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.

Related links


  1. From post: "Where is Sungei Pandan Kecil? I really can't figure it out from the online maps."

    The location ("Sungei Pandan Kecil?") that you annotated on the Google Earth map is correct. The name of the canal is labelled on Google Maps.

    Also, the names of major waterways are listed on PUB's 'Water Level Sensors' map -- the waterway names are shown on the respective sensor-icon pop-ups.

    Google Street Views of Sg Pandan Kechil (Nov 2011):
    * Trapezoidal section: Looking downstream from West Coast Highway
    * Trapezoidal section: Looking upstream from West Coast Rd
    *U-shaped section: Looking upstream from AYE towards the NUS' Kent Vale staff condo

    35 years ago (prior to coastal reclamation & redevelopment of the West Coast area in the late 1970s), the original Sg Pandan Kechil was a meandering tidal stream that reached West Coast Rd during high tide. (Similar: The earth-bank Sg China at the Admiralty Park mangrove remnants.)

    The original landscape around Sg Pandan Kechil was mangrove -- followed by coconut plantations north of the back-mangrove. West Coast Park itself was completed in 1979 on reclaimed land.

    Today, you may still be able to see 1 or 2 active mud lobster mounds "exploding" out of the cowgrass verge along Sg Pandan Kechil. I'd often spotted such mounds at this spot opposite Kent Ridge Sec Sch, though I'm not sure if any mud lobster still bother nowadays, as the mounds are regularly mowed down or kicked by itchy-legged people.

    These historical maps (1898, 1911, c. 1975) show how Sungei Pandan (sometimes labelled as Sg Pandan Besar) & Sg Pandan Kechil looked like, before they were drastically altered by urban development between 1969 & 1979.

    These maps at NHB's CARDS archive show how Sg Pandan Kechil remained unchanged between 1958 & 1976.
    * 01 Apr 1958 Town Map Sheet
    * 1966 Town Map Sheet 2/207
    * 01 Jan 1976 Town Map Sheet 3/207

    Note: The upper-left stream Sg Buntu Kechil as shown on the above maps is known as Sg Bangkok today -- ref: PUB's water-sensor webpage. This is the concrete canal beside Tanglin Sec Sch (Google Street View: looking downstream).

    From post: "If this canal is widened, it might affect the 'Pond' at the Park which is a tiny patch of mangroves called the Marsh Garden"

    The lack of a buffer zone between the Sg Pandan Kechil canal & the Marsh Garden makes it very likely that a widened canal would destroy part or possibly all of the Marsh Garden & its pond.

    Unless PUB can significantly increase the canal's capacity by changing the existing trapezoidal section (between AYE & the sea) to a U-shaped profile, as well as deepen it at the same time. However, the result would look rather ugly, esp. during low tide. Imagine a deep concrete box-gorge.

    Incidentally, the little sluice drain leading from the marsh pond to the canal has a high population of the edible, brackish snail Faunus ater. In fact, the sluice drain, marsh pond & adjoining Sg Pandan Kechil are the only places in S'pore where this snail (tentatively designated as native) can be found.

    * Status and distribution of Faunus ater (Linnaeus, 1758) (Mollusca: Cerithioidea) in Singapore -- Nature in Singapore 4: 115-121 (26 May 2011), NUS

  2. From post: "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."

    Deja vu on ALL counts ...

    On 12 Nov 2011 (Sat, 1:45 - 5:30pm), the same thing happened to the upstream sections of Sg Pandan Kechil between Clementi Woods to AYE, resulting in floods along AYE (near NUS), Clementi St 1 (near Clementi Woods Park) & the vicinity of Varsity Park Condo (West Coast Rd).

    * Screencaps from CNA's video-report of the canal bursting its banks at ~2:30 pm
    * Photo of the overflowing canal near Clementi West St 1 (Blks 611 & 612, beside AYE) at ~2:30pm

    NEA said: Highest amount of rainfall (91.4 mm) fell at Kent Ridge at 5.30 pm.

    PUB said: Downpour peaked at 1:20-1:50pm (52.6mm), 105.2mm/hr.

    PUB's double twist: "the flash floods were due to the intense rainfall and high tide of 3m, causing the Sungei Pandan Kechil canal to overflow."

    In reality, when the canal burst its banks at ~2pm, the tide had been receding for >2 hours. Below were the predicted tides on 12 Nov 2011 as stated on NEA's website:-
    * 11:20 am: HIGH (3.0m)
    * 05:58 pm: LOW (0.4m)

    Media reports archived at WildSG News:
    * Flash floods hit western Singapore (CNA - 12 Nov 2011)
    * Flash floods hit West Coast, condo carpark (ST - 13 Nov 2011)
    * Flash floods hit condo carpark (AsiaOne - 13 Nov 2011)

    Public feedback about the Sg Pandan Kechil being a flooding hotspot & PUB's reply as archived at WildSG News:
    * Tackle flood worries (ST Forum - 15 Nov 2011)

    * Singapore: Additional measures as monsoon looms (ST Forum - 22 Nov 2011)

    Excerpt: "[PUB] will also investigate every flood incident to determine the follow-up actions required. In the case of Clementi Woods, PUB is working with the National Parks Board to install additional drainage outlets within the park, and this is expected to be completed in two weeks."

    Uniquely SG Logic: Improving park's longkangs => Increased capacity of adjoining undersized canal ! :)

    The below pages provide photos of what happened at the West Coast- Clementi West- NUS Kent Ridge areas, when Sg Pandan Kechil overflowed during receding mid-tide on 12 Nov 2011.

    * S'pore: flash floods hit western & eastern regions (Earth Changes & the Pole - 13 Nov 2011)

    * Long-Term Flood Control Plan in Pipeline (CNA - 15 Nov 2011)
    -- The unlabelled photo shows the overflowing Sg Pandan Kechil beside Clementi West St 1, beside AYE near NUS. MEWR minister did not mention this incident during the media conference. Instead, amongst other things, he spoke about using smartphone apps as part of flood control (by the public). :)

  3. From post: "When a dam is placed across a canal/sungei, does it mean that it is no longer 'subject to tidal influence'?"

    Actually in SG, the dammed waterbody (canal or reservoir basin) is less subjected to tides (ie. keep sea out), but more on calculated "risk-taking" by humans hoping to keep the freshwater in.

    When it is raining upstream within the catchment (esp. highly urbanized ones with minimal water-retaining capacity), the technician manning the downstream tidal/ sluice gates is faced with this dilemma:

    * Should some/ all of the tidal gates be opened to release the backed-up water, before the upstream situation worsens ? Trade-off: Wastage of PUB's "precious" (ie. impounded freshwater).

    * OR: Should the tidal gates be kept shut, in the hope that upstream rains would weaken in time ?

    Meanwhile, senior personnel are alerted to remotely-taken decisions via real-time SMSes ... eg. 0/9 Marina Barrage gates, 4/8 Pandan tidal gates, etc. operated (ie. open).

    From post: "So if the tide is high, [...] the sluice gate can't be opened to drain the water, even if there is a huge flood of water incoming from rainfall."

    All of Marina Barrage's 9 crest-gates can be kept shut during high/ rising tides, while giant pumps are operated to force the impounded water out to the sea.

    However, there will always be a lag caused by the substantial difference between the rate & volume that can be pumped out via the 9 gates (if all are open) vs. that of water rapidly pouring in from the large catchment.

    But the more crucial issue is not tidal related: PUB traditionally prefers to hoard the impounded freshwater. The previous SOP was to keep the tidal gates shut for as long as possible despite during heavy rain upstream, even when the tide is low.

    Way before Marina Barrage was fingered for the "once in 50 years" floods at Orchard Rd, Alexandra, etc., there is already an established occurrence of regular floods at the Commonwealth & other upstream Sg Ulu Pandan areas from the late 1970s to the present, esp. when these areas became increasingly urbanized. And guess what ... the Sg Pandan sluice-gates were competed in 1975.

    Below is a flashback to the first reported incident of how the said sluice-gates at Sg Pandan caused unprecedented serious flooding at the silted Kampong Sg Pandan in Nov 1976.

    * Locked Tidal Gate is blamed for Flood: Family cheers up with a cup of coffee (ST - 02 Nov 1976)photo1, photo2

    * PUB: Tidal gates were open during the storm (ST - 03 Nov 1976)
    – PUB claimed that "all the 4 gates" at the dammed Sg Pandan had been opened. (Note: PUB omitted to say that Sg Pandan has 8 sluice-gates, so only 50% of the gates were in operation during the Sumatra squall.)

    In those years, the national solution to flooding complaints was to clear the affected "squatters" in the name of urban development. Now that S'pore is so modern & built-up, similar measures are not practical.

    More recently (ie. within the last 1.5 years), I notice that staff manning the Sg Pandan sluice-gates have been trying to keep the water-level in Sg Pandan (AYE) close to but below 75% of the maximum depth.

    Unlike previous years that often saw Sg Pandan (AYE) reaching 90-100% depth even during light-moderate rain, PUB has since then been quite conscientious about quickly opening the sluice-gates as soon as the 75% mark is close to being breached.

    For the few incidents where water-level rose to 90 or 100% (& inevitably accompanied by reports of upstream floods), the joke is that the technician that day was perhaps asleep or a temp/ newbie who wasn't aware that it isn't safe to let Sg Pandan cross the 75% depth threshold.

  4. Thank you so much Pat for this fascinating and very educational background information!

  5. By 2014 Q1/Q2, there will be a new 2m-high steel crest-gate at Sg Pandan Kechil just downstream of the West Coast Rd culvert -- see PDF diagram. The gate will permanently keep the tide (high or low) out from the upstream channel.

    * PUB to build tidal gate as interim solution for AYE flood problem (ST - 07 Sep 2013)

    Incidentally, the depth of the canal at that spot is 2.3m. At high tides (2.7 - 3.0m), the water-level there usually ranges from 1.6 - 1.8m in depth, taking up 50-63% of the canal's capacity at that section.

    However, can we in good conscience blame the recent & increasingly frequent inundations at AYE (9.6km) on "rising" tides & "more intense" rainfall ?

    For instance, did anyone not notice that ALL of the formerly vacant grassland plots hugging AYE (9.6km) & the NUS-Clementi Rd flyover have been completely built over during last 8-9 years -- w/o any corresponding upgrading of the wider drainage system at the impacted site ?

    Take a look at the below summary. Sheer coincidence ? Or foregone conclusions resulting from the various acts of man ?

    When a 53-ha rainwater sink is destroyed & rapidly replaced by developments installed over construction-compacted clay soils, where can the rainwater possibly go ... other than swamp over the various artificial hardscapes ?

    1) Clementi Woods Park (12 ha):
    -- Mid-2000s: NParks removed numerous mature "undesirable species" trees (namely Acacia auriculiformis), no replacement tree-planting

    2) NUS High School (4.7 ha):
    -- Previously: Vacant grassland
    -- Construction: 2004 - Dec 2005

    3) NUS Cultural Centre, Museum & Music Conservatory (~6.2 ha, beside Clementi Rd):
    -- Previously: Vacant grass patch with 4 nos. 1/2-storeyed brick buildings
    -- Construction: Mar 2004 - Jul 2006

    4) NUS Kent Vale staff housing (~11 ha, opposite NUS High Sch):
    -- Previously: Vacant grassland
    -- Phase 1 construction: Mar-Nov 2011
    -- Phase 2 completed: Aug 2012

    5) NUS University Town (19.2 ha):
    -- Previously: Warren Golf Course (May 1962 - end 2000), then vacant grassland (2001-2007)

    -- Phase 1 construction: Jan 2008 - Jul 2011;
    => AYE (9.6km) flooded during rain on 12 Nov 2011 during receding mid-tide

    -- Phase 2 completed: Apr 2012 (2 residential colleges added)
    -- Phase 3 completed: Jan 2013 (EduSports complex)
    => AYE (9.6km) flooded during rain on 08 Feb 2013 ~1hr before low tide

    Recent Major Floods at AYE (9.6km) ...

    * 12 Nov 2011 Sat (1:45-5:30pm)
    -- High tide 3.0m @ 11:20am, Low tide 0.4m @ 5:58pm
    -- Sg Pandan Kechil's upstream section beside AYE overflowed during rain; Clementi West St 1, part of AYE (9.6km) & Varsity Condo (West Coast Rd) compound flooded

    * 08 Feb 2013 Fri (2:13pm)
    -- High tide 2.6m @ 9:20am, Low tide 0.2m @ 3:25pm
    -- AYE (9.6km) partly flooded during rain

    * 05 Sep 2013 Thu [8:15-9:30am)
    -- Low tide 0.5m @ 5:13am, High tide 2.6m @ 11:30am
    -- Sg Pandan Kechil's upstream section along AYE overflowed during rain; all lanes of AYE (9.6km) submerged & closed to traffic; Kent Ridge MRT Station, parts of NUS Sci & Engineering faculties also flooded

  6. Thanks once again Pat for this awesome analysis and info!



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