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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
769
DOI: 10.4408/IJEGE.2011-03.B-084
DEBRIS FLOWS IN MOZI GULLY, CHINA FOLLOWING
THE 2008 WENCHUAN EARTHQUAKE
P
enGCHenG
SU
(*,**,***)
, f
anGQian
WEI
(**,***)
, z
unlan
CHENG
(**,***)
& J
inGJinG
LIU
(*,**,***)
(*)
Key Laboratory of Mountain Hazards and Surface Process, Chinese Academy of Sciences, Chengdu, 610041
(**)
Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy,
Chengdu, 610041
(***)
Graduate School of Chinese Academy of Science, Beijing, 100039, China
the debris-flow potential moved into a new active pe-
riod (C
ui
et alii, 2008; t
anG
et alii, 2008). In the past
two rainy seasons, debris flows have been produced
from the area impacted by the Wenchuan earthquake
(x
ie
et alii, 2009; t
anG
et alii, 2009). Mozi gulley,
located in Yinxing, Wenchuan County, Sichuan Prov-
ince (Fig. 1), was seriously affected by the Wenchuan
earthquake, and debris flows have been active in this
gully since the earthquake. During July through Sep-
tember, of 2008 and in September of 2009, five debris
flows blocked the Mingjiang River, resulting in flood
inundation of a village on the left bank. To analyze
ABSTRACT
Numerous landslide that provided abundant mate-
rial for the mobilization of debris flows were triggered
in the Mozi gully on the right bank of Mingjiang River
in response to the M8.0 Wenchuan earthquake of May
12, 2008. Following the earthquake, debris flows oc-
curred during July through September of 2008 and again
in September of 2009. Five of these events blocked the
Mingjiang River. This paper estimates the volume of
landslide material contributed to the valley by comparing
remote sensing images before and after the quake, and
analyzes the resultant landslide dams to estimate the vol-
ume of material necessary to block the channel.
Return periods of debris flows with varying
peak discharges are estimated based on empiri-
cal formulas and field surveys of the valley.
K
ey
words
: Mozi gully, wenchuan earthquake, de-
bris flow, landslide, landslide dam
INTRODUCTION
The M8.0 Wenchuan earthquake of
2008 triggered a number of landslides and
avalanches which provided abundant mate-
rial for debris-flow mobilization. With the
change in the availability of solid mate-
rial, the critical rainfall necessary for trig-
gering debris flows was lower than before
earthquake, the frequency and intensity of
potential debris-flow activity increased and
Fig. 1 - Location map of Mozi valley and environs
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P. -C.SU, F wEI, Z. CHENG & J. LIU
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
the causes and characteristics of this post
-earthquake debris-flow activity in Mozi
gully, field investigations and interpretation
of remote sensing and aerial imagery col-
lected over different time periods were used
to estimate the volume of material contrib-
uted to Mozi gully by earthquake shaking.
Combined with information on the regional
geological structure, topographic and geo-
morphologic setting, and rainfall conditions,
this paper provides a preliminary analysis of
the tendency for debris flows to be generated
from the Mozi gully in the future.
STUDY AREA
The Mozi Valley is a tributary of Min-
jiang River, located in the Yiwanshui village,
45km from Wenchuan (Fig. 1). Geologically,
the valley lies just 12km northwest of Wen-
chuan earthquake epicentre, and is bounded
by the Beichuan-Yingxiu fault on the south-
east and the Maoxian-Wenchuan fault on the
northwest (Fig. 2). According to the Chinese
Earthquake Administration (CEA) the valley
is located in the area of intensity X and has
been completely devastated. The Mozi val-
ley drainage basin is 7.30 km
2
in area, with a
5.20 km long stream (Fig. 3). Basin relief if
2569 m, ranging from 3556 m at the top, to
987 m at the basin outlet. The gradient of the
mainstream channel is 49.3 percent.
Outcrops in the valley are mainly Pro-
terozoic intrusive rocks, dominated by me-
dium grained granite (γo
2
(4)
) that has strong
resistance to weathering. The lower part of
the valley is dominated by a Tertiary quartz
diorite (δ
2
(3)
) and a Tertiary gabbro (v
2
(3)
) out-
crops locally south of the main stem channel
(Fig. 4). The middle and upper portions of
the valley are dominated by a Quaternary
plagioclase granite (γo
2
(4)
) with some medi-
um-grained biotite granite outcropping in
the upper reaches. A few outcrops of a Terti-
ary quartz diorite overlain by quartz schist
and limestone (Pthn
3
) are observed in the left
tributary to the main tem. Quaternary sedi-
ment and gravels are mapped at the junction
of the Mozi gully and the Mingjiang River.
Fig. 2 - Regional geological map around Mozi valley
Fig. 3 - Elevation map of Mozi gully
Fig. 4 - Geologic map of Mozi gully
(*)
(*)
The Second regional geological surveying team, Guanxian
geological map (1:200000) of regional geological surveying
report, China, 1975)
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DEBRIS FLOWS IN MOZI GULLY, CHINA FOLLOWING THE 2008 WENCHUAN EARTHQUAKE
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
771
The climate of Wenchuan County is divided into
southern and northern climate zones with boundary of
Suopodian town. The northern zone is semi-arid cli-
mate of south temperate zone, and the southern zone,
which Mozi valley located in, is a humid climate of
north subtropical zone (CCWC, 1992). The valley also
receives the highest precipitation in west Sichuan Prov-
ince. The average annual rainfall measured at Xiasuo-
qiao village and Yingxiu town between 645.2mm and
1253mm, with 49 (315 mm) to 52.9 (726 mm) percent
of the rainfall occurring in July and September (Fig. 5).
INFLUENCE OF THE EARTHQUAKE
A comparison of satellite remote imagery taken be-
fore and after the Wenchuan earthquake, indicates that
the landscape of the Mozi valley has been significantly
altered by the earthquake (Figg. 6 and 7), The scars
left by landslides, avalanches, and rockfalls triggered
by the earthquake occupy approximately 73.2 percent
of the valley area. Almost all of the channels are filled
with sediment, and some are blocked by the deposits
of particularly large failures (Fig. 7). Field surveys in
December 2008 and May and July 2009 and aerial pho-
to interpretation identified up to eight large landslide
dams in the valley, the largest of which is estimated to
contain 7.8x10
5
m
3
of material (Tab. 1, Figg.8 and 9).
DEBRIS FLOWS FOLLOWING THE EAR-
THQUAKE
CAUSES AND PROCESSES
Up to 80 surges of debris flow were documented in
Mozi gully between July and September of, 2008 and
in September, of 2009, five of them blocked the main-
stem Mingjiang River. The largest debris flow occurred
Fig. 5 - Distribution of monthly average rainfall in measured
in Yinxiu town and Xissuoqiao village near Mozi gully
Tab. 1 - Dimensions of landslide source areas (scars) and resultant landslide dams positions of locations of landslide scars and
dams, Position of locations of landslide scars and dams in the mainstem, and degree of stream blockage in Mozi gully
Statistical table of dams caused by large collapses and landslides in Mozi gully
Fig. 6 - Remote image of Mozi gully from June 26, 2005
(Before the Wenchuan earthquake)
Fig. 7 - Remote image of Mozi gully from June 3, 2008
(after the wenchuan earthquake)
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P. -C.SU, F wEI, Z. CHENG & J. LIU
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
mm of rain, on August 6 dropped a total of 54.8 mm,
and between September 23 and 26, the accumulated
rainfall was 197.6 mm (Fig. 11). Rainfall accumula-
tions at Mozi gully itself are expected to be somewhat
higher than these measurements due to alpine effects.
The local observers Huo Dequan, who lived in the
house of A Point in Fig. 13, told that the large debris
flow happened on July 14
th
, August 6
th
, and September
on July 14
th
, 2008 when the river was completely
blocked for 15 minutes (Fig. 10). According to the Du-
jiangyang weather station (located 15 km from Mozi
gully), between July 1 and September 30, 2008, 605.8
mm of rain fell over a seven day period, with intensities
greater than 25 mm/24 h. The largest daily accumula-
tion was 91.9 mm, which occurred over 24 hours on
September 23, 2008. Storms on July 14 dropped 70.1
Fig. 8 - Aerial image Showing two of the eight landslide dams in Mozi gully after the earthquake (Scale:5000)
(2008.6, National Geomatics Center of China)
Fig. 9 - Photograph of a landslide collapse on the left bank of Mozi gully (2009.7.5)
Fig. 10 - Photograph showing the Mingjiang River partially blocked by debris flow from Mozi gully, 2008
Photograph taken in (December, 2008) The river was completely blocked for only 15 minutes
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DEBRIS FLOWS IN MOZI GULLY, CHINA FOLLOWING THE 2008 WENCHUAN EARTHQUAKE
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
773
24
th
in 2008, as well as on September
13
th
, 14
th
, 20
th
and 26
th
in 2009 (Figg.
12 and 13). The day of debris-flow oc-
currences had heavy 24-hour rainfall in
the area, which exceeded 25 mm.
On August 17, 2009, debris flows
travelled out of Mozi gully and buried
about 500 m of the National 213 high-
way, and the existing V-shaped channel
was turned into the U-shaped (Figg. 13,
14, 15, 16 and 17). The deposits from
these debris flows formed a mound with
an average thickness of 20 m and width
of 400 m, giving an estimated volume of
9x10
5
m
3
. The density of the debris flow
measured from samples of the deposits
was 1.95 g/cm
3
. With this high value, the
debris flows are expected to be highly
viscous and move as a series of surges,
as reported by eye-witnesses.
In this event, debris flows gen-
erated from Mozi gully completely
blocked the Minjiang River, forming
a dam. The dammed water rose 15 m
above the streambed and inundated a
village located 3 km upstream from the
confluence of Mozi gully and the river
(Fig. 18). In December of 2008 the
water was 8 m deep and the backwater
extended 1km upstream (Fig. 19).
Fig. 13 - Photograph showing Mingjiang River blocked by debris flow from Mozi gully in 2009(front edge of
deposits have been removed). Photograph taken on July, 2009
Fig. 11 - Rainfall measured at Dujiangyang meteorology sta-
tion from July to September of 2008
Fig. 12 - Rainfall measured at Dujiangyang meteorology sta-
tion in September, of 2009
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P. -C.SU, F wEI, Z. CHENG & J. LIU
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
where B
w
is the width of the mainstream channel, and
H
w
is its depth. The slope downstream the dam is the
initiation angle of debris flow, 14º; Φ
S
is the internal
ANALYSIS OF THE DAMMING PROCESS
MAGNITUDE OF THE DEBRIS FLOW
The volume of debris flow damming Minjiang
River can be estimated empirically using the fol-
lowing formula (z
Hou
et alii, 1991):
Fig. 14 - Debris flow deposits in downstream reach of Mozi gully
Fig. 15 - Comparison of landform change before and after debris flow events in Mozi gully
Fig. 17 - Sketch map of Mingjiang River blocked by de-
bris flow from Mozi gully in December, 2008
Fig. 16 - Remote image of Mozi gully after de-
bris flows on August 17, 2009
(1)
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DEBRIS FLOWS IN MOZI GULLY, CHINA FOLLOWING THE 2008 WENCHUAN EARTHQUAKE
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
775
rial transported by debris flow is assumed to be more
than one third of the total, so there have been more
than 1.2 million of sediment carried out of the valley
in the rainy season after the earthquake.
As for the discharge, we have measured three
cross-sections in the valley (Figg. 13, 20, 21 and 22),
with parameters listed in Table 2. Those cross-sections
are used to estimate a peak discharge of the debris flow.
friction angle of the material; V
c
is the smallest vol-
ume of material know to block the river. According
to field surveying by GPS Trimble R8, Φ
S
= 25º, B
w
=
115 m, H
w
= 12 m, and then V
c
= 5.1× 10
4
m
3
.
It follows that the five events blocking the river
should have a volume larger than V
c
= 5.1× 10
4
m
3
.
Field surveys indicated that there is still approximate-
ly 80 × 10
4
m
3
of sediment left in the valley. The mate-
Fig. 18 - Submerged houses in lake upstream from landslide dam at confluence of Mozi gully and the Mingjiang River in
December of 2008
Fig. 19 - Lake formed by landslide dam at confluence of Mozi gully and the Mingjiang River in December, 2008.
Blue arrow indicates direction of flow
Fig. 22 - Channel cross section
III — III´ (Fig.13) (in m)
Tab. 2 - Shape parameters of cross section and estimated velociies and peak discharges of debris flows in Mozi gully
Fig. 20 - Channel cross section
I — I´ (Fig.13) (in m)
Fig. 21 - Channel cross section
II — II´ (Fig.13) (in m)
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P. -C.SU, F wEI, Z. CHENG & J. LIU
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
The discharge is estimated as the product of
the velocity of the material passing through the
cross-section and the cross-sectional area. Velocity
is calculated by the following formula, which has
been widely used in the southwest of China (C
Hen
et alii, 1983; z
Hou
et alii, 1991):
U
c
= (l / n
c
) H
c
2/3
I
c
1/2
where U
c
is the velocity, n
c
is the roughness of the
streambed,H
c
is the average depth of flow, and I
c
is
the hydrological slope of the flow, usually equal to the
channel gradient. The results are listed in Table 2.
According to hydrology stations, the perennial av-
erage discharge of the Minjiang River near the junc-
tion with Mozi gulley is about Q
H
=363 m
3
/s, which is
smaller than the estimated debris flow peak discharg-
es. This is the reason for the blockage.
DEBRIS FLOW DISCHARGES FROM
VARYING RAINFALL RATES
Debris flow discharge can be estimated by com-
bining flood water discharge and the entrained sedi-
ment (C
Hen
et alii, 1983; z
Hou
et alii, 1991). The
flood water discharge is calculated by
For complete confluence,
and for partial confluence,
where:
Here Q
p
is flood discharge (m
3
/s) under rainfall
of frequency P, Φ is the runoff coefficient, i is the
rainfall intensity (mm/h) and S is 1-hour rainfall
(mm), F is the drainage area of the valley (m
2
), τ is
confluence time (h), μ is an empirical coefficient for
flood, τ is the runoff parameter, i.e. the infiltration
rate during runoff (mm/h), n is the index, τ
0
is time
of confluence at Φ = 1, t
c
is duration of runoff (h), m
is the confluence parameter, J is the average gradient
of the mainstream channel (‰), L is the mainstream
length (km). Seven parameters are to be determined:
F, L, J, S, n, m, μ. The first three can be drawn from
the topographic map; S and n are independent pa-
rameters, and values of m, μ from are experience pa-
rameters, which can be get from regional flood refer-
ences (WREPDS, 1984). Detailed values are shown
in Table. 3, and computed results are in Table.4.
Discharge of debris flow can be derived from the
flood discharge calculated above by combining the
content of sediment:
Q
c
= Q
p
(1 + φ) D
where Q
p
is the water discharge and Q
c
the debris
flow discharge; D is the blockage coefficient, taken as
1.1~3.0 for the situation; φ is the sediment coefficient,
defined as φ = (γ
C
- 1) / (γ
H
- γ
C
), with γ
C
and γ
H
the unit
weight of flow and solid grains. Field test indicated
that γ
C
= 1.95 g/cm
3
, γ
H
= 2.7 g/cm
3
. Debris flow dis-
charges in response to rainstorms with return periods
of 10, 20, 30 50 and 100 (Probabilities of 10, 5, 3.3, 2
and 1percent) years are listed in Table 4.
In summary, we estimate that about 1 million
m
3
of sediment was carried out from the valley of
Mozi in 2008, and more than 30 Mm
3
of sediment
remains in the valley. This material will continue to
supply debris flows for a long time.
(2)
(3)
(4)
(5)
(6)
Tab. 3 - Coefficients used for estiamtion of peak
water discharges in Mozi gully
Tab. 4 - Debris flow discharges estimat-
ed from Mozi gully for varying
rainstorm frequencies
(7)
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DEBRIS FLOWS IN MOZI GULLY, CHINA FOLLOWING THE 2008 WENCHUAN EARTHQUAKE
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
777
of which depends on the triggering rainfall. For example,
a debris-flow discharge of approximately 250 m
3
/s is es-
timated in response to a 10-year -recurrence rainstorm,
and of 880 m
3
/s for a 100-year-recurrence rainstorm.
Furthermore, the dam residual in the mainstream has
raised the water level and still dammed a certain volume
of water, and the right bank downstream the valley has
been strongly eroded. This stored material are very likely
to form debris flows and dam the river again, this is the
great threat of the valley to the local people and properties.
ACKNOWLEDGEMENTS
This research was financially supported by the
Research Fund for Commonweal Project of the
Ministry of Water Conservancy (200801032), the
national Science and Technology Program of Chi-
na (2008BAK50B04), International collaboration
program of the Ministry Science and Technology
(09B2420420), and the Research Fund for Common-
wealth Projects (Meteorology) (GYHY201006039).
CONCLUSIONS
The Wenchuan earthquake turned the Mozi val-
ley into an active source for debris flows. Many debris
flows occurred in the rainy season after the earthquake
in 2008, and analyses of these events leads to the fol-
lowing conclusions:
1. The landform has been drastically changed by the
earthquake. About 73.2 percent of the valley is
covered by the scars of landslides and avalanches.
Approximately 30 Mm
3
of material provided
abundant sources for debris flow.
2. Debris flow in the valley is typically of high den-
sity and high viscosity. The density is estimated as
1.95 g/cm
3
; the peak discharge on July 14, 2008 is
estimated to have been approximately 541 m
3
/s,
with a return period of 50 years. The critical volu-
me to block the river is 5.1×10
4
m
3
, meaning that
there must be many blockages in the future.
Debris flows will continue to be an active process
because of the abundant material in the valley, discharge
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