Moving to

I'm moving to Mostly.

I plan to use that site as a "self-marketing website" of sorts and to manage content in a way that I would otherwise not be able to do on blogger alone.

This blog will stay, ostensibly for more provisional ideas prior to refinement. I'll be gradually moving content (I still like) over to the other website. =)

Monday, June 6, 2011

On a Dam, Heavy Rain and Floods: Engineering Ramblings of a non-Civil Engineer

In the wake of one of many recent 50-year floods, Minister for the Environment and Water Resources (MEWR), Vivian Balakrishnan, said that all planning norms have to be reviewed "taking into account the very high probability that our weather patterns have shifted". (Note: it's not his fault. He's only just been appointed to helm MEWR.)

On the matter of the floods, I find it bewildering (or telling) that no one in government has publicly asked (and answered) the most basic systems-level question: "What has changed?" The first answer to that question is that the Marina Barrage was built.

Construction of the dam started in 2005 and was completed in 2008. Government estimates have that it would reduce flood-prone area in Singapore from 150 ha to 85 ha. At this point, I am suspicious at the validity of that estimate. I'm no civil engineer, but let's just talk things through.

Collection of water in the Marina Reservoir itself, is not a factor that encourages flooding even in high intensity rain like the 65mm in 30min of 5 June 2011. A crude estimate of the area of the reservoir is 1.5 million square meters, ensuring it collects about an average of 54 cubic meters of water per second in such rain, which is just over a sixth of the capacity of the pumps in the Barrage (280 cubic meters per second), so there is no problem dumping out just that amount of water. But what about when all the water from the CBD comes in? I'm not sure what multiple to use, but it appears that the CBD and all the area channeling water into the Marina reservoir may have more than ten times the area of the reservoir.

While it sounds like even an open channel (~350m wide) would not be able to discharge as much water as the pumps, it is notable that the Barrage raises the water level of the Marina basin and that of the water table of the central area. If I were to hazard a guess at outcome of the possibly complex impact on the drainage system brought about by the addition of the Barrage, I'd say that due to the raised water level (relative to pre-2005 levels), the Barrage may well reduce the rate at which rainwater can be removed in the drainage system (through the reduction of the "linear hydraulic head loss" or "slope of the water surface", reducing flow rates) and through percolation into the ground (since the water table has been raised). One might guess that (the limited) percolation into the ground provides some buffer time (whose magnitude I'm not sure of) so the drains will be able to remove a decent amount of water. On the possible flow rate again (determined by the "linear hydraulic head loss"), the opeartion of the pumps might be able to increase it, but the effects would most certainly take time to propagate back towards the town area. Thus, by some magic hand-waving, it is not clear that the post-2008 situation is less flood prone.

I do hope that all this was considered in the feasibility study for the Barrage. Call me skeptical, but I do not think the "vision" of a lawyer named Lee should take precedence over sound engineering analysis. (... which this is not, exactly, though the questions asked may be featured in such.)

But before blaming the Barrage, let us try to look for more direct causes.

Singapore has had storms before, and a cursory comparison via looking out the window does not seem to support any marked increase in intensity. While the numbers do indicate that the June rains have been rather intense, though it is not clear that "climate change" has occurred. Even with a change in weather patterns, there is no reason to believe that these numbers would not increase fairly continuously without any huge jumps. ("Chaos" in weather does not work as an explanation unless Singapore is in a special "attraction" basin for storm clouds, which, if so, would be apparent in large changes in annual rainfall.)

Some relevant numbers:

  • 5th June 2011: 124mm in the Central Area (with 65mm within 30min)

  • 16th June 2010: 100mm within 2 hours (leading to flooding in Orchard Road

  • More numbers from 2010

  • Average rainfall in June: 161.2mm; Average rainy days: 13. (Averaged over more than a century.)

  • 20 Dec 2006: 24-hour rainfall recorded was 366 mm, third highest 24-hour rainfall recorded in 75 years.

  • 1969: second highest recorded 24-hour rainfall (467 mm)

  • 1978: highest recorded 24-hour rainfall (512 mm)

Absent any other information, we should then ask the question of whether over the past years, during periods of relatively high rainfall, drains were filled to near capacity. If this were the case, it would explain the discontinuous effect where a non-drastic increase in rainfall leads to so many (statistically speaking) 50-year floods. It would then clearly not be due to the construction of the Marina Barrage. The question then becomes, if this were the case, why wasn't anything done to prevent flooding? Drains filled to near capacity clearly implies that a reasonably probably variation in rainfall could easily lead to a flood. Was a cost-benefit analysis done or was this just not noticed?

If, on the other hand, during periods of high rainfall, drains were not anywhere near filled to capacity, there is a stronger case that the Marina Barrage is the cause. Again I ask, albeit more specifically, did the team that built it assess the impact of the dam on drainage. Some of this could have been easily done by numerical simulation.

Whatever the likely cause, a lot of economic damage has been done by the floods and there are questions to be answered.


Xtrocious said...

From a non-engineer...

I think that the recent construction of massive underground malls in the Orchard Road area may also have contributed to the lessened ability of the ground to absorb water...

Hence even if there were more "uncovered" grass patches, the natural storage ability would still be greatly reduced...

Anonymous said...

Sounds rational and logical. Hopefully it will stimulate discussion towards a acceptable solution.

convexset said...

I think MEWR should step forth to talk about what went into the decision to build the barrage, or what the exact principles by which it operates are. If it truly is the case that it was built primarily because a powerful man thought it was a good idea, then no sound engineering analysis will come up... which would be disappointing.

I have a sense of what the nature of power is, so it would come as no surprise if powerful people can push through decisions that are otherwise poor.