Thursday, 22 July 2010

More on 50-year floods

A reader recently pointed out that a "50-year flood" is not literally a once-in-50-years flood. Quite educational for me, actually.

So that made for a really good segue to a full-page article on page A8 of today's ST (July 22, 2010), headlined "100mm: Expect floods if this much rain falls in an hour". Here are my key takeaways from the piece:

  • Flooding can be expected when more than 100mm of rain falls in an hour.
  • Out of those 10 occasions, 8 resulted in floods.
  • Out of those 10 occasions, 2 were in 2009, 3 in 2008, 2 in 2007 and 3 in 2006. In other words, we've been getting the so-called 50-year flood two or three times a year. If that is the case, then based on the USGS explanation of a 50-year flood, we probably need to re-define what a 50-year flood means in our context.
  • The highest one-hour rainfall ever recorded was 148mm in November 1995.
  • The last 10 such occasions highlighted in the article (which seemed to exclude the recent floods) ranged from 101mm to 137mm.
  • By comparison, the 16 June storm that flooded Orchard Road saw 101.6mm in 2 hours. Averaged over 2 hours, that was about half the rainfall of the top 10 recorded rainfalls from 2006 to 2009.
  • On the other hand, the more recent storm on 17 July brought down 114mm at Lower Pierce Reservoir, but 194mm at Poole Road in Tanjong Katong.
  • PUB lists 52 low-lying, flood-prone zones. Presumably, these make up the 66 hectares of flood-prone areas today (yes yes, down from 3,178 hectares in the 1970s, as PUB, Dr Yaacob and others have reminded us). And yet, the recent areas flooded, such as Orchard Road, Opera Estate and Jalan Boon Lay, are not on that list.


Having read the articles, some questions came to mind.

Did Dr Yaacob have the statistics at hand, when he described the first, 16 June storm as being "once in 50 years", when the statistics clearly show that it wouldn't even have made the top 10 over the past 5 years?

What has changed, for Orchard Road to suddenly become a flood-prone zone now? The (new) official line is that Stamford Canal could not handle the amount of rain coming down. But the statistics show that in absolute terms, the amount of rainfall coming down on 16 June was not truly an outlier. So could it have, as some have speculated, been due to all the construction and development work, that has changed the run-off patterns such that a lot more water went into the Stamford Canal than previously? Or some other reason? Instead of saying that the Stamford Canal could not drain the amount of water going into it and then thinking solely on widening the canal, why are we not asking why the amount of water going into the canal was so much more compared to before?

Does the "66-ha of flood-prone areas" statistic still hold true today? What is PUB's methodology in identifying these areas, and updating this list?

Did the National Climate Change Committee even look at flooding as a possible result of climate change? Because looking at its website, that did not seem to be a focus area at all. So much for scenario planning.

It's of course true that Singapore cannot be completely flood-proof. But I don't think anyone is really asking for that. Talking about half of Singapore becoming canals is a straw man argument that ignores the real point.

I think what has shocked and upset people, are the scope and location of the floods that have been occurring, and the Government's cavalier response so far. Frankly, if the flooding had been limited to the recognised flood-prone areas (i.e. those on PUB's list), I'm pretty sure there would not have been the amount of backlash that has arisen.

But they are not, they are in new areas, including very highly visible Orchard Road, and the Government did not know what was going on and seemed more interested in maintaining its not-to-be-blamed Teflon coating than in digging in and figuring out what really happened -- the first step to finding a solution. That was what was most disappointing.

Finally, I just wanted to say kudos to Victoria Vaughan, the ST journalist who wrote the story in today's ST. I suspect she knew exactly what she was doing, laying out all the facts for discerning readers to figure out what the real questions are. If only she could (or had the editorial leeway to) ask those questions in the article itself.

34 comments:

The said...

Interesting article on the 100-year flood. What it says about the once in 50 years flood is that the chance of it occurring in any one year is 2%. So, to have 3 occurrences in 5 weeks (not 50 years), the probability is 0.02 x 0.02 x 0.02, or 0.0008%, or a 1 in 125,000 chance. Now, compare this to the chance of winning the first prize in a 4D draw - which is 1 in 10,000. And don't forget we are talking about 5 weeks. For the full year, if there are more floods, the odds will be exponentially minute.

Bottom line is, the flood data in Singapore is hopelessly out of date. I suspect the main reason is the rapid urbanization.

The Stamford Canal was adequate when the tallest building along Orchard Road was the Cathay Building and the tallest building along Shenton Way was the Asia Insurance Building. Since then, many building have popped up along Orchard Road.

Take the Ion. It used to be a huge piece of land with lush vegetation. When a piece of permeable land is developed, it means that that piece of land is now concreted over. Rainwater, instead of percolating through to the ground, is now run off as surface water into the drainage system. Multiply that by the tens or hundreds of projects over the decades and you can see that unless the drainage system keep pace with developments, its capacity will not be adequate.

The key question is - when was the Stamford Canal last upgraded.

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kenleejj said...

One of the key factors that led to the Orchard Road floods could be Stamford Canal's inability to deal with the extra run-off resulting from recent new property developments.

Did the government not foresee that the construction of these new projects would have led to flooding? Did it not project the likely increase in surface run-off which would have resulted from this construction?

If the answer to both questions is "no", I'd like to know why. The government must ensure that risk projections and related studies are properly carried out before construction commences on all major projects, whether in flood-prone areas or not.

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