Description of the project area
The IRIS pilot area on the river Traun covers the lowest river section, with a length of about 8.5 kilometres, from the weir of the Kleinmünchen hydropower station to the mouth of the Danube.
The Traun originates in the Totes Gebirge mountain range in the Styrian Salzkammergut and flows into the Danube after crossing the Northern Limestone Alps, the Prealps and the Upper Austrian Alpine Foothills near Linz. With a length of 153 km, it is the second largest tributary of the Danube in Upper Austria after the Enns.
Hot Spots & Highlights
The project area is located in the European nature reserve Traun-Donau-Auen. In the middle of the intensively used central area, a species-rich, near-natural ecosystem has been preserved. It is dominated by softwood and hardwood floodplain forests, but also hosts numerous small stagnant waters with diverse amphibian fauna as well as breeding, resting and hibernation habitats for rare and endangered bird species.
The project area is of high importance for aquatic ecology. Although it is a residual water stretch, due to its location between the Traun and Danube hydropower dam chains there are suitable habitats for current-loving fish species of the lower Traun and the adjoining Danube section.
Need for action
The Traun is heavily regulated in the project section and shows a massive bedload deficit due to the weir of the Kleinmünchen small hydroelectric power station and the upstream chain of dams. In the event of flooding the regulation in combination with large volumes of water lead to very high flow velocities causing a considerable deepening of the Traun up to 2 metres in the project area.
In the hinterland, too, the mean groundwater level has dropped by 0,8 m since 1980 as a result of the deepening of the river bed, leading to a progressive decoupling of the water body and the water-bound terrestrial habitats. In addition, the fish habitat is severely limited by regulation and hydropower use. It causes significant structural deficits in the riverbed and massively reduces habitat diversity. Accordingly, the current ecological status of fish is only moderate.
The aim of the project is to develop a plan of action coordinated between flood protection, water ecology, nature conservation and other interest groups and to implement the results with pilot measures. The goal of the measures is on the one hand, to stop the deepening of the riverbed as far as possible and to re-establish a gravel bed in the river section. On the other hand, water and land habitats should be better interlinked and lacking habitats such as gravel banks with pioneer and succession areas typical for the floodplain should be initiated.
Outlook and time plan
By the end of 2021, a River Development and Risk Management Concept (GE-RM) coordinated with interest groups and also in consideration of requirements of the Danube will be available for the Lower Traun. Starting in 2022, detailed planning and implementation of a pilot measure will follow.
The measure is expected to be implemented on the left bank of the river Traun. The current focus is on widening and restructuring the riverbed.