The Swabian Gate Weir was constructed in 1873 on the Dreisam River in Freiburg/Breisgau, Germany. The historic installation was listed for monument protection, but its decaying state began to threaten the river. It became especially problematic for the migration of fish, such as the salmon stocks which were being reintroduced to the ecosystem. To improve the site’s ecological performance while preserving the weir’s historical character, a fish ladder was constructed.

A geosynthetic clay liner (GCL) was essential to the works. A special GCL had to be utilized under the revetment to act as a water barrier and seal the protected structure. This required heightened puncture resistance and several of the GCL panels had to be installed under flowing water.

SECURING THE FISH LADDER & PRESERVING HISTORY

The EU Water Framework Directive stipulates ecological continuity, but the old 2.5m high and 42m long Swabian Weir Ramp with its 10 steps could not be surmounted by fish. While fish bypasses were used in other locations 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, the federal road B31 on both sides of the site, and residential buildings. Planners had no other choice but to re-build the structure.

A 115m long and approximately 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 armor stones in setstone and transom construction.

The actual fish ladder channel is 98m long and has 21 basins with a height difference of 15cm each. This allows fish and invertebrates to overcome the height difference of 3.3m upstream.

GROUNDWATER SEAL BENEATH THE FISH LADDER

It was important for the designers of UNGER ingenieure Freiburg to seal the building downwards in order to reduce surface water losses (exfiltration from surface water or infiltration into groundwater). This was especially critical at low water levels in the river and in groundwater. NAUE Bentofix® BZ 13-B GCL was installed between the existing subsoil and the revetment. It was the only GCL able to meet the conditions specified in the tender, including the main requirements of exhibiting a very high puncture resistance and having the ability to be installed underwater.

Bentofix® BZ 13-B is a needlepunched GCL and is a combination of a GCL with an additional sand protection and ballast layer. The additionally integrated sand layer acts as a ballast for underwater installation and protection layer for rock placement.

The supply order came from Walther Keune-Bau, Freiburg, and the laying technique was agreed on between all parties. Bauberatung Geokunststoffe (BBG)–the geosynthetic consultant–prepared the installation map.

The GCLs were installed under the revetment from the middle of the second basin to the upper end of the sole structure. Towards the side slopes, the panels were led directly under the upper edges of the crossbars and the foot protection.

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They were laid directly on the subgrade with an overlap of 1 m in flow direction. Due to the firmly bonded nonwoven layers, no additional protection measurements were necessary.

The GCL panels were covered with a 10cm gravel-sand layer 0/32mm, followed by a 30cm gravel-scrap mixture 0/120mm to 0/ 150mm and the revetment.

The entire bottom structure was constructed in two sections, half on one side and half on the other, as the river could not be diverted outside the water cross-section due to the lack of space. The dam was laid approximately 2.50m off-centre and then converted to the finished revetment for the second construction phase. The GCLs are therefore located in the flow direction approximately in the middle of the river with an overlap of approximately 2m. The panels were then installed in the middle of the river.

One challenge was the narrowness on the construction site. There was no storage space, so every Bentofix® GCL roll was only taken from the storage yard of Walther Keune-Bau to the construction site just prior to installation.

The fish ladder project was executed between June and August 2019.

Learn more about NAUE’s geosynthetics and engineering services at www.naue.com.


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