Description of the project area
The IRIS project area for the River Development and Risk Management Concept Isel and Drau extends over a total length of 52,8 km from the Isel Gorge near Bobojach in the municipality of Virgen to the mouth of the Isel into the Drau in Lienz and from there along the Drau to the border with Carinthia. Together, the catchment areas of the two rivers cover almost the entire area of East Tyrol.
The Isel is a glacially influenced river that originates from a glacier gate of the Umbalkees at the upper end of the valley. The water flows through a deeply incised Kerbtal and has created several waterfalls over time. The course of the river Isel in Virgental is deeply incised and flows through several gorges (Iselschlucht). At Matrei in East Tyrol, the tributary Tauernbach flows into the Isel. The Isel valley then widens and finally, after almost 60 km, the Isel flows into the Drau in Lienz. At the mouth into the river Drau, the Isel has a catchment area of around 1200 km². The most important tributaries of the Isel are the Tauernbach, the Schwarzach and the Kalserbach, these tributaries almost double the catchment area of the Isel on a river stretch of only 800 m length. The mean water flow of the Isel at the estuary into the Drau is approximately 39 m³/s, the HQ100, a flood event that occurs statically once every 100 years, is around 770 m³/s.
With a total length of 749 km, the Drau is the fourth longest tributary of the Danube. It originates from a spring at Toblacher Feld (South Tyrol), a few kilometres west of the Austrian border. In its further course it runs through a wide valley past Innichen and crosses the border to Austria at Arnbach. Below Heinfels there is a weir at, which diverts part of the water into a reservoir of the Amlach hydroelectric power station. From there, the water is channelled through a tunnel to Amlach and used to generate energy. Below roads, the Drau flows through a narrow valley with several alluvial cones. Only a few kilometres upstream Lienz, near Leisach, the valley opens up.
The Isel flows into the Drau in Lienz. The Drau then flows through the Lienz valley to a further narrow valley section, the Carinthian Gate. The catchment area of the Drava River on the border with Carinthia is around 2080 km². The most important tributaries of the Drau are the Sextener Bach, the Villgratenbach, the Isel and the Debantbach. The mean water flow at the border to Carinthia is around 60 m³/s and the HQ100 is around 968 m³/s.
Hot Spots & Highlights
In 2015, the province of Tyrol reported the area “Osttiroler Gletscherflüsse Isel, Schwarzach und Kalserbach” as Natura 2000 area to the European Commission. Natura 2000 sites aim to protect endangered native wild plant and animal species and their natural habitats. In particular, the tamarisk populations on the gravel banks of the three Alpine rivers make the area particularly worth of protection. The entire IRIS-project area on the Isel is located in this Natura 2000 area.
Along the river Isel between Lienz and Matrei in East Tyrol and the river Drau between Lienz and the Carinthian border, there are popular cycle paths along the riverbanks. The section of the Isel between Huben and Lienz is considered by to be a water sports enthusiast’s paradise for rafting and kayaking.
Need for action
With regard to flood protection, the district’s capital Lienz represents the hotspot in the project area. Here according to the currently valid Isel hazard zone plan an HQ100 flood leads to flooding of the urban area. Apart from that, only a few objects along the Isel are at risk of flooding, which is mainly because there are already eight widened river stretches in the Isel valley. These widening stretches were originally designed as basins for the retention of bedload. In order to ensure the functioning of these basins, regular maintenance work is necessary. These maintenance actions must now be reconciled with the designation of this section of the Isel as a Natura 2000 site.
In the Drau project area there are long stretches where the existing flood protection dams, which were built after the floods of 1965 and 1966, will be flooded during an HQ100 flood event. As these dams are not designed to be overflow-proof, bursting of the dams cannot be ruled out in the event of flooding. This section also requires measures for improving ecology, since the ecological objectives cannot be reached due to morphological deficits.
In the course of the River Development and Risk Management Concept – in addition to the integration of existing flood protection measures – possibilities for optimising the retention of bedload in the Isel valley will be developed. In this way, interventions in the river system and necessary maintenance measures can be minimised. An integral part of these considerations is also the elaboration of a Natura 2000 management plan, which will ensure the future maintenance work on the Isel in this conservation area.
For the river section of the Drau a concept will be elaborated, which on the one hand will restore the state of the art of existing flood protection structures and at the same time will ensure the achievement of the ecological objectives in this section of the river.
Outlook and time plan
Work on the River Development and Risk Management Concept for the Isel and Drau rivers started in spring 2019. The focus here is on surveys in the field of nature conservation and fish ecology as well as the investigation of the bedload balance of the Isel and the condition of the existing flood protection structures.
In 2020, the involved technical administrative departments will develop the integrative guiding principles and the concept of measures. It will be discussed and coordinated with the affected communities, infrastructure providers and other stakeholders. The completion of the River Development and Risk Management Concept is planned for the beginning of 2021.