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As the mighty Peace River flows from northern British Columbia (BC) to Alberta, its waters carry incredible potential to help fill our need for clean energy.

The Site C project is the third project on the BC side that taps into the that potential. When complete, its generators will make enough power to run 450,000 homes each year for the next century. Together, the three Peace River projects will generate around 40% of BC Hydro’s power.

Situated about 7 kilometres southwest of Fort St. John, the Site C project has been under construction since 2015. The expected completion date is 2025. As the third project on the river system, Site C gains significant efficiencies by taking advantage of water already stored in the Williston Reservoir behind the W.A.C. Bennett Dam. This means that Site C will generate approximately 35% of the Bennett Dam’s energy, but with only 5% of the reservoir area.

AtkinsRéalis is the engineer of record for the core structures, including the diversion tunnels, excavations and earthfill dam, roller-compacted concrete (RCC) buttress, generating station and spillways. Over the course of this project we provided engineering services, delivering feasibility studies and supervising geotechnical investigations, completed detailed design and provided technical support throughout construction.

What we’re building

The project’s design is strongly influenced by the complex geological and hydraulic requirements at the site. It includes the following key components:
 
  • Stabilizing the north bank via a large excavation of unstable materials and flattening of slope.
  • Constructing two temporary cofferdams across the main river channel and two 12.5 m diameter diversion tunnels to allow for the construction of the earthfill dam
  • An earthfill dam approximately 1,050 m long and 60 m high above the riverbed. The dam comprises a central impervious core, core trench excavated to shale bedrock, a grout curtain along the floor of the core trench, filters on either side of the core, gravel drains on the downstream side of the core, and outer shells of sands and gravels.
  • An approach channel with an impervious lining along the channel and an 8 m high berm running along the middle.
  • An 800-m-long RCC buttress to enhance seismic protection. The RCC buttress supports the valley's south wall, provides an abutment for the earthfill dam and is a foundation for the generating station and spillway.
  • Six power intakes and penstocks with vertical lift gates.
  • A generating station comprising six 183 MW vertical Francis generating units.
  • A temporary diversion consisting of twin tunnels and cofferdams.
 

 

Innovation and challenges

 
The historical design layout for the main project structures required large, deep excavations in challenging geological conditions on the right bank. The rock exhibits weak near-horizontal bedding plans, sub-horizontal cross-cutting shears, and isostatic rebounds, which provide stability and potential differential movements of the proposed structures. Our design addressed this firstly by rotating the axis of the concrete structures perpendicular to the river, parallel to the valley slope, and secondly by developing the concept of the RCC buttress so the horizontal hydraulic thrust on the headworks is carried down an inclined concrete buttress to the rock below the riverbed to by-pass the weak bedding planes in the riverbed and valley walls. This arrangement also avoids differential swell and rebound beneath the structures.
 
Because the power intakes, powerhouse, and spillway all sit at right angles to the direction of the Peace River flow, how the water gets to the spillway is a unique and very challenging aspect of the Site C project. It required extensive use of physical hydraulic models (PHM) as well as computational fluid dynamics (CFD) models. We designed the approach channel around the right abutment of the main earthfill dam to be 800 m long, 24 m deep, and over 200 m wide to handle flood flows to the spillway with reasonable flow velocities and approach conditions that could be managed with carefully designed geometry of the spillway piers. The channel is large enough to keep maximum velocities below 5 m/s for the PMF and to about 1 m/s for standard generation flows. We addressed the oblique approach flow conditions to the power intakes by adjusting the intake submergence and adding anti-vortex devices that resulted in acceptable hydraulic conditions under all potential operating scenarios.
 
An impervious liner on the approach channel's invert is sealed to the headworks structures to limit water ingress into the bedrock beneath the approach channel and under the RCC buttress. A berm in the base of the approach channel will facilitate future liner maintenance.
 
The diversion tunnels were used for the minimum downstream flow releases during the initial reservoir filling. Tunnel #2 was outfitted with orifices just before reservoir filling to increase energy dissipation and thus reduce tunnel capacity to permit the reservoir to fill with normal inflows from upstream.

 

 
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Why it matters
Site C is a legacy project that will be an important source of clean energy for the next 100 years.
  
We all feel the power of water when standing next large rivers, lakes or the ocean, and we are fortunate to live in a time when technology has progressed to the point where harnessing that potential into large and multi-faceted hydroelectric projects is possible. Not only does Site C maximise energy investments made decades ago, but it also creates more clean energy to power our future. 
 
Designing and building projects that matter—and discovering and innovating together along the way—is what inspires us. We take a lot of pride and satisfaction in knowing that Site C will help power the next four generations and that we are part of the team making it possible.

 

Why it matters
Site C is a legacy project that will be an important source of clean energy for the next 100 years.
  
We all feel the power of water when standing next large rivers, lakes or the ocean, and we are fortunate to live in a time when technology has progressed to the point where harnessing that potential into large and multi-faceted hydroelectric projects is possible. Not only does Site C maximise energy investments made decades ago, but it also creates more clean energy to power our future. 
 
Designing and building projects that matter—and discovering and innovating together along the way—is what inspires us. We take a lot of pride and satisfaction in knowing that Site C will help power the next four generations and that we are part of the team making it possible.

 

8 %
increase to BC Hydro's supply
3 rd
dam and hydroelectric generating station on Peace River
5,100 GWh
of electricity each year
1,7 million
electric vehicles powered per year in British Columbia
6
183 MW Francis units
1,100 MW
of capacity

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