River continuity under difficult boundary conditions–useful steps and practical hints for designing a fish way

The basic objective of the European Water Framework Directive (WFD) (2000/60/EC) is to achieve good surface water body ecological status, which is defined by the respective quality of four components–macrozoobenthos, macrophytes including phytobenthos, phytoplankton and fish. An essential prerequisite for a largely undisturbed development, in particular for fish and macrozoobenthos, is the water body's longitudinal continuity.

For hundreds of years, man has used river systems for diverse purposes. To meet these satisfactorily barriers have been installed within the river courses, inter alia, in the form of large weir structures. This is the case, in particular, in densely populated urban and industrial regions like the wider parts of the Ruhr catchment in the central region of the German state of North Rhine-Westphalia (NRW), where there are a large number of such transverse structures. The Ruhr is a 219 km long tributary of the Rhine having an average discharge at its mouth in Duisburg of about 80 m³/s. From here to the North Sea is 253 km along the Rhine. The Ruhr is an important source of drinking and industrial water for about five million people. To safeguard that task Ruhrverband operates more than 800 water management installations across the Ruhr Basin, including eight reservoirs and five impounded lakes.

For some time now, the Ruhrverband has been considering how to restore river continuity in its basin (Weyand et al. 2006), and how to address and solve the issue in a structured and targeted way, in agreement with the weir operators concerned. In line with its task of ensuring reliable water quality and quantity management in the catchment, Ruhrverband has built weirs at impounding lakes and reservoirs. For the latter there is still consensus in Germany that the dams’ complex structure makes the continuity issue less topical and urgent at these locations, but that continuity should remain high on the agenda for all other weir sites. So Ruhrverband has moved forward on this route and completed upstream fish passes at several barrages, in successive project stages.

To ensure that fish (specifically the so-called long-distance migrators) can reach their spawning, growth and living habitats, riverine continuity must be maintained, especially in the lower reaches of the rivers. Continuity in the Ruhr is still heavily restricted by several ‘built-in’ barriers. In particular, the large weir barrage at Lake Baldeney impounding reservoir is an insurmountable obstacle for fish, blocking their up-river journey, leaving aside their possible passage through the existing lock, which is not readily detectable or manageable by fish.

The Baldeney Weir is 31.1 kilometer upstream of the confluence of the Ruhr and Rhine. It is the fifth weir in a chain of 16 obstacles in the lower part of the river downstream of its confluence with its main tributary, the river Lenne. The average flow rate at the weir is 75.9 m³/s, ranging from 17.8 m³/s at mean minimum discharge up to 588 m³/s at mean flood. The normal water level just downstream of Baldeney reaches 43.0 m msl (m msl represents metres above mean sea level), corresponding to a water depth of about 2.9 m. During flood events it rises to 44.7 m msl.

Following the work of Huet (1959) and Illies (1961), the Ruhr at Baldeney Weir is classified as belonging to the Barbel zone. Characteristic fish species include Barbel (Barbus barbus), Roach (Rutilus rutilus), Chub (Leuciscus cephalus), Minnow (Phoxinus phoxinus), and European perch (Perca fluviatilis). They are accompanied by diadromous fish species like salmon (Salmo salar), sea trout (Salmo trutta f. trutta), sea lamprey (Petromyzon marinus), and Eel (Anguilla anguilla).

Dumont & Hoffmann (2011) came to the conclusion, in a feasibility study, that–from a purely functional and constructional point of view–there are several suitable design variants for a fish pass. These encompass, among other things, the construction of a technical vertical slot pass on the right river bank. Its implementation would have been too costly and complex, however, due to the confined space and difficult site surroundings–e.g., various power lines and sewers, needing support and safety works. Another solution might be construction of a close-to-nature diversion channel (pool and boulder pass), which would, however, have to run on top of a high level and through a deep cut, for topographic reasons, and which would also have high demands for land belonging to others. The study also dealt with incorporation of a slot pass in an old reverse-pumping station that is now closed. This might also be a structurally demanding solution, and even more unfavorable from an operational point of view. The installation of a fish lock or a fish lift at the same location was also considered.

Discussion of these fish way construction proposals for Baldeney Weir, held with the competent NRW ministry (Düsseldorf), have not yielded any decision as to which solution should be preferred. On the other hand, discussion of the pros and cons of the different options showed a need for further clarification, especially in relation to the optimum entrance position, the most suitable type of construction, and way in which the fish passage should be installed and integrated, to match the conditions of the existing barrier structure and its surroundings.

The establishment of an expert committee was agreed with the aim of elaborating a solution acceptable to all concerned. The committee's membership included representatives from those directly involved–Ruhrverband and RWE AG (the latter as operator of the hydropower plant)–and from the competent local and federal authorities (the Ministry for Climate Protection, Environment, Agriculture, Conservation & Consumer Protection of NRW, and the State Agency for Nature, and Environment and Consumer Protection) and the district government. The decision finding process has also been supported by expertise and assistance provided by the scientific community.

A common visual inspection of the structural, operating and topographic conditions of the weir and power plant on Lake Baldeney, a detailed study of all available documents, and extensive discussion of the insights gained, led all parties to agree that it was impossible to make an unambiguous statement, on the basis of these efforts, about the proposed fish pass’ entrance position. The working group recommended conducting further investigations, however, that would provide qualified evidence about the attraction flow needed for the fish pass and its traceability within the dominant turbine flows. On that basis, it is possible to identify potential migration corridors that might form in accordance with existing local underwater conditions and turbine operations, and the amount of attraction flow, and to deal separately with the selection and assessment of the type and construction method of a site-specific fish pass.

The committee suggested using hydrodynamic-numerical flow simulation tests on the basis of different entrance variants as a suitable method for determining the best location for the fish pass. The results obtained in this way were produced as flow vectors and velocities at different cross-sections. Visualization of these results allows fish ecologists to evaluate and determine which of the pre-set and computed entrance variants are detectable by fish. Colored imaging is considered a suitable form of visualization for the purpose, generating a so-called ‘etho-hydraulic signature’ of the computation results, which can be interpreted in an interdisciplinary way by the experts involved (Adam & Lehmann 2013).

However, the final decision about using this variant still depends on the results of the forthcoming physical and etho-hydraulic modeling required to provide evidence of its suitability at Lake Baldeney. The tests will be carried out at the Karlsruhe Institute of Technology. Fish generally need the presence of a readily perceivable attraction flow in the tailwater area of a transverse structure, in the direction of the entrance to the fish pass. That is agreed to be at least about 1% of the individual river discharge, which may come up to about 1.4 m³/s at Baldeney Weir, as the maximum turbine capacity is 140 m³/s. With the variant preferred for Lake Baldeney, it is still to be confirmed that the attraction flow required for the fish does not compete with the outlet flow from the lift chamber, which is only about 0.25 m³/s. On this basis, it was determined that it would make sense to install an entrance basin or pool in front of the lift system discharging the total attraction flow needed for fish pass traceability into the downstream water body. The entrance basin should be designed so that the additional discharge exceeding the outlet flow from the lift chamber does not comprise a concentrated, directed jet, but rather produces a diffuse current, via a trough spillway, as an overflow close to the entrance chamber walls. This can be achieved by installing a specifically designed spillway crest and roughening the wall surfaces.

As it is essential to know how the flow patterns generated in the entrance basin will influence incoming fish, a specific model configuration was tested to determine whether the fish would be attracted by the overflowing water and, with that, be distracted from the guiding corridor or even show avoidance behavior. Although the scope of these tests has been rather limited, so far, the results suggest that water inlet via a trough spillway will probably not have a negative impact on either fish behavior or the attraction flow pattern created in the entrance basin.

In the final step, a full-scale model was built representing the entrance scenario and the proposed fish lift. It was intended to reproduce the flow velocities and water depths occurring naturally round the cylindrical lift chambers. With this assembly it is possible to observe, document, and finally to analyze statistically the behavior of fish as they approach the planned structure, swim into the lift chambers, reside in them and then swim out into the backwater. Some six hundred fish, representing twelve typical Ruhr species, will be ‘in action’.

The results to date suggest that the new fish lift concept is suitable in principle for the restoration of upstream continuity in the Ruhr River. Should the remaining tests confirm this, Ruhrverband will bring this fish way project to fruition as soon as possible.

The situation at Baldeney Weir and hydropower station shows clearly that standard solutions cannot be used to tackle continuity problems at structures with design capacities of this order, because of the complexity of the topographic, structural, operational and hydraulic conditions. Hence, Ruhrverband considers the chosen approach of bringing together professionals and experts from different disciplines, and pooling their expertise, a good method of arriving at a solution that takes adequate account of the interests of all parties. Everyone involved is convinced that it will be possible to achieve a result that meets the ecological requirements of both the WFD and the fish, in the end, as well as benefitting the project in terms of economic viability.

Our special thanks go to the Ministry for Climate Protection, Environment, Agriculture, Conservation & Consumer Affairs of the German Federal State of NRW, which substantially co-funded this solution-finding approach and the implementation of the investigations required. Sincere thanks also go to all institutions and persons participating in the roundtable for their active, constructive and open-minded co-operation. These are in the matter of research: Institute for hydraulic engineering and water resources management at the TU Darmstadt, in the matter of project planning: Ingenieurbüro Floecksmühle (engineering consultants) and Bugefi (environmental planning, water management and fisheries), and in the matter of fishing: Westphalian and Lippe Fishery Association and Ruhr Fishery Cooperative.