Minister for Environment and Water Resources Masagos Zulkifli stated that it is not feasible to be building drains to accommodate every extreme rainfall event as this would require massive land take and much higher costs.
Speaking during Parliament on Monday (5 February), Mr Masagos responded to questions filed by five Members of Parliament amid heightened public concerns over the recent flash floods.
MP for Nee Soon GRC Mr Kwek Hian Chuan Henry asked whether the flood prevention systems will be able to prevent flash floods similar to those that occurred on 8 January 2018 after the completion of the drainage upgrades in 2019, what are the guiding principles used to determine the drainage capacity needed in an area, and whether these include balancing the potential collateral damage versus the investment cost.
MP for Fengshan SMC Ms Cheryl Chan Wei Ling asked with regard to the flash flood on 8 January 2018 at Bedok North Avenue 4 (a) whether the ongoing enhancement drainage works at Bedok Canal was one of the reasons that limits the water capacity that can be stored or discharged through the canal during heavy downpours and high tides, if so, what are the interim measures to prevent potential flash floods at this area until enhancement works are completed in 2019, and what measures are in place to ensure the public living and working near any canals are alerted before possible water overflow occurs.
MP for Tampines GRC Ms Cheng Li Hui asked what current mechanisms are in place to notify motorists of impending flash floods in order to redirect traffic from affected areas and whether such mechanisms are can be further enhanced to achieve broader and more effective notifications.
NCMP Mr Dennis Tan Lip Fong asked what caused the heavy flooding at the nine locations in the eastern part of Singapore on 8 January 2018, whether each location has a history of flooding, whether existing drainage works is a cause, what measures have been taken to assist members of the public who were affected by the flooding, and what measures will be taken to prevent similar flooding.
MP for Bukit Batok SMC Mr Murali Pillai asked how does PUB ensure that existing drains and canals are kept free of debris that have potential to create constrictions or bottlenecks, so as to alleviate flooding.
The minister noted that on the morning of 8 January 2018, the prevailing Northeast Monsoon, aggravated by the development of a Sumatra squall gave rise to intense rainfall across several parts of the country with the highest recorded total rainfall that morning was 118.8mm, which means that about half of Singapore’s average monthly rainfall in January fell over just four hours.
He said that eight of the nine locations that experienced flash floods that morning are low-lying and therefore susceptible to flash floods. Two of these, Tampines Road (opposite Jalan Teliti) and Arumugam Road, have a history of flash floods.
According to the minister, PUB is already carrying out drainage improvement works in these eight locations to enable the drains there to discharge more water in a shorter time when completed.
Ms Masagos said that during drainage improvement works, PUB ensures that the drainage capacity of the area is maintained at the original level before works commence, noting that drainage works did not cause the flash floods in these locations.
He then added that for the ninth location at Tampines Avenue 12, apart from intense rainfall which exceeded the drain’s design capacity, a temporary construction access road built by the contractor had obstructed the earth drain in an adjacent worksite, which aggravated the flash flood.
The minister said that PUB has worked with the developer to improve the drainage at the earth drain and this will help to improve the situation at Tampines Avenue 12 in an event of intense storm. The longer-term measure is the permanent drainage system which will be built in tandem with the upcoming development project at the adjacent worksite.
He stated that the flash floods were caused by the intense rainfall temporarily exceeding the existing design capacity of the drains.
"Although the flood waters affected only certain stretches of the roads, and subsided within 15 to 60 minutes, we acknowledge that members of the public were inconvenienced and a number of cars were stalled. When flash floods occur, we urge members of the public to exercise caution and avoid traversing in submerged areas even if they appear safe. Flash floods are usually of short duration and it is better to wait for them to pass before continuing the journey," he said, adding that to aid the public, PUB, together with Traffic Police and LTA, will help divert vehicles at the earliest possible.
The minister then said that PUB takes into account terrain, the extent and type of developments in the area, catchment area served by the drain, as well as the design rainfall intensity over the catchment, which is standard international practice.
"However, with climate change, we can expect more intense rainfalls to be the norm in future," he stressed, citing a study by the Potsdam Institute for Climate Impact Research who found that flood risks from rainfall changes will increase in the next two to three decades, due to global warming from greenhouse gases already emitted into the atmosphere.
"The need for adaptation is significant, even in countries with good infrastructure such as Singapore," he added.
Mr Masagos then said that PUB has raised the drainage design standards since 2011, so that the drains can handle up to 45 percent higher rainfall intensities, adding that these new standards were endorsed in 2012 by a Drainage Expert Panel comprising local and international specialists.
According to the minister, PUB has commenced and completed drainage improvement works at 327 locations since 2012.
"There are another 73 locations undergoing similar works, with 22 more planned this year. The Government has invested $1.2 billion on these works, and has set aside another $500 million for the next two to three years," he added.
Mr Masagos noted that Bedok Canal, which serves some of the affected areas, is being widened at a cost of $128 million from its existing width of 38 metres to 44 metres, wide enough to accommodate an expressway of 10 lanes, 5 lanes each way.
"Even with this widening, we cannot guarantee that there will be no floods in future, as rainfall events of even higher intensity could still occur that exceed the design capacity. This is especially so with climate change," he added.
He then said that to deal with the most extreme historical rainfall events, the Bedok Canal would need to be widened to at least 62 metres, displacing the Bedok Park Connector and community spaces adjacent to the Canal, and possibly even affecting the surrounding residential areas.
"This will look like a 16-lane expressway," Mr Masagos noted.
"Given competing needs for other land uses such as housing, parks and roads, we have to be practical in our drainage expansion in Singapore. We have to design with practical considerations and not for extreme conditions all the time for all places," he said.
"Building our drains for extreme conditions would mean that much of the capacity would be extremely costly, but not needed most of the time. We will, however, ensure our critical infrastructure is well-protected from such extreme rainfalls. This is achieved through the “source-pathway-receptor” approach which was developed based on the recommendation of the Drainage Expert Panel to implement a holistic range of interventions," he added.
The minister then said that under this approach, other than improving the pathways, or drains, the Government also manage the source, which refers to where the rain falls, adding that measures include detention tanks to slow down surface runoff and reduce the amount of stormwater entering the drains during peak rainfall.
"For instance, the Stamford Detention Tank and the Stamford Diversion Canal are constructed to protect Orchard Road against floods. Built beneath a nursery and coach bay of the Tyersall Learning Forest, the Stamford Detention Tank will temporarily hold excess stormwater from the drains. After the rain subsides, the stormwater will be pumped back into the drains for subsequent discharge into Marina Reservoir," he said.
Mr Masagos then stressed that PUB stipulates buildings to have receptor features, such as higher platform levels, crest protection and flood barriers to prevent floodwaters from damaging them for areas that are most at risk of flooding.
Apart from structural interventions, PUB places equal emphasis on non-structural measures such as the maintenance of drains. PUB and NEA’s Department of Public Cleanliness carry out regular inspections to remove debris, litter and leaves from the drains.
"We encourage everyone to keep our environment litter-free. Any obstruction in the drain reduces its capacity and impedes water flow, which could lead to or aggravate a flash flood. We urge developers and contractors to ensure that the drains in and around their work sites function well," he stated.
He then encouraged members of the public to provide feedback on drain conditions through PUB’s MyWaters app or MSO’s OneService App or call PUB 24-hour Call Centre.
"Our approach to flood management is a comprehensive one that seeks to alleviate flood risks while minimising disruption to the public. We will continue to take all necessary steps to enhance flood resilience across Singapore, and keep the public informed of flash flood incidents," the minister ended.