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Cities are built of concrete. The buildings and roads are all made up of this non-absorbing and impermeable material. In a city, therefore, rainwater is usually speedily drained away via sewers. Under extreme weather, however, the drainage system may be overloaded as large amounts of rainwater come in within a short period. In recent years, we have seen more and more extreme rainstorms and storm surges, globally due to climate change. We are facing an increasing risk of flooding and even loss of human life and property in cities. Given this situation, the concept of “sponge city” – managing rainwater discharges by the natural principles – is emerging around the world.
Sponge is a common daily article that is soft and can easily absorb and store water. A “sponge city” incorporates the functions of a sponge in its design – absorbing excessive water during rainstorms, to attenuate flooding risk by regulating rainwater discharge. After filtration and purification, the absorbed water can be utilised as a water resource, unlike typically, with rainwater is discharged into the sea and wasted.
In practice, there are six principles of a sponge city: infiltration, detention, storage, purification, reuse and discharge. Infiltration means making road surfaces more permeable; detention increases water retention such as on green roofs; storage refers to enhancement of water storage capacity like temporary underground stormwater tanks; purification aims to clean rainwater with vegetation boosted by greening facilities; reuse rainwater for toilet flushing, irrigation and car cleaning, etc; discharge is the final step of draining rainwater through the sewers.
Hong Kong has introduced the concept of “sponge city” in new development areas. For example, the newly built Anderson Road Quarry site includes facilities such as underground stormwater storage tank, green roof, and porous pavement. A first-of-its-kind lake park was also built for flood control and leisure purposes. The artificial lake is used for temporary storage of stormwater which can be utilised for irrigation.
In the plan for the forthcoming extension of Tung Chung new town, the concept of “sponge city” was also adopted, including river revitalisation, attenuation ponds and porous pavements. A section of Tung Chung River will be transformed into River Park, the revitalised river will use the natural river bed to facilitate infiltration. In bad weather with rainstorms, the park will turn into a channel for flood attenuation.
A sponge city offers numerous benefits, including reducing flooding risk, increasing water resources, alleviating heat island effects by the increase of greenery, which also provides extra leisure grounds for the public. The concept makes a city safer and more comfortable, and is most suitable for new town developments in Hong Kong. Early and comprehensive planning of infrastructure development will allow Hong Kong to become a more livable city.