Consideration of three-dimensional flow and transport phenomena in morphological models of inland waterways

Thomas Wenka, Andreas Schmidt, Thomas Brudy-Zippelius

Wednesday 1 july 2015

11:45 - 12:00h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 9A. Sediment - Transport

The close interaction of geometric, granulometric and fluid mechanical effects and their in-fluence on bed-forming processes in alluvial waterways require the highest possible temporal and spatial resolution approach. Particularly the three-dimensional effects of the flow close to construction works and along river bends play a crucial role. It is quite obvious that 3D flow models allow a better prediction of morphological processes, contradicted, however, by the much higher modeling effort, especially regarding practical problems. This conflict led to the investigation of the potential of three-dimensional flow modeling to describe morphological processes better than by 2D depth-average models. Further the study, implementation and application of different shear stress approaches stood in the foreground of a R&D project of which the main results are presented in this paper. Based on well-documented flume tests in meander models of different curvature ratios 2D and 3D simulations have been performed. For the modeling of a practical problem a stretch of a German inland waterway was selected. The “2D sediment transport model Middle Lower Rhine" (Rhine-km 730.0 to 776.5) was reduced to the core region of the curve and reverse curve between Rhine-km 739.0 and Rhine-km 749, 0 and complemented by a 3D model. The comparison of the 2D and 3D models has shown that with a specific parameterization used in the 2D depth-averaged model the effects of the secondary motion can be sufficiently taken into account in regions of curvature. Due to the direct simulation of such effects and a better approximation to the in-situ data, however, the higher computation cost of the 3D model can be justified in certain cases. Therefore, both model approaches should in particu-lar be operated in river sections with strong curvatures. In this way, the results of the simpler 2D model can be evaluated and further adjusted to the specifics of the river reach.