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Singapore to look into underground reservoirs

SINGAPORE — Flood water from intense storms here that would have otherwise gone to waste could eventually be turned into another water and even power source.

People exercise at MacRitchie Reservoir. Could Singapore soon have underground reservoirs? TODAY file photo

People exercise at MacRitchie Reservoir. Could Singapore soon have underground reservoirs? TODAY file photo

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SINGAPORE — Flood water from intense storms here that would have otherwise gone to waste could eventually be turned into another water and even power source.

National water agency PUB is exploring using underground space to collect and store excess storm water. It will call for a tender for a 24-month study on the technical and economic feasibility of developing an underground drainage and reservoir system.

The study will explore design options for the system, such as stormwater conveyance tunnels to transport water to an underground reservoir cavern. There could also be a pumped storage hydropower station that could convert kinetic energy of the water flowing in the tunnels to electricity.

Expected to be completed by the end of 2017, the study will also include a geological survey to find a location with suitable soil and rock properties.

Announcing this at the Singapore International Water Week (SIWW) Technology and Innovation Summit yesterday, Minister for the Environment and Water Resources Vivian Balakrishnan said Singapore receives about 2.4m of rainfall a year. But because of a shortage of land to capture and store rainfall, much of it is discharged into the sea. At the same time, prolonged droughts dry out the current reservoirs, leading to water shortage.

“Climate change increases the probability of both intense rainfall and prolonged periods of drought,” said Dr Balakrishnan in a speech. “ ... Consequently, an underground drainage and storage network will mitigate the impact of climate change and enhance Singapore’s drought resilience, whilst overcoming our land limitations.

“We will be able to take the excess rainfall from the storms, convey it to underground storage, harvest its energy as it percolates through those tunnels and reuse that energy later when we need to pump the water out again.”

Civil and environmental engineering experts TODAY spoke to said the caverns could be as big as five million to 50 million cubic metres, and finding the most cost-effective way of building this underground system would be the biggest challenge.

“There are no special challenges compared to typical tunnelling and underground excavation work,” said Associate Professor Tan Soon Keat, director of Nanyang Technological University’s Maritime Research Centre. “The challenge is more on the balance sheet.”

Some experts felt the reservoir should be designed as one big central cavern, while others said smaller, multiple caverns spread across an area was more cost-effective. They also differed on how deep the reservoirs should be.

Professor Lui Pao Chuen, who is adviser to the National Research Foundation and who first floated the idea in 2012, said there could be multiple caverns, starting with a first “module” of five million cubic metres and having its capacity increased over the years.

Prof Tan said keeping it relatively near the surface at 30m to 40m underground would be a good depth to move the stored water freely.

“The longer it (takes water) to pump up, the more friction you have to overcome,” he added. “Whereas in a shallower surface, there is still friction but it is manageable.”

On the other hand, Assistant Professor Chew Soon Hoe from the department of Civil and Environmental Engineering at the National University of Singapore said it may be more cost-effective to build a big central cavern. Conveyance tunnels to transport water from different areas can be built cheaper at shallower depths, he said.

A permanently half-filled underground reservoir, if located at the fringes of the more densely-populated central areas of the island, would also be useful for collecting excess storm water and alleviating floods where economic costs would be the highest. But that would require a sizeable cavern and greater depths of 100m to 150m underground, Asst Prof Chew added.

While more power is needed to bring up water from greater depths, the hydropower that could be generated from the system could come in handy. “If you need to supply power, it’s very costly,” he said. But if the water going in can generate power supply, then “the newly-generated power to pump the water up to the reservoir … would be ideal”.

Given that the caverns would require rock that is not porous, experts suggested potential sites in central and north-eastern Singapore. Prof Lui said the rock mass west of Bukit Timah Expressway may offer a few potential sites.

As for concerns about the extent of underground digging done on the island, Asst Prof Chew gave the assurance that engineers would be able to mitigate any impact on the surroundings with proper design.

“The underground still promises a great opportunity for bigger exploration ... We can afford to build a lot more at a greater depth,” he added.

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