E3S Web of Conferences 415, 01028 (2023)
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https://doi.org/10.1051/e3sconf/202341501028

Measurements of debris flow entrainment and dynamics
Hervé Vicari1,2*, Charles W. W. Ng3, Steinar Nordal4, Vikas Thakur4, W. A. Roanga K. De Silva3, Haiming Liu3, andClarence E. Choi5
1Norwegian Geotechnical Institute (NGI), Natural Hazards section, Oslo, Norway
2Current affiliation: WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
3Hong Kong University of Science and Technology (HKUST), Hong Kong
4Norwegian University of Science and Technology (NTNU), Civil and Environmental Engineering, Trondheim, Norway
5Hong Kong University (HKU), Hong Kong

Abstract. The mechanisms of debris flows and their interaction with mitigation structures are still not wellunderstood. Among the research challenges, only few entrainment measurements are available in literature,as entrainment is often masked by deposition on top. In this paper, we present a simple, cheap, and effective
method to measure the entrainment depths. Flume experiments have therefore been performed to assess theinfluence of the initial debris flow volume and of an upstream flexible barrier on entrainment. To better
understand the debris flow dynamics, the flow basal stresses have been measured. A high degree ofliquefaction at the base of the debris flow is observed. A mitigation measure to reduce entrainment has also
been studied. A compact flexible barrier was installed in the upstream part of the channel and is observed todeflect the flow along a curvilinear path. High normal stresses are measured at the base of the overflow,which are caused by the additional centrifugal stresses from the overflow. The results from the flume testssuggest that the flow interaction with an upstream flexible barrier may significantly influence the debris
flow dynamics both upstream and downstream of the barrier.

1 Introduction
Debris flows typically increase in scale by entraining
soil, fluid and boulders along the flow path. The
entrainment process may significantly influence thedebris flow dynamics, due to the increase of the debris
flow volume [1] and to the generation of excess porepressures in the erodible bed [2]. To understand andmodel debris flow entrainment, reliable measurementsof the entrainment depths are needed. Some methods
have been proposed in literature to measure debris flow
entrainment. For instance, laser technologies have been
applied [3] to measure the net erosion (difference
between erosion and deposition depth). In this case,however, entrainment cannot be distinguished fromdeposition. Electronic sensors have been successfully
applied to measure the entrainment depths [4,5].
However, they may be complex and expensive to installin the field. In this work, we describe a simple but
effective methodology to measure the entrainment
depths, which is applied in large-scale flumeexperiments.
The development of excess pore pressures within thedebris flow may also significantly affect the flow basal
stresses and therefore the dynamics of debris flows [6].
Hence, flow basal stresses are measured in the largescale flume tests and compared to values in literature.
Furthermore, the presence of mitigation measures suchas flexible barriers may also significantly affect thedebris flow dynamics. In this work we reportmeasurements of the flow basal stresses affected by acompact flexible barrier.

2 Methodology
2.1 Physical flume modelling
Large-scale flume experiments were conducted by
Vicari et al. (2021) to study the entrainment of a wet soil
bed by a debris flow [7]. Fig. 1 shows the 28 m-long
flume [8] which was used to perform the tests. Initial
volumes of wet debris of 2.5 and 6 m3 were placed in astorage container. The debris could be triggered by dambreak, by opening a 1 m-tall door. The debris material
was then flowing along a 15 m-long 2 m-wide channel
inclined at 20°. The base of the initial part of the channel(9 m) is fixed (non-erodible), which allowed the flow todevelop and acquire a typical elongated shape. Wet soil
was placed over the last 6 m of the inclined channel andhas a thickness of 120 mm. The inclined channel ends ina horizontal 4.4 m-long runout section. At the end of therunout section, a terminal flexible barrier was placed toarrest the flowing mass. Instrumentation was placed in
the channel to measure flow depths and velocities, basal
stresses, and entrainment depths along the erodible bed.
Three tests were performed (Table 1), The first two testswere executed without any upstream barrier, with initial
volumes of 2.5 and 6 m3 respectively. In the third test, a0.6 m-tall flexible barrier was placed at 4.3 m from the* Corresponding author: herve.vicari@slf.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/).

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gate, to study the effect of an upstream flexible barrieron entrainment reduction.

in place) (Fig. 2d), which allows the calculation of theentrainment depth, e:e = ne ‧ tn

(1)

where tn is the thickness of each nut (in the experiments,5 mm). The deposition depth is also measured after eachtest, as the debris thickness above the remaining nuts.
Fig. 2e shows an erosion column after a test. A similar
instrumentation to measure entrainment was also
successfully used in small-scale experiments by [9,10].
Fig. 1. Flume model.

3 Test results and discussionTable 1. Test program.
Release flow
volume (m3)Flexible barriers
setupV2.5-B1

2.5

Terminal flexible

V6-B1

6

Terminal flexiblebarrier

V6-B2

6

Upstream + Terminalflexible barriers

Test ID

3.1 Entrainment depths
Fig. 3 shows the measured entrainment depths for thethree tests. The entrainment is typically higher at thestart of the erodible bed, due to a ploughing entrainment
mechanism [11,12]. Entrainment depths for test V6-B1
are higher compared to the entrainment depths of test
V2.5-B1, which is due to a higher initial volume (6 m3vs. 2.5 m3 respectively).
Conversely, in test V6-B2, the lowest entrainment
depths are measured. Indeed, the upstream flexible
barrier retained part of the initial volume (1.2 m3),
dissipated energy and reduced the flow velocity.
Furthermore, the upstream flexible barrier was observedto split the flow into two distinct surges, which eroded
significantly less bed material compared to the other twotests.

2.2 Instrumentation to measure theentrainment depthFig. 3. Measured entrainment depths in [7].

3.2 Flow basal stresses
A load cell was installed at the base of the flume bed at
3.4 m from the gate to measure both the normal andshear stresses. The load cell was connected to a surface
plate which is roughened through epoxy and sand. On
top of the load cell, an ultrasonic sensor measures theflow depth normal to the slope.
Fig. 4a and 4b show the measured flow depth andbasal stresses for tests V6-B1 and V6-B2 respectively.
In test V6-B1, the measured normal stress is observed tohave a similar trend and shape compared to themeasured flow depth. The normal stress can bemodelled by considering the static equilibrium of adebris flow column perpendicular to the channel bed:

Fig. 2. Erosion column designed by [7].

A technique has been developed in [7] to measure theentrainment depths along the erodible bed (Fig. 2)
allowing to differentiate entrainment from deposition.
Columns of nuts are installed through a bolt into theerodible bed prior to its construction (Fig. 2a). The soil
bed is then prepared (Fig. 2b). Afterwards, the bolts areremoved, and the nuts are therefore free to be entrainedwith the erodible bed (Fig. 2c). After the test, the numberof nuts entrained, ne, at each column location is counted(difference between initial number of nuts and nuts left

σn,stat = ρf g hf cosθ(2)

where ρf is the debris flow density (measured equal to2155 kg/m3, based on the flow composition), g is theacceleration due to gravity, hf is the flow depth and θ is2

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the slope angle (20°). Therefore, the calculated normal
stress is approximately equal to:
σn,stat ≅ 20 hf

Instead, in test V6-B2, the flow measured normal
stress is significantly higher compared to the measured
flow depth. To explain the measurement, the kinematicsof the flow impacting the upstream flexible barrier isshown in Fig. 4c. It is observed that a curvilinear
overflow develops, which induces centrifugal stresses
normal to the topography and therefore to the load cell.
The centrifugal stress may be calculated as proposedby [13]:(3)

with hf measured in [m] and σn,stat in [kPa]. Hence,reading the flow depth on the left y-axis provides thecalculated theoretical normal stress according to Eq. 3.
It can be observed that for test V6-B1, the theoreticalprediction of the normal stress is quite in agreement withthe measured normal stress.

σn,centrifugal = k ρf hf vf2(4)

where k is the curvature of the flow path and vf is thevelocity of the overflowing layer. Based on video
analysis of the experiments, the following values of theflow parameters are assumed: k ≅ 1.1 m-1, hf ≅ 0.2 m, vf≅ 5 m/s. Using these values, σn,centrifugal ≅ 12 kPa iscalculated. Adding the centrifugal component to thecalculated static basal stress (σn,stat ≅ 8 kPa) results in a
total normal stress of approximately 20 kPa which issimilar to the measured normal stress in test V6-B2. This
result shows that a compact upstream flexible barrier
may significantly alter the flow dynamics.
The measured shear stress in Fig. 4a and 4b does notfollow the same trend as the measured flow depth andnormal stress. A frictional behaviour is assumed tomodel the flow basal shear stress through the MohrCoulomb model:
τf-b = σn tan(φf-b)(5)

where φf-b is an apparent friction angle at the base of theflow, which implicitly accounts for the effect of porepressures on shear strength reduction (notice indeed thatthe total normal stress σn is used in Eq. 5). φf-b istherefore calculated from Eq. 5, based on the measured
shear and normal stresses, and shown in Fig. 5. At theflow front, high values of φf-b are calculated (15° to 40°),
which suggests faster dissipation of excess porepressures. Instead, the body of the flow is characterized
by a lower apparent friction angle φf-b (5° to 15°), whichsuggests significant liquefaction. This observationagrees with the experimental results by [14].

Fig. 4. Measured flow basal stresses and flow depth in [7] for(a) test V6-B1; (b) test V6-B2. (c) Run-up impact mechanismon the upstream flexible barrier for test V6-B2.

Fig. 5. Calculated flow basal apparent friction for test V6-B1[11].

3E3S Web of Conferences 415, 01028 (2023)
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The average apparent friction angle is calculated as
𝜑̅f-b ≅ 9°. Table 2 compares the flow basal apparent
friction angle φf-b calculated from different studies(flume tests and in situ data). In general, the measured
φf-b is quite lower compared to the effective friction
angle of poorly sorted soil material which is typical ofdebris flow (φ’ ≅ 30° - 40°). This observation suggests
the significance of liquefaction during the debris flow
dynamics. In table 2, back-calculated values of φf-b forreal debris flow events using a depth-averaged model
are also reported. The range of back-calculated apparent
friction is similar to the measured values of φf-b, whichsuggests that flume experiments are relevant to study themechanical processes of real debris flow events.

2.

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A. R. K. De Silva, H. Liu, C. E. Choi, Can.
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8. C. W. W. Ng, C. E. Choi, U. Majeed, S. Poudyal,W. A. R. K. de Silva, Fundamental Framework toDesign Multiple Rigid Barriers for Resisting
Debris Flows, 16th Asian Reg. Conf. Soil Mech.
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MPM Simulations of Debris Flow Entrainment,
Modelling Boulders Explicitly, in Geohazards 8,12-15 June 2022, Québec, Canada (2022).
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Berti, J. Geophys. Res. 115, F03005 (2010).
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Table 2. Literature values of debris flow basal apparent
friction.
Study

Debris flow
basal apparent
friction, φf-b (°)[15] – Illgraben debris flow channel~6[14] – sand+gravel mixture on roughbed[14] – sand+gravel+mud mixture on
rough bed
[14] – sand+gravel mixture on
smooth bed~ 19~0~0[7] – This study~9[16] - Back-calculation of several
full-scale debris flows using a depthaveraged model3 - 134 Conclusions
The following conclusions can be drawn:- A simple but effective instrumentation, “erosioncolumns”, has been designed and tested. This techniqueoffers the possibility to instrument real debris flow
channels, which may improve our understanding on theentrainment mechanisms.
- The presence of a compact flexible barrier increases
the flow basal stresses upstream of the barrier owing to
the formation of a curvilinear flow.
- The measured flow basal stresses are used to calculate
the flow basal apparent friction angle, which suggests a
significant liquefaction in the flow body.
The authors are grateful to Centre for Research DrivenInnovation (CRI) KLIMA2050 and INTPART project“Landslide Mitigation of Urbanized Slopes for SustainableGrowth”, funded by the Research Council of Norway. Theauthors are also grateful for the financial sponsorship from theResearch Grants Council of Hong Kong SAR, China (AoE/E603/18). W.A.R.K. De Silva gratefully acknowledges thesupport of Hong Kong PhD Fellowship scheme (HKPFS)
provided by the RGC of HKSAR.

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