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.
Le Tocq should resign as a Guernsey deputy says Lindsay de Sausmarez
Deputy Lindsay de Sausmarez says there is only one option to remove disgraced politician Jonathan Le Tocq, and that would be after his sentencing in late January.
L'Ecume II: Lewis Carr jailed for 20 months for fatal collision at sea
The former second officer in control of the Commodore Goodwill when it collided with a trawler, killing three fishermen, has been sentenced at Jersey's Royal Court.
Guernsey's former Chief Minister admits making indecent images of children
Serving deputy Jonathan Le Tocq pleaded guilty in Guernsey's Royal Court this afternoon, 4 December, to all 15 charges against him.
Guernsey Deputy and wife arrested and questioned by police
Guernsey's treasury minister has revealed he and his wife are subjects of an investigation following a complaint alleging harassment.
Guernsey Post expect to deliver 750,000 parcels this Christmas
Guernsey Post says that islanders' appetite for online shopping is increasing.
Alderney to get a Royal Visit on 15 December
The Duchess of Edinburgh will be in the island to mark the 80th anniversary of Homecoming.
Two airlines bid to operate Guernsey's essential air links
Both Aurigny and Loganair want to fly the Guernsey to Jersey and Guernsey to Southampton routes.
Guernsey overtakes Jersey as top Channel Island for UK Sea Travel
Guernsey’s market share of ferry passengers from the UK to the Channel Islands has exceeded 50% for the first time.
