The residual flow and main run-of-river hydropower plants (HPP) Schiffmühle are located on the 35 km long river Limmat in Untersiggenthal and Turgi near Baden, some 27 km downstream of Lake Zurich. Between lake Zurich and Schiffmühle there are seven HPPs, namely in flow direction Letten, Höngg , Dietikon, Wettingen, Aue, Oederlin and Kappelerhof. There are three more power plants between HPP Schiffmühle and the junction with river Aare, namely Turgi, Gebenstorf and Stroppel. Altitudes of the lowest and highest points of river Limmat are 330 m and 406 m, respectively. The surface area of the whole catchment amounts to 2384 km2, of which 0.7 % are glaciated.

On river Limmat, the mean monthly discharge increases from March to June and then decreases from July to October. The annual discharge in 2015 was 89 m3/s, while the long-term average is 101 m3/s (1951-2015).

Schiffmühle hydropower plant

There are two run-of-river Schiffmühle HPPs on river Limmat in Untersiggenthal and Turgi near Baden, namely the main powerhouse and the residual flow HPP. In the scope of FIThydro, the residual flow HPP is the case study HPP. This HPP has an installed capacity of 0.5 MW and a mean annual output of 1.9 GWh. It operates with a bevel gear bulb turbine.

The other HPP Schifmühle with the main powerhouse is located downstream of the run-of-river Test Case HPP.

Layout of the HPP Schiffmühle

Layout of the HPP Schiffmühle

Country Switzerland
River Limmat
Operator Limmatkraftwerke AG
Capacity 0.5 MW
Head 2.97 m
Design discharge 14 m³/s
1 Bevel gear bulb turbine

The Operator: Limmatkraftwerke AG (LKW)
LKW produces environmentally friendly and local electric from four main and two residual flow hydropower plants on river Limmat between Baden and Turgi. The company is owned by the Regionalwerke Holding AG Baden (60%), a local utility company, and the regional power company AEW (40%). The Regionalwerke AG Baden is responsible for the operation of the HPPs and all technical and energy management issues. The administrative and financial management are performed by the Axpo AG. The average annual energy output is around 91 GWh. The company fulfills the standards according to ISO 9001 and the production of renewable energy is certified by TÜV SÜD Erzeugung EE.

Challenges at the Test Case

Pressures on the water body's ecosystem
The river Limmat is located in the Rhine river catchment, which was historically one of the most important Atlantic salmon rivers in Europe. The upstream migration of Salmons (Salmo Salar) in the Rhine catchment became almost impossible due to transverse structures such as hydropower plants. In the past few years most of the HPPs at the Limmat river have been equipped with state-of-the-art fish upstream passage facilities. However, downstream migration measures and sediment management strategies are not realized in any case. Furthermore, the river Limmat is highly influenced by HPPs and densely populated areas and considered as a heavily modified water body. The river has a moderate ecological potential. Various measures for sediment control, fish migration, flow changes, habitat in-channel and morphology off-channel have been implemented in the water body.

Fish population
The river Limmat is located in the Rhine river catchment, which was historically one of the most important Atlantic salmon (Salmo Salar) rivers in Europe.  The following species (see below) face potentially mortality during downstream migration, or difficulties during upstream migration in the Limmat catchment. All of the existing fish species (at least 22 species) in the Limmat River are facing potential mortality during the downstream migration. Some of the most important species are: Eel (Anguilla anguilla), Brown trout, Common barbel (Barbus Barbus), Grayling, Spirlin, Nase, Chub, Bleak.

Downstream migration
At HPP Schiffmühle (Dotierkraftwerk ‑ residual flow HPP), an angled fish guidance structure with horizontal bars, i.e. Horizontal Bar Rack (HBR), has been implemented to protect and guide fish to the downstream of the HPP in 2013. The rack is positioned parallel to the main flow to have a lateral intake. The HBR has a length of 14.6 m and a spacing of 20 mm between the bars, which are positioned in a vertical angle of 90°. At the end of the rack there is a bypass for the fish with three openings in a vertical chamber in different water depths (close to the bottom, central and close to the surface). From there a 25 cm wide pipe bypasses the fish downstream, letting them out at about 0.20 m above the downstream water level. The discharge at the bypass is 170 l/s.

For monitoring downstream migrating fish, 1 PIT-tag antenna is installed at the bypass.

Upstream migration
HPP Schiffmühle has a combination of a nature-like and a technical fish pass (vertical slot) for upstream migration. The nature-like fish pass entrance is located approx. 36 m downstream of the turbine flow outlet. The technical fish pass entrance is located 2 m downstream of the turbine flow outlet. The outlet of the technical fish pass is merged to the nature-like fish pass at an elevation of 336.83 m a.s.l. The total discharge in the fishway is 0.5 m3/s.

To monitor upstream migration and fish behavior in the migration facilities, 5 PIT-tag antennas are installed in the technical vertical slot fish pass and in the nature-like bypass.

The HPP Schiffmühle supplies up to 14.6 m3/s of turbine water and 0.67 m3/s of the water in the fish ladders (upstream and downstream) to the downstream river reach as e-flow. Moreover, during high river discharges, additional water is supplied over the frontal weir at the HPP and over the side weir along the power canal to the residual flow reach.

Sediment management
An innovative vortex tube for bed load transport connectivity, sediment flushing through weirs and upstream and downstream fish migration facilities have been in operation at the residual flow HPP Schiffmühle.

Sediment continuity moderate
Continuity (fish migration) moderate to high
Habitat moderate

Fish migration
22 fish species known, e.g. barbel, sprilin, trout, grayling
Migration device: Horizontal Bar Rack-Bypass System

Research objectives and tasks

The planned studies at HPP Schiffmühle will address various aspects of upstream and downstream fish passes, downstream habitat and sediment transport. The findings of the studies will have a wide range of applications for other similarly sized HPPs and answer the fundamental questions on the fish behavior at fish passes.

Research tasks

  • Field campaign: hydraulics, habitat, attraction flow and Lateral Line Probe in fish ladder
  • 3D numerical model of HPP area
  • Fish monitoring
  • Bed load monitoring at vortex tube
  • Habitat and sediment modelling

Main FIThydro partners involved in the research at the Schiffmühle HPP

Research topics FIThydro partners
Downstream fish migration, assessment of hydraulics and fish guidance efficiency of Horizontal Bar Rack Bypass System ETHZ, FCO, LKW, AF-Consult
Assessment of effectiveness of the natural fishway and the technical fish ladder (vertical slot) for upstream migration; attraction flow inf fish ladder ETHZ, FCO, LKW
Habitat modelling and attraction flow SJE, TUT
Ecological and morphological downstream impact of sediment management at HPPs NTNU, ETHZ

Research objectives
Upstream & downstream migration
Efficiency of Horizontal Bar Rack Bypass System
Attraction flow
Residual flow
Downstream habitat
Sediment management

Update and results

The flow conditions at the entrance of the nature-like fishway of the residual flow HPP Schiffmühle is more attractive for all monitored fish species than the flow condition at the entrance of the vertical slot fishway close to the draft tube outlet. However, the fish entrance efficiency is higher for the vertical slot fishway. Overall, the passage efficiency of the fish pass system is high (> 80 % for most species monitored), indicating an appropriate design and a good functionality.

Regarding downstream fish migration, velocity measurements and fish monitoring results show that the attraction flow to the bypass of the fish protection and guidance structure (Horizontal Bar Rack-Bypass System) is inefficient and needs to be upgraded and optimized. The results indicate that the design, location and operation of a fish guidance rack-bypass system is of prime importance for a successful implementation and a high guidance efficiency. Using a 3D numerical model, alternative bypass designs in terms of layout and inlet location have been investigated.

Sediment and shelter measurements by NTNU

Sediment and shelter measurements by NTNU

Bathymetry and velocity measurements using ADCP at HPP Schiffmühle

Bathymetry and velocity measurements using ADCP at HPP Schiffmühle

Measuring flow from a fish's perspective using the Lateral Line Probe

Measuring flow from a fish's perspective using the LLP

Bathymetry and velocity measurements using ADCP in the headwater channel

Bathymetry and velocity measurements using ADCP in the headwater channel

picture rights: Areal views, Layouts of HPP and fishways, sediment management, research updates © ETHZ; pictures of the fis passes © Jeffrey Tuhtan