La place de l'hydrogéomorphologie dans les études d'inondation en France méditerranéenne / The contribution of hydrogeomorphology in flood hazard assessment: a review of the situation in southern France

Abstract

Abstract The hydrogeomorphological method for delimiting flood risk zones in France was developed some twenty years ago. It is based on a simple principle: the outer limits of a stream! s flood plain materialize the outer envelope of past floods. These limits are determined with the use of aerial photographs and field surveys of micro-topography, as well as analyses of deposit granulometry and colour. Indications of present or past land use (fields, location and distribution of archeological sites, houses and farm buildings, roads) are also useful. This field- based method long remained ignored but being reliable, easy to use and inexpensive, it has now been incorporated into the package of methods recommended by French risk prevention plans (PPR). It therefore complements more widespread approaches such as the costly hydrological-hydraulic method. The many recent catastrophes which have occurred over the past twelve years in the Mediterranean regions of southern France, in Nîmes (1988) and in the Aude département (1999) demonstrated both the inadequacy of the hydrological- hydraulic method and the reliability of the hydrogeomorphological method. The hydrogeomorphological method can, however, be improved by setting observations for the present period against information on the more ancient Holocene evolution of flood channels. Furthermore, its intrinsic limits, which make it unable to provide a precise determination of the water depths and flow rates used in PPRs, emphasize the need for developing a hybrid method that is as effective and inexpensive as possible. Such improvements are well under way.