NUMERICAL ANALYSIS OF THE EFFECTS OF ENGINEERING MEASURES ON THE LOWER DANUBE

摘要

In large rivers conflicts between anthropogenic uses and ecological sustainability are increasing due to the growing number of different interests. The Danube, as part of a pan-European inland transport link between Rotterdam (North Sea) and Constanta (Black Sea) represents a high priority navigation fairway. At the same time the Danube is important from an ecological perspective, e.g for sturgeon spawning and habitat. The investigation area of this study represents a widely intact bifurcation zone in the Lower Danube. Due to a significant erosion of the river bed in the northern arm a successive change in discharge distribution towards this arm occurs. This results in a decrease of water depth in the southern arm, requiring a detour for navigation of about 110 km on the way to the Black Sea. In this study different theoretical hydraulic engineering measures have been assessed with the goal of increasing the discharge in the southern arm. This is expected to lead to necessary water depths for navigation, thereby also considering a minimized impact on fish migration (in particular sturgeons). On the basis of a 3D hydrodynamic numerical model the impacts of the different measures were predicted and analysed. The objectives of the 3D modelling task were thus given by (i) determination of hydrodynamic variables (i.e., flow velocities, water surface elevations, turbulence properties); (ii) determination of sediment transport characteristics (i.e., suspended sediment concentrations) and the associated alteration of hydromorphology; (iii) analysis of the impact of the simulated hydraulic parameters on fish migration; due to the construction works planned. The hydrodynamic simulations were performed using the model RSim-3D, while the morphodynamic processes were computed employing non-uniform transport equations embedded in the newly developed sediment transport model iSed. The results show that transverse structures in the northern arm, dredging 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 discharge distribution, water depths for navigation and fish migration might be a modification of the river course.