E3S Web of Conferences 415, 07011 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341507011

Defining Protection Works Against Debris-Flow Hazards:
Industrial Standard, Tailor-made or Haute-couture?
Guillaume PITON1*
1Univ. Grenoble Alpes, INRAE, ETNA, 38400, Grenoble, France

Abstract. Despite centuries of forestry and empirical soil conservation works as well as decades of researchon debris flow processes, defining protection strategies against debris flows remains complicated. We arefar from guidelines resembling an industrial standard to define the type, location and shape of debris flow
protection works. We believe that this particular step will likely never be fully standardized because design
engineers must have a certain degree of freedom to tailor the protection works to the astonishing varietiesof catchments, geologies and geomorphologies, and the associated complex emerging debris flow regimes.
Some catchments are have such specific features that defining an adapted protection strategy requires
innovation: this is not even “tailor-made” but perhaps “haute-couture”. In this paper and the associatedkeynote lecture, we propose a possible way to approach the problem by firstly focusing on the sedimenttransport connectivity and the channel malfunctions leading to debris flow deposition; and secondly to selectstructures that prevent the latter, as well as to adjust the channel capacity to the debris flow supply.

1 Introduction
Debris-flows are ubiquitous in mountainous areas,
involving rapid mass movements delivering large
amounts of sediment, e.g. mud, gravel, cobbles andboulders, from hillslopes, gullies and steep creeks tovalleys [1]. They cause damage and casualties each year
[2, 3]. Societies have developed and implemented soil
conservation measures and torrent control works tocurtail erosion and prevent debris-flows for a very longtime. Indeed, these operations have been organized andoperated through large-scale plans for more than 150
years [4].
Soil bioengineering and afforestation have been used
for centuries to fight diffuse soil erosion and emerging
gullying processes [5]. Debris-flows often emerge invery active erosion areas sometimes as part of otherlandslide processes [1]. However; defining mitigation
works against these fast, poorly-predictable processes is
not simple. Many types of structures have been usedover the decades, including hillslope revegetation,
drainage systems, check dams, open check dams, bankprotections and dikes [6]. Most of these structures haveone of three main objectives: (i) preventing sedimentsupply; (ii) changing some features of the sedimenttransport process; and (iii) guiding flows [7].
General presentations of these various measures canbe found in the literature with many sketches, picturesof existing structures and sometimes basic design
criteria [8–13]. A few attempts were made tostandardize some topics of the planning, structuraldesign and maintenance of torrent control works [14–16]. However, the functional planning and design, i.e.
the selection of the type, location and shape of thestructure to be used, escaped this standardization.
Meanwhile, many tests and trials are still performed andstructures with strange, huge or surprising shapes are
sometime built in particular catchments [see e.g. 6, 17,18]. In general, this step of functional design ofmitigation measures remains difficult. Many questions
do not have clear answers, although they seem quite
simple. For instance, how can we select the type ofprotection measures in a given catchment? How todesign them? Or, more generally: why can-it besometimes appropriate to innovate using original
structures, while tried-and-tested solutions defined byeventual industrial standards are suitable in othercatchments?
If the structures used in torrent control are much
more diverse (or weird) than in flood control, it isprobably because they mostly seek to influence theprocesses involved in the sediment cascade which takemany forms and cover a wide spectrum of dynamics.
One could also speak about controlling or interferingwith the sediment connectivity [19]. The study ofsediment connectivity and of solid transport processes ismaking huge progress. The side-effects andconsequences of curtailing soil erosion and disrupting
sediment transfer to the downstream fluvial networks
are thus now much better understood than some decades
ago [20]. This enlightened knowledge of the sedimenttransport process under frequent and extreme events
enables us to define structures that are better adjusted tothe peculiarities of each catchment. Masterplans andstructures are then “tailor-made” to the sites.
In this short paper and the associated keynote
lecture, we propose a possible way to guide theselection, location and definition of hazard mitigation* Corresponding author: guillaume.piton@inrae.fr© 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, 07011 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341507011

measures, notably by using results from hazard
assessments to adjust the mitigation plans to theelementary processes leading to channel malfunctions,i.e. to debris flow deposition and avulsion. In essence,
the idea is to perform a detailed analysis of theconnectivity of sediment transport and to identify theprocesses leading to dis-connectivity. Once thisassessment has been performed, defining mitigation
measures is mostly a matter of modifying the processesor channels with structures so that the process intensity
meets the channel capacity. The approach is describedin further detail in a book chapter to be published [21].

3 Tailoring the strategy to prevent channel
malfunctions
3.1 General approach
Once the main channel “weak points” are identified, thegeneral approach consists of adding structures tomitigate malfunctions in three main ways. (i) By
decreasing the event magnitude using measures thatdiminish the catchment sediment supply. (ii) Bychanging the processes (the presence of boulder andtrunks, discharge, volume), making some undesirable
functions less likely, and thus also the associatedcascading hazards. Some targeted changes can beachieved by using specific open check dams. (iii) By
increasing the channel capacity using guiding structuresthat direct flows in natural areas.
Table 2 provides a qualitative synthesis of the mainfunctions and also lists the structures that can be used toachieve these three main objectives. Figure 1 provides asynthetic diagram showing conceptually how thevarious functions change the magnitude of theprocesses, using frequency curves and/or the associatedpeak discharge and flow depths. The effects of thevarious types of measures are highlighted and compared
to a given reference event of volume V0 of return period
T0, having a peak discharge Q0 and overflowing thechannel of depth hbanks. The figure is conceptual and thus
strongly simplifies reality. In a way, it adjusts theapproach of [22] to debris flows and is inspired by [7].

2 Channel malfunction analysis
Debris flow hazard assessments seek to identify wheredebris-flows might deposit, cause avulsion, or
impact/bury infrastructure, assets and natural or
cultivated areas. Such assessments will necessarily try
to identify which channel sections are too narrow andwhich crossing structures are too small to allow thepassage of the flow fronts and the boulders and large
wood. The flow capacity of channels is also appraised,
as well as the areas of chronic or episodic deposition that
include slope breaks and widened areas or confluences.
When sufficiently detailed, such studies can bereanalysed to identify the triggering factors or processes
leading to massive deposition, overflowing and avulsionof debris-flows. These triggering factors are hereafter
called “channel malfunctions”. The main types ofchannel malfunctions are identified in Table 1. The
associated controlling processes and the driving
parameters that could be used to predict whether or notthese malfunctions are likely to occur is also provided.
Emphasising which channel malfunctions are likely
to emerge and drive debris flow hazards on a given site
is an interesting step to perform before defining themitigation strategy. Doing it after a given debris flowrelated disaster is sometimes not straightforward, but webelieve that in most cases, it is possible to identify whichof them are the best candidates. Several malfunctions
can emerge in cases where the channel is strongly illdesigned or overly-constrained e.g. by urbanisation.

3.2 Prevention of sediment supply
Measures preventing the supply of sediment to thechannel have cumulative effects that can be considered
as merely reducing the frequency of events of thevolume V0 so that its return period become less frequent(T1 > T0). The main structures used to do so are soil
conservation measures, check dams stabilizing channelsor consolidating hillslopes, along with drainage systems
and diversion work that bypass sediment sources.
3.3 Changing the processes with open check dams
Knowledge on the functioning of open check dams has
improved sufficiently to define the type and shape ofstructures that are able to achieve clearly defined
functions [23–25].
Debris flow breakers and large wood trapping
structures are useful to prevent obstructions due to large
boulders and large wood (type-B and type-W
malfunctions, see Table 1), thus preventing overflowing
for discharge Q0’ < Q0 for smaller volumes V0’ < V0 that
happed more frequently T0’ < T0 (Figure 1b).
Barriers with large slits or slots enable a debris
buffering function, i.e. to reduce the peak discharge of agiven volume. If overflow occurs for a given peakdischarge Q0, it occurs for a larger event supply V2 > V0
which is also less frequent T2 > T0.
Structures merely trapping a given volume ofsediment, i.e. having a debris deposition function,
reduce the event volume by the amount that is trapped.
Such structures should have a bottom outlet to allow
frequent events to pass through [24]. The case of water
retention, not included in Figure 1, is covered by [22].

Table 1. Main types of channel malfunctions.
Type
Type-B :Boulder jam
Type-W :Large woodjamType-Q:
Discharge
excess
Type-S:
Deposition atslope breakType-V:
VolumeexcessProcess
Jamming of abridge, culvert ordam by clusters ofboulders
Jamming of bridge,
culvert or dam bywoody debris
Overflowing by
mere excess of peakdischarge
Deposition related
to insufficienttransport capacity
Headward
deposition due to an
excess of sedimentvolumeDriving
parameters
Boulder diameter/ opening size
(width or height)
Large wood
length / opening
width
Peak discharge /channel capacity
Channel slopes
Volume supplied/ channel
buffering
capacity2

E3S Web of Conferences 415, 07011 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341507011

Table 2. Main types of structure functions.
Generalobjective
Prevent
sediment
supplyChangecertainfeatures ofdebris-flows(type, size,discharge,
volume)Guidedebris flows

Function

Effect on the processes

Soilconservation
Channelstabilization
Hillslopeconsolidation
Diversion
Transformation
Wood filtration
Debris buffering

Prevent erosion on hillslopes using vegetation protective
capacity
Prevent long term incision and extended channel erosion
eventsDecrease sediment supply by decreasing landslide androck avalanche movements
Divert water from erosion-prone areas
Break debris flow fronts by trapping boulders
Trap large wood pieces
Transiently store debris to reduce solid peak discharge

Debrisdeposition

Trap a certain volume of sediment

Water retention
Conveyance
Deflection

Transiently store water to reduce water peak discharge
Convey flows along a chosen pathDeflect flows toward less vulnerable areas

Typical structures used toachieve the function
Afforestation, fascine, terrasses,
soil bioenginnering
Check dams, channel lining
Drainage systems, high or seriesof check dams
Tunnels, by-pass channels
Debris flow breakers
Racks, Austrian-dams
Open check dams with large slots
Open check dams with small slots
or with grills, flexible barriers,
large check dams
Flood retention large dams
Dikes, bank protections, groynes
Diversion berms

Figure 1. Conceptual multi-panel diagram representing the initial channel functioning in black and changes of processes in colors

3

E3S Web of Conferences 415, 07011 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341507011

O., Earth Surf. Process. Landf. 42(1), 91–108(2017).
5. Evette, A., Labonne, S., Rey, F., Liebault, F.,
Jancke, O. and Girel, J., Environ. Manage. 43(6),972–984 (2009).
6. Hübl, J. and Fiebiger, G., Debris-flow hazards andrelated phenomena, Springer, Berlin, Heidelberg,
445–487 (2005).
7. Hübl, J., Water 10(12), 1718 (2018).
8. Eisbacher, RH., Geosci. Can. 9(1), 14–27 (1982).
9. Kronfellner-Kraus, G., Sabo Gakkaishi - J. Jpn.
Soc. Eros. Control Eng. 35(3), 33–44 (1983).
10. Ikeya, H., Bull. - Int. Assoc. Eng. Geol. 40(1), 15–33 (1989).
11. Chatwin, S. C., Howes, D. E., Schwab, J. W. andSwanston, D. N., A Guide for Management ofLandslide-Prone Terrain in the Pacific Northwest,(BC Ministry of Forests, 1994).
12. VanDine, D. F., Debris Flow Control Structures
for Forest Engineering, (Res. Br., B.C. Min. For.,
Victoria, BC, Canada, 1996).
13. Bergmeister, K., Suda, J., Hübl, J. and RudolfMiklau, F., Schutzbauwerke gegen
Wildbachgefahren: Grundlagen, Entwurf undBemessung, Beispiele, (Ernst & Sohn, 2009).
14. NILIM., Manual of Technical Standards for
Establishing Sabo Master Plans for Debris Flowsand Driftwood, (National Institute for Land andInfrastructure Management , Tsukuba, Japan,2007).
15. Rudolf-Miklau, F. and Suda, J., Ital. J. Eng. Geol.
Environ. (201103), 1083–1091 (2011).
16. Hubl, J., Nagl, G., Suda, J. and Rudolf-Miklau, F.,
Int. J. Eros. Control Eng. 10(1), 47–55 (2017).
17. Liu, F. Z., Xu, Q., Dong, X. J., Yu, B., Frost, J. D.
and Li, H. J., Landslides 14(6), 2089–2104 (2017).
18. Moase, E. E., Guidance for debris-flow anddebris-flood mitigation design in Canada,(Master’s Thesis, Simon Fraser University, BC,Canada, 2017).
19. Marchi, L., Comiti, F., Crema, S. and Cavalli, M.,
Sci. Total Environ. 668, 389–399 (2019).
20. Rinaldi, M., Piégay, H. and Surian, N., Geophys.
Monogr. Ser. 194, 95–113 (2011).
21. Piton, G., D’Agostino, V., Horiguchi, T., Ikeda, A.
and Hübl, J., Advances in Debris-Flow Scienceand Practice, Chap. 6.1 (Springer Verlag, 2022).
22. Bokhove, O., Kelmanson, M. A., Kent, T., Piton,
G. and Tacnet, JM., River Res. Appl. 35(9), 1402–1414 (2019).
23. Piton, G. and Recking, A., J. Hydraul. Eng.
142(2), 1–23 (2016).
24. Moldenhauer-Roth, A., Piton, G., Schwindt, S.,
Jafarnejad, M. and Schleiss, A. J., Int. J. SedimentRes. 36(1), 136–150 (2021).
25. Piton, G., Goodwin, S. R., Mark, E. and Strouth,A., J. Geophys. Res. Earth Surf. 127(5) (2022).

3.4 Guiding flows in selected areas
For a given flow depth, guiding structures, i.e. dikes andbank protection directing flow along a chosen path, ordeflection berms sending flows to low vulnerability
areas, enable the discharge capacity to be increased toQ3 > Q0. The associated volume that can be handled
without damage is then increased to V3 > V0, thus also
reducing the frequency of overflowing to T3 > T0.

4 Concluding remarks
A comprehensive approach adapted to a topic ascomplex as the definition and design of debris flow
mitigation measures cannot be completely presented infour pages. The present extended abstract is thus a quick
and simplistic overview of one possible approach. The
general idea is that design engineers should not only
have a view of the magnitude and frequency of thedebris flow supplied by the catchment (within normallimitations), but must also have a detailed understandingof the way that debris-flows pass, deposit, overflow andcause avulsion along the downstream channel.
Defining mitigation measures is then a matter of
selecting the measures that will modify the magnitudeof the event, and/or modifying the process or thechannel, so that malfunctions emerge more rarely. In thesimplest cases, standardized structures will be sufficient.
In large catchments were several geomorphologicalprocesses interact, a tailor-made strategies should bedefined. In some weird sites with specific features andcomplex emerging processes, this definition requires
innovation (the haute-couture case). This extended
abstract presents the approach quickly. Tables 1 and 2
are useful for simple classifications of the types ofmalfunctions and the main functions that structuresmight achieve. Figure 1 is also a synthetic diagram
showing how these types of structures conceptuallychange the debris flow frequency, volume, peakdischarge and peak flow depth. When using several
types of structures, such effects will be combined andadded. We hope that this short contribution can help tochoose an approach to address the complicatedchallenge of defining mitigation strategies.
The author would like to thanks Dr. Saoirse Robin
Goodwin for her review of this paper and Prof. Marina Pirulli
for inviting him to present this work at DFHM8 and fororganizing and hosting the conference.

References
1.

2.
3.

4.

Laigle, D. and Bardou, E., Reference Module in
Earth Systems and Environmental Sciences,Elsevier, 61–84 (2022).
Hilker, N., Badoux, A. and Hegg, C., Nat. Hazards
Earth Syst. Sci. 9(3), 913–925 (2009).
Skermer, Nigel. A. and VanDine, Douglas. F.,
Debris-flow Hazards and Related Phenomena, 25–51 (Springer Berlin Heidelberg, 2005).
Piton, G., Carladous, S., Recking, A., Liebault, F.,
Tacnet, J., Kuss, D., Quefféléan, Y. and Marco,4