Réévaluation des apports moyens de matières en suspension de l'Arve au Rhône

The Arve River is the main contributor of the suspended particulate matter (SPM) flux to the Rhone River between Lake Geneva and the city of Lyon. A reliable evaluation of the Arve River average delivery is crucial for the knowledge and management of sedimentary fluxes in this section of the Rhone and through its hydroelectric installations. This study aims to re-evaluate the average fine suspension flux of the Arve by crossing historical data with reference SPM fluxes for the years 2012 to 2017 derived from continuous turbidity monitoring at the Geneva-Bout-du-Monde station of the Rhone Sediment Observatory. This dataset allowed evaluating the two flux calculation methods implemented by the Swiss National Hydrological Service for periods with discontinuous SPM measurements: the flow exceedance method and the two-segment sediment rating curve. Using the best combination of these methods, the average interannual flux between 1965 and 2017 was estimated at 0.68 Mt/yr with a standard deviation of 0.29 Mt/yr. This value is consistent with the average flux measured by the turbidity station between 2012 and 2017 (0.63 Mt) and with historical monitoring for the year 1890 (0.71 Mt); nonetheless, it is well below the previous estimate proposed by the Etude globale Rhone (2000) report: between 1.0 and 3.6 Mt/year, which probably included the graded suspension of coarse silt and sand, very poorly known. From bathymetric and grainsize surveys of the deposits, it is estimated that the Verbois reservoir captures approximately 0.1 Mt/year of coarse particles from the Arve River, and about half of its SPM inputs.