Impacts des houles exceptionnelles sur le littoral ivoirien du Golfe de Guinée

Abstract

This study integrates the im pacts of swells with significant m orphologic im pact in the analy sis of the coastal sedim ent budget and of coastline m obility ; in this case, the swells occurring in August 2 007 on the coast of the Gulf of Guinea, east of Abidjan. Swells were not often taken into account in the ev aluation of the m obility of that coastline, for which all modifications (retreat and accretion) need to be taken into account. For this study , we carried out a diachronic analy sis of aerial photographs (1 97 9-1 989) and we established a series of topographic profiles (2 007 -2 009), allowing to quantify v olum es of deposited or eroded coastal sedim ents. The application of these m ethods shows that the coastal erosion rate at Port-Bouet bay and Assinie beaches, which generally does not exceed 1 .5 m /y , is exacerbated ov er short periods by strong swells from the South Atlantic Ocean. Som e of them are of seism ic origin such as those that subm erged and eroded part of the Ivorian coast during the night from 1 3 to 14 August, 2 007 . The coastline retreated between 2 .3 and 1 8 m in one tidal cy cle, causing significant dam ages in Port-Bouet (Abidjan Bay ) where a dense population and sev eral econom ic activ ities are located. Considering the long-term evolution of these 2 coastal segm ents, this swell which strongly eroded the beaches in Iv ory Coast in August 2 007 appears as an exceptional ev ent, as volum es of sand eroded from all the beach profiles were significant. The studied coastline is not only continuously eroding, but it has also a high sensitiv ity to exceptional swells.