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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
283
DOI: 10.4408/IJEGE.2011-03.B-033
THE CHARACTERISTICS OF GENESIS AND EVOLUTION OF
DEBRIS FLOW FANS BASED ON FIXED POINT OBSERVATION
J
insHan
zHanG
(*)
& x
inGJu
sHen
(**)
(*)
Key Laboratory of Mountain Hazard and Surface Processes, Chinese Academy of Sciences, Sichuan Chengdu 610041,
China; Institute of Mountain Hazards and Environment, Chinese Academy of Sciences& Ministry of Water Conservancy,
Sichuan Chengdu 610041, China; Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
(**)
College of Tourism and History & Culture, Southwest University for Nationalities, Sichuan Chengdu 610041, China
both longitudinal and transverse sections are largely
determined by fluidic properties of debris flows. The
greater the unit weight is, the larger the slope of debris
flows on both longitudinal and transverse sections is.
K
ey
words
: Jiangjia Gully, fixed point observation, debris
flow fan, genesis and evolution
INTRODUCTION
Debris flow fan is usually relatively flat in land-
form, hence an ideal place for human residence and
human activities. For this reason it turns to be a major
vulnerable place where debris flow-induced disasters
are often observed. In order to make use of debris flow
fans in a rational and effective way, it is significant
to make an exploration of its initiation and evolution.
Up to now two approaches have been adopted to
study accumulation of debris flows. For the first ap-
proach, static relations are usually established between
its features and eigenvalues. b
ull
(1964) studied the
exponential relationship between watershed area and
area of debris flow fan, t
anG
(1990) reviewed longi-
tudinal and transverse section features of debris flow
fan, l
iu
(1995) argued that watershed area and gradi-
ent of main gully are two major morphological param-
eters for evolution of debris flow fan, s
oRRiso
-v
alvo
et
alii (1988) analyzed correlations between patterns and
morphologies of debris fan and its physical conditions,
f
Ranzi
& b
ianCo
(2001) explored the relations be-
tween watershed area and accumulative volume. After
ABSTRACT
The Liziao Gully and Huajiaopo Gully are 2
tributaries of Jiangjia Gully in its middle reaches, a
well-known gully for its debris flows of high frequen-
cy. The two gullies are extremely similar in features
of their drainage basins, including high frequency
of debris flows and rapid formulation of debris flow
fans. During field studies, process of movement and
accumulation of debris flow fan are observed, and
sampling and analysis of debris flows with different
features are conducted. Terrain of debris flow fans and
their longitudinal and transverse profiles are measured
prior to and post rainy season each year. By analyz-
ing evolution process, planar features, longitudinal
and transverse section, accumulation rate and other
parameters of debris fans in the 2 tributary gullies dur-
ing 2006-2008, following conclusions are obtained:
(1) Influenced by geological and lithological features
in its drainage basin, Huajiaopo Gully is larger in unit
weight and grain size of its debris flow, and its accu-
mulation fan is formulated in Mound-shape. Whereas
debris flow in Liziao Gully is smaller in unit weight
and grain size, with debris flows moving on the fan
and its accumulation is in a linguiform. Further evolu-
tion of debris flow fans is finished with Mound-shaped
accumulation and channel shifting. (2) In case of ac-
cumulation in main channel, debris flow fans in tribu-
tary gullies are formed by aggradation and aggrada-
tion height is uniformly distributed at upper and lower
parts of the fan. (3) Slopes of debris flow fans on
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
By fixed point observation of real debris flows in
its evolution of debris flow fan, time-series evolution
characteristics of debris flow fan could be derived and
served as a contrast with those obtained through spatial
surveys. This approach could also be used for verifi-
cation and modification of evolution model of debris
flow fan based on simulative and mathematical models.
Thus fixed point observation should not be ignored in
studies of debris flow fan. Due to incidental nature of
debris flows, observations and studies of evolution of
real debris flow fan have been limited. A good example
is studies done by C
ui
(1996), who made fixed point
observations and samplings of debris flow fan of debris
flows in Chaqing Gully, a tributary of Jiangjia Gully
for three years from 1989-1991 and proposed to predict
evolution of debris flow fan boundary by using GM (1,
1), an approach used in systems studies.
In Jiangjia Gully, about 10-20 debris flows are
triggered each year although a maximum of 28 de-
bris flows was recorded. As a gully of highly fre-
quent debris flows, Jiangjia Gully has been an ideal
place for observation and studies of debris flows.
Liziao Gully and Huajiaopo Gully, two tributaries
of Jiangjia Gully with similarly high frequency of
debris flows, are selected as the study area. Constant
observations and surveys of debris flow accumula-
tion have been conducted and data for evolution of
debris accumulation acquired in order to analyze ev-
olution characteristics of debris flows such as planar
morphology, longitudinal and transverse section and
accumulation rate.
A SKETCH OF STUDY AREA
Huajiaopo Gully and Liziao Gully, two tributaries
of Jiangjia Gully in the circulation area of the water-
shed, are located on the left and right side of the main
gully with their debris flow fan being separated by the
main channel at 103˚09′12″۫۫۫E, 26˚145′9″۫۫۫N (Fig. 1).
With similar physical features (table 1), the two gul-
lies, located at the same place of main gully, could be
compared in evolution of their debris flow fans, which
may be both quickly mobilized and readily observable.
TOPOGRAPHICAL FEATURES
Huajiaopo Gully and Liziao Gully could be divid-
ed into three sections: clear water convergence sec-
tion, debris flow genesis and circulation section and
accumulation section. The first section is relatively
an overall analysis of factors affecting the initiation of
debris flow fan, C
Hen
et alii (2005) proposed that hin-
terland watershed area, channel gradient, width of main
river valley and energy of main river and other factors
might have substantial impact on evolution of debris
flow fan, and pointed out that a positive exponential re-
lationship could be set up between areas of debris flow
fan and watershed, and there is a negative exponential
relations between debris fan and channel gradient, as in
the case of Xiaojiang watershed. All these studies based
on relationships between debris flow accumulation and
watershed features could be used for statistical forecast
for judging characteristics of debris flow accumulation
and determining accumulation morphology and evolu-
tion phases based on debris flow fan. However such
approach based on geographical data could be only
used to obtain static features of a debris flow fan, which
could not reflect evolution process of debris flow fan.
For the second approach, an experimental or
mathematical simulation of accumulation of debris
flows is often applied. t
ian
et alii (1993) presented a
calculation formula for accumulation area of viscous
and low-viscous debris flows; l
iu
(1990) discussed
the relations between watershed area and debris flow
discharge, between debris flow discharge and maxi-
mum length of debris flow fan, between areas of wa-
tershed and debris flow fan, and each relationship
formula is established; w
anG
et alii (2000) set up a
relationship formula between accumulation area and
downstream flow slope and fluid intensity by doing
small flume experiment. Numerical simulations of de-
bris flow accumulation process have been extensively
conducted. m
izuyama
& y
azawa
(1987) discussed
the debris flow depositional process and simulated a
large scale debris flow event occurred in 1938; t
anG

(1994) established a mathematical model for predict-
ing risk area of debris flow based on a flood numeri-
cal simulation model; w
anG
&
fei
(1998) proposed a
debris flow model based on two-facial fluid theory and
established a corresponding equation and numerical
solution. w
ei
et alii (2003) established a momentum
model for debris flow risk zoning by using numeri-
cal simulation and GIS techniques and applied it in
studies of debris flows in Chacaito Gully of Venezuela
in 1999. These studies might be either conclusions of
experimental results or a simulation of accumulation
boundary of a debris flow. Due to constraint of data,
they are only similar on qualitative terms.
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THE CHARACTERISTICS OF GENESIS AND EVOLUTION OF
DEBRIS FLOW FANS BASED ON FIXED POINT OBSERVATION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
285
GEOLOGICAL FEATURES
Being located in the Deep and Great Fault Belt
of Xiaojiang River, the study area is characterized
by tensely folded tectonics, extensively distributed
joints and weathered rock with geological metamor-
phization. The dolomite of Dongying Group of Sinian
system in Huajiaopo Gully, which is discordant with
and above sandstone and slate of Kunyang Qunshang
Group, is white-colored and presents a medium and
thick-layered, hard and compact texture. Under im-
pact from fracture, joints and later sliding, stratum has
been split into boulders or gravels. Stratum in Liziao
Gully is mainly composed of black sandstone and
slate, which is soft in property and can be weathered
into laminae or gravels. The middle and lower reaches
of both gullies are well developed with landslides and
rock falls on both bank slopes, which are sources for
genesis of debris flows.
The deposits accumulated in both gullies are
different in grain grading for their different stratum
properties. Grains in Huajiaopo Gully are thoroughly
coarser, which is indicated in grain grading curve of
deposits as having a higher peak for coarse grains and
a lower peak for fine grains. The median diameter of
grains in deposits of Huajiaopo Gully is 10.3 mm,
which is larger than that in Liziao Gully (6.4 mm)
(Tab. 2).
flat above an altitude of 2000 m with a slope of 15°.
The second section is steep in relief with an altitude of
1400-2000 m and a slope of over 25° and some areas
having a slope of 45°, where rock falls and landslides
are frequently triggered and debris flows initiated and
circulated. The third section, which is below altitude
of 1400 m, has a flatter landform with a slope of 8°
for most part, where debris flows are accumulated. A
comparison of the two gullies indicates that Huajiaopo
Gully is smaller in channel gradient and average slope
than Liziao Gully since there is a larger proportion of
clear water section in Huanjiaopo Gully (Tab. 1). It is
however similar for both gullies in their middle sec-
tions where debris flows are mobilized.
Fig. 1 - Location
and
map of Hua-
jiaopo Gully and
Liziao Gully
Tab. 1 - The basic parameters of Huajiaopo Gully and
Liziao Gully
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
to warm-temperate semi-humid climate with an an-
nual rainfall of 700-850 mm (H
e
, 2003). The rainfall
collected at Dongchuan Station could be used as an
indicator of characteristics of precipitation of Hua-
jiaopo Gully and Liziao Gully. The precipitation data
collected at Dongchuan Station from 2006-2008 show
that annual average precipitation is about 500 mm, of
which more than 80% is concentrated during rainy
season (from May to September) and about 60% is
distributed during three seasons from June to August.
Under control of such precipitation pattern, most de-
bris flow events in tributaries of Jiangjia Gully are ob-
served during the rainy reason each year. During our
observation period precipitation is lower than average
annual precipitation for years.
ANALYSIS OF OBSERVATION DATA
Accumulation process of debris flow fans
Viscous debris flows with high unit weight have
been observed in Huajiaopo Gully. When fluid mass
arrives at the debris flow fan, surface roughness will
become greater as slope turns to be flatter. Mound-
shape accumulation will appear when flowing resist-
ance has been met. Succeeding debris flow may con-
tinue to move and accumulate on the mounds, or goes
other ways or directions with less resistance when it
reaches where it is higher than its surrounding, and
accumulates there. A debris flow fan will appear fol-
lowing numerous avulsion-accumulation processes.
Such Mound-shape accumulations and avulsions on
the debris flow fan of Huajiaopo Gully are clearly
manifested.Compared with those in Huajiaopo Gully,
the debris flows in Liziao Gully are smaller in unit
weight and higher in fluidity with thinner stacks for
each debris flows, hence presenting a lobate shape
PRECIPITATION
Huajiaopo Gully and Liziao Gully are both locat-
ed in the circulation section of Jiangjia Gully where
Dongchuan Debris Flow Observation Station of Chi-
nese Academy of Sciences (Dongchuan Station) is
based. Most part of the two gullies is situated at an
altitude between 1300-2200 m, an area of subtropical
Tab. 2 - Grain grading analysis of Huajiaopo Ditch and Liziao Ditch deposits
Fig. 2 - Multiple stacking of the Mound-shaped accumula-
tion fan of debris flows in Huajiaopo Gully
Fig. 3 - Frequent avulsions of debris flows on accumula-
tion fan in Huajiaopo Gully
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THE CHARACTERISTICS OF GENESIS AND EVOLUTION OF
DEBRIS FLOW FANS BASED ON FIXED POINT OBSERVATION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
287
Survey of aggradation has been conducted for 6
times in Huajiaopo Gully. Of all surveys for aggra-
dation events, three surveys reveal obvious aggra-
dation event, with aggradation height for each time
respectively being 0.42 m, 0,56 m and 0.58 m. There
are another three times of survey for aggradation,
of which two times show aggradation events, with
height of the higher aggradation being 0.09 m, one
time shows scouring with its scouring height being
0.12 m. In considering accuracy of survey, it could
be concluded that there has been no obvious aggra-
dation for latter three times.
There has been 4 times of aggradation survey in
Liziao Gully. Of all surveys for aggradation events,
three surveys reveal obvious aggradation event, with
each aggradation height respectively being 0.42 m,
0.32 m and 0.34 m. The fourth survey shows a scour-
ing for 0.1 m, which could be regarded as having no
aggradation for accumulation fan.
(Fig. 4). Although stacking and avulsion is evident
during such lobate-shape accumulation processes,
the stacking is much thinner as compared with that in
Huajiaopo Gully.
TOPOGRAPHICAL SURVEY
In order to study evolution characteristics of de-
bris flow fan in Huajiaopo Gully, a large-scale topo-
graphical survey of the debris flow fan was done in
2006 and, for purpose of comparison a survey of de-
bris flow fan in Liziao Gully was also conducted in
2007. For each gully, surveys were arranged in June
and September, the starting and ending months of
rainy seasons. When a significant change of accumu-
lation fans was observed, there would be an additional
survey. On dates 2006-08-15 and 2006-09-02 during
observation period, a survey of accumulation fan in
Huajiaopo Gully was done with each survey respec-
tively marked No 0601 and No 0602. Since 2007 sur-
veys were conducted for both gullies and there were
altogether 5 surveys with dates being 2007-06-21,
2007-09-11, 2008-06-26, 2008-07-16 and 2008-08-
25, each being marked No 0701, 0702, 0801, 0802
and 0803. Topographical surveying was done using
total station, and a map at scale of 1:500 was pro-
duced. Since the average scoring and aggradation
layer of deposit for each debris flow is less than 10
cm in thickness and the fan is in Mound-shape, the
change may not be revealed on the map at this scale.
In order to reflect the morphological characteristics of
debris flow fan more accurately, fixed cross section
surveys were added after topographical surveys were
conducted (Fig. 5).
SCOURING AND AGGRADATION FEATURES
OF ACCUMULATION FANS
Previous observations of transverse section of
main Gully indicate that there has been a constant
aggradation since 2002 at the main gully where ac-
cumulation fans of both tributaries are joined (C
ui
et alii, 2004, 2006). During 2006-2008, main gully
was still in aggradation process with the smallest
height of aggradation being 2.15 m in 2006, the
highest being 3.34 m in 2008 and the total height
being 8 m (Fig. 6).
When the main gully is being constantly silted,
there has been aggradation processes for debris flow
accumulation fan in both tributary gullies (Fig. 7).
Fig. 4 - Lobate-shape accumulation of debris flow fan in
Liziao Gully
Fig. 5 - Locations for cross section surveys
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
Fig .6 - Scouring and aggra-
dation of the main
channel in 2006-
2008
Fig. 7 - The average scouring
magnitude of each survey
Fig. 8 - Longitudinal profile of debris flow fans in Huajiaopo Gully and Liziao Gully
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THE CHARACTERISTICS OF GENESIS AND EVOLUTION OF
DEBRIS FLOW FANS BASED ON FIXED POINT OBSERVATION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
289
Fig. 9 - Longitudinal profile
changes of debris
fans in Huajiaopo
Gully and Liziao
Gully
Fig. 10 - Transverse sec-
tion of debris flow
fan in Huajiaopo
Gully
Fig. 11 - Transverse sec-
tion of debris
flow fan in Liziao
Gully
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
CROSS SECTION FEATURES OF ACCUMULA-
TION FANS
Longitudinal section
All cross section lines of accumulation fans sur-
veyed at different time for both Huajiaopo Gully
and Liziao Gully have generally been in parallel and
there has no obvious change for longitudinal gradi-
ent, but a gradual rise in cross section lines has been
observed (Fig 8), which indicates that there has been
a sustained and balanced accumulation on the cross
section, The longitudinal gradient of accumulation fan
in Huajiaopo Gully is larger than that in Liziao Gully
(Fig. 9).
Transverse section
The transverse section of accumulation fans of
debris flows in Huajiaopo Gully and Liziao Gully
takes the shape of convex which is similar to ac-
cumulation fans of other debris flows (t
anG
, 1990)
(Figg. 10 and 11). Calculation of slopes at both sides
of the two gullies shows that slopes in Huajiaopo
Gully are larger than those in Liziao Gully. For both
gullies the slope at the upstream side of main gully is
larger than that at the downstream side of main gully.
Put in other words the slope at right side of Hua-
jiaopo Gully located at the right bank of main gully
is larger while the slope at left side of Liziao Gully
located at the left bank of main gully is smaller.
Since 2007 both gullies have shown an increasingly
larger slope for the side in the downstream direction
of main gully and a smaller slope for the side in the
upstream direction of main gully, which indicates an
increasing variation for slopes of both sides for the
two gullies (Fig. 12).
RESULTS AND CONCLUSIONS
Debris flow fan in Huajiaopo Gully is accumu-
lated in a Mound-shape while that in Liziao Gully
is accumulated in a lobate shape. Geolithological
properties has determined that debris flows in Hua-
jiaopo Gully are larger in unit weight and grain size,
and debris flow fans are accumulated in a holistic and
Mound-shape, whereas debris flows in Liziao Gully
are smaller in unit weight and grain size and debris
flow fan is formulated in a linguiform. Further evolu-
tion of debris flow fan is completed by Mound-shaped
accumulation-channel avulsion. This result may be
different from some studies done in the past. Experi-
mental studies by t
akaHaski
(1986) show that accu-
mulation fan of debris flows is in circular or elliptical
shape in its planar morphology when its unit weight
is large and that the accumulation fan will be in strips
when unit weight is small. Numerical simulation stu-
dies by y
anG
(2003) also show a similar result. The
reasons for variation in morphology of debris flow
fans may be that evolution of debris flow fans is ini-
tiated at different stages. The evolution of debris flow
fans as revealed by observation data in this study may
be initiated at a later stage when a debris flow fan has
already taken its shape while past experiments and si-
mulations were done at initial stage of evolution of
debris flow fans.
When accumulation has already occurred in the
main gully, debris flow fans in tributary gullies would
be formulated by aggradation, where debris may be
uniformly distributed at top and bottom of the fan.
Observation data reveal that when the main channel
was scoured deep, water flow in tributary gullies may
incise debris flow fans and voluminous debris flow
may move along deep ravines until it reached the main
Fig. 12 - Change of transverse section
gradient of debris flow fan in
Huajiaopo Gully and Liziao
Gully
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THE CHARACTERISTICS OF GENESIS AND EVOLUTION OF
DEBRIS FLOW FANS BASED ON FIXED POINT OBSERVATION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
291
channel (k
anG
, 2004; C
ui
, 1999). The ravines on the
debris flow fans may not be silted unless a big debris
flow in tributary gullies is mobilized. In case that the
main channel is silted, the front part of debris flow fan
in tributary gully may be gradually covered by debris
carried from main gully. As a result the debris flow
fan may rise higher. With decline of gradient of debris
flow fan, tributary gully may be soon filled with debris
and succeeding debris flows may frequently change
its channels. It is observed that there have been over
ten avulsions for one debris flow event, which may
result in an even aggradation pattern on debris flow
fans. As is shown on longitudinal map, accumulation
fan has a uniform aggradation pattern on its upper and
lower part.
Slopes of debris flow fans on both longitudinal
and transverse sections are largely determined by flu-
idic properties of debris flows. The greater the unit
weight is, the larger the slope of debris flow fans. In
Jiangjia Gully watershed, there has been a higher ratio
of coarser grains for debris flows with a larger unit
weight. Due to a larger resistance, such debris flows
may stop at a place with a steeper slope and so a stee-
per accumulation fan may be formed.
ACKNOWLEDGEMENTS
This research was financially supported by the
National Natural Science Foundation of China (No.
40871024). Professor Cui Peng provided insightful
comments on earlier versions of this manuscript.
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ui
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Statistics