Climate change can lead to more flash floods

Amid the north-east monsoon and record rainfall events over the past few months, flash floods have hit Singapore on at least three occasions. 

The most recent occurred in the evening of Dec 29, when several places including Dunearn Road, Bukit Timah Road near King Albert Park, and Balmoral Road were inundated.

Cars were stranded along many of the affected roads.

The rainy north-east monsoon season, in particular, made November Singapore’s wettest since rainfall data collection began in 1980. This wet phase of the monsoon is expected to persist until late January 2025.

“We are unable to predict the intensity of these showers far ahead of time, as rainstorms can develop quickly in Singapore’s climate,” said Mr Maurice Neo, director of the catchment and waterways department at national water agency PUB.

“However, extremely heavy rainfall over a short period can sometimes exceed the capacity that the drains are designed for. That is when flash floods – small and localised floods that typically do not last for more than an hour – may occur,” he added.

On Dec 29, PUB helped several motorists and property owners by moving their vehicles away from flood waters, pumping water from the basements of residential and commercial buildings, and handing out flood protection devices such as flood bags and barriers, said Mr Neo.

The rainy season, which lasts through January, is not expected to let up any time soon. The La Nina climate phenomenon, which brings wetter and cooler conditions to the region, is forecast to return some time between now and March 2025, causing rain to extend beyond the monsoon season.

The Meteorological Service Singapore’s website shows that the Republic is currently on “La Nina watch”.

 

According to Singapore’s third national climate change study, climate change is also expected to bring heavier, more intense and frequent storms. This could lead to more flash flooding.

On Nov 22, a deluge triggered two flash floods in Yishun and Potong Pasir.

The month before, on Oct 14, one of the heaviest downpours to hit western Singapore in over 40 years brought flooding to places like Yuan Ching Road, Bukit Timah and Tampines.

These three episodes were among the heaviest rainfall events since 1978. Dec 29’s 134.6mm of rainfall – recorded in the central region between late afternoon and evening – comprised 41 per cent of Singapore’s average monthly rainfall in December and fell within the top 1 per cent of maximum daily rainfall records since 1978.

Between 1981 and 2020, there were more than 80 extreme rainfall events in the top 1 percentile, according to calculations by Assistant Professor He Xiaogang from NUS’ department of civil and environmental engineering.

The Bukit Timah area has been flood-prone for decades, and drainage improvement works have been carried out along the Bukit Timah Canal and Bukit Timah First Diversion Canal over the past 10 years.

There are also ongoing works to widen and deepen a 900m stretch of Bukit Timah Canal from Rifle Range Road to Jalan Kampong Chantek to enhance flood protection in the area, which will be completed by 2026, said PUB.

But there is only so much drain widening and deepening that can be carried out. The authorities have started to take more measures to get the public better prepared for floods.

In November, in the lead-up to the monsoon, PUB launched its first national campaign to prepare the public for flash floods.

On Dec 29, a brick retaining wall at the front of two conserved terraced houses in Cairnhill Road also collapsed amid the wet weather.

In response to queries from The Straits Times, the Building and Construction Authority (BCA) said its engineers responded and assessed the two houses to be structurally sound.

 

A retaining wall is designed to hold back soil where there is a difference in ground levels, so that one side is higher than the other.

BCA said the adjacent buildings and their retaining walls are also in safe condition. The units affected are 128A and 128B.

The BCA spokesperson also said that it will serve orders to the owners of the two units to appoint professional engineers to conduct a detailed investigation, advise on immediate precautionary measures and recommend permanent fixes.

The remaining historic buildings in the Cairnhill area are mostly two-storey terraced houses, with many of them built in the 1910s and 1920s, according to the Urban Redevelopment Authority’s website. They received conservation status in 1989.

Every year before the monsoon, PUB would distribute flood protection gear to homes and businesses in flood-prone zones and hot spots. This includes inflatable flood bags and various flood barriers. As at early November, around 16,000 of such items were distributed.

 

Beyond preparing for floods, Prof He said that there is scope to improve early warning systems for extreme rainfall events. But early warning in tropical urban environments requires a multi-faceted approach, he added.

“We need to enhance our understanding of how different climate and weather systems interact across multiple scales, from large-scale (weather phenomena) to local weather conditions. We also need to assimilate diverse data sources,” he said.

Flood prevention tips for homes:

• Always ensure drains and drainage systems within your compound are well-maintained and choke-free, and that flood barriers are in good working condition.
• Install flood barriers when there is a risk of flooding to protect your property from the rising water level.
• Move essential items to higher ground.
• Turn off all main switches and disconnect electrical appliances in the event of evacuation.
• Do not touch electric switches or appliances if you are wet or standing in water.