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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
165
DOI: 10.4408/IJEGE.2011-03.B-020
FIELD INVESTIGATION AND DYNAMIC ANALYSIS FOR DEBRIS FLOW
IN WEIJIA GULLY OF BEICHUAN COUNTY (CHINA) AFTER THE WEN-
CHUAN EARTHQUAKE
x
iaoJun
ZHOU, P
enG
CUI & J
ianQianG
ZHANG
(*)
(*) Institute of Mountain Hazards and Environment / Key Laboratory of Mountain Hazards and Earth Surface Processes
Chinese Academy of Sciences, Chengdu 610041, China; pengcui@imde.ac.cn
of magnitude 8.0 struck the Wenchuan area, north-
western Sichuan province, China, which caused a
large number of mountain hazards, such as rock falls,
avalanches, landslides, debris flows. On 24 September,
floods and debris flows broke out in many areas due
to the heavy rain. Roads and bridges were destroyed;
temporary shelters were washed away and a great
amount of farmland was buried. Especially the debris
flow in Weijia gully and Hushiban gully buried two-
third of Beichuan middle school and threatened more
than 300 people at the resettlement areas. The heavy
rain and debris flows caused 42 people dead and more
than 4000 people were besieged because of the road
interruption (t
anG
et alii, 2008, 2009). With the devel-
opment of economy in mountain areas and impaction
of global climate, the risk of debris flows is more and
more serious. Therefore, it is crucial to understand the
characteristics and mechanism of debris flows.
Debris flows are important geomorphic agents in
mountainous terrain and can greatly affect mountain
stream ecosystems (s
wanson
et alii, 1998). A debris
flow is a natural flow of liquefied geomaterials moving
down a slope or a stream in mountainous regions (d
ai
et alii, 1999; C
Rosta
, 2001; z
Hou
et alii, 2001; l
i
et
alii, 2004; C
Hen
et alii, 2006). The liquefied materials
are usually a mixture of water, soil and rock particles,
with a high concentration of solid particles. Many fac-
tors affect the debris flow movement, such as slope of
channel, rainfall intensity, and solid materials. Debris
flow is usually taken as Bingham flow (J
oHnson
&
ABSTRACT
A great amount of mountain hazards were trig-
gered by the Wenchuan earthquake, such as rock falls,
avalanches, landslides, debris flows and dammed
lakes. Heavy rain on 24
th
September 2008 initiated de-
bris flows in Weijia Gully of Beichuan County, which
resulted in serious damage to infrastructures and life-
lines. In this paper, the debris flow was investigated
and analyzed. The results show that solid loose mate-
rials, microtopography and rainfall are the main rea-
son for the event. The total loose mass generated by
the collapse and landslides reaches about 2.69×10
5
m
3
,
which provides abundance material. The conditions
of the surface water infiltration, runoff and flow con-
centration are changed by strong surface disturbance
and large-scale destructive vegetation damage, which
is beneficial to the formation of the erosion and flood
peak. The rainfall was 272.7 mm from 23 to 24 Sep-
tember, 2008 in the study area, and the maximum rain
reached 41 mm one hour. Velocity, peak discharge,
sediment discharge and impulsive force were calcu-
lated. The results were compared with investigation,
which provide reference for the design of debris flow
protection.
K
ey
words
: wenchuan earthquake, debris flow, field investi-
gation, dynamic characteristics
INTRODUCTION
On 12 May 2008, a devastating mega-earthquake
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X. ZHOU, P. CUI & J. ZHANG
166
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
hazards points were found, including 158 landslides,
43 sinkholes, 23 debris flows and 49 potential disas-
ters. 78 geo-hazards affected large areas, which mad
20 residents destroyed, 75 roads damaged and 25 river
channels blocked.,
GEOLOGY AND GEOMORPHOLOGY
A large number of mountains are in Beichuan, and
the direction is about NE-SW. The county can be di-
vided into three geomorphological units (Fig. 2). Ter-
rain in northwest is higher than southeast, and the al-
titude decreases 46 m/km from northwest to southeast.
Chaqi Mountain is 4769 m, which is the highest point
in this county, and Xiangshuidu Mountain is the lowest
point which is only 540 m. The valley slope is generally
higher than 25°; some can be up to 40° or even higher.
Beichuan County (Qushan town) is located in
southeast of Beichuan Qiang Autonomous Aounty
County and Jian river flow through the town, which
belongs to one of the most serious disaster area af-
ter the Wenchuan earthquake. It is also located in the
junction between front and back Longman mountain
fold belt, and Beichuan- Yingxiu fault (central fault in
Longman mountain) across it, which the alignment is
NS, trend is NE. The lower Cambrian Qingping group
(including siltstone, sandstone, chert, calcium phos-
phate rock) exposed above the fault, and upper Car-
boniferous Huanglong group and Chuanshan group
(including limestone, crystalline limestone) and lower
Permian (including limestone, argillaceous limestone
and shale) are exposed below the fault. Due to the
lithological complexity, geo-hazards are prone to oc-
cur. According to the field investigation, a total of 13
geo-hazards were found before Wenchuan earthquake,
including 8 landslides, 3 sinkholes and 2 debris flows.
After the earthquake, there are 20 geo-hazards points,
including 13 landslides, 5 sinkholes, 2 debris flows
CLIMATE
Beichuan County is located in the transitional
zone from Sichuan basin to eastern Tibet plateau, be-
tween the western edge of eastern subtropical humid
monsoon climate zone and the Plateau dry-hot valley
climate. The highest annual rainfall is 2340 mm, maxi-
mum daily rainfall is 101 mm and maximum rainfall is
42 mm per hour. The extreme maximum temperature
is 36.5 Celsius degree (on Aug. 1972); minimum tem-
perature is -4.8 Celsius degree (on Dec. 1975). Figure
R
aHn
, 1970), Bagnold dilatant fluid model (Takahashi,
1978), generalized viscoplastic flow (C
Hen
, 1986) or
other kinds of flows (o′b
Rien
et alii, 1993; l
iu
, 2002).
This paper studies the debris flow of Weijia gully,
analysing the formation and calculating the velocity,
peak discharge, sediment discharge and impulsive
force, and compares the results with field surveys to
test their applicability for engineering design andpre-
diction of debris flow.
GENERAL SETTING
STUDY AREA
Beichuan Qiang Autonomous County is in the
northwest of Sichuan, 160 km away from north of
Chengdu (Figure 1). It belongs to a transition zone
from Pingqiu of Sichuan basin to plateau, the altitude
is from 540 m to 4,700 m and the area is about 2,869.2
km
2
, the longitude is 103° 44' ~ 104° 44', and latitude
is 103° 44' ~ 104° 44'. It is 92 km long from east to
west, and 59 km from north to south. A total of 75
geo-hazards were found before the Wenchuan earth-
quake including 48 landslides. After the earthquake,
emergency investigation was carried out. 273 geo-
Fig. 1 - Location of study area
Fig. 2 - Geomorphological units of Beichuan County
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FIELD INVESTIGATION AND DYNAMIC ANALYSIS FOR DEBRIS FLOW IN WEIJIA GULLY OF BEICHUAN COUNTY (CHINA) AFTER
THE WENCHUAN EARTHQUAKE
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
167
by a total of 17 landslides reached about 2.69×10
5
m
3
,
which provided abundance materials for debris flow.
In history, debris flows occurred three times in Weijia
gully, which were in 1955, July 24
th
, 1992, August 12
th
,
1995 respectively. For the latest debris flow from Weijia
gully, Xijia gully and Huashi ban gully on September
24
th,
2008, a volume of about 340,000 m
3
was rushed out
and 144,097 m2 debris flow fan was formed, which bur-
ied a large area of Beichuan County (Fig. 4). The solid
materials of Weijia gully rushed into Beichuan middle
school, formed a 400 m wide, about 300 m long, about 3
m to 5 m high debris flow fan.
THE INVESTIGATION OF DEBRIS FLOw IN
wEIJIA GULLY
There is one main trunk and two branches for the
Weijia gully (Fig. 5). The main trunk is about 1.5 km
3a shows the mean monthly rainfall and temperature
from 2000 to 2009. The mean annual rainfall is 929.6
mm, and the mean annual temperature is 13.3 Celsius
degree (no data recorded from June to November in
2008 because of earthquake). The rainfall concentrates
in June to September, accounting for 73.8% of annual
rainfall. Figure 3b shows the annual rainfall from 2000
to 2009 in Beichuan, indicating the distribution of rain-
fall in the study area is non uniform.
INVESTIGATION AND ANALYSIS OF DE-
BRIS FLOW IN WEIJIA GULLY
Weijia Gully is located in the southwest of Bei-
chuan County, nearby the right bank of Jianjiang River.
It is about 450 m far from the county. A large number of
landslides, debris flow and dammed lakes were triggered
by the earthquake. In the gully, the loose mass generated
Fig. 3 - Rainfall amount from 2000 to 2009 in Beichuan
Fig.4 - The activity of debris flow and its hazards in Beichuan County after the earthquake a) The Beichuan County
after the earthquake (but before the debris flow). b) The Beichuan County after the earthquake (and also after
the debris flow)
Fig. 5 - Debris flow in weijia gully a) The trunk of weijia gully. b) the branches of weijia gully
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X. ZHOU, P. CUI & J. ZHANG
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
TOPOGRAPHY
Topography is an important factor in controlling
debris flow. There are three types of cross section in
Weijia gully (Fig. 6). The source of Weijia gully is at
1350 m, the entrance is about 650 m and the average
slope gradient of the channel is 116‰..
PROVENANCE
According to the investigation after the earthquake,
loose solid materials are about 2.69×105 m3, the area
was about 7.0×104 m
2
Generally there are three ways
providing solid materials for the debris flow. (1) Slopes
slide and loose solid materials deposit in the channel,
long, the left branch is about 0.155 km, and the right
branch is about 0.156 km. The GPS of debris entrance
is 104°26′27″E, 31°48′48″N, the height is about 650
m, the area is about 0.52 km
2
. Bedrock is exposed at
the source area, the rock strike angle is NW281°, ten-
dency is NE11°, and dip angle is 59°. Solid materials
are mainly from the landslides after the earthquake,
surface area of landslides is up to 0.226 km2, which
accounts for 42% of catchments basin of debris flow.
Its volume is about 2,700,000 m
3
according to the
emergency investigation
A volume of 300,000 m
3
of the main slip mass,
which comes from the landslides front, participated
in forming the debris flow (t
anG
et alii, 2008, 2009).
The slope on both sides of the gully is from 28° to
71°, the vegetation covered very well on the right
side, but it serious damaged on the left side. Accord-
ing to the interview, the rain was very heavy in many
places on September 23
rd
, 2008. When the debris
flow broke out, produced loud noise, the ground vi-
brated, and flood sharply rise accompanied with slits
and stones.
ANALYSIS OF THE FORMATION OF DEBRIS
FLOw FOR wEIJIA GULLY
There are many factors affecting debris flow ini-
tiation and development. The loose soil, rainfall and
micro-topography are the main ones
Fig. 6 -
Cross sections of weijia Gully
Tab. 1 - Basic values of cross sections
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FIELD INVESTIGATION AND DYNAMIC ANALYSIS FOR DEBRIS FLOW IN WEIJIA GULLY OF BEICHUAN COUNTY (CHINA) AFTER
THE WENCHUAN EARTHQUAKE
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
169
Where
ν
c
is velocity (m/s),ρ
H
is solids density (g/cm3), m
c
is
density of debris flow (g/cm
3
), c m is external resist-
ance coefficient m
c,
= 75 H
-0.425
cm , H is average soil
depth of cross-sections; I is soil surface slope gener-
ally using vertical slope (‰); α is internal resistance
coefficient,α= [ρ
H
Φ+ 1 ]
1/2
is correction factor.
According to emergency investigation, the peak
discharge was up to 260 m
3
/s and velocity was 4.1
m/s (t
anG
et alii, 2008, 2009). And the calculated
value was 4.2 m/s.
THE DISCHARGE OF DEBRIS FLOw
Discharge not only reflects the intensity but also
determines the engineering structures. Discharge is
usually calculated by two methods, one is rainstorm
method, and the other is morphological investiga-
tion method. Based on the climate characteristics of
Weijia gully, rainstorm method is used to calculate
the discharge. First of all, assuming that debris flow
and storm occurred at the same time and the same
frequency, then calculating the flood discharge by
the hydrology method, at last, choosing the block-
age coefficient, the discharge can be calculated. The
formulas are as follows, theparameters and results
are in Table 3.
(2) slopes suffer serious erosion due to the continuous
heavy rain and result in a great number of clay com-
ing into the channel, (3) cracks and porosity of soil
increase because of earthquake. With the infiltration
of surface water, Quaternary deposits and weathering
layer are also easily imported into the channel.
RAINFALL
Rainfall is another major factor inducing the debris
flow. On the one side, the surface of slopes became un-
stable after the rainfall, and it easily caused landslides;
on the other side, the deposits along the channel are
washed out by the conflux, which made the loose solid
materials come into the channel continuously. Accord-
ing to the statistical analysis of the rainfall data in this
area, some characteristics can be obtained. (1) Rainfall
is abundant. The annual mean rainfall is 929.6 mm;
the maximum annual rainfall is 2340 mm (in 1967).
(2) Rainfall is concentrated. The rainfall from June to
September amounts to about 70% of the annual value.
The maximum monthly rainfall is in August, which
can be up to 280.8 mm. (3) The intensity of rainfall is
strong. The maximum monthly rainfall is 977.6 mm;
the maximum daily rainfall is 179.6 mm, and the maxi-
mum hourly rainfall is up to 42 mm from 2000 to 2009.
Especially during September 23 to 24, 2008, the rain-
fall reached 272.7 mm in the study area, the maximum
hourly rainfall was 41 mm. It could not infiltrate the
soil in short time. Therefore, abundant rainfall provides
dynamic conditions for the debris flow.
DYNAMIC CHARACTERISTICS OF DE-
BRIS FLOW IN WEIJIA GULLY
Based on field investigation and making an anal-
ogy of the dynamic characteristics in the history, de-
bris flow of Weijia gully is analyzed.
THE VELOCITY OF DEBRIS FLOw
Formulas in calculating the velocity are mostly
empirical, which should combine the regional charac-
teristics. The dry density of soil in Weijia gully is 1.59
g/cm3, fine particle content is between 1.68% and
2.36%, the water content is between 30% and 50%.
So, density of debris flow is between 2.06 and 2.38 g/
cm3, it belongs to viscous fluid. The formula used in
this paper is recommended by the institute of Railway
Academy of Science and Southwest Academyof Sci-
ence. (k
anG
et alii, 2004)
Tab. 2 - The basic parameters and results of velocity
calculation of debris flow occurred on 24
th
Sep-
tember
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X. ZHOU, P. CUI & J. ZHANG
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
Where i is the maximum mean rainstorm inten-
sity, mm/h,
n= 1.285 lg(H
1/6 p
/ H
1/p
), q is the feature coeffi-
cient of the drainage basin,
m is the confluence coefficient, m = 0.221 q
0.204
, is the
runoff yield coefficient ,μ = 3.6F
-0.19
. According to the
contour map of rainstorm of Sichuan (P = 5%), H
1/6 p
(the designation rainstorm) can be calculated, then, H
1/6
(the maximum 10 minutes rainstorm) and H
1
(the
maximum 1 hour rainstorm) can be looked up in the H
1/6
and H
1
contour map respectively, C
V1/6
( the varia-
tion coefficient of the maximum 10minutes rainstorm)
and C
V1
( the variation coefficient of the maximum 1
hour rainstorm) also can be looked up in the C
V 1/6
and
C
V1
contour map, different C
V1/6
and C
s
= 3.5 C
V1/6
, C
V1
and C
s
= 3.5 C
V 1
correspond to different K
p
, which
can be looked up in the coefficient table of Pearson3
curve. Q
c
is the flux of debris flow when the frequency
is P, m
3
/s, Q
p
is the flux of flood, m
3
/s, Φ is sediment
correction coefficient of debris, ρ
c
is the density of
debris flow, g/cm
3
, ρ
w
is the density of water, g/cm
3
, D
c
is the blockage coefficient, when the channels block
weakly, D
c
takes from 1.1 to 1.4; when block gener-
ally, D c is 1.5 to 1.9; when block seriously, D
c
is
2.0 to 2.5 when channels block very seriously, D
c
is
2.6 to 3.0. (w
anG
, 1996; t
He
d
ePaRtment
of
w
ateR
R
esouRCes
of
s
iCHuan
, 1984)
The peak discharge of debris flow is 184 m
3
/s. For
the Weijia gully, the channels are very narrow, so it is
difficult to discharge such a large flux of debris flow
and it certainly causes great damage.
DEBRIS FLOw VOLUME AND SEDIMENT DIS-
CHARG
E
Debris flow volume can be calculated assuming
that debris flow and storm occurred at the same time.
According to the investigation of the latest debris flow
occurred on September 24th, 2008, the local residents
told that debris flow broke out at 5:00 am and lasted
more than 60 minutes. So sediment discharge was cal-
culated as follows. Debris flow volume w
c
is
Sediment discharge W
s
is
Considering duration of debris flow is 60 min
and flux equal to 184 m
3
/s, debris flow volume can
be calculated as 18,848 m
3
and sediment discharge is
12,958 m
3
.
IMPULSIVE FORCE OF DEBRIS FLOw
The calculation of impulsive force contains two
aspects, one is for the fluid and the other is for the
single rock. Many methods can be used to calculate
the force, but most of them are modified by some
theoretical formulas. Only the whole impact force is
calculated because huge rock is rare in Weijia gully.
where δ is dynamic pressure of debris fluid (Pa), g is
gravity acceleration, 9.8 m/s
2
, α is the angle between
the thrust face and impulsive force of debris flow (°),
λ is shape coefficient, square is 1.47, rectangle is 1.33,
circle is 1.00. Taking α =80°, λ =1.47, δ can be calcu-
lated as 5.3×10
4
Pa.
DISCUSSION AND CONCLUDING RE-
MARKS
Due to the strong motion resulting from the
earthquake, a large number of landslides induced and
abundant unconsolidate materials generated for de-
Tab. 3 - Parameters and results of discharge calcula-
tion
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FIELD INVESTIGATION AND DYNAMIC ANALYSIS FOR DEBRIS FLOW IN WEIJIA GULLY OF BEICHUAN COUNTY (CHINA) AFTER
THE WENCHUAN EARTHQUAKE
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
171
which provides abundant material for debris flows for
a long time. The great changes of river basin consti-
tute the conditional combination of the debris flow
formation. Therefore, the intensity of post-earthquake
debris flow increases rapidly and debris flow became
one of the most serious risk sources after the Wen-
chuan earthquake.
Debris flow is a multi-phase mixture of solid, liq-
uid and gas, it is difficult to obtain dynamic character-
istics of debris-flow quantitatively. In the future, the
calculation method should be developed by coupling
the loose soil supply process and the rainfall process
together which may yield more accurate result.
ACKNOWLEDGEMENTS
This work was financially supported by State
Key fundamental Research Program (973) project
(2008CB425802, 2011CB409902).
bris flows. Debris flows will be a major hazard in the
Wenchuan earthquake area for many years. There-
fore, field investigation and dynamic analysis for
debris flow is a critical element for post-earthquake
reconstruction. A case study of debris flow in Weijia
gully shows that:
The rainstorm method was applied to calculate the
debris-flow discharge. The peak discharge was calcu-
lated as 184 m
3
/s, with error of 29.2% compared to the
measured value. So the rainstorm method should be
modified. The theoretical formula was applied to cal-
culate the velocity. The results show that the velocity is
about 4.2 m/s. The error between the calculated value
and the field-measured value is 2.44%, indicating that
the result calculated by the formula is acceptable.
Abundant rainfall provides dynamic conditions
for the debris flow. The total of the loose mass gen-
erated by the landslides reaches about 2.69×105 m
3
,
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