Revision as of 18:27, 20 May 2014

Lower Traun

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Key features of the case study

The study site of the 6th order alpine stream has a total length of 5,5 km. Hydrological classification corresponds with a moderately nival discharge regime with a distinct annual character. The nowadays heavily modified river within its remains of riparian forests flows through a monotone canal-like channel with a corresponding degraded aquatic community. The start of sustainable river regulation measures was set in 1885. Dams were raised in order to prevent the self steering widening of the water caused by flood events. Several bayous were cut off from the main river bed. The downward gradient was increased by straightening the Traun river over the whole reach of the study site. The transport capacity of the new river bed was optimized hydraulically. The river width was reduced dramatically. The river bed was lowered around several meters over the period of 100 years cause of the increased transport capacity. The whole river restoration project is planned to be fulfilled as gradual development. The first section with a total length of 2,3 km which is presented as case study has already been completed in 2011. Projects for the remaining sections have not yet been concluded but they are planned to be implememted over the next years.

Site description

The Traun is a right tributary of the Danube, which it meets near the City of Linz in Upper Austria. With a total length of 73 km and a total catchment area of approx. 2770 km², the Traun river is one of the largest rivers in Austria.

Measures selection

Success criteria

Ecological response

Hydromorphological response

Monitoring before and after implementation of the project

Socio-economic aspects

Contact person within the organization

Extra background information

As natural running waters are subject to self-steering dynamic processes in the lapse of time several site descriptions and historical maps of the Traun stream course and dthe river bed morphology were analysed. The stream course of the Traun river out of 17th century local maps, maps from 1763 – 1787 (Josephini land survey), 1805 – 1869 (Franciscan survey map), from 1869 – 1887 (Franciscan - Josephini land survey) and from the river regulation project from 1885 has been digitized and combined with the present state of the actual aerial view.

References

Related Measures

• Reduce erosion
• Trap sediments
• Reduce anthropogenic flow peaks
• Shorten the length of impounded reaches
• Increase flood frequency and duration in riparian zones or floodplains
• Link flood reduction with ecological restoration
• Manage aquatic vegetation
• Widen water courses
• Allow/increase lateral channel migration or river mobility
• Shallow water courses
• Modify aquatic vegetation maintenance
• Initiate natural channel dynamics to promote natural regeneration
• Remove bank fixation
• Recreate gravel bar and riffles
• Develop riparian forest
• Adjust land use to develop riparian vegetation
• Remove bank fixation
• Improve backwaters
• Lower river banks or floodplains to enlarge inundation and flooding
• Reconnect backwaters and wetlands
• Remove hard engineering structures that impede lateral connectivity
• Construct semi-natural/articificial wetlands or aquatic habitats
• Set back embankments, levees or dikes
• Restore wetlands
• Retain floodwater

Related Pressures

• Groundwater abstractions
• Artificial barriers downstream from the site
• Alteration of instream habitat
• Sand and gravel extraction
• Embankments, levees or dikes
• Impoundment
• Alteration of riparian vegetation
• Channelisation / cross section alteration