GE-RM Danube

 

The Danube: a river that unites

The Danube is the largest river in Austria. With a total length of almost 3,000 kilometres, it is also the second longest river in Europe. From its source to its mouth, it flows through a total of ten countries – more than any other river in the world. This brings many challenges, but also great opportunities.

Within the scope of the GE-RM Danube, the segment of the Danube in the province of Upper Austria was examined in more detail. Due to its size and the complex framework conditions, the development of an integrative concept of measures for the Danube has been a great challenge. After all, the Danube is one of the most important waterways in Austria and Europe. In addition, the Danube in Upper Austria is strongly influenced by energy production and the associated power plant operation.

GE-RM Danube

The Danube: a river that unites

The Danube is the largest river in Austria. With a total length of almost 3,000 kilometres, it is also the second longest river in Europe. From its source to its mouth, it flows through a total of ten countries – more than any other river in the world. This brings many challenges, but also great opportunities.

Within the scope of the GE-RM Danube, the segment of the Danube in the province of Upper Austria was examined in more detail. Due to its size and the complex framework conditions, the development of an integrative concept of measures for the Danube has been a great challenge. After all, the Danube is one of the most important waterways in Austria and Europe. In addition, the Danube in Upper Austria is strongly influenced by energy production and the associated power plant operation.

Flood risk and ecological status of the river

Following the preliminary study, the results of the inventory were used to close knowledge gaps regarding flood risk and water ecology and to identify sectoral deficits. After a comparison with the existing objectives, based on this, a concept of measures was developed.

For this purpose, the Danube was divided into five sections, which are based on the reservoirs of the power plants (see figure). In addition, evaluations of existing measures and proposals for potential measures are available for the border section in the Jochenstein reservoir.

Eine Karte mit den Renaturierungsgebieten des Life-Iris-Projekts.

Detailed reports for download (Only DE)

All details on the inventories, the existing deficits as well as the results of the target definitions and the definition of action needs can be found in the detailed reports.

River ecology deficits on the Danube

The river ecology of the Danube in Upper Austria is heavily influenced by the series of hydropower plants. The backwater causes problems that apply to all described sections and are therefore described here by way of introduction. Particularly problematic is the loss of key habitats such as distributaries, gravel banks, islands, accessible tributaries, old river branches and floodplains. Reduced flow velocity, impaired bank and bed structure (bank gradient, stone accumulation, fine sediment deposition) and generally low dynamics also cause problems from an ecological point of view. In general, the decoupling of the Danube from its surroundings has a negative impact on the ecosystem.

Aschach reservoir section

 

Deficits

Fish passability

Currently, the Aschach power plant is an obstacle to migration for aquatic life and is classified as impassable. The Aschach power plant has the highest drop height of over 15 metres and, with over 40 km, the longest reservoir of all Austrian Danube power plants. In order to make the power plant passable for aquatic life, the drop height must be compensated by installing a fish passage. Although the construction of such a passage is particularly complex due to the space available, a suitable measure is to be implemented by the VERBUND by 2027.

Tendency to silting up

From 1963 to 2002, almost 24 million m³ of fine sediments were deposited in the Aschach reservoir. During the major floods of 2002 and 2013, more than 6 million m³ were mobilised at short notice and discharged into downstream reservoirs and floodplains.

Opportunities

Extensive measures

The Aschach reservoir is located in a so-called water gap (see also photo near Schlögen). Therefore, there are naturally only a few tributaries and oxbow lakes in this area. After regulatory measures and the construction of power plants, the number of tributaries and oxbow lakes has been further reduced. The large amounts of deposited sediments offer the opportunity to use the material for ecological riverbank design. Since the 1980s, extensive biotope and bank structuring measures have been implemented in the backwater area of the power plant, which partly compensate for the deficits in this area. A total of 24 measures with a total length of almost 12 km have already been implemented in this section.

Ottensheim-Wilhering reservoir section

 

Deficits

 

River Structure

The Ottensheim-Wilhering reservoir is located in a basin where backwater dams are required. Therefore, the few tributaries (Pesenbach, Aschach, Innbach) had to be diverted into the tailwater. This section of the Danube is particularly poor in structure and contains only very few high-quality habitats. However, the construction of the dynamic bypass arm for the Ottensheim-Wilhering power plant has brought great improvements. The 14.2 km long bypass arm fulfils an important ecological function, as it is both a fish migration aid and a high-quality habitat. Incidentally, it is also the longest fish migration aid in Europe.

Opportunities

 

Flood Protection

In the future, the municipalities in this section will be protected much better against floods. Currently, the municipalities in the Eferding Basin are in the process of submitting plans based on the existing general project, in which different protection concepts were compared in terms of their effectiveness. The planning envisions the construction of dams that are set back from the watercourse and thus do not have a negative impact on the river habitat. These measures are intended to prevent damages like those caused by the flood in 2013.

Ein Luftbild das Hochwassers in Becken in Österreich aus dem Jahr 2013.

Asten-Abwinden reservoir section

 

Deficits

 

Moderate Ecological Potential

This section extends from the water gap of the Linzer Pforte breakthrough to the Linz Basin and is characterized by adjacent traffic routes, settlement areas and thus bank reinforcements, which do not allow for dynamic river development. The hydromorphological pressures, the very low fish population, the absence of the index species Huchen and an insufficient age structure of the fish populations make for a moderate ecological potential.

Ein Luftbild des Abschnitts StauraumAsten der Donau in Österreich.

Opportunities

 

Nature and Human Use

Linz, the third largest city in Austria, is located in this section. Due to the intensive urban use, industrial facilities and the Linz ports, the interaction with the Danube is particularly high here. Furthermore, the Danube’s potential as a local recreation area plays an important role. A good balance between nature and human use is therefore of great importance. Large-scale renaturation measures have already been implemented near Ottensheim and Wilhering. Part of the measures is also the Marktau island tributary system, which is to be extended within the scope of LIFE IRIS. This benefits, for example, the common nase (“Nase” means “nose” in German. Nases prefer flowing waters where they stay directly in the current mainly on gravel banks. They are very sociable animals and are often found in large shoals. Other aquatic species also benefit from the renaturation measures and the improved habitat quality.

Wallsee-Mitterkirchen reservoir section

 

Deficits

 

Unbalanced Sediment budget

Interference in the sediment balance is a fundamental problem of hydropower plants. Natural bedload transport is largely prevented and floods lead to increased mobilisation of fine sediment. The Wallsee-Mitterkirchen reservoir is particularly affected by this. Over the observation period from 1982 to 2019, gravelly bed material has shifted to the central reservoir area and contributed to bed deepening and structural poverty in the head of the reservoir. Overall, however, significantly more material has left the reservoir during the period than has been replenished from above. Dredging is carried out to maintain flood protection and fairway depths for navigation. Especially in this reservoir, massive dredging for gravel extraction has been carried out in the past, leaving the section downstream of the Mauthausen bridge in a particularly structurally poor condition. In addition, however, the chain of power plants on the Enns and the expansion of the Enns harbour mean that no bedload reaches the Danube from this important alpine feeder. The resulting sediment deficit amounts to about five million cubic metres of solid material. This corresponds to about 500,000 truckloads.

Ein Luftbild von Kraftwerk Wallsee-Mitterkirchen an der Donau.

Opportunities

High potential for action

With the Enns, Gusen and Aist, the Wallsee-Mitterkirchen reservoir has important tributaries. The head of the reservoir, i.e. the upstream section of the reservoir, is comparatively long with a high number of falls and has a high potential for measures. High-quality habitats for juvenile fish, especially for flow-loving gravel spawners, have so far only existed locally and are a result of structuring projects that have already been implemented. An expansion of such projects and a near-natural design of the river could further improve the habitat quality in this section. The near-natural state of the Danube in this section and the up to 3 km width of the river basin before the regulation of the Danube can be imagined with the help of the Josephinian map from 1775.

Ybbs-Persenbeug reservoir section

 

Deficits

 

Fish passability and fish population

Like the Aschach power plant, the Ybbs-Persenbeug power plant is currently not yet passable for fish and other aquatic organisms. Therefore, it is planned to install a fish migration aid by 2027. In general, this section is also characterised by low fish stocks and a partially unfavourable population structure of the key species common nase, barbel, dace, nerf and bream. Another problem, as in the entire course of the Danube, is the high dominance of neozoa, i.e. animals that were not originally native to the area.

Opportunities

Ein Luftbild der mäandrierenden Donau in der Nähe der Insel Wörth.

High Structural Value

The Ybbs-Persenbeug reservoir has a comparatively high structural quality in the extensive head of reservoir between Wallsee and Ardagger. Although there are hardly any flow-through tributaries left, which were typical especially in the Machland, there are still large areas of connected and isolated oxbow lakes. Due to the subsequent constriction in the Strudengau, there are high level fluctuations between high and low water and corresponding residual dynamics.

Living in harmony with the river – flood protection Machland North

The flood protection measures “Machland Nord” are also located in this section of the river. “Machland Nord” is one of the largest flood protection projects in Europe and was created in response to the flood of the century on the Danube in 2002. The measures protect almost 23,000 people from floods. In addition to technical solutions and settlements, the project also uses the optimisation of floodplains and retention areas. Measures that work with nature also have a positive effect on the ecosystem. Floods, for example, are again having the biotope-building effect they have always had. As the picture shows, the concept proved its worth during the 2013 floods.

Measures

The concept of measures is based on the described deficits and opportunities and was developed by the entire team in an interdisciplinary approach. In the process, corresponding types of measures were selected and located from a nationwide standardised list of measures. The individual measures or bundles of measures are described in a separate report in the form of profiles and their locations are shown on maps of measures. Each measure or bundle of measures was assessed from a technical point of view with regard to its relevance for the achievement of objectives and assigned a corresponding priority. The preparation process was concluded with a public participation event, at which feedback on the concept of measures was collected and taken into account. In total, 169 potential measures were defined for the Danube.

Overview of Measures for the Danube in Upper Austria

The 169 measures defined in the GE-RM Danube concern all five described sections and the right river bank in the Jochenstein reservoir. The number of measures and the length of the defined measures per reservoir are shown in the diagram.

All proposed measures from the GE-RM Danube are presented by means of fact sheets and a comprehensive report.


Photo credits:

Aschach power plant, Ybbs-Persenbeug power plant & Ottensheim fish migration aid: Verbund
Wallsee-Mitterkirchen power station: Hans Koberger
Flood Eferdinger Becken 2013: Austrian Armed Forces/KERMER
All common nase images: Clemens Ratschan/TB Zauner
Insel Wörth: Marktgemeinde Neustadt an der Donau