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.
Influencers flown to Guernsey for tourism-promoting content
22 influencers with a reach of over 2.5 million people are Visit Guernsey's latest strategy to tempt holiday makers to the Bailiwick.
Water restrictions now unlikely for Guernsey
Storage levels begin to recover but people are asked not to waste water.
Turtle looked after in Guernsey is on its way to the Azores
The post Brexit paperwork has been processed meaning Barnacle Bill will soon be swimming in her more usual environment.
Senior deputy makes second appearance in Guernsey's Magistrates Court
Guernsey politician Jonathan Le Tocq has been remanded in prison for a further three weeks.
Is Guernsey "chronically online" when it comes to sexual health?
The impact of social media on sex and relationships is the focus of this year's Sexual Health Week.
Guernsey's Chief of Police shares three year plan
Guernsey's new Chief of Police has outlined his three-year plan to improve the safety and security of the Bailiwick.
Islanders rally together to donate a bus to a children's charity
A Guernsey woman has united a community of generous individuals and organisations to gift a bus to Bright Beginnings.
Disabled islanders lead inclusion training for Guernsey deputies
Ten Guernsey Deputies have signed up for a Disability Confidence Training course.
