Skimming walls (fixed or floating)

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Figure 1: Skimming wall at Bellows falls power station

Skimming walls (or surface mask) can be used to deflect those species that migrate in the surface layers, such as salmonid smolts. This device is inefficient for bottom-oriented species, such as eels. A guide wall must cover a certain depth to generate a sufficient repelling effect and must be installed at an angle to the channel intake to guide fish to a bypass entrance located at its downstream end.

Such a device has been installed on the East Coast of the USA at the Bellow Falls power station (Connecticut River, maximum turbine discharge 410 m3/s). The wall extends vertically halfway down the 9 m deep water column, at an angle of 40° across the channel and guides fish towards a sluice gate and a channel leading to the tailrace. An auxiliary bypass located near the bar rack is intended to accommodate fish which have passed under the wall. The efficiency of the guide wall was estimated at 84%, with 10% of the smolts passing through the secondary bypass and 6% through the turbines. The efficiency of the device is probably closely related to the angle of the wall with respect to the flow and above all to the depth (4.5 m) at which the wall is submerged (Odeh , et al., 1998).

In France, the application of skimming walls is reserved to large HPP only, where other solutions are not feasible.

Methods, tools, and devices

During planning

Planning of skimming walls requires 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 skimming wall must be planned according to the power plant geometry and construction works must be adapted to physical forces and the hydropower scheme.

During implementation

The physical installations of skimming wall must be planned according to the power plant geometry and construction works must be adapted to physical forces and the hydropower scheme.

During operation

A skimming wall doesn’t need maintenance as cannot be clogged

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
Current meter
Differential pressure sensor base artificial lateral line probe, iRon
Particle image velocimetry (PIV)
Radio frequency identification with passive integrated transponder (PIT tagging)
Radio telemetry
Sediment simulation in intakes with Multiblock option (SSIIM)
Visible implant elastomer
Relevant test cases Applied in test case?
Anundsjö test case Yes
Bannwil test case -
Ham test case Yes


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 Never
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 All
Section in the regulated system measure is designed for -
Upstream of hydropower plant
River type implemented -
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


  • Odeh , M und Orvis, C. 1998. Downstream Fish Passage Design Considerations and Developments at Hydroelectric Projects in the North-east USA. 1998.