Work has started on a £1.5m flood defence scheme to protect some of the most vulnerable areas of Gloucester.
A reservoir is being built to catch overflow water from a brook in Longlevens, parts of which were devastated by the floods of 2007.
The Environment Agency (EA) scheme - capable of holding more than 10 Olympic swimming pools of water - is expected to be working by August 2011.
The flood risk to more than 350 homes will be reduced.
In normal conditions the reservoir will be empty, filling when river flows increase and there is a risk of flooding.
This will significantly lower the water levels in Horsbere Brook which burst its banks four years ago.
Flood banks will be created to the west between the flood storage area and the highway.
The EA has bought nearly 50 acres of farmland to create the reservoir and will be working with Gloucestershire County Council to create 15 hectares of biodiversity habitat and improved public access.
Anthony Perry, from the EA, said: "This will be a historic day for people living along the Horsbere Brook."
Stan Waddington, the council's cabinet member for environment, said: "The Horsbere scheme is the largest of six major flood risk schemes being achieved with the Environment Agency."