Catalogue of measures for floodplains – LIFE Riverscape Lower Inn

By implementing the measures of the LIFE Riverscape Lower Inn project, VERBUND is making an important contribution to achieving the European objectives of the Water Framework Directive. In addition, the land restoration of river structures creates new habitats and supports the river morphological development of floodplains.

The designed measures in the dynamic sections of the storage areas still connected to the flow regime of the Inn are creating watercourse habitats which in future will be able to take on the function of habitats that are quantitatively and qualitatively limited by silting processes in the side areas. 

Apart from these conservation objectives, the designed measures are also making a big contribution to achieving the objectives of the Water Framework Directive of the European Union and the national watercourse management plans based on them in the detail waterbodies of the Inn (good ecological potential).

The objective of the measures is to restore lost river structures and improve the fish habitat. Measures in the area of the mouths of tributaries of the Inn (Enknach, Stampfbach, Mühlheimer Ache) will also be developed, as these watercourses are also extremely important for fish ecology as spawning grounds for various species that live in the Inn. 

Project area
The measures on the border between Upper Austria and Bavaria encompass the two storage areas of the Inn power plants at Ering-Frauenstein and Egglfing-Obernberg as well as the barrage root of the Inn power plant at Schärding-Neuhaus.  

In the Twentieth Century, the development of the Inn into a continuous barrage chain fundamentally changed the interaction between the river’s floodplain and the original energy gradient. Accordingly, the water levels during different flow events are extremely important for the development of measures in the Ering-Frauenstein and Egglfing-Obernberg storage areas.
A residual gradient and water level fluctuations still exist in the areas around the barrage root, meaning that guideline-compliant structures can also be constructed here; this is why the development of measures for the implementation concept primarily focuses on these areas. 

Historic river morphology
Before the changes, the Lower Inn was marked by anthropogenic interventions of two morphological river types, the breakthrough sections and the furcation sections in the reservoir locations. These furcation sections were subject to a high morphological dynamic, which was characterised by permanent change.

The channel morphology of the project area was characterised by primary and secondary courses with permanent flow, overgrown and unvegetated, stable and unstable islands. The high morphological dynamic was characterised by a continuous, progressive shift in alignment, which led to erosion on the steep slopes and alluvial deposits on the sliding slopes. On the sliding slopes, this formed shallow rising bank gradients, on which various stages of succession from the pioneer sites to soft floodplain formed.

Over the length of the Inn, the channel morphology was characterised by scour-ford sequences; scours arose on the steep banks and fords in the transitional areas between river bends and in the branching areas. 

Due to control measures, damming and the construction of the Inn power plants, aspects of the historic river morphology were lost.


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The progressive silting of areas bounding the Inn storage areas seriously limited the expansion of some watercourse habitats in recent decades. With the planned dredging measures, new habitats can be created which will functionally counteract this loss. 

The measures are focused on the oldest sedimentation areas where
there are hardly any open areas of water and thus on what are structurally the weakest sections of the storage areas at present. Implementation will therefore considerably strengthen internal coherence of the conservation area.

In terms of a mosaic-cycle concept, silting in the still largely timber-free areas will be reduced in stages, in order to develop
habitat complexes that at their core comprise a deeper area of water connected with the Inn
which is surrounded by shallow areas of water of varying depth and shape. As the areas lie in a section of the storage area with even stronger fluctuations in water level, shallow water areas dry out in a
natural cycle. These areas are formed with gradients so that the desired habitat mosaic is retained for a certain period of development. By restoring a permanent connection to the Inn, these systems can be used as habitat again all year round.

For fish fauna, deep-water areas can fulfil the function of winter habitat, as a place of retreat during flood events and as habitat for stagnophilic species. These deep-water areas are additionally a feeding habitat for diving water birds.

Areas of shallow water are covered by waterborne vegetation again as a consequence of the expected formation and are suitable as habitat for larvae, for juvenile fish and for the reproduction of phytophilic fish. 
In addition, feeding habitats are provided especially for limicoles and habitat created for pioneer plants.

Dredging on the right bank downstream of the Mühlheimer Ache 
Foreshore areas exist between the Inn and the flood protection embankment downstream of the mouth of the Mühlheimer Ache. The foreshore areas downstream of the mouth of the Mühlheimer Ache between the Inn and the Mühlheimer dam are covered on the one hand with riparian forests, while several old course systems are located along these areas on the other. These old course systems are subject to a slow, constantly advancing silting process through the depositing of fine sediments and through organic material. Moreover, in some areas, there is no permanent connection with the old course systems and the Inn. Only unusual flooding events briefly create a connection with the Inn.

The goal of the measures planned here is to restore the functionality of the old course systems. Extensive dredging work and the creation of deep connections to the Inn or to the Mühlheimer Ache are necessary for this.  

M25 Dredging of the Aufhausner Lacke at river km 43.4-43.0 L
The Aufhausner Lacke old course system is located in the accompany foreshore and is subject to a slow, constantly advancing silting process. This silting is caused by the deposit of fine sediments (flooding events) and by organic material. The continuity of the connection with the Inn is only seasonally possible due to the silting processes. An extensive connection with the Inn only exists during unusual flooding events of the Inn. 

Dredging is taking place on an area of approx. 1.6 ha, as is the restoration of a deep connection with the Inn. Year-round use of the old course system as habitat is ensured by the creation of the permanent connection between the Inn and the Aufhausner Lacke.

Removal of bank protections (“bank dismantling”)
With regard to dismantling the bank, existing block safeguards will be removed up to about 2 m below the low-water level. This measure will ensure a continuous transition between water and land and enable an inherently dynamic development of the renatured bank in the medium term.

A flat bank gradient in the area of the mid-water line will be created through initial sediment redepositions. The flat bank gradients will additionally be structured with river wood.

The structuring with river wood in the lower and mid-water area is leading to the formation of a dynamic bank zone and interlocking of the shoreline. The wood structures contribute to the formation of sustainable bays which fulfil the function of habitat for juvenile fish in combination with the river wood. 
In the case of the location along the outer curve or the location on the steep slope, the higher hydraulic loads will form steep crumbling banks, which offer habitat for cave breeders (sand marting, kingfisher). Additional river wood can be counted on from the extensive riparian forest areas and the anticipated further and own development. 

Due to the fluctuations in water level that still prevail in the area of the barrage root, various stages of succession from pioneer sites to soft floodplain can form along the rising, flat bank gradients. 
Gravel banks with a flat bank gradient are valuable juvenile fish habitats and also provide habitats for gravel breeders (little ringed plover, sandpiper). During higher flow events, flat gravel banks can be used as spawning grounds for rheophilic species of fish.

The armour stones removed from the bank protection will be reinstalled as local disruptive elements (groynes) in the proximity of the bank. Due to local flow diversion, these groynes promote side erosion and thus contribute to bank restructuring or promote the self-dynamic morphological development of the renatured bank in the medium term.

Gravel fillings can be created using the excavated material arising from the dredging activities. This results in higher flow speeds in the area of the flat gravel bank. 
The Enknach empties into the Inn on the right bank at river km 58.34. The goal of the measures on the Enknach is to restore continuity between the Enknach and the Inn and to improve the watercourse habitat in the mouth section of the Enknach. 
To this end, downstream of an existing pipe culvert through the Braunau flood dam, the river bed or the water level of the Enknach should be raised by gravel fills to create a steeper gradient. This will create a more pronounced character of a flowing watercourse in the mouth section of the Enknach and significantly reduce the effects of congestion caused by the Inn on this section of the watercourse. 
In addition, two river bed sills should be installed immediately downstream of the pipe culvert to prevent erosion of the gravel bottom and thus retain continuity through the pipe culvert over the long term.

The Stampfbach empties into the Inn on the right bank, downstream of the Inn power plant at Ering-Frauenstein, at river km 47.8. The goal of the measure is to improve the watercourse ecological conditions of the Stampfbach and to restore and improve year-round continuity of the newly created habitat as a reproduction site for rheophilic species of fish in the Inn. 
At the same time, the approx. 150 m-long, rectilinear, monotonous mouth section (pipe culvert at the mouth into the Inn) will be structurally upgraded by relocating and lengthening the course. The new course of the channel should have oscillating, winding lines. This is intended to ensure retention of the asymmetrical cross-section as well as the scour-ford sequences with deep points in the outer curve and shallow banks in the inner curve.
The timber that accumulates due to the required gradients will be built into the watercourse with rootstock as river timber structures. The connection to the Inn will be designed to be passable all year round.

The Mühlheimer Ache empties into the Inn on the right bank at river km 44.42. Over a length of around 650 m, a structuring and curving of the mouth section of the Mühlheimer Ache is envisaged, in order to make the currently rectilinear, heavily regulated and structurally weak river course much more attractive.
In the lower section of the mouth, a realignment of the course will be initiated by dismantling the bank protection and removing sedimentary deposits. The accompanying woody plants, if they have to be removed for the dismantling measures, will be built in as river timber structures or set back as sprouting rootstocks with stems. The removed armour stones can be reinstalled as groyne structures to promote side erosion.
The upper section of the mouth will be accompanied by a check dam on the right and left banks. Here, the removed bank protection will be reinstalled as a covered protection at the base of the slope so as not to jeopardise the integrity of the embankment. Through initial sediment redeposition and the installation of groynes and deadwood structures, oscillating lines with scour-ford sequences can form within the expanded profile.