Heavy rain plus high tide cause flooding

This photograph of students clinging on to a fence to avoid walking through a flooded area in their school has been making the rounds on the Internet.

SINGAPORE - Thursday's floods were the result of the unfortunate coming together of two forces of nature - heavy rain and high tide.

The rain was particularly heavy as a result of "the convergence of winds over the region", according to the National Environment Agency.

It fell heaviest near Kent Ridge, reaching 102.8mm in just over an hour. The nearby Sungei Pandan Kechil canal was supposed to direct the rainfall out to sea, but it was already filling up fast because of the high tide.

The ensuing lack of drainage led to flood waters rising to a height of half a metre across 90m of the Ayer Rajah Expressway (AYE), where all four lanes leading to the city were closed for about 40 minutes.

National water agency PUB's chief engineer of drainage planning, Mr Ridzuan Ismail, said on Thursday that the canal was due for an upgrading and that a preliminary study had already been done.

It had overflowed before. In 2011, the flooding led to the basement carpark of Varsity Park Condominium along West Coast Road becoming inundated, leaving cars in bumper-high water.

Then in February this year, intense rain in the area led to the closure of one lane of the AYE.

Yet upgrading of the canal in this problematic area will not happen immediately. Mr Ridzuan said that it will take a further nine months to determine the scope of works needed, including how much the canal's capacity can be expanded and what construction method to use.

At the earliest, work will only begin in the first half of 2015, and it may take a few years depending on the scale of work and site conditions, he added.

Meanwhile, the heavy rain also caused trouble in other parts of Singapore. Water rose to a height of half a metre along 100m of Commonwealth Avenue, closing two lanes of the road.

Maxwell Road, Cuscaden Road, Alexandra Road and Lorong Kismis were hit with flood water reaching a height of up to 0.2m.

PUB data showed that drainage improvement work in these areas is either ongoing or will soon begin - with the exception of Alexandra Road.

The work in Commonwealth Avenue, for instance, will cost $13 million and involves putting in a new 50m-long culvert and enlarging the existing road drain. It will be completed by 2015.

Assistant Professor Vivien Chua from the National University of Singapore's civil and environmental engineering department said these areas could have faced an increased risk of flooding because the surrounding area was built up, leaving a lack of permeable surfaces to absorb the rain.

Another alternative solution, she added, could be to use detention basins. The West Coast Park near the Sungei Pandan Kechil canal, for example, could house a "dry pond" which stores water during heavy rainfall and high tide conditions, but can be drained and used for recreational purposes during drier weather.

Mr Ridzuan said that PUB also has an ongoing drainage improvement programme to alleviate floods in hot spot areas and to increase flood protection. In 2010, PUB identified 22 canals and waterways that needed upgrading.

"However, we cannot possibly design our drainage system to cater to the most extreme of storms, due to limited land space," he noted.

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