A hole has appeared behind the end of the German wall at the eastern side of L'Ancresse, near the slipway.
The States of Guernsey has fenced the area off, and work will start next week to fill it.
Under the latest States resolution, Environment and Infrastructure are to keep repairing the crumbling World War Two defence work for a number of years.
A £150,000 repair plan was agreed in April.
Members of the public are being asked to avoid the area and not pass the barriers.
"Tropical nights" forecast for the Channel Islands
Daytime highs in the 30s and nighttime temperatures around 20°C are forecast for the Channel Islands.
Guernsey rabbits at risk from two deadly diseases
The GSPCA is warning rabbit owners of two fatal diseases spreading through Guernsey.
Exhibition to celebrate 90 Years of Guernsey's Ambulance Service
Nine decades of Guernsey's Ambulance & Rescue Service will be commemorated this summer.
Guernsey sewage pipe repairs "unlikely to be a quick"
Swimmers in Guernsey may need to avoid the waters around Soldier's Bay for some time.
Guernsey FC to fight for promotion at Victoria Park
The Green Lions are getting ready to host Peacehaven & Telscombe FC in the Southern Combination Football League Play-off Final.
Move to return St Julian's House to Guernsey's emergency shelter
Homelessness charity 'At Home in Guernsey' says it is crucial St Julian's House has free beds for emergency situations.
Jersey and Guernsey included in UK-Gulf trade deal
Jersey and Guernsey have been included in a landmark UK deal with Gulf states that is being hailed a 'significant' boost to the islands' economies.
St Pier says States put investigation before his experience in P&R seat vote
Deputy Gavin St Pier says public perception, not who is best for the job, influenced decisions.
