Estuarine, Coastal and Shelf Science (2001) 53, 25–38 doi:10.1006/ecss.2000.0685, available online at http://www.idealibrary.com on The Rhone River Plume in Unsteady Conditions: Numerical and Experimental Results C. Estournel a , P. Broche b , P. Marsaleix a , J.-L. Devenon b , F. Auclair a and R. Vehil a a Laboratoire d’Ae ´rologie, UMR 5560, 14 avenue E. Belin, F-31400 Toulouse, France b Laboratoire de Sondages Electromagne ´tiques de l’Environnement Terrestre, UPRESA 6017, Universite ´ de Toulon et du Var, BP 132, F-83957 La Garde Cedex, France Received 26 November 1998 and accepted in revised form 18 May 2000 Surface current maps obtained in the plume of the Rhome river by two VHF radars gave rise to comparisons with simulations produced by a primitive-equation model. Two situations were investigated: one in steady conditions, the other with a variable wind. The model gives a good estimation of current direction and velocity not only in the plume itself but also in the surrounding water. The model reveals the mechanisms involved in the plume dilution and more generally in the renewal of surface water near the coast when the wind changes direction. The sensitivity of the model results to the upper boundary condition for turbulent kinetic energy is also shown.  2001 Academic Press Keywords: river plume; ROFI (region of fresh water influence); Gulf of Lion; Rhone plume; VHF radars; 3D coastal modelling Introduction River plumes have been the subject of numerous studies over the past decade. One reason is that rivers constitute the principal means by which continental input is transported to the ocean. Such input, particu- larly nutrients and pollutants, is constantly increasing and poses a threat to the coastal environment. At river outlets, circulation has very specific characteristics linked to the contrast in density between river water and sea water. It is essential therefore to take such characteristics into account when considering the fate of the discharged material. The characteristics include freshwater surface flow, the low degree of mixing with sea water, strong current velocities and marked sensi- tivity to wind (Chao & Boicourt, 1986; Chao, 1987). However, these characteristics are greatly influenced by tides which increase vertical mixing and thus the dilution of freshwater and which alter the position of the plume on either side of the river mouth during a cycle (Ruddick et al., 1994). The Rhone river plume (Gulf of Lions, north-west Mediterranean) has been the subject of several studies based on remote sensing observations (Forget et al., 1990; Demarcq & Wald, 1984; Devenon et al., 1992; Broche et al., 1998) and modelling (Estournel et al., 1997; Marsaleix et al., 1998; Arnoux-Chiavassa et al., 1999). The weakness of tides and of landward undercurrent in the river mouth leads to a plume which is diluted only slowly and whose surface currents often exceed 50 cm s 1 allowing the plume to spread out over several dozen kilometres in a day or so. More recently, attention has been paid to the validation of existing models: Estournel et al. (1997) drew up a comparison using satellite images of sus- pended matter in three different situations character- ized by variable winds. The comparisons are limited to the shape of the plume as the model does not correctly take into account the complexity of the behaviour of particles such as sedimentation velocity changes due to flocculation or biological processes. Furthermore, no information was available on the vertical structure of the plume. ON the other hand, Arnoux-Chiavassa et al. (1999) compared surface current fields and vertical profiles of temperature and salinity. These comparisons are nonetheless limited to steady wind conditions. The purpose of this article is to improve the work begun by Estournel et al. (1997), by comparing the results of modelling with current fields measured using a system of two VHF radars. Two situations were chosen: the first corresponds to a moderate Rhone flow in near-constant wind conditions. The aim is to show that in simple conditions, the model provides a good estimation of plume position and particularly current velocity both within the plume 0272–7714/01/070025+14 $35.00/0  2001 Academic Press