Saturday, 16 October 2010

Fanapi Flooding Claims

On Wednesday 6th, Shern Jian-chuan 沈建全, a professor of marine environmental engineering at Kaohsiung Marine University in Nanzih had an editorial piece in the Taipei Times. It dealt with the subject of flood control following Typhoon Fanapi, which hit Niaosong, Nanzih, Ciaotou and Ganshan districts of Kaohsiung particularly hard - all districts with which, like me, Shern Jian-chuan will be quite familiar. To set the scene, the professor issues the following claims about the impact of Fanapi:
"As Typhoon Fanapi swept across eastern and southern Taiwan on Sept. 19, it brought extremely heavy rainfall of 500mm to 800mm or more in six hours to Gangshan (岡山) and Ciaotou (橋頭) townships in Kaohsiung County and Nanzih (楠梓) and Zuoying (左營) districts in Kaohsiung City. That is well in excess of the 480mm deluge that can be expected to occur once every 200 years. In fact, we could expect this kind of volume to occur in these areas only once every 500 years. Such heavy rainfall is bound to cause flooding, whatever city it falls on."
I see no reason to question his claims as to the measurements of rainfall - having seen such flooding myself in precisely these areas over several years, I can well believe those figures. However, the claim that such quantities of rainfall have a 1/500 chance of occurring seems to me highly questionable. That probability statement cannot possibly have been derived purely from a rigorous data analysis, since there is insufficient data - recorded history itself on this island only goes back about 400 years. It can only have been derived, therefore, from a statistical model based on an assumption that rainfall in Taiwan is normally distributed. But since normal distributions are rarely found in nature, I'm disinclined to regard such statements as trustworthy. I have tried to contact Shern Jian-chuan to check his facts, but in the meantime I've looked at the Central Weather Bureau's own statistics on rainfall in Kaohsiung - unfortunately the website only gives statistics for the last decade which is far too little (presumably Shern Jian-chuan has access to a larger data set). Anyway, between 2001 and 2010, the mean rainfall in the Kaohsiung area for the month of September is 302.2mm with this September's outlier a massive 853mm. For that rainfall pattern to fit a normal distribution, there would have had to have been a year in which Kaohsiung received -248.6mm of rain: so clearly, Shern Jian-chuan would have to have access to a much larger data set from which he can claim a mean rainfall value much larger than 302.2mm (i.e. 550.8 or higher) - if, that is, he is using a model based on a normal distribution of rainfall to derive those probability statements.
"What’s more, global warming may cause extra-heavy downpours to happen more and more often."
And look at that - that's just bluster, casually thrown out without a hint of supporting evidence whatever, on the expectation that it will be swallowed down without question. And this, a year after the climategate scandal - which, disgracefully, received scarcely any media attention at all in Taiwan. I hope to have a reply from professor Shern in the next week or so.

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