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Bentonite mats allow fish to migrate

Case Studies, News | August 1, 2021 | By:

The geosynthetic-based fish ladder (left) at the Schwabentorwehr in Freiburg im Breisgau, Germany. Photograph courtesy of NAUE GMBH & Co. KG

The Schwabentorwehr (Swabian Gate Weir) in Freiburg im Breisgau, Germany, was in disrepair and insurmountable for fish. With the help of bentonite mats, the fish now have a free run again.

The historic Schwabentorwehr on the Dreisam River was a federally listed building for monument protection. However, the 1873-constructed building was not only in disrepair but was an obstacle to the migration of fish. To improve the site while preserving its historic character, a natural fish ladder was added to replace the ten steps of the weir’s ramp, which was impassable for fish. A bentonite mat/geosynthetic clay liner (GCL) was installed under the revetment to act as a water barrier and seal the protected structure.

The European Union (EU) Water Framework Directive stipulates ecological continuity. While other locations used fish bypasses along the river, the layout of the Schwabentorwehr in the middle of Freiburg im Breisgau prevented this. There were a public footpath, a cycle path, federal roads (one on each side of the site) and residential buildings. Planners had no choice but to rebuild the structure.

It was important for the designers to seal the building downward to reduce surface water losses. Essential requirements for the sealing material were a very high puncture resistance and having the ability to be installed underwater.

Based on the specific requirements, the designers opted for Bentofix BZ 13-B (bentonite mats/GCLs) from NAUE GMBH & Co. KG. The bentonite mats are needlepunched, reinforced, versatile geocomposites with immediate sealing efficiency. They combine two geotextile outer layers and a core of high-swelling powder sodium bentonite clay. The bentonite mats encapsulate an additional sand layer. Both the bentonite and the sand layer are fully needled and erosion-proof. The result is a uniform, multidirectional, shear-resistant hydraulic barrier with self-sealing and rehealing characteristics. 

Bentonite mats were installed between the existing subsoil and the revetment. The bentonite mats were laid directly on the subgrade with an overlap of 3.3 feet (1 m) in the flow direction. The bentonite mats were then covered with a gravel-sand layer, followed by a gravel-scrap mixture and the revetment.

Now, fish can overcome the height difference of 10.8 feet (3.3 m) upstream. The new fish ladder is particularly crucial for the salmon resettlement program. 

SIDEBAR: Project Highlights

Fish ladder for a small dam in Germany

OWNER: City of Freiburg im Breisgau, Germany

LOCATION: Dreisam River in Freiburg im Breisgau, Germany

CONTRACTOR: Walther Keune-Bau GmbH & Co. KG, Freiburg im Breisgau

DESIGN ENGINEER: UNGER ingenieure, Freiburg im Breisgau



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