Coastal groynes, Cayeux-sur-Mer, FR | 2015

Building new groynes and replenishing the pebble ridge to combat erosion and flooding in Picardy

The coastal area of the Bas Champs in the town of Cayeux-sur-Mer in the Bay of Somme in Picardy has long been facing strong erosion of its pebble barrier, the only protection against marine incursions for a low-lying region of 4,300 hectares.

The project involved installing 24 new coastal groynes in continuation of the existing 80 groynes, as well as replenishing the pebble barrier over a length of approximately 2.2 km between Amer Sud and the north of the town of Cayeux. The construction of these new groynes on the seafront of Cayeux will effectively combat beach erosion, and thus potential flooding of the town, by stabilizing the existing pebble barrier.

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The long-term success of this project can only be effective with regular maintenance of the site, which includes replenishing the beach with pebbles. The groynes were executed with steel sheet piles PU 12, PU 18 and PU 22, 3.8 to 10.0 m in length, in grade S 355 GP steel. The steel sheet piles are covered with a reinforced concrete beam and exotic wood (Azobé) edge beams.

The construction work of the new groynes also included replenishing pebbles before and throughout the work. Nearly 250,000 m3 of pebbles were thus filled in. After the execution of each groyne, the project owner proceeded to place a pebble fill necessary for the proper maintenance of the works. This specific contract was awarded to a local company.

The 24 new groynes are spaced 90 m apart along the beach of Cayeux, the last 6 of which are progressively set back towards the backshore area. The groynes have a length of 85 to 90 m and a width of 90 cm, except for 3 double groynes, which are 5.16 m wide. 

One of these double groynes is used as a boat ramp, another provides access for emergency services and pedestrians, and the last must withstand erosion on its north face. The single groynes consist of a single sheet pile curtain crowned, while the double groynes consist of two sheet pile curtains crowned.

The steel sheet piles were delivered in several batches between January and October 2014. Note that the specifications severely limited work during the summer season of 2014. The groynes were inaugurated in June 2015, while the entire project was completed and inaugurated as planned in December 2015.

The total cost of the work amounted to 18 million Euros. The project was partially funded by the European FEDER fund.