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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
705
DOI: 10.4408/IJEGE.2011-03.B-077
DEBRIS FLOWS IN MEIGU COUNTY OF SICHUAN (SW CHINA):
GEOMORPHOLOGY AND HAZARDS
Xilin LIU
(*)
& Dan ZHANG
(**)
(*)
School of Geography and Planning, Sun Yat-Sen University, Guangzhou, Guangdong 510275, P.R.China
(**)
Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, P.R.China
m
oRton
, 1989; i
veRson
et alii, 1997; C
oe
et alii, 1997;
C
annon
et alii, 2001; w
ilkeRson
& s
CHmid
, 2003;
G
RiffitHs
et alii, 2004; G
odt
& C
oe
, 2007). A lot of
studies around the world in recent years showed that
the runoff-dominated and fire-hose effect processes
are triggered by rainfall especially for short-duration,
moderate to high intensity rainstorm or thunderstorm
that results in overland flow from unsaturated, con-
solidated older colluviums, and steep bedrock cliffs
(
ilkeRson
& s
CHmid
, 2003; C
Hen
et alii, 2006; G
odt
&
C
oe
, 2007; m
oRton
et alii, 2008; P
elfini
& s
antilli
,
2008; C
oe
et alii, 2008; b
RaysHaw
& H
assan
, 2009).
For the triggering of debris flows, precipitation is an
important but not the only critical factor, topography,
lithology, erosion, sediment entrainment and vegetal
cover are essential for debris flow initiation (G
odt
&
C
oe
, 2007; s
CHneideR
et alii, 2010). Rainfall-induced
debris flows, therefore, form a wide range of geomor-
phologic characteristics in onset zones, incised chan-
nels and cone deposition and present a hazard that is
being increasingly recognized. Many researchers have
gone into the prediction of debris flows’ occurrence
(b
atHuRst
et alii, 1997; G
ianneCCHini
et alii, 2007)
and estimation of their magnitude and run-out dis-
tance (H
unGR
et alii, 2007; m
illeR
& b
uRnett
, 2008)
in order to reduce associated risk.
The purpose of this case study is to document the
debris flows activities associated with a 1 June 2005
rainstorm and hail with short-duration and high in-
tensity in Meigu county of Sichuan province, China.
ABSTRACT
A torrential rainstorm on June 1, 2005 in Meigu
county of Sichuan province, SW China, resulted in the
generation of debris flows from two drainage basins.
Map (1:50000 scale) of topography and depositing
materials, coupled with the data from meteorological
station and field investigation, are used to evaluate the
geomorphology and hazard characteristics of debris
flows. The return period of debris flows is 20 years
and its magnitude is small-moderate scale. Rainstorm
and hail with short duration and high intensity in the
upper reaches of drainage basins are the triggering fac-
tor for debris flows. Unstable side slope in low-middle
reaches and a large number of scattered materials in
channels provide source materials for debris flows.
Grain-size curves of debris flows are multiple peaks.
The high clay content, boulder piling up boulder, mud
crack show that the debris flows are viscous. Debris
flows occurred abruptly, 9 local residents were killed
or missed, 8 were injured and great property was lost
K
ey
words
: debris flows, hazard processes, Meigu county
INTRODUCTION
Debris flows represent a remarkable geomorpho-
logic hazard. They occur in diverse physiographic and
climatic environments (l
aRsen
et alii, 2006). Three
processes of debris-flows initiation have been identi-
fied in literatures: infiltration-triggered, runoff-domi-
nated and fire-hose effect (J
oHnson
& R
odine
, 1984;
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X. LIU & D. ZHANG
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
well. The outcrop consists of Jurassic purple rock,
Triassic limestone and purple rock, Ordovician red
sand-shale stone (e
ditoRial
C
ommittee
of
z
HaoJue
C
ounty
- a
nnals
of
s
iCHuan
P
RovinCe
, 1999). Both
drainage basins are on the east limb of an anticline
along Kangding and Yunnan, and being eroded in-
tensely by the water system of Jinsha river. Elevations
in the study area range from about 1000 m to 3000 m
with large relief.
The climate of study area is typically of plateau
climate, with dry-cold winter and wet-warm summer.
According to the climatic regionalization, this area
belongs to Yalongjiang temperate zone in the west of
Sichuan province with mean annual temperature of
11.4°C and mean annual rainfall of 814.4 mm. The
southwest monsoon prevails during mid-May to mid-
October. Because of the thermal influence, frequent
rainstorms usually fall in the night with high intensity
during the rainy season.
The soils distributed in the study area mainly
include paddy soil, moisture soil, yellow-brown soil.
There are thick soil-layer and abundant heat-water, the
soil fertility is rich. Local residents usually plant rice,
corn, cabbage, garlic and so on. They can reap twice
a year in their fields. Unfortunately, the increasingly
human disturbances such as grazing and deforestation
have severely depleted the forest cover in Caimoluo
and Naituo drainage basins. As a result, there are no
extensive tree covers except a few scattered small
shrubs and herbaceous plants. Water loss and soil ero-
sion are serious in study area.
DEBRIS FLOW INITIATION PROCESSES
Field observation indicates that the debris flow
initiation process is similar with a process called
“fire-hose effect” (J
oHnson
& R
odine
, 1984), when
overland flow of water resulting from intense rainfall
reaches the base of bedrock slopes, it mobilized the
solid material and eroded deep gullies into the heads
of the deposits. Base on the field investigations of
rainfall, geomorphology, stratum lithology, geologic
structure and human activity, the debris flow initiation
processes can be described in four aspects.
INTENSE RAINFALL AND HAIL
The hydrometeorologic conditions for debris flow
initiation may vary widely. Rainfall intensity, accumu-
lated rain during the storm event and antecedent pre-
At about 1:00 in the morning, debris flows occurred
almost at the same time both in Caimoluo gully and
Naituo gully near Maluo village in Luoeyigan town in
the southeast of Meigu county. During the disaster, 9
local residents were killed or missed, 8 were injured
and great property was lost. In this paper, the physical
and geologic settings of study area are presented and
followed with an analysis of the triggering rainfall.
Base on field investigation of topography, lithology,
human activity and grain-size analyses of deposition,
the initiation processes and geomorphology and haz-
ard characteristics of debris flows were discussed. The
objective of this paper is that the baseline information
presented hereby can be used in future regional mod-
eling efforts to assess debris flow hazard
STUDY AREA
The Caimoluo gully and Naituo gully that we sur-
veyed for this study are located in the right bank of
Liutong river, they are ranked the first-branch trench-
es. Liutong river is the first tributary of Jinsha river.
The distance between both gullies is 0.5 km. Base on
the Global Position System (GPS), Caimoluo gully is
positioned at 28°06.394′N, 103°01.681′E and Naituo
gully is positioned at 28°06.461′N, 103°01.859′E.
Both gullies belong to Maluo village of Luoeyigan
town, which is the boundary between Meigu and
Zhaojue county (Figure 1).
Caimoluo and Naituo drainage basins are situated
in the north and south trending tectonic belt between
Sichuan province and Yunnan province which is char-
acteristic of widespread folds and faults. The rocks
are weathered intensively and the joints are developed
Fig. 1 - Sketch map of the topography of debris flow
gullies
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DEBRIS FLOWS IN MEIGU COUNTY OF SICHUAN (SW CHINA): GEOMORPHOLOGY AND HAZARDS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
707
in its drainage basin, especially in the upper reach, be-
cause of its shorter length and smaller drainage area.
This can be regard as evidence that it was the intense
rainfall and hail provided enough water for the debris
flows of Caimoluo gully and Naituo gully. In a word,
the rainfall and hail with short duration and high inten-
sity was important reason why debris flows occurred.
TOPOGRAPHIC AND GEOMORPHOLOGIC
CONDITION
According to on-site measurement by GPS and
the topographic map with the scale 1:50000 (Fig. 1;
Fig. 3), the topographic and geomorphologic data of
Caimoluo gully and Naituo gully can be obtained
(Tab. 1). It can be seen that the topography and geo-
morphology of Caimoluo gully and Naituo gully have
two favorable conditions. For one thing, this topo-
graphic configuration is conducive to a rapid concen-
tration of runoff, and when combined with intense
rains, it can lead to high peak discharge and erosion
rates. For another thing, it provides potential energy
for debris flows.
STRATUM LITHOLOGY AND GEOLOGIC
STRUCTURE CONDITION
The strata in Caimoluo and Naituo drainage ba-
sins are mainly Jurassic purple rock, Triassic grey
rock and purple rock, Ordovician red sand-shale
stone. Though the lithology on both sides of Caimoluo
cipitation are important factors for triggering debris
flows (C
amPbell
, 1975). Rainstorm with short dura-
tion and high intensity is crucial for the debris flow
occurrence within study area. At about 10 o’clock in
the night of May 31, hails suddenly struck the upper
reach of Caimoluo and Naituo drainage basins. An
hour later, heavy rain fell also mixed with hail, the
flux of both gullies increased rapidly. Heavy rain kept
on about two hours before debris flows occurred. Ac-
cording to the records from meteorological station of
Meigu county, on June 1 debris flows occurred, the
daily rainfall amount is 29.3 mm and during the past
30 days from May 2 to May 31 before debris flows oc-
curred, the total rainfall was 140.8 mm (Fig. 2).
Rainstorm runoff formed in the upper reach of
both gullies, eroded solid materials in the channels
and deposits piled up in the lower valley slopes. When
slope became saturation, the stability lowered and a
series of collapses began to occur, then a great deal of
solid material such as gravels, loose soil was engulfed
by the floodwater. As a result, at about 1 o’clock on
the dawn of June 1, debris flows were triggered.
A shorter gully without name between Caimoluo
gully and Naituo gully is worthy to be mentioned
whose main channel length is 2.77 km. Base on field
investigation, it was found that the unconsolidated ma-
terials and topographic are all similar to the study gul-
lies. But there was no debris flow occurrence on June
1. The reason may be that there was not enough rainfall
Fig. 2 - Daily and cumulative rainfall during May 2005
recorded at Meigu county station
Fig. 3 - Longitudinal profile of the debris flow gully beds
Tab. 1 - Morphometric characteristics of both
gullies
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X. LIU & D. ZHANG
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
The deforestation and farming activities are increas-
ingly serious, which result in the number of trees and
bushes is less and less in two drainage basins. As a
result, the soils are eroded seriously and the ecologi-
cal environment is deteriorative increasingly. It is just
these unreasonable human activities that accelerate
the debris flows occurrence.
Moreover, the houses of local residents and
other infrastructures such as electricity and commu-
nication facilities were built near debris flows gullies
where is the best prone to be destroyed when debris
flows dashed out. Therefore, the unreasonable dis-
tribution of buildings and constructions is also one
important reason why the loss caused by debris flows
was so disastrous.
DEBRIS FLOWS CHARACTERISTICS
DRAINAGE BASIN CHARACTERISTICS
A classic debris flow landform includes well-
defined starting zones, incised channels, levees, and
cone deposits (z
immeRmann
, 1987). The mid-section
consists of a steep, confined channel with relatively
steep sidewalls, and often a fan or cone at the lower
end where the gully intersects a valley floor (b
Ray
-
sHaw
& H
assan
, 2009). The ease of identifying gul-
lies, however, is offset by the more complex nature of
debris flows frequency and magnitude. For example,
some debris flows channels have a U-shaped profile
with a flat channel floor and nearly vertical walls, and
are flanked by debris-flow levees. While some chan-
nels have steep-sided ridge-like levees produced by
material tumbled from flows that had a convex upper
surface that was higher than the channel walls (m
oR
-
ton
et alii, 2008).
Both Caimoluo gully and Naituo gully can be ob-
viously divided into source area, transportation area
and deposition area, the debris flows are typical gully-
type. As to the transportation area, both banks of that
gully is the same as that of Naituo gully, the slope
characteristic and the supply mode of solid materials
are different. The bedrock on left bank of both gul-
lies is exposed, whereas the rocks and soils on the
right bank usually fall into the channels in the way
of collapse because of the well-developed vertical
joints (Fig. 4). For example, a large-scale landslide
is located on the right bank in the middle reach of
Naituo gully, potato and walnut have been cultivated
on the landslide. There are a lot of cracks which are
about tens of centimeters width on the landslide (Fig.
5). More and more solid materials fall into the chan-
nel in the way of collapse and slide. Therefore, the
cracked strata, lithology, as well as geologic structure
in Caimoluo and Naituo drainage basins contribute
to the debris flow occurrences by providing source
materials
UNREASONABLE HUMAN ACTIVITIES
Tens of villages distribute in Caimoluo and
Naituo drainage basins such as Yibowo village,
Zhazhi village, Naituo village. Local residents main-
ly live by farming and grazing. Because study area is
situate at the district boundary between Meigu and
Zhaojue county, the cultural and educational level
fall behind and local residents hardly have any pro-
tection consciousness for the ecological environment.
Fig. 4 - Loose materials on the right slope of Caimoluo gully
Fig. 5 - Landslide surface cracks on the right slope of
Naituo gully
Fig. 6 - Serious blocking in the gully bed of Caimoluo gully
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DEBRIS FLOWS IN MEIGU COUNTY OF SICHUAN (SW CHINA): GEOMORPHOLOGY AND HAZARDS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
709
from both banks fall into the channels constantly and
continually. Fractured bedrocks on the sidewalls pro-
vide abundant material for the development of debris
flows. As a result, both banks in channels were eroded
seriously and the riverbed was blocked badly. When
the next heavy rainstorm is coming, another debris
flow may occur because of the blocking and flood
overflowing in the channels. Thus, the debris flows
have the further development tendency.
DYNAMICAL CHARACTERISTICS
Base on the on-site survey of mud trace which
is a strong evidence of dynamic characteristic, some
indexes such as the maximum width and height of
mud trace, the maximum area of the cross section
and slope gradient were measured (Tab. 2). Accord-
ing to the improved formula which is adopted to
calculate debris flow velocity of Dongchuan, China
(C
Hen
et alii, 1983):
where V
C
is debris flows velocity (m/s). MC is chan-
nel roughness coefficient. a is collision coefficient,
for viscous debris flow, a=0.01~0.02; for turbulent
debris flow, a=0.02~0.03. R is hydraulic radius (m).
I is channel gradient. The velocity and flow of debris
flows in both gullies are calculated respectively. The
results are showed in Table 2.
DEPOSITING CHARACTERISTICS
Debris flows have powerful carrying ability and a
great deal of large volumes of boulders, gravel, sand
and clay can be carried down to the lower reach of
gully. Deposits piled up in the gully mouth are called
“debris fans” (w
Hite
, 1981; m
adole
et alii, 1998).
In the study area, plenty of clay, sand, gravels and
even boulders were piled up on the gully month, thus
two deposition fans were formed. The length, width,
and gradient of the fans were measured with equip-
ments and accordingly the volume of run-out depos-
its was calculated. The results are showed in Table 3.
Much of the debris volume reaching the gully mouth
comes from materials entrained in channels, the size
are unstable, especially for the right one. The channels
are jammed badly with abundance of solid materials
(Fig. 6). As seen in Figure 6, the sidewall of chan-
nel exhibits extensive areas of freshly exposed where
we can see the red soil, steep to near vertical walls.
And there likely was less than 0.5 m material mantle
on the surface, which is one of reasons why the solid
materials are loose. Therefore, one clear characteristic
of drainage basin is that the transportation area is also
an important supply source of solid materials, which
provide lots of loose materials for debris flows so as to
increase the magnitude and damage ability.
As showed in some studies, the channel gradient
was the first significant parameter, with steeper chan-
nels being more likely to produce debris flow (C
annon
et alii, 2001; b
RaysHaw
& H
assan
, 2009). The mean
channel gradients (in degrees and percent slope) for
failures that did result in a debris flow were 29±6° and
57±13% (b
RaysHaw
& H
assan
, 2009). In Caimoluo
gully and Naituo gully, there are steep slope gradient
which are 169% and 195% respectively (Figure 3) and
perennial flow water in channels, the velocity are 1.3
m/s and 0.4 m/s respectively, the flow are 0.2 m
3
/s and
0.02 m
3
/s respectively.
RETURN PERIOD CHARACTERISTICS
The occurrence of debris flows is closely con-
nected with many factors, such as topography, geol-
ogy, soil, vegetation, precipitation, as well as human
activities. Some authors think that debris flows events
in the high mountain environment cannot be easily
correlated with daily precipitation data, it can be as-
sumed that the debris flows are triggered by 20-30 mm
of rainfall, probably concentrated in a short time, such
as a few hours (w
ilkeRson
& s
CHmid
, 2003).
According to the past documented record, a
large-magnitude debris flow ever occurred in 1982 in
Caimoluo gully. So we can infer that Caimoluo gully
is an lower-frequency debris flow gully, its return pe-
riod is 20 years approximately. What’s more, the loose
soil and gravels have been sliding and collapsing on
both sidewalls of Caimoluo gully and Naituo gully
after debris flows occurred. And the solid materials
Tab.2 - Mud trace and dynamic parameters in
both gullies
(1)
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
range from clay to boulder. Huge boulders were carried
to near gully mouth by debris flows. The size of the
largest one is about 4.5×4×3 m
3
(Fig. 7) in Caimoluo
gully and that is about 4×3×1.7 m
3
in Naituo gully.
Debris flow deposits are on unsorted and stratified.
Depositing geomorphology mainly are gravel islands,
lateral accumulation mounds in channel and surge de-
posits, fan-shaped gravel deposits on the toe. Other
micro-topography could be seen such as the boulder
piling up boulder, mud crack, which demonstrate that
the debris flows are viscous.
Grain samples are source soils derived from right
slope, in-channel debris flow accumulation in the mid-
dle reach of Caimoluo gully, landslide earth derived
from right bank in the lower reach of Naituo gully, all
these were collected and tested by geo-technology in
the laboratory. Under the preconditions that the water
content accounts for 10% and the density is 1.65g/
cm3, the test results of the source soils from Caimoluo
gully are as follows: internal frictional angle 31.3º, co-
hesive force 12 Kp
a,
liquefaction limit of the viscous
grains 33.2%, plasticity limit 19.8% and the plasticity
index 13.4. The peak of grain-size distribution curve
leaned to right. According to soil texture classification
published by the United States Department of Agri-
culture (b
Rady
, 1974), the weight content of fine silt
(0.005―0.002 mm) and clay (<0.002 mm) accounts
for 19.3% (Fig. 8). As to the in-channel debris flows
accumulation in the middle reach of Caimoluo gully,
under the preconditions that the water content accounts
for 7.7% and the density is 1.62g/cm
3
, the test results
are as follows: the internal frictional angle 38.2º, co-
hesive force 12 Kp
a
. The weight content of fine silt
(0.005-0.002 mm) and clay (<0.002 mm) is 2.2% and
the grain-size distribution curve has three typical peaks
appearing on the percentage content of silt, sand and
gravel respectively (Fig. 8). The landslide lying in the
right bank of the lower reach of Naituo gully was in-
duced by debris flows, and it also provides a great deal
of solid materials for debris flows. According to the
geotechnical testing on the accumulation of the land-
slide front, on the precondition that the water content
accounts for 7.4% and the density is 1.76g/cm3, the in-
ternal frictional angle and cohesive force are 35.8º and
5 Kp
a
respectively. The grain-size distribution curve
has only one peak (Fig. 8).
DISASTER CHARACTERISTICS
Disaster characteristics of debris flows in Meigu
county can be summarized as follows. Firstly, they are
different from those of most rainstorm-induced debris
flows. There was almost no precedent precipitation the
day before debris flows occurred in locality. The de-
bris flows were triggered by rains and hails which fell
only two hours earlier before debris flows occurred.
Secondly, some large boulders from the upper reach
of both drainage basins can be seen on the fans. Thus,
it can be inferred that the transport distance of debris
flows are as many as several kilometers (Fig. 9). Some
trees’ rinds were decorticated by debris flows (Fig. 10).
So it is clear that the destructive energy of debris flows
is powerful. Last but not least, the disaster occurred at
midnight and kept on less than half an hour, however,
9 local residents were killed or missed, 8 were injured
and great property was lost, as well as buildings, farms,
Tab. 3 - Deposition fans characteristics in
both gullies
Fig. 7 - Large boulder carried by the debris flow in Cai-
moluo gully
Fig. 8 - Grain-size curve of source soil and debris flow
deposits in study area
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DEBRIS FLOWS IN MEIGU COUNTY OF SICHUAN (SW CHINA): GEOMORPHOLOGY AND HAZARDS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
711
tions against natural disaster, for example, they built a
floodwall with the height of 2 meter, unfortunately, the
floodwall did not take effect during this catastrophe.
Therefore, some technological and civil engineering
measurements should be taken into account in order to
mitigate debris flows hazards in Meigu county.
Several suggestions can be adopted in the study area
from this study case. Firstly, the important interaction
between human and natural environment indicates that
local residents should improve protection consciousness
for the ecological environment, excessive deforestation
and farming must be prevented. Secondly, local resi-
dents can move out of the dangerous area of the debris
flows and the infrastructures should be put in the safe
region with finance support by governments. Thirdly,
the meteorological monitoring in areas prone to debris
flows activity could help predict debris flow hazards.
Fourthly, a variety of geomorphic methods, including
repeat photography, dendrogeomorphology, lichenom-
etry, plant succession, and stratigraphic analyses, could
be employed in the study of debris flows so as to under-
stand the mechanism and process of debris flows better.
ACKNOWLEDGEMENTS
This study was financially supported by the
National Science Foundation of China (Grant No.
41071186).
roads, bridges, communication facilities and hydro-
power facilities were damaged seriously (Fig. 11 and
12), vegetal cover and surface soil on the slopes and in
the channels were washed away by flood, which result
in the ecological environment further deteriorated.
CONCLUSIONS AND SUGGESTIONS
Debris flows are significant hazards in mountain-
ous areas, especially in southwestern China. The debris
flows in Meigu county were triggered by short-duration
heavy rainstorm and hail, the topography, lithology and
human activity also play an important role in debris
flow initiation. In both gullies, the details of debris flow
characteristics can be obtained by a series of field inves-
tigation. Multiple evidences such as boulder piling up
boulder, mud crack and clay content demonstrate that
they are viscous debris flows. The viscous debris flows
broke out rapidly and violently, so it has destroyed the
houses, farms, roads, bridges, communication facilities
and even lead to loss of human life. This is a catastro-
phe caused by natural hazard, whereas could arose high
attention for the whole society.
Historically, debris flows ever occurred in Caimoluo
gully and Naituo gully, there are distinct fans trace on
the mouth of both gullies and the trail of transport and
deposition in the channels. Local residents have little
consciousness of the hazards, though they taken precau-
Fig. 9 - Large boulders carried by the debris flow in
Naituogully
Fig. 10 - Big trees debarked by the debris flow in Naituo
gully
Fig. 11 - Destroyed house by the debris flow
Fig. 12 - Destroyed communication facilities by debris
flow
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X. LIU & D. ZHANG
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
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ditoRial
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ommittee
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