Hydrodynamics at a large river confluence of the Sava and Danube rivers in Belgrade

This study focuses on the hydrodynamics of the Danube and Sava rivers confluence. It concerns a confluence with a low width to depth ratio (~20) for its scale and within the confluence there is a large bed discordancy, meaning a bed level difference between the tributary channel and the main channel. Previously a PhD research has been done on the confluence in which the confluence has been analyzed numerically using a SSIIM2 model and in which Acoustic Doppler current Profiler (ADCP) measurements have been done. The conventional way of processing the ADCP data is by transforming the four radial velocities as measured by the four beams of the ADCP into a velocity vector containing the streamwise, cross-stream and vertical velocity. However, caused by among others the bed discordancy, the flow in the confluence is thought to be inhomogeneous. Therefore, a newly proposed method on processing ADCP data has been used in this study. This method predefines a mesh onto a transect of a river and combines the radial velocities that are measured within this mesh into a vector containing the streamwise, cross-stream and vertical velocity. This method depends less on the assumption of homogeneity within the flow. With these two processing methods and the numerical model the flow structure and bed shear stress within the confluence have been investigated, which should give a better understanding of the hydrodynamics of the Sava and Danube Rivers confluence. Firstly, it has been investigated to what extent the two methods could give different results for this confluence by investigating to which extent the assumption of homogeneous flow is reduced by the new method. Secondly, for the new and conventional method is has been determined how much data is needed to capture the turbulence in the data. This is done for different mesh cell widths to determine the mesh cell width that is required to capture the turbulence. Next the resulting secondary flow patterns from the new and conventional method were compared and differences were analyzed. These secondary flow patterns have afterwards been compared to the patterns that resulted from the numerical simulation, which were performed for the same conditions as the observed in the ADCP measurements. Also, the bed shear stress from the numerical simulations have been compared to estimates based on the ADCP data. Lastly, the observed flow structures have been compared to other studies on confluences. It has been found that, based on the accuracy of the data and on the used mesh cell width of ten meters, the new method does not outperform the conventional method. The same result has been found when comparing the two methods on the secondary flow field they produce, and the average flow velocities over the transect. The lack of differences between the methods seems to be caused by the location where the measurements have been collected. These measurements were collected downstream of the confluence where less inhomogeneity of the flow is expected. The flow field that resulted from the two methods is a large helical cell produced by the bend in the confluence, with a small counter rotating cell in the transects of ADCP measurements nearest to the confluence. This cell that is attributed to the curvature of the channel had a larger size in the ADCP data compared to the similar cell that was visible in the output of the numerical model. The numerical model also showed streamwise velocity values which remained more constant when moving to the bed, compared to the streamwise velocities derived from ADCP data. This could implicate an underestimation of the roughness of the bed in the numerical bed. This is also indicated by the bed shear stresses, which show lower values for the numerical model compared to the estimation based on the ADCP data. The absence of clear back-to-back helical cells is also seen for other large-scale confluences, however these confluences mostly possess much larger width to depth ratios (>100). Since this ratio is much smaller for the Danube River and Sava River confluence, the large-scale effects are thought not to cause the absence of helical cells. Based on other research on confluences the large bed discordancy in this case seems to be the reason of the absence of the back-to-back helical cells which is typical for confluences.