ST : Work to expand canals next year

Nov 25, 2009
Work to expand canals next year
Capacity of Bukit Timah Canal will be doubled from Wilby Road to Sixth Avenue
By Amresh Gunasingham

A DIVERSION canal which overflowed last Thursday and flooded parts of Bukit Timah Road will be expanded over two phases in the next three years, said the national water agency yesterday.

The tender has closed for the first phase, which will be a $20 million facelift to double the capacity of the main Bukit Timah Canal at its juncture with the diversion canal. This is the portion stretching from Wilby Road to Sixth Avenue.

A floodgate located at the Sixth Avenue junction to divert water from the main canal into the three-decade-old diversion was unable to contain rising water levels from 92mm of rain being dumped over half an hour. The rainfall is the second-biggest in the last two decades over the same period, following the 96mm in November 1995.

This caused water levels in the main canal to rise and breach the low-lying stretch from Third Avenue to Coronation Road.

PUB said construction would probably start in the third quarter of next year, and take two years.

The second phase will see the 3km diversion canal, stretching from Sixth Avenue to Sungei Ulu Pandan, widened in parts from 11m to more than 20m. It will also be deepened from the present 4m. Work on this phase is expected to start in two years.

The PUB will also install a sensor system along the Bukit Timah Canal near Blackmore Drive next week, to provide early warning to police and nearby developments if the canal's water level is rising.

Last Thursday, at the flood's height - between 1pm and 2pm - the floodwater was knee-deep, throwing traffic in Bukit Timah into chaos.

Property and cars were damaged as three underground carparks were partially submerged.

The diversion canal was built in 1972 and sized according to how much development there was in the area as well as economic considerations, said Mr Tan Nguan Sen, director of catchment and waterways at PUB, yesterday.

'We did not want to build oversized canals that would not be used to their full capacity.'

Since then, housing and infrastructure developments have burgeoned on the road, a key factor in the volume of water flowing over the surface.

Land scarcity limits the size of drainage systems so they cannot cater to every extreme event, said Mr Tan.

To cope with last week's volume of rainfall, the diversion canal would have to be 30m wide.

The buildings which had their basement carparks submerged are hoping to avoid a similar occurrence in future.

Corona Ville condominium, one of the three, has a drainage system linked directly to the diversion canal which overflowed.

PUB is in discussions with its management to install pump systems in the carpark. It is also talking to the Holland-Bukit Timah GRC about installing physical barriers such as sandbags to prevent water flowing into other developments.

The flood comes three years after one of Singapore's worst floodings in recent history. In December 2006, 345mm of rain fell over a 20-hour period, the third-highest in the last 75 years. Parts of the island from Thomson Road to Yio Chu Kang were submerged and landslides were triggered in Mandai Road and Bukit Batok West Avenue 2.