The Test Case is located on the small river Mo in the northern part of Sweden. The hydrology of the river Mo is typical for north Sweden, with a lot of snow in the winter, a spring flood when the snow melts, reduced flow during the summer and often an autumn flood before the next snow.

The catchment area is 820 km². Mean precipitation for the whole catchment area is approx. 727 mm per year, with a mean flow at the Baltic sea of 27.9 m³/s and 10.2 m³/s at the HPP Anundsjö.

Anundsjö hydropower plant

The HPP is impounding a small reservoir, releasing the water through the turbines and a tunnel back into the river. It has an installed capacity of 5 MW. The inter-annual discharge is 10 m³/s with a low-water flow of 2.21 m³/s.

A small reservoir of 0.5 ha is located upstream of the HPP Anundsjö. Further upstream are areas of spawning grounds, which are not directly affected by the HPP. Directly downstream of the outlet, a small water body is located, where the river is divided into two. During the winter, a larger reservoir further upstream provides the HPP with water.

Layout of the HPP Anundsjö

Layout of the HPP Anundsjö

Country Sweden
River Mo (Moälven)
Operator Statkraft
Capacity 5 MW
Head 61 m
Inter-annual discharge 10 m³/s
1 Francis turbine

The Operator: Statkraft
Statkraft is a leading company in hydropower internationally and Europe’s largest generator of renewable energy. The Group produces hydropower, wind power, solar power, gas-fired power and supplies district heating. Statkraft is a global company in energy market operations. It is Norway's largest and the Nordic region's second largest power producer. Read more

Challenges at the Test Case

Pressures on the water body's ecosystem
Due to two dams downstream of the HPP, for which the fish passage efficiency is unknown, the continuity is moderate. Most parts of the rapids have been modified for timber floating, with only a few being restored. Furthermore, the hydrology directly downstream of the HPP is affected by short-time regulation and variations of the upper water level in the small reservoir by up to 0.5 m. A recurring problem in Sweden is pollution through Mercury, which is released from the ground through logging activities, causing a moderate pollution of the water body.

Fish population
The fish species that can be found in the Mo are: salmon, see trout, pike, perch, grayling, brown trout and crayfish.
During the summer from July to October, the power plant is shut down twice a week for an hour, releasing the discharge through the gates in order attract salmon into the residual area.

Upstream migration
The upstream migrating fish encounter the outlet, located 4 km downstream the fishway. The attraction flow is represented by the minimum flow, 0.8 m3/s between the 15th of July and the 5th of October; 0.25 m3/s the rest of the year. At the end of the 4 km long residual area the upstream migration continues in the fish way. Through the dam there is a fish ladder made in concrete.

Downstream migration
The downstream migration device consisted of a floating guidance, attached right next to the intake, leading the fish into a smolt trap and then out to the downstream side of the dam. The new license of 2012 states that this device shall be placed out 1/5 every year and water released in the smolt trap shall be 0.25 m3/s. This solution was evaluated together with the authorities after the first year, 2016, and the conclusion was that it did not work as intended. 2017 a new solution was tested, with fishing nets, leading the smolts to the fish way instead. The water to the smolt trap should instead go through the fish way and the date for placement is moved to a flexible time due to when ice is gone from the reservoir.

In the new license from 2012 the requirements are release of water in fish way between the 15th of July and the 5th of October 0.8 m3/s and the rest of the year 0.25 m3/s. During the same period one hour every Tuesday and Thursday the power plant must shut down and release the discharge through the gates in order to attract salmon up in the residual area.

Continuity moderate
Hydrology moderate
Morphology moderate
Pollution moderate

Fish migration
salmon, see trout, perch, grayling, brown trout, crayfish
fish way with fish ladder

Research objectives and tasks

The overall question at the HPP Anundsjö is how FIThydro can improve the situation for fish by different measures and means around the HPP. This includes the situation for the smolt, which are mainly arriving at the HPP after being released in the upstream area. It also includes the situation of juvenile fish of different species as well as the possibility for salmon to pass on the way to spawn and back (influenced also by the HPP located downstream of Anundsjö and influencing especially the number of migrating fish.

Research tasks
The research tasks and field studies conducted at the Anundsjö HPP are:

  • Bathymetry and velocity in reservoir
  • Bathymetry of the fishway and the entrance with GPS and drone flights for Structure for Motion (SfM)
  • Bathymetry, flow velocity data and drone flights for SfM and habitat in bypassed river reach

Main FIThydro partners involved in the research at the Anundsjö HPP

Research topics FIThydro partners
Smolt affected by the current in the reservoir and at the turbine inlet CNRS, NTNU, SWECO
Flow and turbulent structure in the fish pass NTNU
Flow, sediment and resulting habitat availability in the residual flow area NTNU, SWECO
Fish way entrance attraction: Ability to pass and attraction at the fish pass entrance NTNU, SWECO, TUM

Research objectives
Upstream migration & fishway function
Downstream migration pathway
Habitat and sediments in residual flow
Smolt behaviour in intake reservoir

Update and results

The work focused on the reservoir (CNRS, SWECO, NTNU), the bypassed river reach with the fishway (NTNU) and the junction of the hydropower plant tunnel outlet and the river (TUM, SWECO, NTNU). A telemetry study with smolt covered even a larger area including a stretch of the river upstream the reservoir and the downstream area until the bay of Örnsköldsvik (INBO). Scenario modelling applying different mitigation measures in a Bayesian network have been applied to show cost-effective ways of mitigating negative impacts (SINTEF). The hydropower scheduling model SHOP has been applied to show the lost or gained income from power production with different scenarios of releasing eFlows (SINTEF).

ADCP was used to map the bathymetry of the reservoir, serving as input for hydrodynamic modelling. Combined with telemetry study of smolts, it is clear that the current configuration of a floating debris collector and a guidance device actually prevents smolts from finding a safe downstream migration pathway. The telemetry study shows that only 1 smolt managed to find a safe way for downstream migration when released in the bypass section just below the dam. The one-hour release of water to the bypassed reach of river twice a week, do not reach the confluence of the hydropower plant outlet and the river within an hour, and the attraction effect is not very clear. Water velocities seem to be too low to allow migration into bypassed reach without additional release of water. Detailed measurements of water velocities by ADV, and the use of drones and the Structure-from-motion-method to create bathymetry was used to validate the new double-averaging method of hydrodynamic modelling in 3D. The study to include costs of lost power production showed that it would be more efficient to release eFlow during 2 hours at night time instead of releasing it twice during 1 hour per week.

picture rights: © Wolf-Dietrich Marchand, Markus Förster, Sweco