Trois Villes test case

Fact box: Trois Ville
Country	France
River	Saison
Operator	Société Hydroélectrique de Gotein
Capacity	0.16 MW
Head	5 m
Inter-annual discharge	22.3 m³/s
Turbine(s)	1 Kaplan turbine

Introduction

The hydropower plant (HPP) Trois-Villes is located on the Saison in the southwest of France. The Saison is 54 km long. It flows in southwest of France near the Pyrenees. It flows into the Gave D’Oloron, which then flows in the Adour River. Altogether, there are 3 HPP on the water body and one fish farm.

The hydrology of the Saison is characterized by sustained flows in winter, high water levels in spring due to snow melting and a low water period from August to October. At Trois-Villes the mean interannual discharge is estimated at 22.3 m3/s.

About the hydropower plant

Trois Ville is run-off-river HPP with a 1.3 km long bypass reach. It has an installed capacity of 0.16 MW. The turbine on site is a Kaplan Turbine (Maximum turbined flow: 4.1 m3/s, Rated head: 5 m)

The Operator: Société Hydroélectrique de Gotein

The HPP Trois Villes is operated by the Société Hydroélectrique de Gotein, which is a small company responsible for operating two small HPP.

Pressures on the water body's ecosystem

The hydrology of the Saison is highly altered by the hydropower plant of Sainte-Engrâce located more than 25 km upstream, which runs with hydropeaking. The continuity is altered by the several weirs and HPPs along its course.

Test case topics

Fish population

The river is dominated by cyprinids and salmonids.

Downstream migration

To prevent fish from passing into the turbines an inclined bar rack is located in front of the hydropower plant with a width of 6.36 m, a bar spacing of 20 mm and an angle of 26° to the horizontal.

Upstream migration

At the dam, the fishes can use a pre-barrage fish pass to go upstream (2014). In the fish pass, a flow of 1 m3/s is maintained. The fish pass has three pools of 44, 49 and 62 m² size. The dissipated energy in the pools is between 72 W/m3 in low flow water periods and 196 W/m3 when the discharge is 2 times the mean interannual discharge (?44 m3/s).

At the power plant a baffle fish pass was built in 2014. It has a slope of 16% and maintains a flow of 150 l/s. It consists of a succession of 5 portions of baffle (0.5 x 7.5 m) connected by pools of 3 x 2 x 1 m. It allows salmonids to go upstream. There is a specific device for eels: it is composed of 2 portions of 3 and 9 m length and a slope of 25 to 30 %, connected by resting pools (1.5 x 1.0 x 0.5 m). The flow allocated to the fish pass is only a few liters per second. These devices target the fish using the tailrace channel to go upstream.

E-flow

In France, the law of 2006 (LEMA) imposes an e-flow of 1/10 of the mean inter-annual discharge before 1rst January 2014. They are two options for the environmental flow in the bypass section. In Trois-Villes, the discharge value is 2.5 m3/s, it’s bigger than 1/10 of the mean interannual discharge and doesn’t come from a biological study. It comes from a previous regulation (decree from January 1990) and wasn’t change.

Research objectives and tasks

The test case site of Tois-Villes is a small HPP with a fishfriendly water intake. It is planned to assess the efficiency of this water intake. Hydraulic modelling will be done for the entrance of the HPP to assess the influence of the bypass and undertake modifications of the flow conditions. Through further hydraulic modeling of the water course, the effect of the power plant on the habitat will be assessed.