Promenaders will notice a dark black shape in the sea off Lancing during the coming months.

The vessel in question will be either the Mibau or the Armour Rock, two giant barges chartered to ship in more than 47,000 tonnes of rock from Larvik in Norway.

Each of the barges carries 20,000 tonnes of a granite called syenite, which will be transferred to smaller barges that can carry 4,000 tonnes.

The smaller barges will be beached at low tide and a bulldozer will move the rock on to the shingle, where it will be repositioned to form a total of 42 breakwaters, replacing wooden groynes, currently piled up on the foreshore awaiting removal.

Work has already started at the Shoreham Beach end and will progress past Widewater Lagoon to East Worthing, finishing opposite Brooklands leisure park.

The syenite will be reinforced by thousands of tonnes of seabed-dredged shingle, which will be used to build up vulnerable sections of the beach.

The Worthing-based Environment Agency said the £10 million scheme would provide improved defences for 3.3km of coastline, helping to safeguard more than £94 million worth of property and assets, including 1,300 homes and 90 commercial premises.

Concern has been voiced about the work's impact on Widewater, a unique wildlife reserve where one of Britain's rarest animals, a sea anemone called Edwardsia ivelli, may still survive in the water.

The agency's Jo Hunt said: "In recent years Widewater has experienced low water levels and this has damaged the fragile habitat.

"Apart from the sea anemone, it is home to several other rare species including Lesne's earwigs, lagoon cockles, bilge bugs and hoglice. Many birds, including some very rare species, also regularly visit.

"A piping system will be installed to promote higher water levels in the lagoon during the summer months.

"This will provide a more stable environment that will lead to considerable improvements to the whole area for residents, for wildlife and for visitors.

"The Edwardsia ivelli is transparent and in the shape of a star and has not been seen for several years. With the increase inflow of water, it is hoped the creature may make a reappearance."