GE-RM TRAUN

Flussabschnitt von Traun

The Traun: A river space close to the city

The Traun rises in Ausseerland in Styria and then covers a distance of 153 km as a tributary of the Danube. The mouth of the Traun lies in the urban area of Linz. This is also the study area for the GE-RM Traun. The GE-RM Traun focuses on the lowest 8.5 kilometres from the Kleinmünchen power plant to the mouth of the Traun into the Danube. The urban character and the high degree of regulation of the Traun are special here, but at the same time the large remaining floodplain areas and its associated potential.

GE-RM TRAUN

The Traun: A river space close to the city

The Traun rises in Ausseerland in Styria and then covers a distance of 153 km as a tributary of the Danube. The mouth of the Traun lies in the urban area of Linz. This is also the study area for the GE-RM Traun. The GE-RM Traun focuses on the lowest 8.5 kilometres from the Kleinmünchen power plant to the mouth of the Traun into the Danube. The urban character and the high degree of regulation of the Traun are special here, but at the same time the large remaining floodplain areas and its associated potential.

Overview of flood risk and ecological status

The inventories resulting from the preliminary study, the inventories are used to close knowledge gaps regarding flood risk and river ecology and to identify sectoral deficits. After a comparison with the existing objectives, a concept of measures was developed on this basis.

Short report GE-RM Traun

All details on the inventory, the existing deficits as well as the results of the target definitions and the needs for action can be found in the detailed reports (in German).

Deficits and opportunities

Deficits Traun

Altered hydrology and sediment budget

In the section under consideration, the discharge of the Traun is relatively low. The diversion of the Traun at the Traun weir of the Kleinmünchen power plant (see photo) creates an almost six-kilometre-long residual flow section. In this area, the discharge of the Traun is lower than its natural discharge due to the use of the power plant. At the same time, the backwater of the Danube power plant Abwinden-Asten extends into the area where the Traun flows into the Danube. This creates an untypical flow character, which leads to strongly changing conditions for aquatic organisms. In addition, the bedload and sediment balance in the Traun is strongly altered. Particularly in the dammed area upstream of the power plants and in the residual flow section, riverbed incision occurs. The result is a loss of flowing water character, deterioration of the habitats for aquatic organisms, but also a drop in the groundwater level, which leads to decoupling and the drying out of the adjacent floodplains.

Vogelperspektive auf das
Gra

Water temperature

For several decades, the average water temperature on the Traun has been rising steadily. This trend is basically due to climate change, but is amplified by other factors (such as discharges into the Traun, river regulation or lack of shading). In the considered section of the Traun, the low discharge contributes significantly to warming. In particular, species that depend on low water temperatures are constantly losing their habitat as a result.

Fish population

The three fish species Grayling (shown in the photo and recognisable by the distinctive dorsal fin), Barbel and Nase, which are specific to the Traun, were detected in all existing surveys. Associated species that prefer higher flow velocities, such as Brown trout, European Chub, minnow, gudgeon, hazel, bullhead and loach, were also detected in almost every fish monitoring. Accompanying species that prefer calmer water areas with higher turbidity, such as bleak, roach, ide or pike, on the other hand, were hardly or not at all detected. The demanding mid-distance migrants huchen and burbot were also only recorded with a single specimen (huchen) or not at all (burbot).

Aesche mit charakteristischer Rückenflosse

Opportunities

Alluvial forest

Despite the intensive human interventions in this section of the Traun, a wide floodplain forest with its typical species has been preserved. This habitat is also the focus of the NATURA 2000 site “Traun-Donau-Auen”. If appropriate measures are taken, the water balance of these riparian forests, which depend on recurrent flooding, can be brought back into balance. The silver willow, a typical species in the floodplain forest of the Traun, also benefits from this. It prefers regularly flooded, nutrient- and base-rich soils.

Flood risk

A balanced sediment matter balance offers great advantages not only from an ecological perspective, but also in terms of flood protection. In particular, severe incision of the riverbed can have a negative impact on the stability of dams. However, excessive sediment accumulations also increase the risk of flooding, as the available runoff profile becomes smaller and outburst floods occur earlier. In this sense, the entire human-environment system benefits from the planned measures on the Traun.

Concept of measures Traun

The concept of measures is based on the described deficits and opportunities and was developed by the GE-RM team in an interdisciplinary manner. In the process, appropriate types of measures were selected from a nationwide standardised list of measures. The individual measures or bundles of measures are described in a separate report in the form of fact sheets and located in maps.

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 was assigned a priority. The creation process was concluded with a public participation event, at which feedback on the concept of measures was collected and taken into account.

Example of concept for measures: Traun

As the River Development and Risk Management Concept for the Traun only covers a relatively  short section, the concept for measures is very detailed. The figure provides an insight into the concept of measures, which at the same time forms the basis for the pilot measure and already envisages very concrete changes to the river course of the Traun.

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Fotonachweise:

Auwald Traun: Harald Pfleger
Kraftwerk Kleinmünchen: LINZ AG