Fish ladder

Fish migration is crucial for the health of aquatic ecosystems. Where structures such as hydroelectric power plants cross rivers, these can represent obstacles to the natural migration of fish, affecting ecological connectivity. To address this at the hydropower plant in Landau on the River Isar in southern Germany, a fish pass required to be constructed – the first of four planned along this section of river. Inros Lackner designed the structure on behalf of Uniper. The project complies with the requirements of the European Union's Water Framework Directive (WFD), which, among other things, promotes healthy aquatic ecosystems and the preservation of biodiversity.

Impacts on aquatic ecosystems
Many bodies of water in Europe are severely affected by human activities such as the construction of hydroelectric power plants and weirs. These structures disrupt habitats and impede the migration of fish and other aquatic organisms. This threatens biodiversity because important habitats such as spawning grounds or feeding areas can no longer be reached. In Landau on the Isar, the hydropower plant and the associated weir-like sill structure form such obstacles. In order to restore ecological connectivity at both locations, Uniper commissioned Inros Lackner to implement a fish ladder at the power plant and another at the sill. The Landshut Water Management Office contributed to the financing. The power generation facility consists of a four-channel weir and a three-turbine powerhouse with a water pressure head of 7.4 metres. About 500 metres downstream is the sill structure, which was built to stabilize the power plant’s tailwater. It consists of a sheet pile wall that protrudes 0.6 metres above the riverbed. Beyond that, the ground drops away after a flat stretch of about 15 metres, forming areas with water up to 3 metres deep.

Technical implementation
The fish ladder in Landau combines a bypass channel, a slotted pass and a rough-channel pool pass to make it easier for fish to overcome the eight-metre height difference in water levels. The design took into account the flow conditions, the water depths and the needs of the river’s various fish species – especially the huchen, which is characteristic of the region. The bypass channel was placed close to the hydropower plant to reflect the natural migration route of the fish. In the lower section of the river, a combination of a rough-channel pool pass and a rough-surface ramp connects the cut-off channel to the main river. The pools, which are connected by small weirs and channels, facilitate continuous water flow while enabling fish to swim upstream without having to struggle against strong currents. The pools also provide resting areas for the fish. The fish ladder has a length of 360 metres and features entry and exit points near the hydropower facility, with entry at the slotted pass in the lower part of the river and exit through a crossing structure in the upper part. An additional 26-metre-long section connects the Isar directly with the cut-off channel and improves connectivity at the sill structure.

Sustainable use of rivers
Construction of the fish ladder began in November 2022 and was completed in September 2024. The resulting restoration of ecological connectivity at the Landau hydropower plant and the associated sill structure protects the region’s aquatic ecosystems. The fish ladder thus enhances the sustainability of the way the river is used, harmonising ecological, economic and social needs.