GE-RM
LEITHA
Leitha: Finding new perspectives!
The Leitha originates from the confluence of the rivers Schwarza and Pitten in Lower Austria, runs through the Vienna Basin and the Pannonian Plain, and leaves Austria for Hungary at Nickelsdorf in Burgenland.
The GE-RM Leitha considers the entire project area over a length of about 102 km. In former times, the Leitha formed meanders in many areas, but these have almost completely disappeared due to river regulation and straightening. In addition, the share of stretches with residual flow is very high at almost 50% of the total river length.
Due to water diversions, low residual flows, but also due to natural infiltration into the groundwater, the Leitha occasionally falls dry in certain sections, which causes ecological problems. At the same time, the flooded areas in case of a flood event are large due to the flat landscape.
GE-RM
LEITHA
Leitha: Finding new perspectives!
The Leitha originates from the confluence of the rivers Schwarza and Pitten in Lower Austria, runs through the Vienna Basin and the Pannonian Plain, and leaves Austria for Hungary at Nickelsdorf in Burgenland.
The GE-RM Leitha considers the entire project area over a length of about 102 km. In former times, the Leitha formed meanders in many areas, but these have almost completely disappeared due to river regulation and straightening. In addition, the share of stretches with residual flow is very high at almost 50% of the total river length.
Due to water diversions, low residual flows, but also due to natural infiltration into the groundwater, the Leitha occasionally falls dry in certain sections, which causes ecological problems. At the same time, the flooded areas in case of a flood event are large due to the flat landscape.
Overview of flood risk and ecological status
The inventories resulting from the preliminary study are used to close knowledge gaps that exist with regard to flood risk and river ecology. The aim is to identify sectoral deficits and comparing them with the existing objectives in order to draw up a concept of measures based on them. The diagram provides a summary of the deficits of the river Leitha.
Deficits and opportunities
Deficits Leitha
Flood risks
Due to the flat and partly extensive foreland, the flood areas of the Leitha are very large. Although sufficiently dimensioned flood protection measures have already been implemented for a large number of local and urban areas along the Leitha and these measures generally provide protection against flood events up to the so-called 100-year event, some settlement areas are nevertheless severely affected by floods. These are found in particular in the municipalities of Ebenfurth, Eggendorf, Katzelsdorf and Lichtenwörth.
It should also be noted that some of the flood protection structures along the Leitha are already old. This leads to an increased risk in the event of an overload and large areas with residual risk. The number of people potentially affected by a 100-year flood event is around 4.500 in the study area.
Flooding events of the river Leitha have concerned the population for a long time. The photo shows the flood in Leithaprodersdorf in 1965, which led to flooding in large parts of the municipality. Even though flood protection measures have been implemented in Leithaprodersdorf in the meantime, this does not apply to all communities along the Leitha.
The picture shows a straightened section of the Leitha, which is characterised by the absence of important habitats.
Fish habitats
From a fish ecological point of view, the deficits to be mentioned are the lack of indicator fish species and accompanying species as well as a disturbed population structure of the detected species. These deficits show that the Leitha cannot fulfil its function as a habitat for fish reproduction and as “nursery” for many fish species over long stretches. Especially the straightening and the effects caused by river regulation measures, which result in high flow velocities and uniform water depths, cause problems for the ecology.
Important habitats (areas with reduced flow velocity for juvenile fish, gravel banks as spawning sites, etc.) for various fish species of different age stages are missing. In order to improve the fish ecological status of the Leitha, morphological improvement of river stretches is necessary in the first place. The creation of a sinuous river course and widenings can promote lateral erosion and increase the overall river dynamics.
In river stretches where it is not possible to change the river course, fish habitats can be created through so-called instream measures (construction of groynes and scour holes, shelters).
The administrative map, which drawn for Lower Austria between 1867 and 1882, shows the Leitha in a still more natural course in many areas with many meander bends. At the same time, the map already contains the planned water course, which was implemented within the framework of the regulation works.
River continuity
Interruptions of the river continuity (weirs, transverse structures that cannot be passed by fish, residual water stretches that dry out) restrict the migration of fish. In many sections, migration is totally prevented. This deficit is another important point. Therefore, the removal of migration obstacles that cannot be passed by fish, such as the riverbed sill downstream of Gattendorf, is a focus in the concept of measures of the GE-RM Leitha. The restoration of passability is a prerequisite for morphological habitat improvements to be effective. In addition to an increase of the residual flow, local measures for the reduction of infiltration can be implemented in the residual water stretches of the Leitha by creating fish rescue pools. These rescue pools are modelled on natural deep spots (so-called scour holes, see picture).
Opportunities
Riparian forests of the Leitha
Despite the major changes in the river landscape of the Leitha, its riparian forests – which also represent one of the most important protected assets in the Protected Area Leithaauen – are among the most important riparian landscapes in eastern Austria. Especially in the lower reaches of the Leitha, the surrounding land is strongly influenced by the high groundwater. Thus the Black alder, which copes well with waterlogging, forms an extensive floodplain forest (groundwater forest) along the Leitha. Black alder floodplain forests are therefore considered to be an outstandingly representative and distinctive ecological unit on the Leitha.
Unique species
Very rare species can be found not only directly on the Leitha, but also in its catchment area. One example is a butterfly species called the False ringlet. The species is one of the most endangered day butterflies in Europe. In Lower Austria, the butterfly only occurs in a single population on a wet meadow – not far from the Leitha River. Targeted management at catchment level can also support the habitat requirements of such species.
Another special feature is the diversity of different species of dragonflies, which find optimal habitat conditions on the Leitha and the adjacent water-influenced areas. An example of a particularly endangered species is the Green snaketail, which has its habitat in the area of one of the Leitha pilot measures, which is of course taken into account in the implementation of the measure.
Concept of measures for the Leitha
Overview of measures on the Leitha
Measures for reduction of flood risk
Measures to reduce and avoid flood risks can be very different. The most effective measures are combinations of structural and non-structural measures. In the GE-RM Leitha numerous proposals for measures to protect the population against floods were defined. These range from the preservation of existing retention areas to the restoration of existing facilities and the protection of objects. In the planning process, these measures were coordinated with ecological objectives in order to use synergies.
Measures to improve continuity
Fish migrate naturally. This enables them to use different habitats. For example, different habitat characteristics are important for reproduction than for growth or nutrition. Therefore, fish migrate within river systems to find the optimal conditions for their needs. Today, numerous artificial barriers impair fish migration. In order to allow fish and organisms a migrate again, numerous measures to improve river continuity were defined on the Leitha.
Measures to improve river morphology
To gain land for settlements and agriculture, many of our rivers have been straightened and regulated. As a result, the typical morphology of the river is restricted and therefore important habitats in the water and on land are usually missing. For example, gravel banks and islands are crucial for so-called gravel spawners (i.e. fish that depend on gravel for spawning) or for birds that breed in these places. A wide range of the proposed measures focus on improving the morphological conditions of the water bodies. These measures also partly support the objectives of flood risk management.
Measures to improve the sediment balance
A well-functioning sediment balance is of decisive importance for both, the flood risk management and the river ecology. If, for example, accumulations of bedload occur at narrow points in settlement areas, floods can occur. However, with interventions like for example dredging, the sensitive ecosystem and aquatic life will be disturbed. For this reason, measures to establish a balanced sediment budget were defined in the GE-RM Leitha.
Example of a concept of measures:
Lichtenwörth to Ebenfurth
The figure provides an insight into the concept of measures for the river Leitha in the area from Lichtenwörth to Ebenfurth.
In this area, there is still a flood risk; the areas that are not sufficiently protected against flooding are highlighted in the map.
To benefit from their retention effect, those floodplains that lie outside the settlement areas are to be preserved as much as possible.
Regarding river ecology, a bundle of measures is planned in this area including structural improvement, the restoration of the Leitha to its original river type, and the creation of the pools to save fish when the Leitha runs dry, which are described above. Another focus is on giving the Leitha more space again. For this purpose, a development corridor has been designated considering the existing general conditions, which indicates the space required to restore the original river type. Specific measures, such as the reconstruction or removal of obstacles to migration and the increase of the discharge, complete the bundle of measures in this area.
Fotonachweise:
Hochwasser-Bild: Freiwillige Feuerwehr Leithaprodersdorf
Fischrettungs-Pool, Leitha bei Katzelsdorf: Land Niederösterreich
Fotonachweise:
Hochwasser-Bild: Freiwillige Feuerwehr Leithaprodersdorf
Fischrettungs-Pool, Leitha bei Katzelsdorf: Land Niederösterreich