A large city surrounded by mountains like Hong Kong is susceptible to both flash floods in midmountain areas and local flooding in low-lying areas when a large rainstorm occurs. Severe rainstorms tend to be more frequent under a changing climate. Hence, it is essential to simulate rainfall-runoff processes in the urban area and evaluate flood coping strategies. This paper presents a model that is capable of simulating the overland flow process and underground drainage hydraulics simultaneously, based on detailed topographic, rainfall, and drainage network information about the northern Hong Kong Island. The model was calibrated with a flood-extent map and further validated with a flood event in June 2008. Using the model, the flood hazards associated with flood coping projects in three phases were simulated. Although these flooding control measures were intended to meet a major societal need for alleviating flooding problems and are shown to be effective in alleviating local flooding, the densely urbanized area may still suffer flooding when severe storm events occur. Hence, new strategies are needed to cope with the urban flood risk under more critical storms.