Migration barrier removal
Not only is a hydropower dam a barrier in itself, it also changes the upstream river reach into a more lake-like habitat. Increased fish mortality due to predation at downstream migration barriers and dammed reaches may in some cases result in greater mortality than turbine passages (Jepsen et al. 1998; Koed et al. 2002). If connectivity for is to be restored, it should initially be considered whether the migration barrier can be removed. It is often the best and most long-term solution if the goal is to recreate connectivity. Here, the solution focuses on power-regulated rivers where dams will basically be maintained but also in power-regulated rivers there are possibilities for removing obstacles. In particular, smaller weirs have been removed in residual flow reaches or minimum flow reaches with great success (Fjeldstad et al. 2012).
Planning of fish barrier removal will start with mapping and surveying of the barrier itself and the river reach upstream and downstream of the barrier. 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. The physical adjustments should then be planned according to the hydraulic calculations, assuring a stable bottom substrate and hydraulic conditions suitable for fish migrations
Physical implementation of migration barrier removals 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 barrier, such as rocks and boulders, 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
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 very site-specific.
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
- Fjeldstad, H-P, Barlaup, B.T., Stickler, M, Gabrielsen, S.-E. and Alfredsen, K. 2012. Removal of weirs and the influence on physical habitat for salmonids in a Norwegian river. River Research and Applications; 28, pp. 753-763.
- Jepsen, N., Aarestrup, K., Økland, F. and Rasmussen, G. 1998. Survival of radiotagged Atlantic salmon (Salmo salar L.) – and trout (Salmo trutta L.) smolts passing a reservoir during seaward migration. Hydrobiologia 371/372: 347-353.
- Koed A, Jepsen N, Aarestrup K, Nielsen C (2002) Initial mortality of radio-tagged Atlantic salmon (Salmo salar L.) smolts following release downstream of a hydropower station. Hydrobiologia 483: 31−37