Other measures to improve downstream fish migration
- 1 Introduction
- 2 Methods, tools, and devices
- 3 Relevant MTDs and test cases
- 4 Classification table
Water spill over dams and gates, partly combined with periodic stop of power plants, have been used to bring down migratory fish past power stations. The method is suitable for periodic fish migrations, such as salmon smolt during spring floods and for eels during autumn floods. These migrations often coincide with flooding where the water flow exceeds the power plant capacity. At run-of-river power plants in North America, this method is frequently used to pass salmon smolts, both for Atlantic salmon and species of Pacific salmon. In Germany it has been used to pass eels (Adam 2000, Egg, etc. 2017). Water spill past turbine intake is considered a safer alternative to turbine passage, but also the use of spill water fish injuries can occur. The loss is still low (2% after Coutant and Whitney 2000). It should be noted that the relationship between water spill and fish migration is not linear, i.e. that increased spill does not necessarily mean a proportional increase in migration past the dam. More important for its efficiency is the placement of the dam relative to main stream and power intake (Coutant and Whitney 2000, Fjeldstad et al. 2011).
Partially submerged guiding walls
Installations that guides fish to a bypass and which only partially descend into the water column are referred to as guiding walls. Walls have been made of concrete, metal and plastic and are installed floating or at the surface to guide fish. At the same time, guiding walls can help stop driftwood and ice. They are less suitable for bottom migratory fish. The method works only for parts of the migratory fish, but it is relatively affordable and easy to install, even at existing power plants. There are commercial suppliers that also offer floating guiding walls (e.g. tuffboom.com). The solution can help to increase the effectiveness of other measures to promote migration.
In Austria, a system of horizontal tensioned steel wires is tested which acts as a grid when the steel wire is tensioned. Low wire spacing makes the system suited for guiding fish down to smolt size. The grid is set up vertically, i.e. as a β-rack. At the end there is a bypass that guides fish safely downwards. Besides the physical barrier itself, it is expected that vibration in the steel wire has a deterrent effect on the fish. The system can be cleaned by reducing the tension on the steel wires and allowing the flow of water to remove drift goods such as leaves, branches, waste and the like. The solution is promising but not sufficiently tested and therefore not recommended as best practice.
Guiding fish towards safe migration paths using air, sound, light and electrical barriers
Fish are detrimental to a series of measures, including flashing lights, noise, air bubbles and electrical power. This can be used to control fish from dangerous migration routes, such as intake for turbines. The measure initially only works if the fish has alternative migration routes, for example escape routes and safe bypasses. The guiding impact can be improved with attraction measures such as increased water flow to a bypass (repulse and attract). Common for repulsion measures is that they usually only work for parts of migrating fish, that they work less well by flooding and are selective in terms of species and sizes. Fish may also be accustomed to deterrence measures over time, so that they get lower efficiency.
Design varies between the methods and refer to the literature below. The most important deterrence methods are:
- Electric screens
- Strobe light
- Bubble curtains
- Acoustic deterrence, e.g. BAFF (bio-acoustic fish fence), which is a perforated tube located below the surface that emits sound and compressed air, which will act as a repellent on fish.
Fish-friendly turbines are optimized in geometry and operation so as to avoid damage to fish as much as possible. Differences from ordinary turbines are low rotational speed, large diameter and small spaces between turbine blades and turbine housing (Ebel 2013). Because of. higher investment costs, fish-friendly turbines are relatively rarely built. However, there are examples of conventional turbines with reduced spacing between minimum gap runner and different designs (Alden 2008, Meijnen and Grünig 2013, Fisher et al. 2000).
Hydraulic screws are suitable for smaller water discharges and drop heights, and are generally fish-friendly, but depending on the type there is a risk of damage due to spaces and sharp edges (Ebel 2013). A further development of the hydropower screw that functions for down- and upstream migration is the Archimedes double screw. This uses a part of the energy to pump water and fish upstream in the inner screw, which has no moving parts. Monitoring of this type of hydropower screw has in part not shown damage to migrating and migrating fish in the Grigull study (2015) but damage to fish cannot be excluded in hydropower screws (Ebel 2013). At Ham Power Station in the Albert Canal (11m head) 50-70% of the fish were damaged (Johan Coeck, pers.comm.), Which was attributed to both the design and the pressure altitude. In assessing such turbines, additional effects should be weighed, such as effects on lids and lures, selectivity for certain species and sizes, as well as potential for predation at intake and outlet.
The solution can help to promote migration in special cases, but further testing and assessment of overall passivity is required before it can be recommended as pattern practice.
Shaft power plant
Shaft power plants have a horizontal and very fine trash rack and a bypass option immediately at the intake on top of the water entering the turbine. The concept consists of a concrete box, which is covered with a horizontal and fine trash-rack aligned directly with the riverbed, a specially developed rack cleaner and a conventional Kaplan turbine with a permanent magnetic generator sitting in the shaft (Geiger et al. 2018). The cross section of the intake can be enlarged quite easily and with minor additional costs, causing low velocities in the inlet section (e.g. 0.3 m/s) which allow even small fish and fish with low swimming abilities to freely swim over the intake. Coarse sediments can also pass over the trash-rack, while finer sediments will pass through the turbine.
Planning of any downstream fish migration systems require a comprehensive study on fish behaviour to assess when and where fish move on their downstream migration. This can be conducted by telemetry experiments or visual observations with video or sonar technology. River hydraulics at the intake must be modelled by 2D or 3D CFD software if needed. The physical installations of the trash rack and bypass corridor(s) must be planned according to the powerplant geometry and construction works must be adapted to physical forces and the hydropower scheme. Fish-friendly turbines is planned by the manufacturer. Attraction and repulsion measures is planned in collaboration between fish migration experts and product suppliers.
Construction of fine-mesh racks requires heavy lifting equipment and both fixing and placing of the structure needs to be done when the hydropower plant is brought to full stop. Installation of racks and a bypass system requires a suite of skilled labour on steel, concrete and wood works. Fish-friendly turbines is installed by the manufacturer. Attraction and repulsion measures is installed in collaboration between fish migration experts and product suppliers.
Depends on system used.