Fish-friendly pumps: How an innovative hydraulic system combines flood control with nature conservation
Technology successfully adapted to nature: Since 2024, a fish-compatible pump developed by KSB has been protecting fish in Kudensee, Schleswig-Holstein (Germany). Thanks to its rounded edges and optimised incidence angles, fish such as eels are able to safely swim through the pump on their migration routes.
Kudensee is a 2.46 km² nature reserve in Schleswig-Holstein, comprising a wide expanse of water framed by tall reed beds and green wet meadows. The lake is an important spawning and nursery ground for numerous species of fish(opens in a new tab). In addition to bream, a particularly large number of eels can be found here, as the dense belt of reeds serves as a hiding place for this critically endangered species. Once the eels become sexually mature, they set off for their spawning grounds in the Sargasso Sea.
Until recently, they faced a perilous obstacle right at the beginning of their journey: the Kudensee low-lift pumping station. As Lake Kudensee lies below sea level, the pumping station is necessary in order to pump water from the lake into the nearby Kiel Canal. For the eels, it is the only way out of the lake. However, when they passed through the pumping station, many were killed by the pump impellers.
The Kudensee low-lift pumping station pumps water over a dyke into the Kiel Canal to regulate the water level in the Kudensee nature reserve.
Kudensee is a prime example of a general issue that arises with water protection measures. Migratory fish such as eels and salmon travel long distances over the course of their life, yet many regions require low-lift and other pumping stations to ensure flood control and safeguard low-lying coastal and marshland areas. Fish suffer harm in pump systems when they collide with the edges of impellers or diffusers, are subjected to strong shear and deformation forces, or when rapid pressure changes cause a sudden expansion of their swim bladder.
Pumps with axial or mixed flow impellers in particular entail all of these risks, as they have high blade tip speeds, narrow clearances, unfavourable incidence angles and strong clearance flows. Eels are especially affected: Due to their long, snake-like bodies, there is a high probability that they will be cut or crushed by the edges of the impellers. A study by the Belgian Instituut voor Natuur- en Bosonderzoek (INBO) found that around 97 percent die(opens in a new tab) when they try to swim through a pumping station with a centrifugal pump.
In 2020, KSB started working together with a consortium of pump and turbine developers, fluid dynamics specialists and research institutes in the field of biology, including the "Laboratory of Fluid Dynamics and Technical Flows" at Otto von Guericke University Magdeburg and the Institute of Aquatic Ecology and Fish Biology (IGF) in Jena. The group's objective was to develop a fish-friendly pump for the Kudensee low-lift pumping station. The Deich- und Hauptsielverband Dithmarschen (DHSV) made the pumping station available as a test site because it wanted to assess the suitability of the fish-friendly pumping technology in practice. In addition, an existing pump needed to be replaced with a more powerful pump set, without having to make structural modifications to the building.
The consortium faced one key challenge: Fish-friendly pump geometries need to eliminate sharp edges, create more uniform flow conditions, reduce shear forces, smooth out pressure profiles and lower relative velocities in critical transition areas. However, these improvements soon clashed with the demand for high efficiency, optimum suction performance and integration into the existing building structure. Successful development therefore required systematic optimisation of multiple objectives under realistic operating conditions.
The consortium’s work was made easier by the fish safety standard NEN 8775(opens in a new tab) introduced by the Netherlands in 2020. It provides a clear two-stage assessment system that combines model-based forecasts with practical tests. The standard specifies standardised fish species for the tests, together with numbers and observation periods. For each test, at least 100 animals per fish species and appropriate control groups are required. This enables biologists to measure both acute mortality and sublethal effects with sufficient statistical certainty.
Hydraulics specialists from KSB and Otto von Guericke University combined traditional hydraulic design methods, computational fluid dynamics (CFD) and an analysis based on the discrete element method (DEM) to create a virtual model of the fish. These “digital fish” had defined sizes and densities. The virtual animals moved through the flow field calculated using CFD.
The model calculated hydrodynamic forces, pressure differences and potential collisions using Newton’s laws of motion. This provided a precise picture of the paths taken by the virtual fish, where they came into contact with components, and where critical peaks occurred in the physical conditions experienced by the fish. Drawing on this combined information, the designers were able to identify zones with an increased risk of injury in advance, for example at the impeller inlet, in the diffuser clearance or at the discharge. The developers progressively made these zones less hazardous, long before the first prototypes were built.
The tried-and-tested SEZ tubular casing pump series was used as the basis for the new fish-friendly hydraulic system from Bremen. KSB has been using this type series for decades to transport large volumes of water in applications such as dewatering, irrigation and cooling water supply. The spatial constraints of the Kudensee low-lift pumping station significantly limited the possible geometries and the diffuser design, making the precise interplay between the hydraulic design, digital simulation and subsequent validation even more important.
Prior to the tests with live fish, an interdisciplinary team of computer and electronics specialists, hydraulic engineers, design engineers and biologists used passive sensor devices as substitutes for fish to record the various physical conditions experienced by the fish. These “artificial fish” were circulated through the system under clearly defined operating conditions and provided measurement data which the developers used to calibrate their numerical models and provide the basis for approval of the live tests.
Artificial fish bodies and sensor models were used to investigate flow paths and physical conditions within the pump prior to the tests with live fish.
The most important test for the newly developed pump was finally a test with live fish. The Tierschutzkommission (German Animal Welfare Commission) authorised the test because the team was able to credibly demonstrate, through simulations and sensor tests, that a high fish mortality rate was not to be expected. In fact, thanks to optimisation of the design using CFD, DEM and sensor tests, the actual mortality rate was significantly lower than expected. To ensure comparability, the tests were conducted in accordance with the NEN standard.
The overarching aim for the development of the simulation and sensor methods is to eventually eliminate the need for live fish testing. During the live fish tests, the team studied six species of fish, including three particularly sensitive species: bream, European eel and roach.
The studies showed a mortality rate of less than 10 percent after 48 hours, as well as high recapture rates. The results thus not only meet the requirements of applicable standards such as NEN 8775, but also set new benchmarks for the protection of fish during pump operation. This is particularly highlighted by the results for bream, which, due to their considerable length and body height, often suffer significantly higher loss rates in pump systems that are also marketed as “fish-friendly”.
In August 2024, the fish-friendly SEZ F pump(opens in a new tab) was installed at the Kudensee low-lift pumping station. Since then, it has enabled eels to safely migrate to their spawning grounds. The findings from Kudensee demonstrate that effective fish protection can be achieved not only through bypass solutions such as fish ladders, but also directly within the pump by adjusting its mode of operation.
