Optimizing a Natural Shaped Fishway -- Comparisons between Physical and 3D-Numerical Modeling

This paper deals with comparisons between Physical and Numerical Model Investigations for dimensioning an outlet of a Natural Shaped Fishway which should re-establish fish migration with very specific boundary conditions. River Leine is segregated by weirs acting as barriers for the migrating aquatic fauna. To facilitate the ascent for the aquatic fauna, the Fishway's outlet was located downstream at the outside bend of River Leine. In this outside bend. River Ihme flows into River Leine generating an eddy urging fish not to swim at the undercut slope. Therefore, this eddy had to be suppressed. Physical modeling helped finding á suitable solution to optimize the outlet of the Natural Shaped Fishway and to generate a distinct leading flow featuring frame conditions for the design of Fishways, but with a distinct smaller amount of flow inside the Fishway. Flow velocities were limited based upon swimming capabilities of the local fauna. The junction between Fishway, River Leine and River Ihme was designed with a vertical Current Deflection Wall (CDW) between Fishway and River Ihme. The cross-section of the Fishway was optimized varying cross-profiles ending up in a rectangular profile reducing friction and inducing fauna-adequate current velocities in the Fishway. Most important, a leading flow was generated distinguishable for migrating fish with a discernible current into River Leine leading fish into the weir's backwater and ensuring restoration of linear passage. In this case study, the results of the Physical Model Investigations were confirmed in a 3D Numerical Model Star-CD. It was shown, that exact and narrow-meshed bathymetric data for 3D-Numerical Simulations is essential for good results, especially in areas of higher turbulence. Based upon results of the corresponding physical and numerical model tests it was possible to obtain a suitable solution for this Fishway and future investigations on Natural Shaped Fishways.