"The speed and force of the water...it's an absolutely abnormal situation," said Portuguese Infrastructure Minister Miguel Pinto Luz, who visited the site after the collapse on Wednesday.

No injuries were reported, he said.

However, the minister noted it would likely take "weeks for this infrastructure to be operational again".

The section of motorway where the viaduct was located had been closed to traffic in both directions after the partial breach of a dike on Wednesday.

The collapse occurred a few hours later.

About fifteen trucks loaded with riprap -- stones and rubble meant to protect against erosion -- were later brought in to reinforce the breach.

The collapse comes after deadly storms have battered Portugal for the past two weeks, pummelling the country's infrastructure.

Also on Wednesday, a train derailed near the central city of Abrantes, after hitting debris that had fallen onto the tracks.

No one was injured, but several railway lines remain suspended in Portugal.

Following the passage of storms Kristin, Leonardo, and Marta -- extra-tropical cyclones which dealt significant damage -- Portugal is on alert again due to the risk of flash floods caused by heavy rainfall.

The Iberian Peninsula is on the front line of climate change in Europe and is experiencing increasingly long heatwaves and more frequent and intense episodes of heavy rain.

"In just these two days, the rainfall has been equivalent to 20 percent of Portugal's average annual rainfall," Environment Minister Maria da Graca Carvalho stated Wednesday.

Under fire from critics, Portugal's interior minister Maria Lucia Amaral resigned Tuesday.

The government, still facing public pressure, will face questioning from parliament on its handling of the crisis on Friday.