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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
173
DOI: 10.4408/IJEGE.2011-03.B-021
DEBRIS FLOW ANNUAL FREQUENCY AND SEDIMENT DELIVERY
VARIATIONS COMPARED TO RAINFALL CHANGES
OVER THE LAST 40 YEARS (JIANGJIA GULLY, CHINA)
J.Q. ZHUANG
(*, **)
, P. CUI
(**)
& y.G. GE
(**)
(*)
School of Geological Engineering and Surveying of Changan University Key Laboratory
of Western China Mineral Resources and Geological Engineering, Xi’an, 710054, P. R. China
(**)
Institute of Mountain Hazards and Environment/Key Laboratory of Mountain Hazards and Earth Surface Processes,
Chinese Academy of Sciences, Chengdu, 610041, P. R. China
coefficients are 0.4454 and 0.4737, respectively. The
work can provide a scientific basis for the long-term
forecast and prevention of debris flows.
K
ey
words
: debris flow transportation sediment; precipita-
tion change; responded; Jiangjia Gully
INTRODUCTION
Global climate change, which has caused a series
changes, including precipitation, temperature increases,
and more frequent natural disaster events, has become
one of the world’s most critical issues. Global warming
makes precipitation increase in certain regions, which
is then accompanied by an increase in floods in terms
of their frequency and intensity. David Nolan of Miami
University led a study group that concluded that global
warming will lead to fewer tropical cyclones, but the in-
tensity increased significantly through the model simula-
tions. The MK trend test of the frequency and intensity
of extreme precipitation events of the Yangtze River re-
gions showed that the frequency of extreme precipitation
events increased significantly (w
anG
et alii, 2005). At
the same time, the past 50 years of land surface proc-
ess observation shows that climate change has caused
the frequency of block movement activities to increase
(e
vans
& C
laGue
, 1994; e
ybeRGen
& i
meson
, 1989).
Therefore, many researchers began to study the impact
of climate change in terms of creating natural disasters.
Since there are differences in the regional responses to
global change, some scholars have studied the relation-
ABSTRACT
Natural hazards occur more frequently due to
ongoing global climate change, which has increased
the impact of precipitation. Debris flows and their
relative activities have also changed over the past 39
years at Jiangjia Gully, a typical debris flow valley
with high-frequency debris flows located in the Yun-
nan Province of China. This paper concentrates on the
responses of sediment transportation induced by de-
bris flows at Jiangjia Gully to rainfall change, using
statistical analysis of the observation data of debris
flows and rainfall. The results showed that: (1) the an-
nual precipitation and rainy season precipitation both
decreased in fluctuation over the past over 40 years
and experienced two high rainfall stages and one low
rainfall stage; (2) the days of the daily precipitation
that exceeded 20mm, 30mm, and 50mm changed in
dissimilarity, and the days with over 20mm of daily
precipitation increased slowly and with over 30mm
and 50mm both decreased slowly; (3) the sediment
amount transported by debris flow generally increased
with just a little fluctuation in the past 40 years, which
was consistent with the change in annual precipita-
tion and the days with over 20mm of daily precipita-
tion; and (4) the sediment transported by debris flow
had a good relativity to annual precipitation and the
days with over 20mm of daily precipitation. The fre-
quency of debris flow occurrence has a good relativ-
ity to the rainy season precipitation and the days with
over 20mm of daily precipitation, and their correlation
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J.Q. ZHUANG, P. CUI & Y.G. GE
174
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
amount transported by controlling the precipitation and
temperature changes (n
eaRinG
, 2001; z
HanG
& G
aR
-
bReCHt
, 2002). The dry-hot valley is the sensitive area of
globe change and is the region of frequently occurring
debris flow (C
ui
et alii, 2005). Documents about this rela-
tionship have showed a clear connection between precip-
itation and debris flow magnitude. This paper reveals the
responses of sediment transportation induced by debris
flows to rainfall change and provides a scientific basis
for the prevention and mitigation of debris flow hazards.
STUDY REGION SETTING
The Jiangjia Gully (JJG) Ravine with a trunk chan-
nel length of 13.9km and covering a total area of 48.6
km
2
(Figure 1) is a tributary of the Xiaojiang River. It is
located in the Xiaojiang fault zone in the northeast of the
Yunnan Province of China (N23°13′-23°17′, E103°6′-
103°13′). Complex geological structures, fragile rocks,
numerous landslides, and abundant rainfall foster fre-
quent debris flows. The highest annual record was 28
events in 1965. In 1961, the Dongchuan Debris Flow
Observation and Research Station was set up, which is
a facility of the Institute of Mountain Hazards and En-
vironment at the Chinese Academy of Science. Since
1987, the station has performed regular observations
and collected systematic data about debris flows. There
is an estimated 1.23×1010 m
3
of loose sediments stored
in the valley (C
ui
et alii, 2005). There are 12-20 debris
flows in every rainy season (May-October). Annual
sediment yield in JJG is 2.0 million m
3
on average while
a maximum of 6.6 million m
3
occurred in 1991. In the
ships between the debris flow occurrence and precipita-
tion changes in different regions and received different
results. v
an
s
teiJn
(1996) studied the relationships be-
tween the debris flows in northern Europe and found that
the rainstorm debris flow occurrence increase was main-
ly caused by the intensity of the rainfall increase, but
the impact degree of climate change to debris flow fre-
quency and magnitude was still not conclusive. J
omelli
et alii (2004) studied the relationship between gully
debris flow and climate change and found that the rela-
tionship did not have a significant correlation. R
ebetez
et
alii (1997) studied the relationship between debris flow
and climate change and concluded that the occurrence
frequency of the large-scale debris flow was increasing
significantly, but the occurrence frequency of the small-
scale debris flow has been decreasing significantly since
the late 1980s. Global change in large-scale regional oc-
currences is a hot spot (k
umaR
& P
aRikH
, 2001; P
HilliPs
et alii, 2009), but the responsiveness has been different
within the large-scale region, so it is more reasonable to
explore the response of small regions (watershed) to glo-
bal change. For a single river basin, the rainfall changes
are the main factor in the occurrence of the debris flow
occurrence, and the periodic of debris flow frequency re-
flects its response to the periodic annual rainfall change.
It is an important means of disaster prevention to be able
to predict the occurrence probability of debris flow ac-
cording to precipitation change over the last decade.
The sediment amount transported by debris flow and
frequency of debris flow occurrence is a symbol of debris
flow magnitude, and climate change affects the sediment
Fig. 1 - Location of study area
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DEBRIS FLOW ANNUAL FREQUENCY AND SEDIMENT DELIVERY VARIATIONS COMPARED TO RAINFALL CHANGES
OVER THE LAST 40 YEARS (JIANGJIA GULLY, CHINA)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
175
number of debris flow events and sediment amount
transported by debris flow and precipitation data was
studied, using spearman correlation analysis to reveal
the response law of debris flow to climate change.
PRECIPITATION AND DEBRIS FLOW
ANNUAL TIME SERIES ANALYSIS
The JJG debris flow was mainly triggered by rain,
especially by rainstorms that occurred between June
and September every year. The sediment amount trans-
ported and the annual numbers of debris flow events,
which are the key parameters of the debris flow, are the
most factor response to precipitation change. Analyz-
ing the trend of the sediment amount transported, the
annual number of debris flow events, and the precipi-
tation data are the first steps toward revealing the re-
lationship between debris flow and precipitation. This
can assist with obtaining the change characteristics of
the debris flow and precipitation, especially the change
characters of rainstorms during the past forty years.
PRECIPITATION ANNUAL TIME SERIES
Figure 2 shows the change characteristics of an-
nual rainfall and rainy season rainfall over the past 40
years. The trend of annual rainfall and rainy season
rainfall showed a slight decrease in the fluctuation with
large variations. The variation coefficients were 0.157
and 0.203, respectively; the maximum annual rainfall
and rainy season rainfall are 1130.8 mm and 922.4 mm,
respectively, in 1968, and the minimum annual rainfall
and rainy season rainfall are 514.5 mm and 318.4 mm,
respectively, in 1989. The precipitation can be divided
into three stages, using the five-year moving average
curve (Fig. 2). The first stage is rainy year with higher
watershed of JJG, the rainy season
is the period from May to October
and the average annual rainfall is
800 mm from the maximum of 1130
mm to the minimum of 514 mm.
The precipitation in the rainy season
amounts to more than 85% of the
annual precipitation. Rainstorm and
thundershowers occur frequently
during the rainy season. The debris
flows in JJG are rain-induced. Of-
ten, ten to twenty minutes of high
intensity rainfall can initiate debris
flow with and average density debris
flow of 2.0t/m
3
(k
anG
et alii, 2007). The discharge of
each debris flow is from hundreds to thousands of cu-
bic meters per second (k
anG
et alii, 2007). The annual
number of debris flow events from 1965 to 2004, which
is regarded as the annual frequency for rare debris flow,
occurs beyond the rainy season. The precipitation data
from 1965 to 2004 was obtained from the Huili National
Weather Station, which is 18km away from the JJG. The
long-term record provides a good opportunity to analyze
debris flow frequency because of the lack of such long-
duration observation data in other areas and invalidation
of post-event investigation methods, such as dendro-
chronology and lichenometry, in estimating annual fre-
quency in high-frequency ravines like Jiangjia.
DATA AND ANALYSIS APPROACH
Precipitation data: The precipitation data is from
the Huili National Weather Station, which is 18km
away from the JJG. The precipitation data is daily rain-
fall data from 1965 to 2004.
The debris flow data: The debris flow data con-
tains the annual number of debris flow events, and
the sediment amount transported by debris flow is
from the Dongchuan Debris Flow Observation and
Research Station, which is a facility of the Institute
of Mountain Hazards and Environment at the Chinese
Academy of Science. The debris flow data is annual
data from 1965 to 2004.
Analysis approach: The changing tendency of the
precipitation and the annual number of debris flow
events and sediment amount transported by debris flow
was studied, utilizing five-year smoothed trend analy-
sis for climate change. This tends to be a tedious proc-
ess. Additionally, the relationship between the annual
Fig. 2 - The Change in Annual Rainfall and Rainy Season Rainfall
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J.Q. ZHUANG, P. CUI & Y.G. GE
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
THE SEDIMENT AMOUNT TRANSPORTED AND
ANNUAL NUMBER OF DEBRIS FLOw EVENTS
ANNUAL TIME SERIES
Figure 4 shows the change characteristics of
the sediment amount transported over the past 40
years. The trend of the sediment amount transported
showed a slight increase in fluctuation with large
variations with a variation coefficient of 0.693.
The maximum sediment amount transported was
659×10
4
m
3
in 1991, and the minimum sediment
amount transported was only 26×10
4
m
3
in 1993.
Figure 4 shows the five-year moving average trend.
Before 1971, the sediment amount transported
showed a decrease and fluctuation increase between
1972 and 1995, and the sediment amount transport-
ed showed a quick decrease since 1995. The change
characteristics are consistent with the precipita-
tion change. The sediment amount transported is
three years earlier than the precipitation in the first
change year and consistent with the precipitation in
the second change year.
The trend in the annual number of debris flow
rainfall from 1965 to 1974; the second stage is dry year
with lower rainfall from 1975 to 1994; the third stage
is rainy year with higher rainfall from 1995 to 2002;
and the decrease in the precipitation from 2003.
The previous studies have revealed that extreme
rainfall is important for the occurrence of debris
flows and that the debris flows are mainly caused
by extreme rainfall events. The statistics were for
the number of days of daily rainfall greater than 20
mm, 30mm, and 50mm. The trend in the number of
days of daily rainfall greater than 20 mm showed a
slight increasing trend, but the trend in the number of
days of daily rainfall greater than 30 mm and 50 mm
showed a slight decrease (Fig. 3). Figure 3 shows
the five-year moving average trend in which, before
1975, the number of days of daily rainfall greater
than 20mm, 30mm, and 50mm showed a rapid de-
crease and fluctuation with lower rates between 1975
and 1995. Here, the number of days of daily rain-
fall greater than 20mm, 30mm, and 50mm showed
a quick increase since 1995. This trend is consistent
with the precipitation calculations.
Fig. 3 - The Change in Daily Rainfall
Greater than 20mm, 30mm,
and 50mm
Fig. 4 - The Change Characteristics of
Sediment Amount Transported
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DEBRIS FLOW ANNUAL FREQUENCY AND SEDIMENT DELIVERY VARIATIONS COMPARED TO RAINFALL CHANGES
OVER THE LAST 40 YEARS (JIANGJIA GULLY, CHINA)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
177
THE RESPONSE OF DEBRIS FLOW TO
PRECIPITATION
Precipitation affects the debris flow characteris-
tics by influencing the watershed hydrology process,
material weathering, and soil evolution (n
eaRinG
,
2001; z
HanG
& G
aRbReCHt
, 2002).
THE RESPONSE OF THE SEDIMENT AMOUNT
TRANSPORTED TO PRECIPITATION
The sediment amount transported is the symbol of
debris flow magnitude and an important factor of debris
flow prevention and assessment. The response degree
and characteristics of the sediment amount transported
by debris flow to precipitation can be obtained by stud-
ying the relationship between rainfall and the sediment
amount transported. The response of sediment amount
transported to precipitation is obvious with the same in-
ter-annual oscillation trend. The correlation coefficient
between the sediment amount transported and rainy
season precipitation is 0.5341 (p<0.01) higher than
the correlation coefficient, which is 0.5032 (p<0.01),
between the sediment amount transported and annual
events showed a slight decrease in fluctuation with
large variations (Fig. 5), and the variation coeffi-
cient was 0.527; the maximum annual number of
debris flow events was 28 in 1965; and the mini-
mum annual number of debris flow events was two
in 1993 with two stages. Before 1971, the annual
number of debris flow events showed a rapid de-
crease and a fluctuation decrease after 1972 (Figure
5). The change trend was different between the an-
nual number of debris flow events and the sediment
amount transported and precipitation change. The
annual number of debris flow events was controlled
by precipitation, which then affected the sediment
amount transported.
The "abnormal" phenomenon was rationality
plotted, using the cumulative curve of the sedi-
ment amount transported and the annual number
of debris flow events (Fig. 6). The annual number
of debris flow events decreased, but the sediment
amount transported increased. The result was that
the magnitude of debris flow increased and its dam-
age intensity also increased.
Fig. 5 - The Change in the Annual
Number of Debris Flow
Events
Fig. 6 - The Cumulative Curve of the
Sediment Amount Transport-
ed and the Annual Number of
Debris Flow events
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J.Q. ZHUANG, P. CUI & Y.G. GE
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
precipitation, using S-related analysis. This is because
the debris flow mainly occurs during the rainy season.
The correlation degree is different between the sedi-
ment amount transported and the days of daily rainfall
that are greater than 20mm, 30mm, and 50mm. In this
case, the correlation coefficients were 0.4408 (p<0.05),
0.3387 (p<0.05), and 0.1860 (this did not pass the test
reliability), respectively. The response degree of sedi-
ment amount transported to days of daily rainfall great-
er than 2 mm is higher than the daily rainfall that was
greater than 30 mm and 50 mm.
The region in the southwest monsoon climate is
where rainfall events are concentrated in summer (the
rainy season). The extreme rainfall events (daily rainfall
that is greater than 20mm) is a key factor of debris flow
occurrence, which affects soil erosion and landslides
number and size and then influences the sediment amount
transported (z
HuanG
et alii, 2009). In short, the debris
flow occurrence and magnitude are controlled by rainfall;
the change of the precipitation will lead to changes in the
sediment amount transported by debris flow.
THE RESPONSE OF THE ANNUAL NUMBER OF
DEBRIS FLOw EVENTS TO PRECIPITATION
The accumulation of material and the hydrologi-
cal process within the catchment are affected by the
inter-annual variability of rainfall and then influence
the debris flow occurrence (C
ui
et alii, 2008). The
response relationship between the annual number of
debris flow events and the annual and rainy season
precipitation resulted in correlation coefficients of
0.444 (p<0.05) and 0.4454 (p<0.05), respectively.
The relationship between the annual number of de-
bris flow events and the days of daily rainfall greater
than 20mm, 30mm, and 50mm were studied because
the debris flows are mainly caused by extreme rain-
fall events. In this case, the correlation coefficients
were 0.4408 (p<0.05), 0.3387 (p<0.05), and 0.1860
(this did not pass the test reliability), respectively.
The response degree of the annual number of debris
flow events to the days of daily rainfall greater than
20 mm is higher than that of daily rainfall greater
than 30mm and 50mm. Hence, it can be concluded
that the debris flow is mainly caused by daily rain-
fall greater than 20mm, and the critical rainfall daily
rainfall is small mainly due to the geological factors
that make the loose material in the catchment move
easily (C
ui
et alii, 2005).
CONCLUSIONS
Global climate change affects the evolution and oc-
currence of natural disasters, which makes it important to
determine a means of predicting how these global changes
in order to provide relief and assistance. The following
conclusions have been determined from this research
study:
1. The annual precipitation and rainy season precipi-
tation both decreased in fluctuation over the past
40 years and experienced two high rainfall stages
and one low rainfall stage. The two high rainfall
stages appeared from 1965 to 1974 and from 1995
to2002, respectively, and the low rainfall stage oc-
curred from 1974 to 1995. The abrupt change of
rainfall occurred in 1974, 1995, and 2002, respec-
tively. The days of daily precipitation that excee-
ded 20mm, 30mm, and 50mm changed in dissi-
milarity, and the days with over 20mm of daily
precipitation increased slowly while those with
over 30mm and 50mm both decreased slowly.
2. The sediment amount transported by debris flow gene-
rally increased with little fluctuation during the past
39 years, which was consistent with the change in
annual precipitation and the days with over 20mm of
daily precipitation. The frequency of debris flow oc-
currence generally decreased during the past 39 years
and reduced quickly before 1971 and in fluctuation
after 1971. The annual number of debris flow events
decreased, but the sediment amount transported incre-
ased. The result is that the magnitude of debris flow
increased and its damage intensity also increased.
3. The sediment transported by debris flow had a good
relativity to annual precipitation and the days with
over 20mm of daily precipitation with correlation
coefficients of 0.4408 and 0.5341, respectively. The
frequency of debris flow occurrence has a good re-
lativity to rainy season precipitation and the days
with over 20mm of daily precipitation had correla-
tion coefficient of 0.4454 and 0.4737, respectively.
ACKNOWLEDGEMENTS
This research project was supported by the Nation-
al Key Fundamental Research Program of China (973)
(2008CB425802) and Project Group of Knowledge
Innovation Program of Chinese Academy Sciences
(KZCX2-YW-Q03-5).The authors wish to thank Mr.
Hong Yong, Mr. XiaoYu Li, Dr. Kaiheng Hu and Mr.
Lai-zheng Pei for their contributions.
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DEBRIS FLOW ANNUAL FREQUENCY AND SEDIMENT DELIVERY VARIATIONS COMPARED TO RAINFALL CHANGES
OVER THE LAST 40 YEARS (JIANGJIA GULLY, CHINA)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
179
REFERENCE
C
ui
P., C
Hen
x.Q., w
anG
y.y., l
i
y. (2005) - Jiangjia Ravine debris flows in south-western China. In: J
akob
& H
unGR
(eds),
Debris flow Hazards and Related Phenomena. Springer, Berlin Heidelberg.
C
ui
P., z
Hu
y.y. & C
Hen
J. (2008) - Relationships Between Antecedent Rainfall and Debris Flows in Jiangjia Ravine, China,
In: C
Hen
C.l. & R
iCkenmann
d. (eds), The Fourth International Conference on Debris Flow. Springer Press, Wellington.
e
vans
s.G. & C
laGue
J.J. (1994) - Recent Climatic Change and Catastrophic Geomorphic Processes in Mountain Environments.
Geomorphology, 10:107-128.
e
ybeRGen
f.a. & i
meson
a.C. (1989) - Geomorphic Processes and Climatic Change. Catena, 16: 307-319.
J
omelli
v., P
eCH
v.P. & C
HoCHillon
C. (2004) - Geomorphic Variations of Debris Flows and Recent Climatic Change in the
French Alps. Climatic Change, 64: 77-102.
k
anG
z.C., C
ui
P., w
ei
f.Q. & H
e
s.f. (2007) - Data collection of observation and research of debris flow in Dongchuan, Chinese
Academy of Sciences. Science press, Beijing.
k
umaR
k.s.k. & P
aRikH
J. (2001) - Indian agriculture and climate sensitivity. Global Environmental Change, 11 (2): 147-154.
n
eaRinG
a.m. (2001) - Potential changes in rainfall erosivity in the U. S. with climate change during 21st century. Journal of
Soil and Water Conservation, 56 (3): 229-232.
P
HilliPs
o.l., a
RaGao
l.e.o.C., l
ewis
s.l.
et
Alii
(2009) - Drought sensitivity of the Amazon rainforest. Science, 323:1344-1347.
R
ebetez
m., l
uGon
R. & b
aeRiswyl
P.a. (1997) - Climatic Change and Debris Flows in High Mountain Regions: The Case
Study of the Ritigraben Torrent (Swiss Alps). Climatic Change, 36: 371-389.
v
an
s
teiJn
H. (1996) - Debris Flow Magnitude-Frequency Relationships for Mountainous Regions of Central and Northwest
Europe. Geomorphology, 15: 259-273.
w
anG
y.
J
., J
ianG
t. & s
Hi
y.f. (2005) - Changing trends of climate and runoff over the upper reaches of the Yangtze River in
1961-2000. Journal of Glaciology and Geocryology, 27 (5): 709-714.
z
HanG
x.C. & G
aRbReCHt
J.d. (2002) - Precipitation retention and soil erosion under varying climate, land use, tillage, and
cropping system. J. Am. Water Resource. Assoc., 38: 1241-1253.
z
HuanG
J.Q., C
ui
P., G
e
y.G., & H
onG
y. (2009) - Relationship between rainfall characteristics and magnitude of debris flow.
Journal of Beijing Forestry University, 31 (4): 78-84.
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