Engineering Measures to Reduce Flash Floods at Bukit Timah Canal and Ulu Pandan Canal Areas

The following question stood in the name of Mr Christopher de Souza –

To ask the Minister for Sustainability and the Environment (a) what caused the flash floods in and surrounding the Bukit Timah-Dunearn Canal and Ulu Pandan Canal on 17 April 2021; (b) whether the overflow of water from the canals onto the roads and the park connectors, respectively, can be prevented; and (c) what engineering measures will be put in place to mitigate against such overflows in the future such as to make the areas safe for residents, pedestrians and motorists during heavy downpours.

Deputy Speaker, Sir, on 17 April 2021, the Sumatra squall, coupled with convergence of surrounding winds arising from the influence of typhoon "Surigae" near the Philippines gave rise to intense and prolonged rainfall across Singapore. The highest rainfall that day was at Ulu Pandan with 170.6 millimetres of rain. This is the highest daily rainfall recorded in April since 1980 and is equivalent to 110% of Singapore’s average rainfall for the whole month of April, falling in a single day. This rainfall resulted in flash floods occurring at Bukit Timah and Dunearn Roads near Sime Darby Centre and a stretch of the Park Connectors along Ulu Pandan Canal.

With your permission, Deputy Speaker, may I display some slides on the LED screens.

This is a map of the stretch of roads that I mentioned in my reply. Just a bit of landmark, Sime Darby Centre is the block marked in black.

The water levels in Bukit Timah Canal were very high due to the intense and prolonged rain, impeding discharge from roadside drains along Dunearn Road and Bukit Timah Road into the canal. This resulted in floods of around 150-millimetre depth at Dunearn and Bukit Timah Roads, and around 50-millimetre depth on the pedestrian sidewalks adjacent to the roadside drains. All affected roads remained passable to traffic and the flash floods subsided within 30 minutes. PUB Quick Response Teams were on site directing traffic and assisting affected residents by issuing inflatable flood bags and portable flood barriers to prevent floodwaters from entering their homes.

The Public Utilities Board (PUB) had completed the upgrading of the Bukit Timah First Diversion Canal – this is the stretch marked in blue – between Bukit Timah Road and Clementi Road in September 2019 at a project cost of $300 million to alleviate flood risks along Bukit Timah Road and Dunearn Road. Members who have been on the Rail Corridor hike would not miss this impressive infrastructure. It is somewhere after you have crossed Bukit Timah Railway Station and another 500 metres or so, you will come to this stretch, which opens up to the area at Old Holland Road.

So, it is impressive because it is large and deep. And I think it has helped significantly on this day of 17 April, because the flood on this day is nowhere near the earlier floods that we had seen along the same stretch of roads when lesser rain fell.

Water level sensors on this stretch – the sensor is placed where the yellow triangle is located – installed along the upgraded stretch of Bukit Timah Canal from Jalan Kampong Chantek to Sixth Avenue, just upstream of the expanded Bukit Timah First Diversion Canal, showed the section of the canal was 50% full, 50% full, on 17 April with no flash floods along that stretch despite the heavy rainfall.

PUB started the upgrading work on the remaining section of Bukit Timah Canal between Rifle Range Road and Jalan Kampong Chantek – this is the stretch marked in red – where the localised flash flood occurred, and is expected to complete it in 2024. When completed, we expect the top water level in this section of the Canal to be lower by over one metre for the same rainfall intensity experienced on 17 April.

Another stretch, which is along Ulu Pandan Canal, a low-lying 300-metre stretch – as marked in red – of the Park Connector next to the Ulu Pandan Canal was also flooded on 17 April even though major roads – the major roads are Clementi Road in grey, Clementi Avenue 6, Commonwealth Avenue West and the Ayer Rajah Expressway – served by the Canal did not experience flooding. PUB will raise this low-lying section of the Park Connector when PUB reconstructs the Ulu Pandan Canal in tandem with future developments.

Since 2011, the Government has invested almost $2 billion in drainage works and will invest another $1.4 billion over the next five years to enhance our flood resilience. Drainage design standards were also raised in 2011 to cater for higher intensity rainfall, as part of PUB's broader efforts to address the impact of climate change.

It is, however, not practical to expand our drains to accommodate every extreme rainfall event as this would require massive land take and much higher costs. PUB will put in place cost-effective measures to minimise flood risks and better manage flood events. For instance, PUB is currently upgrading its rainfall forecasting radar technology, which will help improve prediction of locations where heavy rainfall might occur and enhance response time to potential flash floods.

PUB also works closely with LTA and Traffic Police during heavy storms to monitor road conditions and notify motorists of flooded roads to avoid through electronic road signages, online and social media platforms and radio broadcasts. Members of the public can be alerted on flash floods by subscribing to the National Environment Agency's and PUB's weather forecast and water level SMS alerts or access the mobile app, "myENV", for updates on impending heavy rain and the water level in drains at designated locations.