At the test case Las Rivers on the Ariège in France, scientists from CNRS/Institute Pprime and from OBF conducted hydraulic measurements and fish tests to investigate the efficiency of downstream migration solutions for Atlantic salmon smolts and silver eels. The Las Rives test case is a medium hydropower plant (HPP) with a fish friendly water intake where an inclined bar rack with several technical solutions for downstream migration was tested.

At Las Rives hydropower plant, a new downstream migration device was built in 2015 in order to protect downstream migrating fishes (atlantic salmon smolts and silver eels) from passing through the turbines. This device is located at the head of the headrace channel. It is an inclined bar rack (barspacing 20 mm, inclination 26° to the horizontal) combined with 3 outlets at the top of the bar rack. The downstream migration discharge (1.35 m³/s) is collected in the downstream migration duct and is controlled by a weir. Then the discharge is evacuated to the bypassed reach through a channel.

The effectiveness of downstream migration solutions

The effectiveness of the fish-friendly water intake was studies in the framework of the project FIThydro and of the project EFIGRI during two downstream migration periods for Atlantic salmon smolts (spring 2017 & 2018) and silver eels (autumn/winter 2017/18 & 2018/19). The efficiency of the inclined low bar spacing trashrack has been validated by radio telemetry for smolts (81% of efficiency) and eels (100% of efficiency). No cumulative effect on different successive HPPs has been shown during these tests. Depending on the river discharge, the fishes use bypasses as well as the overspill weir.

Passage routes of downstream migrating Atlantic salmon smolts at Las rives, overall in spring 2017 and 2018 (© Tetard S., 2019)
Passage routes of downstream migrating silver eels at Las Rives, in autumn/winter 2017/18 (blue) and autumn/winter 2018/19 (red) (© Tomanova S., 2019)

ADCP measurements were conducted at four cross-sections for different flow discharges, that show well-predicted normal and tangential components of the upstream velocity. This has also been validated by 3D modelling highlighting the attractiveness of the three bypasses, and was confirmed by bypass discharge measurements

A safe landing

The landing conditions of the fishes at the foot of the control weir ending the downstream migration duct has also been studied and showed some fish damages (about 30%) which should bring on some modification of the landing zone. Pressure measurements have been carried out to link the total pressure and the weal water depth.

Zone of landing (left). Water jet trajectory (top right) and pressure measurement by placing a plate on the bottom with a pressure taps (bottom right) 2019.
Calculated trajectory of the jet in actual setting

You can find more information on the test case and field work in Las Rives on the Las Rives Test Case site.

Laurent David (CNRS/Pprime)


Lemkecher, F.; David, L.; Courret, D.; Chatellier, L. (2018). Field measurements of the attractivity of bypasses for fishfriendly trashrack. Riverflow 2018, 5/8 september, Lyon.

Tetard S, Roy R, Tomanova S, Courret D, Mercier O, Richard S, Mataix V, Lagarrigue T, Frey A, De-Oliveira E. (2019). Effectiveness of fine-spaced low-sloping racks to protect Atlantic salmon smolts at four hydro powerplants in south-western France. American Fisheries Society & The Wildlife Society 2019 Joint Annual Conference, 2019, Septembre 30th, Reno, 2019.

Tomanova S, Courret D, Mercier O, Richard S, De-Oliveira E, Mataix V, Lagarrigue T, Frey A, Tetard S. (2019). Efficiency of downstream passage devices to protect migrating silver eels assessed with radiotelemetry. 5th International Conference on Fish Telemetry, Arendal, Norway, 2019, June 24th-28th., 2019.