In freshwater systems, radionuclides are found under dissolved phase and fixed phases on suspended matters and bottom sediment. The exchange between the fixed phases depends on the masses fluxes characterized by sedimentary dynamics. This work aims at improving the approaches for sedimentary dynamics in the radioecological transfers modelling. Considering the different matters present in a river (nature, form…), the bibliographic study shows that dynamic and radionuclides interactions are far from being homogeneous. Consequently, the radionuclide transfers are to be considered heterogeneous in space and time and the modelling needs to consider specifically the main classes of matter.

The work is divided according three parts.
The first one presents a radioecological water quality model (code TRACE) developed to merge sedimentary and radioecological models via a multi-classes approach.
The second part exposes the experimental protocols used to adjust the parameters of this code. On the basis of the experimental flume of the Centre d’Océanologie de Marseille and a decantation system, these protocols allow, for in situ samples, to identify the main matter classes and to determine their sedimentary parameters (critical erosion shear stress, settling velocities…).
The last part presents an application on the longitudinal distribution of the 137Cs masses activities in the bottom sediment of an irrigation canal in the south Rhone river area. The observations show that the granulometric sorting that appears naturally in the canal is the cause of a longitudinal increase of the finest particles contribution to the bottom sediment composition. In the same time, this evolution comes with an increase of the 137Cs activities. These observations confirm the strongest affinity of this radionuclide for the finest particles and the interest of a multi-class approach.