Drava - Kleblach
Drava - Kleblach
Key features of the case study
As part of the LIFE Nature project “Restoration of the wetland and riparian area at the Upper Drau River” several measures were implemented at the site of Kleblach. Bank stabilization structures were removed and the river bed was widened. Lateral erosion increased the sediment input and initialized the development of gravel / sand bars and islands. Furthermore the river bed incision decreased. One of the former side arms was reconnected to the river for annual flooding, and a second side arm was widened to a width of 30 m, creating diverse instream structures and increasing aquatic habitat diversity.
Site description
The Austrian river Drava (in German “Drau”) is part of the Danube catchment. The source of the river is in the Italian municipality Toblach where it continues its flow 264 km along the border between the Central Alps and the Southern Alps and across the Austrian federal states Tyrol and Carinthia. In Osijek (Croatia), the river joins the Danube River after 707 km in total. The site of the restoration project is located near the village of Klebach in Carinthia. At this point the Drava has a discharge basin of roughly 2500 km². The average annual flow of the river near the site is 63m³/s. The river at the site was historically braided with gravel banks in the channel. Until approximately 140 years ago, the Upper Drava was a free flowing, meandering mountain river with numerous braiding stretches due to alluvial cones of the tributaries. In this dynamic river system with its annual floods and high bed load transport, the river course frequently changed. A braiding river - floodplain system with large gravel banks, grey alder, willow wetlands, and wetland meadows characterized the valley.
Drivers and Pressures
The Upper Drava is the last free-flowing, not dammed stretch of the river Drava. Nevertheless, human activity has changed the river and its floodplains. The first substantial human changes began with the building of the railroad line through the Upper Drava valley in 1868. In the following years, river-engineering channelized the river in order to reduce flood risk, as well as to expand intensive agricultural land use and settlements. When the river was forced into a single main channel, the river dynamics were restricted and the number of side arms, gravel banks, wetland water bodies, and other natural habitats decreased. Due to the regulation processes and reduced sediment supply by the tributaries caused by torrent control structures, the river bed deepened. This river bed incision resulted in a sinking groundwater level, furthermore agricultural lands and the remaining wetlands went dry. The stability of bank protection structures also suffered from the erosion.
Measures selection
At the site of Kleblach on a length of 1,3 km bank stabilization structures were removed and the river bed was widened up to 45 m in several sections. A second side arm was created with a length of 500 m and a width of 30 m. These measures aimed the stabilisation of the river bed and the groundwater level, the creation of gravel banks and the increase of instream and bank structures. One of the former side arms was reconnected to the river for annual flooding. This reconnection of floodplain water bodies with the main channel should prevent aggradation processes and provide habitats for juvenile fish and stagnophil fish species. Additionally agricultural land was purchased for the establishment of new floodplain forests.
Success criteria
No information found.
Ecological response
The within the LIFE-Nature project restored river stretches comprise different habitat types according to the EU Habitats Directive (e.g., dynamic gravel banks (3220) and tamarisk and willow pioneer communities (3230, 3240), which will further develop to alluvial forests (91 E0)). These sites provide:
- Improved spawning habitats for amphibians and fish, in particular the Danube salmon (Hucho hucho), Souffia (Leuciscus souffia), European bullhead (Cottus gobio), Ukrainian brook lamprey (Eudontomyzon mariae), and Atlantic stream crayfish (Austropotamobius pallipes);
- Extended habitats for 140 bird species, including 51 red listed species, and resting places for migrating birds crossing the Alps. Typical species that will benefit include the common kingfisher (Alcedo atthis), the common sandpiper (Actitis hypoleucos), the grey wagtail (Motacilla cinerea), the lesser spotted woodpecker (Dendrocopos minor), and the golden oriole (Oriolus oriolus);
- Extended habitats for two plants which were both nearly extinct in Austria: German tamarisk (Myricaria germanica) and the dwarf bulrush (Typha minima);
In the framework of the project, an additional 5 hectares of water bodies in the floodplain area were created. These areas connect habitats for amphibians, such as the Italian crested newt (Triturus carnifex) and yellow-bellied toad (Bombina variegata). They provide important habitats for small fish species, such as the Amur bitterling (Rhodeus sericeus) and crucian carp (Carassius carassius) and improve valuable food supply for the white stork (Ciconia ciconia). The European otter (Lutra lutra) re-settled in the region, also benefitting from the LIFE-Nature restoration measures.
Hydromorphological response
The restoration measures which were implemented on the Upper Drava show on the basis of the monitoring results positive morphological developments. In detail the following effects could be observed:
- Stabilisation (mostly elevation) of the river bed
- Widening of the cross sections and sediment input due to bank erosion
- Development of diverse morphological structures like e.g. gravel banks / islands, pools, riffles and runs
- Higher diversity of the flow velocities and the substrate types
- Interaction between the restored sites and the regulated sections (temporary increase of the river bed erosion in downstream regulated sections possible)
- Long term success of river bed widenings depend on the superior bed load balance in the catchment (decreasing sediment input reduces the effects of the widening)
On the site Kleblach the first years (2002-2004) were dominated by high hydromorphological dynamics. Processes like local instream sedimentation of gravel banks, intense bank erosion in the side arm which lead to sediment input, could be monitored. With a mean sedimentation in the main arm of 0,35 m in 2011 a decrease of the river bed erosion could be registered.
Water quality response
No information found.
Monitoring before and after implementation of the project
Socio-economic aspects
Contact person within the organization
Extra background information
References
Related Measures
- Increase flood frequency and duration in riparian zones or floodplains
- Link flood reduction with ecological restoration
- Widen water courses
- Remove bank fixation
- Remove or modify in-channel hydraulic structures
- Recreate gravel bar and riffles
- Adjust land use to develop riparian vegetation
- Remove bank fixation
- Lower river banks or floodplains to enlarge inundation and flooding
- Reconnect backwaters and wetlands
- Restore wetlands
Related Pressures
- Hydropeaking
- Hydrological regime modification
- Artificial barriers downstream from the site
- Artificial barriers upstream from the site
- Alteration of instream habitat
- Sand and gravel extraction
- Embankments, levees or dikes
- Loss of vertical connectivity
- Impoundment
- Alteration of riparian vegetation
- Channelisation / cross section alteration
