Croydon Council said its plan to use an underground footway in Purley to hold around 600,000 litres of water was approved after its "primary" pond at Purley Depot overflowed on Friday night.
Water is being pumped into a pedestrian underpass in Croydon to protect around buildings under threat. The Purley Cross underpass will be turned into an emergency pond as hundreds of thousands of litres of water is pumped into the area.
Authorities hope the move will protect around 400 buildings which are under threat from flooding in Kenley and Purley. They are also trying to protect a water treatment works which supplies tens of thousands of homes.
The people who died after being hit by a train in south London this morning have been named as Donna Oettinger, 41, and her three-year-old son Zachary.
British Transport Police said that the woman killed after being hit by a train in south London was believed to be in her thirties.
Her son, who also died following the collision, is thought to have been three years old.
Senior Investigating Officer Detective Chief Inspector Iain Miller said:
The woman and child were killed around two miles south of Purley in south London in the Riddlesdown area near Kenley
A British Transport Police (BTP) spokesman said:
A woman and a child have been killed after they were hit by a train near Purley in south London.