Easton Broad flood protection scheme starting this month
Work is to start later this month on a 1,300ft (400m) long, 8ft (2.5m) high sea defence embankment in Suffolk to protect freshwater reed beds.
The bank near Southwold is aimed to cut the risk of brackish water polluting Easton Broad during tidal surges.
Staged construction over several years will be across existing reed-beds.
The earth and clay embankment will also act as a barrier to the risk of tidal flooding to the B1127 road between Reydon and Wrentham.
Mark Johnson, from the Environment Agency which is funding the work, said: "Later this month we will build a trial part of the embankment across the reed bed.
"Depending on the weather, the formation layer to this first section should be completed in November.
"We will be monitoring this foundation to the embankment and continue with construction to complete the embankment over the next few years.
"We are taking a phased approach to allow time for the soft ground to adjust to the new embankment and to minimise any effect on the wildlife in this highly valuable conservation site."
Easton Broad, which is a nationally and internationally designated nature conservation area, is located about two miles (3km) north of the resort of Southwold.
Mr Johnson said: "Easton Broad comprises a sand and gravel frontage at the sea that is shaped by the tide into a ridge, with an area of open water behind it that is brackish and saline.
"Behind this open saline water is the UK's second largest freshwater reed bed.
"The Easton River flows through the reed bed and then passes under the sand and gravel ridge by a pipe to the open sea.
"We have requirements to take appropriate steps to prevent deterioration of internationally designated nature conservation areas."
Easton Broad, which is home to freshwater birds including the bittern, is protected from the sea by a shingle beach which is regularly breached during high or surge tides.