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

Idea of the '50-year flood' has been misunderstood
A picture posted on Facebook shows cars being stranded along the Ayer Rajah Expressway during one of the recent flash-floods.

SINGAPORE - 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.

Dr Yaacob was referring specifically amount of rainfall that caused the flood although his 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.

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.

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