Parliament​ary Question on flash floods in Orchard Road

Dear Dr Balakrishnan,

I refer to your 9 Jan 2012 parliament replies to questions on Orchard Road flash floods: http://app.mewr.gov.sg/web/contents/contents.aspx?Yr=2012&contId=1514.

You explained that the three recent floods over the Orchard Road area are part of a larger, longer rainfall change in Singapore. By plotting Singapore’s maximum one-hour rainfall over the last 30 years, you concluded together with the expert panel that Singapore is experiencing increasing rainfall intensity.

If rainfall intensity is indeed the culprit for our recent floods, shouldn’t there be a worse flood in 1995 since it recorded a higher maximum rainfall intensity of about 145 mm in 1 hour compared to 130 mm in 1 hour in 2010? Similarly, 2007’s maximum rainfall intensity of about 135 mm in 1 hour is also more than that in 2010. No massive flooding of Orchard Road in 2007 either.

While the slope of the trend line looks rather steep on your graph, it is merely 10 mm rise over 11 years (1987 to 1998) or less than 1 mm rise per year. Are you saying that one extra mm of rain going from 2009 to 2010 led to the 2010 Orchard Road floods? The trend line alone is insufficient to conclude that we are facing a situation of increasing rainfall intensity. What is the statistical significance of the trend line? What is the strength of the correlation between rainfall intensity and year?

You asked the PUB to assume we will continue to have storms exactly similar to the last three episodes. Have you asked the PUB whether we had similar storms prior to the last three episodes? Your case will be so much stronger if you can show Singaporeans that the last three episodes were the only such storms Orchard Road ever experienced in the last thirty years.

You concluded by saying that the weather has changed. What better way than to blame it on the weather. But the fact is that while Singapore weather can suddenly change from day to day, the underlying trend doesn’t suddenly change from year to year. It doesn’t suddenly change from no floods in 2009 to massive floods in 2010. Only human activities can bring about such rapid changes.

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