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.
Possible job cuts at RBSI in Guernsey and Jersey
The bank says it is simplifying operations as finance becomes increasingly digital.
Guernsey students beat England's A level pass rate
Guernsey students performed slightly better in A-Levels than their English peers.
Guernsey community unites out west for solar eclipse
Thousands of islanders basked in the August heat to watch the once-in-a-generation partial solar eclipse.
19 months of traffic lights on busy Guernsey road
The Charroterie will have temporary lights from the end of August till March 2027 to enable 70 new flats to be built.
New Dean of Guernsey has Alderney connections
Matthew Trick will be installed as the Dean of Guernsey in late September having recently been Alderney's locum priest.
Guernsey's youngsters concerned by the high cost of living
The latest Public Health report shows a significant number of 16-24 year olds are put off staying on the island because of cost pressures.
A solar eclipse and meteor shower over Guernsey tonight
Guernsey's west coast will experience 94.5% totality as the moon obscures the sun.
Average Guernsey home selling for more than £600,000
Rent prices slow but are still up 34% over five years.
