Extreme weather events are becoming more frequent, which can make urban areas extremely unsafe.

In China's Hong Kong Special Administrative Region (HKSAR), the majority of public transportation has resumed after earlier rains delivered by Typhoon Haikui pounded the area, flooding roads and public spaces.

What many people might not realize is that they took place despite Hong Kong's reasonably good drainage system. The Hong Kong Western Drainage Tunnel (HKWDT) has been completed in order to minimize flood damage to the most densely populated areas in Hong Kong. According to the Drainage Services Department (DSD), with a total length of 10.7 kilometers and an excavation diameter of 8.2 meters, the tunnel is the longest drainage tunnel in Hong Kong.

The tunnel spans deep below through the western spine of Hong Kong, designed to reduce Hong Kong's periodic flooding concerns. The tunnel collects excess surface runoff from the Mid-Levels area and above, and distributes it directly into the sea, avoiding the densely populated areas below.

Happy Valley Underground Stormwater Storage Scheme (HVUSSS) is the first flood prevention project in Hong Kong to use a Movable Weir and a Supervisory Control and Data Acquisition (SCADA) system to monitor tidal levels, water levels inside the storage tank, and box culverts immediately upstream and downstream of the tank in real time. The Movable Weir is positioned based on real-time monitoring data so that stormwater overflow from the box culvert to the storage tank can be triggered at the best time.

According to DSD, the construction on the HVUSSS comprised a 60,000 cubic meter underground stormwater storage tank underneath the Happy Valley Recreation Ground, a 650-metre-long box culvert, and a stormwater pump house with a peak flow rate of 1.5 cubic meters per second.

There's also a lot to learn from other cities. Rotterdam, in the Netherlands, has been constructed on river deltas and hence are vulnerable to floods. During times of heavy rain, "water squares" employ public open spaces to retain flood waters. On dry days, these urban sinks can be used as hangout spots, and in heavy rain, they can store large volumes of water, keeping flood water away from properties.

This is very similar to the approach taken in Sydney's Victoria Park in Australia, where parks are all at a lower elevation than surrounding streets. This allows the temporary storage of stormwater.

Large-scale rainwater catchment systems, used in Denmark, efficiently collect rainwater and discharge it into an artificial lake, where it undergoes a water treatment system to meet standards, and then enters the homes through a separate plumbing system for use in washing machines and toilets.

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