10th Asian Regional Conference of IAEG (2015)

Engineering measure and its effect for Shaofanggou landslide type debris flow
Bu Xiang hang, Tang Chuan( State Key Laboratory of Geohazard Prevention and Geoenvironment Protection，Chengdu University of Technology，Chengdu 610059)
E-mail:309351649@qq.com.
Abstract
By analyzing the motion process, the gully characteristics, landslides blocking point as well as
the high depth of cutting, We noted that dealing with clogging and cutting are of most importance in abasin comprehensive management project. Building the check dams helps to weaken the kinetic
energy of debris flow heading down from the upper reaches. A new type of drainage channel which iscomposed by anti-slide piles, sidewalls and ribbed slabs, will control the landslides blocking point andprotect the “toe cap” in the middle reaches of the gully. A series of check dams with opening sizes andaqueduct will intercept the coarse particle and discharge debris flow smoothly. Finally, by comparing
the figure of accumulated rainfall, solid matter participation of “2010.8.14” and “2013.7.10”, thedynamic reserves and gully channel characteristics pre and post control measures. Specifically,
comprehensive engineering measures have good effect to the Shaofanggou landslide type debris flow.
Provide a reference for engineering measures for landslide type debris flows in the earthquake areas.
Key words: landslide type debris flows, drainage channel, check dam, Wenchuan earthquake, Effectevaluationof about 204.3 × 104 m3. The slide direction tends to
196°, the altitude is 983─1544m. The channel was
blocked completed by LS01 landslide.
Rainfall is the most important triggering factor for
initiating debris flows (Tang.2011). On August 14,
2010, Shaofanggou debris flow was triggered by anintense rainfall. However, with the failure of the LS01
landslide dam, there would have an instant dramaticincrease in the discharge. Ground investigations of thedeposition area indicate the run-out of the debris flow
was some 250,000 m3 a relatively high volume fromsuch a small catchment. The landscape changed
radically, particularly in the outlet of the gully (Fig.1).

1. Introduction
Although mitigation measures were constructed ina few gullies such as the Wenjia and Bayi Gullies,
disastrous debris flows still occurred because themitigation measures installed in those areas proved to
be inadequate (Wang et al. 2012; Wang et al. 2013b).
Thus, developing new techniques based on the debris
flow characteristics in the quake-stricken area is a
critical task. In this paper, debris flow hazard
mitigation measures are evaluated based on an analysisof the formation mechanism of Shaofang gully, which
was blocked by a large-scale slope and characterized
by a small catchment and a high gully slope. A new
layout and designs of engineered structures were
proposed to mitigate the potential debris flow hazard
and were shown to be effective in protecting a highway
located downslope. These new mitigation techniques
can provide a reference design for landslide type debris
flows hazard mitigation in similar locations.

3. Present-day channel characteristics
3.1 Source materials characteristics
The huge amounts of landslide debris produced by
the strong earthquake, that is the prerequisite of debris
flow event in the Wenchuan earthquake area
(Tang.2009). The Shaofang catchment had larger
debris storage, with a total volume of about 251.9 × 104
m3. On the 4th of August 2010 a debris flow volume ofthe solid materials that rushed out of the gully wasestimated to be approximately 25 × 104m3,
approximately 10% of the total volume. After debris
flow event, types, number and volume of solidmaterials had some changes in Shaofang gully. The
total volume reduced to approximately 223.9 × 104m3 ,still, it is huge, with its dynamic reserve about 54.7×104m3. The key LS01 had about 46.9× 104m3, it wasvery easy to block channel again. Above all,

2. The study area
Shaofang gully locates the northeast of Yingxiu andon left bank of Minjiang River, with the main stream of
1.58Km long and catchment area of 0.61Km2. The
gully catchment elevation is 888m-1902m asl,
representing an elevation difference of 1014m. Its
basin is willow-leafed in shape, and the longitudinal
gradient is 464.97‰. Most of the slopes are steep,varying from 30°to 80°(Fig.1). LS01 body is about 480
m long, 200 m wide and 18 m thick with a total volume

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10th Asian Regional Conference of IAEG (2015)

present-day source material characteristics were similar
to the formers, the LS01 materials occupied most partof them (Fig.2).

Huang (2012) combined with actual situation ofdebris flows along Minjiang river, and calculated the K
by inverse calculation. When K=1, analysis result ismostly approach to reality. Tang (2010) discussed theestimation method of the sediment yields from debris
flows with various return period rainfall in Wenchuan
earthquake area, the equation is flowing:−=(19 ∙ ∙ /72)−

3.2 Geomorphological characteristics
Catchment longitudinal gradient was still steep,after debris flow event. By comparing gradients of six
channel segments before and after debris flow, theaverage longitudinal gradient increased from 427.72‰
to 464.97‰. There is one exception: the sixth
segmentation (F-G) decreased slightly, the others
increased (Table.1). As a result, the vertical and middle
erosion were mainly in upper and middle parts of thegully, and the loose accumulation were mainly in lower
part. On the whole, longitudinal gradient increased inupper and middle parts of the gully, it is beneficial toinitiate of debris flow. Longitudinal gradient decreased
in lower part, it is of no beneficial to initiate of debris
flow, but it helps to for accumulation of rainfall andloose materials. After debris flow event, there were twosteep tributaries formed, especially in the middle partof the No.2 tributary was deposited by an abundance ofloose accumulation, with about 1-2 m above the groundin height.
After debris flow event, there were still anabundance of loose accumulations remaining the gully,
with dynamic reserve about 54.7× 104m3. And the key
LS01 had about 46.9× 104m3. The gradient of eachsegmentation channel ranged between 277.3 and682.2‰, the average gradient of main channel
increasedfrom
427.72‰
to464.97‰.
Above-mentioned reasons, the present-day channel
characteristics are similar to before, and still highly
prone to debris flow hazard under a certain rainfall
intensity.

When Ws is the outflow volume of debris flow, γh is thedensity of materials, γw is the density of water, T is the.
duration of debris flow.

So, an equation for calculating the outflow volume
is proposed when the Minjiang river at the critical stateof being partially and not blocked by Shaofanggou
debris flow (Z=0.5), the following result derived fromthe debris-flow data set of Table 2:

=

(

)

(

)

4

= 3.2 × 10 m3

In order to protect the safety of the hingway and thelives and property of locals, a debris flow mitigation
project was found in the Shaofang Gully in October2010. The design standard of the debris flow mitigation
measure was to resist rainstorm-induced debris flows
with a 50-year return period, and ensure the outflow
volume must be less than 3.2× 104 m3
4.1 Engineering layout
In practice, as seen in Fig.2 and Fig.3, theengineering layout of debris flow prevention wasapplied in Shaofang gully with integrated methods
including slope toe protection and stabilization ，sediment trap works in gully, aqueduct over the open
tunnel．

4. Debris flow mitigation engineering in the
Shaofang gully

4.2 Check dams
Three check dams were adopted for controlling theinitiation of remaining loose accumulations, andweakening the kinetic energy of surge in the upper part(the No.2 and No.3 segmentations) of Shaofang Gully,
as shown in Fig. 4. They were all constructed using
concrete, the sequence of the dams is as follows : thefirst dam is 5.0m high and 28.5m wide; the second dam
is 5.0m high and 19.7m wide; the third dam is 5m highand 22.6m wide.
The average gradient of the upper part was larger
than before about 27.25‰（Table.1）, and there werestill a lot of loose accumulations remaining in gully,
the structure of bank slopes were loose, with 35-55°in
gradient. Steep slopes and gully bed provided terrain
conditions for loose debris potential energy
transformed into kinetic energy, and forming
high-speed debris flow head (Tang.2009). Vertical andlateral erosion of debris flow is much larger than clear
water. So block-silting method can prevent erosion of
fluid, reduce the average gradient of main channel,
lower impact force of debris flow head, and weaken the

The maximum sediment load of main river may
be exceeded due to the over loaded confluence ofdebris flow, which causes main channel blockage andresults in a disaster chain (Cui.2013). The main river
can be either partially or completely blocked by adebris flow. Therefore，the capacity of sediment load ofmain river should be considered in planning adebris-flow mitigation. Tang (2005) proposed an
equation for estimating the degree of blockage:
2
Z=
When Z is the discrimination value of blockage (Z≥1.0,
the river is completely blocked. Z=1.0~0.5, the river ispartially blocked. Z≤0.5, the river is not blocked), B is thewidth of the main river, Qn is the peak discharge of bebris
flow, Qz is the discharge of the main river, K is the correction
coefficient (K=1.0~1.5), γn is the density of the debris flow,
β is the direction angle between gully and main river.

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10th Asian Regional Conference of IAEG (2015)

pressure of engineering in the middle and down parts.

gentler than before about 23.7‰（Table.1）. Bedrock
are common on both sides, that provide better
conditions to dam abutment. When debris flow transit
the three check dams, water and stone can be separated.
The mean particle size of debris flow becomes smaller,
thereby decreasing the impact forces of the large stones
to the open tunnel and highway, and draining the fluid
into designated area of Minjiang River. This methodthus makes use of both the siltation and hazard
mitigation functions of check dams. The function ofengineering measures and variation of sources were
illustrated in table3.

4.3 A new type of drainage channel
Drainage channels are important. They are widelyused in engineering owing to their structure areconsisted of local materials. The styles of drainage
channels are limited in debris flow gully for limited
space (Okubo et al. 1997; You et al. 2011; Takahisa
2008; Chen 2014). A neo-drainage channel that willcontrol the landslides blocking point and protect the“toe cap” in the middle reaches of Shaofang gully. It
will prevent unstable source from being eroded bydebris flow, reduce the volume of materials whichmaybe taken part in fluid.
The neo-drainage channels are made of reinforced
concrete with anti-slide piles ,step-ribbed slabs andsidewalls on both sides of them (Fig.5). and connecting
the ribbed slabs and sidewalls with anti-slide piles
through the embedded rebar so that the sidewalls
would close with soil and stone ,prevent the rainfall
from eroding foundation and stabilize the overall
structure. In the paper, a total 465m long channel wasconstructed in the No.2 tributary and the middle part of
Shaofang gully, with 146 anti-slide piles (Fig.3).
Loose composition materials of LS01 front arevery poor to concrete, which will be failure to block thechannel while a heavy rain poured. One possible source
supply model of LS01 blocking channel is supplied(Fig.6). Source supply order as follows: First of all,fluid scour loose materials of LS01 foot; Then, fluid
carry material at the bottom of the slope to flow,
causing the upper part of the slope to participate in theactivities of debris flow; After that, sliding resistanceget dramatically plunged ,causing an unbalance theupper body of slope and finally a blockage.
Let us analysis the mode from the perspective ofprevention and control to analysis the model, if theLS01 foot is steadied by the neo-drainage channel, it
will prevent the occurrence of step 2 and 3. As well,
the No.2 tributary and “shoe toe” can be controlled bythat. A lot of active source could be steadily stopped,
the scale and probability of debris flow can besignificantly reduced.

5. Effect of the debris flow mitigation engineering in
the Shaofang gully
The mitigation engineering was accomplished in
May 2012. On July 2013, Mozigou and Huaxigou extra
large-scale debris flows were triggered by a rainstorm
near Shaofanggou.
The “2013.7.10” duration rainfall was larger than“2010.8.14”, but the “2013.7.10” outflow volume ofdebris flow was about 33 times less than “2010.8.14”(Table.4). That reflected comprehensive control project
had a certain effect. The outflow volume of debris flow
(0.72×104m3 ) was less than Minjiang river carrying
capacity (3.2× 104 m3). So the “7.10” debris flow did
not cause a disaster to Minjiang River. Field
investigation, Three check dams were constructed in
upstream, two dams were fully deposited, the other one
was deposited about two thirds, and they stopped
source all about 2.07×104m3. The neo-drainage channel
stopped source about 44.28× 104m3 in midstream.
Three check dams with opening sizes were stopped
source about 0.5× 104m3.
The fig.7 shows the change of upstream channel
after constructing check dams. Longitudinal grade ofgully reduced from 346.87‰ to 251.8‰, making fluid
run slowly and dissipate the energy of debris flow. So
debris flow materials were under control, the erosion
rate greatly reduced, and vegetation restored.
Vegetation would be served as botanical check dams(Cui.2013)．Not only control gully erosion，but also
protect structural check dams by load share. Looking
into the vegetation which are rooted on LS01
accumulations with new-built neo-drainage channel,
the restoration of vegetation grows to 70% which result
directly from the gully erosion control and theprotection of slope toes (Fig.8).Here is the contrast
analysis on the sediment from upstream anddownstream areas in table 5.
In this manner, the particle size of the sediments issmaller downstream than upstream. And the closer the
distance from gully mouth, the smaller the mean
particle size of specimen. This outcome indicates thatcheck dams with opening sizes are rational.

4.4 Check dams with opening sizes
Three check dams were constructed with opening
sizes in downstream are mainly for intercepting thecoarse particle and smoothing the drain fluid. The fluidwhich passed dams was harmless and discharged
smooth into Minjiang River by aqueduct. They were all
constructed with concrete, the sequence of the dams islisted as follows : the first dam is 17 m high with amaximum opening of 1.0 × 1.2 m (width × height); thesecond dam is 13 m high with a maximum opening of
1.0 × 1.2 m (width × height); the third dam is 17 mhigh with a maximum opening of 1.0 × 1.2 m (width ×
height).
The average gradient of the downstream was3

10th Asian Regional Conference of IAEG (2015)

6. Conclusions

Chinese).

(1)A rainstorm induced large-scale landslide typedebris flow hazard occurred in Shaofang Gully on 14
august, 2010. The outflow volume of debris flow was
estimated to be 250,000 m3.The disaster destroyed
highways, and blocked the part of Minjiang River.
Eighty percent of outflow volume was created from the
LS01.
(2) Shaofang Gully is a typical landslide type debris
flow gully, abundant source materials were induced byearthquake. LS01 blocked the channel with landslide
dam under “2008.5.12” earthquake action, with the
failure of the landslide dam, an instant dramaticincrease took place in the discharge by the “8.14”
intense rainfall.
(3) After debris flow event, the total volume ofsource materials was approximately 223.9 × 104m3,
dynamic reserve was about 54.7× 104m3. LS01 has thevolume of dynamic reserve was about 48.72× 104m3,
with an potential blocking. The average longitudinal
gradient increased to 464.97‰. It is still highly prone
to debris flow hazard under a certain rainfall intensity.
(4) Three check dams were adopted for controllingthe initiation of remaining loose accumulations, andweakening the kinetic energy of surge in the upper partof Shaofang Gully. The neo-drainage channel using
prefabricated reinforced concrete includes anti-slide
piles, step-ribbed slabs and sidewalls on both sides of
the channel in the middle reaches of Shaofang gully.
They developed based on the characteristics of debris
flow. A series of check dams with opening sizes wereused to settle out debris flow particles of various sizes
and separate water from stone along the gully channel.
The impact forces of a debris flow can be reduced
using this method.
(5) These mitigation measures in the Shaofang
Gully effectively resisted a debris flow triggered byrainfall on July 10, 2013. Moreover, it effectively
protected the highway and minimized losses. Field
study, longitudinal grade of gully reduced from 346.87‰
to 251.8‰, vegetation restoration rate of LS01 was at
about 70%, the closer the distance from gully mouth,
the smaller the mean particle size of specimen. Those
outcomes indicate that mitigation measures are rationalfor Shaofanggou landslide type debris flow.

Wang, Z.Y., Qi, L.J., Wang, X.Z., (2013b). Debris
flow

controlwithenergydissipation

structures-experiences from Wenjiagou. Journal of
Hydraulic Engineering, 43(3):253-263 (in Chinese).
Tang,C.,2010. Activity tendency prediction of rainfall i
nduced landslides and debris flows in theWenchuan
Earthquake Areas.Journal Of Mountain Science.2010,28(3) :341~349(in Chinese).
Wang, T. Shi, J. S.，Wu, S. R.，2010. Formation
mechanismofWenjiagou

high-speed

and long -runout debris flow avalanche triggeredby
Wenchuanearthquake.

JournalofEngineering

Geolo-gy18(5): 631-644 (in Chinese)．
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Southwest Jiaotong University (in Chinese).
Hu, X.W., Huang, R.Q.,2009. Analysis of blocking
river mechanism of TangJiaShan landslide anddam-breaking mode of its barrier dam. Chinese
Journal of Rock Mechanics and Engineering 28(1):181-189 (in Chinese).
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flows on 13 August 2010 in the Qingping area,
southweatern China: The combined effects of astrongearthquake

andsubsequent

rainstorms.

Geomorphology 139-140 (2012) 559-576.
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Area，Sichuan. Geological Review58(4):709-716 (inChinese)．
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10th Asian Regional Conference of IAEG (2015)of typical debris flow in Wen ChuanEarthquake zone.

8:103-107.

Chengdu University of Technology.

Takahisa M., (2008). Structural countermeasures for

Okubo,S., Ikeya, H.,Ishikawa, Y., Yamada, T., 1997.

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earthquake area. Engineering Geology.

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drainage channel of viscous debris flow. J. Mt. Sci.
Table.1 Comparison of longitudinal gradient before and after bebris flow event
Seq. No.
Tributary

Before debris flow（‰）

After debris flow（‰）

1

/

577.4

2

/

726.5

1 (A-B)

623.9

2 (B-C)

443.3

Average

462.1

Average

Main gully

3 (C-D)

211.2

longitudinal

277.3

longitudinal

Segmentation

4 (D-E)

355.4

gradient

395.9

gradient

5 (E-F)

526.9

427.72‰579.7

464.97‰6 (F-G)

405.6

682.2

381.9

Table.2 Date set of Shaofanggou debris flow used for calculating the outflow volume
Gully nameT (s)Qz (m3/s)

Z

B (m)

γn (t/m3)γh（t/m3）

γw（t/m3）β (°)

Shaofang

3600

400

0.5

80

1.95

2.27

1.0

90

Table.3 Engineering measures and its effect (Unit: × 104 m3)
Designed
ChannelEngineering
measuresvolume ofMain function

blockedsource

Upstream（C-D）

Check dams

Weaken the kinetic
energy of surge

Now volumeof blocked
sourceDynamic
reservebeforeengineering

Dynamic
reserve afterengineering

2.73

2.07

2.58

1.51

53.11

44.28

49.10

4.83

3.35

0.50

2.98

2.48

Control the LS01
Midstream

Neo-drainage

blocking point +（D-F）

channelprotect the“ shoes toe ”

Downstream（F-G）

Check dams with

Separate water

opening sizes

and stone ++ aqueduct

discharge smooth5

10th Asian Regional Conference of IAEG (2015)

Table.4 Rainfall and outflow volume
Rain time

Duration rainfall

Outflow volume(mm)（×104m3）

2010.8.14

162.1

25

2013.7.10

169.6

0.72

Table.5 Specimen particle size analysis
Number of specimen
Distance from gully
mouth(m)Particle size (mm)

1

2

3

4

5

6

10

30

50

70

90

980

10.28

24.23

52.97

91.85

112.65402.9

Specimen were taken from sediment surface; The first to fifth and the sixth specimen were taken from deposition basins ofcheck dams in downstream and upstream areas respectively.

Fig.1. Aerial photograph, taken on August, 2010 showing the LS01 and depositional zone
after “8.14” debris flow event

Fig. 2 The distribution of landslide-debris in Shaofang gully catchment

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Fig. 3 Site plan and photographs of the mitigation measures in the Shaofang Gully

Fig. 4 Sedimentation schematic of check dams

Fig. 5 Sedimentation schematic of a drainage channel

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Fig. 6 One possible source supply modelFig. 7 The change of upstream channel prior to (a) and after (b) constructed check dams

Fig. 8 Contrast of LS01 prior to (a) and after (b) constructed neo- drainage channel. Photographs are taken on
September 2010 and on September 2014 respectively. Red dashed line is the top boundary of LS01.

8