Post on 12-Jan-2016
Catastrophic debris flows triggered by 13-14 August 2010
rainfall in the the Wenchuan earthquake-affected area
—lessons learned for reducing risks
Prof. Dr. TANG Chuan
State Key Laboratory of Geohazard Prevention
Chengdu University of Technology
◆ Strong earthquakes not only trigger serious,
coseismic landslides but can also lead to increased
post-seismic slope instability for a long period of time .
◆ After the Wenchuan earthquake, an abundance of
loose landslide debris was present on the slopes and in
the gullies. The debris later served as source material
for rainfall-induced debris flows.
◆ In addition, numerous extension cracks were
induced on hill slopes near the earthquake area, and
these cracks led to additional landslide and debris
flowsactivity during subsequent, heavy rains.
Tang et al. (2011) Landslides induced by the Wenchuan earthquake and a subsequent strong rainfall event: a case study in Beichuan area. Engineering Geology, doi:10.1016/j.enggeo.2011.03.013
Case 1: Qingping debris-flow event Qingping area is situated in Mianzhu County of Sichuan Province. It is about 80 km to the northeast of the epicenter, in a central position of the fault zone of this earthquake.
文家沟滑坡
滑源区
韩家大坪
Main deposition(3000x104m3)
Deposition(2000x104m3)
1300 平台
碎屑流流
通堆积区
Source area
280m 高的陡坎
Source
韩家大坪
1300 平台
280m
Wenjiagou landslide induced by the Webchuan earthquake
The total volume of the deposit is about 50 million m3 。 From an elevation of 2402 m, it rapidly fell 883 m onto the mounth of the gully and destroyed a village, resulting in 48 deaths.
Deposits of rock avalanche
An overview of the initiation zone of the Wenjia landslide. The strata
of the source area of the landslide is mainly composed of the
Devonian dolomite. The main scarp has a height of 445 m, which was
formed along a set of joints of the dolomite mass.
Catastrophic debris-flow event on August 13, 2010 in Qingping area
Wenjia gully and Mianyuan river
Mianyuan river
文家沟
Qingping area prior to the catastrophic event on August 13, 2010
老大桥
幸福大桥
清平乡场镇
Taken on 23 Dec., 2008
Qingping area after the catastrophic event
清平乡场镇
幸福大桥
老大桥
Photo on 14 Aug., 2010
Qingping area after the main shock (2008.12.23)
Qingping area after debris-flow event on Aug.13 (Photo taken on:08.22.2010)
Many houses were buried by Debris flow
Photograph showing the initiation area in erosive channel developed on
landslide deposits, with increasing channel scour in the downstream
direction.
The presence of scoured channel on landslide deposits concentrates the surface water runoff and hence provoked channelised flow. As results, the loose material was transmitted into a debris flow.
A check dam with 150 m long and 8m high was destroyed by debris flow
18 small check dam were distroyed
Rainfall records for post-earthquake debris flow events of the Wenjia gully
Event dateAntecedent
rainfall ( mm/d)
Rainfall intensity( mm/h)
Event volume( 103 m3)
2008-9-24 88.0 11.5 ( mm/10min) 500
2010-7-31 92.6 35.5 ( mm/h) 100
2010-8-13 82.6 37.4 ( mm/h) 3,500
2010-8-19 127.9 31.9 ( mm/h) 300
2010-9-18 51.9* 16.5 mm/30min) 170
* Rainfall data from an newly installed rain gauge at the Wenjia gully with an elevation of 1300m asl
Distribution of hourly and accumulated precipitation between Aug. 12
and Aug. 13, 2010. The debris flow in Qingping area occurred on August
13 between 00:00 and 01:00am at a rainfall intensity of 37.4 mm/hr and
after antecedent rainfall of 82.6 mm.
0
5
10
15
20
25
30
35
40
45
12
Au
g-1
4
15
16
17
18
19
20
21
22
23
13
Au
g-0
0 1 2 3 4 5 6 7 8 9
10
11
12
13
14
Hour
Ra
infa
ll (m
m)
0
20
40
60
80
100
120
140
160
180
Ac
cu
mu
lati
ve
ra
infa
ll (m
m)
Debris flow occurred
Calculation of discharge and volume of debris flows
B( m)
b (m)
H(m)
Q#
(m3/s)Q*
(m3/s)T(s)
W#
(104m3)W*
(106m3)C W#
S
(106m3)
8.13 215 130 5 1530 - 9000 275 3.50 0.72 1.98
9.18 - - - - 220 3600 16 0.17 0.72 0.11
Note: # Calculated value ; * Measured value
Yu B, Ma Y, Wu Y F. 2010. Investigation of severed debris flow hzards in Wenjia gully of Sichuan Province after the Wenchuan earthquake. Journal of Engineering Geology 18:827-836 (in Chinese).
The head of debris flow on Septemer 18, 2010 in Wenjia Gully
Inventory map of
debris flows triggered
by the rainstorm event
on the 13 August 2010
Many other debris flow occurred during August 13-14 ,2010
Xiaogangjian Landslide dams
On 14 August 2010, a total of 21 debris flows were triggered by heavy rainfall around the town of Yingxue, located near the epicentre of the Wenchuan earthquake.
Case 2: Yinxiu debris-flow event
0 0.5 1.0km
±
DF6DF3DF5
DF2DF1
DF4
a
DF6
DF3DF5
DF2
DF1
DF4b
±
0 0.5 1.0km
DF2Shaofang gully
DF3Xiaojia gully DF5
Wangyimiao gullyDF6
Mozi gully
DF1Hongchun gully
DF4Baijialin gully
c
a: TM image taken on March
31, 2006 before the
Wenchuan earthquake
b: Aerial photograph taken
on May 18, 2008 after the
Wenchuan earthquake
c: Aerial photo taken on
August 15, 2010 after the
rainstorm event
Alluvial fan prior to and after the debris-flow event
This catastrophic flood event claimed the lives of 56 people. More than 5,500 residents at high risk were forced to evacuate.
The Yingxiu town was flooded
Debris-flow dam
Original river channel
Hongchun gully
Shaofang gully
a
Newly reconstructed Yingxiu town was
flooded due to the debris flow dam.
Flood water-depth was estimated at 2.5-
3.0 m and flood duration lasted 7 days.
Newly reconstructed Yingxiu town was
flooded due to the debris flow dam.
Flood water-depth was estimated at 2.5-
3.0 m and flood duration lasted 7 days.
The photograghy shows how the flood water with a high concentration
of sediment flowed into the buildings and streets of the inner town
Aerial photograph taken on Aug. 14, 2010 shows the drainage area of the Hongchuen debris flow and the earthquake induced landslides.
The Yingxiu-Beichuan fault just runs through the Hongchun gully.
The overflow eroded the gully on large landslide deposits in the Hongchun gully. In the source area mainly granitic rocks are exposed, which are deeply fractured and highly weathered, and massive coseismic landslides were developed on the slope.
Channel features after outburst of landslide dam (LS2) in the debris flow gully
0
5
10
15
20
25
30
35
12A
ug-0
0 3
06
9
12 15 18 21
13A
ug-0
0 3 6 9
12 15 18 21
14A
ug-0
0 3 6 9
12 15 18 21
Hour
Rai
nfal
l(mm
)
0
50
100
150
200
250
Acc
umul
ativ
e ra
infa
ll(m
m)
Debris flow occurred
Distribution of hourly and accumulated precipitation between Aug. 12 and Aug. 14, 2010. The debris flow in the Yingxiu area occurred between 02:00 and 03:00 am since 16.4 mm/hr of rainfall and the antecedent rainfall of 162 mm was recorded.
Triggering rainfall
Characters of debris-flow volume, velocity, and discharge
debris-flow volume A quick field measurement with hand-held GPS and
laser Rangefinders were conducted on the debris-flow
fans to estimate their volume and the dimensions of
their runout zones. The volume of debris flow deposits
on the fan account for 0.7 million m3
From these data it was estimated that the mean velocity
and peak discharge of the Hongchun debris flow
reached 8.7 m/s and 746 m3/s, respectively.
◆ Prior to this catastrophic event, debris flows had been
recognized locally in the region, but their potential for
widespread and devastating impacts was not fully
appreciated.
◆ The assessment of debris flow hazard in the earthquake
area had largely overlooked the effects of cascading natural
disaster chains, such as the formation of debris-dams,
dammed lake, and flood.
Lessons learned for reducing risks
◆ It should be noted that identification of the areas that might
be inundated by future debris flows and estimates of flow
volume are required to quantify debris flow hazard. Because
many pre-existing alluvial fans are being utilised or
considered as settlement areas, the risk due to debris flows
has dramatically increased.
◆ For mitigation of debris flows risk, apart from engineering
measures, also non-engineering measures are required such
as land-use zoning to regulate and restrict the use of
hazardous areas and relocation to safer places of people
currently living in areas susceptible to debris flows and
related flooding.
Thank you for your Thank you for your attention!attention!