GE-RM
ISEL – DRAU
River development and risk management for a liveable river basin
The project area studied in the GE-RM comprises the river Isel between its confluence with the Drava and the Isel Gorge near Bobojach, as well as the Drava between the border with Carinthia and the mouth of the Isel.
The Isel was nominated as a Natura 2000 site in 2015. As there is no management plan for the area in place yet, the vegetation and the animal groups amphibians and birds were surveyed during the GE-RM inventories in such a quality that they can later also serve as a basis for a future Natura 2000 management plan.
GE-RM
ISEL – DRAU
River development and risk management for a liveable river basin
The project area studied in the GE-RM comprises the river Isel between its confluence with the Drava and the Isel Gorge near Bobojach, as well as the Drava between the border with Carinthia and the mouth of the Isel.
The Isel was nominated as a Natura 2000 site in 2015. As there is no management plan for the area in place yet, the vegetation and the animal groups amphibians and birds were surveyed during the GE-RM inventories in such a quality that they can later also serve as a basis for a future Natura 2000 management plan.
Overview 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 graphic provides a summary of the deficits on the Isel and Drava:
Deficits and opportunities
Deficits Drava
Flood risk
A 30- or 100-year flood event could affect 50 properties with higher-value uses in the section of the Drau between the mouth of the Isel and the border with Carinthia. All municipalities are afflicted. The section from the mouth of the Isel to the mouth of the Debantbach is designated as an “area of potentially significant flood risk” in the National Flood Risk Management Plan.
Condition of the flood protection structures
Some of the protective structures, in particular dam structures, are in a poor to very poor state of preservation.
River development
The ecological status according to the National River Basin Management Plan 2021 is poor.
Reasons:
- Large (hydro-)morphological deficits in the entire study area due to regulation and bank protection measures.
- Two tributaries with non-passable mouths.
- Monitoring in 2019 revealed a poor condition for biological quality element fish
Illustration: The map of 1820 shows the Drau as a wide, branched river with several side arms and large gravel banks that occupy almost the entire valley floor. The Drau was regulated in the course of railway construction in 1870.
Opportunities for birdlife
Data research and field surveys carried out on birdlife show that more than 90 bird species occur on the rivers Isel and Drava. The “indicator species” for mountain rivers, the Sandpiper (see figure) and the Little Ringed Plover are of particular importance. In 2019, 32 (!) territories of the Sandpiper and 1 territory of the Little Ringed Plover were identified, with an upward trend. This positive development should be supported by the creation of further habitats.
Deficits Isel
Flood risk
Along the lower section of the Isel the urban area of Lienz, the Lienzer Pfister, 2 individual properties in Oberlienz and the settlement area in St. Johann (Weirer) are affected in the event of a 30 to 100-year flood.
The section from the mouth of the Isel to the Schlossbrücke is designated as an ” area of potentially significant flood risk “. On the middle part of the Isel, the settlement area on the right bank in St. Johann, the commercial area and sports centre in Matrei and the settlement and commercial area in Kaltenhaus and Proßegg are affected. In the upper part of the Isel, the settlement area of Auerfeld, the Matrei swimming pool and a single property in Ganz (all in Matrei i.O.) are at risk in the event of a flood.
Condition of the flood protection structures
In Lienz, Ainet and St. Johann, in Matrei i.O. as well as at the mouth of the Schwarzach and Virgen/Gries, dams and longitudinal structures are in a moderate or poor state of preservation.
River development
The assessment is divided into the sections lower, middle and upper Isel.
The ecological status on the lower Isel is classified as “moderate to poor”, on the middle Isel as “moderate”. There are morphological deficits in the entire section. Numerous transverse structures and tributaries are not passable and interrupt the continuity for aquatic organisms. Monitoring in 2019 revealed a moderate or poor status for fish on the lower Isel. On the Middle Isel, poor or moderate status was classified in St. Johann and Matrei i.O.
On the upper Isel, the ecological status is classified as “moderate”. Exception: the Isel Gorge, which is in good status. Major morphological deficits exist in Matrei and locally in Virgen. One transverse structure and two tributaries are not passable. Fish monitoring in 2019 revealed a moderate status in Matrei and a good status in the Tauernbach.
Opportunities for amphibians
During the amphibian surveys conducted in 2019, 5 of the 8 amphibian species native to East Tyrol were identified:
Common toad, Grass frog, Fire salamander, Yellow-bellied toad and Alpine newt. Occurrences of 3 further species (Alpine salamander, Great crested newt, Pond newt) are possible.
Hotspots are the “Glanzer Auen” on the right bank of the Isel near Oberlienz and the Brühl in Matrei i.O., populations of Grass frog and Alpine newt with particularly high numbers of individuals have been found in the Virgen Valley.
Measures
Overview of measures Drava – Isel
39 measures for reduction of flood risk
Measures to reduce and prevent flood risks can vary greatly. The most effective measures are combinations of structural and non-structural measures. In the GE-RM Drau-Isel, a total of 39 measures were defined to protect the population from floods. These range from the preservation of existing retention areas to the restoration of existing structures and the protection of individual objects. In the planning process, these measures were coordinated with ecological objectives in order to use synergies.
11 Measures to improve river 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 to migrate again, 11 measures to improve continuity are proposed.
53 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 river’s typical morphology is restricted and therefore important habitats in the water and on land are missing. For example, gravel banks and islands are crucial for so-called substrate spawners (i.e. fish that depend on substrate for spawning) or birds that breed in these places. 53 of the defined measures focus on improving the morphology of water bodies, whereby these measures also partly support the objectives of flood risk management.
5 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, 5 measures to establish a balanced sediment budget were defined in the GE-RM.
Example of a concept of measures: Matrei in East Tyrol
The map section shows the concept of measures for the water body area around Matrei in East Tyrol. In order to reduce the flood risk and ecological deficits, the concept provides for a wide variety of measures, some of which complement each other.
Additional flood protection measures are planned mainly in the form of detached dams and object protection measures, the use of retention areas and the rehabilitation of existing facilities. From an ecological point of view, the focus is on restructuring, improving the continuity of the watercourse and its tributaries, and restoring the watercourse to its original morphological condition.
Details on the measures can be found in the GE-RM summary report (in German):
Fotonachweise:
Headerbild: BBA Lienz
Hochwasser Drau: EZB
Flussuferläufer: Ragger
Hochwasser Isel: REVITAL Senfter
Amphibienwelt: H. Angerer
Maßnahmen: BBA Lienz
