Many regions of the world are exposed to different types of hazards. The occurrence of multiple risks poses a huge difficulty in the environment. Intense rainfall in mountainous regions like Hong Kong can trigger multiple hydrological or geologic hazards, such as floods, landslides, and debris flows. In previous studies, there is limited attention on multi-risk assessment and comparison of the effects of multi-hazards and separate hazards on infrastructures and people.

Hence, the primary objective of this study is to assess the risk that arises from multi-hazards; landslides, debris flows, and floods in Hong Kong under extreme rainstorms with the intensity of 29%, 44%, 65% and 85% of the 24-h probable maximum precipitation. The risks to people and buildings have been analyzed qu...[ Read more ]

Many regions of the world are exposed to different types of hazards. The occurrence of multiple risks poses a huge difficulty in the environment. Intense rainfall in mountainous regions like Hong Kong can trigger multiple hydrological or geologic hazards, such as floods, landslides, and debris flows. In previous studies, there is limited attention on multi-risk assessment and comparison of the effects of multi-hazards and separate hazards on infrastructures and people.

Hence, the primary objective of this study is to assess the risk that arises from multi-hazards; landslides, debris flows, and floods in Hong Kong under extreme rainstorms with the intensity of 29%, 44%, 65% and 85% of the 24-h probable maximum precipitation. The risks to people and buildings have been analyzed quantitatively. The buildings considered are residential, schools, hospitals, commercial centers, temples, factories, community centers, government, and church buildings. The buildings affected by the three types of hazards and integrated multi-hazards were identified by combining the intensity of the hazard (maximum flow velocity, maximum flow depth, deposition depth) and exposed buildings, and presenting them on a map using GIS. The potential number of people at risk in the affected buildings is estimated based on the number of floors, the number of flats, and the average number of people living in a flat during the time of the event. The results indicated that, among all structures and facilities in Hong Kong, residential buildings appear to be the most vulnerable and could lead to the highest potential loss of life under all the rainfall scenarios analyzed. Because of the steeper slope terrain, the western and southern parts of Hong Kong Island are more vulnerable to multiple hazards, while the north-eastern part may experience fewer multiple hazards. In all, flooding is the least hazard affecting all building types. Under landslides, the highest potential loss of life at 44%PMP is only about one half of the highest potential loss of life in the 65 and 85%PMP cases. The risk escalates as the storms become more intense. Under all %PMPs considered, church, community center, and temple show less vulnerability to hazard effects; and under 29%PMP, the hazards reveal the little impact on people and buildings.

Comparing the impact of the separate hazard scenarios with the multi-hazards, records show that each of the separate hazards has a lower impact on population and buildings than the risk that arises when the multi-hazards are combined.

Overall, there are large numbers of residential buildings impacted by all four hazards (landslides, debris flows, floods, and integrated multi-hazards) compared, because these areas are the heavily populated parts of the region. In general, residential buildings can be classified as highly affected types of buildings while schools, commercial, factories, hospitals, and government are classified as moderately-affected types of buildings and churches, community centers, and temples can be categorized as slightly-affected building types.

Stress testing has been carried out considering engineering measures such as flexible and rigid barriers. The risks to people and buildings have been evaluated considering the two cases: when engineering measures are applied and without measures under rainfall scenarios of 29%PMP and 85%PMP. It is observed that the impact on people and buildings is reduced when the barriers are applied compared to when there are no engineering measures applied. The multi-risk assessment technique serves as a vital tool in decision making in infrastructure design, management of disasters, and overall risk, construction plans, and preparation for emergency response.