Monographie de la prise d'eau de Seignosse sur la côte Aquitaine

Ingénieurs à S.O.G.R.E.A.H.

Résumé

This article describes a novel arrangement developed by SOGREAH on behalf of the Satel Tourist Development Company for supplying sea water to large-scale bathing areas located on the low ground between the dune and the Forest of Seignosse on the south-west coast of France. The first bathing pool, opened in 1968, covers an area of 5,000 square metres. Its volume is 5,000 cubic metres and depth varies from 40 cm at the shallow end to 1.8 m at the deep end. Another pool, covering 3,500 square metres, is to be opened shortly. Various alternatives for supplying these pools with sea water were contemplated. Extensive studies showed that one of the most economical solutions would have been to sink a 45 m well by the edge of the beach. However, as the water turned out to contain iron, this solution had to be abandoned in favour of erecting a metallic structure to enable continuous pumping of sea water, even at low tide. This 160 m long structure consists of two lines of vertical, Larssen IV S-type piles driven into the beach. Horizontal metallic caissons connect the tops of the piles, the caissons also serving as piping guides. On the seaward side, the structure is equipped with three vertical caissons 1.2 m in diameter and 2 cm thick which reach 18 m down into the sand. Each perforated caisson is equipped with a 350 cu.m/hr Flygt 2250 pump. The pumps. which can be raised or lowered as beach level demands, discharge into fibre-glass-reinforced PVC 20 cm diameter pipes which run inside the horizontal caissons at the top of the stockade. Sand pumped with the sea water is removed by SOGREAH-type cyclone separators followed by a settling basin which removes fines together with lighter-than-water contaminants such as hydrocarbons. A tank is provided for dosing chemicals required for eliminating algae and other micro-organisms likely to develop in the bathing pool. Treated water is fed into the pool via a 40 cm pipe. An outlet pipe to the sea is also provided, thus making possible frequent water renewal. Cost of the sea-water pumping station is about 1.3 M Francs. Pool cost is about 1.1 M Francs. As the pumping plant is designed for supplying four or live pools, its price will not amount to more than about 20-25 % of total investment. The metallic pile and caisson structure stood up exeeedingly well to the very heavy storms which occurred during winters 1969-70 and 1970-71.