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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
293
DOI: 10.4408/IJEGE.2011-03.B-034
EFFECTS OF DEBRIS FLOWS ON CHANNEL MORPHOLOGY
AT JIANGJIA RAVINE, YUNNAN PROVINCE, CHINA
x
inG
-
Hua
ZHU
(*,**,***)
, P
enG
CUI
(*,***)
& J
in
-
sHan
ZHANG
(*,**,***)
(*)
Chinese Academy of Sciences, Inst. of Mountain Hazards and Environment - Chengdu 610041, China
(**)
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
(***)
Chinese Academy of Sciences, Key Lab. of Mountain Hazards and Earth Surface Process - Chengdu 610041, China
INTRODUCTION
Debris flows, widespread in mountainous areas,
are characterized by high capacity of sediment trans-
port, catastrophic occurrence, high-concentrated sedi-
ment, wide range of grain size, high velocity and short
period of movement (w
u
et alii, 1990; w
u
et alii,
1993; IMDE, 2000). When thousands million tons of
debris are transported from upper to lower channel,
strong erosion or deposition always occurs and dra-
matically changes the channel morphology and threats
people’s lives and property along the channel.
At present, some scholars have studied the in-
fluence of debris flows on channel morphology and
gained some achievements. Several scholars consid-
ered that the erosion in upper channel is related to soil
erosion in upper stream and they provided several
computational methods to quantify channel erosion
or deposition (C
aine
, 1976; d
ietRiCH
& d
unne
, 1978;
b
enda
, 1990). Others found that the erosion or depo-
sition in debris flow channel had relation to the scale
of debris flow (C
endeRelli
& k
ite
, 1998; R
iCken
-
mann
et alii, 2003). With the development of 3S tech-
nology and mathematical theory, new analytical and
numerical simulation methods are attracting increas-
ing attention (f
oRmann
et alii, 2007; d
anie
& k
elly
,
2008). However, many present studies are based on
some specific single debris flow events or laboratory
tests data, and the available research data are not con-
tinuous and systematic. Therefore, the conclusions
gained from these studies may not match with the
ABSTRACT
Debris flows, widespread in mountainous areas
of China, transport large amounts of sediment in
small intervals of time. To investigate the effects
of debris flows on channel morphology, we chose
10 cross-sections to monitor periodic series of mul-
tiple debris flows triggered by summer rainstorms
at Jiangjia ravine, Yunnan province, southwestern
China. The 10 sections were distributed in the up-
per, middle, and lower sections of the trunk drain-
age to record erosion and deposition. Based on the
observation data from 1999 to 2006, the effects of
debris flows on channel morphology in the upper,
middle and lower channel were analyzed respec-
tively. It can be found that the channel morphol-
ogy evolvement in upper, middle and lower channel
were not uniform, as the erosion or deposition of a
specific section depends on the local conditions of
the section and scale of the debris flow. The analy-
sis shows sediment discharge, maximum velocity
of debris flow and channel gradient are three key
factors in erosion/deposition volume calculation in
channel. Based on the erosion/deposition characters
of Jiangjia ravine, a simple computational model is
presented to calculate the erosion or deposition of
the debris flow channel.
K
ey
words
: debris flow, sediment transportation, erosion and
deposition, channel morphology
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X. ZHU, P. CUI & J. ZHANG
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
the erosion section, the debris flow transport section
and the deposition section (C
ui
et alii, 2005). The main
rock types in Jiangjia watershed are grey black slate
and sandstone, and they are susceptible to be weath-
ered into clod and sheet. Weathered rock and Neozoic
deposits formed the thick gravel soil layer in the water-
shed, often 30-80 m thick mantling on bedrock. What’s
more, as Jiangjia Ravine is at the west side of Wumeng
Mountain and is influenced by the warm and humid
airflow from Indian Ocean, the climate in this area can
be divided into dry season and rainy season. The rainy
season is from May to October and the precipitation
is up to 500~1000 mm, which accounts for more than
85% of the annual precipitation in this area.
Jiangjia ravine becomes the ravine with the most
frequent occurrence of debris flow in Xiaojiang al-
ley for the abundant of loose sediment, steep stream
slopes and plentiful precipitation.
CHARACTERS OF DEBRIS FLOW SEDI-
MENT TRANSPORTATION
Jiangjia Ravine is one of the most famous debris
flow ravine in China. Dongchuan Debris Flow Ob-
servation and Research Station (DFORS) was set up
in the mid-60s of last century, and some researchers
began to observe and record debris flow events since
then. Based on the continuous and systematic obser-
vation data of nearly 50 years at DFORS, Figure 1
shows the annual debris flow discharge in Jiangjia Ra-
vine. By analyzing the observation data of debris flow
discharge in the 50 years, we can conclude as follows:
1) Debris flow events occur at Jiangjia ravine eve-
ry year, which transport a large number of sedi-
ment into Xiaojiang River. According to statisti-
cs, there were 465 debris flow events in Jiangjia
fact. To investigate the effects of debris flows on chan-
nel morphology, we set 33 cross-sections to monitor
periodic series of multiple debris flows triggered by
summer rainstorms in Jiangjia ravine. The drainage is
the site of the renowned Dongchuan Debris Flow Ob-
servation and Research Station (DFORS). Analyzing
the observation data, some scholars get the following
conclusions: the debris flow channel morphology will
dramatically evolve along with debris flow events;
the channel morphology evolvement in upper, middle
and lower channel are not entirely consistent even in
the same debris flow event (fenG et alii, 2005; Cui
et alii, 2006; C
Hen
& H
e
, 2006).
In this work, 10 cross sections, distributed in up-
per, middle and lower channel, were chosen to moni-
tor debris flow in Jiangjia Ravine. Based on the data
of field observations from 1999 to 2006 in Jiangjia
Ravine, the channel morphology evolvement in upper,
middle and lower channel was analyzed combined with
debris flow events in the 8 years respectively. As the
erosion or deposition of a specific section depends on
the local conditions of the section and debris flow char-
acteristics, we found the sediment discharge of debris
flow, the maximum velocity of debris flow and channel
gradient were the three key factors. Combined with the
erosion or deposition characteristics of Jiangjia Ravine,
a simple computational model is presented to calculate
the eroded/deposited volume in debris flow channel.
THE STUDY AREA
Jiangjia Ravine is at Dongchuan section of Yunnan
Province and at the right bank of the Xiaojiang River,
which is a tributary of Jinsha River. The area of water-
shed is 48.6 square kilometers and the length of main-
stream is 13.9 kilometers with three main lateral chan-
nels - Menqian Gully, Duozhao Gully and Dawazi Gully.
As the whole valley is located in the east side of
Xiaojiang fault zone, the stratum is strongly draped
and broken. Therefore, a large number of landslides
and collapses distribute in the area, which can provide
about 1.23 × 10
10
m
3
of loose sediments. In addition,
the terrain in Jiangjia Ravine is very steep. The lon-
gitudinal gradient reaches to more than 0.35 in upper
channel while about 0.06~0.09 in lower channel. The
gradients at both sides of the valley are also very steep
and the average gradient is up to 43°. Based on the
differences of morphologic characteristics, the main
debris flow channel can be divided into three sections:
Fig. 1 - The annual sediment discharge in Jiangjia Ravine
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EFFECTS OF DEBRIS FLOWS ON CHANNEL MORPHOLOGY AT JIANGJIA RAVINE, YUNNAN PROVINCE, CHINA
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
295
In this work, the observation data of 10 cross sec-
tions are used to analyze the characters of channel
morphology evolvement. Figure 2 shows the distribu-
tion of related observation cross sections in Jiangjia
Ravine. As drainage canal was built in 2003 and
channel was diverted subsequently, there were only 4
years observation data of D17 and D19 from 1999 to
2002. The data about D11 was from 2000 to 2006 and
the field observation data of the rest 6 cross sections
are from 1999 to 2006. M3 and M61 are arranged to
monitor the erosion and deposition changes in curved
channel in detail, which is not analyzed in this work.
As the interval between two debris flow events is
too short sometimes, a survey could not be done right
after each debris flow event. Therefore, only 40 sur-
veys were carried out even there were 67 debris flows
events in Jiangjia Ravine in the 8 years.
THE ANALYZING METHOD OF OBSERVATION DATA
The erosion or deposition always changes the eleva-
tion of lowest point and area of cross section. By point-
ing the observation data in Cartesian coordinate (The X
axis represents width in cross direction while Y axis rep-
resents height), the erosion or deposition of a cross sec-
tion can be reflected in this coordinate. The changes of
the lowest point elevation and cross section area can re-
flect channel morphology evolvement to a certain extent.
Ravine from 1965 to 2006. The most frequent
year is 1965 with 28 times while the less fre-
quent year is 1993 with 2 times.
2) There are differences and periodicity about the
interannual variation of sediment discharge. The
amount of sediment transportation in Jiangjia Ra-
vine is very huge and the average sediment tran-
sportation through DFORS is 6.91×10
6
t each year.
However, the annual discharge shows differences
and periodicity with a period of 5 to 9 years.
3) Debris flow mainly occurs in rainy season, espe-
cially from June to September.
Debris flow can transport large quantities of sediments
from upper channel to lower channel, which dramatically
changes the channel morphology. Therefore, analyzing
the sediment transport in Jiangjia Ravine will contribute
to the study of channel morphology evolvement.
THE STUDY OF CHANNEL MORPHOLO-
GY EVOLVEMENT IN JIANGJIA RAVINE
METHODS
To study the effects of debris flows on channel mor-
phology in detail, we have monitored the debris flow ac-
tivities at 33 cross-sections in Jiangjia Ravine since 1999.
In each cross section, a fixed point on the left channel is
selected for the measuring instrument and another point on
the right of the channel is marked for surveying reference.
Fig. 2 - The distribution of observation cross-sections in Jiangjia Ravine
(D11 was used from 2000 to 2006, D17 and D19 were used from 1999 to 2002, and others were used from 1999 to 2006)
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
but erosion in several local areas. In figure 1, it can be
seen that the debris flow was relatively active in 2001
and sediment discharge was up to 2.88×10
6
m
3
, which
was the largest scale in these 8 years. The sediment dis-
charge was 1.49×10
6
m
3
in 2002 and declined quickly
since 2002. Therefore, it can be concluded as follows: 1)
the channel morphology in upper channel changes along
with the changes of debris flow sediment discharge; 2)
there is a limit value of debris flow scale in channel mor-
phology evolvement. The channel may be scoured when
the sediment discharge reach or exceed this limit value,
otherwise the channel may be lift by sediment deposi-
tion; 3) the limit value of sediment discharge in Jiangjia
Ravine is between 1.49×10
6
m
3
and 2.88×10
6
m
3
.
CHARACTERS OF EROSION AND DEPOSIION
IN MIDDLE CHANNEL
D9, D11, D13 and D15 are chosen to reflect
the middle channel morphology evolvement. In this
work, the cross section D11 is taken as an example to
analyze the characters of middle channel morphology
evolvement in the 7 years.
Comparing the elevation curve of 2006 with the
curve of 2000 in figure 4, it can be found that the chan-
nel elevation was lift in the 7 years in general with
the maximum lifting height of 15.3m. However, the
erosion occurred in several local areas for some years.
In figure 1, it can be seen that the scale of sediment
discharge was up to 2.88×10
6
m
3
, but the elevation of
middle channel was lift slightly in 2001 which was
different from upper channel. Compared with the mor-
phology evolvement of upper channel, the characters
of erosion and deposition in middle channel are shown
as follows: 1) the extent of erosion and deposition was
larger in middle channel than that in upper channel
In addition, we define channel erosion as nega-
tive changes and deposition as positive changes. In
analyzing process, A
i
is defined as the changes of
cross-section i, D
i
as the horizontal distance between
adjacent sections and V
i
as the quantity of erosion-
sedimentation between adjacent section. V
i
can be
calculated as follow equations:
where A
i+1
is the area changes of the cross sections
i+1. If the sum of A
i
and A
i+1
is above zero, we use
Eq.1 to calculate the eroded volume; otherwise Eq.2 is
used. Accumulate the eroded or deposited volume of
each corresponding channel, the eroded or deposited
volume of Jiangjia Ravine can be calculated.
CHARACTERS OF EROSION AND DE-
POSITION IN UPPER, MIDDLE AND LO-
WER CHANNEL
As the complexity of debris flow deposition and
erosion, the channel morphology evolvement is nota-
bly different in upper, middle and lower channel even
in the same debris flow event (C
Hen
et alii, 2006).
Therefore, the channel morphology evolvements in
upper, middle and lower channel are analyzed along
with debris flow events in the 8 years respectively.
In this work, 10 cross-sections are chosen to
record the channel morphology evolvement. In these
10 cross-sections, D1, D3, D5, D7 are chosen as to
reflect the channel morphology evolvement in upper
reaches. Likewise, D9, D11, D13 and D15 are chosen
to reflect the evolvement in middle reaches while D17
and D19 are chosen in lower reaches.
CHARACTERS OF EROSION AND DEPOSITION
IN UPPER CHANNEL
D1, D3, D5 and D7 are chosen to reflect the upper
channel morphology evolvement. Figure 3 shows the
elevation changes of cross-section D5 from the early
1999 to the end of 2006. The elevation changes can
reflect morphology evolvement of upper channel in
these 8 years to a certain extent.
As we can see from figure 3, the channel elevation
had been lift 18m since 1999. Moreover, the cross-
section was obviously eroded in 2001 and quickly el-
evated by almost 10m in 2002. In the rest other years,
the elevation of this section was mainly slow in general,
(1)
(2)
Fig. 3 - The erosion and deposition changes of D5 from
1999 to 2006
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EFFECTS OF DEBRIS FLOWS ON CHANNEL MORPHOLOGY AT JIANGJIA RAVINE, YUNNAN PROVINCE, CHINA
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
297
even in the same debris flow event. The cumulate de-
posited volume of D11 was about 31.2% larger than
that of D5 and the cumulate amount of erosion of D11
was 27.4% larger than that of D5. 2) The middle chan-
nel was lift slightly in these 7 years in general, which
was different from upper channel.
CHARACTERS OF EROSION AND DEPOSITION
IN LOwER CHANNEL
In this work, D17 and D19 are chosen to reflect
the lower channel morphology evolvement. As drain-
age canal was built in 2003 leading channel diversion
subsequently. Therefore, the observation data of D17
and D19 was only 4 years from 1999 to 2002. Figure
5 shows the elevation changes of cross-section D19
from the early 1999 to the end of 2002.
As we can see from figure 5, the elevation cross-sec-
tion D19 was lift in 1999 with an average lifting height
of 3.6m. Meanwhile, the lower channel was diverted in
1999, which lead the gradient of lower channel up to
13%. As the debris flow activity was not active in 2000,
the channel was eroded slightly in 2000. However, the
debris flow was relatively active in 2001 and the chan-
nel was eroded about 16.7m. H
e
Y.P. (2003) once intro-
duced channel gradient balance to analyze the erosion and
deposition changes of D19. The channel gradient balance
means that the channel slope has a capacity of self-adjust-
ment by the erosion and deposition of the partial or whole
channel. Based on this analysis, we hold these following
points: 1) the equilibrium condition will be broken as the
change of gradient in lower channel and the channel will
re-adjust the gradient by eroding or depositing to achieve
new equilibrium condition; 2) the process of the self-
adjustment will feed back from lower channel to upper
channel until new equilibrium condition is realized.
THE COMPUTATION MODULE OF CHANNEL
EROSION AND DEPOSITION
As the characters of erosion or deposition in up-
per, middle and lower channel are not uniform even in
the same debris flow event, whether a specific channel
will erode or deposit depends on debris flow characters
and the channel characters. Therefore, a range of fac-
tors related to erosion and deposition should be consid-
ered, such as debris flow characters (density, viscosity,
composition, flow depth, and speed) and channel char-
acters (composition of banks, mobility of components,
and shape). However, we just chose three key factors
including the sediment discharge of debris flow, the
maximum velocity of debris flow and channel gradient.
From field observations, deposition quantity in
the channel increased with the sediment discharge in
a certain range. If the debris flow discharge is high
enough, the channel will be eroded. In other words,
there is a limit value of debris flow scale between
erosion and deposition in channel morphology
evolvement. The channel may be soured when the
sediment discharge reach or exceed this limit value,
otherwise the sediment may deposit in channel. The
maximum velocity of debris flow, which is the typi-
cal dynamic parameters, has a feeble positive cor-
relation with quantity of erosion or deposition sedi-
ment in channel. There is a paving process in the
channel during the early stage of debris flow. A re-
sidual layer remained afterwards which contributed
to protection of the channel bed (w
anG
et alii, 2001).
If the speed of debris flow increased to a level suf-
ficient to break the protection of the residual layer,
erosion happened. The effects of a debris flow on the
upper channel and the lower channel may not be the
same, therefore eroded/deposited volume may differ.
Fig. 4 - The erosion and deposition
changes of D11 from 1999 to 2006
Fig. 5 - The erosion and deposition changes
of D19 from 1999 to 2002
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X. ZHU, P. CUI & J. ZHANG
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
1) Based on the observation data and model experi-
ments, quantifying the relationship among the se-
diment discharge, the maximum velocity of debris
flow and the gradient of debris flow channel. Sub-
sequently, the explicit mathematical expression of
equation 3 can be educed to calculate the quantity
of erosion or deposition.
2) As we can see from the characters of erosion and
deposition in upper, middle and lower channel,
the upper channel is eroded while the lower chan-
nel is lift in general. In other words, there must be
a channel segment or cross-section which keeps
deposition-erosion equilibrium. The local condi-
tions of channel and the related debris flows cha-
racters are the boundary conditions and solutions
of equation 3, which can be used to verify the ex-
plicit mathematical expression of equation 3.
3) There are many other factors affecting channel
morphology evolvement, such as debris flow
characters (density, viscosity, composition, flow
depth) and channel characters (composition of
banks, mobility of components, and shape). These
factors should be appropriately considered in the
study of channel morphology evolvement.
4) The channel morphology evolvement registers as
self-adjustment of channel gradient under the in-
fluence of debris flow. In the view of dynamics,
the self-adjustment of channel gradient is sedi-
ment transportation from upper channel to lower
channel based on principle of minimum potential
energy. Therefore, the kinematics and kinetic the-
ory should be used to analyze it.
ACKNOWLEDGEMENTS
The work was supported by the State Key Funda-
mental Research Program (No. 2008CB425802) and
State key project (No. 2006BAC10B04), Ministry of
Science and Technology, PRC.
Some cross sections were eroded while the others
were deposited. At the same time, erosion and depo-
sition will adjust the overall channel gradient.
By analyzing the relationship among quantities of
erosion or deposition sediment, scale of debris flow,
debris flow velocity and channel gradient, the compu-
tation module can be expressed as equation 3:
where ΔV
j
is the quantity of erosion-sedimentation, D is
the eigenfunction of debris flow, C is the eigenfunction
of channel conditions, V
T
is the quantity of sediment
discharge, V
T
* is the limit value of debris flow scale, v
is the velocity of debris flow, J is the channel gradient.
CONCLUSIONS AND DISCUSSIONS
In this work, we have analyzed the characters of
channel morphology evolvement in upper, middle and
lower channel based on observation data from 1999 to
2006. The conclusions can be drawn as follows: 1) the
eroded/deposited volume is closely related to the scale
of debris flows. 2) The characters of erosion or deposi-
tion in upper, middle and lower channel are not uniform.
The extent of erosion and deposition was larger in mid-
dle channel than that in upper channel. 3) The gradient
may change with the variation of the base level of erosion,
which will break the equilibrium condition. Erosion and
deposition will adjust the channel gradient to achieve new
equilibrium condition. What’s more, the process of the ad-
justment always feed back from lower to upper channel.
By analyzing the relationship between quantity of
erosion or deposition sediment, scale of debris flow, de-
bris flow velocity and channel gradient, the computation
module of channel erosion can be expressed as follow:
ΔV
j
= F(D(V
T
,v),C(J))
In future work, the following aspects need to be con-
sidered in the study of channel morphology evolvement:
(3)
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e
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