The transport of substances and fluids with varying physical properties in the hyporheic zone may be influenced by structural measures, such as river regulations, renatura- tion, or hydroelectric power plants. Understanding this process is critical to predicting the impact on this biologically valuable habitat for fish spawns, insects, and a large variety of microorganisms. The objective of this work is to investigate buoyancy-induced effects on mass and momentum exchange within the hyporheic zone. It is hypothesized that destabilizing buoyancy forces increase hyporheic exchange, whereas stabilizing buoyancy forces may decrease hyporheic exchange. To investigate this, the Navier-Stokes equation already implemented in the research code MGLET was extended by a Boussinesq approximation. The code was extensively validated against cases from the literature. During this process, the spatial accuracy of scalar computations near an immersed boundary could be improved. Within the double-averaging framework, different processes contributing to bed-normal scalar transport were identified. Further, buoyancy effects were considered in budget equations for temporal and spatial velocity fluctuations. To assess the impact of buoyancy forces on hyporheic exchange under realistic conditions, the Rayleigh number, a dimensionless parameter, was calculated using values from an experimental study. The obtained, yet preliminary, results suggest that Rayleigh numbers of 200 and above can influence the exchange. Particularly, the spatial variance of the bed-normal velocity field is increased, which is likely to emphasize the role of locally upwelling and downwelling flow.     «

The transport of substances and fluids with varying physical properties in the hyporheic zone may be influenced by structural measures, such as river regulations, renatura- tion, or hydroelectric power plants. Understanding this process is critical to predicting the impact on this biologically valuable habitat for fish spawns, insects, and a large variety of microorganisms. The objective of this work is to investigate buoyancy-induced effects on mass and momentum exchange within the hyporheic zo...     »