14.1 Introduction

To foster and conserve fish populations, large efforts in re-establishing connectivity and restructuring rivers have been made by the Hydropower sector. Most of these efforts aimed to pursue the common goal of reaching the good environmental potential according to the water framework directive (WFD) in heavily modified water bodies. According to the sectors experiences, a “best environmental option” should be implemented and chosen as a solution to reach the good ecological potential. The focus of the plant owners is to identify key habitats for specific species and establish these along the rivers, tributaries, oxbows and especially in river like bypass channels. The attempt is to offer accessible habitats for the whole live cycle of fish starting from spawning, to the juvenile phase up to the adult stage. The consideration of habitat issues is of uppermost importance for the conservation of fish populations. The restoration of longitudinal connectivity without consideration of habitat issues leads to sub-optimal solutions in river restoration.

A systemic approach should also be pursued in guidelines for fish passage. The focus on hydraulic design parameters in most of the available guidelines often results in technical solutions as it is easier to adhere to the hydraulic guide values implementing technical fishways such as vertical slot passes. The focus on fishway hydraulics also may shift attention from other important issues that must be addressed to reach the goals of the WFD, such as habitat availability. The goal should be to foster all endangered rheophilic and lithophilic species and thus to identify and implement the fluviatile aspects necessary for their protection and promotion.

14.2 Ecological Measures in Impoundments of River Power Stations

At the Inn River in Germany and along the border to Austria, VERBUND is operating a cascade of run off the river hydropower plants. The first plant started operation in the beginning of the 19th century. However, the deterioration of the river started much earlier related to navigation, flood protection and the development of agricultural areas. The Inn River has been straightened and banks have been fixed, which enhanced incision of the river bed. The construction of hydropower plants also aimed to stabilize the river bed. Modifications like impounding, bank protection and further flood control works have completely changed the river system. The siltation processes and the high sand transport formed secondary flood plains and oxbows with a high primary productivity. Over time, major floods and the ongoing sedimentation left only minor structures with almost no lateral connectivity to oxbows.

To counteract these processes the operator VERBUND, together with authorities and planners developed a large scale restoration scheme for the Inn River in Germany. The focus of the first measures was along the impoundments of the hydropower plants Wasserburg, Teufelsbruck and Gars (start of operation in 1938) with the goal to identify measures to foster and protect existing fish species. The focus was not only on rheophilic potamodromous fish species, but also on the improvement of the fish biomass as fish play a major role in the existing bird sanctuaries especially with regard to birds prey. A decline of the fish population has been observed by local fisherman since the last three decades.

In a first step former and existing habitat elements were identified and classified to certain habitat needs of populations. A so-called “fish habitat concept” was the overall approach to identify possible measures. The aim was to identify the main habitat needs for all different stages of development. Starting from spawning, juvenile up to seasonal habitat use e.g. floods and winter conditions. A major challenge in the analysis was, that each hydropower plant and the respective river stretches provided differing natural constraints due to the local conditions (geometry) such as gorge type, or wide artificial wet lands (reed and willows) but also due to river training and the high fine sediment loads of the Inn River.

The German Federal Water Act (Wasserhaushaltsgesetz, WHG) demands concepts and measures to protect fish population as a requirement to operate hydropower plants. Large hydropower plants with low head and large Kaplan turbines have lower mortality rates than small plants. Additionally, technical solutions, in this scale, are not available (Reckendorfer et al. 2017). Further, existing solutions such as small trash rakes implemented at the existing inflow structures would either increase velocity or, in the case of the Inn River, with high sand transport and high loads of large driftwood, would make existing hydropower production almost impossible.

Thus, alternative approaches and measures for fish protection had to be developed and applied along the River Inn. After analysing the historical conditions and discussions with stakeholders and decision makers, VERBUND and a team of involved experts decided to implement as many key habitats to the system as possible and furthermore add certain key habitats into the bypass systems, i.e. the connectivity measures have been combined with several main habitat components (flow, gravel, shallow parts, ponds etc.). In large rivers such as the Inn most of the eco-morphological structures and components are difficult or almost impossible to reconstruct or maintain within the main river. It is known that key habitats show a substantial effect on fish population whereas technical measures such as smaller trash rakes lack their verification on a population level.

Flood protection necessities, land and forest use, ownership and sanctuaries are constraints to be identified prior to the concept phase of implementation of restoration measures. In the concept phase starting with the historic river system, the preconditions due to flood protection and impounding were identified, including also secondary floodplains with their ecological functions. These floodplains had a variety of warmer shallow lagoons connected to the main river. The possibility to add these former structures into the existing system were investigated by the project team. These secondary wetlands play a major role for the bird sanctuaries as feeding grounds due to their high primary and secondary productivity.

As gravel is not any longer transported through the series of plants a concept to compensate for the lack in gravel transportation and its function was also necessary. The main ecological functions of gravel for fish such as providing spawning grounds and nurseries can be provided in connected and restructured tributaries, at shores with removed bank protection and in newly created bypass rivers.

At different sections of a reservoir of a run-of-river plant different measures are identified (Holzner et al. 2014; Loy et al. 2014) and might be possible to improve the ecological situation (Fig. 14.1).

Fig. 14.1
figure 1

Schematic view of possible measures to add habitats to existing reservoirs (water level variation (0–4.5 m; upper axis) and flow velocity (0.35–2.5 m/s; lower axis) for different design discharges (HQ10, HQ5, MHQ, MQ und NQ))

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14.2.1 Immediate Vicinity of the Hydropower Plant; Significant Low Flow Velocities and Little Variation in Water Level

Possible measures:

  • Creation of artificial or desilted existing oxbows with lateral connectivity to the river; mainly dredging work, creating raw sand areas and improvement of negative stagnant reed front, deep and shallow water conditions and adding temporarily change

  • Allow possible permanent flooding (connected from downstream, due to sand freight) of longitudinal areas that were formerly dry, sand removal from oxbows deposited during extreme floods

  • Longitudinal connectivity; either technical solution in gorge like sites or if possible long bypass systems with gravel bed, flow variation, river like conditions in flow variation (water level).

14.2.2 Middle Part of the Reservoir; Minor Flow Variation and Temperate Seasonal Variation in Water Level

Possible measures:

  • Desilt existing oxbows and allow lateral connectivity to the river; mainly dredging work similar Sect. 14.1

  • Allow and add water level and floodplain variation especially during higher floods at shallow areas—different measures in the flood plains

  • River shore and flow variation due to newly constructed structures: groins, gravel, stones from existing historic bank protection works and add shallow areas and terrestrial succession

  • Connect and desilt tributaries to allow lateral connectivity and add gravel function.

14.2.3 Head of the Reservoir/Downstream Part of the Next Plant; High Flow Variation, High Seasonal Variation in Water Level and Therefore Almost Natural Variation (Water Level, Flow Conditions)

Possible measures:

  • Artificial islands, shallow gravel bars (gravel function), inflow of bypass structure, add oxbows with lateral connectivity, add floodplain characteristic with deep and shallow water depth conditions and change of water level

  • Remove shore protection, add river shore and flow variation with added structures; groins, gravel, trees, stones from existing historic bank protection works, add shallow areas during floods and allow terrestrial succession

  • Connect and desilt tributaries to allow lateral connectivity and add gravel function.

The main focus of the implemented measures (Fig. 14.2) at the existing reservoirs was to allow and foster dynamic fluviatile changes and lateral connectivity to oxbows and tributaries. Especially the heads of the reservoirs often allow to meet the requirements of flow velocity and water level dynamics. Additionally, due to the historic river bed incision there are often minor restrictions in respect of flood protection. Therefore, the elimination of bank protection, the reconnection of existing and the creation of new floodplain areas and islands as well as the introduction of gravel bars into the system is possible. For these new alluvial structures, constructed from existing alluvial depositions, it is important to meet the aim of possible shallow gravel bars with shallow areas reaching up to high floods. Along the 250 km river Inn in Germany only around four to seven locations are yet identified to allow this sort of large scale restructuring. Restrictions are mainly flood protection risks, infrastructure, availability of land, settlements and influenced agricultural land. An important design criteria for the River Inn is to cope with the deposited sand in the flood plains and their natural deposition during and after floods. Therefore, an artificial furcation zone needs to be designed in respect of sheer stresses that only minor sand deposition occurs. Sand is part of the natural river system of the Inn and its transport and deposition is accepted and supported. For artificial backwater zones—an upstream connectivity—would result in a fill up within one season. The existing deposited sand has been used to create protection structures around the artificial oxbows or was added to the River Inn, as transport and deposition on land is almost impossible. The Inn system does transport the sand on a regular basis without any recognisable change or environmental impact.

Fig. 14.2
figure 2

All habitat components need to be available and reachable to meet the whole set of requirements of the life cycle. Nature like bypass channels can play a major role to add components to the anthropogenic changed river system

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More than seven years after the implementation of the first structures some of them still have the function as expected, others were dominated by hydrological events with its sand impact, some show regular natural variation in structure and habitat and some need regular maintenance to guarantee connectivity or spawning ground conditions. In a natural river system with its variation in hydrology and sediment transport some approaches targeting the aquatic zones became later terrestrial dominated. However, in an integrated approach the major factor and aim is, allowing changes within the system to happen. Sometimes the river dominates the system, sometimes the anthropogenic changes are so dominant that process oriented maintenance should be considered on a regular basis. But such an approach becomes difficult to implement if protected species might be affected, either by the maintenance measures or by the natural changes in the zoonosis.

14.3 Near Natural Bypass Systems as Key Element of an Integrated Approach

Upstream connectivity for fish migration is a main requirement to meet the WFD criteria in Europe. Guidelines have been developed mainly using the experience in salmon rivers and at small HPPs in (northern) Europe. Especially guidelines on the findability of the entrance such as flow velocity the “competing” discharge, and the location of entrance are difficult to be reasonably implemented at existing plants and local site conditions at large alpine rivers. For example, in the Inn the mean summer discharge varies from 500 to 1,000 m3/s (downstream Salzach River confluence) and high turbulences and flow velocities can be observed downstream of the plant (up to 200 m). Therefore, the guidelines on entrance location can seldom be met. Existing guidelines only can provide a rough basis for the design at large rivers. Furthermore, the combined effects of connectivity and the creation of the habitat requirements of the fish species to the river system is recommendable and the following aspects and criteria should be fulfilled.

The main design criteria for bypass-systems on the Inn River are:

  • Only potamodromous fish species (no salmon and no eel)

  • All present fish species (around 40–50) shall reach the habitat seasonally to fulfil their life cycle

  • All required habitats within the reservoirs should be available, reachable and should meet the quality requirements to fulfil the life cycle

  • To reach spawning areas, add juvenile habitats, food habitats as well as flood and winter habitats are essential.

General layout for bypass channels and connectivity:

  • Connection up and downstream (genetic cross over and compensation migration especially after floods); mainly juvenile and small fish, adult fish

  • Lateral connectivity to adjoining tributaries, streams, oxbows and small pools—habitat variety for different age classes

  • Spawning grounds with substrate variety, shallow areas typically for a natural river reach and typical habitat layout, including mainly juvenile habitat aspects

  • If no constraints (adjoining owner, ownership, geometrical conditions) exist: variation of discharge, duration of flow, water levels, sediment transport—mainly river like design criteria slope, depth, velocity and sediment variation

  • Detectability of entrance due to attractive structures, flow conditions at specific site and prioritisation of habitat aspects versus pure connectivity criteria.

Starting with the first sites in 2013 until now all experts involved in the layout and design as well as the experts at the authorities agreed in the local adaption of the guidelines design criteria to meet the overall aim—to increase and add a large number of habitats necessary—to the Inn River system. A variety of different types of bypass channels (size and discharge), technical structures and adaptions to the local site conditions were implemented. For experts without local knowledge it is often difficult to understand the local constraints (land use, ownership, geometrical constraints (gorges) and third party influence during the approval process, fears from flooding etc.) which restrict some originally intended design criteria. However, even small bypass channels with constant discharge can provide missing habitats for key fish species for all life-stages. Since 2015 all aspects of our bypass channels, habitat variations and instream structures are systematically evaluated by the Technical University Munich (Nagel et al. 2019). Spawning rates, numbers of juveniles, number of species and change of species, habitat use over time, are evaluated in the project lasting in total 10 years. In the following 5 years an individual tracking and tracing of more than 20.000 fish is commissioned to be installed and evaluated to get further information of travel distance, habitat use (bypass and special habitats) and the seasonal habitat use of the Inn fish population. In 2027 a connectivity of almost all plants from Switzerland to Vienna is reached.