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Restoring the Danube’s natural river dynamics
Background
Large sections of the Danube and its major tributaries have been hemmed in by artificial riverbank stabilisation, cutting off side channels, depriving floodplain forests of floodwater and leading to a constant deepening of the main river bed, which in turn causes water tables under the floodplain to fall.
Removal of bank stabilisations gives the river the possibility to restore its normal dynamics of flooding, erosion and sedimentation and to gradually repair the damage to its habitats and the floodplain. Two large-scale LIFE projects (Donau-Auen and Donau-Ufer) along the Danube downstream from Vienna, paved the way.
Based on a concept to reconnect the various stagnant bodies of water in the floodplain with the main river, the LIFE-Nature project Donau-Auen worked out detailed technical plans and carried out the first such reconnections at Orth and Untere Lobau. These entailed lowering the river dyke at several points and changing weirs across the mouths of side channels so that more water will flow into these side channels and the floodplain forest and stagnant waters.
Clearly, this revitalised the floodplain ecosystem. Wetland habitats and their denizens, such as the fish Umbra krameri which was considered extinct in Austria until its rediscovery in 1992, profited. Paradoxically, it also served the very practical purpose of protecting Vienna against floods. Instead of following conventional wisdom and strengthening the dykes, the river was given more room to sprawl. The dykes closest to the river were lowered so that the old floodplain itself becomes a retention basin, and floodwaters back up to dykes further inland.
A comprehensive concept for river management
The LIFE project had an important demonstration and learning effect. Before LIFE, the beneficiary had tested the method on small sections along the river, but the project gave the opportunity to realise large-scale revitalisation measures. They proved successful and now, after the project, all water engineering measures along the Austrian Danube must not only be checked for their nature impact, but must also be in line with the ambitious “Gesamtkonzept” (Comprehensive Concept) for the river Danube eastwards of Vienna.
According to this Concept, within the next decades half of the artificial riverbanks will be dismantled, the floodplain hydrology restored, and the shipping channel on the Danube simultaneously maintained. LIFE enabled the beneficiary to test new hydrological restoration actions, which were directly included in the “Gesamtkonzept” and therefore became the standard for river restoration measures along the Austrian Danube.
In the Donau-Ufer project, the “Gesamtkonzept” is being implemented further. All artificial elements strengthening the bank are being removed from a 2.8 km pilot section of the Danube opposite the town of Hainburg, so that erosion and accretion processes will again lead to the formation of natural riverbank structures. Some 24 000 m³ of stone blocks will need to be removed!
Within a few years, once the work is completed, the river is expected to erode the steep bank to form a gently shelving shallow shore, with occasional cliffs where the bank has collapsed. These and other elements of the floodplain landscape which have become rare will once again be formed directly along the main course of the river Danube.
Complementary projects along the Danube and its tributaries
A LIFE-Nature project upstream of Vienna, the Wachau project, includes a pilot action to recreate gravel banks and islets in the Danube, using 400,000 m³ gravel dredged from the shipping channels annually in that river section. These gravel structures will serve as spawning ground for the fish fauna and as resting and breeding ground for aquatic birds. If the pilot action is successful, this recycling of dredged gravel for habitat restoration will become river authority policy.
Again, the tributaries are not ignored. The LIFE project March-Thaya elaborated a comprehensive regeneration concept for the March river and began its implementation, providing a kick-start for the reinstatement of dynamic river processes (erosion, sedimentation, meandering…) over a section of 10 km along the Slovak border. It created nature-like riverbank structures (providing new habitats for many typical plant and animal species) and improved the connectivity between cut-off meanders and the main river, providing standing water refuges for fish during flood events.
Along the Lafnitz, the LIFE-Nature project there will revitalise 6 oxbow lakes, and three others will be reconnected with the main watercourse. This will bring currently isolated populations together again and create new habitat as spawning grounds for fish, nurseries for amphibians and foraging habitats for birds. Besides the river itself, the Lafnitz project will allow the floodplain area to develop its own characteristic mosaic of flowing and standing waters, muddy banks, pioneer vegetation and floodplain forests.
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