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The Rosetta Stone Project – Integrating experimental results
on debris flow mechanics across the scales: next results
Elisabeth T. Bowman1*, Amanda Fawley2, Roland Kaitna3, Shuai Li4, Brian McArdell5, Jim McElwaine6, Nicoletta
Sanvitale7, W. Andrew Take2, Lisa Tauskela2, Alex Taylor-Noonan2, and Yuting Zhao1
1

Department of Civil & Structural Engineering, University of Sheffield, Sheffield, United Kingdom
Department of Civil Engineering, Queen’s University, Kingston, Ontario, Canada
3
Institute of Mountain Risk Engineering, University of Natural Resources & Life Sciences, Vienna, Austria
4Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu, China
5
Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Birmensdorf, Switzerland
6
Department of Earth Sciences, Durham University, Durham, United Kingdom
7Department of Civil, Chemical, Environmental, and Materials Engineering, University of Bologna, Bologna, Italy
2Abstract. We present the combined efforts of a research network designed to address the many challenges
in the experimental modelling of debris flow phenomena. The approach has been to use apparatuses ofdifferent functional arrangement and at different scale with identical and commonly sourced flow materials
from the highly idealised (dry, coarse and uniform) to the highly complex (well graded, segregating, fluid
saturated). Here we briefly present some key findings of the network and point to the research questions thatare currently being addressed. This complementary view of experimental debris flows helps to constrain
methodological artefacts/scale effects and to identify key processes responsible for the diverse appearance
and often high mobility of debris flows.

1 Introduction
Debris flows consist of poorly sorted (polydisperse orwell-graded) solids that are generally fluid saturated,
however, the majority of flume tests conducted in thelaboratory focus on much more simple arrangements –at the extreme these may be dry granular flows of
uniformly sized particles. This reticence to test fluid
saturated flows is understandable as these flume tests
pose significant additional challenges, including
complex pore pressure effects due to coupled stressdilatancy effects (e.g. negative or positive excess porewater pressures during shearing), often contradictory or
confusing basal pore pressure measurement data, andthe potential for a greater influence of scale effects overexperiments conducted on dry material.
The Rosetta Stone International Experimental
Network on Debris Flows commenced in August 2017[1] as a partnership between five institutions (TheUniversity of Sheffield, UK, Queen’s University,Canada, IAN-BOKU, Austria, WSL, Switzerland andDurham University, UK.) with the objective to use thecomplementary expertise and experimental facilities
available (flumes of varying size, a vertical drum, and ahighly instrumented field site) to explore and address thechallenges of physically modelling of fluid saturated
flows over a range of pore-sized materials and geometricscales.
The core idea of the network was that every
participating laboratory would have their own approachto analysing the data (e.g. geotechnical engineering,fluid mechanics, granular physics), but would test
identical materials purchased in the same lot from asingle supplier, to act as a ‘Rosetta Stone’ to translate
between viewpoints, notation, and approaches. Over thepast five years the network has worked together toexplore saturated granular flows in materials ofincreasing complexity, commencing with the mostsimple of material arrangements and proceeding to themost complex that can be accommodated in theapparatuses, informed by the field condition aspresented at the well-instrumented and documented
Illgraben site in Switzerland [1] – (i) dry, uniform,
particles of relatively large size, (ii) saturated, uniform
particles of the same size, (iii) saturated well-gradedflows at specified water content, (iv) variable watercontent tests of both uniform and well graded flows.
The objective of this extended abstract andassociated presentation is to provide an overview of thefindings of the network, including highlights describing
advances in experimental testing methods targeting both
dry and fluid saturated granular flows, before exploring
the observations of complex pore pressure effects insaturated granular flows, and concluding withoutstanding research questions.

2 Experiments
2.1 Advances in experimental testing required
for saturated flows
2.1.1 Release Box
The typical arrangement of laboratory flumes include a
release box with a rotating or vertically lifting gate at thetop of the flume to expose an unsupported height of thesource volume to initiate landslide motion (Fig. 1a). In
saturated materials, particularly comprising particles of* Corresponding author: e.bowman@sheffield.ac.uk© 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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fine sands or silts, the initial packing of the saturated
granular material is of extreme importance. If the void
ratio of the soil is denser than the critical state void ratio,
the shearing of the soil upon release can result in a
dilatant response, leading to negative excess pore waterpressures to such an extent that the saturated material
does not release. Saturated dam break tests intransparent soil conducted by [2] to explore thisphenomenon has shown that the behaviour of dilatantcolumns is limited by the pore air entry value and hence,the particle size of the soil, and is particularly
problematic for finer and more densely packed soils.
The limit equilibrium model of stability proposed by [2]illustrates that the size of the flume (e.g. the unsupported
height of the release box) and the particle size of thematerial have significant impacts on the initial
triggering of instability.
These results indicate that the preparation of a loose
source volume (i.e. contractive on shearing) is essential
for flume tests on fluid saturated flows. Two approaches
were developed in the network to achieve this,depending on the scale of the flume. In small scale flume
testing (e.g. University of Sheffield flumes withvolumes up to 8L) it is possible to ensure that saturated
soils remain loosely packed by careful stirring by hand.
In large scale testing (e.g. Queen’s University flumewith volume up to 1 m3) it is not possible to do this. In
the latter case, an upward hydraulic gradient system has
been developed for the flume release box to liquefy thesand following placement to ensure it is in a contractive
state following reconsolidation prior to the opening of
the release box.

an overriding flow. As illustrated in this cartoon, porepressure responses of replicate sensors installed at thesame location of the flume can yield highly
contradictory and internally-inconsistent responses. In
the extreme example depicted in Fig. 2, sensors may
diverge between recoding a positive or negative porepressure despite being at the same nominal location inthe flow. Research is underway to examine the reasons
behind this behaviour – and to devise a method by which
the “true” pore pressure response may be captured withconfidence.

Fig. 2. Schematic of pore pressure response of basaltransducers under shearing flow.

2.1.3 Image processing
Granular flow profiles are able to be determined at thesidewalls in experimental flumes, drums and conveyors,
via the analysis of images taken at sufficiently highspeed and resolution to obtain measures such as flow
velocity and granular temperature. Typical image
processing methods include Particle Image Velocimetry(PIV) – in which interrogation regions of image“texture” are followed between frames to establish
displacements (Fig. 3 – top) and Particle Tracking
Velocimetry (PTV) in which particle centroids areidentified via thresholding and then tracked between
frames.
In general, the success of the different techniques inmeasuring velocity profiles is well established,however, there are problems with the majority ofmethods in the correct determination of granulartemperature [3] – with both over and underestimation
possible. Within the network, research has been
conducted to develop a method that benefits from theunique strengths of both PIV and PTV and avoids their
weaknesses [4]. The new method, termed “hybrid PIV”
uses the texture (PIV) of individual particles (PTV) tofollow their motion discretely (Fig 3 – bottom).
Comparison with numerical results shows that thismethod is better able to capture the correct granulartemperature of shearing granular systems across both
flume and drum experiments.

Fig. 1. Potential release issues in saturated debris flow
experiments: (a) release box arrangement (b) stability ofunsupported face for dense soil as a function of particle size.

2.1.2 Measurement of basal pore pressure
Pore pressure is notoriously difficult to measure in ashearing saturated granular flow. Fig. 2 shows typicalindicative responses of pore pressure transducers to such2E3S Web of Conferences 415, 02003 (2023)
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The influence of water on the mobility of flows is
not clear. It has been suggested that excess porepressures are key to the high mobility of field scaledebris flows (e.g. [1]), however these may not exist insmaller laboratory flows [5]. In addition, the travel angleof non-fluid saturated natural flows (rock and snowavalanches) is shown to vary with volume [6].
Here, parametric studies on both coarse, uniform
materials under dry and wet (variable moisture content)
conditions and at different scales (large and small
flumes with variable source volumes) allows theinfluence of pore pressure to be systematicallyexamined. Fig. 4 shows the possibilities – withcomparison made against the well-defined results of dryflows. That is, (i) the travel angle of a dry granular flow
may not be expected to vary significantly with volume;(ii) the addition of fluid could have no effect on thetravel angle due to the large particle size, or (iii) could
have a potentially non-linear monotonically increasing
response due to positive pore pressure, or indeed
capillary effects may be induced which could dominate
at small scale, and then be eliminated at large scale
resulting a non-monotonic effect. These possibilities arebeing examined via systematic variation of watercontent in small flume experiments.

Fig. 3. Regular grid-type PIV (top) tracking; Hybrid PIV
tracking (bottom).

3 The importance of pore pressure

Fig. 4. Potential response of travel angle to variation in watercontent.

In providing a valuable critique of laboratory modellingof debris flows compared with field scale processes
Iverson (2015) has stated [5] that “similarity of form
does not imply similarity of process”. That is, it may be
expected that larger flows will develop and sustain porepressures that in turn reduce interparticle friction andthereby produce longer runout under Coulomb resistive
friction than smaller flows, which may instead, be
expected to develop resistance due to fluid viscosity,
surface tension and particle cohesion.
The veracity of this idea can be addressed byexamining differences in behaviour of flows at different
scale (e.g. large and small flumes) and materials.

An alternative approach to constrain the effect offluid on the mobility of debris flows is to conduct
experiments in a re-circulating drum environment. Suchtests with dry and saturated granular materials enable an
examination of the influence of moisture on the flowgeometry, resistance to flow (via total stress and porepressure measurements at a section and via overalltorque) and internal shearing behaviour (via image
analysis at the side walls) to be undertaken. The
advantage of this approach is that the steady flow ofdifferent material volumes can be observed over a long
period of time and measured flow parameters can bestatistically analysed.
A metric to compare the mobility of drum flows withexperiments of identical material in straight flume
configurations is the deviation angle of the centre ofmass from the vertical (Fig. 5). Our observations focus
on the pore pressure in coarse uniform flows, compared
to well-graded (poorly sorted) flows; and how this3.1 Mobility and water content
Flume testing allows the measurement of “mobility” as
described by travel angle (arctan H/L), where H is theheight of the centre of gravity of the commencement offlow release to the length L of the centre of gravity ofthe deposit.

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Fig. 6. Potential response of travel angle to variation insource volume.

moderates flow resistance. We also examine the scaledependence of dilation of the flows in both wet and dryconditions.

4 Conclusions
Over the past five years the Rosetta network [8] has
worked together to explore granular flows in materials
of increasing complexity, commencing with the mostsimple of material arrangements (dry, uniform, coarse)
to the most complex (fluid saturated, well-graded, at a
range of specified water contents).
We present highlights that describe advances inexperimental testing methods targeting both dry andfluid saturated granular flows, and explore theobservations of complex pore pressure effects insaturated granular flows. Outstanding researchquestions are:
• How can we ensure that the measurement of porepressure in laboratory and field tests are accurate(Fig. 2)?
• What is the influence of water content on themobility of debris flows and how is this affected by
the material composition (Fig. 4)?
• How does source volume – both expressed as avariation in flow mass within a given experimental
flume and as differences between flumes ofdifferent scale – affect the mobility of debris flows
and how is this linked to pore pressure response insaturated flows (Fig. 6)?

Fig. 5. Schematic of BOKU drum with typical granular flow
arrangement – centre of mass varies with drum rotation
speed, soil type, fluid content and overall mass

3.2 Mobility and scale
Flume tests of a similar configuration (30° slope
angle, fixed basal roughness) conducted at large
scale (2 m wide, 8.7 m long) and small scale (100mm wide, 1.4 m long) with otherwise the samematerials, enable scale influences on mobility to beassessed for both coarse granular flows (under dryand saturated conditions) and well gradedmaterials. Scale effects may exist due to volumetricresponses (soil dilation, suction, and viscosityinfluences) for tests conducted on a given flumeand between apparatuses of different scale.
Fig 6 shows how the travel angle might be expected
to change with the source volume for large and small
flumes. Results on monodisperse (uniform) coarse flows
within large flume experiments are available in [7] – andthese indicate a clear dependency of travel angle withfluid content for these larger flows. Comparison withsmaller flume flows is also underway.

This research was made possible by the support of aLeverhulme Trust International Network grant, IN-2016-041“The Rosetta Stone Network: Physical testing towards acommon understanding of debris flow”. The authors also wish
to thank the technical support teams at the different institutes
who have made the experimental work possible.

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