Flow structure and grain motion assessments of large river widening in a physical model using ultrasonic Doppler velocity measurements

Local river widening aims to reduce the flood risk and enable the self-morphodynamic development of the river. However, a large amount of transported sediments settles due to the flow velocity reduction in the widening. Assessing the flow and the grain motion is therefore a key factor to the sustainability of a local river widening project. The grain motion depends on the ratio between tractive forces and resisting forces, which can be evaluated through the local shear stress. The most common method to estimate shear stress in uniform flows is to determine the shear velocity based on the logarithmic distribution of velocity over depth. This involves knowing the velocity profile and in turn the flow structure. In the present study, morphodynamic tests are conducted to explore the hydrodynamics and the grain motion of a local river widening in the framework of the 3rd correction of the Rhone River, the largest flood protection project in Switzerland so far. The ultrasonic velocity profiler method is used to measure velocity profiles at two selected cross-sections. The obtained velocity profiles allow for the assessment of local shear stress on the mobile riverbed. The results show a non-uniform distribution of the flow and the shear stresses. The flow conditions at the preferential channel are more favorable to grain motion compared to those at the river's edges. [GRAPHICS] .