Science Talk

How we may tame storm water

The Stamford Diversion Canal, which is under construction, is part of a holistic approach to protect areas such as Orchard Road from floods.
The Stamford Diversion Canal, which is under construction, is part of a holistic approach to protect areas such as Orchard Road from floods.PHOTO: PUB

The hundreds of aspiring Wayne Rooneys who practise at Opera Estate Football Field are oblivious to the fact that they conduct their kickabouts above a massive stormwater detention tank. Similarly, the thousands of visitors to Singapore's sole Unesco World Heritage site do not know that tour buses are spewing them out above an even more gargantuan stormwater tank.

The Singapore Botanic Gardens' new coach park serves as a roof for the Stamford Detention Tank - essentially a very large rectangular concrete box that extends 28m into the ground, and big enough to temporarily restrain 38,000 tons of rainwater, enough to fill about 15 Olympic pools.

Both the Stamford Detention Tank and the Stamford Diversion Canal - which will eventually empty the contents of the tank into the Singapore River - are nearing completion and will be our insurance against a repeat of the Orchard Road floods of 2010 and 2011.

It can rain a lot in Singapore.

More than 124mm of rain fell in a few hours on June 5, 2011. Singapore's average annual rainfall is around 2300mm. So on that fateful day, the equivalent of three weeks' worth of precipitation got dumped just upstream of our famous shopping street in one go, with dire consequences.

No number of conventional drains can cope with that kind of rainfall intensity. Hence a new approach was required to ensure that there would be no encore of the deluge five years ago.

  • About the writer

  • The Stamford Diversion Canal, which is under construction, is part of a holistic approach to protect areas such as Orchard Road from floods.Mr Ng Joo Hee, 50, is chief executive of PUB, Singapore's national water agency.

    He is responsible for the supply of potable water, the reclamation and treatment of used water, and the management of storm water in Singapore.

    He also holds a concurrent appointment as the deputy secretary (special duties) in the Ministry of the Environment and Water Resources.


To tame storm water effectively, the method that Singapore's national water agency, PUB, has since adopted is a far more holistic one that embraces much more than "plain vanilla" drains and canals. This so-called "source-pathway-receptor" approach dictates that flood protection does not just encompass drains and canals (the pathways), but also upstream areas generating stormwater run-off (the source), and parts downstream which might be flooded (the receptors).

The whole idea is to slow the accumulation of storm water at every stage, even during the heaviest bout of rain, by detaining, retarding, delaying and diverting it, so that it does not overwhelm our drainage infrastructure.


Source measures are best explained by referring to any greenfield location that has been given over to development. Much of the rain that falls on a field or vegetated area will soak into the ground. Once an area is built on, however, it becomes impervious and starts generating surface run-off instead.

It is well established that imperviousness is positively correlated to the frequency of flood recurrence. The trick then is to reduce the run-off coefficient, or the fraction of rainfall that is converted to run-off, at its source.

Since 2014, PUB has required real estate developers to implement measures to reduce run-off from their plots by up to 35 per cent. The less imaginative may just put in a detention tank, but green roofs and bioswales (landscape elements used to slow, collect and filter storm water) can also help, and are prettier to boot.

Good examples include the garden on top of Orchard Central Mall, the Housing Board's Greenwood Sanctuary @ Admiralty, and the Balam Estate rain garden.


It doesn't just rain a lot in Singapore. We expect that, increasingly, it will come all at once. Climate change may also parch Singapore but, at the same time, studies suggest a trend towards higher rainfall intensities, and an increasing frequency of high-intensity rain events.

Do more intense storms call for bigger drains? Upsizing every drain and canal is impossible, and would be prohibitively disruptive and expensive. And by themselves, larger storm drains will not keep Singapore's streets dry. But upgrading drainage infrastructure in tandem with source and receptor enhancements just might.

After the floods of 2010 and 2011, PUB raised drainage design standards markedly in preparation for more intense storms. The new standards apply equally to public and private developers. On our part, PUB embarked on a drain improvement binge. Since 2012, we have commenced works, large and small, to improve and expand drainage at more than 300 locations all over our island. Most of these have already been completed.

The re-naturalisation of the Kallang River in Bishan-Ang Mo Kio Park, previously an ugly concrete canal, must be the most well-known of these projects. Although most people would look at it as parkland beautification, it was, first and foremost, drainage improvement.

The Kallang River is Singapore's longest river, connects Lower Pierce Reservoir to Marina Reservoir, and is a crucial stormwater pathway. With her own flood plain in Bishan-Ang Mo Kio Park, that stretch of the Kallang River is now able to provide more flood protection than ever before.


When we get to the receptor end, storm water mitigation usually comes in the form of more "brute-force" applications, like raising an entire road or the platform of a new building. The idea here is to give flood waters no chance of affecting transport or dwelling.

This may sound somewhat extreme but for certain topographies, they are the best and only way. For naturally low-lying areas, such as parts of Bedok and Siglap, which are inevitably vulnerable to flooding when heavy rain coincides with high tides, raising platform levels is the most obvious means of flood protection.

More generally, it just makes sense for property owners to have the entrances to their basements and underground facilities fully shielded from flood water.

The best illustration of receptor mitigation is perhaps the Marina Barrage. Curiously, after the Orchard Road floods, public blame was assigned to the newly completed barrage, so much so that this became something of an urban myth.

Nothing is further from the truth. The barrage did not and could not have caused Orchard Road to be flooded. Marina Barrage was conceived of and functions as a flood alleviation scheme - to eliminate the influence of high tides on drainage in low-lying areas in the city and to release excess storm water from its catchment. Pop into the visitor centre at the barrage and this becomes amply clear to anyone watching the scale model there in motion.


Everyone has a stake and a role in flood protection. Drains and canals are not public bins and we must never throw trash into them because they need to be free flowing in order to carry storm water away, and because this storm water will eventually be turned into our drinking water.

The Orchard Road floods and the damage they caused have a silver lining however: As harbingers of climate change, they spurred early action. The predicted climate effects - record-high temperatures, irreversible sea level rises, and significantly dryer weather but with increased frequency of the most intense rain - are frightening and, if ignored, will absolutely devastate both our water system and drainage infrastructure.

Our only hope, as always, is to be clear-eyed about the challenge facing us and respond in an intelligent and coordinated fashion. The only way that we may possibly defend ourselves against the droughts and the floods to come is for all parties to work hand in hand.

A version of this article appeared in the print edition of The Straits Times on November 18, 2016, with the headline 'How we may tame storm water '. Subscribe