A stress test of urban system flooding upon extreme rainstorms in Hong Kong

Abstract

Many cities face high flood risks due to intense rainstorms and dense urban environments. Quantifying the degree and duration of flood impact to urban systems under extreme rainstorms is essential for long-term flood risk management and short-term emergency response. This study aims to develop a novel stress testing framework for evaluating urban system flooding upon extreme rainstorms in Hong Kong. The stress testing framework is based on a pipe-surface coupled hydraulic model for detailed flood simulation in urban settings and a dynamic flood impact evaluation method for assessing interruptions to urban transportation and building systems. The proposed stress testing method is applied to assess the flood impacts in Kowloon, the most densely populated area in Hong Kong. Detailed urban settings such as engineering drainage facilities and buildings are incorporated into the flood simulation model, and the model is validated both qualitatively and quantitatively by field observations. Flood hazard scenarios are investigated under design rainstorms of different return periods (20, 100, and 1000 years), and the impacts on transportation and building systems are evaluated with the new process-based impact evaluation method. The results of the case study indicate that underground mass transit railway (MTR) stations and residential buildings will be more affected than highways and other types of buildings in Kowloon: up to 56% of MTR stations and 43% of residential buildings may be affected under the 1000-year rainstorm, with inundation durations of 8 and 7 h for the MTR stations and residential buildings, respectively. To ensure public transportation services and reduce potential exposure of residents during extreme rainstorms, mitigation measures should be taken to reduce the affected percentage and duration of MTR stations and residential buildings. The proposed methods contribute to enhanced societal flood preparedness, warning, and response.