Effect of a reservoir release on the morphology of a gravel bar: Field observations and 2Dh modeling

In the context of a reservoir release, this paper aims at understanding the impact of a flood wave on the morphology of a gravel bar.A comprehensive field survey was conducted in the Arc River (French Alps), a highly regulated gravel-bed river. To face reservoir silting dam managers perform sediment release by flushing a series of 3 reservoirs once a year. The area of interest is located in the mid part of the river. Morphological field campaigns were performed before and after the event. Both campaigns contain topography measurements, aerial pictures and grain-size survey. Independently, during the day of the reservoir release, flow measurements were carried out using current meter and Large-Scale PIV. Observations indicate a gross trend of the reach to erosion during the flushing event. The upstream part of the main flow channel was eroded, transverse connecting channels across the bar were strengthened or were modified, and the secondary flow channel on the left side remained unchanged. Furthermore large areas of fine sediment deposits were identified and measured. Water flow in the reach was simulated with the two-dimensional model Rubar20. 2Dh modeling gives us additional information on velocity fields during the whole event. Comparison between calculation results and LS-PIV flow measurements leads to good agreement between measurements and calculations.