Alluvial channel survey and modeling in a Po river reach dominated by alternate bars.

The DHI MIKE21C two dimensional model was used to evaluate flow and suspended sediment concentrations in a natural river reach. The 5km long reach is a part of the river Po near Boretto, where the river bed is dominated by alternate bar development. The results of the model are compared to data from a detailed survey carried out with Doppler acoustic backscatter instruments in years 2005-2007. The shallow water hydrodynamics is coupled with a sediment transport routine fitted for long term simulation, including some three dimensional flow characteristics (quasi 3D). The goal of the study was to investigate numerical results for a better understanding of fluvial processes and to identify uncertainties in the results of the long term 2D simulations that will follow. The vertically integrated approach accounting of helical flow seems not to properly simulate the secondary flow that appears to be important in the observed grain size sorting and alternate bars development, both that processes need to be included in future simulation. The outcome of the study will improves the quality of long term prediction of a 2D simulation.