E3S Web of Conferences 415, 02021 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341502021

Growth of Debris Flows by Soil Bed Erosion: Effects ofFrictional and Hydrodynamic Shear Stresses
Pengjia Song1, and Clarence Choi1*
1Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, ChinaAbstract. Debris flows are complex solid-fluid mixtures, which travel downslope under the influence ofgravity. The destructive potential of a debris flow is governed by its volume and speed, both of which canbe significantly enhanced by the erosion of soil bed material along the flow path. Existing friction-induced
erosion theories simplify a debris flow as an equivalent fluid that induces frictional shear stress on a soilbed. Erosion occurs when the frictional stress exceeds the soil strength. Research also shows the importanceof hydrodynamic stresses on the erosion of soil beds. However, the mechanism of soil bed failure under theinfluence of frictional and hydrodynamic shear stresses is not clear. In this extended abstract, someexperimental flume test results are presented. The tests simulate the erosion of soil beds by dry sand flow
and water flow to evaluate the effects of frictional and hydrodynamic shear stresses imposed on a soil bed,respectively. Findings from this research can be used to improve estimates of erosion of debris flows inmountainous regions, which will strengthen hazard assessments.

1 Introduction
Debris flows surge downslope under the influence ofgravity. They are often reported to cause fatalities anddamage infrastructure [1]. The destructive potential of adebris flow is governed by its volume and velocity,which may be enhanced by the erosion of soil bed
material [2]. Therefore, understanding the mechanics ofsoil bed erosion is crucial to improved predictions of thetravel distance.
One of the most important aspects of existing
theoretical representations of soil bed erosion isidentifying the principal driving stress. The commonly
adopted depth-average soil bed erosion models [2-3]simplify a debris flow as an equivalent fluid, which
induces frictional shear stress at the flow-bed interface.
Consequently, bed material fails and is eroded when theinterface frictional stress is larger than the strength of
the soil. Research [4-5] shows that the hydrodynamic
shear stress exerted by the fluid phase of a flow also
causes soil bed erosion [5-6].
Research in granular mechanics shows that thefrictional shear stress originates from the enduringcontact between solid particles, while the hydrodynamic
shear stress is caused by a fluid shearing against asurface [6]. Given that debris flows are complex
mixtures of fluid and solids [1], the hydrodynamic andfrictional shear stresses both play a vital role in erosion.
However, the mechanism of soil bed failure under theinfluence of hydrodynamic and frictional shear stresses
remains elusive.
In this extended abstract, details of physicalexperiments that model soil bed erosion caused by

predominately frictional shear stress and hydrodynamic
shear stress are described.

2 Methodology
2.1 Physical modelling of soil bed erosion
Physical modelling was carried out using a flume (Fig.
1) to compare the soil bed erosion caused by thefrictional and hydrodynamic shear stresses. The flume
has a length of 2.5 m and a width of 0.2 m. A storage
container with a length of 0.5 m, width of 0.2 m anddepth of 0.5 m is installed at the upstream end of theflume. Flow material with a volume of 0.02 m3 is stored
in the container for each experiment, and the material isreleased by triggering the pneumatically-controlled
gate. The released material accelerates down the 1.3-mlong rigid section of the channel bed and then erodes thesoil prepared in the 0.7-m-long and 0.17-m-deep
erodible section of the channel. A thin layer of Toyoura
sand is affied to the surface of the rigid section to createa similar roughness as the erodible bed, which consistsof Toyoura sand with a volumetric water content of 5%.
Two experiments are carried out with a slope of 30°.
One experiment use water as the flow material, which isexpected to exert predominantly hydrodynamic shear
stress on the soil bed. The other experiment uses dry
Toyoura sand as the flow material, which is expected toexert predominantly frictional shear stress on the soil
bed. Toyoura sand has a grain size ranging from 0.1 mm
to 0.3 mm, which is fine enough to exclude the influenceof collisional stresses induced on the soil bed [7].

* Corresponding author: cechoi@hku.hk© 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, 02021 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341502021

2.2 Instrumentation

flow in contact with the soil bed can destroy the matricsuction within the soil bed and decrease the soil
resistance. Thus, the hydrodynamic shear stress canerode a significant amount of soil bed material.

An ultrasonic sensor is mounted directly above theinterface between the rigid and erodible sections in eachexperiment. The ultrasonic sensor has a sampling
frequency of 2000 Hz and is used to measure the flow
depth. A high-speed camera is installed at the side of the
flume to capture the flow kinematics on the erodible bed
during each experiment. The camera captures images ata rate of 1000 frames per second. The images areanalysed using Particle Imaging Velocimetry (PIV) [8]to deduce the velocity fields of the flow.
To measure the erosion depth, 12 erosion columns
are installed in the erodible beds (Fig. 2). The erosion
columns are washers stringed through a threaded
column and the column of washers is prepared to havethe same height as the erodible bed before each test. Just
before each experiment, the screw columns are removed
without disturbing the washers. When the flow overrides
the soil bed, the washers near the surface are carried
away. Then, the erosion depth can be deduced from thedifference in of the columns before and after eachexperiment. The washers forming the erosion columns
are 304 stainless steel, which has a friction coefficientof 0.58 (Bernoulli et al., 2018). The internal frictionalcoefficient is 0.6, which is slightly higher than that of
the washers.

Fig 3. Comparison of the erosion depth by sand flow andwater flow

4 Discussions:
Existing studies on the erosion of soil bed by debris
flows rely on an equivalent fluid assumption, whichassumes that debris flow imposes frictional shear stressat the flow-bed interface. However, this study showsthat an equivalent fluid assumption is highly idealised.
Moreover, the infiltration of water into the soil bed isoverlooked in current studies. Infiltration changes thematric suction of the soil bed, which affects its shearstrength.

5 Conclusions:Physical experiments are carried out to compare theerosion caused by flows with predominately frictionaland hydrodynamic shear stresses. Some findings can bedrawn as follows:(1) Toyoura sand flow, which impose predominately
frictional shear stress on a soil bed, barely induces any
soil bed erosion. However, significant erosion isobserved in the experiments with water flow, which
predominately impose hydrodynamic shear stress on thesoil bed.
(2) Under the influence of matric suction, thestrength of soil bed is larger than the frictional shear
stress imposed by the dry sand flow. As such, frictional
shear stress is not effective at eroding the soil bed.
(3) Water flow destroys the matric suction within thesoil when in contact with the soil bed. The strength of
the soil bed decreases, leading to erosion.

Fig. 1. Side view of the flume. All dimensions are in meter.
(Courtesy of the Journal of Geophysical Research: EarthSurface)Fig. 2. Top view of the erodible bed with erosion columns
without soil. (Courtesy of the Journal of GeophysicalResearch: Earth Surface)

The authors are grateful for the generous financial
sponsorship from the Research Grants Council of Hong
Kong (General Research Fund Grants 16210219).

3 Test results
The profiles of the erosion depth in the two experiments
are compared in Fig.3. It is evident that Toyoura sandflow does not erode the soil bed much. The negligible
erosion may be from the large shear strength of the soil
bed contributed by matric suction. However, significant
soil bed erosion is observed in the experiments withwater flow. The erosion depth is most obvious near theinterface between the rigid and erodible beds. Then, theerosion depth decreases with the distance from theinterface between the rigid and erodible beds. The water

References
1.
2.

2

D. Bernoulli, S.C. Cao, J. Lu, M. Dao, Surf. Coat.
Technol. 339, 14-19. (2018)
M.J. Froude, D.N. Petley, Nat. Hazards Earth Syst.
Sci. 18, 8 (2018)

E3S Web of Conferences 415, 02021 (2023)
DFHM8

3.
4.
5.
6.
7.

8.
9.

https://doi.org/10.1051/e3sconf/202341502021

M. Pirulli, M. Pastor, Géotechnique 62, 11 (2012)
R.M. Iverson, C. Ouyang, Rev. Geophys. 53, 1(2015)
J. Du, C.E. Choi, J. Yu, V. Thakur, J. Geophys. Res.
Earth Surf., 127, 3 (2022)
M. Rinaldi, B. Mengoni, L. Luppi, S.E. Darby, E.
Mosselman, Water Resour. Res., 44, 9 (2008)
B. Andreotti, Y. Forterre, O. Pouliquen, Granular
media: between fluid and solid. (Cambridge
University Press, 2013)
P. Song, C.E. Choi, J. Geophys. Res. Earth Surf.,
126, 5 (2021)
N. Sanvitale, E.T. Bowman, Granular Matter, 18, 3(2016)

3