A multi-technique approach for evaluating sand dynamics in a complex engineered piedmont river system.

A multi-technique approach is proposed to study sand dynamics in an engineered piedmont river. Only a few studies focused on such systems and innovative methodological protocols still need to be designed to better understand sand transport in piedmont rivers where bedload dynamics has been largely modified with nowadays a residual sand transport on a fixed gravel matrix. The proposed methodology is based on an analysis of bathymetry and turbidity measurements and on modelling, including the development of sediment rating curves and 2D numerical modelling, and using sediment budgeting for cross-validation. Its application to the Isère-Rhône confluence (France) provided some insights of sand fluxes in this complex river system where few sediment flux data are available. Indeed, a substantial amount of sand sporadically reaches the downstream part of the Isère River because of the presence of a series of dams, and jeopardizes navigation and flood management at the confluence. Based on the analysis of the 2015 flushing event, it was found that the sediment transport capacity was reached during the event whereas sand supply can be considered as null when dam bottom gates are closed. Suspended load of sand was prevailing downstream of the last dam but quickly settled down at the confluence. The sand deposit was eventually evacuated from the confluence during the small floods occurring after the flushing event with a minimum discharge of approximately 500 m3 /s in the Isère River and 1000 m3 /s in the headrace canal of the Rhône River. The presented methodology can be transferred to other sites with similar issues.