Quantification of continuous sand flux time-series downstream of a dam during a flushing event

Sand dynamics is an important issue in harnessed gravel-bed rivers where sand deposits can locally impact river management for ecological or safety purposes. However, sand flux is very complex to evaluate continuously in such rivers because of the strong spatial and temporal variability of the sand concentration throughout a river cross-section and along the river, and also because of the supply-limited aspect of sand. Sand transport capacity formulas are not applicable for such rivers. This paper introduces some methods easy to apply and based on the concentration index, i.e. assuming a possible relationship between the sand concentration at a specific position of a river cross-section and the section averaged concentration. These methods that use regular pump samplings and turbidity measurements are applied on the Lower-Isère, France, downstream of a series of dams during a flushing event. During this 2 week-event, we estimated a sand flux between 1.3 and 1.7 Mt depending on the method and site used. The concentration index method appeared robust and so very useful for a continuous evaluation of sand fluxes but the index concentration must be measured at a location where the turbulence intensity is high enough so that sand suspension can be observed and it must validated with regular sand gaugings. Eventually, we showed that the sand supply allowed the system to reach its sand transport capacity for approximately 3 days after a delay of 2 days after dam openings.