Sponge City: Rainwater put into good use

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Thanks to earlier investment in flood management infrastructure, Hong Kong suffered from relatively limited damage when Typhoon Hato struck the region last month. However, the city needs to turn its attention to more sustainable ways of managing excess rainwater in the long run.

(Photo credit: Chris Lusher)

The first Hurricane Signal No.10 since 2012 was hoisted when Typhoon Hato, a Category 3 tropical cyclone, struck the Pearl River Delta Region on 23rd August. Hato dumped up to 25 centimetres of rain per hour and caused a storm surge of over 1.3 metres, which also coincided with a period of high tide. Albeit most of Hong Kong was largely unaffected by flooding, with the exception of low-lying areas such as Heng Fa Chuen, Tai O and Lei Yue Mun.

Flood management in Hong Kong comes in the form of hard engineering techniques that efficiently discharge water during times of storm surge and intense precipitation. Four major drainage tunnels divert surface runoff from the mountainside away from the urban core, and three underground reservoirs have a capacity to store excess rainwater from a rainstorm with an intensity of a 50-year return period.

However, more needs to be done. Rising global average temperatures mean that sea levels are rising and extreme weather events are happening more frequently. Rather than playing a game of catching-up with mother nature, cities should adapt to the changing environment. Sponge City is a concept that defies the traditional notion of cities being composed of impermeable surfaces. Rather than simply draining out excess precipitation into the sea, cities can act as a “sponge” that absorbs rainwater—imitating natural processes of infiltration.

Typhoons and monsoons can soon become our friend instead of foe. Recycling rainwater is the first step to making Hong Kong more sustainable. We receive almost 2,400 millimetres of rainfall annually, but the majority of this body of water is wasted through evaporation or drained into the Harbour. Meanwhile, we have signed on a HK$13.4 billion deal to import 80% of our water from Guangdong Province for the next three years.

Instead of relying on multi-billion dollar trade deals and investment in large-scale infrastructure projects, a Sponge City can be created from the bottom-up. This can begin with enabling buildings to generate their own water supply, with the long-term goal of making them self-sufficient. Singapore has already begun with a complex rainwater collection system at their Changi International Airport. The airport, which is one of the world’s busiest, captures 28% to 33% of its water from the skies. Elsewhere across the city, green roofs and capture systems are installed in high-rise buildings.

In Berlin, an entire neighbourhood was built with the natural water cycle in mind. Rummelsberg, a residential area in the eastern part of the city, has a dense cluster of apartment blocks not dissimilar to many neighbourhoods in Hong Kong. But instead of piping away rainwater that falls atop these roofs down to the street, extensive green roofs temporarily absorb the water. It then flows down to the courtyard, which sits on top of a layer of permeable soil.

At street level, swales collect excess water from roads and pavements, which are paved with more permeable materials. As a result, Rummelsberg is capable of handling heavy rainfall even without the presence of conventional drainage systems.

By retaining rainwater in soil and vegetation, cities can counter the urban heat island effect. Evapotranspiration from extensive greenery can help substantially lower street level temperatures, acting as natural air-conditioning and reducing energy consumption. Taking this one step further, popularisation of urban farming can help reduce the carbon footprint of the modern food production system, which is highly mechanised and industrialised.

The benefits of transforming Hong Kong into a Sponge City are plentiful. Aside from mitigating flood risks, urban areas can become a more pleasant environment to live and work in. Achieving this though would require active government and community involvement.

The government can first target public housing estates and new development areas, which are technically and legally less challenging to manage. Unlike the dense urban core, where average sizes of building plots are relatively small and tightly packed together, planned new towns were built with large open communal spaces. These spaces have the potential to be transformed into an urban oasis by creating actual greenery and urban wetlands (not a large paved concrete plaza scattered with a few trees that is so commonplace across the city).

In the city centres of Hong Kong Island and Kowloon, the government can consider increasing the permeability of paved spaces. Roads can be laid with porous asphalt or pervious concrete, which are able to bear frequent traffic whilst allowing rainwater to infiltrate into the ground. Pavements can be built with interlocking permeable concretes or permeable clay bricks, relieving pressure from roadside storm drains.

The final stage would be getting the private sector on board. The biggest hurdle to overcome is the dilemma between retrofitting dilapidated old buildings and complete redevelopment. Many of the city’s ageing buildings were built in an era when “sustainable design” was unheard of. Installing green roofs and rainwater collection facilities may require large amounts of investment. It would take enormous political will and effort to convince developers and the public that the cost of eco-friendly design is worth it in the long run.