Cinématique d'un littoral sableux à partir de photographies aériennes et de cartes topographiques. Exemple du littoral d'Argelès-Plage à Saint-Cyprien (Roussillon, France)/Kinematics of a sandy shoreline deduced from the analysis of aerial photographs and topographic maps. Example of the beach extending from Argelès-Plage to Saint-Cyprien (Roussillon, France)

Extended English abstract Time-lapse analysis of aerial vertical photographs and topographic maps represents a useful tool to evaluate the kinematics of sandy shorelines at a temporal scale of several decades. This method is a useful aid to the formulation of guidelines on beach management. However, a visual comparison of aerial photographs is often misleading and a more accurate approach is needed. A method is herewith proposed and used to assess the recent behaviour of the beach which stretches between Argelès-Plage and Saint-Cyprien, on the western Mediterranean coast of France. The method is based on data processing of different kinds of documents. The so-called Géo-Image software was used to process aerial photographs taken between 1942 and 1994 as well as the topographic map of 1936. This data processing was absolutely necessary to compare the documents, especially the photographs. As a matter of fact, these documents are differently scaled, oriented and distortioned. Thus, they were corrected using polynomial models. The 1987 topographic I.G.N. map was enlarged from the 1/25000 to the 1/4000 scale and considered as a prime reference for all the photographs. However, errors may still remain. The 1987 topographic map is itself approximate. Residual errors are expected from the calculation of the polynomial models. The exact position of the shoreline can be also uncertain. These errors were analysed and quantified in order to determine a confidence interval. Such a process is essential for a rigorous interpretation of the data concerning the shoreline schift. This confidence interval was estimated as ± 12 m. Evolution during the last sixty years of the coastal sector under scrutiny shows a contrasted behaviour. It was actually controlled by the exceptional flood (estimated return period : three centuries) of the Tech river which took place in October 1940. A solid discharge of about 15 Mt was suddenly supplied to the sea, inducing dramatic changes in the coastal configuration, particularly in the area around the mouth of the river.Résumé L'évolution récente du littoral sableux d'Argelès-Plage à Saint-Cyprien a été appréhendée par l'analyse diachronique de photographies aériennes verticales et de cartes topographiques. La méthode utilisée repose sur le traitement informatique des documents. Elle permet une comparaison précise de la position du trait de côte d'un document à l'autre. Néanmoins, comme toute méthode de ce type, elle comporte inévitablement certains risques d'erreur, notamment lors du processus de correction des photographies. Ces risques sont analysés et quantifiés de manière à définir une marge d'erreur. Une telle démarche est en effet indispensable pour conduire avec rigueur l'interprétation des résultats. Ces derniers montrent d'importantes variations de la position du trait de côte lors des soixante dernières années, en particulier de part et d'autre de l'embouchure du Tech, petit fleuve côtier qui a joué un rôle majeur dans l'évolution du littoral étudié.Durand Paul. Cinématique d'un littoral sableux à partir de photographies aériennes et de cartes topographiques. Exemple du littoral d'Argelès-Plage à Saint-Cyprien (Roussillon, France)/Kinematics of a sandy shoreline deduced from the analysis of aerial photographs and topographic maps. Example of the beach extending from Argelès-Plage to Saint-Cyprien (Roussillon, France). In: Géomorphologie : relief, processus, environnement, Juillet 1998, vol. 4, n°2. pp. 155-166