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ORIGINAL ARTICLE
Relationships between debris flows and earth surface factorsin Southwest China
Fangqiang Wei Æ Kechang Gao Æ Kaiheng Hu ÆYong Li Æ James Smith Gardner
Received: 20 May 2007 / Accepted: 17 August 2007 / Published online: 1 September 2007
� Springer-Verlag 2007
Abstract Southwest China, including the Provinces of
Yunnan, Guizhou, Sichuan and Chongqing, is a region with
serious debris flow hazards, where 7,561 debris flow sites
have been identified. Based on the data from these sites, the
distribution regularity of debris flows was analyzed. Earth
surface factors that may influence the formation of debris
flows were analyzed from the viewpoints of energy and
material conditions. Four major earth surface factors were
selected: relative relief, stratigraphy, fault density and land-
use conditions. With the support of GIS, the research
region was divided into 125,177 grid cells and for each cell
data for the four factors were collected. Based on this
information, the distribution of quantity and the occurrence
probability of debris flows and the role of each factor were
statistically analyzed. The results should be helpful for the
assessment of debris flow hazards and debris flow fore-
casting in the research region.
Keywords Debris flow � Earth surface factor �Debris flow distribution � Southwest China
Introduction
Debris flow is one of the hazards, which generally occur in
mountainous regions. Being influenced by many factors
related to topography, geology, and climate, the debris flow
formation mechanisms are complex and there is a lack of
generally accepted theory. This has limited the progress of
research relating to debris flow prediction, which still remains
at the level of analyzing statistics of the critical rainfall,
especially for the regional prediction of debris flow (Cui et al.
2000). However, debris flows result from interactions
between rainfall and a variety of earth surface conditions.
Although critical rainfall is crucial, its value is limited
without the added consideration of earth surface conditions.
Much of the past research has focused on the relation-
ships between topography feature and site hydrology
(Muller 1973; Patton 1988; Willgoose 1994). Li et al.
(2002) carried out a statistical analysis of the topographical
features of debris flow sites including relative difference of
elevation, drainage area and gradient. Santos et al. (2006)
studied the significance of morphologic features in a
regional debris flow disasters assessment. However, these
researches did not take into account other earth surface
factors such as relative relief (local difference in elevation),
stratum, fault density and land-use conditions and their
relationship to debris flows. This paper examines these
F. Wei
Key Laboratory of Mountain Hazards and Surface Process,
Chinese Academy of Sciences, Chengdu 610041, China
F. Wei (&) � K. Hu � Y. Li
Institute of Mountain Hazards and Environment,
Chinese Academy of Sciences, Chengdu 610041, China
e-mail: [email protected]
K. Hu
e-mail: [email protected]
Y. Li
e-mail: [email protected]
K. Gao
South China University of Technology,
Guangzhou 510641, China
e-mail: [email protected]
J. S. Gardner
Natural Resources Institute,
Clayton Riddell Faculty of Environment,
Earth and Resources, University of Manitoba,
Winnipeg, MB, Canada R3T 2N2
e-mail: [email protected]
123
Environ Geol (2008) 55:619–627
DOI 10.1007/s00254-007-1012-3
relationships based on data from 7,561 debris flow sites in
Southwest China. The results of this research are intended
to be helpful for the assessment and prediction of debris
flow hazards and disasters.
General conditions in the research region
Southwest China is a region that experiences frequent and
serious debris flows. The region is located between E97�300–E110�150 and N21�50–N34�200, including Sichuan, Yunnan,
Guizhou and Chongqing. Its area is 1.142 million km2 with a
population of 201.6 million. The following is a brief
description of the physical conditions.
Topography
Bounding the east Qinghai-Tibet Plateau, the research
region is composed of undulating ground with remarkable
relief, higher in the northwest and lower in the southeast.
Based on the macro-topography characteristics, it can be
divided into several typical geomorphic units: the moun-
tains and plateaus of western Sichuan, the Hengduanshan
mountains, the Yunnan-Guizhou Plateau, the Sichuan
Basin and the Qinba mountains (Fig. 1).
The mountains and plateaus of western Sichuan, an
eastward projecting borderland of Qinghai-Tibet Plateau
including the Ruoergai Plateau and Minshan mountains, are
in the northwest of the research region. Ruoergai Plateau, a
part of Qinghai-Tibet Plateau, has a generally flat surface at
altitudes between 3,500 and 3,800 m. The average altitude
of the Minshan Mountains is greater than 4,000 m, its
relative relief is large and there are active glaciers present.
The Hengduanshan Mountains area covers approximately
600,000 km2 with an average altitude of 4,000–5,000 m.
The Sichuan Basin is at a low altitude of 700–1,000 m, is
surrounded by mountains and forms the densely inhabited
core of Sichuan Province. The Chengdu plain in the western
part of the basin is flat, the middle part of the basin is hilly
area with low relative relief, and parallel ridges or hills
dominate the eastern part of the basin. Most parts of Yun-
nan-Guizhou Plateau are rugged and broken, eroded by
Changjiang River and its tributaries, except on flatter pla-
teaus in the middle and eastern part of Yunnan and the
northwest part of Guizhou. The Qinba Mountains include
the Qinling and Dabashan Mountain, with respective alti-
tudes of 2,000–3,000 m, and 1,000–2,000 m. They contain
many deeply incised valleys due to intense erosion.
Geology
The research region, covering the first and second topo-
graphical stairs of China, has a complex geologic structure
with intense neotectonic movements and frequent earth-
quakes. The Qinghai-Tibet Plateau is controlled by tec-
tonic systems of collision and eta-type structure systems.
The Yunnan-Guizhou Plateau and Sichuan Basin are
controlled mainly by Neocathaysian tectonic systems and
the Qinba Mountain area is controlled by the latitudinal
tectonic systems. The Hengduanshan Mountains area is
controlled by eta-type structure and longitudinal tectonic
systems.
Climate
The southeast and southwest monsoons, with air masses
from the Pacific and Indian Oceans, respectively, bring
abundant precipitation to most areas of the research
region. The dry season and wet season are distinctive with
more than 80% of the annual precipitation is in the wet
season (May–October). The rainfall patterns also vary
greatly with the monsoon and the large-scale topography.
The Yunnan-Guizhou Plateau, Sichuan Basin and Qinba
Mountain areas belong to the subtropical monsoon cli-
matic regime, with the mean annual precipitation being
about 1,000 mm, decreasing from east to west. Because of
the south–north strike of all the mountains and rivers in
southwest Yunnan, the warm and moist air masses move
northwards along the rivers and brings copious rainfall,
with the mean annual precipitation being as great as
1,500–2,800 mm.
Debris flow distribution in the research region
Influenced by the terrain, geology, precipitation and human
activities, debris flows develop and are distributed widely
in the research region. There are 7,561 recorded debris flow
sites (Fig. 1). Based on the debris flow distribution in
Fig. 1, the characteristics of debris flow distribution under
various conditions may be drawn as follows.
In the transitional zones of large geomorphic units
The geologic structure is complex with active seismicity
and the relative relief is great in the transitional zones
between large geomorphic units. The great relief produces
increased orographic precipitation. Together, these factors
provide favorable conditions for debris flow development.
In this region, the transitional zones from the Qinghai-Tibet
Plateau to the Yunnan-Guizhou Plateau and the Sichuan
Basin, as well as the mountainous areas around Sichuan
Basin, are areas where intense debris flow activity occurs.
620 Environ Geol (2008) 55:619–627
123
In areas with intense stream incision
and great relative relief
Areas with intense stream incision also are characterized by
crustal uplift, complex geologic structure, great relative relief
and steep terrain, all of which are favorable for debris flow
development. Thus, debris flows usually are found in areas,
such as the Hengduanshan Mountains and along the north to
south trending rivers in southwestern Yunnan as well as in
the Yalongjiang, Anninghe, Daduhe valleys, the downstream
reaches of the Jinshajiang, and the upstream reaches of the
Minjiang, the Jialingjiang and the Bailongjiang.
In the fault belts and seismic zones
Fault belts include areas with complex geologic structure,
intense neotectonic movement and frequent seismic activ-
ities. In these zones, the bedrock is disrupted and broken,
the stability of slopes is low, and stream incision along the
fault lines is strong. Under these favorable conditions,
debris flows are common. In addition, seismic activity
usually induces large-scale landslides. The debris flow
activities may be active for long periods after an earth-
quake event because the landslides provide abundant
unconsolidated material for debris flows formation.
In the areas with heavy and intense rainfall
High intensity and prolonged rainfall are major triggering
factor for debris flows. Thus, debris flows are common in
areas exposed to the upslope flow of monsoonal air masses.
Examples of such locations are the Panzhihua-Xichang
area in southwest Sichuan, the eastern part of Longmen-
shan Mountain, and the north and eastern parts of Sichuan
Basin, all with a mean annual precipitation of greater than
1,200 mm. Also the Dayingjiang valley in the southwest
Yunnan, with a mean annual precipitation of 1,345–
2,023 mm is characterized by intense debris flow activity.
Fig. 1 Terrain and debris flow
distribution in Southwest China
Environ Geol (2008) 55:619–627 621
123
The key earth surface factors affecting debris flows
Three conditions are necessary for debris flow formation:
energy, material and a water source. Since water sources
mainly refer to rainfall, they are not included as an earth
surface factor, leaving the energy and material conditions
to be considered.
Energy conditions
The energy conditions are determined primarily by topo-
graphy including relative relief and slope gradient. The
former is responsible for the potential energy and the later
for the energy transformation slope for the movement of
debris flows. In our analysis, they are measured by grid
cells of the same shape and size and are thus correlated. For
simplicity, the relative relief is selected as the key factor to
describe the energy conditions.
Material conditions
Availability of unconsolidated material is necessary for
debris flows to occur. The amount of unconsolidated material
is the key material condition because it not only determines
whether a debris flow could initiate but also influences the
critical quantity of water (i.e., rainfall) required to trigger the
debris flow. The amount of stored unconsolidated material is
difficult to estimate directly and quantitatively for each grid
cell or site in a large region. However, it may be reasonably
estimated by assessing the factors that influence the pro-
duction of unconsolidated material.
Geologically, the structure and stratigraphy are two
factors that directly influence the production of unconsol-
idated material. In particular, joints and faults provide
weakened zones in the bedrock while the strata of various
lithology respond in differently to weathering. They impact
independently and thus both should be taken as key factors
for evaluating the material conditions.
Ground cover is another factor of influence. The accu-
mulation rate of unconsolidated material tends to be less in
well-vegetated areas than in poorly vegetated or barren
areas. Human activities and land uses influence ground
cover, soil structure and the supply of unconsolidated
material through mining and road construction. Vegetation
ground cover may be estimated from a vegetation map.
Human activities are not easily reduced to a single index.
Both, however, may be reflected in the land-use condition,
which can be estimated from a land use map.
In summary, the main factors influencing debris flow
formation are relative relief, stratigraphy, geological
structure including fault zones and land-use index acting
in combination under given moisture (i.e., rainfall)
conditions.
Relationships between debris flows and the earth
surface factors
Methodology
Each earth surface factor may influence debris flows in
several ways. Statistical analyses are employed to describe
the frequency of debris flows under variable conditions for
each factor. As the drainage area of the debris flow sites
varies from less than 1 km2 to more than 100 km2, it is not
suitable to use the site as the unit of statistical analysis.
Rather, a set of grid cells was superimposed on each site
and the cell was used as the unit of analysis. First, the size
of the grid cell has to be determined. With 7,651 debris
flow sites (i.e., valleys) in question, nearly 80% are 10 km2
or less. This result is similar to that of Li et al. (2002) for
the whole of China, and that of Wei et al. (1999) in Sichuan
Province. Thus the statistical grid cell was fixed at 9 km2
and the research region was divided into 125,177 grid cells.
The distribution of debris flows with respect to each earth
surface factor was analyzed using GIS (Arc GIS 9.0 pub-
lished by ESRI). This analysis included the number
distribution of debris flow sites and the probability of debris
flow sites occurring with different conditions of each earth
surface factor. The former is a simple numerical measure
and the later is derived with the following equation.
Pi ¼Ni
Sið1Þ
where Pi is the probability of a debris flow site occurring
with an earth surface factor condition, Ni is the number of
debris flow sites distributed in the grid cells with an earth
surface factor condition, and Si is the total number of grid
cells with an earth surface factor condition.
Data acquisition
Relative relief
A digital elevation model (DEM) of the research region
was constructed using GIS and a 1:250,000 topogaphic
base map. Data describing difference of elevation or rela-
tive relief for each grid cell was captured using GIS.
Faults
A geological map of 1:200,000 scale was used as the base
map and transformed into a vector geological map by GIS.
622 Environ Geol (2008) 55:619–627
123
The faults are represented by lines and their range of
influence is a belt. The density of faults, D, is described as:
D ¼Pn
i¼1 Li
Að2Þ
where Li is the length of each fault crossing a grid cell, n is
the number of faults crossing a grid cell, and A is the area
of the cell. This can be calculated with the distance and
density tools of GIS based on a digital geological map.
Strata
On the geological map, each stratigraphic unit is described
and the primary lithology of each is listed. Using to dif-
ferent lithologies in the research region, the stratigraphic
units were divided into five categories to reflect their
hardness and resistance to weathering (Table 1).
Land use
A digital land use map (1:100,000 ) made in 2000 was
selected as the base map. There are many land use types
and they can be classified into seven categories based on
their influence on the generation of unconsolidated mate-
rial. They include forested land, grassland, farmland,
hydrologic basin, industrial and residential land, bare and
barren land as well as glacier and flood land, beach, and
desert land. In a single cell there may be different types of
land use and we define the land use index I as:
I ¼X7
i¼1
aiAi ð3Þ
where Ai is the area of each land use type in a grid cell, ai is
the weight for different types, which can be determined by
Table 2 based on their contribution to unconsolidated
material.
Relationships between debris flows and each
earth surface factor
Based on the above factors and the data from 7,561 debris
flow sites, the relationships between debris flows and each
earth surface factor were statistically analyzed with the
following results:
Relationships between debris flows and relative relief
The relative relief of each of the 125,177 grid cells in the
research region is classified into categories with a 50 m
interval. The number of debris flow sites in each category
is counted, as shown in Fig. 2.
The distribution of debris flow sites by category is
skewed and unimodal and may be described by a gener-
alized exponential distribution. The special case is a
Table 1 Categories of the stratums in the research region
Category Lithology
Sedimentary rock Magmatic rock Metamorphic rock
1 Dolomite, charcoal grey and
coarse lamellar limestone,
concreted, siliceous limestone,
cherty limestone
Thick-bedded rhyolite, thick-
bedded andesite
Quartz, quartz vein, diabase,
diabase vein
2 Quartzy sandstone, siliceous
conglomerate, bleached
limestone, quartzy siltstone
Fine and medium-grain granite,
diorite, gabbro, andesite, basalt,
tuff, rhyolite porphyry, basic
igneous rock, ultrobasic rock,
alkaline granite, diabase,
porphyrite
Marble, quartz schist, amphibolite,
serpentine
3 Sandstone, siltstone, marlite, sandy
and siliceous mudstone,
conglomerate
Volcaniclastic rock, porphyritic
coarse-grained granite, syenite
Schist, slate, granulite,
metamorphic basalt,
metamorphic liparite,
metasandstone, gneissose
4 Shale, semi-consolidated
mudstone, peat, coal-bearing
strata, semi-consolidated rock,
unconsolidated sandstone
Volcanic debris Phyllite
5 Quaternary loose deposit (loess, alluvial deposition, diluvial deopsition, slope wash and moraine), mild clay, clay, clay sand
Environ Geol (2008) 55:619–627 623
123
Gamma distribution or Weibull distribution (Li et al.
2002). In the research region, 60% of the sites are dis-
tributed in the grid cells with relative relief between 400
and 1,300 m. However, this may not signify that relative
relief of 400–1,300 m is the most favorable for debris
flows. For further understanding, we consider the proba-
bility of debris flow sites occurring in various categories of
relative relief using Eq. 1. The result is shown in Fig. 3.
As shown in Fig. 3, the debris flow site occurrence
probability increases with an increase in relative relief.
That is to say, grid cells with large relative relief are
favorable for the development of debris flows. However,
when the relative relief reaches about 2,150 m, the prob-
ability reaches its maximum value and then drops abruptly,
the reason being that the terrain becomes too steep to retain
and store unconsolidated material.
The relationship between the probability and the relative
relief can be described by a model as:
y ¼ a1 e� x�b1
c1
� �2
þ a2 e� x�b2
c2
� �2
ð4Þ
where y is the probability and x is the relative relief. Eq. 4
is a model of fit with the tool of Matlab. The fitting curve is
as Fig. 4 when a1 = 0.1239, b1 = 2,194, c1 = 103.2,
a2 = 0.09039, b2 = 1,395 and c2 = 1,031 with 95% confi-
dence bounds.
Relationships between debris flows and stratigraphy
The strata are divided into five categories as shown in
Table 1. The distribution of debris flow sites among these
categories is illustrated in Fig. 5 with the third category
showing the greatest number of sites. The probability of
debris flow sites occurring in the stratigraphic categories is
calculated by Eq. 1 and shown in Fig. 6. In Fig. 6, the
probability increases with the reduction of stratigraphic
hardness and resistance to weathering, indicating that weak
and less resistant rock is more favorable for debris flows.
However, except for the fifth category, the increase is not
obvious for the other four stratigraphic categories. The
reason of this phenomena maybe is that the influence of
stratum on debris flow development is not like that of
relative relief assuredly or the classification of stratum is
unreasonable.
The relationship between the probability and the strati-
graphic categories can be described by a model as:
y ¼ a ebx ð5Þ
where y is the probability and x is the stratigraphic cate-
gory. Eq. 5 is a model of fit with the tool of Matlab. The
fitting curve is as Fig. 7 when a = 0.02065 and b = 0.2987
with 95% confidence bounds.
Table 2 The weight of each land-use type
Land-use
type
Forest
land
Grass
land
Farm
land
Hydrologic
basin
Land for industry
and residence
Bare and barren
land, glacier
Flood land, beach,
gobi, salt lick
Weight 0.05 0.15 0.25 0 0.2 0.25 0.1
0
50
100
150
200
250
300
350
400
450
0
400
800
1200
1600
2000
2400
Relative relief (m)
yellav wolf sirbed fo reb
muN
s
Fig. 2 Distribution of debris flow sites by relative relief category
0.00
0.05
0.10
0.15
0.20
0.25
0
400
800
1200
1600
2000
2400
Relative relief (m)
Prob
abili
ty
Fig. 3 Probability of debris flow sites by relative relief category
624 Environ Geol (2008) 55:619–627
123
Relationships between debris flows and faults
Fault densities are classified into different categories with
an interval of 0.001 km/km2. The distribution is shown in
Fig. 8 and is irregular but concentrated in the cells with
moderate fault densities. The probability of debris flow sites
by fault density is estimated by Eq. 1 and the distribution
shows an obvious regularity (Fig. 9). The probability
increases with an increase in fault density and decreases
when the fault density exceeds 0.04 km/km2. The reason
why the probability decreases when the fault density
exceeds 0.04 km/km2 maybe is the broken earth surface is
eroded or deposited to a basin or a wide valley with small
slope.
The relationship between the probability and the fault
density can be described by a model as Eq. 4 where y is the
probability and x is the fault density. The fitting curve is as
Fig. 10 when a1 = 0.1635, b1 = 0.03966, c1 = 0.00264,
01 2 3 4 5
500
1000
1500
2000
2500
3000
3500
stratigraphic categories
yellav wolf sirbed fo reb
muN
s
Fig. 5 Distribution of debris flow sites by stratigraphic categories
0.001 2 3 4 5
0.02
0.04
0.06
0.08
0.10
0.12
stratigraphic category
ytilibaborp
Fig. 6 Probability of debris flow sites by stratigraphic categories
Fig. 7 The fitting curve of the probability and stratigraphic category
0.00
0.01
0.01
0.02
0.02
0.03
0.04
0.04
0.05
50
0
100
150
200
250
Fault density (km/km2)
yellav wolf sirbed fo reb
muN
s
Fig. 8 Distribution of debris flow sites by fault density
Fig. 4 The fitting curve of the probability and relative relief
Environ Geol (2008) 55:619–627 625
123
a2 = 0.1043, b2 = 0.03388 and c2 = 0.01507 with 95%
confidence bounds.
Relationships between land use and debris flows
The index of land use for each grid cell is calculated by
Eq. 3 based on Table 2. The distribution of debris flow
valley numbers with different indexes is shown in Fig. 11
as near normal with most sites concentrated between 0.09
and 0.16. The probability of debris flow sites occurring
under different land-use indices is estimated by Eq. 1 and
shown in Fig. 12. The probability generally increases with
an increase in the land use index. This reflects that large
index of land-use is favorable for the development of
debris flow. However, when the index of land-use is greater
than 0.18, the probability decreases with the increase of
land-use index. The reason of this is that most of the grid
cells are occupied by farmland, land for industry or resi-
dence and bare land with small slope when the index of
land use is greater than 0.18.
0
0.00
0.01
0.01
0.02
0.02
0.03
0.03
0.04
0.04
0.05
0.05
0.05
0.1
0.15
0.2
0.25
0.3
Fault density (km/km2)
Pro
babi
lity
Fig. 9 Probability of debris flow sites by fault density
Fig. 10 The fitting curve of the probability and fault density
0
50
100
150
200
250
300
350
400
0
0.02
0.04
0.06
0.08 0.1
0.12
0.14
0.16
0.16 0.2
0.22
land-use index
syellav wolf sirbed fo reb
mun
Fig. 11 Distribution of debris flow sites by land use index
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0
0.02
0.04
0.06
0.08 0.1
0.12
0.14
0.16
0.18 0.2
0.22
land-use index
ytilibaborp
Fig. 12 Probability of debris flow sites by land use index
Fig. 13 The fitting curve of the probability and land use index
626 Environ Geol (2008) 55:619–627
123
The relationship between the probability and the land
use index can be described by a model as Eq. 4 where y is
the probability and x is the land use index. The fitting curve
is as Fig. 13 when a1 = 0.07375, b1 = 0.1607, c1 = 0.0591,
a2 = 0.03022, b2 = 0.0702 and c2 = 0.03795 with 95%
confidence bounds.
Conclusion
Relative relief, stratigraphy, fault density and index of land
use are the main earth surface factors influencing debris
flow location. Using the data of 7,561 debris flow sites in
the research area, the distribution of debris flow valleys is
found to be near normal with respect to each factor, except
for the fault density. In general, the probability of debris
flow valleys occurring in each factor increases with the
increase of the value of the factor. However, except for
stratum, the probability has an inflection point. For relative
relief, the inflection point maybe caused by the terrain
becomes too steep to retain and store unconsolidated
material when the relative is more than 2,150 m. For fault
density, the inflection point maybe caused by the broken
earth surface is eroded or deposited to a basin or a wide
valley with small slope when the fault density exceeds
0.04 km/km2. And for land use index, the inflection point
maybe caused by most of the grid cells are occupied by
farmland, land for industry or residence and bare land with
small slope when the index of land use is greater than 0.18.
The relationships between the probability and relative
relief/fault density/land use index can be described by
y ¼ a1 e� x�b1
c1
� �2
þ a2 e� x�b2
c2
� �2
with different fitting con-
stants while the relationship between the probability and
stratum can be described by y ¼ a ebx.
Acknowledgments This research was supported by the Know-
ledge Innovation Program of Chinese Academy of Sciences (KZCX3-
SW-352).
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