L'Agly, fleuve côtier des Pyrénées orientales : évolution sur le long terme et dynamique actuelle

Abstract

A research work on Agly fluvial system and its drainage basin (1,045 km 2 ) is presented following a systemic approach. Various spatio-temporal scales allow to get a long-term view of evolution and dynamics of this Mediterranean coastal river, since its gradual sinking in a mid-Miocene planation surface. Selective erosion and structural features account for angular channel network, while quaternary sediment deposits depend on morphoclimatic conditions. Flash-floods are induced by heavy storm events and are a determining factor in fluvial dynamics. A major obstacle to social development, these flash-floods have been considered as the scourge of country ever since XVIII th century. Modelling flood wave propagation reveals importance of relief and channel network characteristics for transfer processes. A quantification of sediment transport by Agly river underscores a very low bed load transport, whereas suspended sediment (TSS) and total dissolved solids (TDS), in about equal amounts, account by far for major part of transport. Total sediment yield for entire basin can be estimated at 200·10 3 t·year -1 . The fluvial geomorphology shows a recent deepening of inner channels, related both to human activities and to natural processes : This river metamorphosis has occurred in second half of XX th century and has been so short, thal it could be considered as an epiphenomenon in river's long story. The dam and dikes built against aiguats (combined storm events and floods), must not lead to overlook natural events : Mediterranean region will always remain dangerous for human activities because of its sudden, heavy downpours and its powerfull coastal streams.