Etude dichronique et historique de l'évolution du trait de côte de la baie de Tanger (Maroc)

The diachronic analysis of air photos and Corona satellite images shows that the erosion of the sandy coast of Tangier bay occurred at variable speeds between 1972 and 1997. The period between 1972-1981 shows generalized erosion with a rate of 4 m/year. The period of 1981-1997 registers a restoring of the average trend with the formation of three sectors, the first one in erosion with an average rate of 1,5 m/year, the second sector is stable and the third in accretion. Because of its remoteness from the production zones, the sandy beach evolves in a closed system. In fact, on the one hand, the construction of the harbour jetty intercepted the longshore drift generated by the west swell and, on the other hand, the development of the western hillside immobilized the sandy materials that do not contribute anymore to the current solid contribution. Finally, the longshore drift which redistributes the pebbles brought by the oued Halou is broken by the construction of a defence system. The fluvial discharges are henceforth immobilized between the breakwater and the eastern groyne and do not contribute to the sedimentary dynamics. This situation aggravates the impoverishment of the involved sedimentary stock. At present, the erosion that drew sediments of the beach from the
most easterly beach to accrete that western portion can’t find more materials because of the erosion and of the quasitotal occupation of the beach. In fact, the western beach not nourished by sediments, will see its vulnerability increase gradually. The repairing of the beach further to successive storms is more difficult. This reality is already felt in 2006.
On the other hand, a portion of the clayey coastal cliff of Tangier bay, considered as stable, sustains significant erosion. The mean rate of cliff retreat, recorded between 1981 and 1997, is estimated at 1,5m/year. The reconstructionof the natural coastal system and the monitoring of the successive developments in the bay show that the anthropization of the coast has unbalanced the system in an irreversible way.