Large-scale Urban Drainage Modelling for Hong Kong Island

The rising likelihood and severity of flooding caused by climate change heightens the demand for detailed and precise hydrodynamic models. Flood modeling commonly involves analyses done either in a simplified watershed-scale 1D model, a detailed local-scale 2D model, or a combination of both approaches. However, the laborious setup and high data requirements hinder detailed watershed-scale modeling, particularly in urban areas like Hong Kong, where intricate drainage systems pose additional challenges. Alternative methods have been developed to consider underground drainage capacity such as subtracting the water volume held by pipes from the surface runoff and representing it with an equivalent infiltration rate. However, these methods do not give sufficient information on the flow movement underground. Enabled by Hong Kong’s extensive datasets, this study attempts the integration of the digitized urban drainage system into a watershed-scale hydrodynamic model. Hong Kong Island, with a land area of 99.5 square kilometers, is set as the study area. This domain covers 35,259 conduits, 32,622 junctions, and 31 rain gages. A case study is adopted using the September 2023 black rainstorm event to demonstrate the model’s capability to map surface flood inundations and describe the dynamics of the underground drainage system at once. Observed flood depths during the event are then used for results validation. Large-scale urban drainage models like this may aid decision makers in flood risk assessment and emergency action planning.