Fish-passable structured ramps

Numerous cross structures on our streams and rivers represent obstacles for fish that they want to overcome during their migration against the direction of flow. The guideline 509 from the DWA (Fishways and fish-passable structures - design, dimensioning, quality assurance) for the fish-passable design of these structures exists since 2014. This guideline is now being revised.

Photographed from a bridge upstream, the picture shows, a so-called structured ramp. The ramp consists of 15 rows of large stones lying across the stream. Each row is about 20 metres long and about 25 centimetres higher than the previous row. The rows have a distance of about 6 metres. Between the rows, the stream bed is deeper, creating pools filled with water. In the rows of large stones, there are gaps through which water flows and fish can swim. To the left and right on the banks of the stream you can see bushes and behind them forest.
© BAW-IWB

Cross structures that can impede fish migration are essentially hydropower plants and structures that are intended to keep the riverbed and thus the water level at a stable height. Where these structures cannot be removed, it is necessary either to build appropriate fish passages or to redesign the cross structures so that they are passable for fish.

The legal basis for the required fish passability of cross structures in the countries of the European Union is based on the so-called Water Framework Directive.

The fish ladder shown consists of two parallel concrete walls. Between the two walls, transverse walls also made of concrete form individual water basins so that the water flows down in a staircase shape. The cross walls have a vertical opening that goes down to the bottom so that the fish can swim upwards. The view is against the direction of flow.
© BAW-IWB

The picture shows a so-called slot pass for a fish ladder next to a hydropower plant.

The pool pass consists of two parallel concrete walls that first make a slight curve in the ground plan and then a 180-degree curve follows. Crossbars made of rows of large stones create pools filled with water. From pool to pool, the water flows down in steps. On the left, you can see an embankment with a meadow and further back some trees and several houses. Behind the fish ladder is a large building (the hydropower plant).
© BAW-IWB

The picture shows a so-called basin pass for the fish ladder next to a hydropower plant.

The guideline, which exists since 2014, is now being completely revised by a DWA working group consisting of members from Germany, Switzerland and Austria, taking into account the findings of recent years.

Our contribution is essentially to work on the sub-volume on fish-passable ramps. This means the design and construction of gently sloping structures in rivers and streams, which often fulfil numerous water management functions. Examples are the stabilisation of the riverbed, flood protection, the elevation of the groundwater level, the interconnection of the watercourse with its accompanying floodplains. These ramps, unlike fishways at power plants, are also affected by floods and must be able to withstand these floods.

For so-called structured ramps, we developed a dimensioning system to calculate the flow depths during floods and to dimension the stone sizes both economically and stably. Further information can be found in the downloadable poster.

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