Baffle fishways, including Denil, passes is also referred to as countercurrent fishways since it consists of special deflectors that lead to helical countercurrent currents, high energy conversion and reduced water velocities in the main pass . Fish species with high swimming capacity can swim right up in the water stream and a baffle fishway can be constructed relatively steep for salmon (gradient 0.2-0.25). However, the remaining current conditions are still turbulent and often water velocities are above 2 m/s. It has been shown that baffle fishways are unsuitable for most fish species and juvenile fish, including all carp fish, eels, white fish and grayling (DWA 2014). AG-FAH (2011) indicates that baffle fishways have not proven to work in practice. Armstrong (2010) writes that specially designed low gradient Denil fishways can work for several species, but they require certain hydraulic ramifications and are therefore not suitable for varying water discharges (Armstrong et al., 2010). The baffle fishway can be used at particularly steep fishway sites and may be suitable for adult salmon and trout for limited space and steep terrain. However, in most cases, other fishway types should be chosen.
Planning of a baffle fishway will start with mapping and surveying of the barrier itself and the river reach upstream and downstream of the barrier, including information about the hydropower scheme. Surveying must also be conducted in the area of the river bank where the fishway is planned, including geological surveying. Geographic data should be handled in GIS software for further planning and analyses. The design of the fishway should be conducted with conventional hydraulic- and civil engineering calculations and drawing. All material used in a fishway must be planned to withstand physical strain from water, floods and frost. Monitoring facilities should basically be planned in the upper part of the fishway.
Physical implementation of bafle fishways requires heavy machinery suited for the river size and its surrounding terrain, such as excavators and lorries. Work with explosives is relevant in most cases and blasted rocks and transportation of material out from the site is common. Surplus rocks should not be disposed at site because of pollution risk. The construction phase includes construction of concrete formwork, casting of concrete and iron reinforcement work.
Injuries on baffle fishways from physical wear must be monitored and repaired in order to secure regular fish migration. Maintenance work normally require hand-tools more than heavy equipment but casting of concrete is typical. Depending of the site, removal of sediment, branches, logs and floating debris in pools and fishway entrance is common. Monitoring systems require regular inspection, depending on product and system.
Relevant MTDs and test cases
|Fish species for the measure
|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
|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
|Technology readiness level
|TRL 9: actual system proven in operational environment
|Cost of solution
|See cost table
- AG-FAH. 2011. Grundlagen für einen österreichischen Leitfaden zum Bau von Fischaufstiegshilfen (FAHs). Bundesministerium für Land- und Forstwirtschaft, Umwelt und Wasserwirtschaft, Wien. 87 pp
- DWA - Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall. 2014. Merkblatt M-509: Fischaufstiegsanlagen und fischpassierbare Bauwerke ‒Gestaltung, Bemessung, Qualitätssicherung. Hennef, 334 pp.
- Armstrong, G., Aprahamian, M., Fewings, G., Gough, P., Reader, N. and Varallo, P. (2010). Environment agency fish pass manual. Environment Agency, Bristol, UK.