10th Asian Regional Conference of IAEG (2015)

Initiation mechanisms of huge debris flows in the Wenchuan earthquake area
W. Hu(1), X.J. Dong(1), Q. Xu(1), G.H. Wang(3), T.W.J. van Asch(2), P.Y. Hicher(4)(1)State Key Laboratory of Geo-Hazard Prevention and Geo-Environment Protection,Chengdu University of Technology, Chengdu, 650023, People's Republic of China
Email:513933225@qq.com(2) Faculty of Geosciences, Utrecht University, Heidelberglaan 2, 3584 CS, The Netherlands(3) Disaster Prevention Research Institute, Kyoto University, Japan.
(4) LUNAM University, Ecole Centrale de Nantes, CNRS UMR 6183 GeM, FranceAbstract
The frequency and the magnitude of the huge debris flows highly increased after theWenchuan earthquake around the epicenter area in S-W of China. The field investigation
reveals that runoff during the rainstorms plays a major role in generating debris flow on theloose deposits, resulted from the debris avalanche during the earthquake. However, themechanisms of these runoff generated debris flows are not well understood due to thecomplexity of the initiation process. Especially, it is interesting to explore the hugemagnitude of these post-earthquake debris flows. To better understand the initiation
mechanism, we performed flume tests to simulate the initiation process in the laboratory.
With the help of a 3D laser scanner, the whole processes of the initiation of debris flows
were clearly monitored. It was found that the run-off incision caused an accumulation ofmaterial down slope, which, after saturation, failed as shallow slides transforming theprocess in a second stage into debris flows. After this initial phase, the debris flow volume
increased rapidly by a chain of subsequent cascading processes starting with collapses ofthe side walls, damming and breaching leading to a rapid widening of the erosion channel.
By comparing the erosion amount, the latter mechanism is much more important than theformer one. The latter process is called “damming and breaching effect” in this article.
Compared with other erosion processes, the “damming and breaching effect” is found to be
the main reason for the initiation and huge magnitude of debris flows in the post-earthquake
area.
Keywords: post-earthquake, debris flow, initiation mechanism
1.

Introduction

concentrated run-off (flash floods) erosion inchannels filled up with sediments which may besupplied by landslides from the slope. Such debris
flows contain a small fine fraction (less than 10-20%
silt and clay, tang et al 2011) compared to soils
involved in landslide-induced debris flows, and thesource materials have a much higher hydraulicconductivity. Tognacca and Bezzola (1997), Cannon
et al. (2003) and Berti and Simoni (2005) studied
debris flows initiated by channel-bed mobilization,
which is only a part of the initiation process of debris
flows confined in channels. A framework that
describes adequately all the processes involved in theinitiation of debris flows is still missing (e.g., Cannon
et al., 2001; Berti and Simoni, 2005; Coe et al., 2008a,
2008b). Moreover, the former studies have focused

The 2008 Wenchuan Earthquake in SW China inthe Sichuan Province generated many landslides,
which delivered a huge amount of loose deposits. It
caused a dramatic increase in debris flow occurrencein the subsequent years. The mechanism of thesedebris flows has not yet been completely understood.
The mechanism is complex due to the interaction ofdifferent processes such as run-off induced gully
erosion in initially unsaturated granular deposits,
damming and breaching effects caused by theinstability of the side wall of the gullies.
Debris flow initiation can be generally subdivided
into two mechanisms: 1) Failure of shallow landslides,
which transform into debris flows and 2)

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10th Asian Regional Conference of IAEG (2015)

mainly on sediment erosion. More recently, Kean etal (2013) concluded that the debris flow initiation by
runoff can be grouped into two categories: mass
failure of the channel sediment by sliding along a
discrete failure plane and grain-by-grain bulking byhydrodynamic forces. The former initiation process
requires a sudden large impulse of sediment to beadded to and/or entrained within the water flow, suchas from the failure of the sediment-filled bed of thechannel or failure of the channel banks caused bychannel erosion. The latter process has been
motivated by some sediment transport experiments insteep flumes (e.g., Tognacca et al., 2000). In some ofthese experiments, a debris flow surge is produced byhydrodynamic forces eroding individual particles atthe surface rather than by sliding along a failure planeat a certain depth. But there are other mechanisms
involved in the initiation process. From field study,
several huge erosion gullies were found after thedebris flows in Wenchuan earthquake area insouthwest of China. These huge erosion gullies makeus rethink about the mechanisms and the wholeprocess of the initiation of debris flow. The aim of
this paper is to explore the interacting processes
involved in the generation of these debris flows. For
this purpose, a series of flume tests simulating
erosion and incision by run-off water were conducted.
By using a laser scanning technique, a sequence ofprocesses related to the initiation of debris flows wasmonitored with the aim to understand which process
plays a major role in the initiation and enlargement of
the erosion gully for future prevention and mitigation.

runoff. The intensive runoff produced the debris flow
and a huge erosion channel. The incision of thematerial was caused by gully erosion. The wideningof the gully must be caused by slope instability,
looking at the hollow shaped slope forms which aretraced by the slumping. However the complete insightin the sequence of processes leading to the initiationof debris flows and the formation of this huge erosion
gully are not clear. The flume tests were conducted tosimulate the whole initiation process and special
attention was paid to study the formation of huge
gully.
3.

The experiment set-up and test
procedures

3.1 Numerals
The flume was instrumented in order to obtain thehydrological changes and the deformation in thedeposits during the initiation process. The equipment
was described in details in Hu et al. (2014). Thematerials for the flume tests were collected at the topof the so called 1300 platform in the Wenjia gully,
which formed the loose source material for the debris
flow. There is a difference in grain-size distribution
between the in situ materials taken from different
locations.
4. Test resutls
About 10 tests were carried out with different
slopes varying from 12 to 35 degrees, which
provoked the same initiation process. Tests with aslope of 28 degrees were selected for the presentationof this process. Two tests were carried out with thesame initial conditions of a 28-degree slope and arunoff discharge of 0.0003 m3/s. One test wasstopped several times to obtain 3D scanning of theerosion channels. Another one was carried out
without any pause to obtain the erosion curve during
the initiation of the debris flow. A study of the videosof these two tests shows that the pauses did notchange the initiation mechanism nor the process.
The 3D scanner results are shown in Figure 1. Thedevelopment process of the debris flow can bedivided into 6 stages.
StageⅠ: a small gully was formed at the top of
the slope due to runoff surface erosion. The wash-out
particles were deposited in the lower part, as shownin Figure 1a. This is due to the decrease of theseepage force. Most of the run-off water infiltrated
into this slope deposit, decreasing the shear force ofthe remaining runoff water which consequently
entrained only some fine particles. At this stage, nodebris flow was initiated (Figure 1a).
StageⅡ: The water continued to infiltrate into theslope deposits. The finest particles were washed out
due to the seepage force of the infiltrated water.
According to a laser particle size distribution
analyzer these particles appeared to be smaller than2.

The post-earthquake debris flows
in the SW of China
A total of 72 gully debris flows were induced by
the most serious rainfall event on the 24th ofSeptember 2008 in Beichuan County, close to theearthquake epicenter, causing 42 casualties (Tang etal., 2009). The position of these debris was shown inFigure 1. In addition, a heavy rainstorm on the13th–14th of August 2010 near the town of Yingxiu,
located at the epicenter of the Wenchuan earthquake,
triggered many landslides and channelized debris
flows. On the 13th of August 2010, numerous debris
flows occurred along the Qingping section of theMianyuan River, (Tang et al 2012, Xu et al 2012).
The Wenjia gully debris flow located in the Qingping
section of the Mianyuan River was the largest one
among these debris flows. The loose source materialof this debris flow was deposited by a rock avalanche,
which occurred during the Wenchuan Earthquake.
The high energy of the rock avalanche was able toentrain the shallow, loose, soil material along its flow
path increasing the amount of fines in the depositedmaterial with a total volume of more than 70 million
m3. On the 13th of August 2010, a heavy rainfall
generated in the gully a huge amount of the surface

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10th Asian Regional Conference of IAEG (2015)

0.05mm. The washing out of the fine particles
decreased the density and hence the shear strength ofthe deposited material and made the material moresensitive to static liquefaction. After about 5 minutes
from the beginning of the test, a complete
fluidization of the deposited soil mass occurred andthe mass transferred into a debris flow. A gully wasformed at the toe of the slope due to this fluidization
process, as show in Figure 1(b).
Stage Ⅲ : The erosion gully continued to be
deepened and widened due to the sediment erosionand entrainment. (Figure 1(c)).
Stage Ⅳ: As a result of continued water infiltrationand seepage into the soil, both sides of the erosion
gully became unstable and small landslides wereinitiated and the gully was dammed by the landslidedebris (Figure 1(d))
Stage Ⅴ: There was a quick rise of water level
behind the landslide dam, which became quickly
saturated. After about 20 seconds, the landslide dam
breached by combination of overtopping and internalerosion. The debris from the breached landslide damtransferred into a larger debris flow. The process instage four and stage five is called “damming andbreaching” in this article. This process increased
largely the volume of the debris flow and widened
and deepened the erosion gully as shown in Figure1(e).
Stage Ⅵ: “The process of damming and breaching”
continued and contributed to the enlargement of theerosion gully. Sediment erosion also played a role,
however much less than the “ damming andbreaching”. Finally, a large erosion channel wascreated, as shown in Figure 1(f).
By comparing the digital map of different erosion
stages with the initial slope topography, an isopath
map for the different stages was obtained as shown inFigure 5. Positive contour numbers mean cumulative
deposition with the units of centimeter and negativenumbers means cumulative erosion with respect tothe initial slope surface. The change in topography
from Figure 1(a) to Figure 1(b) was caused by theshallow landslide of the deposited material at the toeof the slope. From Figure 1(b) to 1(c), thetopographic change was caused mainly by the bed
and side erosion. From Figure 1(d) to 1€, it wascaused by the breaching of the dammed materials.
From 1(e) to 1(f), it was caused mainly by thecontinuous “damming and breaching effect” andpartly by bed erosion.
The above observations show that damming,
breaching and fluidization of the loose deposits
generated peaks in the solid concentration and playeda key role in the initiation and the development of thedebris flows.

tedl(a) StageⅠ(b) StageⅡ(c) Stage Ⅲ(d) StageⅣ(e) StageⅤ(f) StageⅥ

Figure 1 Digital Elevation Model (DEM) for initiation of debris
flow. (a) StageⅠ: Superficial erosion and deposition of loosematerial. (b). stageⅡ. Shallow landslide in the deposited materials
and initiation of debris flow. (c) Stage Ⅲ: bed erosion. (d). stage
Ⅳ: instability of the side wall of the erosion channel and damming.
(e). stage Ⅴ. Breaching of the dammed material and enlargementof thechannel. (f).stage Ⅵ. Continuous breaching and damming
effect and enlargement of the channel.

5.

Conclusions
The “damming and breaching effect” highlighted
by our experiments shows again the complexity ofthe processes related to the supply of material for theinitiation and development of debris flows and the
big challenge to model these processes.The long
chain of processes showed by the flume tests withrunoff erosion (lateral erosion and incision), slope
failure, damming and breaching and in addition bed
failure (not simulated in our experiments) was
usually simplified by equations taking into accountonly a part of these processes (among others Bertiand Simoni(2005), Iverson et al (2011) Tognacca, etal (2000) Cui et al. (2013), Kean et al (2013)) or bysimplifying the supply of material to debris flow with3

10th Asian Regional Conference of IAEG (2015)

erosion rates related to flow velocity and flow height(Eglit and Demidov 2005, McDougall and Hungr
2005, Van Asch et al. 2014).

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