Part of the EFRE-funded flood protection project in Brixen (Section 5), an innovative riverbed sealing measure was implemented in the Eisack to protect the city centre from flooding while improving the river corridor hydraulically, ecologically and in urban-planning terms.
There are various reasons for rising water levels within the Eisack’s cross-section. These include increasing urbanisation, higher flood discharges due to glacial melt, increasing rainfall intensity, diminishing retention areas and a depletion in water-absorbing substrates (e.g. snow or fine-grained soil).
High water levels in Brixen pose a dual flooding risk: if the Eisack overflows its banks, the town centre will be flooded directly. But the town is also threatened by potential rising ground water level.
Three measures created capacity for a 1-in-100-year flood event: the concrete floodwall was raised, the river cross-section was widened, and the riverbed was deepened. As a result, the low permeability sediment was reduced to a level that could not guarantee a separation of water level of Eisack and surrounding groundwater. To ensure sealing of the deepened riverbed, Bentofix® Green biodegradable* bentonite mats were installed. These mats use bentonite powder as the sealing medium, while the carrier and cover geotextiles are made from biodegradable* raw materials.
This provides the encapsulation of the bentonite to create a geosynthetic clay liner (GCL), enabling straightforward installation. The use of degradable raw materials avoids the release of non-degradable polymers into the river ecosystem (e.g., through bedload transport or hydraulic abrasion). While the fibres can degrade in the Eisack’s environment, the bentonite remains in the riverbed, maintaining its sealing function even after the geotextiles have degraded*.
Another challenge was the tightly scheduled construction sequence: most riverbed works had to be completed during low water levels between New Year and Easter. Only during this period was it possible, as shown in Figure 2, to carry out the construction work on one half of the cross-section at a time by means of a temporary dam in the middle of the river. Subsequently, the water-carrying section and the construction site swapped sides of the river. Among other things, the quick and easy installation of the biodegradable* Naue Bentofix® Green geosynthetic clay liners made it possible to keep to this tight schedule.
Biodegradable* GCLs are an effective and environmentally friendly solution for temporary base sealing in river engineering projects. They build on the advantages of conventional bentonite mats by adding even stronger ecological properties. In short, Bentofix® Green delivers reliable hydraulic sealing and construction efficiency (simple handling, rapid installation), helping projects meet environmental standards without compromising performance. Their use in the Brixen project demonstrates the potential of these building materials in the context of integrated flood protection measures.
We would like to take this opportunity to thank Fabio de Polo and Philipp Walder from Civil Protection Agency of the Autonomous Province of Bolzano, Mountain Basins Functional Area and Simon Berger from the University of Innsbruck, for their excellent cooperation on this project.
Bentofix® Green seals the deepened riverbed and helps maintain separation between the Eisack river and the surrounding groundwater.
After the geotextile components degrade*, the bentonite remains within the riverbed and continues to provide the required sealing effect.
Simple handling and rapid placement helped the project team complete the riverbed works within the limited low-water construction period.
No. 01221
