# THE KUDENSEE PROJECT: DEVELOPMENT OF A LESS-DAMAGING REPLACEMENT PUMP FOR SUSTAINABLE WATER MANAGEMENT

Stefan Hoerner, Dennis Powalla, Islam Abdelghafar, Emeel Kerikous, Falko Wagner, Jeffrey A. Tuhtan, Roland Schumacher

► To cite this version:

Stefan Hoerner, Dennis Powalla, Islam Abdelghafar, Emeel Kerikous, Falko Wagner, et al., THE KUDENSEE PROJECT: DEVELOPMENT OF A LESS-DAMAGING REPLACEMENT PUMP FOR SUSTAINABLE WATER MANAGEMENT. 2026. [hal-05507409]

HAL Id: hal-05507409

https://hal.science/hal-05507409v1

Preprint submitted on 12 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# THE KUDENSEE PROJECT: DEVELOPMENT OF A LESS-DAMAGING REPLACEMENT PUMP FOR SUSTAINABLE WATER MANAGEMENT

Stefan HOERNER¹,², Dennis POWALLA², Islam ABDELGHAFAR²,³, Emeel KERIKOUS²,⁴, Falko WAGNER⁵, Jeffrey TUHTAN⁶, Roland SCHUMACHER⁷

¹ Laboratory of Geophysical and Industrial Flows, CNRS, Grenoble-INP, University Grenoble-Alpes, France
email: stefan.hoerner@univ-grenoble-alpes.fr

² Institute of Fluid Dynamics and Thermodynamics, Otto-von-Guericke University Magdeburg, Germany
email: dennis.powalla@ovgu.de
email: islam.abdelghafar@ovgu.de
email: emeel.kerikous@ovgu.de

³ Hull International Fisheries Institute, University Hull, United Kingdom

⁴ Mechanical Power Engineering Department, Faculty of Engineering-Mattaria, Helwan University, Cairo, Egypt

⁵ Institute of Aquatic Ecology and Fish Biology, Jena, Germany
email: falko.wagner@igf-jena.de

⁶ Department of Computer Systems, Tallinn University of Technology (TalTech), Tallinn, Estonia
email: jeffrey.tuhtan@taltech.ee

⁷ KSB Supreme Service GmbH Bremen, Germany
email: roland.schumacher@ksb.de

## ABSTRACT

Marshlands along Europe's northwestern coast require numerous pumping stations to prevent flooding and rewetting. However, conventional axial pumps cause significant damage to entrained fish due to collisions with impeller blades and built-in structures. At Lake Kudensee, a nature reserve in northern Germany, migratory fish must pass through a pumping station to reach the Kiel Canal, which connects North Sea and Baltic Sea. This makes the location critical for nature conservation. The classic axial pumps installed at the station in 2003 cause significant damage, which is why one unit was replaced with a newly developed, less-damaging pump. The new pump design was developed by pump manufacturer KSB in collaboration with mechanical engineers and aquatic ecologists. It was based on an iterative process involving classical hydraulic design rules with computational fluid dynamics (CFD) coupled with the discrete element method (DEM), or "virtual fish," to evaluate flow, collision risks, and biological thresholds. Critical impact areas were identified in the impeller, stator, and outlet. These findings led to design adjustments, such as modifying the impeller geometry and improving the stator by redesigning the blades and reducing their number. Prior to field testing with live fish, an approach inspired by the 3R principle was used, comprising analytical blade impact models and tests with various sensor probes. This approach confirmed low mortality rates for fish passing through the pump and indicated safe hydraulic conditions. Subsequent validation tests with live fish, involving six species — including sensitive species such as bream and roach — showed high recapture rates and very low mortality rates (48 h) significantly below 10% for all species tested. However, the results are preliminary. This project demonstrates that combining advanced numerical modeling with biological criteria can significantly reduce fish mortality in retrofitted pumping stations. The Kudensee pump could serve as a reference case and benchmark for future less-damaging hydraulic machinery development and reduce the reliance on live fish testing.

**Keywords:** Fish passage; hydraulics, ethohydraulics; turbomachinery; downstream passage
# 1. Introduction

The marshlands along Europe's northwestern coastline necessitate continuous pumping to prevent the rewetting and flooding of the landscape. These pumps are numerous. Along the tidal flats of the German north-sea coast, approximately 1,000 pumping stations regulate the water level.

The axial pumps utilized on the dykes are typically smaller in size compared to the water turbines installed along waterways in run-of-river power stations. A smaller turbomachine is usually associated with higher rotational speeds. In this combination, the risk of injury and death for living beings entrained is significantly higher than for slow-rotating, large machines. While a smaller size leads to an increased collision risk, thinner blades with higher rotation rates and thus collision velocities increase the severity of collisions (Amaral *et al.* 2011, Saylor *et al.* 2019). In consequence, classical axial pumps can have a tremendous impact on freshwater fish populations if the animals are forced to pass these pumps due to entrainment or for migration purposes. Very high mortality rates have been reported for different species repeatedly by various groups and studies (Bierschenk *et al.* 2019) with up to 97 % for European Eel (*Anguilla anguilla*) (Buysse *et al.* 2011). In consequence, less-damaging pump development and operation often primary refers to one of these most endangered migrating fish species, e.g. (Evans *et al.* 2024). This objective is both consequential and achievable. Eels have been observed to demonstrate a remarkable capacity to withstand impacts from blade collisions at velocities of up to 13 m/s (Saylor *et al.* 2020b). This is well above the 4 m/s threshold that has been determined for severe impact damage for other fish (Saylor *et al.* 2020a). Various manufacturers have previously proposed technical solutions for less-damaging pumps. However, a thorough examination of these pumps has revealed that their purported less-damaging benefits may not be as extensive as initially promoted, when assessed in field studies (Bierschenk *et al.* 2019; Bruneel *et al.* 2024).

## 1.1. The Kudensee Case Study Site

The Kudensee is a lake in the north German marshlands and part of a 2.46 km² nature reserve. The lake is home to several species of fish, including the European eel, the bream (*Abramis brama*), the roach (*Rutilus rutilus*), the perch (*Perca fluviatilis*), the gudgeon (*Gobio gobio*), the pike (*Esox lucius*) and the zander (*Sander lucioperca*). In particular, the eel population is considered well preserved (Hartmann & Spratte 2006). Due to its low elevation of 1.67 meters below sea level, Lake Kudensee has no direct connection to the North Sea. Migratory fish must pass through the pumping station that drains into the Kiel Canal. This makes the pumping station a bottleneck and an essential component for preserving the biodiversity and fish populations of this nature reserve and its spawning grounds. The pumping station built in 2003, originally consisted of three identical classical axial pumps with a total discharge of 2 m³/s each<fnref n="1" /> (DHSV). These pumps must be considered a cause of significant damage to entrained fish. A rough test with rainbow trout (*Oncorhynchus mykiss*) and zander (*Sander lucioperca*) killed before this test (n=3 for each species, body length 23-37 cm) showed severe damage, amputations, and decapitations in all fish carcasses.

# 2. Retrofitting of the Pumping Station

As part of a retrofit measure, one of the existing pumps was replaced with a less damaging pump technology. The very compact design of existing pumps posed a particular challenge. It was achieved by means of a pre-swirl regulator, which generates a counterflow on the suction side of the pump, combined with a four-bladed runner and a very short stator. The latter consists mainly of the bearing hub for the impeller with five straight blades of 20 cm length. The pumps are operated with a fixed rotational speed of 500 rpm.

The retrofit pump design was a trade-off in between a maximum fish survivability, the constraints from the installation requirements and the hydraulic characteristics for a safe water management. The data sheet for both pumps, both existing and retrofit versions, is shown in Table 1.

<table><caption>Table 1. Hydraulic data of the existing and retrofitted pump</caption><thead><tr><td></td><td>Existing</td><td>Retrofit</td><td>Unit</td></tr></thead><tbody><tr><td>Discharge (base load/max)</td><td>-/2</td><td>1.3/1.8</td><td>m³/s</td></tr><tr><td>Hydraulic head</td><td>1.5 <sup>1</sup></td><td>1.5/3</td><td>m</td></tr></tbody></table>

<footnote n="1">¹ https://www.dhsv-dithmarschen.de/?view=article&id=145:schoepfwerk-kudensee</footnote>
<table><tr><td>Rotational speed (base load/max)</td><td>-/500 (fixed speed)</td><td>346/495</td><td>rpm</td></tr><tr><td>Number of blades (rotor/stator)</td><td>4/5</td><td>2/2</td><td>-</td></tr></table>

# 3. Methodology

## 3.1. Retrofit Pump Design Process

The pump design was an iterative process in close collaboration of the pump manufacturer KSB and researchers from the Institute of Aquatic Ecology and Fish Biology Jena, and mechanical engineers from

University Magdeburg and University Grenoble-Alpes. After the definition of a hydraulic design, the flow field and relative collision risks were determined using computational fluid dynamics coupled with discrete element method (CFD-DEM) simulations. Biological thresholds defined by the biologists allowed us to assess the hydraulic conditions. Combined with cumulative collision impact intensities for runner, stator, and housing, critical regions were defined. Based on the recommendations, an improved design was developed subsequently evaluated until the final significantly improved version was found.

<figure><img src="image_2.png" /><figcaption>Figure 1: Design process for the less damaging pump technology</figcaption></figure>

## 3.2. Numerical flow and collision assessment

The numerical approach with coupled CFD-DEM simulations is well established and has already been used in several studies, for example in (Richmond et al. 2014, Romero-Gomez et al. 2019). The specific setup used in the present study corresponds to the methodology described in detail in (Powalla et al. 2022). The hydraulic conditions were determined using classical CFD simulations. Once stable flow conditions were established, neutral buoyancy, known as virtual fish surrogates (ViFi), were injected into the flow field. An unsteady Reynolds-averaged Navier-Stokes flow model was used together with a k-ω-SST turbulence model and an adaptive y+-wall model. Siemens StarCCM+ software package was used for the study. Spatial discretization was based on a polyhedral grid (base size 0.05 m) with orthogonal prismatic cells (minimum cell size 0.005 m) in the boundary layer. The entire grid consisted of 7.93M cells. The numerical schemes were second-order implicit (time) and second-order upwind (flow variables). The time step was set to $1.683 \times 10^{-4}$ s, which corresponds to approximately 0.5° of runner rotation.

The DEM configuration consisted of neutrally buoyant spheres with a diameter of 0.044 m and rubber material properties (Arlon Thermabond 99180T010) from the standard material database. The contact and resistance models were used with the default settings. Hydraulic conditions were determined based on the basis of the individual ViFi traces. The velocity profiles, acceleration, pressure gradients, and total pressure along the paths were statistically analyzed to maximize the benefit from the large number of available data sets ($750 > n > 500$) for each design iteration. The cases were assessed without (passive ViFi) and with swimming activities (active ViFi) following the concept introduced by (Powalla et al. 2022).

## 3.3. Biological Thresholds

Biological thresholds were defined on a literature review. A significant research gap on pump-specific mortality thresholds has been found on pump-specific mortality thresholds. Laboratory data and turbine-specific studies were used to provide the parameters necessary for a biological assessment.

# 4. Final Design and Field Validation

## 4.1. 3R Inspired Pre-Assessment

To assess the effectiveness of the design approach, a prototype was constructed and the final design was field tested. The Dutch standard for fish safety assessment, NEN 8775-2020, was followed to ensure the comparability of results. Due to ethical concerns of placing a sensitive living being in a potentially very harmful environment, a multistage assessment inspired by the 3R principle was performed (Hoerner et al. 2024) prior
to the live fish experiments proposed by NEN 8775 to estimate
the risk of injury and mortality. The concept was to stop any
live animal tests if the results of alternative methods or
preliminary results during the test phase indicated a damage
rate greater than 30% for the species in question. A direct
comparison of the damage rates of existing pumps to the
improved design would have been of great interest. However,
it has been decided to withdraw any live fish experiments on
the existing systems due to ethical concerns as the preliminary
tests with dead fish showed 100% damage rate.

## 4.2. Numerical Assessment

Following the thresholds in the table above, damage from
hydraulic conditions was not expected.

The collisions were tracked using the force accumulated at the
boundary surfaces. This approach does not allow

quantification of the intensity of the individual collision, but it is suitable for determining critical areas with high accumulated forces. This information was used to modify the impeller design. In addition to analytical impact models (Van Esch & Spierts 2014) that focus exclusively on impeller design, other critical areas were found in the stator and outlet. As a result, the stator was redesigned with a reduced number of stator blades and a smoother curvature.

<figure><img src="image_3.png" /><figcaption>Figure 2: The DEM coupling allows to determine high risk for collisions</figcaption></figure>

## 4.3. Blade-Strike Modelling

Following the recommendations of NEN 8775 the mortality risk was estimated using an analytical model prior to animal tests. These models introduced in the 1950s estimate the collision risk by comparing the time scale for fish to pass a rotor, as a function of volume flow rate and the length of the fish, with the time scale for a rotor blade to pass over the gap between the blades. Subsequently, the mortality probability is given as the probability of a strike and its severity, which calculates either by a simple tweaking factor in the early versions, or more recently based on fish species, blade thickness, and rotational runner speed. The model introduced by (Van Esch & Spierts 2014) was used as suggested by NEN 8775. The results predicted a mortality risk between 6% and 29% for the species to be investigated.

<figure><img src="image_4.png" /><figcaption>Figure 3: The DEM based ViFi allows for a statistical assessment of the hydraulic design</figcaption></figure>

## 4.4. Sensors Tests
Sensor tests were performed twice by TalTec: once in a lab setting at the manufacturer, when the pump prototype was tested for efficiency and performance. RAPID (Robust Autonomous Pressure and Inertial Device) sensors<fnref n="2" /> were injected into the pump for various operation conditions, with changing hydraulic head, and thus flow rate for multiple rotational speeds to obtain the characteristic curves and estimate the collision risks. These results suggested less than 16% damage rate for the operation points of interest, when following the hypotheses that the majority of collisions above a threshold of 400 g could be potentially severe, which is based on the findings from lab tests on a blade strike test rig for this sensor device (Kösters et al. 2025a, Kösters et al. 2025b).

In a second setup, multiple sensor types were used prior to the live fish tests. The results are still not finally processed, but a first analyze suggested similar results on site with 10% collisions before the Nadir point (just before runner entry) and 23% post-Nadir point (33% total). Damage from hydraulic conditions was not expected.

A 400g assumption is a very conservative threshold for this specific sensor, only indicating a collision with probably more than 3 m/s strike velocity (Kösters et al. 2025b). On the basis of lab experiments on live fish, strike mortality probability has been determined for multiple species for different collision speeds. Although it is still negligible or low for 3 m/s (e.g., still 0% for large brown trout (Saylor et al. 2020a)), it increases substantially with the strike velocity (Saylor et al. 2020a). Following the data, it was concluded to continue the experiments, as a substantial share (> 30%) of the strikes must be considered to be not fatal.

## 4.5. Live Fish Tests (Preliminary Results)

The fish tests were performed in November 2025; only preliminary results are available. The following six species were assessed: European eel (Anguilla anguilla), bream (Abramis brama), roach (Rutilus rutilus), perch (Perca fluviatilis), gudgeon (Gobio gobio) and rainbow trout (Oncorhynchus mykiss). 100 fish were injected into the pump, while the control group also comprised 100 fish for each species. The health and injuries of all fish were evaluated before and after the experiments. Following the methodology of Wagner et al. 2022, 97 bream were additionally equipped with non-invasive dorsal fin mounted archival biologgers from Taltech.

The recapture rate was 94-100%. Direct mortality never exceeded 2 % for all investigated species. Delayed mortality at 48 h was well below 10 %. Data analysis is ongoing. Mortality must be compared with the delayed mortality in the control group and injuries must be compared with the monitored damage before testing to obtain final results.

## 5. Conclusions

A less-damaging pump design for refurbishing existing pump station has been developed using a novel iterative CFD-DEM based approach; virtual fish have been used to assess hydraulic conditions and critical zones for collisions. The pump design turned out to be very successful, with 48 h mortalities well below 10% for all six investigated fish species. These very low mortality rates comprise species such as bream (Abramis brama), which are known to be very sensitive to collisions and showed mortality rates above 70% in other less-damaging pump designs (Bruneel et al. 2024).

Current numerical models lack the capacity to provide reliable quantitative predictions of fish mortality during the passage of a hydraulic machine. However, they are well suited for conducting a qualitative comparison of different designs, scenarios, or operation points. A well-documented baseline case would allow one to refer to this design and determine if the machine or scenario at hand would be better or worse than this benchmark case. The Kudensee pump prototype is intended to become such a benchmark case and would allow us to replace live-fish tests in the future.

## Acknowledgements

The authors thank the Haupt-und Deichverband Dithmarschen, namely Jörg Daniels, Pascal Bedey, Jan-Ole Kruse, and the other staff members involved in the project for their continuous support and involvement, which allowed us to finish the project successfully. The authors also thank in particular the staff members at IGF Jena and Fluss for the work during the live-fish tests and all other persons involved in the project. The authors are grateful for the funding the obtained in multiple involved projects: The RETERO project funded by German Ministry of Education and Research (BMBF, grant number 031L0152A) and the StrikeSense project funded by the Deutsche Bundesstiftung Umwelt (DBU, grants number 39399/01 & 39399/02) for method development. Jeffrey Tuhtan's contribution was

<footnote n="2">² https://biorobotics.pages.taltech.ee/edf/en/</footnote>
funded in part by Estonia's Research Council (ETAG) grant no. PRG2198 "MultiFlow - Multiscale Natural Flow Sensing for Coasts and Rivers". The numerical work for the pump design and the validation measures were financed by KSB Supreme Service GmbH.

## Ethics Statement

Live fish experiments were performed under the license no. 58-9/24 issued by the Ministry of Agriculture, Rural Areas, Europe and Consumer Protection in Schleswig-Holstein.

## References

Amaral SV, Hecker GE, Dixon DA (2011) Designing leading edges of turbine blades to increase fish survival from blade strike. EPRI-DOE, Conference on Environmentally-Enhanced Hydropower Turbines, EPRI Report No. 1024609.

Bierschenk B, Pander J, Müller M, Geist J (2019) Fish injury and mortality at pumping stations: A comparison of conventional and fish-friendly pumps. In: Marine and Freshwater Research 70.

Buysse, D, Mouton AM, Stevens M, Van den Neucker T, Coeck J. (2014), Mortality of European eel after downstream migration through two types of pumping stations. Fisheries Management and Ecology, 21: 13-21.

Bruneel, S, Pauwels IS, Broos S. et al. (2024) A fish-friendly axial flow pump turns out to be eel safe, roach unfriendly and bream unsafe. Scientific Reports 14, 30234

Evans, OJ, Norman J, Carter LJ. et al. (2024) Rethinking fish-friendliness of pumps by shifting focus to both safe and timely fish passage for effective conservation, Scientific Reports 14, 17888

Hartmann U, Spratte S, Seen - FischArtenKataster Schleswig-Holstein (2006) Ministerium für Landwirtschaft, Umwelt und ländliche Räume des Landes Schleswig-Holstein (in German)

Hoerner S, Kösters WI, Abbaszadeh S, Wagner F, Tuhtan JA (2024) Towards a reliable and validated toolbox to replace live fish tests for the assessment of injury and mortality during downstream passage, IAHR 15th International Symposium on Ecohydraulics and Fish Passage. Quebec City, Canada

Kösters WI, Tuhtan JA, Hoerner S, Kruusmaa M, Abbaszadeh S (2025) Sensor Probes for Fish Passage Safety: Evaluating Strike Severity Metrics and Data-Driven Prediction (Preprint at https://hal.science/hal-05314322)

Kösters WI, Tuhtan JA, Efimov D, Kruusmaa M, Hoerner S (2025) An open laboratory blade strike rig to evaluate the risk of injury and mortality to fish and to test passive sensors, Sustainable Energy Technologies and Assessments 81, 104427.

Powalla D, Hoerner S, Cleynen O, Thevenin D. (2022) A numerical approach for active fish behaviour modelling with a view toward hydropower plant assessment. Renewable Energy, 188, 02

Richmond MC, Romero-Gomez P. (2014) Fish passage through hydropower turbines: Simulating blade strike using the discrete element method. IOP Conference Series: Earth and Environmental Science, 22(6):062010

Romero-Gomez P, Lang M, Michelcic J, Weissenberger S. (2019) Particle-based evaluations of fish-friendliness in Kaplan turbine operations. IOP Conference Series: Earth and Environmental Science, 240:042016

Saylor R, Fortner A, Bevelhimer M. (2019): Quantifying mortality and injury susceptibility for two morphologically disparate fishes exposed to simulated turbine blade strike. In: Hydrobiologia 842.

Saylor R., Sterling D., Bevelhimer M., & Pracheil B. (2020). Within and Among Fish Species Differences in Simulated Turbine Blade Strike Mortality: Limits on the Use of Surrogacy for Untested Species. Water, 12(3), 701

Saylor R, Bevelhimer M, Pracheil, B. (2020) Fish injury and mortality caused by simulated impacts from turbine blades, DOI:10.21951/1755094

Van Esch BPM, Spierts ILY (2014) Validation of a model to predict fish passage mortality in pumping stations. Canadian Journal of Fisheries and Aquatic Sciences, 71(12):1910-1923

Wagner F, Busch A, Buysse D, Hoerner S, Kenndorf M, Pauwels I, Rößger T, Salqueiro Roth M, Schletterer M, Stamm J, Toming G, Tuhtan JA, (2022) A new, non-invasive fish backpack biogetter to measure the physical conditions experienced by swimming fish during downstream passage, Fish Passage 2022: International Conference on River Connectivity, Richland, USA