SINGAPORE: While the national water agency, PUB, takes into account factors such as terrain and rainfall intensity of the area when designing drains, it is "not feasible" to build them to a capacity that accommodates every extreme rainfall event.
Minister for Environment and Water Resources Masagos Zulkifli said this in Parliament on Monday (Feb 5). He was responding to questions from MPs who asked about flash floods that occurred at nine locations in eastern Singapore on Jan 8.
PUB previously said the highest recorded total rainfall that morning was 118.8m - about half of Singapore's monthly average in January in just over four hours.
Speaking in Parliament, Mr Masagos reiterated that flash floods on the day were due to intense rainfall that temporarily exceeded existing design capacity of drains.
And with climate change, he said Singapore can expect more intense rainfall "to be the norm in future".
"To prepare for this, PUB has raised drainage design standards since 2011, so that our drains can handle up to 45 per cent higher rainfall intensities," Mr Masagos said.
Mr Masagos said PUB has also commenced and completed improvement works to drains at 327 locations, with another 77 undergoing similar works.
MASSIVE LAND, HIGHER COSTS TO ACCOMMODATE EVERY EXTREME RAINFALL
But Mr Masagos cautioned that drainage design needs to be done with practical considerations, and not for extreme conditions "all the time for all places".
Building drains to accommodate "every extreme rainfall event" would require the Government to acquire massive amounts of land and much higher costs, he said.
"Bedok Canal, which serves some of the affected areas, is being widened at a cost of S$128 million from its existing width of 38m to 44m, wide enough to accommodate an expressway of 10 lanes, five lanes each way," Mr Masagos said.
But even with this widening, he said that there is no guarantee that there will not be floods in the future.
That is because climate change could result in rainfall events of even higher intensity that could exceed design capacity, he said.
"To deal with the most extreme historical rainfall events, the Bedok Canal would need to be widened to at least 62m, displacing the Bedok Park Connector and community spaces adjacent to the canal," he said.
Mr Masagos said expanding the canal could even affect surrounding residential areas, and end up looking like a 16-lane expressway.
He said authorities have to design drainage taking into account competing needs for housing, parks and roads.
"Building our drains for extreme conditions would mean that much of the capacity would be extremely costly, but not needed most of the time."
Instead what authorities will do is ensure critical infrastructure is well-protected from extreme rainfall.
Mr Masagos said this can be achieved through PUB's source-pathway-receptor approach, a holistic approach that also includes developers having to implement on site measures to slow down stormwater run-off from inundating public drains during heavy rainfall.