GE-RM Enns-Styria

The Enns River: Renaturalization and Flood Protection

The project area studied within the GE-RM comprises the entire river section of the Enns River within Styria from Mandling at the border to Salzburg to Altenmarkt near St. Gallen at the border to Upper Austria. The river section, which is about 130 kilometres long, is characterised by great diversity. While sections in the Gesäuse area are almost natural, humans have massively altered the course of the Enns and especially the Enns Valley in many places. Power plant influences are also clearly visible in many sections of the river.

GE-RM Enns-Styria

The Enns River: Renaturalization and Flood Protection

The project area studied within the GE-RM comprises the entire river section of the Enns River within Styria from Mandling at the border to Salzburg to Altenmarkt near St. Gallen at the border to Upper Austria. The river section, which is about 130 kilometres long, is characterised by great diversity. While sections in the Gesäuse area are almost natural, humans have massively altered the course of the Enns and especially the Enns Valley in many places. Power plant influences are also clearly visible in many sections of the river.

Overview of the flood risk and the ecological status of water bodies

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.

The graphic provides a summary of the deficits on the Enns River in Styria. The schematic representation alone shows how different the situation is in the various sections of the Enns.

Detailed reports to download (only DE)

All details on the assessment can be found in the detailed reports. In addition, very extensive maps were produced on the Enns in Styria, which can also be downloaded.

Section Mandling to the mouth of the Sölk River

 

Deficits

Water morphology & flood risk

Originally, the river Enns had an oscillating course in this section, which, however, was fixed or straightened over large stretches by human intervention (see photo of the regulated Enns in the local area of Schladming). This has increased the gradient and flow velocity. Despite this regulation, there is insufficient flood protection in some areas. Especially in the municipality of Schladming, the flood risk is still high. The villages of Tunzendorf, Pruggern and Stein an der Enns are at risk of flooding as well.

Discharge and hydrology

The discharge behaviour of the Enns is strongly influenced by power plant operation from the Mandling power plant onwards. The operation of the power plant leads to strong fluctuations in river discharge within a few hours. In combination with a large number of authorised water withdrawals, this can lead to extremely low discharge rates, especially in the winter months, often resulting in ecological problems.

Opportunities

Surprisingly high fish population

Although this section is severely influenced by humans and there are only a few so-called key habitats such as shelters and habitats for spawning or juvenile fish, the fish population is surprisingly good in some sections. This is partly due to the revitalisation measures already implemented. At the Salzburger Siedlung, for example, the course of the Enns River was widened and renaturalised (see photo). This has created important habitats. Measures that reduce surge loads, i.e. the influence of the power plants on the discharge of the Enns, and create further key habitats can further strengthen the fish population. This also has a positive effect on sections with low fish populations.

Section from the mouth of the Sölk to the entrance to the Gesäuse area

 

Deficits

Regulation stretch & power plant use

Historically, the Enns had a meandering course in this section, which, however, was almost completely straightened in the course of the Enns regulation. What remained was a comparatively monotonous river. Another challenge here is the high fluctuation of discharge due to the operation of the Sölk and Salza power plants. From a hydrological point of view, this results in a highly unnatural situation.

Fish population

In this section of the Enns, four stretches were surveyed. In terms of fish ecology, two stretches were found to be in an unsatisfactory condition and two stretches were found to be in a poor condition. However, the poor assessment is not only due to the generally low number of fish, but also due to the unfavourable age structure of the population, the species spectrum and the lack of index species.

Opportunities

Diverse Iris Meadows

Iris meadows are characteristic for the Styrian Enns Valley. However, the moderately moist, nutrient-poor areas have been endangered for centuries by agricultural use and the creation of residential areas and traffic routes. Due to the regulation of the Enns, the connection between the river and the surrounding countryside is restricted and the natural water balance disturbed. Renaturalisation measures along the Enns could bring the water balance back into balance and thus ensure that in the future millions of iris blossoms will continue to bloom every year in May.

LIFE+ Enns – Foundation stone for positive developments

As part of the LIFE+ project Enns, some still existing cut-off arms of the Enns were reactivated and reconnected. In addition, wood groynes and smaller river widenings were used to create a high structural variability with different habitats. The renaturalised areas (e.g. upstream of the mouth of the river Salzam, see photo) primarily fulfil an ecological function, but also enjoy great popularity as local recreation areas. In order to permanently compensate for existing deficits, such measures must be continued and expanded.

Section Gesäuse Area

 

Deficits

Fish population

The river section in the Gesäuse area has the greatest diversity of habitats in the Styrian Enns. It is therefore all the more surprising that the fish population in this section is extremely low. Several factors are responsible for this: in the mid-1990s, an invasion of cormorants (see photo) – a predator of most fish species – caused the fish population to collapse. Since then, the population has not been able to recover. This is mainly due to human impact and the noticeable effects of the Sölk and Salza power plants. In addition, many refuges in the feeder rivers are inaccessible due to obstructions, making it difficult for fish to migrate to this section of the Enns.

Opportunities

National park & sustainable tourism

The Gesäuse National Park has now been in existence for 20 years and protects a valuable landscape and habitat. Many different species, some of them endangered, occur in the Gesäuse. Many of these species depend on a healthy river. The dipper, for example, depends on clear, flowing waters rich in oxygen (see photo). However, the national park not only protects important ecosystems, but also makes it possible to experience them in a nature-friendly way. This attracts visitors and also brings economic opportunities for the region.

Section of the hydropower plant stretch from Gstatterboden to the provincial border

 

Deficits

Migration obstacles

The passability for fish and other aquatic organisms is insufficient in this section of the Enns. Only the Gstatterboden weir is equipped with a functioning fish ladder. At the other weirs, the height and space conditions are very difficult (see photo of the Landl power plant), so that only complex technical solutions for restoring passability can be considered. However, so far such measures have not been implemented.

Kraftwerk an der Enns in Österreich.

Flood risk

The current discharge analysis shows that several properties in Hieflau and Altenmarkt would be affected by a 100-year flood event. Measures to reduce the flood risk should bring about an improvement here. But even after the implementation of measures, a small risk remains. In order to be prepared for an extreme event, additional measures of raising public awareness and disaster prevention are necessary.

Measures

The concept of measures for the Enns in Styria is based on the described deficits and opportunities and was developed by the entire project team in an interdisciplinary manner. In the process, suitable 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 located in 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.

Overview of measures for the Styrian Enns

Symbol Hochwasserschutz

Measures to reduce the flood risk

Measures to prevent flood risks can be very diverse. The most effective measures are combinations of structural and non-structural measures. In the GE-RM for the Enns River in Styria, 37 measures were defined to protect the population from floods. These range from the preservation of existing retention areas to the rehabilitation of existing facilities and the object protection. In the planning process, these measures were harmonised with water ecology objectives in order to exploit synergies.

Symbol Verbesserung der Durchgängigkeit

Measures to improve passability

Fish migrate by nature. This allows them to use different habitats. For example, different locations are important for reproduction than for growth or feeding. That is why fish migrate within river systems to find the optimal conditions for their respective needs. Today, numerous artificial obstacles interfere with this. In order to enable fish and other aquatic organisms to migrate as freely as possible again, 8 measures that focus primarily on reconnecting feeder rivers have been defined for the Styrian Enns.

Symbol Hydrologischer Verbesserungen

Measures to reduce the impact of power plants

Hydropower is one of the crucial pillars of Austria’s energy production. However, hydropower plants can have serious environmental impacts. Therefore, the assessment of the ecological dimension is essential in the planning, operation and modernisation of power plants. In the GE-RM for the Styrian Enns, 7 measures were defined to reduce negative environmental impacts. The focus is on measures that reduce the impacts of the operation of the power plants and increase the residual flow discharge.

Symbol Verbesserung des Feststoffhaushaltes

Measures to improve the sediment management

Functioning sediment management is crucial both regarding flood risk management and regarding the water ecology. If, for example, bedload accumulates at narrow points in settlement areas, flooding can occur more quickly. Dredging could solve this problem, but such an intervention massively disturbs the sensitive ecosystem and aquatic life. In order to prevent this, 4 measures for the creation of a well-balanced bedload level were defined in the GE-RM Enns.

Example of a concept of measures

The measures of the GE-RM cover the entire Styrian Enns. The map shown provides an insight into the measures for the Liezen East area, where a combination of flood protection and river development measures has been planned. In order to improve flood protection for properties at risk, walls and dams are planned that are set back from the river and do not have a negative impact on the river. For the river development, the concept provides different variants that can be implemented depending on the general availability. The variant with the largest ecological impact envisages the creation of a new river bend and thus an extension of the river course.

The alternative is a more space-efficient widening of the River Enns. By renaturalising the mouth of the Pyhrnbach stream, the connection to the Enns should be restored. The widening of the Enns and the optimisation of the retention area – the area that can be flooded during floods – east of the mouth of the Pyhrnbach complete the concept in this area and have a positive effect on flood protection and water ecology.

Maps & Reports

All measure profiles and maps on the key measures from the GE-RM Enns are depicted in comprehensive documents.


Photo Credit:

Kraftwerk Landl: Verbund
Salzburger Siedlung: basemap.at