[UPDATED: on 7 September, adding PUB's stop-gap measures on AYE]
The Public Utilities Board (PUB) will build a two-metre-tall tidal gate within six to nine months' time to stem the flow of rainwater into the Sungei Pandan Kechil canal.
Additionally, it will spend the coming months until the end of this year expanding the openings of drains passing beneath the Ayer Rajah Expressway (AYE) to improve the flow of water into and out of them.
These measures were introduced after the AYE was closed for 40 minutes on Thursday morning after the canal overflowed from the heavy rain and rising tide, causing flooding on all four of its lanes.
Running alongside the AYE, the Sungei Pandan Kechil canal receives rainwater from one side of the expressway, and conveys it through drains running beneath the road to the other, where it flows down the canal and into the sea.
The installation of a tidal gate along the canal, therefore, will "prevent high tide from the sea from flowing into the canal, creating a temporary storage buffer for stormwater flow", a PUB spokesperson told Yahoo Singapore on Saturday, adding that its design and other dimensions are currently being worked out.
"This measure will help alleviate the situation during a heavy storm," she added.
In the longer term, however, the agency will upgrade the capacity of the Sungei Pandan Kechil, through widening and deepening both the canal and its accompanying drains, following a projected nine-month design study. The PUB predicts that construction to do this should begin by the first half of 2015.
On Thursday evening, Minister for the Environment and Water Resources Dr Vivian Balakrishnan said it was "not acceptable" for a major expressway to shut down due to flooding.
"We're going to need to do some work there," he said.
"We will need some time, but I am confident that we can make an improvement to the situation... I told PUB we've got to do our best to make sure this doesn't recur in the future."
He was speaking on the sidelines of an event at the National University of Singapore.
Heavy rain fell over the central and western parts of Singapore on Thursday from 8.15am to 9.30am. Commonwealth and Kent Ridge were among the areas hardest hit by flash floods.
Earlier in the day, Dr Balakrishnan posted photos and a note on his Facebook page which said his "regular jogging track was under water".
He said drainage improvement work at Commonwealth Avenue will be completed by June 2015.
"The flooding at AYE will require expansion of the Sungai Pandan Kechil which drains south to the sea and is subject to tidal influence. This is a major project that PUB has been considering. Will have to expedite this," he added.