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.
'Multi-generational' living could ease Guernsey's housing crisis
The Planning Authority says homeowners considering an extension for a family member could be helped by an existing policy.
Guernsey dance school upset by temporary theatre closure
A Guernsey dance school says its summer show could be cancelled after Beau Sejour Theatre announced a two-month closure.
Guernsey's business leaders come out against a plan to pause GST
Associations including Chamber and GIBA say an amendment to the tax debate offers no alternative way of raising public money.
Plans to improve the gene pool of the Royal Golden Guernsey Goat
UK reared goats could be imported into Guernsey this year to diversify the bloodline.
Fog causes flight chaos at Guernsey Airport
A weekend of thick drizzle and hill fog has led to multiple delays or cancellations.
Channel Islanders asked to report injured or dead seabirds
People in Jersey, Guernsey, Alderney and Sark are being asked to report any injured or dead seabirds they see on the beaches.
More than 200 emergency callouts for overflowing cesspits in Guernsey
Guernsey Water says it's dealing with a high volume of emergency calls following days of heavy rain.
Guernsey Library to host Costume Creation event ahead of World Book Day
The Guille-Allès Library is making sure every child can dress up this World Book Day.
