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

The Agly river: long-term evolution and present fluvial dynamics of a coastal stream

CNRS - UMR 6042 Géodynamique des Milieux naturels et anthropisés, Université Blaise Pascal, 29, bd Gergovia, 63057 Clermont-Ferrand

Abstract

A research work on the Agly fluvial system and its drainage basin (1,045 km²) 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 the 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 the fluvial dynamics. A major obstacle to social development, these flash-floods have been considered as "the scourge of the country" ever since the XVIII^th century. Modelling the flood wave propagation reveals the importance of relief and channel network characteristics for the transfer processes. A quantification of the sediment transport by the 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 the major part of the transport. Total sediment yield for the entire basin can be estimated at 200·10³ t·year^-1. The fluvial geomorphology shows a recent deepening of the inner channels, related both to human activities and to natural processes : This river metamorphosis has occurred in the second half of the XX^th century and has been so short, thal it could be considered as an epiphenomenon in the river's long story. The dam and dikes built against "aiguats" (combined storm events and floods), must not lead to overlook natural events : the Mediterranean region will always remain dangerous for human activities because of its sudden, heavy downpours and its powerfull coastal streams.