05 September 2013

Purple rain and Sungei Pandan Kecil

'Purple' rain hits the built up part of Singapore and there's massive flooding, roads not passable to ordinary cars, traffic jams, trees falling.
Rain Areas Animation from the NEA website
Within hours, Minister Vivian Balakrishnan posts on facebook: "flooding at AYE will require expansion of the Sungai Pandan Kechil which drains south to the sea and is subject to tidal influence."


Minister Vivian posted this:

"This morning's heavy storm brought more than 100 mm of rain to the Kent Ridge area with major flash floods especially at AYE and Commonwealth Avenue. Both the Sungai Ulu Pandan and Sungai Pandan Kechil overflowed their banks.

Drainage improvement work at Commonwealth Avenue will be completed by June next year.

The flooding at AYE will require expansion of the Sungai Pandan Kechil which drains south to the sea and is subject to tidal influence. This is a major project that PUB has been considering. Will have to expedite this."

Where is Sungei Pandan Kecil?
I really can't figure it out from the online maps. If a reader knows more, please do leave a comment.

There is the very large Sungei Pandan, which at the upper reaches divides into Sungei Ulu Pandan and Sungei Pandan. Further upstream, these become merely 'canal'. 'Sungei Pandan Kecil' doesn't show up on the wikipedia entry of 'List of Rivers in Singapore'.
Click on image for larger view.
There are two 'canals' that drain off the AYE and Commonwealth Avenue areas. One of them on the west end of West Coast Park has been referred to as Sungei Pandan Kecil, although on the NParks map, this is labelled 'canal'.
Canal at the western end of West Coast Park
from the NParks website.
If this canal is widened, it might affect the 'Pond' at the Park which is a tiny patch of mangroves called the Marsh Garden, I last visited here more than 10 years ago.

What does "subject to tidal influence" mean?
It probably means that the level of the water in this "Sungai Pandan Kechil" goes up and and down with the tides. So at high tide, the water may not drain as rapidly into the sea. If there is heavy rain at high tide, then rainwater is likely to back up and result in flooding.

Today, the peak rainfall did NOT coincide with the highest tide. Peak rainfall was at about 0830hrs, when the tide was predicted to be about 1.5m. The highest tide today is predicted at nearly 3m at noon. In Singapore, the highest daily predicted tide is about 2.5 to 3m. [Why am I saying 'predicted'? Because the tide tables only give predicted tide heights. Actual tide heights as measured at various locations are a different thing altogether and may not match predicted tide heights (as I have found to my personal dismay on low tide trips).]

When a dam is placed across a canal/sungei, does it mean that it is no longer "subject to tidal influence"?
Hmm, good question. Hopefully, someone can help with a proper answer.

But looking at the issue from a logical ordinary person's point of view: I do know that all dams have a sluice gate, to release water from the dam so that the water doesn't back up and cause flooding upstream. I guess this is done now and then on some sort of schedule for all dams.

Obviously, to drain the dam water, the sluice gate can only be opened when the water outside the gate (in the sea) is lower than the water inside the gate (behind the dam). So if the tide is high, the water in the sea may be  higher than the water inside the gate and the sluice gate can't be opened to drain the water, even if there is a huge flood of water incoming from rainfall.

So, to my mind, all our canals/sungei/river are subject to some sort of tidal influence even if there is a dam across it.

What is Singapore doing to prepare for flooding due to climate change?

Today's flooding was relatively short in duration. Resulting in some funny photos and comments. But things could get a lot worse with climate change.
Photo from New Nation
In a recent article Singapore will be able to cope with rising global sea levels: Experts NUS Assistant Professor Kua Harn-Wei noted that while the Government has been planning for rising sea levels and temperatures over the past decade, he was more concerned with whether Singaporeans can adapt to the environmental impact of climate change. “Rising sea levels may be just one of the many effects. We are going to get more complicated effects like stronger winds, greater storms and more flash floods. Those are the problems that we need to get ready and prepared for.”

A recent study also found that coastal flooding in cities around the world could cause damage totaling $1 trillion annually by the year 2050 if no mitigating steps are taken. Although Singapore is not highlighted as one of the cities at high risk, the risk remains.

Already today, we experience severe if short-lived storms called the Sumatras. These tend to happen before dawn when most people are safely in bed and thus unaware of them. But a Sumatras happening at rush hour will probably be keenly felt by many people.
Wet weather over Cyrene Reef
Storm building up over Cyrene Reef.
The Singapore Government clearly understands the potential effect of climate change on melting the Arctic ice which, among others, will allow ships to bypass the Malacca Straits and Singapore port. So much so that Singapore has applied and been granted permanent observer status in the Arctic Council.

But it is melting ice, among others, that will contribute to global sea level rise. So what is being done to prepare for sea level rise in Singapore?

In June it was reported that the Building and Construction Authority (BCA), which is in charge of protecting the island nation's coasts, called a tender for a coastal adaptation study - the most extensive one done here yet - to come up with a framework to keep low-lying areas safe. I suggested that in protecting from rising seas, there might be a role for natural shores.

Update on 6 Sep:

Check out the awesome comments made by Pat to this post below which provides lots of fascinating and educational information, links and analysis. Thank you Pat!

Media articles report that authorities say the floods "were the result of the unfortunate coming together of two forces of nature - heavy rain and high tide". PUB says "upgrading of the canal in this problematic area will not happen immediately. 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"
From Straits Times 6 Sep 13.
"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. 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." More.

Update on 7 Sep:

It was reported in the Straits Times that "PUB will build a tidal gate at the Sungei Pandan Kechil canal within six to nine months. The 2m-tall steel gate near West Coast Road prevents high tide from the sea from flowing into the canal, by creating a temporary storage area. "If we can cut out the influence of the tide, it helps buy some time during severe rain." PUB will also consider building a barrage - a larger version of a tidal gate - in the long term."

Pat shares MORE intriguing information and analysis about this PUB proposal in new comments below. Pat's points are also echoed in Development vs environmental protection: Public discussion needed by Tan Wee Cheng, Today Online 7 Sep 13.

Update on 8 Sep:

Chengming Wang shares photos of a survey of the Pandan waterways on facebook and says: "It is not convincing that high tide is a cause of the flooding at AYE. There is sufficiently wide area (about 380 meters by width) at the river mouth where Sungei Pandan Kechil discharges into Sungei Pandan. If the high tide is the reason, West Coast area which also received same amount of rain should have been flooded as well. Someone pointed out that chocking of inlet points to Sungei Pandan Kechil is the main reason for the AYE flooding. Flooding caused by high tide does not subside within short time in 40 minutes."

Update 13 Sep:

In Voices/TODAY, this flooding incident was discussed including mention of the possible impact to Singapore of rising seas which will result in higher tides, and more. This clip appears to be unedited and is very long.



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