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

Jeremy Au Yong Straits Times 15 Sep 13;

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.