Chichester District Council is to spend £125,000 replacing and repairing beacons on five groynes along the Selsey shore.
The groynes form a significant part of sea defence, limiting the loss of sand and shingle to the tides.
But the longer ones can pose a threat to boats and anyone else using the water so have to be marked with beacons.
During a meeting of the full council on Tuesday (November 28), it was agreed that money would be taken from the general reserve to pay for the work.
Jonathan Brown, cabinet member for environmental strategy, said: “Over the decades, the council has contributed to coastal defences, building and maintaining structures including groynes to protect the coast.
“The groynes extend into the sea and pose a threat to navigation so have to be marked with beacons.
“We have 37 of them. These are routinely inspected and five in Selsey are either missing or beyond repair and need replacement.
“If we choose not to replace them, it will pose significant danger to those using the water and would represent a serious liability to the council.”
Three of the groynes are on West Beach and two on East Beach, with each replacement or repair expected to cost up to £25,000.
Explaining the high price, Mr Brown said: “The cost of repairs varies according to location, extent of the damage and the depth – and unfortunately the five in question are all in relatively deep water and will require specialist marine equipment and, potentially, construction divers to repair.
“So the cost is substantial.”
He added that, while the council could often apply for a grant to build sea defences, none was available in the case and no money had been set aside in a specific reserve.