Estimation des flux de matières en suspension lors des chasses hydrauliques de l'Arc de 2006 à 2011

Dam flushing is conducted regularly on the Arc River (Northern French Alps) to remove sediments from dam reservoirs. This paper presents the analysis of five field campaigns carried out during dam flushing events in the upper Arc River basin (in June 2006, 2007, 2009, 2010 and 2011). Measurements of suspended sediment concentrations (SSC) and discharges were performed at several sampling sites along the river channel. To complete the discharge data set, a 1D numerical hydraulic model, MASCARET, was calibrated and used to estimate the instantaneous discharge at any position along the river system. Following the temporal integration of the instantaneous suspended sediment fluxes, a spatial distribution of the suspended load along the Arc and Isère Rivers was obtained. The classical error propagation method was used to quantify errors resulting from the method employed to estimate the integrated fluxes, by taking into account the relative errors introduced by both the measurements and the model. It appears that the response of the river's sediment transport differs from one year to another. The quantity of fine sediments removed from the reservoirs varied from 10, 000 tons in 2007 to 40, 000 tons in 2006. Over the 120 kilometers of river network, the suspended load could either increase significantly (30, 000 tons in 2007) or else remain exactly the same (in 2009 and 2011). The mean relative standard deviation on the integrated fluxes is equal to 11 %. Taking into account these uncertainty values, it appears that the global tendencies are significant, but the local mass balances are more difficult to interpret.