Changing weather patterns causing more flash floods

Sumita d/o Sreedharan and Woo Sian Boon Today Online 13 Sep 13;

SINGAPORE — Changing weather patterns and rapid development have resulted in more flash floods here over the last few years, and though work is under way to tackle the situation, it could take a decade or more before results become apparent, national water agency PUB has said.

Responding to questions from TODAY, the National Environment Agency (NEA) said the amount of rain falling on the island has crept up in recent times — and so have the instances of flash floods, which had initially been described as the result of freak weather.

In 2010, there were 15 days when flash floods occurred. In 2011 and last year, the number was 23 days for each of the two years. So far this year, there have been 14 days.

The NEA said the trend towards heavier rainfall began in the ’80s. “Rapid development and urbanisation, as well as global warming, are likely to be significant factors which may explain this trend,” it added.

The PUB said it has been noting this trend and has put in place a series of drainage projects to control the flash flooding. However, the scale of these projects is such that they will require several years to be built, and results will only be seen down the road.

The most recent incident last Friday led to the unprecedented closure of a stretch of a major highway, the Ayer Rajah Expressway.

In a briefing for this newspaper, PUB Chief Executive Chew Men Leong reiterated that the drainage improvement work is “going to take years”. “It’s not going to happen tomorrow — that’s the difficulty we face here,” he said, citing land constraints.

“We do not have sufficient land to cater to the most extreme storms ... so we cannot eliminate flash floods completely. It is simply not possible but we mitigate. We are confident of increasing resilience and reducing the occurrence of flash floods.”

Since the ’70s and ’80s, when there was widespread flooding, the Government has spent S$2 billion on the drainage infrastructure,

Last year, an expert panel on drainage design and flood prevention measures unveiled a raft of recommendations. Since then, another S$750 million has been set aside to increase drainage capacity.

Following a review conducted between 2010 and 2011, the PUB found that 22 of the 48 major canals here have to be upgraded to cope with the changing weather patterns. The agency is prioritising the upgrading work of 11 canals, including the Bukit Timah Canal and Changi Airport Diversion Canal.

PUB Director of Catchment and Waterways Tan Nguan Sen said that, ideally, the entire drainage system will be ready “within the next 10 years” to alleviate flooding. “But you must understand that it is always a work in progress ... as and when there are new areas discovered, then we will also analyse them and include them in the programme,” he said.

Citing the Government’s move to explore an underground masterplan, Mr Tan said that a possible future solution is to build “deep underground drains” to overcome land constraints.


Among some of the initiatives is a flood forecast system being piloted in the Marina Catchment, a 10,000-hectare area where most flood-prone places are located. The system is part of what Mr Chew described as the “next step”.

“We have to figure out if we can at least have some chance to forecast the floods. I don’t think it will be great advance warning but five to 10 minutes (before the flash floods occur) ... it is already very good,” he said.

Currently, the PUB uses a “source-pathway-receptor” approach to tackle the problem holistically. Source refers to areas where rainwater falls and features such as detention tanks, green roofs and rain gardens can be built to retain rain. “Pathway” solutions include deepening and widening canals while flood barriers, for instance, can be created to protect buildings which are on the receiving end of the run-off.

In 2009, after a big downpour resulted in parts of Bukit Timah being submerged, then Environment and Water Resources Minister Yaacob Ibrahim said the deluge was a freak event that “occurs once in 50 years”.

Nevertheless, an analysis by the Meteorological Service Singapore of hourly rainfall data between 1980 and last year showed that annual maximum hourly rainfall total has increased from an average of 96mm to 117mm. There has also been an increase of 1.5 days per decade in the number of days each year with hourly rainfall totals exceeding 70mm, the level of rainfall that corresponds to the highest 1 per cent of hourly rainfall intensity during the period.

Experts pointed to climate change and lagging infrastructure as possible reasons for the flash floods. Citing a report by the expert panel which convened here last year, Dr Winston Chow, a research fellow at the NUS Department of Geography, noted that there has been “statistically significant” increase in the intensity and frequency of extreme rainfall events.

Dr Chiew Yee Meng, Head of Environmental and Water Resources Engineering at Nanyang Technological University, said scientific research has shown a close relationship between the occurrence of floods and urbanisation. “The increase of paved areas prevents rainwater from infiltrating into the ground, thus increasing the amount of surface run-off that drains into the waterways,” he said.

Mr Chong Kee Sen, Vice-President of the Institution of Engineers in Singapore, pointed out that the presence of, say, a grass patch reduces the amount of rainwater flowing into a drain by as much as 80 per cent.

PUB’s Code of Practice on Surface Water Drainage was updated this year to require new developments to slow down the run-off — by building features such as detention tanks or green roofs — that would be discharged into the public drainage system during heavy storms. Additional reporting by Xue Jianyue

Some measures the PUB has carried out
Woo Sian Boon Today Online 13 Sep 13;

49 CCTVs in areas such as Orchard Road, Bukit Timah and Upper Thomson to provide up-to-date pictures of flooding conditions

158 water-level sensors to monitor drainage network for real-time updates during heavy storms. Public can subscribe to PUB’s water level alert service

A flood forecast system is being piloted in the Marina Catchment area

S$750 million to be spent over the next five years to carry out 20 drainage improvement projects

Levels of existing low-lying and flood-prone roads were raised

Under PUB’s revised Code of Practice on Surface Water Drainage 2013, developers will be required to incorporate on-site measures to control and slow down storm water run-off from their premises

Minimum platform and crest levels were also stipulated for buildings and entrances to underground facilities (i.e. basement car parks and underground MRT stations)

PUB, NEA on weather patterns, impact of drainage improvement work
Today Online 13 Sep 13;

Are weather patterns changing?

National Environment Agency (NEA): Based on an analysis by the Meteorological Services Singapore of hourly rainfall data between 1980 and last year, there has been a general upward trend in the annual maximum hourly rainfall total — from an average of 96mm in 1980, to 117mm last year.

There has also been an increase of 1.5 days per decade in the number of days each year with hourly rainfall totals exceeding 70mm (70mm corresponds to the highest 1 per cent of hourly rainfall intensity for the period 1980 to 2012). Similarly for temperature, Singapore has experienced an average warming rate of 0.25°C per decade since the 1950s, with more frequent warm days and nights.

The upward trend for rainfall extremes over the past few decades in Singapore is consistent with the global trend towards an increase in the number of heavy precipitation events. Rapid development and urbanisation, as well as global warming, are likely to be significant factors which may explain this trend. It is important to remember that we cannot attribute any single event to climate change. What can be said is that the likelihood of these types of heavy rainfall events is increasing across our region because of climate change.

Has drainage improvement work in certain areas led to rainwater run-off spreading to other areas, as some experts have suggested?

PUB: When planning any drainage improvement work, PUB will conduct detailed site investigations, which include carrying out a topographical survey of the area, checking on the existing drainage systems and reviewing the flood protection measures such as the minimum platform levels and crest levels for developments in the vicinity. The proposed drainage improvement work will be designed such that the drain capacity is enhanced and storm water level in the canal/drain will be lower. Computer simulations are also used to help us analyse the flow of storm water, to ensure that the drainage improvement work, while alleviating floods in affected areas, will not make the adjacent areas worse off than before the work was carried out.

What are specific examples where flash floods have been alleviated because of drainage improvement work?

PUB: Some recent examples of completed drainage improvement work are:

Neram Road outlet drain, which has helped to alleviate flooding along Upper Neram Road

Jurong Port Road outlet drain, which has helped to alleviate flooding along the junction of Jurong Port Road and Jalan Tepong

Roadside drain along Lorong 35 Geylang, which has helped to alleviate flooding along Sims Avenue near Lorong 35/Geylang Road junction

Other areas alleviated by the Marina Barrage include Hong Kong Street, Dakota Crescent, as well as parts of Boat and Clarke Quay.

Rising to the occasion
Tackling floods is about making radical change to infrastructure and expanding our drainage network. TODAY file photo
Both pre-emptive and coping measures are needed to stay a step ahead of floods, say panellists in the extended segment of Thursday’s episode of VoicesTODAY ‘Floods: A natural or a man-made problem?’
Today Online 14 Sep 13;


Vivian Chua: A flood prediction system typically has sensors to measure water levels at different areas around the island. The key to such monitoring systems is how we can transmit the information rapidly enough to the public so that they know not to go to places where there may be flooding.

Jose Raymond: If you follow Twitter and Facebook, I think there are many people out there putting up photos and videos about what is happening out there, and I think that is key to us coming together and finding a solution to how we can tackle all this. I don’t think people from the Government are going to be on the ground all the time, and I think citizens can play a part. If there is an area around your home which is constantly flooding every time there is rain, they should bring it up. If there is a safety issue, they need to inform the authorities.

Olivia Choong: I also think it would be useful to have more signs and indicators around Singapore when there is flooding, especially for drivers.

Vivian: (On the impact of climate change) We need to be clear that rising sea levels alone will not lead to more flooding — it is a rise in sea levels combined with greater intensity and frequency of storms that pose a greater risk of flooding. We are studying trends in the Asia-Pacific region. The main purpose of our work is to increase awareness of what is going to happen. If we do not study this at all, the Government is not going to be able to find solutions. From an academic perspective, this is basic research.

Chong Kee Sen: Very little is known about the impact of climate change, which is why we brought the World Engineers Summit to Singapore, to understand the potential impact and look for innovative mitigation solutions. The Building and Construction Authority is also looking at how to protect our shorelines from rising sea levels.

Olivia: I understand that there (will be) widening of canals, and barriers (are being built) ... (but apart from that) I think it is always worth looking at other technologies, even low technologies like storm water harvesting.

The National University of Singapore is also conducting research at the moment about permeable surfaces, like roads and tiles.

Jose: We need to get used to the fact that there will be recurring floods and we need to get used to that kind of life. There must be acceptance from employers and schools and we need to adapt. We also need to understand that we contribute to climate change and we need to take a look at ourselves to see what we can do differently. It is about adapting. If you think your business is vulnerable, take the necessary precautions, just like what businesses at Orchard have done. They need to take the proactive step and not rely on someone to be there for them when needed.


Peter Loon: It can be as basic as making sure people are aware that they should not throw litter into the drains, and having a citizens’ network so that we can keep each other informed. But it is also about making radical change to infrastructure and expanding our drainage network. I acknowledge that rain is a constant phenomenon, but we should be tackling it actively if it seems to be getting worse. This can be something as simple as upgrading and widening drains whenever we start new projects, to cater to more than what is needed now.

We do not want to wait until a fatality happens, and then react to that. (Flooding) affects our economy. When a major arterial road like the Ayer Rajah Expressway floods, it delays people who are getting to work. It is time we put in effort on a concerted level with many agencies to fix this. There are already areas being significantly affected, so we should be improving on the infrastructure and building them on a preventive basis, with the view that things might get worse.

Vivian Chua is an Assistant Professor from the National University of Singapore’s Department of Civil & Environmental Engineering.

Olivia Choong is the founder of Green Drinks Singapore, a non-profit promoting collaboration on environmental issues.

Chong Kee Sen is Vice-President of The Institution of Engineers Singapore.

Jose Raymond is Executive Director of the Singapore Environment Council.

Catch the one-hour extended encore telecast of this week’s VoicesTODAY episode “Floods: A natural or a man-made problem?” at 5.30pm today on Channel 5.

Floods: A natural or man-made problem?
VoicesTODAY asks: Floods - a natural or man-made problem?

Are last week’s flash floods another sign better urban planning is needed? How key is the loss of green spaces? Should S’poreans have to deal with floods as a fact of life? What are other environmental concerns as development proceeds, including underground?

Catch the live telecast of VoicesTODAY tonight (Sept 12) at 9pm on MediaCorp Channel 5 or join the discussion on

Email if you're keen to engage in the live discussion by phone or video chat on Google Hangout

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