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Small-scale study of Debris-Flows Interactions with a Lateral
Debris Basin and Crossings: The Manival Torrent case study
Damien Alliau1, Guillaume Piton2, Guillaume Chambon2, Anne-Sophie Drouet3, Vincent Koulinski4, Damien Kuss3,
Dominique LAIGLE2,*, Vincent Mano6, Clément Misset5, Christophe Peteuil1, Sébastien Roux1, and Pierre Verry6
1 CNR, Centre d'Analyse Comportementale des Ouvrages Hydrauliques CACOH, 4, rue de Chalon sur Saône, 69007 Lyon, France
2 INRAE, UR ETNA, Domaine Universitaire, 2 rue de la Papeterie, BP76, 38402 Saint-Martin-d’Hères Cedex, France
3 SYMBHI, Syndicat mixte des bassins hydrauliques de l'Isère, 9 Rue Jean Bocq, 38000 Grenoble, France
4 E.T.R.M., 581 Route de Mont Carmel, Chef-Lieu, 73700 Les Chapelles, France
5 Office National des Forêts - Direction Forêt & Risques Naturels, 9 quai Créqui, 38 000 Grenoble, France
6 Office National des Forêts - service de Restauration des Terrains en Montagne de l’Isère, 9 quai Créqui, 38 000 Grenoble, FranceAbstract. Small-scale models are useful tools to study the interactions between debris flows and structuresand channels. Small-scale modelling of debris flows remains however complicated because of the complex
rheology and scaling challenges of these geophysical processes. An on-going study of a debris basin and thedownstream channel where two fords and a bridge are located is presented in this extended abstract. The
studied torrent is the Manival catchment, located near Grenoble in France. We present the catchment
features, the scientific questions studied, some preliminary calibration results describing the mixtures usedto model debris flows as well as results from three debris-flood and two debris-flow runs. In essence, themodel highlighted that the structure enable a large share of the bedload transport to pass downstream. Debris
flows can be more or equally trapped depending on their rheology which controls the surges dynamics andthe deposition slope in the debris basin.

1 Introduction
Retention basins are common mitigation measuresagainst debris-flow hazards. Even if thousands of such
structures exist in mountain areas of Europe and Asia,
the interactions between those structures and debris
flows and floods are insufficiently known and many
open questions remain. Indeed field observation of suchinteractions are very rare. As a consequence, structures
are designed empirically with simple criteria and often
induce side effects as excess in sediment trapping
possibly leading to downstream sediment starvation andhigh maintenance costs [1].
Although progressing rapidly, numerical models are
not yet capable to comprehensively simulate the many
processes involved in debris-flow trapping, e.g., flow
spreading, effect of grain segregation, interactions
between boulders and open check dams. Numericalmodels tend to correctly capture some of these effects
while neglecting others [2–4].
Small scale models remain thus interesting options
to study structures and sites with complex topography
and interactions with structures [e.g. 5–7]. In this paper,
we present an on-going study using a small-scale modelof the Manival torrent, namely its debris basin and thedownstream channel that is crossed by two fords. The
first and second sections of this paper present the mainfeatures of the catchment and the model. The last section
describes preliminary experiments predesigning the* Corresponding author:

flow mixtures, as well as two experiments of the study:
more results will be available at the conference.

2 The Manival catchment
The Manival torrent is located in the Chartreuse
mountain range about 10-km north of Grenoble, in the
Northern French Prealps [8]. A more than 300 m-highJurassic marly limestone cliff, along with thick colluvial
deposits below rock-walls are source of debris flows. Its
steep slopes (mean channel slope: 0.16 m.m-1; Melton
Index: 0.62) and its high potential sediment supply makethe catchment pretty active. The Manival torrent
experiences bedload transport, debris floods and debris
flow with, on average one major event every three years.
Its catchment has a drainage area of 3.6 km² at the inletof the modelled debris basin and downstream reach.
Two monitoring stations are located in the torrent, one
in the debris basin and another upstream [9,10].
To mitigate the associated risks, torrent control
works have been implemented since the 1890s by theFrench torrent control service (ONF-RTM). Nowadays,
the torrent is equipped with more than 160 check dams,dikes and a debris basin (located 45.271°N, 5.832°E).
This basin has a length of about 100 m and is closed by
an open check dam (Fig. 1). The check dams prevent thebed incision and at confining the flows inside a
10 – 20 m wide single channel.

dominique.laigle@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/).

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Fig. 1. Views of the Manival debris basin: A) view from upstream and B) view from downstream (courtesy of O. Lefebvre)

With its initial shape, the debris basin used toexperience chronic sediment deposit during bedload anddebris-flow events. Deposits were driven by its large
width and a very gentle slope (7 % within the basincompared to 12-13 % upstream), in addition to therelatively small openings of the open check dam [11].
As a consequence, the debris basin was modified in2018 to evolve toward a derivation configuration [1]: aguiding channel with a steeper, regular slope was addedin the debris basin; an intermediate interception
structure was built at the upstream end of the guidingchannel; a lateral weir was also built guiding high flows
toward the debris basin which is now located on the leftbank and excavated deeply below the guiding channel
level. The change of design was motivated by an
incision of the channel that could reach up 10 m
downstream of the debris basin [1]. The objective wasto restore sediment connectivity for low intensity events
while keeping a sufficient volume storage for rare
events. During routine events, i.e., low magnitude
bedload transport events, flows are supposed to stay
confined in the guiding channel allowing an almost full
sediment connectivity. During debris flows, flows wereexpected to be redirected toward the side weir and bepartially trapped by filling the basin.
Those adaptations may significantly change theprotection system operation. In addition, the volume of
the 100-year return period debris flow, which isestimated at about 30 000 m³, strongly exceeds thedebris basin storage capacity that was estimated in therange 10 000 – 17 000 m³ depending on the deposition
slope [12]. It is thus anticipated that a large share of thedebris flows could be transferred downstream. Twofords and a bridge located on the downstream channel
may be potential avulsion points because of the lower
level of the lateral banks at these locations. Moreover,the small height of the banks at those places could leadto the uncontrolled flooding of riverine fields and houses
in case of debris flood. The channel has a slope of about12% near the debris basin, but it reduces to about 5%
when approaching the bridge some 2 km downstream.

3 Objective of the study
Considering the context, the local basin agency(SYMBHI) and the torrent control office (ONF-RTM)
decided to perform a small-scale model study: (i) tocheck whether the routine event transfer and thetrapping of the project design event actually occur, andwhether adaptations of the debris basin may optimize
this functioning. Adaptations of the fords and bridges
can also be studied to decrease flow levels anddeposition trends; (ii) to better understand the deposition
mechanisms (spatial and temporal dynamics of filling,effective storage volume, slope of deposition); (iii) tomeasure the ratio of supplied volume to the volume
passing the debris basin and reaching the downstream
channel as a function of the nature and intensity ofevents; (iv) to measure the flow levels at the debris
basin, interception structure and its outlet barrier; (v) toserve as benchmark data to calibrate numerical models
of sediment transfer throughout the basin and thedownstream channel; and finally (vi) to convince
stakeholders about the usefulness of debris basins with
derivation configurations, in order to consider
equivalent adaptations of the numerous debris basin
facing similar problems of excessive trapping [1].

4 The experiments
The small-scale model study is performed at CNR
hydraulic flows and structures laboratory located in
Lyon (France). A small flume (Fig. 3) was first used todefine the debris flow mixtures, later tested on a small
scale model, both of them at a scale of 1:25 (Fig. 2).
For debris flows, the experiments were designed usingthe Froude number similitude, as well as similitude
criteria assuming a Herschel-Bulkley rheology [13],which is assumed to be suitable for viscous muddy
debris flows [i.e. 13], occurring in the Manival torrent.

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A)

B)
Fig. 2. A) Digital view of the small-scale model; B) View from downstream of the debris basin after a “fluid” debris flow run

A

4.2 The small scale model tests

BThe small-scale model has a variable width of 2.5 to
6.3 m and is 22.5 m long (Fig. 2). This length isequivalent to about 550 m at prototype scale. The actual
distance between the inlet of the debris basin and theroad bridge being about 2.2 km, the intermediate
reaches located downstream of the basin wereshortened. Overall, the model represents at prototype
scale 60 m of the upstream channel, 112 m of debris
basin, 95 m of downstream channel equipped with checkdams, 80 m of channel with a ford on a check dam and218 m of the downstream natural channel whose outlet
is the road bridge. The mean slope is about 5% in thislast section, and 12% on the upstream channel.
Several cameras filmed the runs, an ultrasonic sensor
measured the flow level at the open check dam and ascale weighted the mass exported out of the model. The
model banks are made of concrete and are thus nonerodible. The model bed is made of mobile sediment
whose grain size distribution was scaled down fromfield samples (model median grain size: 1.36 mm).
Three debris flood experiments were performed withbedload transport fed by a hopper. They confirmed thetrend observed in the field: namely, the new
configuration with a guiding channel restores thebedload transport connectivity. If no jamming occurs at
the open check dam, only about one third of the
15,000 m³ of bedload supplied by a 100-yrs debris flood
event would be trapped in the debris basin, the rest beingtransferred in the downstream reaches with strong
deposition in the 5 %-steep reach (only about 1,500 m³
is transported downstream of this mild reach). If ajamming of the open check dam occurs, up to 11,000 m³of bedload is trapped in the basin and the remaining
volume is transferred downstream of the modelFig. 3. Calibration flume tests: propagation of A: fluidmixture, B: viscous mixture

4.1 The small flume tests
Mixtures were prepared with various content of water,clay (kaolin) and two sands (grain sizes: 0-1.1 mm and3 mm). A 3 m-long, 0.18 m-wide flume was built to help
calibrating the mixture (Fig. 3). Calibration tests wereperformed releasing 300 kg of mixture varying thecontents of water, clay and sand and measuring flow
wave properties (front velocity, height of the flow andresidual deposit) over a slope of 13% corresponding to
the actual channel slope.
The objective was to define two suitable mixtures
mimicking (i) a rather fluid debris flow (criterion: yieldstress leading to 0.4 m-thick deposit in nature, roughly
0.018 m on the model). This was achieved using thefollowing mass ratio: 31.3% of water, 18.8% of kaolinclay, 50% of 0-20 mm sand and gravel (Fig. 3A). (ii)
Another more viscous debris flow (criterion: yield-stressleading to 1.5 m-thick deposit in nature, roughly 0.06 m
on the model) was achieved using the following mass
ratio:16% of water, 7.5% of kaolin clay, 76.5% of 0-20mm sand and gravel (Fig. 3B).

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including some volume eroded in the bed through a“hungry water” effect.
After the calibration tests using 300 kg of the two
representative debris-flow mixtures, runs were
performed with the same materials for volumes
corresponding to full scale 100 years return periodevents: 30,000 m³ at prototype scale, i.e. about 4 tons ofmaterial on the small-scale model (see e.g. Fig. 2B for
the debris basin at the end of the first debris-flow run).
Each run was performed releasing this volume in two
surges coming from the two mixing tanks each
preparing two tons of material. The two tests show
similarities and differences as listed below (note that theprototype scale is used consistently in all descriptions).
In both cases, (i) the flow was released in 5 – 10 min
leading to peak discharges of 100 – 130 m³/s. Althoughslightly higher than expected, this range is in between
typical values expected for muddy and granular debris
according to the empirical equation of [14]; (ii) the flow
was partially diverted toward the side basin whichtrapped some material upstream of the open check dam;(iii) the open check dam was jammed by coarse grains
almost immediately which was expected since the grain
size (max diameter 20 mm) was more than half of theopening width (20 – 32 mm); and (iv) during the second
surge, the flow depth largely submerged the open checkdam.
The main difference between the fluid and theviscous mixtures were obvious effects associated to thehigher mobility of the first mixture: (i) the debris basin
trapped only about 8,000 m³ of the first while almost
22,000 m³ of the latter; (ii) this capacity is directly
related to the steeper deposition slope of the viscous
mixture (about 13%) as compared to the gentler
deposition slope of the fluid mixture (4-6%); (iii) thefluid mixture was also faster (front velocity ≈ 5 m/s)
than the viscous (front velocity ≈ 1.3 m/s), and (iv) the
fluid mixture transferred debris flows much further
since more than 8,000 m³ of material left the flumeoutlet (and about 3,000 m³ were kept in the feeding
system to protect the downstream weighting systemfrom an overloading) while no debris flow reached theflume outlet during the viscous mixture run: all material
was either trapped in the debris basin or deposited in thedownstream channel.

to calibrate numerical models of both debris flows andbedload transport. These calibrated models will then be
used to study further options and different geometry of
the channel and structures.

References
1.

Carladous, S., Piton, G., Kuss, D., Charvet, G.,Paulhe, R., Morel, M. and Quefféléan, Y., Check
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5 Concluding remarks & perspectives
This small-scale model study of debris flows is thefirst of its kind performed for engineering purpose fortwo decades in France. It is proving very useful
information to better understand debris flow trajectories
over protection structures with complex geometry. In
the next phases, steps of adaptations of the debris basin
and of the downstream channel will be performed.
These adaptions will seek to optimize the debris flow
spreading in the basin to maximize the trapping, and toadjust the ford and bridge geometries to minimize theprobability of avulsion or bank overflowing.
At a later stage, the data acquired during theexperiments (3D scan of the deposit during the flow andat the end, velocities and flow elevations) will be used4