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!