Depending on river and objectives, various measures are implemented in the field of ecological hydraulic engineering. Amongst others, these include:
Large stone armour blocks have the positive effect of stabilising the shoreline, but they also prevent interaction between the river and the surrounding areas, thereby breaking the ecologically valuable transition between water and land. Moreover, the erosive power of the water can no longer wash the sediment away from the shore. This has the negative effect of deepening the riverbed. By removing these stone constructions at various locations along the riverbank, the Danube, March and Thaya have been freed from these tight artificial confines and can now interact once more with the natural riverbank.
Erosion and sedimentation processes have produced natural steep and flat shore areas, which continuously and dynamically change. These new sections of riverbanks are of particular importance to sand martins and kingfishers. In contrast, shallow gravel banks are often used by sandpipers and little ringed plovers as breeding sites. The removal of these stone riverbanks is of course only possible in locations where there is no danger to buildings or people in the region. Renaturalisation of the bank gives the river more space and is known as cross section expansion. As a result of these measures, flood protection is also enhanced.
Water body networking
Water-bodies often become landlocked, cut-off from main-stream tributaries. Progressive silting lead to the progressive slowing of water-flow, eventually resulting in the creation of nothing more than inland fishponds. Since the first tentative attempts to reconnect side arms to the Danube, we have learned one fundamental lesson: it is essential to excavate a sufficiently large funnel shaped inflow. Only when the water has the possibility to flow freely throughout the whole year can a channel that is deep enough to ensure the long-term continuous flow of water be established. These side arms provide valuable habitats for fish species found in free-flowing water. They also represent a key habitat for juvenile fish.
Tributaries are subject to ongoing dynamic changes to the banks and riverbed, which are clearly visible, especially after flooding. Water networks also contribute to flood control, as the sidearm can absorb substantial amounts of additional water. Waterway networks also lead to the emergence of islands with meadows that serve as a habitat for flora and fauna, undisturbed by human intervention.
Optimisation of groynes
The raising of low water levels in the main stream of the Danube is a common objective for both ecology and shipping. One method used to achieve this is the installation of groynes to regulate low water. Groynes are constructed from rows of armoured stone blocks which run perpendicular to the riverbank. The water level is raised during times of low water and is concentrated towards the fairway.
As part of the pilot project at Witzelsdorf, a groyne field was modified for the first time in such a way that a structure originally built for the regulation of shipping also provided environmental benefits. The groynes were lowered in height, greatly reduced in number and were slanted more in the direction of the flow of the Danube. The optimisation of this structure to achieve the desired effects has proven to be extremely difficult, and the project area is therefore continuously monitored and, if necessary, adapted.
Dredging of the riverbed in the area around fords is carried out continuously in order to maintain the waterways of the Danube. The gravel dredged from the navigation channel is utilised to construct gravel banks and islands which protect the shallow waters from the force of waves caused by passing ships. They serve as spawning grounds and a refuge for the Danube's fish fauna and also as habitats for gravel-breeding species of birds.
Gravel structures that are not covered by high water for long periods of time also become home to pioneer vegetation, herbaceous plants and different species of willow. Similar effects are achieved through the creation of "back channels", whereby gravel banks are separated from the riverbank by dredging a deep channel between the two.
Free access to migration routes
Steep gradients in estuaries of feeder rivers represent a barrier for migrating organisms. Many species of fish found in the Danube and March use these feeders as spawning grounds and can no longer reach them. Restructuring and widening of the mouths of such rivers allows these fish and other organisms to once more reach these breeding areas.
Bed load management
In the free-flowing stretches of the Austrian Danube, the current of the river digs continually deeper into the riverbed. Within the past 50 years, this has caused degradation to the riverbed of about one meter on the Danube east of Vienna. This erosion of the riverbed also causes surface and groundwater levels to decline, thereby threatening the delicate ecosystem of the Danube wetlands. The primary reason for this erosion of the riverbed is the disruption of natural gravel transportation along the river by hydropower plants. The channeling of the river into a single riverbed also increases erosional forces.
viadonau is continuously working on measures to reduce this tendency. Gravel dredged during the course of maintenance of the fairway channel is not removed from the river, but transported upstream and then deposited in the river again. Furthermore also bed load traps help gathering gravel for further bed load management. River bank renaturation, re-connection of side arms and the optimization of low water regulation contribute to reducing erosion of the riverbed.
Ecological riverbank stabilisation
The stabilisation of riverbanks doesn't always need to involve the using of armour stones. This is particularly evident on the March and the Thaya where the riverbanks are protected by ecological, near-natural embankments. Tree trunks that have fallen into the river and may cause logjams are removed and placed on a nearby riverbank.