Orchard Road anti-flood measures to meet future needs

Dear Dr Balakrishnan,

I refer to the 23 Jul 2012 Straits Times report of your comments on Orchard Road flood prevention measures. You said those were long-term measures, planned decades ahead and that we always have a long-term view of things.

If we planned decades ahead and viewed things in the long-term, how come we were caught flat footed in 2010 and 2011? In between plans that supposedly last a few decades, do we operate on auto-pilot mode?

It was also reported that the expert panel found that our rainfall has increased by about 30% over the past four decades.

The following annual rainfall data was obtained from the government website [1].

1960 1961 1962 1963 1964 1965 1966 1967 1968 1969
1563 1812 2287 1819 2826 1853 2480 2910 2077 2289
1970 1971 1972 1973 1974 1975 1976 1977 1978 1979
2277 1608 1807 2956 2066 1925 2167 1775 2766 2168
1980 1981 1982 1983 1984 1985 1986 1987 1988 1989
2326 1463 1582 1994 2687 1484 2536 2103 2599 2463
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999
1524 1877 2261 2169 1942 2333 2418 1119 2623 2134
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
2371 2783 1749 2391 2136 1931 2753 2886 2325 1921
2010 2011
2075 2524

It shows that we have five decades of rainfall data but the expert panel only used four. Plotting the data on a graph below:

We see that rainfall has been fluctuating up and down over the last 52 years. If our period of consideration started on a trough year and ended on a peak year, we would more likely conclude that rainfall has increased. But if our period of consideration started on a peak year and ended on a trough year, we would more likely conclude that rainfall has decreased instead. Since our conclusion depends very much on which years we choose to compare, comparing actual rainfall between years is not a satisfactory way of deciding how much rainfall has increased or decreased over time.

Referring to the graph above, we should instead consider the trendline over the entire period which shows a 3.4 mm increase each year or an 11% increase over 52 years, far less than the reported 30% increase over four decades. This 3.4 mm increase per year is gradual and doesn’t explain why Orchard Road was suddenly innundated by floods in 2010 and not during the previous peak in 2007. More importantly, the p-value of this 3.4 mm yearly increase is 0.4. In other words, there is 40% chance that this 3.4 mm increase per year is just a random result. In other words, the result is too highly uncertain for us to be reasonably confident to conclude that rainfall has increased over the last four decades, let alone increased by 30%.

Please correct the expert panel and also inform all Singaporeans of their mistaken conclusion that continues to be propagated till this day.

[1] http://data.gov.sg/

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