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.
Guernsey gives £75k for Islands Unlimited inter-islands sailings
Guernsey taxpayers have given inter-islands ferry service Islands Unlimited a £75,000 grant
Guernsey tax package to raise more States income but protect low earners
GST at 3% combined with benefits and States savings will bring in 59M a year.
Changes to make Guernsey's buses more reliable
Bus operator Stagecoach and Guernsey's States have tweaked the timetable to offer a more effective service.
Aurigny plane makes emergency landing after leaving Guernsey
The ATR diverted to Southampton, and landed without incident, after a windscreen cracked in flight.
New festival part of Guernsey's Seafront Sunday
Fete d'la Mair will unite everyone with a love of Guernsey's varied and beautiful coast.
Guernsey firefighters tackle round-island walk in full kit
A team of firefighters will walk 39 miles around Guernsey today (Saturday 6 June) wearing their heavy kit.
No provider awarded subsidy for seasonal inter-island ferry service
No provider has been awarded a subsidy to operate the inter-island ferry service between Alderney and Guernsey.
Guernsey's key industry 'stable' in a turbulent world
The regulator says finance in Guernsey continues to attract funds, investment and insurance business.
