Numerical and laboratory investigation of the hydrodynamic complexity of a river confluence

The paper deals with the hydrodynamic investigation of the junction of two rivers in north-west Hungary in an urban area. The goal of the investigation was to study the flow conditions by means of a 3D k-ε turbulence CFD model and hydraulic scale model. The project gave the opportunity to compare the outcome of the distorted scale model with the ones of the CFD model in field scale, in the distorted hydraulic model scale as well as in undistorted scale, converting all the results to real field scale by assuming Froude similarity. Overall flow patterns, the effect of distortion and the robustness of the k-ε turbulence model in such complex confluence conditions were analysed. Satisfactory agreement was found except for the region straight downstream of the confluence where significant differences between laboratory and numerical results were seen due most probably to the complex swirling and shearing character of the flow. Large scale vortex formation interacting with anisotropic local turbulence would certainly need differential Reynolds stress turbulence closure

Flow Analysis in River Danube by Field Measurement and 3D CFD turbulence modelling

Spatial complexity of turbulent flow conditions has been investigated by means of ADCP measurements and CFD modelling in river Danube. The study area was a meandering river reach, characterized by shallows and strongly influenced by various river training works. High resolution bed survey and freezing plate sampling provided input river bed data for model implementation. The applied k-ε turbulence model could well reproduce velocity distributions measured in nature. Strong spatial variability of the velocity and turbulent kinetic energy fields demonstrated the necessity of 3D model approach under such fluvial conditions

Morphological Investigation of a Critical Reach of the Upper Hungarian Danube

The aim of this study is to analyze the current morphological changes and to predict the potential future trends of a critical reach of the upper Hungarian Danube River. In this section of the river the morphological features have undergone significant changes, mainly due to human impacts, such as the construction of the Gabcikovo hydropower plant close to the Slovakian border, or the river regulation measures with groin fields along the Hungarian reach. The morphological changes of the river inherently led to river management issues, e.g. the developed gravel bars caused problems for inland navigation. In order to reveal and understand the characteristic morphodynamic processes, results from past bed geometry surveys were assessed, moreover, extensive field measurement campaigns were carried out, including bathymetry survey, flow survey with fixed and moving Acoustic Doppler Current Profiler (ADCP), bed material sampling and bedload measurements. The utilization of up-to-date field data processing methods provided essential information on the current dominant morphological processes and finally, suggestions were made on the stability of the river planform

Acoustic based assessment of cross-sectional concentration inhomogeneity at a suspended sediment monitoring station in a large river

Establishing and operating a harmonized sediment monitoring system along large rivers such as the Danube River is a challenging international task. As an element of such a system, a new monitoring site with state-of-the-art instrumentation is currently under development in the Upper-Hungarian section of the Danube River. The monitoring station will consist of a near-bank optical backscatter sensor and a horizontal acoustic Doppler current profiler (H-ADCP). As previous studies showed, the suspended sediment concentration (SSC) that is continuously measured with near-bank sensors can significantly enhance the temporal resolution of sediment transport monitoring. However, sediment plumes from tributary inflows upstream of the monitoring station can alter the detected near-bank concentrations, eventually biasing the sediment load estimation. Such an influence is likely in the cross-section of the planned monitoring station, therefore, a thorough preliminary analysis of the cross-sectional variation of the SSC was performed, based on expeditionary sediment measurement campaigns. Between 2018 and 2021 24 campaigns were carried out at different hydrological regimes, where physical sediment samplings together with fixed and moving boat ADCP measurements were performed. The cross-sectional variability of SSC and its influence on the sediment load estimations were assessed based on the moving boat ADCP measurements, after calibrating the backscatter signal with more than 500 physical samples. Based on the results, we identified different cross-sectional patterns of the SSC which is apparently governed by: (i) the actual hydrological situation considering both the main river and the tributary, and (ii) the local river morphology. Based on our findings, we suggested a correction method that accounts for the above effects, using which the near-bank SSC can be reliably converted into total suspended sediment load