Without multi-hazard combined effect, flooding may occur only at some low-lying areas or spots with poor drainage in Hong Kong even if a rain episode is intense enough to warrant the Amber Rainstorm Warning signal. In contrast, another rain episode with similar rainfall intensity can pose rather different flood risk in the presence of a tropical cyclone (TC). The more persistent the rain is, the higher the cumulative flood risk. Imagine that the Strong Wind Signal, No.3 has been in force with strong winds generally blowing over Hong Kong (even occasional gales over some areas) for some time. A lot of dead leaves, branches, gravels or debris may have flurried to the ground and be washed down to different locations, resulting in potential blockage to the entrances of drainage systems at such locations. Moreover, as the storm approaches, the sea level may rise significantly above the normal position due to storm surge and/or pounding waves, resulting in potential seawater intrusion and flooding over relatively low-lying areas along the coast (Figure 1). This example illustrates how the impact on flooding due to the combined effect of TC and prolonged rainstorm can be way more severe than the totality of impacts from the two hazards occurring individually.