High in the Coastal Mountains 100 kilometres north of Powell River on British Columbia’s west coast, two new “run-of-river” hydroelectricity stations are together generating 235 MW of electrical power.
The East Toba and Montrose hydroelectric projects were completed in 2010 and Knight Piésold was awarded the Consulting Engineers of British Columbia Lieutenant Governor Award of Excellence for them in 2011.
The most northerly of the two facilities is on the East Toba River and is the largest run-of-river project in B.C. It has a 30.7 m3/s design flow and 581 m gross head. About 17 km southwest is the facility on Montrose Creek, which has a 22.8 m3/s design flow and 466 m gross head.
Knight Piésold Consulting Engineers originally identified the sites and the idea of the combined projects. They obtained the permits for the owners, Toba Montrose General Partnership, and then were hired by Kiewit to do the detailed design under an EPC (engineering-procurement-construction) contract.
The remote site and extreme weather conditions created many construction challenges. These included improving or building new forestry roads totalling more than 60 kilometres. There are 11 major new bridge crossings spanning up to 120 metres, and more than 400 minor crossings and culverts. Also, avalanche mitigation structures had to be built at the Montrose intake and along the high sections of the transmission line.
Since fish are absent within the East Toba and Montrose diversion reaches, it was possible to design the large project to have a low direct impact on fish and fish habitat.
Screens in the World
The intake structures for the plants divert a portion of the natural stream flow to the buried penstocks and ultimately to the turbine generators.
Because of the avalanche risks and high sediment loads at Montrose Creek, its intake structure is fitted with large Coanda shear effect screens. The screens prevent debris and sediment greater than 2.0 mm from entering the penstock and send it over the screens and on downstream. The cleaner water is sheared off into a collection channel and intake box to be delivered to the power turbines.
With an overall length of 60 metres, the Coanda screens are the largest in the world for a hydropower application. They are designed to be self cleaning, with wedge wires that shear a section of the flow.
The team did extensive research and prototype testing on the Coanda screens, including hydraulic testing at Colorado State University. The screens had to be designed to deal with issues like ice formation in winter and the accumulation of leaves in the fall. They also had to meet requirements for long-term maintenance since during the winter months, when avalanche risks are high, the intake needs to run with very little operator intervention.
The East Toba facility has an inflatable rubber weir to divert the water through submerged portals. The flow passes through a trash rack and vortex desander before entering an intake reservoir box.
The 2 and 3-m diameter penstocks that deliver the water from the intakes to the powerhouses are a combination of HDPE in the lower pressure sections, and continuously welded steel penstock in the high pressure sections. The use of Weholite (large diameter HDPE pipe) has been pioneered by Knight Piésold for use in the hydropower industry as a robust economical alternative to steel for lower pressure sections of a water conveyance system. The use of soil restraint for the buried sections of the penstock was also optimized in order to reduce the concrete anchor block requirements.
After leaving the powerhouses, the water returns to the rivers via rip-rap lined tailraces. It is unaltered in terms of temperature, quantity and quality.
Switchyards alongside the powerhouses step up the voltage from 13.8 kV to 230 kV for transmission over a new transmission line which is 150 kilometres long. It connects to the BC Hydro grid at the newly constructed Saltery Bay substation. The transmission line alignment was carefully planned using GIS software and 3-D modelling in order to avoid numerous terrain hazards and to minimize the impacts on the landscape. The line includes both H frame wooden pole structures and cylindrical steel towers.
Local First Nations groups (the Klahoose, Sechelt and Sliammon) were engaged throughout the course of the project, including providing help with site engineering and environmental studies in the early stages. cce
Owner: Toba Montrose General Partnership (Alterra Power Corp and GE Finance)
Lead design engineer: Knight Piésold (Sam Mottram, P.Eng., Egbert Scherman, P.Eng., Jeremy Haile, P.Eng.)
EPC contractor: Peter Kiewit