Assessment of human risks posed by cascading landslides in the Wenchuan earthquake area

Landslides exhibit several typical mass movement processes such as slides, rock falls, anddebris flows. It is possible that one hazard causes a secondary hazard, which in turn triggers atertiary hazard. Such domino effects starting from a slide or a rock fall are called cascadinglandslide hazards. Human risks posed by these cascading landslide hazards can be rather highdue to the possible amplification effects of the chain hazards.;Extreme cascading landslides triggered by strong earthquakes occurred frequently all over theworld and caused severe damage and losses. During an earthquake in Pakistan in 2005,numerous earthquake-induced landslides blocked roads and isolated the earthquake-strickenregion, and 26500 out of the total death toll of 86000 were associated with landslides andsubsequent floods. During the Wenchuan earthquake in China in 2008, approximately 25000lives were lost due to earthquake-induced landslides. In the past five years, landslide activitieswith distinct cascading effects led to over 1600 additional fatalities in the Wenchuanearthquake area.;Cascading landslide hazards are not independent and separate from each other but are closelyrelated. Conventional risk assessment deals with each type of hazard separately. The total riskis considered as the sum of all the individual risks. The interactions among the multiplehazards and the impact of the presence of one hazard on the vulnerabilities to other hazardsare often not considered, which may lead to misjudgement of the risk profile and an inappropriate overall human risk. The principal objectives of this research are to identify theinteractions among cascading landslides, quantify human vulnerability to cascading landslides,and propose a general framework for assessing the human risks posed by the cascadinglandslide hazards.;The cascading landslides distributed along a highway (PR303) near the epicentre of theWenchuan earthquake were identified based on interpretation of satellite images taken in 2008,2010 and 2011, combined with annual field investigations. A total of 20 interaction elementsare identified and illustrated in a matrix form. Supply of material is identified as the mostcrucial interaction among all the elements. The potential interactions among these cascadinglandslide hazards contribute to amplification and overlapping effects that elevate the humanrisks. In time scale, the cascading landslides can occur sequentially or concurrently. In spatialscale, both the amplification and overlapping effects can either be the results of a cascadinghazard chain within one catchment or the consequence of spatial overlapping amongneighbouring catchments.;Understanding the process of human flight during a landslide event is important in assessingthe human vulnerabilities to cascading landslides. A model for simulating human flight froman idealised landslide is developed in this thesis. A questionnaire survey was undertaken alongPR303 to collect information regarding human responses and behaviour in the face oflandslide events. The factors influencing human flight are classified into factors related to theevacuees, the landslide intensity and the flight path. Various components of 'available time'and 'demand time' for escaping from the landslide affected area are treated as randomvariables. Based on this model, probability analysis is conducted to estimate the flight success rates of the persons at risk when fleeing from landslides of various intensities. Sensitivityanalysis shows that the pre-failure time and the response time are the most important factorsin the flight process.;The assessment of the human risk requires consideration of the interactions among thehazards and among the human vulnerabilities to these hazards. A general framework forassessing the human risk posed by the cascading landslide hazards is proposed. Theframework consists of five phases, i.e. definition, multi-hazard assessment, exposureassessment, multi-vulnerability assessment, and multi-risk assessment. A detailed case studyin the Wenchuan earthquake zone, which involves slides, debris flows and landslide dams, is presented to illustrate the multi-hazard human risk assessment methodology. Attention is paidto the quantification of the amplification and overlapping effects due to the interactionsamong two or more hazards. Comparison of human risks shows that the risk will beunderestimated if the interactions among the cascading hazards are not considered. The highlevel of risks of the cascading landslides along PR303 quantitatively reveals the adverseimpact of the Wenchuan earthquake. Effective risk management measures must be taken tolower the risk level.