Monday 16 September 2013

Idea of the '50-year flood' has been misunderstood

By Jeremy Au Yong, The Sunday Times, 15 Sep 2013

For many people, the sight of all four city-bound lanes of the Ayer Rajah Expressway submerged just over a week ago would have triggered a thought along the lines of: "Isn't this sort of thing only supposed to happen once every 50 years?"

This half-century time-frame entered the national consciousness in 2009, when then minister for the environment and water resources Yaacob Ibrahim said the flooding in Orchard Road that year was a "freak event" that happened once every 50 years.

It is a quote that is now dredged up every time there is a flood (and there have been several) as proof of how badly the authorities misjudged the flood risk here.

The idea of a 50-year flood, however, is one that is often misunderstood. The first problem, of course, is that there is no such thing as a 50-year flood and Dr Yaacob would have been better off never referring to what happened in Orchard Road in such stark terms.

Many factors go into causing floods and these are almost impossible to take into account in weather models. While meteorologists can make some forecasts about rainfall, it is difficult to model the conditions of drainage or tree cover at any given point in the future.

So it may make sense to talk about a 50-year rainfall record or a storm of once-in-50-years intensity, but not about whether they would lead to a once-in-50-years flood.

Even then, there is value in being more precise when talking about 50-year storms. While saying that a freak storm happens once every 50 years makes it easy for the layman to understand, it misrepresents the actual frequency of the storm.

A once-in-50-years storm is a phrase not to be taken literally. Weather, unlike say the movement of the planets, does not follow a predictable timetable, which is why we know when the next eclipse will happen but not the next thunderstorm.

So, when an intense storm is described in such terms, it is a description of probability.

For instance, if I look at rainfall data over the past 100 years and discover that a very intense storm happened twice, I would divide the number of occurrences of the storm by the number of years to get the probability of such a storm happening in any given year.

This sum would give me 0.02 storms per year - or a one in 50 chance of a strong storm in any given year. Could it happen two years in a row? It is unlikely, but it certainly could.

But that is not to say that we have just been incredibly unlucky with our flooding. It may very well be that climate change has rendered the initial estimate irrelevant. It could be that for the past 100 years, the kind of rainfall that could produce floods was rarer than it is now.

We won't know whether the past few years mark the start of a new pattern or are an outlier in the data until we can sample a larger set of weather data.

In the meantime, though, there are worthwhile conversations to be had about how Singapore should go about managing its flood risk.

Just what sort of buffer is being used right now in our planning parameters? How much more land would we have to give up for us to deal with 100-year storms?

But to get to those questions, we first need to get past the idea of a flood that happens only once every 50 years.

VoicesTODAY asks: Floods - a natural or man-made problem? -12 Sep 2013

Climate change affects Singapore flood risk
By Melissa Chong, Channel NewsAsia, 22 Sep 2013

The United Nations' Intergovernmental Panel on Climate Change is slated to release the first part of its latest report on climate change on September 27.

Governments around the world will be watching to see what hundreds of climate scientists have to say about the potential impacts for their regions.

As for Singapore, its top concern is likely to be how climate change could affect our flood risk.

The United Nations report is published every five to six years, and highlights the latest findings in the rise of temperatures, sea levels and extreme events like floods.

Results in 2007 were comparable to findings by Singapore's National Climate Change Secretariat. By 2100, sea levels around Singapore could rise by up to 0.65 metres while temperatures could increase by up to 4.2 degrees Celsius.

Singapore's inter-agency Resilience Working Group (RWG), which is tasked to oversee the government's efforts to fight climate change, said the ministries will review their plans based on the latest data.

And of the future challenges they expect is more frequent floods.

Across Singapore, rainfall has been on the rise.

The National Environment Agency said the number of days with intense rainfall has crept up, increasing by 1.5 days per decade.

In 1980, average rainfall was 96 millimetres (mm). In 2012, it rose to 117mm.

Given the complexity of weather systems, the RWG said Singapore cannot fully avert flood risks.

However, much can be done to help Singaporeans to cope with the challenges and mitigate the risk to lives and properties.

In 2010, PUB identified 22 canals that required upgrading as the old design could no longer cope with more intense storms.

"We have increased the design requirements for our drains. We're now designing our drains for more intense rainfall so we need to look at capacity, whether it's able to cope with the more intense rainfall," said Ridzuan Ismail, chief engineer (Drainage Planning) at PUB.

Since works began, sections of the Bukit Timah canal and the Kallang River have been upgraded, providing an increase in capacity of between 15 to 50 per cent.

Other locations such as the Rochor Canal and the Geylang River are also undergoing upgrading work.

The construction has to be done in sections because electricity cables and gas pipes are running underground and these cannot be disrupted. As construction works are going on, they have to ensure that water continues to be carried from the catchment areas toward the Marina Reservoir.

PUB said this project is already 60 per cent completed.

The upgraded canal, which will be about two metres wider and 1.7 metres deeper, forms a new U-shape and can hold more water.

The next canal targeted for upgrading works is the Stamford Canal, where a new diversion canal and detention tank will be constructed.

The plan is to eventually upgrade all 22 canals but works are still in the early stages and PUB is hesitant to pin down a completion date.

PUB has also implemented strategies to ensure the public is prepared for floods, using SMS alerts on heavy rains, live CCTV images of flood-prone areas, and updates through social media.

A new regulation was also set in June 2013. Developers must build structures like rain gardens to slow down the flow of storm water into the public drainage system for developments that are more than 0.2 hectares in land size.

The government has stepped up efforts, especially after central Singapore was inundated by floods two weeks ago.

It remains to be seen if Singapore's resilience can keep up as climate change continues to affect the country.

Singapore could see hotter, wetter days next century
By Feng Zengkun, The Sunday Times, 29 Sep 2013

Singapore should brace itself for hotter and wetter days in the next century.

The National Environment Agency (NEA) yesterday held its own briefing on what a long-anticipated global climate change study by a United Nations panel - which was released on Friday - means for Singapore.

According to NEA's preliminary findings, if the earth heats up by 2100 as expected, very heavy storms in Singapore will "very likely" become even more frequent and intense.

A 3 deg C rise in Singapore's temperature, for instance, means that a very heavy storm, which has a 1 per cent chance of happening in any given year, will be five times as likely to occur.

Heavy storms have already become more frequent here in the last few decades, according to the Centre for Climate Research Singapore (CCRS).

In 1980, there were only five days with rainfall of more than 70mm in an hour. That increased to 10 days last year.

The highest amount of rain which fell in an hour also increased from 80mm in 1980 to 107mm over the same period.

Hotter "temperatures that Singapore encounters occasionally now could become the norm in the future", the NEA added in a press statement.

Singapore has been heating up at a rate of 0.26 deg C every decade between 1951 and last year. This is twice the global trend of a 0.12 deg C rise every 10 years in the same period.

The difference may be partly due to urbanisation here, according to the NEA.

Singapore's average daily maximum temperature between 1972 and last year was 31 deg C, while the hottest recorded was 36 deg C in 1998.

The global study also suggested that both the extreme weather patterns related to the El Nino phenomenon could become more intense.

This may lead to drier weather in Indonesia and worsen the annual haze in Singapore, said CCRS director Chris Gordon, although he added that more detailed studies were needed.

The NEA said it would use the global study's findings in its Second National Climate Change Study, expected to be completed by next year. This will provide local projections for temperature, rainfall, wind and sea levels up to 2100.

The findings will be handed over to the relevant agencies for their own studies on how Singapore can cope with the climate changes. These are expected to be finished by 2016.


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