Coastal erosion, processes and rates: An historical study of the Gironde coastline, southwestern France

The macrotidal (2-5 m range) Gironde coast, situated to the south of the Gi ronde estuary, is dominated by NW swell. The prominent cape Pointe de la Ne gade separates two linear sections of this coastline, with different orient ations and different dynamic settings: a northern section, dominated by a m aximum resultant northerly longshore drift (400,000 m(3)/yr); and the south ern section, by a 630,000 m(3)/yr southerly drift. A synthesis of numerous studies allows erosion over (at least) the past 37 years to be estimated; for certain sections for the past 200 years. The ave rage rate of erosion decreases on both sides of the Pointe de la Negade, wh ere it reached its maximum between 1957-1994 (7.3 m/yr). The northern secti on, protected by dykes and sea defences between the estuary mouth at Pointe de Grave and Soulac, has been stabilised. Today, only the limited area of the Huttes is not protected and is subjected to erosion of 9.6 m/yr; it is a area of great concern. To the south, the average rate of erosion decrease s rapidly-it reached 0.4 m/year in Montalivet-and then was relatively const ant around 1 m/year as far as the Grand-Crohot. The extremity of the Cap-Fe rret spit has been generally accreting to seawards; it has also extended to the south, for the past 200 years. Since 1970, the tip of the spit has und ergone erosion. Erosion of the Gironde coast results from a sedimentary budget in deficit b y some 350,000 m(3)/yr escaping northward (the difference between the north erly longshore drift (400,000 m(3)/yr) and 50,000 m(3)/yr retained in the s ystem by the process of erosion/accretion of the Huttes beach and St Nicola s sandbank) and 630,000 m(3)/yr transported toward the south. Aeolian loss appears minimal and offshore loss, due to relative sea-level rise, is estim ated to be between 600,000 and 900,000 m(3)/yr. The system receives no fluv ial supply of sand and the primary source of sediment originates from coast al erosion, mobilised by longshore drift. Total sediment loss for the Giron de coast ranges between 1,580,000 and 1,880,000 m(3)/yr; this explains the average erosion rate, of between 1 and 2 m/yr. Whilst the longshore transpo rt is quite well known, the onshore-offshore movements, especially those ca used by the present-day sea level rise and/or aeolian action, are still poo rly understood and require further investigation.