As is widely known, the groyne layout affects the three-dimensional flow situation and sediment budget in groyne fields. Lack of knowledge exists concerning the evolution of hydrodynamics and morphodynamics in the main channel of rivers due to distinct groyne layouts. Accordingly, the sensitivity of the stream flow and the river bed in the fairway to groyne geometry were investigated with the help of a three-dimensional hydrodynamic model and a sediment transport model by considering eight variants and seven discharges in a straight reach of the River Danube. The variation of crest elevation and length indicated highest sensitivity, especially at modelled discharges for which the largest geometrical changes due to the modification of the groyne layout were implemented. Within a curved reach larger effects for length variation and smaller ones for the variation of crest elevation were found. These findings can be incorporated into future research and planning of groyne layouts.

For centuries, scientists have been attempting to map complex hydraulic processes to empirical formulas using different flow resistance definitions, which are further applied in numerical models. Now questions arise as to how consistent the simulated results are between the model dimensions and what influence different morphologies and flow conditions have. For this reason, 1D, 2D and 3D simulations were performed and compared with each other in three study areas with up to three different discharges. A standardized, relative comparison of the models shows that after successful calibration at measured water levels, the associated 2D/1D and 3D/1D ratios are almost unity, while bed shear stresses in the 3D models are only about 62–86% of the simulated 1D values and 90–100% in the case of 2D/1D. Reasons for this can be found in different roughness definitions, in simplified geometries, in different calculation approaches, as well as in influences of the turbulence closure. Moreover, decreasing 3D/1D ratios of shear stresses were found with increasing discharges and with increasing slopes, while the equivalent 2D/1D ratios remain almost unchanged. The findings of this study should be taken into account, particularly in subsequent sediment transport simulations, as these calculations are often based on shear stresses.

Shallow water depths in the southern arm of a widely intact bifurcation zone in the Lower Danube lead to a detour for navigation of ∼110 km on the way to the Black Sea at low discharges. In this study, different conceptual hydraulic engineering measures have been assessed with the goal of increasing the water depths in the southern arm, thereby also considering a minimized impact on fish migration (in particular sturgeons). The effects of the different measures were analysed based on a 3D hydrodynamic numerical model (RSim-3D) and a sediment transport model (iSed). The results show that transverse structures in the northern arm, dredging operations in the southern arm as well as a spur dike at the bifurcation might not lead to the desired success. The most promising alternative considering water depths for navigation and fish migration might be a modification of the river course.