Evaluating Metropolitan Hazard Risks under Extreme Rainstorms

Half of the world’s population lived in urban areas at the end of 2008. The United Nations predicted that by 2050 about 64% of the developing world and 86% of the developed world will be urbanized. A recent example is the rapid expansion of the Guangdong-Hong Kong-Macau Bay area, which has a population of over 60 million. In the changing climate, many metropolitans are exposed to multi-hazard risks, such as those caused by Hurricane Katrina in New Orleans in 2005, Hurricane Sandy in New York in 2012 and Typhoon Mangkhut in Hong Kong in 2018. How to cope with natural hazard risks in urban settings under the changing climate is an emerging issue. A large city is a system of many highly interactive sub-systems: social needs, transportation, utilities, communications etc. Five GREAT attributes of smart city systems should be advocated; namely Green, Resilient, Empowering, Adaptable, and Transformative. This paper focuses on the ‘Resilience’ aspect of cities when facing extreme storms, taking Hong Kong as a study city. The impact of recent rainstorms and typhoons on Hong Kong and the observed propagation of hazards among key systems of a city are briefed first. Then plausible rainstorms that may hit Hong Kong in the future are introduced. Subsequently, a numerical platform is presented to simulate the hazardous landslide, debris flow and flooding processes over the Hong Kong territory under extreme rainstorms. Finally, management strategies for coping with the extreme storms are discussed. The study helps identify catastrophic hazard scenarios and the bottlenecks in the urban disaster prevention systems, and assist policy making for improved preparedness and metropolitan safety.