Numerical modelling of the suspended particulate matter dynamics in a regulated river network

Understanding and predicting the propagation, deposition and resuspension of suspended particulate matter (SPM) in river networks is important for managing water resources, ecological habitat, pollution, navigation, hydropower generation, reservoir sedimentation, etc. Observational data are scarce and costly, and there is little feedback on the efficiency of numerical simulation tools for compensating the lack of data on a river scale of several hundreds of kilometers. This paper aims at exploring the use of a one-dimensional (1-D) hydrodynamical model for understanding the source and fate of SPM during complex events. The numerical model was applied to the May-June 2008 flood in the Lower Rhône River, France. This event was a combination of floods of the Isère (including dam flushing operations in the Lower Isère River) and Durance tributaries over a two-week period. The simulation code was used to model the SPM fluxes at a high spatial and temporal resolution using a multi-class approach. Approximately half of the 4.9 Mt of SPM measured at the outlet at Beaucaire were found to come from the Isère River and the other half from the Durance River, whereas previous studies estimated that most of the SPM flux at the outlet came from the Durance River. The amount of SPM trapped within the river network, mainly behind the first hydropower structure downstream of the Isère confluence, was estimated to be 3.7 Mt due to the deposition of the coarsest particles. Such a model proved to be able to compute the interaction of various grain size classes with dams and other structures. In turn, the quality of the results of SPM fluxes and deposition is highly sensitive to particle parameters, especially grain size distribution, and to the operational rules of reservoirs.