Chute Bell x 2.5

Charged with rehabilitating a historic hydroelectric plant in Quebec, Gnivar combined digital and physical scale modelling to find the right design.

When Hydro-Qubec decided to re-open the historic Chute Bell hydroelectric plant on the Rouge River in the Outaouais region, the utility commissioned Gnivar to do it as a turnkey project. This was the first project Hydro-Qubec had assigned as a turnkey, and it meant the engineers had to attain a high performance within a limited budget.

Chute Bell was built in 1918, but had been closed down in 1984. Gnivar was asked to rehabilitate the plant and increase its output by a factor of 2.5, from 3.6 MW to 9.9 MW. Instead of three generating sets, it now has two.

A tunnel excavated in the rock had to be modified and water intake structures had to be altered to avoid head losses and flow changes that could reduce the plant’s generating capacity. There were also concerns that changes to the generating plant would adversely affect conditions downstream.

Gnivar combined digital modelling with a reduced scale physical model built at the cole Polytechnique de Montral. This approach combined the flexibility of computer simulations with the greater reliability of the scale model, and eventually the team found an excellent correlation between the test results. Normally scale models are only built for larger projects because of the expense.

The engineers chose the centre and right sluices for the turbine-alternators and added a submerged deflector wall between the sluices at the downstream water intake to prevent vortices forming in front of the unused third sluice. The angle of a wall was changed to make it parallel to the axis of the penstocks, and the transition between the rock face and the wall was smoothed. The engineers also modified the guard gates at the plant’s intake, and added an ice boom 1.5 kilometres upstream from the intake. The boom allows a stable ice cover to form quickly and reduces the amount of slushy “frazil” ice forming in the turbulent water. As was forecast by the tests, there have not been any significant changes to the sediment conditions nor to the water’s quality since the plant re-opened in the spring of 1999. The project cost $12 million.CCE

Prime consultant: Gnivar Inc., Montreal (Marc-Andr Richard, ing., Francis Gauthier, ing., Jean-Franois Mercier