National water agency PUB will be taking action against construction firm Samwoh for unauthorised drainage works that caused a flood in Pasir Ris last month.
The junction of Tampines Avenue 10 and Pasir Ris Drive 12 is not flood-prone, but Samwoh's actions led to a flood that left cars and motorists stranded for almost two hours on Aug 20, PUB said yesterday.
The Sewerage and Drainage Act provides for fines of up to $50,000 for works affecting the storm-water drainage system, and up to $20,000 for unauthorised alteration of the system.
A 200m section of the traffic junction was affected, with knee-high water in some places. Thirteen vehicles were partially submerged and several motorists needed help to leave their vehicles. One person was taken to hospital.
The flood persisted for almost two hours, an unusually long event, as most flash floods in Singapore subside within 30 minutes.
PUB said in a statement that the drainage system there was "more than adequate to cope with the rainfall intensity" on that day.
Water is channelled to Sungei Api Api, a canal about 1km from the flooded junction, and sensors showed that it was only about 50 per cent full on Aug 20.
The amount of rain was also comparable to rain events on April 5 this year and Nov 22 last year, neither of which gave rise to flooding.
Mr Yeo Keng Soon, director of PUB's catchment and waterways department, said: "This prolonged flooding was entirely man-made. It would not have happened if the contractor had complied with PUB's regulations for drainage work."
PUB investigations showed that Samwoh, which is carrying out road widening along Tampines Avenue 10, had built a diversion drain that was meant to reconnect to the original drain farther downstream.
But there were multiple issues with the plan, which resulted in the Aug 20 flood.
Samwoh had not gained approval to begin drainage diversion, and the unauthorised works impeded the discharge of rainwater into Sungei Api Api, resulting in flood water accumulating at the traffic junction, the lowest point in the vicinity, PUB said.
Samwoh was subsequently instructed by PUB to remove all obstructions to ensure storm water could flow in the drain.
A check on the Land Transport Authority's OneMotoring website showed that the Samwoh project involves the widening of Tampines Avenue 10 to a dual four-lane carriageway with a bus lane in each direction. Work is expected to be completed by the fourth quarter of next year.
Mr Lee Cai Jie, chief engineer for drainage operations at PUB's catchment and waterways department, said during a media briefing on Friday that in road widening, affected sections of the roadside drain have to be reconstructed.
But contractors must seek approval from PUB before starting any drainage works, he said. The submission should include a Drainage Detailed Plan setting out the scope of drainage works, such as the proposed alignment and sizing of the new drain, said Mr Lee.
If this is approved, the contractor must then submit a method statement to propose how and when the drain will be constructed prior to starting work.
Said Mr Lee: "Samwoh had submitted the Drainage Detailed Plan and obtained approval from PUB. But it failed to follow up with the method statement for PUB's approval before proceeding with the drainage modification works at its work site."
The last time that enforcement action was taken against a contractor for obstructing drainage systems under the Sewerage and Drainage Act was in 2017.
Construction company Sato Kogyo was fined a total of $14,000 for causing floods in the Thomson area on Christmas Eve in 2016.
PUB's Mr Yeo said yesterday the agency maintains a network of roadside drains, canals and rivers that convey storm water into reservoirs or out to sea.
He added: "It is imperative that our drains are kept free-flowing, especially as we experience more frequent and intense rainfall due to climate change. This incident should serve as a stark warning to all contractors that any construction works carried out must not adversely affect the storm water."
