10th Asian Regional Conference of IAEG (2015)

Hazard mechanism analysis of Taoguan giant debris flow in
Wenchuan earthquake area on 10 July, 2013
Hu Xiewen1,3, Han Mei 2, Liang Jingxuan3, Diao Renhui3, Pan Cong3, Fang Li3(1) State-province Joint Engineering Laboratory of Spatial Information Technology for
High-Speed Railway Safety, Chengdu, Sichuan, 610031, China(2) School of Mathematics, Southwest Jiaotong University, Chengdu, Sichuan, 610031, China(3) Faculty of Geosciences and Environmental Engineering, Chengdu, Sichuan 610031, China

Abstract
The giant debris flow was caused by heavy rainfall in Taoguan gully, in
Wenchuan Earthquake area on 10 July, 2013. The authors have done field
investigation and experiments to analyze the loose materials, rainfall and terrain
conditions of the Taoguan gully, and studied the process of startup flow anddeposition of the debris flow. The result shows that the damage of the rock mass
triggered by earthquake in the gully is the root cause of such frequent debris flow
after the earthquake. Its Mechanism is actually the startup and superposition of loosematerials from avalanche and landslides, and deposits on slopes and in the gully.
From the development trend, the Taoguan debris flow is now on a high-frequency
developmental stage, and the possibility of breakout of large scale debris flows still
exist.
Key words: Taoguan gully, debris flow, loose material shattered by earthquake,
rainfall, Wenchuan earthquake region1.

and 5%~31% compared with the data before theIntroduction

earthquake. The rainfall threshold for debris flow to
As one of the major performance of geologicalbreakout reduced sharply after the earthquake, anddisasters, debris flow usually threatens human lives

the shaking loose effect made water, rock and soil

and foundational engineering facilities seriously incame to coupling state easily, leading to frequent

a long period after earthquake. According toand crowded outbreaks of debris flows (Huang andincomplete statistics, there were 2,333 debris flows

Fan, 2013). The Wenchuan earthquake produced

breaking out in the Wenchuan earthquake area fromabout 4×109 m3 loose material in the earthquake

2008 to 2012, tripled with the number before thearea, and the average annual sediment of debris

earthquake from 2003 to 2007 (Huang and Fan,flow is about 1.8×108m3, it is speculated that the2013). The strong disturbance on the earth’s surface

debris flow will keep active within the next 20

by earthquake changed the forming conditions ofyears after the Wenchuan earthquake.

debris flow, for instance the loose materials,

On July 10-11, 2013, the Wenchuan to

micro-topography and hydrological conditions has

Yingxiu section of Du (Dujiangyan City) -Wenchanged (Cui et al, 2010). Tang et al.(2009)(Wenchuan City) freeway broke out mass of large

contrasted the rainfall data when debris flow breakdebris flow disasters again after “Aug 13th，2010”

out in Beichuan before and after the earthquake,and “Aug 19th，2012” debris flow, which made theand found that the cumulative rainfall and the

Taoguan gully, became the worst-hit area in thestimulate rainfall intensity reduced by 15%~22%

section. The debris flow gripped huge amount of1

10th Asian Regional Conference of IAEG (2015)solid material along the way after its start, andflow in earthquake area. They can provide early

destroyed 3 check dams and 5 sabo dams, the dams

warning for the earthquake area such as Wenchuan,

were filled up in most parts of the situation, some

Lushan and Ludian in southwest China, and candams were even washed out. The debris flow

also

formed an accumulation area whose width is fromcountermeasures and provide a scientific basis for

80-100m, and the average thickness is 5m in therisk assessment.

range of 3km from the accumulation fan. The

provideguidanceforthedesignof

2. The geological environment conditions of thestudy area

estimated total amount of the output material is up
to 1.1×107m3. Unfortunately, there are about 10
households of enterprise of Wenchuan industrial

Taoguan gully which locates in Yinxing

park in this section, large amount of sedimenttownship in Wenchuan County belongs to thewashed away and buried 8 factories and completely

tributary on the left band of the Minjiang River, theburied a power plant, and threats 696 people’s lives

area of the basin is 50.8 km2(Fig 2 and Fig 3). A

and 192 households’ belongings, the direct

large number of residential and industrial park

economic losses is up to about 32 million USD (Fig.

located in the accumulation fan of the gully, the1).

Du-Wen freeway and G213 state road across thearea in the form of bridge. The Taoguan gully
locates in the Jiuding mountain Cathaysian tectonic
belt of the southern section of the Longmen
mountain Cathaysian tectonic belt in regional
condition, it also belongs to the middle zone
between Yangtze platform and Sonpan-Ganzi
synclinorium. The Maowen fault is the main
regional tectonic fracture in the region, whose
direction is between 30-45° and the dip angle isbetween 45-82°. The Jinning period igneous rocks

Fig.1 Distribution of industrial enterprises andexposes in the strata of the region, and all kinds ofresidential areas on depositional segment of

Quaternary loose accumulation covers surface of a

Taoguan gully after Wenchuan earthquake

portion of the slopes.
The Taoguan gully had been deeply cutted and

Taoguan gully is one of the most representative

eroded, there are lots of high mountains and steep

large scale debris flow in the upper reaches of theslopes in the basin. The highest elevation of the

Minjiang River before Wenchuan earthquake

basin is 3903m, and the lowest elevation is 1002m,

according to historical records. In the early morningand the relative elevation is 2901m. The gully’sof May 12, 1890, Taoguan gully broke-out a debris

shape is as similar as a sector, the main channel’s

flow and caused to a death of thousands of people

length is about 14.2km and there are lots of ditches

among about 300 households, only a few people

in the gully. The main gully’s average inclination issurvived. This event was named “Water-washed197‰, and the slope gradually decrease from

Taoguan” in historical records. This area also421‰ in the source of the gully to 61‰ in thesuffered disasters in 1960, 1982 and 1992 in later

accumulation fan of the gully. The cross section ofyears (Wang et al., 2003).

the valley type conversion from “V” in the

The field investigation and tests of the Taoguan

upstream to “U” in the downstream. The valley’s

debris flow in “7 July, 2013” and the reveal of theslope is so steep that the water flowing fast andmechanism and develop trend of the debris flow

fluctuate rapidly.

will be useful for a better understanding of the

The climate of the Taoguan gully belongs toforming mechanism and characteristics of debris

north subtropical humid climate which has little and2

10th Asian Regional Conference of IAEG (2015)

uneven rainfall. The average annual precipitation is

distributed in slopes and ditches, there are rarely

1253.1mm, the maximum annual precipitation ischannel blockings and the loose material is hard to

1688mm (in 1964) and the minimum annual

startup. But the earthquake induced lots ofprecipitation is 836.7mm (in 1974). The largest

avalanches and landslides in the basin, it iscontinuous 4 months (from June to September)estimatedprecipitation is 853.2mm which accounting formaterials in the gully by field investigation in July

68.2% of the annual rainfall. According to the2013. For example, there is a landslide (whose“Rain and flood calculation manual of small basin

volume is about 4.2×105m3) induced by earthquake

in Sichuan Province”, Taoguan gully’s maximum

blocked the channel and formed a quake lake at a

precipitation in 10min is 10mm, 20mm in 1hour

distance of 6km from the accumulation fan. Theseand 90mm in 24hours.

avalanches and landslide loose materials providethat there are 2.799×108m3

loose

sufficient sources to the outbreak of debris flow
subsequently.
3. Analysis of the forming conditions of the
Taoguan debris on July 10th, 2013
A large number of avalanches and landslides
which induced by earthquake in the basin provide
lots of loose material for the outbreak of debris
flow. This is the most significant feature of theforming condition of debris flow in earthquake
area.
3.1 Rainfall condition
The rainfall data that was measured in Mianci
Fig.2 Geographic location map of Taoguan debris

town weather station in Wenchuan before and afterflow

the debris flow show that the precipitation in theregion is heavy on 8 to 10 July, 2013. The
accumulative precipitation is 67.3mm in the firsttwo days and the precipitation on 10th is up to104mm in north slope, The Taoguan gully is near it
and locates in the south of the mountain. Since therainfall usually increases with the altitude, it can beinferred that the accumulative precipitation in the
Taoguan gully is more than 67.3mm and thestimulating precipitation is more than 104mm in
10th.
Obviously, the rainfall that induced the debris

Fig.3 Schematic diagram of Taoguan debris flow

flow which broke out in “7.10th” ought to be heavy,basin (citation from Google Earth)long lasting and intense. According to previous
research (Tan et al., 1992), the critical rainfall of

Taoguan gully has broken out many debris

debris flow in Longmen mountain area isflows in 1890, 1960, 1982 and 1992 according to

80-100mm and the rainfall in one hour is 30-50mm.

relevant data. Before “the 12 May” Wenchuan

The rainfall condition could be completely satisfied

earthquake in 2008, the loose materials in the basincompare to the precipitation in Taoguan gully.

main was mainly comprised of the deposits

3

10th Asian Regional Conference of IAEG (2015)

after the earthquake. The loose material causes

3.2 Loose materials condition
The field investigation found that there are 100

blockage to the gully, which increases the potentialsites of avalanches and landslides triggered bystartup volume of sediment source in gully. These

earthquake in Taoguan debris flow basin (Fig. 4

provided an advantage for the burst of the mega

73

and Table 1), there are about 4.75×10 m loose

Taoguan debris flow on 10 July.

materials that provided by the avalanches andlandslides in the basin and it is estimated that
7

3.3 Terrian condition

3

29.5% (1.4×10 m ) of them could start up. There8

The shape of Taoguan debris flow in plain view3

are 1.856×10 m of loose materials provided by

is approximately like a banana leaf about 50.86 km2

sediment provenances distribute among 18 channels

in area. The branch gully is well developed, and the73

and branches in the basin in addition, 1.46×10 m

longitudinal gradient of gully is up to 197‰. The(7.9%) loose materials among them could start up.

area of upstream water zone whose has an area of

7

3

Besides, there are 4.68×10 m loose materials that

21 km2 takes 41% of the total area of the valley,provided from 32 slopes and 1.28×107m3 (27.3%)

and itamong them could start up. The total volume of thegully(bottom elevation above 2300m), the upper

83loose materials is about 2.80×10 m , 14.8%
7

scatters in the upstream of Lannitang

and middle reaches of the Dahongyan gully(bottom

3(4.14×10 m ) among them could start up. It is also

elevation above is 2330m), the upper and lower

found that a kind of loose material that should pay

Feishuiya

attention to the loose rock mass was shattered byelevation above is 2400m), the upper and middle

earthquake, which is distributed in the primary andreaches of the Tongpengzi gully(bottom elevation

secondary parts of the watershed. This kind of rockabove is 2400 m) and the upper reaches of themass can’t be classified in to the loose material in

Gaoyanwo gully(bottom elevation above is 2650 m).

general, but parts of the shattered rock mass erodedThis area has a large longitudinal gradient about

and started to participated in the debris flow indeed.

350‰~550‰ generally, and the valley here is like

Viewed from the distribution of materiala narrow "V" in shape, which provides favorable

resources, there is no concentrated trend obviously,

terrain conditions for the surface runoff and thefor the material usually distributed in each branch

startup of debris flow. The longitudinal gradient ofgully and the main gully. Overall, the material

middle reache is relatively steep, and the form of

resources of the upper and lower Taoguan gully isvalley turns from the shape of "V" to "U", the width

more than that in the Gaoyan gully whose has aof bottom usually up to 30~100m. The gradient ofvolume of 5.86 million m³. The volume of loose

both sides of the slope is steep, part of the section

materials in the upper Taoguan gully is 7.92 million(such as the Lannitang downstream) presenting

m³, while it in the whole gully is 14.21 million m³.

steep cliffs, which provides favorable deformation

The loose material resources especially the

gully(theintersection

andbottomspace for avalanche and landslide.

avalanche and landslide deposits increased a lot

Kinds ofmaterials
Avalanche
andlandslide

Table 1 Statistics about different types of loose materials in Taoguan debris flow basin
Total materials Dynamic reserves / million m³,
Basic characteristics/million m³ Ratio of total materials（％）
4.75

1.40, 29.5

Overlanddeposits

4.68

1.28, 27.3

channeldeposits

18.56

1.46, 7.9

Total

27.99

4.14, 14.8

Up to 100 points in all kinds of size along thegully, in which 12 points are over 0.1 million m³
The deposits early distribute on the slope andalong the gully channel, and the causes of theformation are various.
The deposits distribute along the gully channel
early and recently
Estimate dynamic reserves according to the natural
repose angle, erosion modulus of slope and thedepth of erosion of the gully4

10th Asian Regional Conference of IAEG (2015)

Because of the down-cutting and washing, steep

because of the block of the cross of Lianghekou. So

terrain provides a good dynamic condition for thethe debris flow lasted long and rushed out withflood’s strongly erosion on the slope and loose

huge scale, forming catastrophic mudslides, buryeddebris on gully bed. The length of downstream

the corporations and the residential construction invalley is about 4 km, gully width increasing

the period of about 3 km range of the export,

obviously up to 60~130m with a "U" shape, thethreatened Dujiangyan-Wenchuan freeway andlongitudinal gradient is between 60~90‰, which is

G213 state road (Fig. 4 and Fig. 5).

conducive to balance and priority to siltation.
4. Forming mechanism of “7.10”debris flow
4.1 Characteristics of outbreak and movement of
the "7.10" debris flow
According to field investigation, at 4 to 5 a.m
on 10 July, 2013, debris flow firstly broke out
under the cross of Lianghekou. To 6 a.m, No. 1
branch of Taoguan Gully firstly buried the water
plant locates in the upside of industrial park,
blocking the main valley, making the main gully

Fig.4 Destroyed workshop(2013-08-16)

back silting and buried hydropower station. Afterthe barrier dam broken, the debris flow flowed
down through 2 km factory district, to the nearest
Zhongyang silicon industry Shunxin at 8 a.m. on 10
July, continuing flow down 500 m to the residential
area at 8 to 9 a.m. on 10 July. The duration of thefirst stage of debris flow below the cross of theLianghekou is about 2 to 3 hours.
According to the sections of mud marks, thepeak flow of the debris flow was about 430 m3/s. At
10 to 11 a.m. on 10 July, massive debris flow going
through the cross of Lianghekou locates in the
middle of the gully (second wave), arrived at theFig.5

factory at 12 noon. According to mud marks of thebridge on Dujiangyan-Wenchuan freeway

Bare pier washed by debris flow under aLianghekou, the peak flow of the debris flow wasabout 280 m3/s.

4.2

From the above, although there is a gap

Dynamic characteristics of “the 10 July,

2013”debris flow

between two wave of the debris flow, it shows that

The

importantindicatorsofdynamic

the Taoguangou debris flow on 10 July has

characteristics of debris flow include density,

characteristics of long duration, the different peakvelocity, discharge and the total amount of rushed

propulsion and gustiness flow. Taoguangou debris

out once, that are also the important parameters forflow is bounded by the cross of Lianghekou, andthe prevention and cure measurement of project

the outbreak time of the section below Lianghekou(Tang et al., 2009 and Zhang et al., 1996). The

was earlier than the section above Lianghekou.

average density of “7.10” is 1.736 t/m3, adopting

In addition, the main gully of Taoguan debris

the method of mixing weighing on site, and it

flow is long, and along the supply resource lies

belongs to the transitional partial viscous debris

numerous with multiple barrier dam. Besides theflow.

upstream and downstream started up respectively

510th Asian Regional Conference of IAEG (2015)

Table 2 Calculation results of velocity and discharge of “the 10 July” debris flow on different part bymodified flood discharge method
Numberof section

Sediment
correction
coefficientSoliddensity(t/m3)

No. 1-1' section（export）
No. 2-2' section（middle）

Table 3

Φ

Roughnesscoefficient
1/n

Mud Longit
Velocitymark -udinal(m·s-1)(m) Grade Ic

Peak discharge(m3·s-1)
5%

2%

2.65

0.88

15.0

2.40

0.08

4.16

201.98232.22 569.89 655.21

2.65

0.88

10.0

3.40

0.11

4.07

65.19

74.94

5
201.982
232.22

Peak discharge of debris flow（m3·s-1）
Amount of debris flow rushed out once

569.891.0833655.211.2454

Amount of solid once（million m3）

0.50290.5782

By measuring the section area with typicalmud

261.00 300.08

Calculation results of the total run-off volume of “the 10 July” debris flow

Design frequency （%）
Design maximum flood discharge（m3·s-1）

residual

Discharge ofdebris flow
(m3·s-1)
5%
2%

mark

and

the

which belongs to typical gravel bed of mountains，

corresponding

with highly gravel roundness, and the bed material

longitudinal grade, using the rain flood correction

is relatively stable, and is difficult to be restarted bymethod combined with blockage of loose materials

debris flow, so there is little possibility to occur

source (factor of blockage is 1.5), according to themassive debris flow. In fact, the Taoguan Gully is

formula, the velocity and discharge can be obtained

just like that before the earthquake, and thein the typical section of middle circulation area andtorrential gully is the main feature.

export areas (Table 2). According to the results ofAfter the Wenchuan earthquake, avalanches andmud mark survey method, the rainfall frequency oflandslide induced by the earthquake forming very

debris flow here is once in 20 years. Consideringrich loose materials in the trench, which changed

the outbreak time of "the 10 July" debris flow lasted

the geological conditions in the gully. So theabout 2 hours, the amount rushed out once Wc andformation mechanism of the Taoguangou debris

the total solid Ws at the export can be obtained

flow occurred frequently related to the increasing

based on formula (Table 3).

loose materials sharply besides its terrain conditions

According to the accumulation area and its

and continuous rainfall. In terms of the disasteraverage thickness of the "the 10 July" debris flow inmechanism, combining with the field investigations,industrial park of the export, and the size rushed out

the starter mechanisms of the Taoguangou flow canfrom the export to the Minjiang river, the amount ofbe summarized as the following four modes:

debris flow rushed out once could be determined,(1) The shattered-relaxed rock mass in the3which equals to 1.1 million m . Compared with thewatershed area collapses and slides headward（Fig.

results in Table 2, it also shows that the amount6）;

rushed out equals to the size rushed out of the(2) The rainfall acted on the surface of slope

rainfall every 20 years.

forms runoff, leading to erosion and avalanche ofloose materials on the surface of slope, supplied by

4.3

way of overland flow (Fig. 7);

Mechanism analysis of "7.10" debris flow
According to the "Wenchuan county" records,(3) Avalanches and landslides in the 12 May

the basin of the Taoguan gully belongs to

Wenchuan earthquake in 2008 eroded by water

non-torrential rain areas, the main characteristics offlow added to slough and supply (Fig. 8);

rainfall is cloudy and autumnal rain. Before the 12(4) The materials resource in the valley was

may Wenchuan earthquake in 2008, the basin wasundercutted and supplied by quickly gathered water

covered well with vegetation, and gully bed mainly

and debris flow started by materials resource ofconsists of coarse gravel with little sand and clay,

three parts above..
6

10th Asian Regional Conference of IAEG (2015)

Fig.6

The startup mode of retrogressive erosion

Fig.7 The startup mode of debris flow on slope formed byavalanche of shattered rock mass

Fig.8

loose materials

The startup mode of side erosion and toe cut of

Fig.9

The No.5 dam was filled up again after theseismic avalanche and landslide source

reinforcement and restoration in 2004(2014-7-14)

From starting mechanism of the debris flow, the firstsolid materials that flowed into the Minjiang river in thethree parts are the main reasons, accounted for about

previous debris flow is so limited that possibility of

75% of the started quantity of material resources ofdebris flow is very large under the condition ofdebris flow

rainstorm. In fact, the original dam(No.3, No.5)

Visibly, the earthquake damage to rock-soil mass isreinforced and heightened, have been silt up at the time

the root of the frequent outbreaks of debris flow afterof “9 July” local rainstorm in 2014 (Fig. 9), all of theearthquake. The plague mechanism is the startup andfactors above have illustrated that this debris flow is

overlays of each other of avalanche, landslide deposits,

very likely to occur.

overland deposits, and channel deposits. As long as theDebris flow disaster, which happened in the main

started material resources are controlled effectively,

disaster area of Wenchuan, Mianzhu, Beichuan insuch as slope protection, blocked dam and so on, theSichuan province in six years from 2008, has showed

occurrence and development of debris flow should bethey will continue arise frequency in the next 5 years,

effectively controlled.

with the greater active intensity in some gullies. The
Taoguan Gully is a typical representation, and it has

4.4 Analysis of development trend about debris flow

great possibility to generate debris flow disaster underAccording to the geological exploration data, thethe action of general rainfall and flood with large

quantity of mass provenances of the Taoguan Gully

catchment area and extremely rich loose materials.

remains 27.99 million m3 after “the 10 July” debris flow

5.

in 2013, except the dynamo-relaxed rock mass shatteredby earthquake at the secondary watershed and primary

Conclusion(1) Induced by The Wenchuan earthquake, there arewatershed. The sources activated in “the 10 July” debris

amount of deposits accumulated after rockfall or

flow mainly silt at the section at upper reaches andlandslide in Taoguan ditch catchment. In addition, many

lower reaches of the cross of Lianghekou, The quantity
7

10th Asian Regional Conference of IAEG (2015)

dynamo-relaxed rock masses shattered by earthquake at

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Especially

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possibility the occurrence of debris flow would begreatly. The debris flow will be break out frequently inthe next 5 years.

8