SINGAPORE - Rail operator SMRT and regulator Land Transport Authority have taken steps to improve the resilience of the anti-flooding system near Bishan station.

Transport Minister Khaw Boon Wan told Parliament Tuesday (Nov 7) that all existing float switches in the storm water sump pit have replaced.

The pumps themselves have also been replaced with heavier duty models which can handle "water with more sediments."

Parallel float switches have also been installed, so that no one switch determines the activation of pumps.

A new radar-based sensor system has been added to independently monitor water levels in the storm water sump pit.

Also, the sump pump control panel has been "relocated away from the tracks, to enable easier manual access to pump operations if needed during operating hours".

These measures come in response to the MRT tunnel flooding near Bishan station on Oct 7.

The unprecedented flooding incident had shut down a large stretch of the North-South Line for about 20 hours and affected 250,000 commuters.

Investigations found that the storm water pit in the tunnel between Bishan and Braddell MRT stations - designed to collect and pump out rainwater - was likely close to full before the recent flooding incident.

The storm water pit at Bishan has a capacity of over 5,000m3 - about two Olympic-sized swimming pools - and can hold about six hours of continuous heavy rain.

Based on the amount of rainfall in Bishan on Oct 7, the water pit - which should have been empty - would have been only about 13 per cent full.

The water pit was also equipped with three water pumps, that could each pump out 85 litres of water per second, but these were not activated due to the faulty float switches.

The failure of the float switches resulted in water flooding the tunnel between Bishan station and the underground Braddell station. The water had to be manually pumped out through the night, with help from the Singapore Civil Defence Force and PUB.

SMRT later discovered that its staff responsible for maintaining the anti-flood system may have falsified records.

They did not obtain track access during the stated maintenance dates, and the pumps were not activated either.

Due to the maintenance lapse, sludge and debris had accumulated in the lower compartment of the water pit. These are being removed.

Mr Khaw said the Land Transport Authority is still investigating why the float switches failed to function normally on Oct 7.

To prevent a recurrence, SMRT will maintain flood protection systems monthly instead of every quarter, he added.

He said the operator is tightening its flood recovery plans, and will strengthen cooperation with the Singapore Civil Defence Force and PUB through regular exercises.

SMRT is also buying additional emergency equipment like portable pumps, to better respond to such emergencies, he said.