Hydro-Sherbrooke’s Historic Dams

Sherbrooke, located in the southeast region of Quebec, was established at the beginning of the 19th century. With a population of 150,000, it is the principal city in the Eastern Townships region and the seventh largest city in the province of Quebec.

The town is situated at the confluence of the Saint Franois and Magog Rivers, two rivers that were important navigable waterways for the Abenaki aboriginal population. Then, when hydroelectric power was in its infancy, Sherbrooke (formerly the Big Forks) took advantage of the site’s geography to generate electricity, capitalizing on the 30 metre drop, escarpment and gorge along the Magog River.

The Frontenac Power Station built in 1888 in the middle of the Magog River Gorge is the oldest, operational hydroelectric plant in Quebec. Other hydroelectric facilities followed, creating a network of eight power stations and three major substations on the waterways within the municipality. The Paton, Frontenac and Abenaquis power stations are in the downtown area, while the Memphremagog, Rock Forest and Drummond power stations cascade down on the Magog River upstream from the city.

Hydro-Sherbrooke, the municipal utility, produces about 19.7 MW from the hydro-stations, serving about 70,000 customers. Tariffs are the same as Hydro-Qubec charges its customers. The city generates about $121 million in annual revenue from the operations, and $18 million in profits.

Urban renewal

In the past few years, Sherbrooke has embarked on a major urban renewal project called the Cit-des-Rivires. The ongoing work includes a market, new bridge, foot and bicycle paths, and the restitution of a contaminated railyard.

In conjunction with the city’s urban renewal program, Hydro-Sherbrooke began a program to upgrade its power stations and dams. SM Group of Sherbrooke has been the prime consultant with Hydro-Sherbrooke in this ongoing program to rehabilitate century-old civil engineering works. Guy Fouquet is the engineer in charge of the work at SM.

The interdisciplinary team had to find inexpensive solutions and construction techniques for preserving the integrity of the structures and their operational performance. As well, since the power stations control water levels, they are closely tied to the local recreation and tourist activities on Lake Memphremagog and the Magog River. The sections of river between the six dams along the Magog River are used for fishing and some contain spawning grounds. These uses, and the need to maintain relatively stable water levels for boating and other recreational activities, had to be taken into account when setting project work schedules. Respect for the environment is also crucial to obtaining the necessary approvals.

Rehabilitation techniques

The design team members have broad complementary expertise in structural engineering, materials engineering and environmental issues. The work involves:

* monitoring (tomography, drilling, instrumentation, etc.) to assess damage and severity;

* assessing the behaviour of various materials over time and in their physical environment;

* assessing the structural behaviour of works (cracking, displacement, stress concentration, vibration, etc);

* evaluating construction and rehabilitation techniques (draining techniques, injection, reinforcement, etc); and

* complying with environmental quality, dam safety legislation, and federal regulations, in particular the Fisheries Act.

The engineers start by carrying out structural studies that enable them to determine the best solutions and avoid errors of interpretation. In some cases, special instruments are used to understand the behaviour of the structures in various situations. This careful research, for example, has shown that certain cracks that might have been interpreted as due to foundation shifting were actually due to discontinuities in the foundation, while others were due to thermal movements.

The window to complete work is dictated by the time necessary to obtain the required certificates of authorization and the power generation needs of Hydro-Sherbrooke. Depending upon the scale of the work required and the duration to complete it, the repair will be scheduled in one or more phases. In some situations partial repair to a section of a dam or building is all that is needed. In other situations the need for total repair requires new construction. For example, here is the scope of work completed at five dams:

* Weedon Dam. Construction of a reverse filter on the downstream side of the right-side wall in order to prevent or reduce the accelerated erosion of its foundation;

* Drummond Dam. Rebuilding of the spillway structure downstream from the existing crest gate without using coffer dams. Subsequent leveling of the old spillway below the water level of the reservoir;

* Paton Dam. Curetting and restoration of the concrete works of the central wall, and reinforcement and consolidation of the right-side wall with an abutment structure downstream;

* Westbury Dam. Injection to the foundation of the spillway structure using various concrete mixtures and filling materials.

* Frontenac Dam. Inspection work to ensure the global stability of the heritage dam built in the late 1800s.

The intervention techniques depend on the kind of degradation. For example, a cement is selected depending on whether the concrete has becomes granular due to leaching or erosion, or due to chemical interactions from contact with water.

Restoration relies on more than just characterization and benefits from the opinion of specialists in materials engineering. Masonry, steel and wood are frequently used in restoring heritage works.

This unique project that not only preserves heritage structures but also meets the utility’s performance objectives for generating power.

Owner: Hydro Sherbrooke

Prime consultant, dam restoration, environmental: SM Group (Guy Fouquet, ing., Alain Philibert, ing.)

Electrical consultant: CIMA, Gestion Conseil SCP.

Cite des Rivieres, Sherbrooke