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.
User pays when Guernsey roads have to close
From the autumn anyone who needs a temporary road closure will have to pay the admin costs.
Channel Islander abandons Bordeaux as the flames 'arrive at the city's gates'
Officials in the Gironde region have told tourists to stay away and are considering evacuating the city of Bordeaux as the flames 'arrive at the city's gates'.
Annual Guernsey Scarecrow Festival set to raise thousands for charity
£10,000 was donated to five Guernsey charities and organisers hope for a similar amount this weekend.
Channel Islands face 'physical' Poland test
The Channel Islands rugby team play their second ever game this weekend - and this time in Jersey.
Baby born on Guernsey's Marine Ambulance
During an emergency transfer from Sark to Guernsey, Harriet Magell was born.
New interim leader for Guernsey's healthcare system
Teresa Cope has senior management experience in the NHS and has recently led the Isle of Man's care system.
Guernsey's reservoir levels falling after prolonged dry spell
Water storage takes a multi-million-litre hit after successive heatwaves.
Guernsey's Commonwealth flag carrier in early Games win
Guernsey bowlers Alison Merrien and Rose Ogier have won their first match in the women's pairs in Glasgow.
