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.
Hottest summer for Guernsey since records began
Guernsey Met has confirmed that the island had its warmest summer since records began in 1843.
Guernsey considering funding requests following the disaster in Nepal
Overseas Aid says it is monitoring the situation closely.
Private sector interest in taking over Guernsey's Digital Greenhouse
Economic Development Committee say talks and meetings are taking place with potential new operators.
Guernsey's bus network revamped to improve reliability
Changes to Guernsey’s bus timetable, based on public feedback, aim to make services more reliable.
Guernsey university-bound students offered Meningitis B vaccine
Uptake among students of the meningitis B vaccine is described as 'extremely positive', with more than 260 first doses administered.
Australian actor returns to Guernsey with Victor Hugo one-man show
A one-man play will explore how Victor Hugo wrote The Toilers of the Sea while living in exile in Guernsey.
Alderney Children's Centre shortlisted for Nursery World Award
A nursery set up to address a childcare shortage in Alderney has been shortlisted for a national award.
Islands Unlimited cancels sailings until 10 September
Islands Unlimited has now cancelled all inter-islands sailings until 10 September because of a mechanical problem.
