SINGAPORE — Singapore could be hit by flash floods when the annual north-east monsoon returns next month, as government agencies yesterday laid out plans to reduce the risk of them occurring and identified 36 areas where drainage will be improved.

Rainfall recorded at the Changi climate station during the current “inter-monsoon” season, which occurs around October and November, “is already above average” while there have been a few storms, said National Environment Agency Senior Meteorological Officer Chow Kwok Wah.

Flash floods were reported at the junction of New Upper Changi Road and Chai Chee Road after heavy rain in the eastern part of Singapore yesterday afternoon, causing traffic to be held up for over 20 minutes.

The authorities are predicting “slightly above-average rainfall and rainy days” for the north-east monsoon, which is expected to last from the middle of next month till March.

Said Mr Chow: “For rainfall in November, we are expecting slightly above-average rainfall. For the rest of the north-east monsoon season, we are expecting slightly-above average rainfall and rainy days.”

The authorities have warned that when heavy rain coincides with high tides, typically ranging from 3 to 3.4 metres, flash floods could take place in low-lying areas. In addition, there could be two to four episodes of monsoon surges, which will bring prolonged periods of moderate to heavy rain, lasting between two and five days. Singapore usually experiences about 20 days of rain this time of the year, with December and January typically the wettest months.

To minimise incidences of flash floods, national water agency PUB is working with the NEA’s Department of Public Cleanliness to step up drainage maintenance and monitoring to make sure drains are not blocked.

The PUB has also increased its weekly inspections at about 100 construction sites to three times a week to ensure drains are not obstructed. About 6,000 scupper holes and drain inlets — found by the side of roads — at flood areas and hotspots have been replaced with better-designed ones, which have additional vertical openings that enable rainwater to be drained should the main gratings be partially blocked.

To prepare the public for the rainy season, flood advisories have been distributed to 500 residential units and shophouses in flood-prone areas. The NEA will also issue advisories a few days ahead of the wet weather, while warnings will also be given a few hours before the onset of a downpour.

The 36 new locations slated for improvement included the Sungei Pandan Kechil canal, which overflowed after heavy rains hit many parts of western Singapore last month, leading to a shutdown of the Ayer Rajah Expressway (AYE) for 40 minutes during the morning peak-hour commute.

Giving an update on the project, PUB Director of Catchment and Waterways Tan Nguan Sen said the inlet and outlet points of the AYE culvert — which sees three drains meet in an intersection — have been widened to allow more water to pass.

The agency will also be calling a tender next month to install a temporary tidal gate near West Coast Road to create a storage area for run-off during heavy rains while preventing seawater from flowing into the canal. Longer-term measures to deepen and widen the canal will commence in 2015.

Apart from deepening and widening canals the PUB is building a “detention tank” to ease the load of the Stamford Canal.

Work on the detention tank, which has a capacity of 15 Olympic-sized pools and costs S$69.7 million to build, will commence at the end of the year and is expected to be completed in early 2016.

Constructed underground beneath a proposed nursery and coach park at Tyersall Avenue next to the Botanic Gardens, it will temporarily store excess stormwater from the existing drains along Holland Road.

The PUB is currently piloting a flood-forecast system in the Marina Catchment area, aimed at predicting flash floods and providing advanced warning. Drainage improvement works are also ongoing at 176 other locations islandwide.