E3S Web of Conferences 415, 07006 (2023)
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Practical guide for debris flow and hillslope debris flow
protection nets and its application in case studies
Nadine Feiger1* and Corinna Wendeler2
1Geobrugg AG, Aachstrasse 11 8590 Romanshorn, Switzerland
2Canton Appenzell Ausserrhoden, Department of Water, Kasernenstrasse 17 A 9102 Herisau, Switzerland

Abstract. Debris flows and hillslope debris flows endanger people and infrastructures. Technical protection
measures are important elements in addition to spatial planning (adapted use of space like hazard maps) andorganizational measures (warning systems, emergency plan, evacuation). Additional to other rigid
protection measures flexible debris flow nets and hillslope debris flow nets were developed in 2008, and2010 respectively. Such flexible barriers consist of an interception surface of netting spanned between
horizontal support ropes including energy devices and posts for large span width. Since then, they areavailable on the market worldwide. Many projects were successfully installed and already filled by debris
flow events. After more than ten years of experience and demonstrating that these measures work against
debris flow and hillslope debris flow, they are fully accepted as possible measures for integral protection
against debris flow. To provide planners and engineers the experience and know-how of these flexible
protection nets, a practical guide was initiated by the Swiss Federal Roads Office (FEDRO) and Swiss
Federal Office for the Environment (FOEN). Two case studies with planned and installed flexible nets, in
one case for a debris flow and in the other case for a hillslope debris flow, are presented.

1 Introduction
Debris flows are flowing mixtures of solids and water insteep torrent channels and are characterized by a surgelike flow behaviour [1]. Hillslope debris flows occur on
open steep slopes and are smaller in volume [2].
Flexible debris flow barriers were first tested bysmall scale tests in Oregon at the USGS flume in 1998
by Natale et all [3]. Around 10 m3 of material werereleased and caught by a so-called flexible barrier. Initialdesign approaches given by WSL (Swiss Federal
Institute for Forest, Snow and Landscape Research)were published in 2001 by Dieter Rickenmann in aninternal report [4]. This approach was based on theenergy method in which the impact time of the stopped
material by the flexible barrier is the most decisive
parameter like the design of rockfall barriers. This
impact time is hard to estimate and not clearly definedin the report [4]. The findings of [4] were based only on
physical modelling and the question remained whether
the impact time of the debris flow would last only the
first rope load peak or the complete time of filling up thebarrier. For muddy or watery debris flow this impact
time can be rather long because a lot of material ispassing through the barrier before clogging andincreases the rope forces. In this case, forces areoverestimated by this approach.
Initial knowledge of large-scale tested flexible
barriers was collected by chance when impacted rockfall
barriers were loaded by a debris flow or an open hillslide.

Between 2005 and 2008 real scale field tests with aflexible barrier in a real torrent called Illgraben(Switzerland) were performed [5]. A three-year funded
CTI (Commission of Technology and Innovation)
project produced a large series of lab tests resulted in
new knowledge about the clogging effect of grain size
versus ring net size and basal opening versus flow height[6]. The basal opening is the distance between the lower
support ropes and the riverbed and is used to passnormal runoff without large amounts of bedload andwood. These results end up being used in several
flexible barrier designs in Switzerland like the Hasliberg
project [7] or the Hüpach project [8].
Between 2009 and 2012 another CTI funded project
took place with many series of large-scale open hill flow
tests to measure impact pressure in mudflows toimprove the knowledge of the pressure surge model
suggested in [5]. Results of this project can be found in[9] and [10]. Alternative approaches on design offlexible debris flow barriers were determined later in[11].
Up to now, more than 600 flexible debris flow
barriers and 400 hillslope debris flow barriers have been
installed worldwide by the manufacturer Geobrugg AG.
Several of these barriers have already been impacted.
Until 2020, no general guideline for flexible nets
against debris flow and hillslope debris flow wereavailable for planner and engineers to help and makethem more familiar with this kind of structures. There isa need to summarize and standardize the design
approach of these structures for planners and engineers.

* Corresponding author: nadine.feiger@geobrugg.com© 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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2 Practical guide𝐹𝑠𝑢𝑟𝑔𝑒 = 𝑐𝑑 ∙ 𝑣 2 ∙ 𝜌 + 0.5 ∙ ℎ𝑓𝑙 ∙ ρ ∙ g𝐹𝑜𝑣𝑒𝑟𝑓𝑙𝑜𝑤 = 𝜌 ∙ 𝑔 ∙ ℎb′ + ℎ𝑓𝑙 ∙ ρ ∙ g

In December 2020 a FOEN (Swiss Federal Office for
the Environment) and FEDRO (Swiss Federal RoadsOffice) financed project resulted in a practical design
guide for flexible nets against debris flows and hillslopedebris flows [12]. A team of experts collected all former
design approaches and summarized them in thisdocument. The document is subdivided into a practicalpart and a technical part. Additionally, reference
projects were collected and described, and a case study
calculation provided.
The basic principle published design approach from
WSL [5], is still determined as most practical valid
approach, and was therefore used to calculate a casestudy in the practical guide. Most of the so far existingflexible barriers mainly the later explained CE-marked
barriers were designed on that common approach which
is based on real scaled field tests. The most important
service ability aspects were mainly summarized in [13],[14] and help to improve long-term maintenance of
these flexible systems.
Although most of the debris flow and hillslopedebris flow nets are already CE marked this marking isonly valid for special barrier dimensions or specificpressure values. If these values do not fit therequirement of the torrent or the slope parameters thispractical guide will help experts in designing. The mostimportant input parameters for debris flow impact
pressure are the volume, the flow velocity and height,
the density of the flow as well as the flow consistency (mud or granular). To estimate these values either field
investigations and/or numerical simulations or empiricalcalculations are essential according to [15]. The samevalues are needed for the hillslope debris flow barrier
design but obtaining the data is more difficult as only
breakout heights and deposition heights are currently
indicated on Swiss hazard maps. Numerical simulations
for example with RAMMS (rapid mass movement
simulations, developed by WSL) can be helpful toestimate the dynamic parameters.
To determine the most suitable design for a torrent
with certain input parameter all three load cases must be
taken into account: pressure surge, filling up the barrier
and overflow [5], [12]. These load cases are illustrated
in Figure 1. Impact pressure (Eq. 1) [5], [12] iscomposed of hydrodynamic and hydrostatic pressureduring the pressure surge. The hydrodynamiccomponent is calculated by the pressure coefficient ( 𝑐𝑑 ),mean velocity (𝑣) and material density (𝜌). The
hydrostatic component is based on the flow height (ℎ𝑓𝑙 ),gravitational force (𝑔) and material density (𝜌).
Overflow condition is calculated by the hydrostaticpressure with the residual height (ℎb′ ) and the additional
weight of the overflow over the flow height (ℎ𝑓𝑙 ).
Further the retention volume ( 𝑉𝑅 ) of the barrier has tobe obtained. To determine the potential retention
capacity and to define the number of barriers for large
release volumes. The retention volume is calculatedwith the following parameters: residual height (ℎb′ ),mean width of protection net (b𝑚 ), angle between
protection structure and riverbed (𝜀), riverbed angle (𝜃)and angle of material deposition (𝜃 ′ ) (Eq. 3).

𝑉𝑅 = 0.5 ∙(ℎ′ )2b∙ b𝑚 ∙ sin 𝜀 ∙ (sin 𝜀tan 𝜃−𝜃′(1)(2)+ cos 𝜀)(3)Fig. 1. a) Pressure surge consisting of hydrostatic anddynamic pressure; b) Filling process; c) Overflow condition
with acting additional weight on the hydrostatic pressure[12].

3 Case study debrisSouthern California

flow

event

In the following section, a case study from Southern
California is presented. Here, post-wildfire conditions
created a potential debris flow hazard for infrastructure.
In 2020, the 5th September El Dorado Fire burned
around 90 km2 of forest, part of the north flank of San
Bernadino Mountains. Based on the debris flow
analysis, a moderate debris flow hazard was identified
to occur during rainfall intensities of 24 mm/hr (USGSdebris flow modelling). The debris material consisted ofash, wooden and boulder debris. The highway wasprotected by ditches and retention basins. Additionally,two flexible debris flow barriers were installed in two
different canyons for further protection of the highway.
In an additional step, exhaustive structuraldimensioning for the barrier design was done due to therequest that no post would be installed in the riverbed.
Based on the canyon geometry, a standard so-called
UX-barrier, from the manufacturer Geobrugg AG,would have worked. An UX-barrier system includes
posts to accommodate greater span width. The request
to avoid posts resulted in a so called VX-barrier being
installed. This barrier increased the sag of the net

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leading to less residual height and hence less retention
volume.
The volume to dimension the barrier was set to 5000
m3. Based on a 5 m system height, with a residual heightof 3.9 m and cross-section geometry a total retention
volume [Eq. 3] of approximately 3490 m3 could beachieved. Material exceeding this volume would
overtop the barrier. Table 1 summarises the data thatwere used for dimensioning.
Table 1. Dimensioning data debris flow barrier.
Fig. 2. Filled debris flow barrier with inlet downstream.
Parameter

Unit

SymbolValue

Density

3[kg/m ]𝜌

2200

Velocity

[m/s]

𝑣

5.5

Pressurecoefficient[-]

𝑐𝑑2.0Flow height[m]

ℎ𝑓𝑙

0.8

4 Case study hillslope debris flow
event Sicily
Heavy rainfall occurred on October 1st, 2009, in theNorth-eastern part of Sicily (Italy). According to [16]many debris slides, debris flows, and mud flows
occurred. In Giampilieri and Scaletta Zanclea the event
caused victims and 1652 people lost their houses due tothe damage.
Based on the slope stabilization analysis in [16],mitigation measures were planned, and a flexible hightensile mesh nailed into the soil was installed. In theterrain of debris flow and hillslope debris flow
occurrence, flexible barriers were planned. A standard
SL-Barrier (shallow landslide barrier) of themanufacturer Geobrugg AG was installed with a systemheight of 3.5 m and a total length of 25 m to protect anaccess road to the village.
After a rain event in 2011, a hillslope debris flow hit
the SL-Barrier and filled it with approx. 50 m3 (Fig. 3).
The maximum deposited material height was 2.5 m. The
width of material deposition was around 12 m,
corresponding to approximately half of the barrierlength.
Back calculation of the dynamic impact ( 𝐹𝑑𝑦𝑛 ) (Eq.
4) was used to estimate a range of impact pressure.
Density and pressure coefficient can be estimated basedon the field observation after the event. For the velocity
a lower and faster values was chosen to establish amaximum and minimum impact pressure. Based on
dimensioning values in Table 2 a dynamic impact forceof 55 - 152 kN/m2 results.

Since overflow condition were defined based on theretention capacity and release volume all three loadcases (Fig. 1) were analysed. The resulting impact force( 𝐹𝑠𝑢𝑟𝑔𝑒 ) was calculated as 145 kN/m2 [Eq. 1]. The loadcase for the overflow resulted as 𝐹𝑜𝑣𝑒𝑟𝑓𝑙𝑜𝑤 in 94 kN/m2[Eq. 2].
A strong thunderstorm impacted the El Dorado burned
area on July 30, 2021. Estimated rainfall of 25 mm/hr
exceeds the thresholds of 15-24 mm/hr used for thedebris flow model.
Overall, several debris flows occurred in different
canyons along the intersect SR 38. The installed flexible
debris flow barrier was overtopped and did not fail.
However, at one location there was a backfilling effectof the retention basin. Due to the road over height and aclogged inlet underneath the road, the overtopped debris
material was retained and filled up the basin thenreached upstream to the barrier. The second barrier wasimpacted with less volume and therefore, retained mostof the material and got only slightly overtopped. In
Figure 2, the filled barrier consists of ash, debris, andwooden material with the inlet further downstream ofthe barrier.
The barriers were cleaned by an excavator, theenergy absorption elements were replaced, and thebarriers were reinstalled to their original system height.
Since the calculated retention volume is smaller thanthe design volume, an overtopping would be expected.
Installing a second barrier in the same canyon would
help to retain more material further upstream to avoid
the filling of the debris retention basin close to the road.
The practical guide was not published at the initialplanning stage of the project to help engineers andplanners. However, after the event back analysis, thepractical guide supported for further adjustments of thebarrier concept helped with the maintenance concept. It
also helped to understand and verify the dimensioningof the net superstructure.

𝐹𝑑𝑦𝑛 = 𝑐𝑑 ∙ 𝑣 2 ∙ 𝜌(4)Table 2. Dimensioning data hillslope debris flow barrier.

3

Parameter

Unit

SymbolValue

Density[kg/m3]

𝜌

1900

Velocity
maximum

[m/s]

𝑣

10

Velocity
minimum

[m/s]

𝑣

6

Pressurecoefficient

[-]

𝑐𝑑0.8

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References
1.

D. Rickenmann, Methoden zur quantitativen
Beurteilung von Gerinneprozessen in Wildbächen,
Swiss Federal Research Institute WSL,Birmensdorf, Switzerland (2014)
2. FOEN (ed.), Protection against Mass MovementHazards. Federal Office for the Environment
FOEN, Bern, Switzerland (2016)
3. J. S. Denatale, R. M. Iverson, J.J. Major., G.L.
LaHusen, Experimental testing of flexible barriers
for containment of debris flows, Tec. report, US
Geological Survey (1999)
4. D. Rickenmann, Estimation of debris flow impact
on flexible wire rope barriers, Int. tech. report,WSL, Birmensdorf, Switzerland (2001)
5. C. Wendeler, Murgangrückhalt in Wildbächen –Grundlagen zu Plaung und Berechnung vonflexiblen Barrieren, Dissertation Nr. 17916 ETH
Zürich, Switzerland (2008)
6. C. Wendeler, A. Volkwein, NHES, 15, 2597–2604, (2015)
7. C. Wendeler, A. Volkwein, A. Roth, B. Herzog, N.
Hählen, M. Wenger, Hazard prevention using
flexible multi-level debris flow barrier inProceedings. Interpraevent,11th, Dornbirn:
Voralberg, Austria, p. 547-554, (2008)
8. C. Wendeler, R. Brändle, N. Nishimura, L.
Stieglitz, G. Lauber, Design and construction of a
20m high, flexible debris flow barrier in the Swiss
alps, in Proceedings, Interpraevent,14th, Nara,Japan, (2014)
9. A. Bötticher, Flexible Hangmurenbarrieren: Einenumerische Modellierung des Tragwerks, derHangmure und der Fluid-Struktur-Interaktion,
Dissertation TU München, Germany (2013)
10. L. Bugnion, P. Bartelt, B. McArdell, C. Wendeler,
Landslides 9:179–187, (2012)
11. L. Canelli, A. M. Ferrero, M. Migliazza, and A.
Segalini, Nat. Hazards Earth Syst. Sci., 12, 16931699, (2012)
12. C. Berger, M. Denk, C. Graf, C. Wendeler,
Praxishilfe Murgang- und Hangmurenschutznetze,
WSL Bericht Nr. 102, on behalf of FOEN andFEDRO, (2020)
13. N. Feiger, Analyse zur Gebrauchstauglichkeit von
Murgangnetzen nach 10 Jahren Praxiserfahrung,MSc thesis, ETH Zürich, (2019)
14. C. Wendeler, H. Salzmann, N. Feiger. Flexible
debris flow barriers and their serviceability proof,
in Proceedings of Second JTC1 Workshop on
Triggering and Propagation of Rapid Flow-likeLandslides, Hong Kong, (2018)15. D. Rickenmann. Natural Hazards, 19: 47- 77,(1999)
16. M. Maugeri, E. Motta. Geotechnics andEarthquake Geotechnics - Towards Global
Sustainability:169-190, (2011)

Fig. 3. Partially filled shallow landslide / hillslope debris
flow barrier SL-150 (red rectangle) with the starting zone
above the structure.

5 Conclusions
Experience, knowledge, and projects of involving
flexible protection barriers against debris flow andhillslope debris flow have grown over the past 15 years.
The practical guide was created to present acomprehensive overview of the state of the art.
The case study in southern California showed apotential debris flow hazard due to a post-fire storm
event. Based on the debris flow modelling, flexible
debris flow protection measures were planned anddimensioned. The storm event of 30th of July 2021
showed the need for such structures. Further, it pointed
out that the retention volume calculation versus release
volume must be considered carefully. The released
volume during the event was about the size estimatedbased on the debris flow modelling and was used for thedimensioning of the debris flow barrier. Nevertheless,only one single barrier with the given geometry is notcapable of capturing 5000 m3. A second barrier further
upstream could have helped to capture more volume
before it reached the highway.
The case study in Sicily contained a heavy rainfall
event in 2009. Afterwards mitigation measures wereplanned and installed. This barrier was directly tested
after a rain event and proved its functionality.
Both case studies showed the success of flexible
barriers for debris flow as well as for hillslope debris
flow. The determination of the design parameter andlocation of the barriers were not trivial. The practicalguide helps and supports an improved procedure forsuch complex projects and enables the selection of themost suitable measure for a certain hazard situation.
The authors would like to thank the Swiss Federal Roads
Office and Swiss Federal Office for the Environment,
especially Philippe Arnold, Eva Gertsch, and Reto Baumann,
to initiate the Practical guide for debris flow and hillslopedebris flow protection nets. Likewise, the authors thankCatherine Berger, Matthias Denk, Christoph Graf, and Lisa
Stieglitz for their effort and the Swiss Federal Institute forForest, Snow and Landscape Research for the support.

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