Evaluating Metropolitan Hazard Risks under Extreme Rainstorms Limin Zhang 1,2 , Shengyang Zhou 2 , and Suzanne Lacasse 3 1 HKUST Shenzhen Research Institute, Nanshan, Shenzhen 518057, China. 2 Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong, China. E-mail: cezhangl@ust.hk, szhouaq@connect.ust.hk 3 Norwegian Geotechnical Institute, Oslo, Norway. E-mail: Suzanne.lacasse@ngi.no Abstract: 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. Keywords: Natural hazards; resilience; smart city; climate change. 1 GREAT Smart Cities Initiative 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 presently has a population of over 60 million. Recent severe rainstorms and storm surges led to catastrophic consequences in urban areas. For instance, Hurricane Katrina in New Orleans in August 2005 caused extensive failure of levees, leading to approximately 1500 fatalities. Hurricane Sandy in October 2012 in the east coast of the US led to 43 deaths and nearly 90,000 buildings in the inundation zone. In the future, more population and wealth will flux into large cities. How to cope with natural hazard risks in urban settings under the changing climate is an emerging issue. A smart cities initiative was started at the Hong Kong University of Science and Technology, advancing five GREAT attributes of smart cities development, namely Green, Resilient, Empowering, Adaptable, and Transformative (Lo 2018): 1. Green: cities planned with due considerations for life-cycle environmental impacts, natural resources consumption (e.g. air, water, energy, materials) and waste generation and treatment; 2. Resilient: cities developed with infrastructure capacities and system redundancies to manage disruptions without severely affecting their essential functioning; 3. Empowering: cities designed for people, empowering wellbeing, efficiency, innovation, and productive partnerships; 4. Adaptable: cities designed with their infrastructure and systems adaptable to changes in new technology and global warming; 5. Transformative: cities designed for empowering their institutions and citizens to define and reposition their growth directions and strategies. This paper focuses on the Resilience aspect of cities under extreme storms. A large city is a system of many highly interactive sub-systems: social needs, transportation, utilities, communications etc. To enhance the resilience of such a complex system, it is essential to (1) identify future critical storm scenarios considering climate changes, (2) evaluate the urban system response under extreme rainstorms using appropriate hazard process modelling algorithms, (3) assess the multi-hazard risks, and (4) formulate risk mitigation strategies. These steps are known as stress testing (Zhang et al. 2017). In this paper, the impact of recent rainstorms and typhoons on Hong Kong and observed propagation of hazards among key infrastructural systems are briefed first. Then plausible rainstorms that may hit Hong Kong in Proceedings of the 7th International Symposium on Geotechnical Safety and Risk (ISGSR) Editors: Jianye Ching, Dian-Qing Li and Jie Zhang Copyright c  ISGSR 2019 Editors. All rights reserved. Published by Research Publishing, Singapore. ISBN: 978-981-11-2725-0; doi:10.3850/978-981-11-2725-0 key6-cd 75