Residents said they heard a loud blast from the direction of Taff’s Well at about 12:30 BST, and then “huge amounts of water” started rushing down the hillside.
One family who live in the affected area said they were turned back at a police cordon and do not know how badly affected their home may be.
Steven Mordecai, another resident from the area, said he saw a “wall of water” when he went to the back of his garden to see what was going on.
“I grabbed hold of my gate and I was floating. It was so forceful it was unreal,” he said.
