China is exploring the idea of building “sponge cities” to solve the problem of urban flooding. Hong Kong has used the concept in a decade of infrastructure upgrades.

Water-logging has plagued many cities in South China following this year’s frequent heavy rainstorms but not Hong Kong, where some of the excessive rainwater is absorbed by large underground storage tanks.

“The operation mechanism is based on real time water level form four sensors. Any rainfall that excess the handling capacity of the downstream drainage network will be temporarily stored in this water tank. Altogether its capacity is 60,000 cubic meters of water, that’s equivalent to water in 24 standard swimming pools,” said Kan Hon-Shing, chief engineer of Happy Valley Stormwater Storage Scheme.

Other water is intercepted and diverted into the sea through massive drainage tunnels.

“The storm water in the uphill areas will be collected by the drainage tunnel and bypass the urban area and is discharged to the sea directly via the tunnel,” said Robin Lee Hong-Nin, Sr. engineer of Lai Chi Kok Drainage Tunnel Scheme.

The city now has three underground storage tanks and four drainage tunnels operating together with enhanced traditional drainage system.

Hong Kong's location puts it in the path of many tropical cyclones. And its annual rainfall is among the highest of cities around the Pacific. Urban flooding was once a serious threat. But not anymore, thanks to facilities which make the city like a big sponge that can quickly and easily absorb surface water.

The overall flood control strategy has three parts: interception upstream, temporary storage midstream, and upgrading pipes downstream.

With that, flooding has been drastically reduced over the past decade. Hong Kong’s next goal is better water recycling.

“We are moving forward to build resilience of this city and we are thinking more about sustainable drainage system. We are talking about “blue green infrastructure” similar to the sponge city concept in the mainland. We encourage the infiltration, storage of storm water and also we encourage reuse of the water,” said Richard Leung Wah-Ming, Sr. engineer of HK Drainage Service Department.

Examples are: Water infiltrated from the Happy Valley storage tank that has been used to irrigate football pitches. And water from drainage tunnels in Lai Chi Kok is used for flushing toilets and cleaning the roads.