1. Investigations by the Land Transport Authority (LTA) have revealed that a combination of factors caused the flooding in the tunnels between Bishan and Braddell MRT stations on 7 October 2017. The accumulation of water in the tunnels caused regular train services on both bounds of the North-South Line between Ang Mo Kio and Newton MRT stations to be disrupted, from about 5.30pm to 12.28am on 7 October 2017 (close to 7 hours), and from 6.02am to 1.36pm on 8 October 2017 (about 7.5 hours).
Existing flood protection measures
2. Two key flood protection measures are deployed at MRT tunnel portal entrances. First, large storm water pits collect excess rainwater before they can flow into the tunnel entrances. The storm water pit at Bishan has the capacity of about two Olympic-sized swimming pools, and can contain about six hours of continuous heavy rain.
3. Second, the storm water pits are equipped with multiple pumps that siphon water to an external drainage system. These pumps are controlled by a series of sensors (also known as float switches), which automatically turn the pumps on when water reaches specified levels. Should water rise to a critically high level, a separate “alarm” switch (“high water level” switch) would trigger an alarm at the Operations Control Centre (OCC). This would alert duty officers who can then investigate the cause of the alarm, and activate additional measures if necessary (see Annex for switch diagram).
Likely Causes of the Incident
4. Under normal circumstances, rainwater collected by the storm water pit would have been pumped away as soon as water levels rise above the float switches, activating the pumps. On 7 October, all pumps failed to automatically activate despite water reaching very high levels. Subsequent checks found that the pumps in the storm water pit were all in working condition. The root cause of the flood is therefore likely due to failure of both the “stop” and “alarm” float switches. The “stop” float switch had remained open instead of closing when the water level in the storm water pit rose above it, preventing all the pumps from being triggered. The “alarm” float switch also failed to send a warning alarm to SMRT’s OCC when the water rose above it, preventing the OCC from reacting before the flooding occurred. As a result, water accumulated in the storm water pit and eventually overflowed into the tunnels.
5. Based on rainfall data for 7 October, the total volume of rain water collected by the catchment area around the Bishan tunnel portal entrance was approximately 640m3. Had the storm water pit been empty, this volume of water would have only filled about 13% of the pit. Hence, it is likely that the storm water pit was close to full on 7 October – which points to the likelihood that float switches were faulty even before the incident. During recovery efforts, it was also observed that sludge and debris had accumulated in the lower compartment of the Bishan storm water pit, where the pumps and float switches are. This could have affected the effective operation of the float switches.
Safeguards to protect commuters
6. For the safety of commuters, the OCC had switched off traction power to affected tunnel sectors as soon as the flooding was detected. Even if the flooding had gone undetected and water levels had risen to the power rails, built-in safety features would have kicked in to automatically cut the power supply to the tracks. SMRT had also swiftly taken action to halt train services as soon as flooding was detected, to ensure the safety of commuters. Commuters were de-trained at stations and no passengers were on the train that stalled in the tunnel.
7. Said LTA Chief Executive, Mr Ngien Hoon Ping: “LTA takes a serious view of these public transport service disruptions, which are understandably disheartening when they occur. Our commuters are at the heart of our public transport system, so we must work together and do all we can to prevent a recurrence.”