Complete or partial migration barrier removal

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Figure 1: Removal of concrete weir in Norway by use of explosives and excavator.

If connectivity 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 rivers regulated for hydropower production where dams will basically be maintained, but also in other regulated rivers there are possibilities for removing obstacles. Only in the Alpine region of Europe, a large number of dams have been built for river bed stabilization, and many of these have been replaced by block ramps, complete or partial removed. In addition, smaller weirs have been removed in residual flow reaches or minimum flow reaches with great success (Fjeldstad et al. 2012).

Methods, tools, and devices

During planning

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, OpenFOAM or TELEMAC 2D and 3D. 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 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

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 very site-specific.

Relevant MTDs and test cases

Relevant MTDs
3D fish tracking system
3D sensorless, ultrasound fish tracking
Acoustic Doppler current profiler (ADCP)
Acoustic Doppler velocimetry (ADV)
Acoustic telemetry
Bedload monitoring system
Current meter
Radio frequency identification with passive integrated transponder (PIT tagging)
Radio telemetry
Structure from motion (SfM)
Relevant test cases Applied in test case?
N/A -

Classification table

Classification Selection
Fish species for the measure Gravel spawners
Does the measure require loss of power production -
Structural (requires no additional flow release)
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

Relevant Literature

  • 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,