Slope failures, surface erosion and debris flows are typical rain-induced geohazards in high-precipitation mountainous areas, posing high risks to society. They are often triggered by heavy rainfall and thus sensitive to climate change. As extreme rainstorms become more frequent, coping with rain-induced geohazards thus becomes critical in geohazard prone regions such as Hong Kong and earthquake-affected areas in Sichuan, China, as well as other countries/areas such as Italy and Japan. The geohazards can interact with each other and enlarge the final magnitude. Hence, appropriate methods need to be developed for integrated rain-induced geohazards analysis in a regional scale to cope with increasing rain-induced hazard risk. There are many methods to mitigate geohazard risk such...[ Read more ]

Slope failures, surface erosion and debris flows are typical rain-induced geohazards in high-precipitation mountainous areas, posing high risks to society. They are often triggered by heavy rainfall and thus sensitive to climate change. As extreme rainstorms become more frequent, coping with rain-induced geohazards thus becomes critical in geohazard prone regions such as Hong Kong and earthquake-affected areas in Sichuan, China, as well as other countries/areas such as Italy and Japan. The geohazards can interact with each other and enlarge the final magnitude. Hence, appropriate methods need to be developed for integrated rain-induced geohazards analysis in a regional scale to cope with increasing rain-induced hazard risk. There are many methods to mitigate geohazard risk such as engineering and non-engineering measures. Among these measures, vegetation is a natural and environmental-friendly way to mitigate geohazards. In this doctoral research, a new integrated numerical model is developed to simulate the whole process of rain-induced debris flows, multi-hazard interactions, and the effects of vegetation on mitigating erosion and debris flows.

A new integrated simulation model is developed for simulating rain-induced debris-flow initiation, motion, entrainment, deposition and property changes. The model is unique in that it simulates the whole process of rain-induced debris flow evolution and two physical initiation mechanisms (i.e. surface erosion and transformation from slope failures). Previous numerical models with an assumed inflow hydrograph and an inflow location can now be conducted at one go scientifically without subjective assumptions. The model is verified first by three numerical tests, each for an individual component, i.e. rainfall runoff module, infiltration module, slope failure module. Then two real debris flow events, namely the Xiaojiagou debris flow in 2010 and the Tsing Shan debris flow in 1990, are used as field applications of the proposed model. The model can simulate the entire evolution process of rain-induced debris flows, and estimates reasonably well the volume, inundated area and runout distance of the debris flow. The integrated model will serve as a powerful tool for analysing multi-hazard processes and hazard interactions, and the assessment of regional debris-flow risks in the future.

The proposed integrated model can be applied for analysis of multi-hazard interactions considering all the three major hazards (rain-induced slope failures, surface erosion and debris flows) and cascading effects. The integrated numerical model is capable of simultaneously simulating rain-induced slope failures, surface erosion, debris flows, and hazard interactions. The effects of hazard interactions on the final hazard magnitude can thus be investigated. The presence of slope failures would intensify the amount of surface erosion significantly, and the enhanced surface erosion would amplify substantially the final debris flow hazard. The cascading effects of multi-hazards can also be simulated by the model. The whole process of debris flow and the subsequent formation process of debris flow barrier lake are modelled by the integrated model. The presence of the hazard interactions and cascading effects could have a significant impact on the volume, inundated area and runout distance of the debris flow, and the final affected area by the multi-hazards. This requires the past hazard mitigation methods to be reconsidered which might underestimate the possible rain-induced hazard scenarios.

Long-term observations of rain-induced geohazards activities and vegetation recovery were performed in the past ten years in the epicentre area of the 2008 Wenchuan earthquake. Detailed field investigations and satellite image interpretations have been conducted in the study area along an 18 km long highway to study the evolution of the rainfall-induced slope failures and debris flows and the vegetation recovery process after the Wenchuan earthquake. The evidence suggests that vegetation plays a role in mitigating the geohazards. A clear relation is found between vegetation recovery and declining rain-induced geohazards in time and space.

The proposed integrated model is extended to quantify and simulate the mitigation of surface erosion and debris flows with vegetation recovery. A hill slope in the Wenchuan earthquake zone is selected as the study area. Field investigations were conducted to obtain root-soil samples representing the extents of vegetation recovery in different times. The effects of vegetation on resisting soil erosion are quantified by laboratory experiments on field samples and empirical relationship. The vegetation factors can then be incorporated into the integrated model considering rainfall runoff, surface erosion and debris flows. It is found that vegetation effectively mitigates surface erosion and subsequent debris flows.