Bentonite mats allow fish to migrate

The listed Schwabentorwehr in Freiburg was in disrepair and insurmountable for fish. With the help of bentonite mats, the fish now have a free run again.

The historic Schwabentorwehr (Swabian Gate Weir) on the Dreisam River in Freiburg/Breisgau was a 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 historical character, a natural fish ladder was added. A special geosynthetic clay liner (GCL) was installed under the revetment to act as a water
barrier and seal the protected structure.

The EU Water Framework Directive stipulates ecological continuity, but the fish could not surmount the old 2.5m high and 42m long Swabian Weir Ramp with its ten steps. While other locations used fish bypasses along the river, the layout of the Swabian Gate Weir in the middle of Freiburg prevented this. There was a public footpath, a cycle path, a federal road on both sides of the site, and residential buildings. Planners had no other choice but to rebuild the structure.

The entire bottom structure was constructed in two sections, due to the lack of space. The dam was laid approx. 2.50m off-centre and then converted to the finished revetment for the second construction phase.

It was important for the designers to seal the building downwards to reduce surface water losses (exfiltration from surface water or infiltration into groundwater). These losses were especially critical at low water levels in the river Dreisam and in groundwater. 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) from NAUE. Bentofix® geosynthetic clay liners (GCLs) are needle-punched, reinforced composites with excellent and immediate sealing efficiency, versatile in use. They combine two durable geotextile outer layers and a uniform core of high-swelling powder sodium bentonite clay. The Bentofix® BZ 13-B type encapsulates an additional sand layer. Both, the bentonite and the sand layer, are fully needled and erosion-proof. The result is a uniform, multi-directional, shear-resistant hydraulic barrier with self-sealing and re-healing characteristics. The additionally integrated sand layer in Bentofix® BZ 13-B enables the underwater installation due to its weight. It also ensures that the bentonite mat is very robust and has an extreme puncture resistance.

Bentofix® was installed between the existing subsoil and the revetment. The bentonite mats were laid directly on the subgrade with an overlap of 1m in the flow direction. Due to the firmly bonded nonwoven layers, no additional protection measurements were necessary. The Bentofix® GTD was then covered with a 10cm gravel-sand layer, followed by a 30cm gravel-scrap mixture and the revetment.

A 115m long and approx. 19.5 to 23.8m wide near-natural sole structure was erected as a river-wide drainage channel with a basin structure. The revetment of the new weir consists of hydraulic armourstones in setstone and transom construction. The actual channel is 98m long and has 21 basins with a height difference of 15cm each.

Now, fish and invertebrates can overcome the height difference of 3.3m upstream. The new fish ladder is particularly crucial for the salmon resettlement programme.