E3S Web of Conferences 415, 01026 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501026

Experimental study on the effects of local sedimentaccumulation on a debris flow surge in a steep channel
Shoki Takayama1*, Naoya Hayashi1, and Fumitoshi Imaizumi1
1 Faculty of Agriculture, Shizuoka University, 836 Ohya, Suruga, Shizuoka 422-8529, Japan

Abstract. Debris flow surges can terminate in a steep channel of > 15°. However, the termination process
and mechanisms remain unknown. This study conducted small-scale flume experiments to investigate theeffects of local sediment accumulation on debris-flow surges in a steep channel. The experiments
demonstrated that local accumulation of bed sediment terminates a debris flow surge owing to abruptchanges in bed gradients and infiltration of debris flow interstitial water. Subsequently, the mass of theterminated debris-flow surge and bed sediment began to move, triggering a larger debris-flow surge. This
result suggests that predicting the scale of a debris flow arriving downstream requires measuring the
distribution of bed sediment in the debris flow initiation zone.

1 Introduction
Debris flow can reach a gradual channel ofapproximately 2–3° [1] owing to the maintenance ofexcess pore-water pressure inside a debris flow [2] andthe dominance of particle collisional stress among theinternal stresses of a debris flow [3]. However, debris
flow surges do not always flow down to gentle channel
sections but terminate in steep channel sections of > 15°[4]. The termination processes and mechanisms in steep
channel sections remain unknown.
Bed sediment in a steep channel is often in
unsaturated states before and during the descent of adebris flow [5]. A debris flow surge over unsaturated
bed sediment slows down owing to the rapid dissipationof excess pore pressure through the bed sediment poreair [6]. If the bed sediment and debris flow rarely containmud-sized grains, unsaturated bed sediment decreases
the velocity of the debris flow surge because of theinfiltration of debris flow interstitial water [7]. Presenceof unsaturated bed sediment is one of the causes ofdebris-flow surge termination in steep channels.
Single rainfall events can generate multiple debrisflow surges [4]. Field observations in the Ichinosawa
catchment within the Ohya landslide, Japan, showedthat a debris flow surge entrained local bed sedimentaccumulated by preceding surges in a steep channel,
resulting in an increased scale [8]. Field observations in
the Chalk cliffs, United States, showed that thetermination of a debris flow surge formed a permeable
dam (~1 m high and ~3m wide) in a steep channel ofapproximately 14°, and its failure triggered a debris flow
surge [9]. Such local accumulation of bed sediment in asteep channel may affect the descend of a debris flow
surge; however, the effects are still unclear.

Multiple rainfall events, which have large timeintervals (days, months, years), also form local
accumulations of bed sediment in a steep stream. Local
accumulations of bed sediment in a steep stream alsochange over longer time scales affected by terminationof multiple debris flows and sediment supply fromadjacent slopes.
This study conducted small-scale flume experiments
to investigate the effects of local sediment accumulationon the descend of debris flow surges in steep channels.

2 MethodThis study used a straight rectangular flume (3.6 m long,
0.10 m wide, and 0.20 m deep; Fig. 1). The flume
gradient was set at 25°, assuming a steep initiation zone
in the Ichinosawa catchment [4]. The flume walls weremade of transparent acrylic panels, which allowed us toobserve the side views of the descending processes of adebris flow surge. The flume walls were much smootherthan those of the flume bed covered with uniform sandof 1 mm diameter. The dried bed sediment of thetriangular longitudinal profile was placed 1.2 m
upstream from the downstream end of the flume. The
gate was installed 0.30 m downstream from the
upstream end of the flume. Debris flow material (2.0 kg)
was placed upstream of the gate. To begin theexperiments, we opened the gate to trigger a debris flow
surge and simultaneously supplied a subsequent waterflow of 70 cm3 s-1 from the upstream end of the flume.
Although the sediment and water in the gate were notmixed, the channel gradient was steep enough for theseto flow down immediately after the gate release.
Ultrasonic sensors were installed immediately
upstream and downstream of the bed sediment. These* Corresponding author: takayama.shoki@shizuoka.ac.jp© 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, 01026 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501026

Fig. 1. Experimental set-up. (a) Schematic diagram. (b) Bed sediment geometry. (c) Image of the experimental setup.
Table 1. Experimental cases.
Test
wd
D
θd
θu
dm
hu/D
case
(-)
(cm) (°) (°) (mm)(-)
0.3-0.8T1
0.500
0.7-0.8
T2
0.625
3
0.4-0.7T3
0.750
35
2.0-3.0
T4
1
10
T5
2
1.75 0.7-1.6
0.5-1.0T629
0.6-1.0T732
0.500
0.5-0.7
T8
15
3
0.4-1.0
T9
20
35
T10
0.50 0.5-0.8
10T11
0.33 0.4-1.0
wd is the water content of the debris flow material, D isthe bed sediment height, θu and θd are the upstream anddownstream bed sediment slope, dm is the mean particle
diameter of the bed sediment, and hu is the peak flow
depth immediately upstream of the bed sediment.

Fig. 2. Particle size distribution of the experimental
materials. The debris flow material was No.1. The bed
sediment material in T1–T9, T10, and T11 were No. 1,
No. 2, and No. 3, respectively.

3 Results

sensors precisely identified the time at which a debris
flow surge passes through the installation points. These
measurements were used to calculate the average front
velocities of a debris flow passing between the two
ultrasonic sensors (i.e., the average front velocity of adebris flow through the bed sediment). In addition, thefront velocities of the debris flow along the flume were
measured using video cameras recording the side of the
flume.
In the present experiments 11 test cases wereconsidered as T1–T11 (Table 1). Each case was tested
at least thrice. The first, second, and third tests of T1
were referred to as T1-1, T1-2, and T1-3. The same
applied to the other test cases. T1 was used as a standard
test case. The other test cases had different parametervalues from the standard case for the water content of
the debris flow material (𝑤𝑤𝑑𝑑 = 𝑚𝑚𝑤𝑤 ⁄𝑚𝑚𝑠𝑠 , where 𝑚𝑚𝑤𝑤 is
the mass of water and 𝑚𝑚𝑠𝑠 is the mass of solids), bed
sediment height (D), upstream bed sediment slope (θu)(Fig. 1b), downstream bed sediment slope (θd), andmean particle diameter of the bed sediment (dm).This
study used silica sands of No. 1, No.2, and No.3, whichhave the mean particle diameter of 1.75, 0.50, and 0.33mm, respectively (Fig. 2). Silica sand No. 1 was used asdebris flow material. Silica sand samples No. 1, No. 2,
and No. 3 were used as bed sediment materials T1–T9,
T10, and T11, respectively.
When D was 3 cm, debris flow depths upstream ofthe bed sediment were smaller than D (Table 1). When
D was 1 cm, the debris flow depths were larger than D.

In T1, T2, and T6–T11, a debris flow surge terminatedover the bed sediment (3.0 s in Fig. 3). The phreatic
surface in the bed sediment progresses in thedownstream direction because the interstitial water of adebris flow infiltrates the bed sediment. A subsequent
flow accumulated just upstream of the bed sediment(3.5 s in Fig. 3). The mass of the terminated debris flow
surge and the bed sediment began to move, triggering alarger debris flow surge (4.0–6.0 s).
In T3 and T4, a debris-flow surge passed through the
bed sediment without stopping. In T5, two of the four
experiments flowed down with stopping and two flowed
down without stopping.
In the test cases with temporary stopping of a debris
flow surge, the front velocity of the debris flow rapidly
increased just downstream of the gate and gradually
decreased between the gate and bed sediment (Figs. 4a,4b, and 4f–4k). Subsequently, the front velocity
decreased rapidly in the bed sediment. The front
velocity increased downstream of the bed sediment,
although it did not reach the velocity immediately
downstream of the gate. Even in the test cases without
temporary stopping of a debris flow surge, the bed
sediment played a role in reducing the front velocity ofthe debris flow (Figs. 4c, 4d, and 4e).
Debris flow surges flowed down without stopping
when the water content of the debris flow material (wd)
was large and the bed sediment height (D) was low (Figs.
5a and 5c).

2

E3S Web of Conferences 415, 01026 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501026

The average front velocity of a debris flow throughthe bed sediment (u) was higher for a larger watercontent of the debris flow material (wd) (Fig. 5a). u
decreased as the mean particle diameter of the bed

sediment (dm) increased (Fig. 5b) and the bed sedimentheight (D) increased (Fig. 5c). u was smaller for a larger
upstream bed sediment slope (θu) (Fig. 5d). u increased
with the downstream bed sediment slope (θd) (Fig. 5e).

Fig. 3. Experimental debris flow through the bed sediment in T1. Phreatic surfaces in the bed sediment were visible because
the bed sediment was dry initially.

Fig. 4. Experimental results on debris flow front velocity along the flume.

Fig. 5. Relationship between the front velocity through the bed sediment (u) and the experimental conditions (for all test
including replicates).

3

E3S Web of Conferences 415, 01026 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501026

Comparing the test cases in which only bed sediment
geometries differed, u decreased as the longitudinallength of the bed sediment (L) increased.

slope, small grain size, and low permeability, thus canbe washed out before arrival of a debris flow surge .

5 Conclusion

4 Discussion

This study conducted small-scale experiments toinvestigate the effects of local accumulation of bed
sediment on debris flow surges in steep channels. The
experiments demonstrated that local accumulation of
bed sediment terminated a debris flow surge andtriggered a larger debris flow surge later. In theseexperiments, the local accumulation of bed sedimentdecelerated or terminated a debris flow surge owing toabrupt changes in bed gradients and infiltration of debris
flow interstitial water. Subsequently, the mass of theterminated debris-flow surge and bed sediment began to
move, triggering a larger debris-flow surge. These
results imply that predicting the scale of a debris flow
surge arriving downstream requires measuring local
accumulations of bed sediment in the debris flow
initiation zone.
The present experiments were small-scale
experiments using uniform grains for the material of adebris flow and bed sediment. There can bediscrepancies between the experimental findings and theactual phenomena. Therefore, an increase in theexperimental scale and the use of non-uniform materials
are needed to better understand the effects of local
sediment accumulation on debris flow surges in a steep
channel.

The debris flows generated in the experiments werein the range of 0.3 < ℎ𝑢𝑢 ⁄𝐷𝐷 < 3.0 . Our experiments
thus simulated both highly uneven riverbed and a dam
like topography observed at Chalk Cliff [9].
Local accumulation of bed sediment reduced thefront velocity of the debris flow (Fig. 4). In many test
cases, the debris flow surge terminated over the bed
sediment (Fig. 5). Subsequently, the mass of theterminated debris-flow surge and bed sediment began to
move, triggering a larger debris-flow surge. The results
demonstrated that local accumulation of bed sedimentcan increase the scale of a debris flow surge, which isconsistent with the results of field observations in the
Ichinosawa catchment [8].
The debris flow surge was terminated owing to the
bed gradient change and dry bed sediment. Infiltrationof debris flow interstitial water in bed sediment cancontribute to the termination of a debris flow because
the material of a debris flow and bed sediment has highpermeability owing to the lack of mud-sized grains (Fig.
2) [7].
The debris flow surge passed through the bed
sediment without stopping when the water content of thedebris flow material (wd) was large (Fig. 5a), which isconsistent with field observations in the Ichinosawa
catchment [4]. In this catchment, partly saturated flows
had a shorter flow distance than fully saturated flows.
This could be because collisional stress dominates overfrictional stress when a debris flow has rich interstitialwater [10], decreasing the effects of bed gradientchanges on flow resistance.
A debris flow surge that terminated over the bed
sediment began to move after the subsequent flow was
deposited upstream of the bed sediment (Fig. 3). This
process is similar to that observed at Chalk Cliff [9]. In
Chalk Cliff, the dam was formed by the termination of adebris flow surge. The dam began to move after its scale
increased, owing to the deposition of a subsequent flow.
The scale of a dam can continue to increase until thefrictional force within the dam can no longer resist
gravity and pressure in the downstream direction.
The average front velocity of a debris flow throughbed sediment (u) decreased as the longitudinal length of
bed sediment (L) increased because a debris flow surgethat terminated over bed sediment required more time tomove again (Fig. 5f). Bed sediment with a larger L ismore stable to resist movement, and it can store morewater and sediment, delaying the time for a debris flow
surge to move again. In addition, the phreatic surface in
bed sediment with a larger L required more time to reachthe toe of the bed sediment, resulting in a delay in time
for a debris flow surge to move again.
Although experimental bed sediment was driedinitially, natural bed sediment is wetted by water flows.
Unstable bed sediment, which has a large downstream

The research was supported by JSPS KAKENHI ( 18H02235
and 21K18790).

References
1.

K. Hutter, B. Svendsen, D. Rickenmann,
Continuum Mech. Thermodyn. 8, 1–35 (1994)
2. R.M. Iverson, D.L. George, Proc. R. Soc. A: Math.
Phys. Eng. Sci. 470, 20130819. (2014)
3. T. Takahashi, J. Hydraul. Div. 104, 1153–1169(1978)
4. F. Imaizumi, T. Masui, Y. Yokota, H. Tsunetaka,
Y.S. Hayakawa, N. Hotta, Geomorphology 339,
58–69 (2019)
5. J.A. Coe, D.A. Kinner, J.W. Godt,
Geomorphology 96, 270–297 (2008)
6. R.M. Iverson, M.E. Reid, M. Logan, R.G.
LaHusen, J.W. Godt, J.P. Griswold, Nat. Geosci.,
4, 116–121 (2011)
7. S. Takayama, Y. Satofuka, F. Imaizumi,
Geomorphology 409, 108269 (2022)
8. H. Tsunetaka, N. Hotta, F. Imaizumi, Y.S.
Hayakawa, T. Masui, Geomorphology 375,
107529 (2021)
9. J.W. Kean, S.W. McCoy, G.E. Tucker, D.M.
Staley, J.A. Coe, J. Geophys. Res. Earth Surf. 118,
2190–2207 (2013)
10. D. Song, G.G.D. Zhou, Q. Chen, Granul. Matter
23, 73 (2021)

4