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Floodings raise questions on existing design methodology for drainage system

According to the Public Utilities Board, the flash floods that occurred in nine locations in eastern Singapore on Monday morning (8 Jan) occurred in low-lying grounds with drainage works in progress and Meteorological Services Singapore said that the floods arose because rain developed too quickly in the area

PUB's director of catchment and waterways Yeo Keng Soon told media that the drains were working and managed to drain the floods when the rain stopped.

It is noted that all the reported flood areas are low-lying grounds with drainage works in progress except Tampines Avenue 12.

According to PUB, investigations are still ongoing in the area, and should be completed within the next two days. Preliminary investigations did not show any obstruction. Investigations will also be conducted to find out if there were any other contributing factors to the flood.

The ongoing drainage works in the affected areas will be completed by March 2019 and the revamped drains will discharge water faster.

Reports from Mainstream Media accepted the conclusion from PUB and the relevant agencies that the flooding that covered the eastern part of Singapore happened "because 118.8mm of rain was recorded at the Kim Chuan Road rain gauge within just four hours. That amount is about half of Singapore's average monthly rainfall in January."

Explanation for flood, plausible but leaves many questions unanswered

Now if one were to refer to the data collected by the weather monitoring system, the highest rainfall received on Monday was at Tai Seng and captialised on by the authorities. But rather than saying that 118.8mm of rain was recorded at the Kim Chuan Road rain gauge within just four hours, half of Singapore's average monthly rainfall in January. Why don't we look at the rainfall on an hour to hour basis?

Based on the info from the weather.gov.sg, we can see that the peak rainfall during the four hour period is 45mm at 8am for the area.

Also for the other areas that are in the east, the following values are the peak at 8am.

Simei                    - 55mm
Chai Chee            - 50mm
Marine Parade   - 27mm
Tanjong Katong - 25mm
Changi                  - 25mm
Marine Parade    - 24mm

It is weird how the reports go on about low-lying area and heavy rain fall when Clementi which is also a low lying area, had no reports of flooding that day despite having a heavy rainfall of close to 45 mm at its peak, similar to areas that flooded.

Rainfall captured for Clementi

According to Mohd Faisal Yusop who commented on Today Online's Facebook page, "I was there this morning. I could see the water actually overflowed from the canal. The construction of the canal needs to be relooked. Lucky this morning the tide was low. Else it could have been worse." 

Screenshot of tidal info from weather.gov.sg

And true to Mr Yusop's point that when the flooding along Bedok occurred, it was near to the lowest point of the low tide. Had the tide been high, the flooding would have been much severe.

Not to mention, the junction of Upper Changi Road and Bedok North Avenue 4 where the flooding had been recorded and went viral, was located just metres away from a storm drain canal leading to the sea. Given the eye-witness testimony that the water overflowed from the canal, was the dam not opened for the rainwater to be discharged?

And judging from the peak hourly rainfall, it is clear that the drainage system had problems coping with such high volumes of precipitation. There seems to be an issue behind the design methodology for the drainage system.

Has there been a revision to drainage standard and the effect of increased urbanisation in Singapore?

According to a report produced in 2012, by the Expert Panel on Drainage Design and Flood Protection Measures appointed by the Ministry of the Environment and Water Resources on 30 June 2011 to review all flood protection and risk management measures that will be implemented in Singapore over the next decade, wrote:

The panel recognises that the impact of continued urbanisation and future impacts of climate change will eventually put a strain on the existing drainage system. In some cities, there is wide-scale retrofitting of source control to manage storm water runoff.

While PUB already takes into account future land developments when designing drains, land use plans could change. There are currently no requirements to mitigate the adverse effects of further urbanisation. Furthermore, while the adoption of ABC Waters concept is useful in managing surface runoff, its benefits may be limited due to lack of land for large scale implementation. It is therefore recommended that guidelines to regulate new and redevelopment projects be developed, for example, to make provisions for compulsory compensatory storage so as to mitigate adverse effects of further urbanization. Given that flash floods typically occur due to the intensities of rainfall for short periods, the provisions for compulsory compensatory storage or other forms of runoff delays would also help even out the impact of rainfall intensities.

Even if the drain and canals were functioning as they should be, but as the surface run-off – due to the build-up of concrete pavement and structure – has increased many folds due to the relentless drive to push for development, has there been a drastic relook at how Singapore manages its drainage system to prevent flooding?

As documented by the entries in Wikipedia and news reports, flooding is no longer once a fifty-year event and a once a-few-months event which disrupts the lives of citizens and damage the property both private and public. The ministry in charge cannot simply attribute the floods to natural occurrence, but yet, tell the citizens everything that was meant to prevent the floods had been working fine.

PUB attributes low-lying area, tidal influence, and heavy rain as reasons for the occurrence of a flood. But like the SMRT breakdowns, the number of occurrence makes it hard to believe it can be solely be attributed to chance.

It will be a shame to modern Singapore if our country is to expect a flood, everytime the rain gets heavy. Why is there no feat of engineering or policy in place to ensure no flooding incidents would take place since the 2011 floodings at Orchard?