Source : Wikipedia.

LTA announces completion of investigation on the tunnel flooding incident at Bishan

The Land Transport Authority (LTA) has announced that it has completed its investigations into the flooding incident that occurred in the tunnels between Bishan and Braddell MRT stations on 7 October 2017, which affected about 231,000 commuters due to the stoppage of train services on 7 and 8 October 2017.

LTA investigations have concluded that the incident was entirely preventable had the SMRT team responsible for the maintenance of the Bishan storm water sump pit and pump system carried out their duties as prescribed under the maintenance regime.

It said that LTA found the existing flood protection measures at Bishan tunnel portal were designed to be more than adequate to handle the rainfall that fell on 7 October 2017. The storm water sump pit has a capacity of 5,044 cubic metres, which is much more than the estimated 640 cubic metres of rainfall that was cleared from the tunnels between Bishan and Braddell MRT stations.

“This is complemented by three high capacity pumps each capable of removing 85 litres of water per second from the storm water sump pit. These flood protection measures, which are designed with a huge buffer, have safeguarded the Bishan tunnel portal entrance for over 30 years without incident,” it noted.

According to LTA, Independent laboratory tests by Singapore Test Services (STS) also found that the individual components of the Bishan storm water pump system were not defective.

Representatives from LTA, SMRT and the national water agency PUB, participated in the tests and reviewed the results, which confirmed that all five float switches and the pump motor control panel were functional. The three storm water sump pumps at Bishan were also in working condition, as they could be manually activated following the incident.

“Representatives from LTA, SMRT and PUB were present to witness the tests and review the findings,” LTA stated.

LTA noted that based on further laboratory tests by STS, there are three possible scenarios for why the Bishan storm water sump pump system, while functional, failed to work as intended.

These are:

  • The lowest “stop” float switch was likely impeded by accumulated silt and sludge in the storm water sump pit, which prevented the storm water sump pumps from activating;
  • The highest “alarm” float switch, which is wired independently from the other four float switches, could have been impeded by floating debris in the storm water sump pit, which prevented the SMRT Operations Control Centre from being alerted of rising water levels in the pit; and/or
  • The SMRT crew that conducted corrective maintenance for the storm water pump system on 13 July 2017 may have failed to switch the pump controls back to “Automatic” mode after work was done.

“Precisely which of these three possible failure scenarios, or a combination of them, led to the events of 7 October 2017, cannot be established definitively, as the exact conditions cannot be recreated following immediate remedial works to the Bishan storm water sump pit and pump system,” said LTA, adding that this includes the clearing of silt, sludge and debris from the sump pit; and the installation of new pumps and float switches to further mitigate the risk of a reoccurrence.

Nonetheless, the authority stressed that all three possible failure scenarios could only arise as a result of a lack of proper maintenance, audits and supervision.

It noted that SMRT had also separately and independently found that the team responsible for carrying out preventive maintenance works for the Bishan storm water sump pit and pump system was remiss as they had not been carrying out the required works on the purported maintenance dates, and in fact, falsified maintenance records.

LTA said that SMRT has accepted full responsibility for the incident and has since disciplined the staff found responsible.

“LTA and SMRT have also implemented numerous engineering enhancements to improve the robustness and resilience of flood protection measures at Bishan and other tunnel portals to further mitigate the risk of a recurrence,” it said.

LTA said that lessons learnt from the flooding incident were shared today at the 5th Joint Forum on Infrastructure Maintenance.

The forum, which is jointly organised by LTA, PUB, SMRT and SBST, is said by LTA to bring together the collective expertise of agencies involved in the maintenance of critical infrastructure and allow discussion of measures to ensure that Singapore’s critical infrastructure stays resilient to flood risks.

It noted that the forum also highlighted the importance of all building and infrastructure owners carrying out regular preventive maintenance and reviews of the adequacy of their flood protection measures, particularly with the increasing impact of climate change.

“In addition, a joint LTA-PUB Standing Committee was announced to look into the overall design of flood prevention measures and institutionalise knowledge on flood prevention,” LTA said.

Mr Ridzuan Ismail, Director, Catchment and Waterways Department, PUB, said, “We recognise that LTA has worked with SMRT to enhance the resilience of flood protection measures following the incident. We also appreciate collaboration with these agencies in sharing information with other infrastructure owners with our common objective of staying resilient to flooding risks.”