In early January, widespread and continuous rainfall fuelled by the north-east monsoon led to slight flooding of half a lane of Airport Boulevard at Changi Airport.
Fortunately, the water ponding was minor and did not affect traffic, said Ms Cheng Liping, senior manager of master planning at Changi Airport Group (CAG).
But flash floods within the airfield can have a dire impact on safety, aircraft, runway operations and air connectivity.
"We are concerned because drains are located within the airfield. If any of them overflow, it will affect the adjacent (plane) parking stands and runways. Any flooding may cause moving aircraft to skid. That will be quite catastrophic," said Ms Cheng.
To further prepare for flash floods caused by intense rainfall, climate change and rising sea levels, a network of sensors was recently installed across the airport's drain network, which includes Airport Boulevard and the airfield.
Flash floods in Singapore are mainly caused by a deluge of heavy rain that overwhelms the capacities of drains.
Most of Changi Airport's drain outlets lead to the sea.
The seven solar-powered sensors - deployed from the middle of this year and announced yesterday - use radar to monitor water levels in drains that are fixed with security grilles. Each sensor is also fixed with a closed-circuit television camera to monitor the drainage situation.
CAG's engineering and development team is alerted if the water level in a drain exceeds 60 per cent. Workers would then be sent to clear any debris such as leaves, grass cuttings and plastic waste trapped in the grilles, blocking the flow of water out to sea.
One outlet drain located south of the airport's aerodrome is more at risk of overflowing, as its grille bars are more narrowly spaced and more easily trap debris, which impedes water flow. If the drain's water level exceeds 60 per cent, the grilles are automatically lifted to release water like a dam.
By February next year, four more sensors will be added to the airport's drain network. Those four will cover potential flooding hot spots, said Ms Cheng.
She noted that many of the drains have been around since the 1970s, and some of them may not have sufficient capacity to handle more intense rainfall events.
Although Airport Boulevard has undergone drainage upgrading works in recent years, it would be tricky to upgrade drains near one of the hot spots, as it is located at a junction of a taxiway for planes.
"If we close the junction, too many taxiways are affected, and that affects airport operations," explained Ms Cheng.
To better monitor the growing network of sensors, CAG has designed a drain monitoring digital map that allows its staff to easily identify drains that are nearing capacity or may be choked.
The drains are colour-coded on the map, with red showing those that are more than 70 per cent full.
Mr Teng Hwee Onn, senior vice-president of engineering management and systems planning under CAG, said: "This map allows us to deploy our maintenance and cleaning resources in a more targeted, efficient and effective manner."
The sensors are one in a suite of anti-flood measures that the airport has introduced in recent years.
The airport is built on higher ground above mean sea level, and the future Terminal 5 will be built 5.5m above mean sea level. Terminal 4, as well as Jewel Changi Airport, were built on higher ground, and about 1m higher than their adjacent drains.
In 2016, a detention pond with the capacity of 216 Olympic-sized swimming pools - also located at the south of the aerodrome - was built for flood protection.
It holds runoff from Terminal 3's airfield and the airport's public roads that is released at low tide into the sea. The detention pond helps to ease the load off a 16m-wide canal.