Landslides exhibit several typical mass movement processes such as slides, rock falls, and debris flows. It is possible that one hazard causes a secondary hazard, which in turn triggers a tertiary hazard. Such domino effects starting from a slide or a rock fall are called cascading landslide hazards. Human risks posed by these cascading landslide hazards can be rather high due to the possible amplification effects of the chain hazards.

Extreme cascading landslides triggered by strong earthquakes occurred frequently all over the world and caused severe damage and losses. During an earthquake in Pakistan in 2005, numerous earthquake-induced landslides blocked roads and isolated the earthquake-stricken region, and 26500 out of the total death toll of 86000 were associated with land...[ Read more ]

Landslides exhibit several typical mass movement processes such as slides, rock falls, and debris flows. It is possible that one hazard causes a secondary hazard, which in turn triggers a tertiary hazard. Such domino effects starting from a slide or a rock fall are called cascading landslide hazards. Human risks posed by these cascading landslide hazards can be rather high due to the possible amplification effects of the chain hazards.

Extreme cascading landslides triggered by strong earthquakes occurred frequently all over the world and caused severe damage and losses. During an earthquake in Pakistan in 2005, numerous earthquake-induced landslides blocked roads and isolated the earthquake-stricken region, and 26500 out of the total death toll of 86000 were associated with landslides and subsequent floods. During the Wenchuan earthquake in China in 2008, approximately 25000 lives were lost due to earthquake-induced landslides. In the past five years, landslide activities with distinct cascading effects led to over 1600 additional fatalities in the Wenchuan earthquake area.

Cascading landslide hazards are not independent and separate from each other but are closely related. Conventional risk assessment deals with each type of hazard separately. The total risk is considered as the sum of all the individual risks. The interactions among the multiple hazards and the impact of the presence of one hazard on the vulnerabilities to other hazards are 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 the interactions among cascading landslides, quantify human vulnerability to cascading landslides, and propose a general framework for assessing the human risks posed by the cascading landslide hazards.

The cascading landslides distributed along a highway (PR303) near the epicentre of the Wenchuan 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 elements are identified and illustrated in a matrix form. Supply of material is identified as the most crucial interaction among all the elements. The potential interactions among these cascading landslide hazards contribute to amplification and overlapping effects that elevate the human risks. In time scale, the cascading landslides can occur sequentially or concurrently. In spatial scale, both the amplification and overlapping effects can either be the results of a cascading hazard chain within one catchment or the consequence of spatial overlapping among neighbouring catchments.

Understanding the process of human flight during a landslide event is important in assessing the human vulnerabilities to cascading landslides. A model for simulating human flight from an idealised landslide is developed in this thesis. A questionnaire survey was undertaken along PR303 to collect information regarding human responses and behaviour in the face of landslide events. The factors influencing human flight are classified into factors related to the evacuees, 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 random variables. 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. Sensitivity analysis shows that the pre-failure time and the response time are the most important factors in the flight process.

The assessment of the human risk requires consideration of the interactions among the hazards and among the human vulnerabilities to these hazards. A general framework for assessing the human risk posed by the cascading landslide hazards is proposed. The framework consists of five phases, i.e. definition, multi-hazard assessment, exposure assessment, multi-vulnerability assessment, and multi-risk assessment. A detailed case study in 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 paid to the quantification of the amplification and overlapping effects due to the interactions among two or more hazards. Comparison of human risks shows that the risk will be underestimated if the interactions among the cascading hazards are not considered. The high level of risks of the cascading landslides along PR303 quantitatively reveals the adverse impact of the Wenchuan earthquake. Effective risk management measures must be taken to lower the risk level.