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.
PUB working on advance warning
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.
