PROJECT DESCRIPTION
BACKGROUND
The “Alte Donau”, the former riverbed of the Danube in Vienna, Austria, extends to 160 ha and is one of Europe’s largest urban lakes. It is fully embedded in the city landscape and used intensively for recreation and anthropogenic purposes. These activities exert increasing pressure on the environmental quality of the water body, especially as the city’s population continues to grow at a rapid rate. The number of swimmers using the lake has increased by 97% since 1988, and the city’s planners forecasts that some 8000 new residents will move into the lakeside area over the next few years.
Due to this increased stress (and also including uncertainties associated with climate change), there is a need to introduce new urban management measures that will help to safeguard the quality of this water body for the long-term.
OBJECTIVES
The LIFE-URBANLAKE project intended to define strategies to reduce the vulnerability of the “Alte Donau” to the effects of climate change and anthropogenic pressures. The strategies would focus on maintaining the lake’s current good environmental status and bathing water quality. Integrated lake management and risk management systems would be deployed as part of the strategies and these would comprise innovative methods for holistic governance of the water resource, as well as its ecologic and socio-economic environment.
Support to facilitate the putting in place of highly competent staff, and rapid, cost-effective counter-measures for tackling potential disruptions were envisaged. A key goal of the project was also to improve stakeholder support for and participation in the lake’s management.
Specific objectives included:
1. Implementation and demonstration of integrated lake management in an intensively used urban environment, in the context of the city’s governance practices.
2. Reduction of the vulnerability of the “Alte Donau” to climate change impacts and other anthropogenic pressures.
3. Maintaining and ensuring good ecological status (in relation to the Water Framework Directive) and good quality bathing water (Bathing Water Directive) through the implementation of innovative technologies and methods, adapted to the special conditions of the “Alte Donau”s urban environment.
4. Achieving and maintaining a stable aquatic environment in an intensively-used urban environment through sustainable management procedures. 5. Maintaining and improving the socio-economical benefits for the city population and other stakeholders.
RESULTS
The LIFE UrbanLake project helped increase resilience of the Old Danube, a former part of the river Danube that is now a lake, ensuring that its good ecological status could be maintained even amid exceptionally adverse weather conditions. The team carried out actions to re-naturalise the riverbanks and their surroundings, improving biodiversity and their attractiveness to visitors.
A key outcome was the drawing up of an integrated lake management plan and a risk management plan, which seek to connect all aspects of the complex water body from its ecological and leisure value to its maintenance cost. The plans enable the Vienna authorities, which are responsible for the management of the lake, to foresee the consequences of their decisions.
Specific actions included the installation of an innovative submersed soil filter to modify the water quality flowing into the lake. While the increase in lime concentration was lower than expected, the other functions of the soil filter, such as phosphorus decrease and water level management, performed well, leading to a reduction of eutrophication of the Old Danube. The soil filter has several modes of operation that allow it to respond to the water composition. The project team also renovated the outflow weir (Hebergraben).
Furthermore, the project acquired equipment for harvesting the long-growing water weed, including a GPS system, an additional weed harvester and a maintenance boat – leading to a significant improvement in the efficiency of the harvesting. However, unusually warm weather caused an unprecedented growth of macrophytes (e.g. 2 800 tonnes of mowing mass in 2017 compared to around 30 tonnes in 2012 before the project). On one side of the lake, shorter growing macrophytes were planted and after initial failure are now growing well.
Finally, the project team carried out three pilot initiatives to re-naturalise the riverbanks in addition to improving a bathing area that was acquired by the city. They planted trees and carried out measures to protect them and to combat neophytes. Monitoring of beavers, macrozoobenthos, fish, amphibians and macrophytes, as well as the water parameters and the soil filter functions, was also performed.
