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.
Three comedians added to Guernsey Literary Festival line-up
Henry Normal, Geoff Norcott and Helen Lederer will all perform at St James, St Peter Port.
Guernsey's hospital moving away from laughing gas anaesthesia
The Princess Elizabeth Hospital will no longer administer piped nitrous oxide during surgeries in its theatres, in an effort to reduce greenhouse gases.
Guernsey hosts global Indoor Bowls competition
Competitors from 30 countries are travelling to Guernsey for seven days of play.
Guernsey rugby player selected for Scotland under 17s
Fraser Kent will travel to Edinburgh at the end of the month to attend a training camp for the Scotland U17s squad.
Record number of seal pups being looked after by the GSPCA
10 seal pups are currently in the care of the GSPCA, the most in its 151 year history.
Inspection reports Hautes Capelles School in Guernsey as 'good'
Hautes Capelles primary school in St Sampson was rated 'good' in all the five categories assessed.
Route for Guernsey's Liberation Day Cavalcade revealed
More than 100 military vehicles, classic cars and tractors will follow the route around the coast.
Herm CEO stepping down
After nearly seven years leading the Herm team, Craig Senior has announced his family's departure.
