Turbulence Measurements in a Stratified Man-Controlled Estuary, the Adour Case

Abstract : The Adour river is a partially stratified river flowing into the Basque Country coastal waters. The dynamics of estuarine waters and suspended matters, including density stratification, mixing, residence and renewal times, is controlled by the competition between the input of terrestrial waters with strongly varying discharge and suspended load and the meso-tidal fluctuations of the ocean level. Since pycnoclines can form a decisive barrier for exchange processes within a water body, it is important to better understand the processes that govern their evolution in particular for water quality studies. A turbulence-dedicated field campaign was performed in the lower estuary in September 2017. The instrumentation is based on fixed bottom moorings with high frequency point currentmeters and velocity profilers. During the well-mixed phases of the tidal cycles, the flow properties are in good agreement with the standard hypothesis of the canonical turbulent boundary layer in open channel flows. Measurements carried out during the arrival of the salt wedge at rising tide demonstrate the strong influence of density stratification on the flow dynamics. Both turbulent kinetic energy and momentum fluxes are damped by the density stratification, which consequently affect the mixing and resuspension processes near the river bed. A second series of experiments has been performed in order to identify the turbulent properties away from the bed. Instrumented lagrangian drifters have been deployed during the two-layers flow pattern in the estuary. The results again show the important effect of density stratification. The local measurements of small-scale turbulent processes will be placed and discussed in a more general contextaiming to describe the intra-estuary exchanges between river and ocean waters and the fate of dissolved and suspended matters