Nature-like fishways
Contents
Introduction
Nature-like fishways mimic natural side channels and are placed next to migration barriers (Calles et al. 2015), and it has been proven good functioning for migration of most fish species, also for slow swimmers, juvenile fish and benthos (Calles et al. 2013). A nature-like fishway should be the first choice when a barrier cannot be removed or combined with a ramp, and there is enough space for the construction. For downstream migration, this type of fishway can also be used, but usually requiring additional structures in the main river (e.g. physical obstacles and guiding elements for migratory fish). In addition to functioning as an upstream migration corridor, natural habitats are also created in a nature-like fishway.
The entrance to the passage should have a defined attraction flow for fish and should be placed close to the migration barrier and in the migration path of the fish. This increases the likelihood that migrating fish will find the entrance to the passage. The location of the entrance area may vary by different river discharges and different fish species, and this should be considered when designing, for example with V-shaped entrance profile, slots in the tahlweg and high bed roughness. In larger rivers and multi species communities, multiple entrances may be required. Also, at the water intake a sloped design that goes down to the river bottom is beneficial, since such a design can withstand a wide range of river discharge. It is not necessary to use sluices that control the water flow in the circulation, but these may still be practical. However, sluice gates must not close the migration corridor. Therefore, the gates should open from the side so that a vertical slot occurs at opening.
The fishway itself is designed as a river reach past the migration obstacle and is led into or above the dammed area. Many natural-like fishways have been designed as a step-by-step with stable threshold pool sequences. With this the bypass can be relatively short (gradient = 0.1 for salmon). However, it is also possible to design the bypass with lower gradient as riffles, or pool-riffle sequences with dynamic substrate, within erosion-proof boundaries. For salmon and trout, the height difference on individual steps may be up to 50 cm (gradient 0.05-0.1, energy dissipation < 250 W/m3). For adult salmon and anadromous brown trout in steep rivers and large bypasses (> 1 m3/s), the height difference may exceptionally be up to 75 cm if there is a deeper pool below. For grayling and inland trout, height differences of maximum 20 cm (gradient about 0.05-0.08 and with energy dissipation < 150 W/m3) and for carp fishes 10-15 cm (gradient 0.01-0.05 and with energy dissipation < 100-150 W/m3) is recommended (DWA 2014; Dumont et al. 2005).
It is often beneficial to establish border vegetation along the fishway to create shelter for migratory fishes. Dimensioning bypass discharge is selected to create a water corridor with sufficient water depth. For most species this depth will be between 1 m in pools and at least 0,3 m in riffles and over weirs. Dimensioning water discharge in nature-like fishways should be at least 0,5 m3/s for salmon, 0,35 m3/s for grayling and 0,2 m3/s for trout. Fishway discharge is also dependent on the size of the river. In Austria, it is recommended for nature-like fishways corresponding to at least 5% of the average river flow for rivers ≤ 20 m3 / s and at least 1-2% for larger rivers (AG-FAH 2011). We recommend using this as a guideline and the type of river, fish species and water supply in the main periods of migration should emphasized. The measure must be dimensioned to withstand floods (Fergus et al. 2010).
Methods, tools, and devices
During planning
Planning of a nature-like fishway will start with mapping and surveying of the barrier itself and the river reach upstream and downstream of the barrier. Surveying must also be conducted in the area of the river bank where the fishway is planned. This includes measurements of water covered area, water edges and river slope and the bathymetry of the area. Geographic data should be handled in GIS software for further planning and analyses. The construction planning should be supported with simple hydraulic modelling or calculations, such as the models River2D, HEC-RAS 2D or OpenFoam (see Chapter 9.1 for references). The physical adjustments should then be planned according to the hydraulic calculations, assuring a stable bottom substrate and hydraulic conditions suitable for fish migrations.
During implementation
Physical implementation of nature-like fishways requires heavy machinery suited for the river size and its surrounding terrain, such as excavators and lorries. It must be considered how the different parts of the banks, such as rocks and boulder, can be used as elements in the new habitat. Under normal conditions, none or only small volumes of substrate need to be transported to or from the construction site. Here, it is crucial that the labor involved has the relevant experience to make the best decisions while adjusting the physical habitat and that they have the required understanding of the planning documents and purpose of the measures.
During operation
Physical habitat measures in regulated rivers must often be maintained to ensure that functions related to flow and sediments are restored, such as flood events and connectivity of the sediments. The frequency of the maintenance will be site-specific
Relevant MTDs and test cases
Classification table
| Classification | Selection |
|---|---|
| Fish species for the measure | All |
| Does the measure require loss of power production | Operational (requires flow release outside turbine) |
| - | |
| - | |
| Recurrence of maintenance | Irregular at events |
| Which life-stage of fish is measure aimed at | - |
| - | |
| - | |
| Movements of migration of fish | |
| Which physical parameter is addressed | N/A |
| - | |
| - | |
| - | |
| - | |
| - | |
| - | |
| - | |
| Hydropower type the measure is suitable for | Plant in dam |
| Plant with bypass section | |
| Dam height (m) the measure is suitable for | Up to 20 |
| Section in the regulated system measure is designed for | In dam/power plant |
| - | |
| - | |
| - | |
| River type implemented | Steep gradient (up to 0.4 %) |
| Fairly steep with rocks, boulders (from 0.4 to 0.05 %) | |
| Slow flowing, lowland, sandy (less than 0.05 %) | |
| Level of certainty in effect | Moderately certain |
| Technology readiness level | TRL 9: actual system proven in operational environment |
| Cost of solution | See cost table |
