Abstract : This work aim at understanding and measuring aggregation processes and their implications for suspended matter transport and deposition at the fresh water /salt water interface. It combines field experiments in the Rhône river mixing zone, laboratory experiments and numerical simulations of aggregation processes.
Indeed, in order to be able to complete this study, it has been necessary to develop several specific tools. Two new experimental methods for aggregate structure determination have been set up that enable investigation on previously inaccessible samples. These two methods allow, beside a better knowledge of the system, to develop and validate a new aggregation numerical model. This model takes into account the fractal structure of aggregates and its variations. The model is validated using both experiments and bibliographic data.
Field experiments were performed during contrasted hydrodynamic conditions (low water discharge, medium water discharge and small flood). For the first time, measurements of the particle size distributions all along the mixing zone are obtained. It is established that settling, dilution and eventually resuspension control the largest particles (more than 5 microns) evolution. Concerning the smallest measured particles (2 to 5 microns), concentrations are increasing all along the mixing zone. In the first mixing stage, this is mainly du to large particle break-up. It is possible to show that in a second stage i) colloids aggregation can explain this increase only if they do not react with large particles and they have a reactivity higher than the
average value (alpha = 0.009) ii) primary production in the Rhône plume can be a significant mechanism explaining this increase.