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Klebach - Side channel

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Key features of the case study

In the Drava river, near the village of Klebach, a side channel was dug in previously agricultural land. This side channel allows for more natural processes to take place. The aggregation of the side channel has taken place as was hoped, however due to high floods the aggregation is less then originally expected. The fish and macrophytes at the site benefit from the measure. However, the WFD status for the fish is still labelled as poor, despite the efforts. This can be explained by the scale of the measure compared to the scale of pressures on the system. Downstream of the site there are a number of hydro-power plants in the river, to name one such pressure that this site-specific measure has not addressed.

Site description

The Drava river (also known as Drau river in German) is a 6^th order stream. At the project site, in the Upper Drava Valley, it forms the boundary between the Crystalline of the Central- and the Limestone Alps. The site of the project is located near the village of Klebach in Austria. From this point onward the Drava has a discharge basin of roughly 2500 km². The average flow of the river near the site is 76m³/s. The river at the site was historically braided with gravel banks in the channel. The Drava river has been largely regulated from the 1930 onward in the area. Banks were fixated and meanders straightened out. Lateral connection with the floodplains and side arms was removed and these areas found new use as agricultural or forestry lands. In-channel gravel banks were removed artificially or through the changes in hydrology caused by the other alterations to the river. Downstream of the side a number of hydro-power plants were constructed, these significantly disrupted longitudinal connectivity of the river. At the same time these plants do produce 'green' energy for large parts of the surrounding areas.

Measures selection

The Waterauthority of the province of Carinthia in Austria, in an effort to restore some of the natural regime and ecology of the Drava, dug a side channel near the village of Klebach. This side channel was situated on the right hand side of the river in what was formerly agricultural land. On the left hand side groins were erected in hopes of keeping the entrance to the side channel open. Also, these groins were meant to initiate morphological variability in the river. The side channel itself was more then 300m in length and was originally build in two parts. This means that water was also able to enter/exit the channel about half way along it, creating what is best described as an artificial attempt at a braided river. The island that was created by the measure was the remainder of the old river bank with its old vegetation still intact.

Success criteria

The aims of the project were to:

• stop degradation
• initiate a changing river morphology
• diversify the spatial variability of flow velocity and sediment grain sizes
• restore an ecologically sound environment (in the form of habitat structures)

Ecological response

Macrophytes and phytobenthos

Not monitored

Benthic invertebrates

The benthic invertebrates were measured in the main and in the side channel. Due to lower shear stresses in the side channel the invertebrates had both higher population densities and species diversity than in the main channel. Part of this was also attributed to algal growth in the side channel which served as a habitat for the benthic invertebrates.

Fish

Fish species were shown to have significantly higher population densities in the side channel. The side channel was shown to serve as a hatching ground for many fish species. Also, two rheophilic fish species were found at the site, indicating it as an important spawning ground for rheophilic species. Despite these positive effects long term analysis has shown that the fish ecological status is still labelled as Poor by WFD standards. None the less, it has improved compared to the parts of the river where no restoration has taken place. One of the reasons for the Poor status of the area may be related to the scale of the measure compared to the pressures influencing the river. The downstream hydropeaking and longitudinal disruption of the connectivity remain unaltered by the local measure of the side channel.

Other biota

Vegetation

In one year the plant species diversity at the site increased from 48 species to 100 species. Another year later the species diversity had increased to 118 species in total. A form of stabilisation was taking place, likely caused by the gravel bars and islands being just above the average water table. This leads to disturbance during high water and floods, which in turn sets back the succession and thus keeps the system in a fairly pioneer-like succession stage. None the less, the measure has increased the plant diversity in the area.

Birds

A total of 40 different bird species were observed in the area; 8 of these are red-list species in Austria. The birds primarily use the gravel bars and the vegetation structures on it for resting and eating. This type of habitat is largely absent in the surrounding area, making it attractive for birds that pass through the area.

Hydromorphological response

Hydrological regime

The hydrology of the site has not been monitored specifically. However, the digging of a side channel has the obvious effect of taking part of the flow from the main channel and directing it into the side channel. Also, small changes in flow speed at the project site can be expected.

River continuity

Unaffected

Morphological conditions

Due to the lack of bank fixation morphological processes have been partially restored. During normal flow aggradation of the side channel and banks occurred. However, during high flow the groins on the left side of the banks weren't capable of forcing the line of major flow to the side channel. This in turn has led to a degradation of the channel which leads to a preservation of aggradation of roughly 50%. This means that there is a net sedimentation in the channel, as was the aim of the project. The grain size in the main channel has been unaffected. At local scale the grain size in both the side channel and between the groins has shown variability.

Monitoring before and after implementation of the project

Pre-project monitoring has been absent in the area. Instead, space-for-time substitutes were used to discover the effect of the measure. Either the main, or nearby unrestored reaches of the channel were used as pre-restoration reference conditions. Most of the short term investigations on the site were carried out by the Institute for Water Management, Hydrology and Hydraulic Engineering seated at the University for Natural Resources of Vienna. From 1999 to 2003 data on fish was collected at the site for a LIFE+ EU project.

Socio-economic aspects

Ecosystem goods and services

Not affected.

Conflicts and synergies

No conflicts or synergies are known.

Project costs

Unknown

Contact person within the organization

Wasserwirtschaft Kärnten e-mail, Tel.: 050 536-31802 _(Austria)

Extra background information

References

• Peer-reviewed paper: Muhar, S., M. Jungwirth, G. Unfer, C. Wiesner, M. Poppe, S. Schmutz, S. Hohensinner and H. Habersack, 2008. Restoring riverine landscapes at the Drau River: successes and deficits in the context of ecological integrity. In: Habersack H., H. Piègay and M. Rinaldi (Eds.), Gravel-bed Rivers VI – From Process Understanding to River Restoration, Developments in Earth Surface Processes, Elsevier, 779–803. _{(language: english)}
• Peer-reviewed paper: Habersack H. & Nachtnebel H.P. (1995) Short-term effects of local river restoration on morphology, flow field, substrate and biota. Regulated Rivers: Research and Management, 10, 291-301. _{(type: abstract; language: english)}

Related Measures

• Increase flood frequency and duration in riparian zones or floodplains
• Shorten the length of impounded reaches
• Remove bank fixation
• Recreate gravel bar and riffles
• Remove or modify in-channel hydraulic structures
• Remove bank fixation
• Reconnect backwaters and wetlands
• Construct semi-natural/articificial wetlands or aquatic habitats

Related Pressures

• Hydrological regime modification
• Hydropeaking
• Artificial barriers downstream from the site
• Impoundment
• Channelisation / cross section alteration
• Alteration of riparian vegetation
• Alteration of instream habitat
• Embankments, levees or dikes
• Sand and gravel extraction
• Loss of vertical connectivity