100mm: Expect floods if this much rain falls in an hour

Jul 22, 2010

By Victoria Vaughan

IF MORE than 100mm of rain falls in an hour, expect flooding.

National water agency PUB said yesterday that in the last 10 occasions when rain of that intensity fell, it had resulted in floods eight times.

Of those past 10 rainstorms, two happened last year, three the year before, two in 2007 and three in 2006.

Every year, Singapore gets, on average, two storms dumping that much rain in an hour, and nine storms in which the 100mm is spread over three hours.

PUB, responding to questions from reporters after the recent floods, explained that besides the intensity of a storm, the following factors also work together to set up perfect conditions for flooding:

• The type of developments in the area; that is, whether it is built up.
• The kind of surfaces there; that is, whether they are paved or porous.
• The distribution and movement of the rainfall.
• The ground level in relation to the water levels in the drains.
• The capacity of the drains to channel away the storm water.
• Whether there is a blockage in the drains.
  

Of the last 10 most intense rainstorms here, the heaviest happened on July 19, 2007. The monitoring station at the Ulu Pandan water treatment plant recorded 137mm in an hour that day, said the National Environment Agency (NEA).

The recent downpours have not topped that. The June 16 storm which put Orchard Road under water sent down 101.6mm in two hours.

Orchard Road was not even the wettest place that day: Sentosa logged 115mm of rain.
Another recent storm, that on July 17 from Typhoon Conson, brought 114mm of rain to the Lower Peirce Reservoir in one hour.

But a high of 194mm that day was recorded at Poole Road in Tanjong Katong.

The second heaviest of the last 10 intense rainstorms came on April 3, 2008, when 126mm was dumped on Jurong Pier Road. This burst led to flash floods in Dunearn Road near Swiss Club Road, Upper Paya Lebar Road near Bartley Road and Commonwealth Avenue. The storm lasted from 4.20pm to 6pm.

The third heaviest downpour happened on Nov 19 last year, when 121mm was recorded at the Ngee Ann Polytechnic station. The Bukit Timah canal burst its banks and knee-deep water brought traffic to a standstill along several stretches of Bukit Timah - Coronation Road to Third Avenue, Wilby Road to Blackmore Drive and the Sixth Avenue junction.

The downpour took place between 12.50pm and 3.20pm.

From the history books, Dec 2, 1978, still holds the record for the highest amount of rainfall in a day in the last 60 years. That day, 512 mm fell.

Seven people died, including five who drowned, and about 1,000 people were rescued and evacuated by the Singapore Armed Forces and the police in one of the biggest rescue and evacuation operations here, according to the Defence Ministry website.

The highest one-hour rainfall, at 148mm, was recorded on Nov 2, 1995.

vvaughan@sph.com.sg