Rainfall-induced landslides, debris flows and flash floods are common natural hazards in mountainous areas. On 7 June 2008, a severe rainstorm hit Hong Kong, causing more than 1900 landslides, 900 debris flows, and 622 major floods over the entire Hong Kong territory. Under the changing climate, extreme rainfall events are becoming more and more frequent, implying more natural hazards in Hong Kong. These hazards usually do not occur separately, but are highly interconnected, and can interact with each other, causing amplifying or cascading effects. Therefore, it is necessary to develop an integrated platform to simulate possible rainfall-induced multiple hazards and possible hazard interactions.

A new integrated model is developed to simulate rainfall-induced landslides, surface...[ Read more ]

Rainfall-induced landslides, debris flows and flash floods are common natural hazards in mountainous areas. On 7 June 2008, a severe rainstorm hit Hong Kong, causing more than 1900 landslides, 900 debris flows, and 622 major floods over the entire Hong Kong territory. Under the changing climate, extreme rainfall events are becoming more and more frequent, implying more natural hazards in Hong Kong. These hazards usually do not occur separately, but are highly interconnected, and can interact with each other, causing amplifying or cascading effects. Therefore, it is necessary to develop an integrated platform to simulate possible rainfall-induced multiple hazards and possible hazard interactions.

A new integrated model is developed to simulate rainfall-induced landslides, surface runoff, surface erosion, debris flow initiation, deposition, and subsurface drainage pipe flow. The model is capable of modelling the full processes of multi-hazards, from rainfall infiltration to landslides, debris flow initiation, erosion, deposition, property changes, and flow material exchange between surface flow and drainage pipe flow. The model is verified with four cases: unsteady pipe flow with constant head, unsteady pipe flow with varied head, unsteady pipe flow in a drainage network, and surface runoff with urban drainage system. Then the model is applied to simulate the multi-hazard processes during the 7 June 2008 rainfall event over the North Hong Kong Island. The new module can simulate the processes of flow in a drainage network, and flow exchange between surface flow and drainage flow. With the drainage pipe flow integrated, this platform is capable of analysing multi-hazard processes and hazard interactions in both mountainous areas and urban areas, and providing basis for risk mitigation.

To start with, landslides, debris flows, and flash floods are analysed separately with three different models under extreme rainfall conditions. Multi-layer separate hazard maps are generated for the entire Hong Kong Islands. Two initiation mechanisms of debris flows, i.e., initiating at the landslide scars and initiating at the landslide deposits, are investigated and compared. Subsequently, integrated analysis of the multi-hazard processes is conducted with the new model, and new hazard maps are generated. The results are compared with those from separate hazard analyses.

The whole process of hazard propagation is modelled with the model for two cases: the Yu Tung Road debris flow event and the North Lantau Highway debris flood event during the 7 June 2008 rainstorm. The flow processes and transformation processes are well modelled with the model when compared to the conclusions of field investigations conducted by the Hong Kong Geotechnical Engineering Office. The blockage of the local drainage inlet by the deposits is modelled.

Finally, a stress testing framework is proposed for the Hong Kong slope safety system. The proposed integrated model is applied to a small catchment with engineering measures (i.e., urban drainage network and debris-resisting barriers). The performance of the current engineering measures is tested with the model first under extreme rainfall conditions. The bottlenecks are identified through the numerical analysis. Then new engineering measures (i.e., enhanced drainage network and barriers) are proposed and tested under extreme events. The results show the proposed engineering measures can cope with heavy, or even extreme rainfall events.