ABSTRACT A three dimensional CFD model was used to compute the flow and the corresponding bed changes over time in a natural river bed covered with dunes. The initial geometry was taken from high resolution bathymetric data obtained by an interferometric multibeam, and covered a length of 120 meters in the 50 meters wide river. The data set indicated a predominant structure of three dimensional bed forms. Measurements were taken with 24 hour intervals, showing the dune movement. The numerical model computed reasonable velocity field and bed shear stress profile over the dunes. The computation of the dune movements also showed similarities with the field measurements, although the dune shapes were more smoothed out. This may be due to false diffusion, even though a grid of 1.1 million cells was used. The results of the present study contributes scientific knowledge to the application of RANS CFD models in modeling complex river bed morphology, roughness parameterization and near bed shear stress treatment.

RESULTS

CONCLUSION

Flow and sediment transport over naturally formed, three dimensional dunes has been simulated with the means of RANS CFD model. The initial geometry was taken from a high resolution interferometric so-nar scan. The simulated results were tested against another measurement series taken 24 hours later. Al-though the measured bed changes were small, dune characteristics as well as dunes migration speed could be indentified from the data. The results of comparing the numerical model with the field data showed that it is principally possible to compute mi-gration of dunes in a natural river bed. However, for a general statement to be made as well as a complete validation of the numerical, simulation with a finer grid and a higher order discretisation scheme should be done. Initial calculations with more cells in the vertical direction give less smearing of the bed forms and better preservation of their height. This research is currently ongoing and further data will be collected including velocity and discharge measure-ments.