Catastrophic debris flows triggered by 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: /j.enggeo

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(3000x10 4 m 3 ) 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 m 3 。 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 ( )

Qingping area after debris-flow event on Aug.13 (Photo taken on: )

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 date Antecedent rainfall （ mm/d ） Rainfall intensity ( mm/h ） Event volume （ 10 3 m 3 ） （ mm/10min ） ( mm/h ） ( mm/h ） 3, ( mm/h ） * 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, 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. Debris flow occurred

Calculation of discharge and volume of debris flows B（m）B（m） b (m) H (m) Q # (m 3 /s) Q* (m 3 /s) T (s) W # (10 4 m 3 ) W* (10 6 m 3 ) CW # S (10 6 m 3 ) Note: # Calculated value ； * Measured value Yu B, Ma Y, Wu Y F Investigation of severed debris flow hzards in Wenjia gully of Sichuan Province after the Wenchuan earthquake. Journal of Engineering Geology 18: (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

km DF6 DF3 DF5 DF2 DF1 DF4 a DF6 DF3 DF5 DF2 DF1 DF4 b km DF2 Shaofang gully DF3 Xiaojia gully DF5 Wangyimiao gully DF6 Mozi gully DF1 Hongchun gully DF4 Baijialin 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 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

Debris flow occurred Distribution of hourly and accumulated precipitation between Aug. 12 and Aug. 14, 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 m 3

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 m 3 /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.

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