Sunday, November 22, 2009

Bonfires of Trust, Flash-floods of Pain: Bukit Timah Floods 2009

While revising for my natural hazards exam, LIM JIAYI (there i acknowledged you!) frantically told me to read the newspapers about the flooding event at bukit timah road on 19/11/09. I was a little skeptical about how big that flood can be, despite the crazily cold and wet weather this past week, due to the north-east monsoon.

Then, I saw this:

Sorry for my delayed reaction and initial skepticism, BUT THIS IS HUGE.

According to some NEA and PUB statements, from between 1.20pm to 1.50pm, 93mm of rain fell in the Bukit Timah area. That being equivalent 115 OLYMPIC -SIZE POOLS' worth of water. okay, to make another less sensational comparison, our average monthly rainfall is about 200mm. Thus, in 30 mins, HALF A MONTH'S WORTH OF RAIN drenched Bukit Timah.

Let's say we are in the 1960s, and Bukit Timah is all green, vegetated and unurbanized. The flood probably wouldn't have occured. Not on such a big scale anyways.

This is because the soils would have been able to take it the excess water, via a process known as Infiltration (#6 in the diagram). The absorbed-water will then flow laterally underground, through processes known as Throughflow, Interflow and Baseflow (#8 in the diagram), into rivers and streams. The underground flows are slow flows - it can take weeks for the water from the storm event to reach the river.

#6: Infiltration (water enters soil)
#7/8: Throughflow, Interflow, baseflow (lateral flows underground)
#5: Overland flow (self explanat0ry)

But with urbanization and concrete everywhere, infiltration is unable to take place, as concrete simply does not have the capacity to take in water. Thus, instead of travelling to the river by slow underground flows, water now travels through Over-land Flow (#5). Our drainage system helps dissipate the water, but when a storm event like 19/11/09 occurs, it overwhelms the capacity of the drain. This is thus, an Urban Flashflood event.

Ah, the poor Ferraris in the basements.

However, the thing that most people probably didn't pay much attention to is the colour of the flood waters. let's refresh our memory:

According to LIM JIAYI, and i quote, "This looks like Milk Tea."
Yes, it certainly does. Which begs the question, why does it look like milk tea????

It looks like milk tea because of the massive soil erosion that took place when the flood occured. And the eroded sediments were then transported by the storm waters.
This erosion problem isn't just limited to 'this prime residential area' (to quote joseph wong, some business man).

Take a walk by the fields next to Jurong East MRT station, in the NUS campus. It's almost everywhere. The worst case I've seen is probably along the stretch outside Toh Guan Dormitries. So maybe its a problem that the NEA should look into. Because soil erosion is a pretty big hassle, a lot of money probably goes into replanting grass at these area.

Its rather ironic if you ask me, some of the worst cases of natural hazards we have in Singapore such as this flood event often occur a long the 'prime' district. Landslides too, are rather common in Bukit Timah and Hillview area.

Jalan Dermawan (hillview) landslide 2007.

so as my jc teacher likes to say DON'T BUY HOUSES IN HILLVIEW! (HAHAHAHA)

okay back to revision!
love, Serene!


  1. *Geography geek moment*

    I would seriously love to see the storm hydrograph of the canal... the spike in storm flow must be insanely high.

  2. Nice analysis on the flood. I liked the Milk Tea reference. Sarah Chua here btw. (:

  3. Wow, you have a great blog here! I'm a Singaporean doing Geography at Oxford University and your blog just makes me miss Singapore so much!

  4. wow. interesting insights that i never thought of (cus i didn't take geo modules). we shouldn't be buying into what our govt says that we have a very good drainage system. things certainly can be improved.

  5. awesome loads! this is interesting. haha