Work begins on Monday to improve flood defences in the Somerset town of Bruton.
The Environment Agency says the job is essential, even though the town has been relatively flood-free for some 25 years.
The flood storage reservoir on the River Brue is no longer said to be adequate, having been built in 1983.
The embankment now needs strengthening and increasing in height, so that in an emergency, more water can be contained.
Bruton had its last big flood in 1982 but new statutory duties, placed on the Environment Agency, have made the improvements urgent.
The work - based around Sheeptown Farm - will take up to eight months.