Ecologically improving habitats

The current renaturation project on the Lower Salzbach river in Wiesbaden, in the German state of Hesse, is part of a programme of measures to improve the ecology of Hesse’s watercourses. The aim is to upgrade the river from an ecological perspective, creating near-natural habitats that benefit both the environment and the population. The measures are being implemented within the context of the European Water Framework Directive (WFD), which requires all water bodies to be in good ecological condition or have good ecological potential by the year 2027. For the Lower Salzbach, this requires upgrading of the river’s bed and its banks over a length of 1.8 kilometres.

Spatial and hydraulic conditions
The relevant stretch of river was divided into three sections to enable the design to be tailored to different conditions and needs. The risk of flooding presents a particular challenge, with water level increases from 0.2 to 4.4 metres within 30 minutes having been experienced during heavy rainfall in recent years. These extreme fluctuations required forward-looking and dependable planning.

Planned measures
The proposed renaturation measures were designed considering the existing space and flow conditions.

1. Redesign of the riverbed: In areas with little space for near-natural development, the existing mid-level and low-level waterbed will be converted into a 2.8m-wide box profile. The banks will be natural stone walls of Taunus quartzite, and obstruction stones and plants will be introduced to create natural flow diversity.

2. Berm renaturation: In sections with wide paved berms, these will be deepened by 0.6 metres and planted with gravel turf. The riverbed will be provided with armourstone lining and gravel with a depth of 0.4 metres. A 0.5m-wide central channel will optimise flow conditions.

3. Instream measures: To promote the natural development of the watercourse, old tree roots and fallen trees are being introduced in selected areas. These structural elements provide protection and habitat for fish and other animals. Reedbeds and wooden lattice-type retaining walls are used to secure the banks.

Flow modelling for design reliability
Two-dimensional flow modelling was used to simulate the effects of the planned measures on the water level. The model was calibrated using water level records and flood photographs. This made it possible to develop reliable flood protection proposals.

Conclusion
The renaturation of the Salzbach contributes strongly to the ecological improvement of Hesse’s water bodies. The project combines the needs of flood protection with the enhancement of aquatic habitats and shows how sustainable ecological improvements can be achieved even where space is limited.