Comparative study of shoreline evolution on the sandspits of the "Petite Cote" (Senegal): case of Joal and Djiffere

The sandy spits of Joal and Djiffere area, located on the Small Coast of Senegal, present different evolutions. The Joal shoreline's kinematics is complex in contrast with Palmarin-Djiffere where the rate of retreat is gradual especially at the north and south ends of the spit. The global average trend of shorelines retreat digitised after geometric correction (1989 to 2013) gives a retreat rate insignificant for Joal coast and important for Palmarin-Djiffere shoreline (respectively -0,82 m/year e.g.20,4 m and -3,83 m/year e.g. 91,92 m). The quarterly sedimentary balances, obtained by beach profiles survey from January 2012 to September 2013, indicate sedimentary exchanges showing the months of january, february, july and august as periods of erosion, and march and september as mounths of beaches feeding. The dynamic of these coasts is consistent with the swells regime of the south coast of Dakar and with the Sangomar breach development.Original Abstract: Les fleches sableuses de Joal et de Djiffere, situees sur la Petite Cote du Senegal, presentent des evolutions differentes. La cinematique du trait de cote est assez complexe a Joal contrairement a Palmarin-Djiffere ou le taux de recul est graduel, en particulier aux extremites nord et sud de la fleche. La tendance moyenne de recul des traits de cote digitalises apres correction geometrique (1989 a 2013) donne un taux de recul insignifiant de - 0,82 m/an (20,4 m) a Joal et important a Palmarin-Djiffere ( -3,83 m/an soit 91,92 m). Les bilans sedimentaires trimestriels obtenus par suivi des profils de plage de janvier 2012 a septembre 2013 indiquent des echanges sedimentaires permettant d'identifier les mois de janvier, fevrier, juillet et aout comme des periodes d'erosion, et mars et septembre de reconstitution des plages. La dynamique de ces littoraux est en coherence avec le regime des houles de la cote sud de Dakar et de la situation d'ouverture de la breche de Sangomar.