Observation and modeling of sand fluxes in the Rhône River during a flood
Observation et modélisation des flux de sable dans le Rhône pendant une crue
Résumé
The estimation of sand fluxes in large rivers is an important issue because of its significant role on the river morphodynamic. However, these fluxes are very difficult to measure and compute because of their large spatial and temporal variability. Equations can be used to compute the sediment transport capacity but the actual sediment transport may be lower than this capacity when sand supply is limited, especially for gravel bed rivers like the Rhône River, France. This study presents the results of a comparison between recent flux measurement surveys on the Rhône River in Lyon and a simple physically based model for computing the sand transport capacity. Flux measurements were achieved combining ADCP measurements and direct sampling using a Niskin sampler. Seven field surveys were carried out, most of them during a 10-year flood in 2018. The range of investigated discharges varies from 900 m3/s to 2700 m3/s during different phases of the flood. Each experiment is made of a set of 12 samples distributed on a cross section, velocities and bathymetry being acquired with an ADCP. The analyses were carried out by sieving and filtering. On the other hand, sediment transport is estimated using a semi-empirical sediment transport formula for suspended load. The model is based on stage-discharge relationships to estimate the main hydraulic parameters, such as the bed shear stress. This model was implemented on two cross-sections with relatively different hydrodynamic conditions, and results were compared with measurements. It is interesting to observe that the transport capacity is reached for the less dynamic cross-section, which appears to control sand transport over the whole reach.We established a suspended sand rating curve for this cross section and experimental results were correctly predicted by the model. However, uncertainties need to be evaluated before establishing reliable sand fluxes time series and budgets.
Domaines
Sciences de l'environnementOrigine | Fichiers produits par l'(les) auteur(s) |
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