10th Asian Regional Conference of IAEG (2015)

Recent catastrophic debris flows and risk assessment in China(Extended Abstract)
Chuan TANG(1), Min CHANG(1), Xun HUANG(1), and Cees van WESTEN(2)(1) State Key Laboratory of Geo-Hazard Prevention and Geo-Environment Protection,
Chengdu University of Technology, China
E-mail:2401634274@qq.com(2) Faculty of Geo-Information Science and Earth Observation, University of Twente, The Netherlands

Debris flows in mountainous areas are one of themost dangerous geomorphological events, primarily
because of the substantial volumes of material thatcan be deposited on urbanized alluvial fans. As aconsequence of climate change and increase in
exposure in many parts of China, the risk associatedwith debris flows is growing. In recent years, many
serious incidents related to debris flows have beenreported in China. Several debris flows that occurredin China were truly catastrophic. Debris flows thatoccurred at Zhouqu City on 8th August 2010
demolished 5,500 houses and caused the death of
1,765 people along its flow path. In addition, a heavy
rainstorm on 13th-14th August 2010 near the town of
Yingxiu, located at the epicentre of the Wenchuan
earthquake, triggered many hillslope and channelized
debris flows. Among them was the Hongchun gully
debris-flow, which transported huge amounts of
sediment to the Minjiang River to form a debris dam,which in turn changed course of the river, resulting in
the flooding of the newly reconstructed Yingxiu town.
Finally, on 13th August 2010, in response to a highintensity rainfall, numerous debris flows occurredalong the Qingping section of the Mianyuan river,southwestern China. As a result of these debris flow
events，a very large volume of sediment and debris
was transported onto the inhabited alluvial fans,directly causing the destruction of more than 100houses along their flow paths in the Qingping area. A
giant debris-flow, which developed in the Wenjia
catchment，transported a total volume of about 3
million m3 of sediment to the river. It generated a
temporary debris-dam that entirely blocked the river.
Subsequent floodwater, which had a highconcentration of sediment, flowed into the newly
constructed streets houses and streets in Qingping
town. At least 750 houses were inundated or buriedby the flows, and about 5000 residents had to beevacuated by helicopter.
In areas with high demographic density,
protection works often cannot be built because of
economic or environmental constraints, and is it notalways possible to evacuate people because ofsocietal reasons. Debris flow risk assessment (DFRA)
is progressively becoming a requirement for theadministrations in charge of debris flow risk
management, because the risk assessment is one of
the most effective soft measures in natural hazard
prevention, and the base of hazard risk management,
which can be contribute to the construction planningand prevention measure making. Debris flow risk
assessment is still in its developing stage, and China
does not have a standardized risk assessment
procedure. Some case studies show that thesuccessful implementation for regional debris flowrisk assessment. However, there are even no studies
for quantitative risk assessment for debris flows inChina.
This paper focuses on the hazard and risk
assessment methods developed recently forcharacterizing the danger caused by the giant debris
flows in China, mainly defined as follows:(1) Recent catastrophic debris flow hazards in China:
an overview, aims to present the framework ofdebris flow problems in China, socio-economicimpact, major events, regions with major debris
flow problems.
(2) Debris flow inventory mapping, aims to present
the state of the art in the development of debris
flow inventories.
(3) Regional scale debris flow risk assessment, aims
to present the input data requirements and themethods that are used on regional scales to derivedebris flow risk maps.
(4) Methods for local scale and site-specific risk
assessment, shows the application of physically
based models for debris flow hazard and risk
assessment.
The proposed methodology is composed of fourmain parts including data collection, hazard
assessment (susceptibility assessment, analysis oftriggers) consequence analysis and risk assessment.
In this study, the methodology on debris flow risk

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10th Asian Regional Conference of IAEG (2015)

assessment for regional, local, and site-specific scales
in practice are explained by the example of the casestudy of the region of Wenchuan, southwestern China.
A special consideration was given to presents thedynamic run-out modelling focusing on continuum
depth-average models and includes a quantitative risk
assessment using run-out models in a specific study
sites. Through the case study, we aimed at thefollows:• Assessment scale: regional, local, site-specific;• Source of input data: empirical to remote sensing;• Run-out modeling ;• Application of vulnerability curves;• Generation of risk map based on economic losses.
The paper proposes a methodology for debris
flow risk assessment for regional-scale analysis. The
developed debris flow risk assessment methodology
is tested in Wenchuan area, in China. Geographic
information systems and remote sensing techniques
are used to create debris flow factor maps, to obtainhazard maps, elements at risk, and risk maps. Debris
flow scenarios were built basing on different return
periods of rainfall. Hazard maps are obtained byusing a logistic regression model. The vulnerabilities
are assessed by adopting some generalization
approaches. Regional debris flow risk maps wereproduced on a continuous scale where numericalvalues indicate the distribution of risk including theprobability of expected losses.
This paper also presents a quantitative risk
assessment using run-out models in a specific study
sites. The methodology used in the analysis consistedof several components, such as a detailed analysis of
rainfall return periods (10, 50, 100 years returnperiod), modelling of the runout of the debris flows,
and application of debris flow height and impact
pressure vulnerability curves and the generation ofrisk curves based on the economic losses. In the study,
debris flow dynamic characteristics and mitigationplans were considered. Moreover, expected loss
values of representative buildings and roads under
different debris flow conditions were assessed
accurately by combining numerical simulation,
elements at risk database and corresponding
vulnerability functions.

Keywords: debris flows, major events, run-out modeling, vulnerability curve, risk assessment2