E3S Web of Conferences 415, 01020 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501020

Effects of debris-flow and bed composition on erosion andentrainment
Lonneke Roelofs1*, Pauline Colucci1, Eise Nota1, Tom Flipsen1, and Tjalling de Haas1
1Department of Physical Geography, Utrecht University, Utrecht, The Netherlands

Abstract. Erosion and entrainment of material by debris flows determine debris-flow volume growth andtherefore hazard potential. Recent advances in field, laboratory, and modelling studies have distilled twodriving forces behind debris-flow erosion; impact and shear forces. A third factor influencing the (relative)
importance of these forces is the viscosity and abundance of the interstitial fluid in the debris flow and thebed. However, how erosion and these forces depend on the composition of the debris flow itself and thecomposition of the bed remains unclear. Here, we present results of small-scale flume experiments with aloosely packed erodible bed that highlight the far-reaching effects of debris-flow and bed composition on
erosion processes and magnitude. We quantify the effects of gravel, clay, and solid fraction in the debris
flow on bed erosion. In addition, we quantify the effects of water and clay content of the unconsolidated bed
on erosion by a debris flow. We show that debris flow erosion increases linearly when the gravel fraction ofa debris flow is increased, which is linked to an increase in both impact and shear forces. We find that debris
flow erosion, and the related forces, are non-linearly impacted by the clay and water content of the debris
flow and those of the bed. For both the clay content of the debris flow and the bed, an optimum in erosion
exists around a specific clay percentage that does not directly relate to an optimum in either shear or impact
forces. When the water content of the bed and/or the debris flow is increased, erosion becomes largest whensupersaturated conditions occur. These conditions are unrelated to the magnitude of the two erodible forces.
This shows that both clay and water content affect erosion by affecting the transfer of pore pressures fromthe debris flow to the bed. We can therefore conclude that impact and shear forces dictate debris flow erosion
in most cases but that their (relative) importance is significantly altered by the means and effectivity of porepressure transfer from the debris flow to the bed. The latter is highly influenced by the viscosity andabundance of the interstitial fluid of the debris flow and the composition of the bed.

1 Introduction
Debris flows can grow in volume due to the entrainmentof bed material by eroding the underlying bed, either ofthe bedrock and/or unconsolidated substrate. At
locations with an abundance of loose substrate volume
growth can be up to several orders of magnitude [e.g. 12]. If debris flows grow in size their destructive powerincreases and so does their hazard to mountaincommunities [3-4]. The erodible power of debris flows,
combined with consecutive flow activity, is further
suggested to be a primary process in cutting valleys insteep landscapes [5]. On a shorter timescale, the erosion,
reworking, and deposition of sediment by debris flows
is a main driver in the evolution of alluvial and debrisflow fans [6]. To minimize debris-flow hazards on Earthand decipher their ability to change landscapes we aim
to gain a better understanding of the mechanisms ofdebris-flow erosion and the parameters that affect it.
In current hazard prediction, volume growth is often
predicted based on catchment characteristics [e.g. 1,7],
past debris flows [e.g. 8], or on linear regression
between peak discharge and volume [3]. In other words,
intrinsic and autogenic settings are used for predictions.
However, when boundary conditions, such as

precipitation intensity and weathering rates, change due
to a changing climate, these predictions will not always
suffice. Therefore, a solid physical understanding ofdebris flow erosion and entrainment is required.
Despite the importance of erosion and entrainmentof material by debris flows, a unified theory for thisprocess does not yet exist. Multiple numerical models
have been developed describing erosion based on avariety of approaches, from empirical to physics-based,using a variety of key concepts and erosional forces.
Observations from field and experimental studies
suggest two driving forces behind debris-flow erosion;1) basal shear forces [1,2,9] and 2) impact forces[10,11]. A third factor influencing the (relative)
importance of these forces is the abundance andviscosity of the interstitial fluid [e.g. 10,12]. The
composition of the interstitial fluid could potentiallyaffect the forces acting on the bed and/or influence thetransfer of pore pressure from the flow to the bed.
Experiments on debris flow erosion have shown thatthe composition of a debris flow and the erodible bed
affects erosion magnitude. Questions remain on how
exactly different debris flow constituents (gravel, clay,and water) and bed constituents (clay and water) affecterosion magnitude and processes. The objective of our* Corresponding author: l.roelofs@uu.nl© 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, 01020 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501020

study is therefore to unravel the effects of debris-flow
composition and bed composition on debris-flow
erosion and erosional mechanisms. We aim atunderstanding the mechanisms of debris-flow erosion
and aim to assess the erosion potential as a function ofdebris flow and bed composition.
This abstract combines the work by Roelofs et al.
(2022) [13] and work in progress by the same authors.

Along the length of the flume, five distance sensors
were installed that recorded flow depth during theexperiments. From these measurements, we alsoinferred flow velocities and shear stresses (see [13] forresults). In the middle of the flume, just upstream of theerodible bed, a force plate and geophone plate wereinstalled to measure normal forces and seismic groundvibrations, respectively (see [13] for results).
To quantify erosion magnitude and patterns, DoDs
(DEMs-of-difference) were created by capturing preand post-flow channel-bed elevation with a Vialux zSnapper 3-D scanner. Strictly speaking, we only capture
the change in bed elevation and not erosion as sedimentdeposition might also occur.

2 Methods
To study the effects of debris-flow and bed
composition on debris-flow erosion and erosional
mechanisms, we conducted a series of experiments in asmall-scale flume with an erodible, unsaturated, andloosely packed, unconsolidated bed (see [13]). Most
experiments were conducted under a flume angle of 34
degrees, but extra tests were performed under flume
angles of 32 and 30 degrees. Experiments were donewith varying debris-flow compositions and with varying
bed compositions. To account for the effects of naturalvariability, each experimental setting was repeated
twice.

2.2 Debris-flow and bed composition
For studying the effects of debris flow composition we
conducted experiments in which we systematicallyvaried the composition of the debris flow while keeping
the composition of the bed constant. The debris-flow
mixture consisted of four components in varying
fractions; gravel, sand, clay (kaolinite), and water. The
erodible bed in these experiments consisted of sand
(98% of solid weight fraction), clay (2% of solid weightfraction), and water (11% of total weight).
For studying the effects of bed composition we
systematically varied the composition of the bed while
keeping the composition of the debris flow constant.
The debris flow running over the erodible bed ofdifferent compositions consisted of 36 kg of sand, 9.6
kg of gravel, 2.4 kg of clay, and 12 kg of water. The
loosely packed bed consisted again of sand, clay(kaolin), and water, but this time in different ratios.
When clay content was varied, water content was kept
constant at 11% of the total weight. When water content
was varied the clay fraction of the bed was set to 0%.

2.1 Flume setup and experimental procedures
The flume consists of a straight, rectangular channel of
0.3 m wide and 5.4 m long, a mixing tank with a forcedaction mixer, and a custom-made release gate (Fig. 1).
The flume was tilted at the beginning of every
experiment and mixing of the debris flow mixture
happened during the lifting procedure. When the flume
reached the desired angle the debris flow was released
into the flume by opening the custom-made release gate.
In the lower half of the flume, the bottom waslowered by 7 cm to create space for an erodible bed witha length of 2.5 m. The floor of the flume was covered
with sandpaper to simulate natural channel roughness.

3 Results
3.1 Debris-flow composition and erosion
Under increasing gravel fraction debris flow erosion
increases linearly, whereas increasing the total solid
fraction results in a decrease in erosion (Fig 2.a,c).
When the fraction of clay is varied the pattern in erosion
magnitude becomes more complex. With the volumetricclay fraction increasing from 0 to 0.075, erosion
increases. However, erosion diminishes whenincreasing the clay fraction from 0.1 to 0.2 (Fig 2.b).
These erosion trends are reflected in the magnitude of
the forces working on the bed (not presented here, see[13]). Increasing gravel fraction correlates linearly to anincrease in impact and shear forces. Increasing clay andsolid fractions results in a non-linear response in themagnitude of impact and shear forces. This is because
the abundance and viscosity of the interstitial fluid affectthe velocity of the flow, the granular temperature, andthe transfer of pore pressure from the debris flow to thebed. The complex interaction between these factors
results in the non-linear response we observe.

Fig 1 Flume set-up. The yellow cylinder in the top right is thesediment-mixing tank. In the lower half of the flume, beneaththe metal frame, the erodible bed is present.

2

E3S Web of Conferences 415, 01020 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501020

Fig 2 Net change above the erodible bed (cm3) for the three different debris flow composition parameters tested: gravel fraction(a), clay fraction (b), solid fraction (c). Gravel and clay fractions are given as a volume fraction of the total solid volume and solid
content is given as a fraction of the total debris-flow volume. Note that a negative net change implies net erosion and a positive netchange implies net deposition. Figure previously published in [13].

the effects water and clay have on the pore pressure in
the debris flow [13]. Different bed compositions
influence the erosion magnitude of debris flows byenhancing or inhibiting the effective transfer of porepressure into the bed. The means of this transfer
determine if the loading of the bed is drained orundrained, and determines if the upper part of the bed
experiences liquefaction.
When the clay content of the debris flow and/or the
bed is low, the transfer of pore pressure and water fromthe debris flow to the bed is relatively effective. Drained
loading of the bed occurs, and the debris flow loses
water and internal pore pressure quickly. This causes thedebris flow to slow down and little erosion to occur.
Under very high clay content of both the debris flow
and the bed, the transfer of pore pressure and interstitial
fluid from the debris flow to the bed is inhibited. Almost
no interaction between the flow and the bed occurs
under these conditions, leading to minimal erosion.
Between these two extremes lies an optimum fordebris flow erosion. Under a clay content of the debris
flow of 0.075 (volume %) or a clay content of the bed of
0.04 (volume %) erosion magnitude reaches itsmaximum. Under these conditions, pore pressure istransferred from debris flow to bed, but the clay in theinterstitial fluid and the bed inhibits the immediate
dissipation of this pressure to deeper parts of the bed.
This causes temporarily undrained loading of the bed,
instigating high pore pressures in the upper layer of thebed, which decrease frictional forces between the grains,enhancing erosion. When pore pressure is so high thatall shear strength of the bed disappears, liquefaction ofthe upper layer occurs.

3.2 Bed composition and erosion
Increasing the clay fraction of the bed gives a non-linear
response to erosion (Fig 3.a). The trend is similar innature to the non-linear response of erosion to anincrease in the clay fraction of the debris flow (Fig 2.b).
Erosion increases up to a bed clay fraction (dry volume%) of 0.04, after which it decreases again. Increasing thewater fraction of the bed does not influence erosion until
total weight fractions higher than 0.13 are reached.
Hereafter, erosion increases exponentially (Fig 3.b).

Fig 3 Net change above the erodible bed (cm3) with a flume
angle of 34° for the two different bed composition parameters
tested: (a), clay fraction (b), water fraction. Clay fraction isgiven as a volume fraction of the total solid volume and water
fraction is given as a fraction of the bed volume. Note that anegative net change implies net erosion and a positive netchange implies net deposition.

4 Discussion and conclusions
Our results show that the composition of the debris flow
and the unconsolidated erodible bed has a significant
influence on the amount of material a debris flow canerode and the relative importance of the involved
mechanisms (see [13]).
For different debris-flow compositions, theinfluence on erosion is correlated to the combined
effects of the magnitude of the forces working on thebed, impact and shear, and the effectiveness of porepressure transfer to the bed (see [13]). Increasing gravel
content in the bed linearly increases erosion, related to alinear increase in both impact and shear stresses on thebed. Increasing clay and solid content of the debris flow
have a more complex non-linear effect on the forces
working on the bed. The complex response is related toReferences
1.
2.
3.
4.
5.
6.

3

T. Takahashi, An. rev. of fl. mech., 13, 57–77(1981).
O. Hungr, S. McDougall, M. Bovis, Debris flow
hazards and related phenomena, 135–158 (2005).
D. Rickenmann. Nat. haz., 19, 47–77 (1999).
C.A. Dowling, P.M. Santi, Nat. haz., 71, 203–227(2014).
J. Stock, W.E. Dietrich, Wat. Res. Res., 39 (2003).
T. De Haas, D. Ventra, P.E. Carbonneau, M.G.
Kleinhans, Geomorph., 217, 165–18 (2014).

E3S Web of Conferences 415, 01020 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501020

7.

M. Jakob, Debris-flow hazards and related
phenomena, 411-443 (2005).
8. R.E. Giraud, Guidelines for the geologic
evaluation of debris-flow hazards on alluvial fansin Utah, 5, (2005).
9. Frank, F., McArdell, B. W., Huggel, C., Vieli, A.
Nat. Hazards Earth Syst. Sci., 15, 2569-2583(2015).
10. L. Hsu, W.E. Dietrich, L.S. Sklar, J. of Geophys.
Res. Earth Surf., 113 (2008).
11. C. Berger, B. McArdell, F. Schlunegger, J. ofGeophys. Res. Earth Surf., 116 (2011).
12. R.M. Iverson, Rev. of geophys., 35, 245–296,(1997).
13. L. Roelofs, P. Colucci, T. de Haas, ESPL, 1-19
(2022).

4