Du modèle à la nature : l’exemple de la côte Togolaise

Abstract

The low and sandy togolese coast, has undergone a violent erosion action, due to a series of developing errors. In the years 1987-1988, it was the suject of important protection works, aimed at protecting, along 15 kms, the town of Aneho and the phosphate works of Kpémé, the main industrial activity of the country. The preparation of these works, made vital by the importance of the social and economic stakes involved, required very detailed hydrologic and sedimentologic studies, along with the élabora building and exploitation by a research consultancy, of two important physical models (32 X 20 m) aimed at optimizing the protection plan, and at defining the evolution of the coastal profile over a 25 years period. The hydrosedimentary evolution of the togolese coast having been followed through by the University of Benin, since 1985, it is now possible to make a first comparison between the results of this model, and the real evolution that has taken place in real life within a four years period. This comparison shows a good global correspondence between the previsions of built in model and the real life evolution of the coastal profile within the protection plan scope. Nevertheless, a detailed study of the evolution area by area shows that rather noticeable differences can be perceived in the sedimentologic behaviour of the compartments, and above all, of the areas in the downstream drift of the protected zones. Those differences are probably linked to the relatively imprecise quantitative hydrosedimentary observed in real life. They are probably due above all, to the fact that the building of the model relies on the use of data corresponding to medium situations as far as sedimentology is concerned (sedimentary discharge), and as hydrology is concerned (swell cycle), which do not represent the real complexity of one year, or of a short series of years, the conditions of which are impredictible. Moreover, it is likely that the reduction to the different scales cannot really express all the details of a complex evolution.