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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
805
DOI: 10.4408/IJEGE.2011-03.B-088
DISASTER CHARACTERISTICS AND OPTIMAL DESIGN OF DRAINAGE
CANAL OF DEBRIS FLOW FOLLOWING WENCHUAN EARTHQUAKE IN
WEIGOU GULLY IN BEICHUAN COUNTY, SICHUAN PROVINCE, CHINA
y
onG
YOU
(*)
, J
infenG
LIU
(*)
& x
inGzHanG
CHEN
(**)
(*)
Key Laboratory of Mountain Surface Process and Hazards, Chinese Academy of Sciences, Chengdu 610041, China; 2. Institute of
Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China. Email: yyong@imde.ac.cn
(**)
School of Environment and Resources, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China
INTRODUCTION
The Wenchuan earthquake occurred along the
northeast Yingxiu–Beichuan reverse fault. The area
most seriously affected by the earthquake was in the
transition zone between the Tibetan Plateau and the Si-
chuan Basin. Due to the unique topography in the area,
the earthquake directly caused more than 15,000 geo-
hazards, mostly in the form of rockfalls and landslides
(y
in
, 2008; H
uanG
et alii, 2008; w
anG
et alii, 2008). The
quantity, magnitude, and extent of the damage caused
by the geo-hazards were most severe in this area (C
ui
et alii, 2008a; d
onG
et alii, 2008; H
uanG
et alii, 2009).
These geohazards provided considerable loose materials
for debris flows, which are expected to be more frequent
and an active period of about 20 ~ 30 years (C
ui
et alii,
2008b; C
ui
et alii, 2009; x
ie
et alii, 2009). Debris flows
have been one of the primary factors impeding post-
disaster restoration and reconstruction.
Beichuan County, one of the areas most seriously
affected by the earthquake, suffered a sudden rain-
storm that triggered a large number of debris flows
between September 23 and 24, 2008. At that time, a
large debris flow formed in the Weigou Gully, in Guixi
Township, Beichuan County, flooding villages, bury-
ing roads, and silting rivers.
In this paper, we examine the characteristics
of the Weigou Gully debris flow. We use data from
field survey and the interpretation of remote sensing
images to identify the optimal characteristics of the
“trapezoid-v” Shaped Drainage Canal.
ABSTRACT
The Weigou Gully, a branch of Pingtong River,
is located in Guixi Township, Beichuan County. The
area was seriously affected by the Wenchuan earth-
quake and its aftermath. Debris flows occurred on
September 24, 2008, July 14, 2009, and September
10, 2009, threatening the safety of the inhabitants
and their property. Debris flows in Weigou Gully
were mainly supplied by landslides triggered by the
earthquake. The debris flows had characteristics of
high frequency, high bulk density and coarse parti-
cle size. Following the earthquake, the frequency of
debris flows increased. Based on the analysis of the
disaster characteristics, the optimal cross-section de-
sign method for the “trapezoid-v” shaped debris flow
drainage canal of the Weigou Gully is discussed. The
depths (h
1
and h
2
) of the cross-section of the “trape-
zoid-v” shaped drainage canal and the optimal cross-
section parameters using these two measurements are
defined. In addition, several formulas for calculating
the cross-section measurements are deduced under
optimal hydraulic conditions when discharge (Q), de-
bris grain composition (D
50
and D
10
) and longitudinal
ratio (I) are known. Finally, the optimal dimensions
for the cross-section of the Weigou Gully debris flow
canal are determined.
K
ey
words
: wenchuan earthquake, debris flow hazards, drai-
nage canal, optimal design, weigou Gully, Beichuan County
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y. yoNG, l. JiNfeNG & c. XiNGZhANG
806
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
GEOLOGICAL SETTING
The Weigou Gully lies on the eastern edge of the Ti-
betan plateau and is part of the Longmenshan Mountain
geosynclines. This region is characterized by elevations
of up to 7,500 m above sea level and by topographic
reliefs of more than 5,000 m over distances of less than
50 km (d
ensmoRe
et alii, 2007). The Beichuan reverse
fault, located on the boundary between the front and
back Longmenshan Mountain fold belts, passes through
the lower reaches of this gully. It’s an active thrust fault
that trends northwest with a dip of 60°-70°, pushing
the Cambrian stratum above the Silurian, Devonian,
and Carboniferous strata (Fig. 2). The cutting depth
is great and the vertical fault is greater than 1,000 m.
Since they are affected by the impact of the Beichuan
fault, the main geological structures and rocks in this
area trend toward the northeast. The strata in the gully
are mainly Cambrian and Silurian, and a large area of
Cambrian rock has been exposed to the northwest of
the Beichuan fault zone. It shows mildly dynamic meta-
morphism, likely due to the intense dynamic regional
metamorphism associated with the activities of the Bei-
chuan fault. A few Silurian sandstones have been ex-
posed downstream. Strongly weathered rocks provide
abundant material for geological disasters.
METEOROLOGICAL SETTING
The study area has a subtropical moist, monsoon
climate with four normal seasons, a mild climate, and
an average temperature of 15.6°C (Tab. 2). The an-
nual average precipitation is 1,399.1 mm, with an an-
nual maximum of 2,340.0 mm, a daily maximum of
101.0 mm, and an hourly maximum of 32.0 mm. Most
rainfall occurs between June and September, account-
ing for 70-90 % of the annual rainfall (C
ommittee
of
b
eiCHuan
C
ounty
annals
, 1996).
ENVIRONMENTAL SURVEYS
DRAINAGE BASIN SURVEY
Weigou Gully (104°38′36″E, 32°00′19″N), in
Guixi Township, Beichuan County, is a branch of Ping-
tong River, an arterial branch of the Fujiang River. The
gully is characterized by a basin area of 3.11 km
2
, a
gully length of 3.68 km, and a longitudinal slope of 187
‰ (Fig. 1). The altitude of the catchment varies from
630 m. to 1,492 m, with an altitude difference of 862 m.
PHYSIOGNOMY
The Weigou Gully is located to the east of the
Tibetan Plateau and forms part of the transition zone
between the Tibetan Plateau and the Sichuan Basin.
Its physiognomy is complex.
The gully slopes are characterized by a higher el-
evation in the north and a lower elevation in the south.
Gentle land (<25°), mainly located in the middle and
lower reaches, occupies 18.33 % of the total area of
the gully. Steep (25°-35°) and very steep (≥35°) land
occupy 20.90 % and 60.77 % of the total area, respec-
tively. The upper reaches are dominated by steep and
very steep land (Tab. 1).
Neo-tectonic movement has created a strong inci-
sion in the gully, resulting in a v-shaped cross-section
and a steep longitudinal gully slope (187‰). The
steep mountain and gully slopes provided favorable
conditions for the formation of debris flows.
Fig. 1 - Sketch of the weigou Gully catchment
Tab.1. Characteristics of the weigou Gully slope
Tab. 2 - Beichuan county mean temperature
and rainfall between 1971 and 2000
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DISASTER CHARACTERISTICS AND OPTIMAL DESIGN OF DRAINAGE CANAL OF DEBRIS FLOW FOLLOWING WENCHUAN EAR-
THQUAKE IN WEIGOU GULLY IN BEICHUAN COUNTY, SICHUAN PROVINCE, CHINA
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
807
tively. Three houses and one temporary settlement were
buried or destroyed (Figg. 3 and 4) during these events.
A debris flow fan with 300 m long and 200 m wide
formed. Farmland with an area of 20,000 m
2
was buried
and the majority of which cannot be recovered without
considerable difficulty. This has seriously endangered
the lives and livelihood of local residents. In addition,
a considerable amount of sediment was carried into
the Pingtong River by the debris flows, resulting in the
main channel being pushed to the other side (Fig. 4). In
the future, this gully will continue breaking out debris
flows that pose a serious threat to the lives and property
of nearly 100 people. Once debris flows block the main
river, flooding will seriously threaten the roads and vil-
lages located in the lower reaches.
LANDSLIDES CAUSED BY THE EARTHQUAkE
The distribution of the landslides in the gully is
interpreted by using a 1:25,000 aerial photograph tak-
en at the end of May 2008 (Figure 5). Four large land-
slides and more than ten smaller landslides distributed
EARTHQUAkE
The study area belongs to the Longmenshan fold
belt, where neo-tectonic movement has mainly con-
sisted of intermittent uplifts characterized by intensely
cutting, vertical and horizontal displacements. The
Wenchuan earthquake occurred along the fold belt and
extended from southwest to northeast. The maximum
displacement caused by the earthquake was up to 5 m
near Beichuan City. The “seismic ground motion pa-
rameter zonation map of China” (GB18306-2001) was
revised after the earthquake. According to the map,
the seismic peak ground acceleration was 0.2g and the
seismic intensity was VIII in Beichuan County.
DEBRIS FLOW CHARACTERISTICS
HAZARDS
Although there was no record of debris flows in
this gully prior to the earthquake, the Weigou Gully
has since become a typical, viscous debris flow gully.
Three large debris flows occurred on September 24,
2008, July 17, 2009, and September 10, 2009, respec-
Fig. 2 - Regional geological map of the weigou Gully
Fig. 3 - Houses destroyed by the September 24,
2008 debris flow
Fig. 4 - Debris flow fan formed after the Sep-
tember 10, 2009 debris flow
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y. yoNG, l. JiNfeNG & c. XiNGZhANG
808
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
0.10 million cubic meters of solid materials were car-
ried to the outlet of the gully.
3. Decrease in Initiation Rainfall
The accumulated rainfall and the critical initiation
rainfall prior to the earthquake were 320-350 mm and
50-60 mm per hour, respectively (t
anG
et alii, 2008).
However, both decreased significantly after the earth-
quake. Taking the September 24, 2008 debris flow
as an example, the accumulated rainfall was 272.70
mm and the critical initiation rainfall was 41.00 mm.
Following the earthquake, they decreased by 14.80
%-22.10 % and 25.40 %-31.60 %, respectively.
OPTIMAL CROSS-SECTION DESIGN FOR
THE DEBRIS FLOW DRAINAGE CANAL
OPTIMAL OVERFLOwING CONDITION
To mitigate debris flow disasters in the Weigou
Gully, three check dams and five consolidation dams
have been planned in the main channel. These dams
will help stabilize the channel and slope, decrease
loose soils, and minimize the peak discharge of de-
bris flows. Meanwhile, a drainage canal will be con-
structed between the outlet and the Pingtong River to
prevent debris flow overflow fans and protect build-
ings and farms. The optimal cross-section design of
the drainage canal is discussed as follows.
Few studies have considered how to select cross-
sectional shapes and measurements in order to en-
sure optimal drainage capacity. w
anG
et alii (1996)
compared the hydraulic conditions of v-shaped,
circular, trapezoidal, and rectangular cross-section
drainage canals with the same overflow. y
ou
et alii
(2006) compared optimal hydraulic conditions for the
four drainage canal shapes and examined the opti-
mal cross-section design for debris flows in vshaped
drainage canals (y
ou
et alii, 2008). f
ei
et alii (2004)
examined v-shaped and trapezoidal drainage canals,
along the hanging wall of the Yingxiu-Beichuan fault.
The total landslide area was about 1.05 km
2
, account-
ing for 33.80 % of the catchment area. The total land-
slide volume was about 3.28 million cubic meters.
The enormous amount of loose materials will easily
form debris flows with adequate rainfall.
ACTIVITY CHARACTERISTICS
1. High Frequency
The gully has broken out three large debris flows
since the earthquake. Because the catchment contains
abundant loose material, debris flows will occur very
easily with sufficient rainfall. Therefore, the gully will
experience a high frequency of debris flows over the
next 5-20 years.
2. High Density and Carrying Capacity
One sample from the September 24, 2008 Weigou
Gully debris flow was collected for analyzing its grain
composition. Figure 6 shows the particle size distribu-
tion curve. The clay particle (<0.005 mm) content is
3.72 %, indicating that it was a high viscosity debris
flow. Based on data from the field survey, the debris
flow density was 1.90-2.10 t/m
3
. The gravel (>2 mm)
content was 84.58 %, indicating that the debris flow
was dominated by coarse gravel. The carrying capac-
ity of the debris flow was very high. Approximately
Fig. 5 - Aerial image of the weigou Gully after the wen-
chuan earthquake (1:25,000, National Geomatics
Center of China)
Fig. 6 - Grain grading graph for the
weigou Gully debris flow
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DISASTER CHARACTERISTICS AND OPTIMAL DESIGN OF DRAINAGE CANAL OF DEBRIS FLOW FOLLOWING WENCHUAN EAR-
THQUAKE IN WEIGOU GULLY IN BEICHUAN COUNTY, SICHUAN PROVINCE, CHINA
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
809
The greater the hydraulic radius, the smaller the
M and the greater the debris flow drainage discharge.
This represents the optimal hydraulic condition. Thus,
the cross-section configuration parameter (M) can be
used to express the optimal hydraulic condition of the
debris flow drainage canal. Plugging (4), (5), and (6)
into (7), the cross-section configuration parameter (M)
under the optimal hydraulic conditions, we get:
OPTIMAL CROSS-SECTION MEASUREMENTS
1. Related Parameters
The related parameters of trapezoidal-V shaped
cross-sections include the debris flow discharge (Q), the
composition of the material, the longitudinal slope of the
drainage canal (I), and the velocity of the debris flow (V).
The discharge of viscous debris flow (Q) can
typically be confirmed by data or calculated using an
equation. However, because there is no precipitation
rainfall or mountain torrents discharge data for the
Weigou Gully, we must use an equation to determine
Q (z
Hou
, 1991). We can ascertain the flood discharge
for different frequencies in order to calculate the de-
bris flow discharge. The discharges are Q
1%
=559 m
3
/s,
Q
2%
=433 m
3
/s, Q
5%
=221 m
3
/s, and the drainage canal
has a frequency of 5.00 %.
The longitudinal slope (I) can typically be de-
termined based onthe terrain. For the outlet of the
Weigou Gully, the longitudinal slope (I) is 0.08.
Several equations have been presented for calcu-
lating the velocity of viscous debris flows (x
u
et alii
2001, H
onG
1996, m
aRt
et alii 2001, y
u
2001). Here
we use (y
u
2008):
Where V is the average velocity of the debris flow
(m/s), R is the hydraulic radius (m), D
50
is the aver-
age diameter of the particles making up 50 % of the
grading curve (mm), D
10
is the average diameter of
the particles making up less than 10%, and I is the
longitudinal gradient.
2. Optimal Cross-section Design
The continual debris flow discharge equation is:
Q = AV
Plugging (7) into (10), we can solve for viscous
debris flow velocity:
but didn’t present the method used for deriving the
measurements under random slope conditions. Few
studies presented methods for calculating measure-
ments under optimal hydraulic conditions.
The optimal cross-section of a drainage canal
minimizes the passage area of the cross-section (A) or
maximizes the hydraulic radius (R) when the values
of inside section longitudinal gradient (I), roughness
coefficient (n), and design discharge (Q) are fixed. The
measurements for the “trapezoid-v” shaped drain-
age canal are shown in Figure 7. Here, m
1
is the side
slope coefficient (m
1
b
1
/h
1
) and m
2
is the groove
transverse slope coefficient (m
2
b
2
/h
2
).
If h
1
, h
2
and F (F is defined as size parameter,
F=h
1
/h
2
) are known, the overflow area (A) and the
wetted perimeter (P) can be calculated by formulas
(1) and (2).
The optimal cross-section is:
That is:
The overflow area (A) and the wetted perimeter
(P) can be expressed as:
By plugging F=h
1
/h
2
into (4) and (5) we get:
The cross-section configuration parameter (M) for
the trapezoidal debris flow drainage canal is defined as
the ratio of the wetted perimeter (P) to the hydraulic
radius (R). That is:
Fig. 7 - Measurements for the “trapezoid-V” shaped
drainage canal
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
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y. yoNG, l. JiNfeNG & c. XiNGZhANG
810
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
Solving (9) and (11), we get:
When Q, D
50
, D
10
, and I are known, the hydraulic
radius (R) under optimal hydraulic conditions can be
calculated by using (12).
For trapezoidal-V shaped drainage canals, we have:
Plugging (6), (8), and (12) into (13) and (14), we
can determine the measurements of the cross-section
under optimal hydraulic conditions.
In practice, both the slope and transverse coeffi-
cients vary within a certain range. If they varied, m
1
and
m
2
should be recalculated by repeating the above steps.
3. Cross-section Design of the Weigou Gully Debris
Flow Drainage Canal
The slope coefficient of the trapezoidal-V shaped
drainage canal in the Weigou Gully is m
1
=0.20 and
the transverse coefficient m
2
=5.00. Plugging these
values into equations (6) and (8), we get the size
parameter (F=5.02) and the configuration parameter
(M=6.93). Plugging F, M, Q
5%
=221 m
3
/s, D
50
=13.00
mm, and D
10
=0.04 mm into (12), we get the hydraulic
radius under optimal hydraulic conditions (R=2.58
m). Plugging all of these parameters into (13) and
(14), we get h
1
=4.38 m and h
2
=0.87 m (Table 3). The
measurements for the optimal cross-section of the
Weigou Gully drainage canal are shown in Figure 8.
CONCLUSION
Among the environmental changes following an
earthquake, the formation and accumulation of loose
solid materials are the primary factors involved in the
formation of debris flows.
The “trapezoid-v” drainage canal is one of the
shapes most widely used for preventing debris flows.
Shape and size are important parameters. This paper
discusses a method for designing its optimal cross-
section. Firstly, the depths (h1 and h2) of the crosssec-
tion of the “trapezoid-v” shaped drainage canal and
the optimal cross-section parameters using these two
measurements are defined. Then, the formulas for cal-
culating the cross-section measurements are deduced
under optimal hydraulic conditions when discharge
(Q), debris grain composition (D50 and D10) and lon-
gitudinal ratio (I) are known.
ACKNOWLEDGEMENTS
This research was financially supported by
the National Natural Science Foundation of China
(40971014) and the National Basic Research Program
of China (973 Program) (2011CB409903).
(11)
(12)
(13)
(14)
Tab. 3 - Measurements for the optimal cross-sec-
tion of theweigou Gully drainage canal
Fig. 8 - Cross-section of the weigou Gully debris flow drainage canal
background image
DISASTER CHARACTERISTICS AND OPTIMAL DESIGN OF DRAINAGE CANAL OF DEBRIS FLOW FOLLOWING WENCHUAN EAR-
THQUAKE IN WEIGOU GULLY IN BEICHUAN COUNTY, SICHUAN PROVINCE, CHINA
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
811
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