Hydrological improvement of a former floodplain in an urban area: Potential and limits

Abstract

The Lobau near Vienna used to be a dynamic floodplain which was cut off from the Danube River in the course of regulation measures in the 19th century. In order to compensate for deficits in the water budget, a water enhancement scheme for the upstream part of the floodplain, the Obere Lobau, was initiated in 2001. The aims of this scheme were to increase water levels in the floodplain and to discharge nutrients from the area's highly eutrophic water bodies by imposing a controlled surface water exchange through the main side arm. Our study evaluates the effects of this artificial water supply on the hydrological and trophic state of the backwaters. The effects of the water enhancement scheme were partly overlaid by the impoundment of the Danube after the construction of a power plant. Both the artificial water enhancement and the Danube impoundment improved the hydrological and trophic condition of the backwaters through an increased surface or sub-surface water supply. In addition, the water enhancement scheme provided a surface water exchange through the chain of backwaters, creating uniformity among the connected water bodies and leading to a significant decrease in temporal variability. However, spatial and temporal restrictions in the scheme ensured that the individual backwaters could maintain their specific biogeochemical character. Our long-term observations of mitigation effects in degraded urban wetlands demonstrate the need for a steady and well controlled water supply in order to achieve successful long-term maintenance of such systems.