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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
865
DOI: 10.4408/IJEGE.2011-03.B-094
A STUDY ON THE PROPERTIES OF THE 12 MAY WENCHUAN
EARTHQUAKE-INDUCED DEBRIS FLOW
Y.S. HAN
( *)
, J. HAN
(**)
, Y.Y. ZHU
(**)
, Y.P.
KONG
(***)
, F.H. SU
(**)
, L.W. LI
(*)
& P. HUANG
(*)
(*)
School of Architecture and Urban Planning, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China
(**)
Institute of Mountain Hazards and Environment, CAS, Chengdu 610065, Sichuan, China
(***)
China Academy of Transportation Sciences, Ministry of Communications, Beijing 100029, China
K
ey
words
: debris- flow, field observation, development fea-
ture, Niujuan Gully
INTRODUCTION
China is among the countries with the most fre-
quent and severe continental earthquake and debris-
flow occurrences. Since 1990s the China mainland
has experienced many earthquakes and related sec-
ondary mountain hazards which were induced by
earthquake and named as secondary mountain hazards
including debris flows, landslides and rockfalls. These
hazards pose tremendous and lasting hazards to life
and property. In some afflicted areas these second-
ary mountain hazards were even more harmful than
earthquake itself. The May 12, 2008 Wenchuan Earth-
quake (Ms=8.0) occurred in the Longmen mountain
fault belts at the east edge of Tibetan Plateau and
affected more than half of China, with the disaster
area covering about 500,000 km
2
. The main shock
was strong and the intensity at the epicentre reached
XI, causing 87,149 dead or missing. The direct eco-
nomic loss reached 845.14 billion RMB (¥, Chinese
money unit and about 6.57 RMB equal to 1 dollar).
The earthquake and its continuous aftershocks (more
than 30,000 times) severely destroyed the massif of
stricken areas and directly triggered large quanti-
ties of secondary mountain hazards, including rock-
falls, landslides, debris flows, dammed lakes, etc.
By means of remote sensing interpretation and field
investigation, more than 50,000 sites of rockfalls,
ABSTRACT
Debris flows triggered by earthquakes are rec-
ognized globally for their huge destructive power.
Studies on the development features of debris flow
are essential for disaster prevention and mitigation
and the results can be used as a scientific basis for
assuring public security, preventing debris-flow dis-
asters and reconstructing the afflicted areas by earth-
quake. Taking the, Niujuan Gully, as the case study
area, this paper discusses field observations of the
initiation and path of debris flows in the epicentral
area of the 12 May, 2008 Wenchuan Earthquake.
Based on the natural conditions and debris flow dis-
tribution in the study area, a field control network
was established to monitor the gully bed, typical
slopes and channel sections of different kinds of de-
bris flows that initiated from hillslopes and gullies.
We observed debris flows and their change under
different rainfall circumstances, surveyed slope, sec-
tion, channel and gully bed by means of GPS, GPT-
3002 total-station instrument and 3D laser scanning
system and drew digital topographic maps, sectional
maps and profile maps with a 1:50 scale. Based on
the observed data, we probed into the erosion laws
and development features(including erosion, trans-
portation, deposition, etc) of different kinds of debris
flows and analyzed debris flow discharge, erosion-
deposition variation and destructive magnitude. The
findings provided data support for debris flow risk
assessment and monitoring systems.
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Y.S. HAN, J. HAN, Y.Y. ZHU, Y.P. kONG, F.H. SU, L.w. LI& P. HUANG
866
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
oped district on heavy industry in Wenchuan County
and the vital communication line in the West Sichuan,
named as the South Gate of Aba prefecture. However,
it was in ruins except for a few severely damaged
buildings after the earthquake. Niujuan Gully, the epi-
centre of the Wenchuan Earthquake, was selected as
the study area which is located in 1,500 meters south-
west of Yingxiu Town and at the right side of upper
valleys of Minjiang River. The study area, with high
mountains, steep valleys, intensive cutting, cracked
rock mass, complicated geological structures, and de-
veloping faults is prone to rockfall, landslide and de-
bris flow. Large-scale rockfalls and landslides supply
abundant loose solid materials for debris flows. Once
high-intensity rainfall happens, debris flow will occur
on large scale. Debris-flow activities become more
frequent following the earthquake and have the ten-
dency of occurring in every gully of study area.
STUDY AREA
NATURAL BACkGROUND
The Niujuan Gully is located in the transition
zone from Longmen Mountains to Sichuan Basin,
which is in the east edge of Tibetan Plateau and within
the main fault belt of Longmen Mountains. The study
area is dominated by alpine gorges within the cover-
age of E103°25′12″-103°28′31″ and N31°01′21″-
31°03′16″. It is a vital traffic line, close to Wolong
panda protection zone and 78 km away from Chengdu
city. The terrain in the study area has a general trend
of increasing altitude from south to north, with the
difference in relative height more than 1833 meters.
This area ranges in altitude ranging from 860-2693
meters and contains Alpine peaks and river valleys.
The Niujuan Gully has a total drainage area of 10.46
km
2
, an altitude ranging from 860-2693m, 5.8 km of
main channel length and a main channel mean slope
of 3.16 percent The Niujuan Gully includes 6 branch
gullies that include Niujinhang gully, Dao gully, Piao-
chang gully, Qingshui gully, Bai gully and Lianhuaxin
gully. Lianhuaxin gully ,as the epicentre of Wenchuan
Earthquake, is 3.77 km long with a average gradient
of 3.16 percent, with high mountains, steep slopes and
deep valleys (Figg. 1 and 2).
The main active fault belt (Yingxiu-Beichuan
Fault Belt) of Wenchuan Earthquake originates in
Niujuan Gully, goes through Wenchuan, Maoxian
to Beichuan county, inclines to the northwest with
landslides, and debris flows and instable slopes were
found in 51 severely-afflicted counties, which directly
killed more than 20,000 people (H
an
et alii, 2009).
The area of potential slope deformation induced by
loose massifs reached 150,000 km
2
. Because the large
number of rockfalls and landslides could carry abun-
dant materials, large-scale debris flows formed due to
heavy rainfall, causing more serious soil erosion. With
the increasing intensity of stricken-area reconstruction
and human activities after earthquake, debris flows
will become more and more frequent and serious, lead
to massive soil erosion, destroy land and river ecosys-
tem and form an immense threat to the environment,
public security, restoration and reconstruction of the
afflicted areas. Some research findings exist for the
formation and development mechanism of debris flow
induced by earthquake in China and abroad (t
amuRa
1978; e
isbaCHeR
& C
laGue
, 1984; G
ao
& z
Hu
, 1986;
J
ianG
et alii, 1991; m
a
& s
Hi
1996; l
i
et alii, 2001;
l
iu
et alii, 2008; l
iu
et alii, 2009). Debris flows in the
afflicted areas are widely scattered, form and develop
quickly, do great damage and are difficult to monitor,
forecast and prevent. Moreover, debris flows develop
actively and occur frequently within 5 to 10 years after
earthquake (H
an
et alii, 2009). Thus, it is imperative
that study on the development and evolution of earth-
quake-induced debris flow in stricken areas should be
strengthened on the basis of existing research achieve-
ments and technologies, in order to effectivey mitigate
against earthquake-induced debris flows. This paper
studies debris flows in the Niujuan Gully, the epicen-
tre of Wenchuan Earthquake, at Yingxiu Township,
Wenchuan County, Sichuan province, China. Accord-
ing to debris flow development and movement, the
paper identifies earthquake-induced debris flows, ex-
plores debris-flow monitoring methodology and data,
and analyzes the erosion and sedimentation character-
istics of debris flows in the study area and expounds
riverbed evolution. The research findings can be used
as a scientific basis for preventing debris flows, recon-
structing the afflicted areas and assuring key project
and public security.
GENERAL SETTING
Yingxiu township in the south of Wenchuan coun-
ty was one of the most severely-afflicted townships by
the May 12, 2008 Wenchuan Earthquake. Before the
earthquake, Yingxiu Township was the most devel-
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A STUDY ON THE PROPERTIES OF THE 12 MAY WENCHUAN EARTHQUAKE-INDUCED DEBRIS FLOW
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
867
of the total study area. All of the soil erosion in the
study area was due to gravity driven processes and the
total soil erosion reached 201,000,000 kg/a
SOCIO-ECONOMIC BACkGROUND
There were 2 townships, 3 villages and 5 villager
teams (a part of village and the smallest administra-
tive unit), that is, 2 villager teams of Zhangjiaping vil-
lage, Yingxiu Township as well as 2 villager teams of
Caijiagang village and 1 villager team of Hejiashan
village, Xuankou Township. Most of the people in
study area consist of a rural population of Han Chi-
nese and Tibetan people. Wenchuan Earthquake did
fatal socio-economic damage to the study area. Be-
fore earthquake, Zhangjiaping village had two villager
teams, 105 households, 389 people, with one acre of
farmland acreage per person. The annual per capita
income was ¥6000-7000, and on livestock consisted
of about 5 pigs per household and more than 1800
sheep. However, the earthquake caused 52 deaths, and
120 injuries, collapsed houses, destroyed homes and
arable lands, and only 90 sheep survived. Caijiagang
village had two villager teams, 68 households and
279 persons before the earthquake, with 0.5 acres of
farmland per person, an annual per capital income of
¥1000, an average of 2 pigs and 5 chooks per house-
hold and more than 300 sheep. The earthquake caused
16 deaths and, 48 injuries, destroyed more than 100
acres of farmland, killed about 10 percent of the live-
stock, and collapsed houses and schools.
DEBRIS FLOwS IN STUDY AREA
The study area was the epicentre of Wenchuan
Earthquake and it was severely affected by intense
ground motion which caused mountains to collapse,
the earth to fracture and rock-soil mass to crack, and
a strike of 229 degrees and is composed of a series
of piezotropy and compression-scissor fractures and
folded strata. The geological environment of the study
area severely affected by the earthquake is very com-
plex. Moreover, neotectonic movement is very active
and the conditions of terrain, strata, lithology, and ge-
ological structure have changed greatly. The stratum
subjected to rockfall and landslide is widely distrib-
uted and consists of heavilyweathered Archaeozoic
granodiorite, triassic sandstone of Xujiahe group and
surface collapse-landslide clinosol(H
an
et alii, 2009).
The study area is typical of subtropical moist
climates and is one of the most rainy areas in West
Sichuan with multi-annual mean rainfall 1253.1 mm,
annual maximum rainfall 1688 mm, and daily maxi-
mum rainfall 269.8 mm. Moreover, precipitation of
study area mainly concentrates in June to September,
which accounts for 68.2 percent of the annual rain-
fall. The climate is warm and humid, characterized by
raininess, flood and clear seasons. The Niujuan Gully
is part of Minjiang River system, which is 50-120m
wide, has a large drop height in gully bed, contains
plentiful groundwater storage and abundant water
yield, and its branches and tributary gullies have a
dendritic shape. The Wenchuan Earthquake triggered
5 landslide-dammed lakes in Niujuan Gully. The water
supply is mainly recharged from rainfall during flood
period and from snowmelt runoff and groundwater
during drought season. In the study area, large quanti-
ties of secondary mountain hazards were induced by
the intensive main earthquake and aftershocks. In the
rainy season, debris flows occurred more frequently
and caused large-scale soil erosion and damage to the
ecological environment of study area. By means of re-
mote sensing interpretation and field investigation, it
was estimated that the area of soil erosion in Niujuan
Gully was about 3.52 km
2
, which accounts for 33.7%
Fig. 1 - Three dimensional digital terrain map of Niujuan
Gully
Fig.2 - ADS40 remote sensing image of Niujuan
Gully,acquired on May 15, 2008
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Y.S. HAN, J. HAN, Y.Y. ZHU, Y.P. kONG, F.H. SU, L.w. LI& P. HUANG
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
DEBRIS FLOW MONITORING
MONITORING AIMS
According to the distribution and motion features
of debris flows in study area, different slopes, proper-
ties and types of debris flow material and main gullies
were selected as monitored objectives. Combined with
the natural environmental background of the study
area, a network of field observation locations and mon-
itoring stations were set up. We used modern surveying
and mapping technologies to observe the distribution
and motion characteristics of debris flows, changes
to the gully and channel bed geometry, to draw large-
scale digital topographic maps, and to create a three-
dimension digital elevation model. Based on these
data collected, we could study the erosion, movement,
transportation, and material of the debris flow, explore
into debris-flow characteristics and channel evolu-
tion, quantitatively analyze the debris-flow path and
deposit, and to estimate the mechanism of debris-flow
transport and channel evolution. These findings can
provide scientific basis for comprehensive studies on
earthquakeinduced debris flows, for quantitative as-
sessments of debris-flow erosion and deposition, and
for disaster mitigation and risk management.
MONITORING METHODS
Debris flows are not only characterized by be-
ing dense and having high velocity, large discharge,
short duration and wide range of particles, but also
by the ability to transport sediment and erode and de-
posit material. Therefore, debris flows can carry large
quantities of sediment into channel in very short time,
which causes river channels that have evolved for a
long period to suddenly change (C
ui
et alii, 2006). In
this research, the zone where debris flows initiated
from Lianhuaxin gully to the mouth of Niujuan Gully
was selected as the monitoring objective. The GPS
monitoring control network, traverse control network,
levelling network and debris-flow observing sections
were established within the 1,300 m by 110 m debris-
flow path and include the debris-flow initiation zone,
transport zone and deposition zone (Fig. 3). Different
types of slope and gully debris flows were selected and
monitoring stations were set up along the channel bed
and debris-flow slopes. Global position system, GPT-
3002 total station, SDL30M precise level gauge, 3D
laser scanning system and other surveying instruments
were utilized to survey the slopes, sections, gully, and
greatly changed geologic environment of study area.
The earthquake triggered exceptionally serious sec-
ondary mountain hazards including ground fracture,
collapse, landslide and debris flow. Additionally, the
earthquake and secondary mountain hazards created
abundant loosely consolidated materials and landslides
formed 5 dammed lakes in the mid-lower reaches of
Niujuan Gully, which potentially induced large-scale
debris flows due to intense rainfall during the rainy
season. Seven large-scale debris flows occurred on
2008-5-12, 2008-6-27, 2008-9-29, 2008-10-14, 2009-
8- 22, 2010-7-15 and 2010-8-14. The first large-scale
debris flow occurred during the Wenchuan Earthquake
at 14:28 pm, May 12, 2008. The earthquake acceler-
ated a landmass at 1.5g in the Lianhuaxin gully, 1km
to the northwest of mouth of Niujuan Gully. Then the
disturbed landmass and landslides turned into high-
speed debris flows, which traveled more than 70 m/s,
increased in size by a factor of four, and formed a re-
markable path that travelled 3.2 km long and 700m
vertical. In the mid-lower reaches of Lianhuaxin gully,
from Feiyuzui waterfall to the mouth of Niujuan Gully,
the debris-flow deposit was composed of riprap rocks
mingled with clastic soils and had a volume greater
than 4,000,000 m
3
that was approximately 1,100 m
long, 80-100 m wide in channel bed and 40- 60 m
thick. In the upper reaches of Lianhuaxin gully, from
Feiyuzui waterfall to the effusive site, the debris-flow
deposit was mainly composed of fine grained sediment
mixed with gravels and had a volume greater than
3,500,000 m
3
that was approximately 2,100 m long,
50-80 m wide in channel bed and 20-40 m thick. Thus,
abundant solid materials provided large quantities of
source material for rainstorm-induced debris flows.
The 6th large-scale debris flow occurred on July 15,
2010. It was triggered by intense rainstorm with 132
mm rainfall in 45 minutes. The maximum discharge of
this debris flow was 12,173 m
3
/s, the velocity reached
9.7 m/s, 600,000 m
3
of material were transported and
channel deposits were about 420,000 m
3.
The biggest
particle diameter is 9.3 meters, with a volume of 421
m
3
and a weight of approximately 1,120,000 kg in the
channel bed. The largest particle diameter in the de-
bris flow fan was 6.8 meters, with a volume of 165 m
3
and a weight of 438,000kg. The geologic conditions,
tremendous influence of Wenchuan Earthquake and
intense rainfall were the main factors causing debris
flows in study area.
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A STUDY ON THE PROPERTIES OF THE 12 MAY WENCHUAN EARTHQUAKE-INDUCED DEBRIS FLOW
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
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was surveyed and mapped on gullies and their two
side slopes at 0.2m observing intervals. Then the
above observation data were input into geographic in-
formation system (ArcGIS 9.2 issued by ESRI comp.)
to create a high-resolution digital elevation model
(Fig. 5). By spatial overlay analysis, we can monitor
the erosion and sedimentation of debris flows in the
deposition zone and the evolution features of gully
bed and even determine the influence of debris flows
on topographic features of gullies and the sediment-
transport capacities.
SECTION MONITORING
According to the motion features, gully location,
gully bed slope and gully width variation of debris
flow, 10 channel sections were installed and 28 field
stations for observation were selected. Sectionposi-
tioned points were marked out respectively in the up-
per, middle and lower reaches of the debris-flow gully.
By utilizing a GPT-3002 total station instrument, field
observations were conducted on sections of gully bed
at 0.3 m intervals. The digital profile of every observed
section in the gully bed was created from the observed
data. By contrast with the two series of debris-flow
gully profiles on August 22, 2009 and July 15, 2010,
we monitored gully bed variation at every observed
channel bed of the debris flows at each monitoring sta-
tion and for different triggering rainstorms. The meth-
ods for monitoring debris flow included 3-5 year long-
term observation, quarterly regular observation and
emergency observation for debris flow occurrences.
According to field observation data, 1:50 scale dig-
ital topographic maps, profile maps and cross sections
were drawn to study the features of debris-flow ero-
sion, initiation, transport and deposition and to probe
into erosion-sediment change and activities of the
gully bed. The technological procedure for monitoring
debris flow is indicated in Figure 4 and corresponding
accuracy index is indicated in Tables 1,2,3.
MONITORING CONTENTS
GULLY MONITORING
Two debris flows that occurred on August 22, 2009
and July 15, 2010 were monitored. Scanning was done
using a 3D laser scanning system and concentrated on
monitoring sediment level, flow depth and accumulat-
ing scope and volume of debris flows in the deposition
zone of the main gully. By means of a GPT-3002 total
station instrument, 1:50 scale digital topography
Fig. 3 - Layout of field monitoring control network, sta-
tions and sections of study area
Fig. 4 - Technological procedure of monitoring debris
flow
Tab. 1 - Accuracy index to GPS
Tab.2 - Accuracy index to GPT-3002
Tab. 3 - Accuracy index to 3D laser scanning system
Fig. 5 - Digital elevation models from observed data of
two debris flows on August 22, 2009 and July 15,
2010
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Y.S. HAN, J. HAN, Y.Y. ZHU, Y.P. kONG, F.H. SU, L.w. LI& P. HUANG
870
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
section, analyzed the erosion, sedimentation, and de-
velopment of gully, and explored the role of the debris
flow on the topographic evolution of the gully.
RESULTS AND ANALYSIS
GULLY EROSION AND SEDIMENTATION
Earthquake-induced debris flows showed ero-
sion and deposition in the mid-lower reaches of gully,
which changed the debris-flow paths and gully topog-
raphy. It is necessary to note that the erosion and depo-
sition of debris flow in different zones along the upper,
middle and lower reaches of gully is complicated and
varies for each section. According to continuous ob-
servations of 10 selected sections in Niujuan Gully, we
calculated the parameters of debris-flow erosion and
deposition within a certain section of gully at a certain
time and further discuss erosion-deposition features of
the upper, middle and lower reaches of gully.
EROSION AND DEPOSITION IN THE UPPER
REACHES OF GULLY
Section S1 (abbreviated to S1) was selected to an-
alyze the erosion and deposition features of the debris
flow in the upper reaches of gully. As shown from Fig-
ure 6, in 2009, S1 was eroded and deposited to a mod-
erate extent with net erosion .The left side, middle part
and right side of section S1 were eroded to a width of
10 m and to an average depth of 1.9 m with a maxi-
mum depth of 2.1 m. The left side and right side ex-
hibited lesser deposition, with accumulated sediment
thickness of 0.4 m. In 2010, erosion and deposition
occurred at S1 with net erosion. The left side and mid-
dle part of S1 continued to be cut downward in a “V”
shape with a maximum erosion depth of 3.4 m and the
gulch widened further about 4m. The middle-left side
and middle-right part exist smallamplitude sedimenta-
tion with the average relative sediment thickness 0.9
m, with the maximum sediment thickness being 1.3
m deep. By comparison of observations in both 2009
and 2010’s at S1, the earthquake-induced debris flows
carried tremendous amounts of loosely consolidated
materials, violently eroded and deposited material in
the upper reaches and have the general trend of ero-
sion. Furthermore, the magnitude of debris-flow ero-
sion and deposition increases as debris flow activity
and scale increase in 2010.
EROSION AND SEDIMENTATION IN THE MID-
DLE REACHES OF GULLY
Section S4 (abbreviated to S4) was selected to
analyze the erosion and sedimentation features of
debris flow in the middle reaches of gully. As shown
from figure 7, in 2009, material was eroded and de-
posited at S4 to a moderate extent with erosion. The
left and middle parts of section S4 exhibited intense
downward cutting and erosion with a maximum ero-
sion depth of 2.6 m. The right side of S4 was also
eroded with a maximum erosion depth of 1.2 m. The
middle-right part and right side of S4 were dominated
by moderate deposition with an average accumulated
deposit thickness of 0.8 m. In 2010, material at S4
was eroded and deposited to larger extent and still dis-
played net erosion. The middle part and right side of
S4 suffered more intense erosion and continued to be
downward cut into a “V” shape with a maximum ero-
sion depth of 2.1 m. The middle-right part of S4 also
exhibited smaller-amplitude erosion with an average
relative erosion depth of 0.3 m; while the left edge of
S1 extended outward 8.1 m and up to 4.4 m of mate-
rial was deposited. From these observations we can
deduce that the magnitude of erosion and deposition
is controlled by the scale, discharge, and velocity of
debris flow. Small-scale debris flows result in lesser
erosion and deposition. On the same scale, the erosion
and sedimentation of debris flow in the middle gully is
larger than that in the upper reaches of gully.
Fig. 6 - Chart of debris-flow erosion and sediment at S1
section
Fig. 7 - Chart of debris-flow erosion and sediment at S4
section
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A STUDY ON THE PROPERTIES OF THE 12 MAY WENCHUAN EARTHQUAKE-INDUCED DEBRIS FLOW
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
871
figure 8, in 2009, material in S9 was eroded and depos-
ited to a large extent with net deposition. The left side
of section S9 was characterized by significant depo-
sition with maximum deposition thickness of 5.9 m.
The middle part of S9 suffered from sharp erosion and
produced a gulch with an average erosion depth of 2.2
m and a maximum erosion depth of 4.5 m. The right
part of the gully also showed erosion with an aver-
age erosion depth of 1.8 m. In 2010, the magnitude of
debris-flow deposition in the lower reaches of the gully
increased with the scale of the debris flow. The left and
middle parts of section S9 exhibit deposition with an
average relative deposit thickness of 0.8 m and a maxi-
mum relative deposit thickness of 3.8 m. The magni-
tude of erosion on the right side also increases with
debris-flow magnitude with the accumulated erosion
depth of 0.4 m. The result shows that the erosion and
deposition in the lower part of gully is most affected
by the magnitude of the debris flow. When debris flows
occurs on a small-scale, erosion and deposition magni-
tude are also small and for large-scale debris flows, the
magnitude of erosion and deposition is large. Gener-
ally, the debris flow had a trend of deposition. In ad-
dition, at S9, when the debris flow traveled along one
side of channel, sediment accumulated in the middle
of the gully, heightening the channel bed and pushing
successive debris flow material to the opposite side.
This action eroded the gully bank and transferred the
channel bed to the opposite bank to form an inclined
channel bed with one bank higher than the other.
GULLY BED EVOLUTION
EROSION AND DEPOSIT OF GULLY BED
In alpine gorge areas, earthquake-induced debris
flows that carry abundant loosely consolidated materi-
als will be triggered by intense rainfall and have strong
kinetic energy and intense erosion and deposition.
Therefore, such debris flows often remarkably change
the topographic features of gully and gully bed, lead
to gully erosion and deposition, and cause the gully
to change its course. The August 22, 2009 debris flow
was large scale event that rapidly moved along the
middle to left side of the gully bed and eroded the
gully to be 5.2 m wide and 2.6 m deep. Meanwhile,
large quantities of loosely consolidated materials with
0.12-0.35 m grain size were produced from this debris
flow and deposited in different parts of the gully and
gully bed. Accompanied by a flash flood, the July 15,
EROSION AND SEDIMENTATION IN THE LO-
wER REACHES OF GULLY
Section S9 (abbreviated to S9) was selected to an-
alyze the erosion and deposition features of the debris
flow in the lower reaches of the gully. As shown from
figure 8, in 2009, material in S9 was eroded and depos-
ited to a large extent with net deposition. The left side
of section S9 was characterized by significant depo-
sition with maximum deposition thickness of 5.9 m.
The middle part of S9 suffered from sharp erosion and
produced a gulch with an average erosion depth of 2.2
m and a maximum erosion depth of 4.5 m. The right
part of the gully also showed erosion with an aver-
age erosion depth of 1.8 m. In 2010, the magnitude of
debris-flow deposition in the lower reaches of the gully
increased with the scale of the debris flow. The left and
middle parts of section S9 exhibit deposition with an
average relative deposit thickness of 0.8 m and a maxi-
mum relative deposit thickness of 3.8 m. The magni-
tude of erosion on the right side also increases with
debris-flow magnitude with the accumulated erosion
depth of 0.4 m. The result shows that the erosion and
deposition in the lower part of gully is most affected
by the magnitude of the debris flow. When debris flows
occurs on a small-scale, erosion and deposition magni-
tude are also small and for large-scale debris flows, the
magnitude of erosion and deposition is large. Gener-
ally, the debris flow had a trend of deposition. In ad-
dition, at S9, when the debris flow traveled along one
side of channel, sediment accumulated in the middle
of the gully, heightening the channel bed and pushing
successive debris flow material to the opposite side.
This action eroded the gully bank and transferred the
channel bed to the opposite bank to form an inclined
channel bed with one bank higher than the other.
Section S9 (abbreviated to S9) was selected to an-
alyze the erosion and deposition features of the debris
flow in the lower reaches of the gully. As shown from
Fig. 8 - Chart of debris-flow erosion and sediment at S9
section
background image
Y.S. HAN, J. HAN, Y.Y. ZHU, Y.P. kONG, F.H. SU, L.w. LI& P. HUANG
872
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
velocity, gully bending characteristics and gully-bed
boundary conditions. Under different circumstances,
the channel of the gully may take different shapes, in-
cluding a single channel as a debrisflow route, double
channels with two debris-flow routes and wandering
channels with multiple debris-flow routes.
On August 22, 2009, the heavy rainstorm oc-
curred at Niujuan Gully, caused a flood and triggered
large-scale debris flows. The flood was retained by 5
dammed lakes in the upper-middle reaches of main
channel. Short of flood action and influence, the de-
bris flow occurred on a large scale, was limited to the
narrow and steep channel, ran rapidly and accelerated
continuously. When entering into the channel bed
of main gully, the debris flow eroded and deposited
material in the main channel under immense tractive
forces of loose solid substance and created a single
channel configuration. On July 15, 2010, an excep-
tional rainstorm happened at the study area, formed
a flash flood, and triggered a larger-scale debris flow.
The flash flood destroyed 2 dammed lakes in the upper
reaches of main gully and caused a catastrophic flood.
This flash flood and debris flow massively eroded and
deposited material along the channel bed and created
a wandering channel configuration..
DISCUSSION AND CONCLUDING RE-
MARKS
The preliminary study on the features of earth-
quake-induced debris flow suggests a number of con-
clusions that can be summarized as follows:
Special geologic conditions including topogo-
raphy, geology, climate, hydrology, land use, veg-
etable and so on, tremendous influence of violent
earthquake, and intense rainfall are the main factors
that contribute to debris flow occurrence. Debrisflow
initiation and gully evolution can be successfully
monitored using modern surveying and mapping
technologies, including GPS, GIS, total station, 3D
laser scanning system and spatial analysis.
Earthquake-induced debris flows are character-
2010 debris flow was a larger scale event that more
intensely eroded and deposited material. By drawing
a comparison between debris-flow observations made
in 2009 and those in 2010, we can obtain the follow-
ing results (Fig. 9).
With the increasing amplitude and scale of the
debris flow in 2010, the gully bed was eroded more
significantly. The area of gully erosion amounted to
55,989 m
2
, which accounted for 76.54 percent of the
gully bed area. The total volume of loosely consoli-
dated materials eroded from the gully reached 78,919
m
3
and the mean erosion depth was 1.41 m. In parts
of gully bed, the maximum local erosion volume was
10,379 m
3
and the maximum mean erosion depth was
2.54 m. In general, the whole gully was eroded to dif-
ferent degrees, but the upper and middle reaches of
gully were the most eroded.
With the increasing magnitude of debris flows
in 2010, the magnitude of debris-flow deposition in-
creased. The area of gully bed deposition amounted
to 10,121 m
2
, which accounted for 13.84 percent of
the gully bed area. The total volume of loosely con-
solidated material deposited in the gully reached
15,143 m
3
and the mean deposit thickness was 1.57
m. In parts of gully bed, the maximum local deposit
volume was 9,827 m
3
and the maximum mean deposi-
tion thickness was 3.34 m. In general, variability ex-
ists in the amount of deposition in the whole gully, but
the two sides and lower reaches of gully exhibited the
most deposition.
There were still fragmentary unchangeable parts
of gully without debris-flow erosion and sedimenta-
tion and its total acreage amounted to 7,041 m
2,
ac-
counting for 9.62 percent of the total area of gully bed
GULLY-BED CONFIGURATION
When the debris flow travelled through a gully, it
considerably changed the gully-bed and channel con-
figuration (Fig. 10). However, the evolution of chan-
nel configuration was affected by water and sediment
conditions of main gully, debris-flow magnitude and
Fig. 9 - Gully erosion and sedimentation caused by
debris flows in 2009 and 2010
Fig. 10 - The evolution of gully channel configuration
caused by debris flows in 2009 and 2010
background image
A STUDY ON THE PROPERTIES OF THE 12 MAY WENCHUAN EARTHQUAKE-INDUCED DEBRIS FLOW
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
873
and sedimentation, even with hundreds of thousands
cubic meters of sediment at a scene.
The most active and frequent debris flows oc-
cur withinfive to ten years following an earthquake.
Therefore, in order to understand and prevent debris
flows, it is imperative to study the initiation and char-
acteristics of earthquake-induced debris flows and to
make field monitoring measurements and risk assess-
ments of debris flows.
ACKNOWLEDGMENTS
This study is financially supported by the West
Traffic Construction of Science and Technology
(No.2008-318-221-96), the National Natural Science
Foundation of China (No.40901273), the National
Natural Science Foundation of China (No.40801009)
and the Knowledge Innovation Program of Chinese
Academy Sciences (No.KZCX2-YW-Q03-5). We
thank State Bureau of Surveying and Mapping of
China for providing digital line graph. And we also
thank x
ue
J
iao
, z
HanG
y
onGxianG
, l
iu
f
anG
and
l
i
Q
inGlin
, l
iao
l
iPinG
, y
anG
z
HiQuan
& y
anG
w
anke
for their assistance in field investigation and process-
ing data and last thank Liu Lian for her assistance in
revising the paper.
ized by intense erosion and deposition and the mag-
nitude of erosion and deposition has a close bearing
on debris-flow scale, gully location and channel con-
figuration. Erosion and deposition occurs in the upper
reaches of the gully with a general trend of mid-small
scale erosion. Deposition and erosion occurs in the
middle reaches of gully with a general trend of deposi-
tion. Erosion and deposition occur in the lower reach-
es of the gully with a tendency of strong deposition.
Meanwhile, the magnitude of erosion and deposition
along the gully become larger with the magnitude of
the debris flow.
The channel and gully-bed configuration is
changed remarkably and influenced by water-sedi-
ment conditions of the main gully, debris flow mag-
nitude and velocity, gully bending characteristics and
gully-bed boundary conditions. Different sized debris
flows have different effects on the channel and gully
bed, and create different gully configurations. Large-
scale viscous debris flows, bearing less water, pro-
duces the sole channel configuration and the channel
bed has a general tendency towards deposition. Under
the action of great flood, non-cohesive debris flow of-
ten generate wandering multi-route channels and the
channel bed has a general tendency of intense erosion
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