Salt marshes expansion in the "Baie de Somme" since the 17th century: deciphering natural dynamics from anthropic disturbances Bruno Goffé, Dimitri Lague and Philippe Davy Chalk plateau N Abbeville Le Crotoy Le Hourdel Amiens Salt marshes Cayeux Noyelle St Valery sur mer Chalk plateau Abbeville 10 km High Tidal ranges 10 m Baie de Somme 11 m Mont St Michel With a dissymetric tidal flow : the outward flow having a lowest intensity than the inward one, Enhancing the sedimendation processes in bay with sand essentially coming from the Channel An apparent strong progression of salt marshes during the 20th century Active gravel bar Map of Polders (land claims) and Salt marshes progression from 1625 to 2010 1835 to 2010 East bay S. Marshes 1835 to 2010 South 1860 Seabank bay S. Marshes 1910 Railway Bank 1965 dyke 1625 to 1835 Seabanks and polders 1835 Somme Canal Polders and saltmarshes progression rates over 400 years moliere 1625 - 2010 Surfaces ha Total bay 4000,0 1910 (RW bank) 3500,0 1835 3000,0 dykes, East bay canal 2500,0 East bay bas champs 2000,0 Southfond bay de baie 1620 – 1835 South bay Seabanks total baie 1500,0 1000,0 500,0 (Dyke) 0,0 1965 1620 1670 1720 1770 1820 1870 1920 1970 2020 Dates Polders and saltmarshes progression rates over 400 years moliere 1625 - 2010 3000,0Surfaces ha East bay 2500,0 21 ha/yr from 1835 to 1920 progressivly decreasing 2000,0 from about 10 ha/y to 4 ha/yr from 1920 to present bastime. champs Total extension 2500 ha 1500,0 Southfond de baie bay 1000,0 3.5 ha/yr, total extension 1400 ha 500,0 0,0 1620 1670 1720 1770 1820 1870 1920 1970 2020 South Bay trend correspond to a natural steady-state salt marshes progression toward the noth controlled by the northward progression of the offshore gravel bar. The second trend corresponds to a transient evolution progressivly reaching the steady state one after the canal cronstruction that moved the Somme river mouth 14 km westward from Abeville to St Valery in 1835 South bay salt marshes steady-state progression controlled by the long term northward offshore gravel bar progression 2007 South 63m/yr 1920 1780 1993 1900 1780 7m/yr 44m/yr 6 m/yr 1920 1939 1225 north moliere 1625 - 2010 3000,0 2500,0 2000,0 bas champs 1500,0 fond de baie 1000,0 7m/yr 3,7 km 500,0 0,0 1620 1670 1720 1770 1820 1870 1920 1970 2020 2,6 ha/yr East bay salt marshes transient process moliere 1835 - 2010 3000,0 150 years 2500,0 2000,0 1500,0 fond de baie 1000,0 500,0 0,0 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 2020 ? Erosion accretion equilibrium process moliere 1835 - 2010 4000,0 3500,0 3000,0 2500,0 bas champs 2000,0 fond de baie total baie 1500,0 1000,0 500,0 0,0 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 2020 Erosion = Recul Dépôt + Végétalisation = Conquète La conquête 1920 Et les premiers reculs 501 ha , 12 ha/an 1946 La conquête et des reculs discrets 304 ha , 11 ha/an 1975 La conquête ? Ou plutôt du recul généralisé -244 ha , -12 ha/an 1997 des conquêtes beaucoup et des reculs encore 440 ha , 20 ha/an 2007 des conquêtes équilibrées avec des reculs -7 ha , -1 ha/an des conquêtes équilibrées 2010 avec des reculs 5 ha , 2 ha/an Il y a-t-il un lien avec la montée des mers? 13 mm/an Derniers 24000 ans 0,7mm/an 2,5 mm/an Derniers 20 a Derniers 130 ans 1,8 mm/an 0 mm/an 2,7 mm/an.