrate the model
mixture of the debris flow. The calibration is preformedthat the model mixtures show the same flow condition
as in the nature although not all components are thesame as in the nature. The maximum front velocity(𝑣𝐹𝑚𝑎𝑥 ) and die maximum discharge (𝑄𝑚𝑎𝑥 ) are used as
reference values for the calibration of the model
mixture. The model mixture shows the same flow
characteristics as in the nature.

3 Physical model of Meretschi
Due to the complexity of the debris flow and also theprotection structures functionality, the whole multi-level
debris flow basin system has been tested by using aphysical model.
The physical model of the system represents a realarea of about 600 x 180 m with a height 100 m at a scaleof 1:55. The entire structure and detailed individual
structures are depicted in Fig. 2.

4 Debris flow mixture
In order to calibrate the model mixture, the reference

values of the debris flow (Table 1) have been estimatedbased on empirical methods under natural condition [5].
Table 1. Target values for the test surges.
Surgevolume𝑽 (m3)

Maximum
discharge
𝑸𝒎𝒂𝒙 (m3/s)

Max. front
velocity𝒗𝑭𝒎𝒂𝒙 (m/s)

5'000

100 - 105

5.6 - 5.8

10'000175 - 185

6.8 - 6.9

20'000295 - 315

8.0 - 8.3

30'000410 - 430

8.9 - 9.2

40'000510 - 540

9.5 - 10

Fig. 3. The natural size distribution (grey & blue), the derived
size distribution for the model (orange) and the effective usedsize distribution for the tests in model (green) [3].

Fig. 2. The physical model of multi-level debris flow basin
system Meretschi.

Additionally, the grain size distribution is necessary
for the mixture. Especially the max. size of the grains isvery important in this project, because based on thisvalue the grids at the outlet of the sediment traps will beplugged. The grain size distribution has been derived
from depositions of last debris flow occurrences [3].
However, the grain size distribution in physical model
doesn’t have to be exactly the same as in nature,primarily it is important to achieve the target values(Fig. 3). For the fine and viscose part of the mixture weuse special ingredient from food industry.

The following objectives were defined for the model
tests:• The protective functionality of the system has tobe checked and documented depending on theexisting debris flow scenarios for the events witha probability of 0.033, 0.01 and 0.0033 in a year.
• The functionality of spillways and corridors(overload structures) hast to be checked.
• The model structure has to be optimized on thebasis of the knowledge gained from the tests inorder to achieve the two objectives mentioned
above.

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E3S Web of Conferences 415, 06004 (2023)
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https://doi.org/10.1051/e3sconf/202341506004

The model mixture is calibrated for each 5 surge
volumes differently during preliminary tests. More than70 surge tests are performed for the calibration. Thewater content has a strong influence on the flow
conditions of the debris flow [4]. Therefore, in first step
the calibration of the mixture is done by changing thewater content. For measuring the maximum front
velocity (𝑣𝐹𝑚𝑎𝑥 ) at the upper part of the model at least 3
ultrasonic sensors are used (Fig. 4). The maximum
discharge (𝑄𝑚𝑎𝑥 ) can be controlled through adjusting
the opening of the debris container (Fig. 5). If
adjustment of the water content does not lead to thetarget values, the viscose part of the model mixture canbe changed.

In the second phase the entire system is tested
through 6 successive surges with total volume varies
between 56'000 to 130'000 m3 (in natural scale). Thetests show that at least 5 modifications are needed tohave the correct functionality. In Fig. 8 themodifications at the spillway 2 (Überlastfall 2A/B) aredepicted.

Fig. 7. Spillway 2 before (left) and after (right) modificationin phase 1.

Fig. 8. Spillway 2 before (left) and after (right) modificationin phase 2.

The tests show also that after the modifications thecorridors work well and control the overrun volumes. It
shows also that part of the corridors can be neglected in
final design (Fig. 9). Totally more than 80 tests are
performed for the main part.

Fig. 4. The ultrasonic sensors for determining the maximum
front velocity.

Fig. 5. The container of the debris upstream the model.

5 Main tests
Fig. 9. Functionality of the corridors in case of the total volume130'000 m3 after all modifications and not necessary parts inred.

After calibrating the model mixture of the debris flow
surges, the main tests are performed in two phases. Inthe first phase the functionality of the spillways ischecked. This happens with and without filled sediments
traps with fresh and dried debris mixture (Fig. 6). The
tests show that the system has to be modified in 6
positions. In Fig. 7 the modifications at the spillway 2(Überlastfall 2A/B) are depicted.

Fig. 10. Event volume via total volume in different part of themulti-level debris flow basin system Meretschi after allmodifications.

Fig. 6. Prefilled traps with fresh (left) and dried (right) debris)
mixture.

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E3S Web of Conferences 415, 06004 (2023)
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https://doi.org/10.1051/e3sconf/202341506004

Fig. 10 shows the volume of events and the total volume
as well as the functionality of each part after allmodifications. It shows for which event volume, whichpart will be activated.

6 Conclusions
By means of physical model tests, the robustness andfunctionality of the individual structures are to bechecked in order to achieve the protection goals. If
necessary, the structure will be further optimized during
the model tests. For this purpose, the project perimeteris reconstructed in physical model at a scale of 1:55. The
test show that the mitigation structures have to be
optimized in different parts to achieve the functionality.
It shows also that debris flow close to structures has acomplex flow properties, which is still can't besimulated inaccurate way. Therefore, physical models
are very important to understand the flow behaviour
close to such structures.

References
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C. Wendeler, Debris-Flow Protection Systems for
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geoformer AG, Schutzmassnahmen Meretschi,Agarn/Leuk VS (2019)
G. G. D. Zhou, et. al., Landslides 15:695–709(2018)
T. Mizuyama, S. Kobashi, G. Ou, Prediction ofdebris flow peak discharge, in Proc. International
Symposium Interpraevent, Bern, Switzerland, Bd.
4:99–108 (1992)

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