When heavy rain falls in the hills and plains west of Colombo — and in the city itself at the same time — Sri Lanka's capital drowns.

As runoff from the west rushes into the Kelani River, which empties into the sea just north of Colombo, water begins to spill into the network of canals that cross the city.

Gates that were built to stop that flow also mean that any rain falling in Colombo itself cannot escape to the river.