Singapore flooding: Drain system 'must grow along with city'

Singapore needs to find long-term solutions to floods, say experts
Amresh Gunasingham Straits Times 23 Jan 11;

Singapore's drainage system will have to grow with urbanisation if it wants to alleviate floods like the ones that hit Orchard Road unexpectedly last year.

Experts say a review of the country's infrastructure of drains and canals is needed, and long-term solutions have to be sought. This is especially so since past guidelines used here to design drains seem to have fallen short.

For example, an assumption used by planners when deciding on the size and capacity of a drain was to look at the rainfall patterns around the catchment area it serves.

It has been assumed that the rain is spread evenly over the area all year round. But bursts of rain over a short period, a trend climate scientists say is increasingly evident, were not factored in.

Said Associate Professor Tan Soon Keat from the Nanyang Environment and Water Research Institute at Nanyang Technological University (NTU): 'The distribution of rain tends (now) to be around smaller concentrated places, over a shorter period of time and of a larger quantity.'

Given that weather patterns need to be observed over several decades to spot a trend, it could take several decades to build up a database that factors in the changing climate here, said engineers.

Last year's Orchard Road floods provide a stark reminder of the vulnerability Singapore continues to face from random 'acts of God'.

Two intense deluges in June last year flooded the prime shopping belt with a magnitude not seen in decades.

Millions of dollars in goods and property were hit.

National water agency PUB came under intense public scrutiny. Questions were raised about the adequacy of drains to protect the rapidly urbanising landscape here.

It did not help that, in the immediate aftermath of the floods, the agency pointed to a blockage in a section of the 4km-long Stamford Canal that runs through the area as the cause of the floods.

That initial assessment puzzled experts The Sunday Times spoke to. They said the drains here are designed to be large enough to account for debris such as tree branches.

'Even if the drain was choked, the torrent that flows through the canal during a storm would push through (the debris),' said a former PUB engineer, speaking on condition of anonymity.

Most drains in Singapore are designed 'conservatively' to be up to 20 per cent larger than the capacity needed to cope with the heaviest rain recorded in any given catchment area, he added.

Such data are based on weather patterns dating back 100 years.

In fact, despite such built-in reserve capacity, a post-flood review of the Stamford Canal found that its capacity was inadequate to cope with the actual deluges last year.

Thus armed, the PUB hopes a project now under way to raise a 1.4km stretch of Orchard Road that runs diagonally to the canal will help to keep the floods away.

The agency is also working with buildings in the area, such as Liat Towers, to put in place flood barriers that can keep storm water away.

The Government is also reviewing the entire network of drains and canals to 'see what more can be done to increase Singapore's level of protection against flood risks', said a PUB spokesman.

'This review is ongoing and details will be shared at an appropriate time,' she added, declining to reveal more.

In the meantime, planning ahead could, at times, be reduced to a guessing game.

NTU's Prof Tan, who has been consulted by the authorities on drainage projects here, added that the network of drains and canals has to be looked at in totality.

'A catchment area such as Orchard will have a lot of 'subcatchments' which drain into the main drain at certain points,' he explained.

In the event of an unusually intense storm, if the drains' 'downstream and upstream are at a peak, it could result in hydraulic choking', he said. This is akin to three adults trying to squeeze through a door large enough for only one person.

Such potential 'choke points' can occur in areas that have been rapidly urbanised, meaning drains further downstream have to be built large enough to cope with a higher run-off.

'If everything works like a clock, then there are no issues. But some coincidental factors could come into play,' said Prof Tan.

The PUB spokesman explained that these factors had been considered in its review of Orchard Road's drains, leading to the decision to raise a stretch of the road.

Work also started last November to deepen Bukit Timah Canal, a project which is being done in two phases. The first phase involves widening a stretch between Jalan Kampong Chantek and Maple Avenue, and should be completed by the end of next year.

The second phase will see an upgrading of the canal between Bukit Timah Road and Clementi Road. Work should start by the end of the year.

PUB has invested $2 billion in the past 30 years to upgrade drainage infrastructure across the island to alleviate floods. It will be investing $150 million a year for the next five years.

This has helped reduce flood-prone areas in Singapore by 98 per cent, from 3,178ha in 1970 to about 58ha today. It hopes to better this figure to 40ha by 2013.