Geomorphology and dynamics of a traveling cuspate foreland, Authie estuary, France

Abstract

Cuspate forelands or salients occur all over the world in lakes, estuaries and on ocean shores, yet there have been few studies conducted on traveling cuspate forelands (or salients), that is, forelands that migrate or travel alongshore. This paper presents a study of a traveling foreland in the Authie estuary, France, termed the Bec du Perroquet. Historical shoreline changes may be traced from the 1200's AD and the region has experienced both marked intertidal-subtidal accretion extending from the south, and massive erosion in the north since this period. An analysis of aerial photographs from 1947 until the present shows that the original Bec foreland was established at the mouth of the Authie estuary, but gradually disappeared by the 1960's and a new foreland developed in the middle of the northern-central portion of the bay. This foreland was composed of a suite of foredune ridges which have been successively eroded on the northern margin and initiated on the southern margin as the foreland traveled or migrated southwards. As the foreland traveled south, from 1947 to 2009 the northern part of the bay retreated more than 350m, while mid-bay, the coastline retreated ~215m. As the foreland evolves and migrates, incipient foredunes can develop rapidly (e.g. 18 ridges formed in an 11week period), while at other times the ridges form slowly and may be eroded and disappear. Two or more foredune ridges may blend into a single ridge over time depending on the initial degree of vegetation cover on the ridge and swale set. Aeolian processes in dune swales are much more important in this system than in typical prograding foredune plain systems due to the sometimes marked lack of vegetation colonization in the swales following foredune ridge development, and aeolian deflation of the swales (along with blowout development) is important particularly when they become open conduits to the beach as erosion of the NW foreland proceeds. The ages of each of the surviving ridges on the foreland in 2009 have been determined, and the evolutionary path of the ridges ascertained. Formerly intact, relatively stable, continuous ridges evolve to erosional knobs, turrets and nebkha over time. Foredune ridges (and swales) can be extremely arcuate to semi-circular in form where the foreland and especially the spit extension are exposed to a wide range of wind directions and where the shoreline trends through an arc of at least 270°. This study illustrates a remarkable cycling of the formation, destruction and reformation (travel) of a cuspate foreland over a ~50+year period.