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.