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.
Unprecedented demand for cesspit emptying in Guernsey
Record February rainfall sees sewage carts carrying out hundreds more trips.
Food will not be exempt from GST in Guernsey
Deputies have voted through the concept of GST at 5% on food shopping - but were reminded the final vote is not until the summer.
Victor Hugo Centre fund reaches £7.5M
Guernsey's Victor Hugo Centre has reached three-quarters of its £10M target, as a bronze sculpture of the famous French writer is unveiled to the public.
"We want our schools to be smartphone-free environments"
The impact smartphones have on the Bailiwick's youngsters is going to be studied.
Assisted dying will be offered in Jersey
An assisted dying service will be offered in Jersey to terminally-ill adult residents in unbearable suffering who have less than six months to live.
Guernsey Man (23) jailed for abusing teen girl
A man who sexually touched a girl almost a decade younger than him has been handed a jail sentence.
New Guernsey charity to fund gender-affirming items
A new charity has been set up to help transgender, non-binary, and gender-non-conforming islanders access clothes and accessories that boost confidence and happiness.
Bank tax receipts boost the States of Guernsey's fortunes
Public finances have been delivered a £36M windfall but with a warning that it cannot be counted on regularly.
