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Restoration of the dynamics of Rhine alluvial habitats on Rohrschollen Island

Exhaustive summary of the project

Context

Of the many ecosystems found in the great alluvial plains of Europe, the richest and most complex are the gallery forests. They form long corridors stretching along the banks of the rivers flowing from the Alps, occupying land that is regularly flooded. Their virgin forest-like appearance is due to the exceptional wealth of ligneous species (trees, shrubs, large creepers).

These forests are now regressing as a result of the major works that have been carried out in recent decades to domesticate these rivers. Consequently, the four major European rivers flowing down from the Alps (the Rhine, Rhone, Danube and Po) have lost most of their riparian forests.

An island in the Rhine some 10 km upstream of Strasbourg as the crow flies, Rohrschollen island, is representative of the Rhine alluvial forests, an ecosystem that is rare at both national and European scale, characterised by priority habitat 91E0 "Alluvial forests with Alnus glutinosa and Fraxinus Excelsior" and habitat 91F0 "Riparian mixed forests along the great rivers (Quercus robur,Ulmus laevis, Ulmus minor)". Rohrschollen island in particular comprises habitats and species that are listed in Annexes I and II of the Habitats Directive and in Annex I of the Birds Directive. These key habitats however are disappearing.

This is mainly due to the fact that the flooding operations are so rare (the last was in 1999) and their static character (with the water slowly rising from downstream) mean that they are insufficient to have a beneficial effect on the hydro system and its associated habitats.
Indeed, the island is only flooded during flood retention operations carried out by the French and German management authorities. This is leading to the progressive disappearance of the first stages of the plant succession and simplifying the spatio-temporal mosaic as a result of the extension of hardwood forest units and the fact that they are renewed more slowly.

 

Project objectives

This project aims to re-establish a dynamic flood regime on Rohrschollen island, based on the hydrological rhythm of the Rhine.
The main action to be taken concerns the creation of a water intake structure on the southern part of the island, so that the Bauerngrundwasser hydrographic network can be supplied directly from the Rhine. This structure will introduce a discharge of between 5 and 80 m³/s, depending on the natural hydrological conditions of the Rhine. Connections between the hydrographic network on the island and the Rhine will be guaranteed by additional works on both secondary branches of the main stream in order to re-establish effective exchanges with the river.

The project will contribute furthermore to restore the remarkable "gallery" alluvial forests found along the four major European rivers flowing down from the Alps. It will be based on the presence of willow and alder wood habitat found at the Rohrschollen site, which is essential to the regeneration of alluvial forest dynamics, but has become rare in the Rhine valley.

By restoring this key habitat, the project aims also to encourage species that are listed in Annexes I and II of the Habitats and of the Birds Directive. These include the Peregrine falcon (Falco peregrinus), the Great egret (Egretta alba), the Great crested newt (Triturus cristatus) and the Spined loach (Cobitis taenia), which is scarce in the Rhine-Ried-Bruch alluvial sector of the Bas-Rhin département.

See the full programm (french version)

Last update : 29th June 2010
Contact : Ville de Strasbourg, Service des espaces verts et de nature – 1 Parc de l’Etoile 67076 Strasbourg Cedex – Tél. 03 88 60 90 90 – courrier@cus-strasbourg.net