St. Mary LLO Valve Replacement

Alberta, Canada

Project Details


2022 ACECBC Award of Merit

Project Leadership:

Rob McLachlan, P.Eng., Don Sproul, P.Eng.

The low-level outlet (LLO) at St. Mary Dam is a 600-m long, 6-m diameter concrete lined tunnel passing through rock adjacent to the dam. The tunnel was used to divert the river during the construction of the dam in the 1940s; the tunnel entrance is now submerged 53 m below the reservoir’s full supply level.

The low-level outlet is used to discharge riparian flows into the St Mary River downstream of the dam. Riparian flows must always be maintained to provide habitat for fish and other species that are dependent on the St Mary River and to maintain water supply to downstream users. An irrigation tunnel conveys water to the St Mary Irrigation Canal which supplies water to farmers, ranchers, and local towns. The supply of irrigation water is the primary purpose of the St Mary reservoir and the lifeblood of the area’s agriculture industry.

By 2014, the 1.8 m guard valve and the 1.4 m diameter hollow jet valve within the tunnel were deteriorated and leaked badly. KCB was contracted by Alberta Transportation to develop concept-level options and cost estimates for replacement of the guard valves, and an energy dissipating valve. Following these option reviews, KCB progressed the design through the preliminary and conceptual stages. We also assisted with the selection of the three different contractors and equipment suppliers and then managed the construction during the implementation of the work.

The key to the completion of the work was the installation of an isolation barrier upstream of the valves. The isolation barrier had to be installed 53 m under the reservoir surface and 200 m down the 6 m diameter diversion tunnel. Once the isolation barrier was installed, the existing valves were removed 400 m down the downstream end of the tunnel, before the new penstocks and valves could be transported up the same tunnel and installed in place of the original valves. Auxiliary piping, valve control systems and communications in the tunnel were also upgraded as part of the work.

To maintain the required riparian flows during the construction period, KCB designed a 1 km long HDPE pipeline that passed from the St Mary irrigation canal to the at the base of the spillway.


    • No provisions for isolation upstream of the 1.8 m and 0.3 m diameter guard valves
    • Installation of temporary isolation barriers to allow the valve to be replaced
    • Infrastructure inside the downstream end of the diversion tunnel including a powerhouse, bifurcation thrust bloc, penstock, concrete walls and other equipment that had to be worked around.
    • Trashrack at the upstream entrance to the tunnel, silt buildup inside the tunnel and unknown conditions at the bellmouths upstream of the guard valves, poor underwater visibility.
    • Maintaining riparian flows to provide habitat for fish, birds, and other species in the St. Mary River, as well as for the water supply to farmers, rangers, small towns downstream


  • Cutting of trashracks using remotely operated vehicles (ROVs)
  • Three-dimensional sonar-based underwater survey
  • Design and fabrication of custom isolation barrier, installation of barrier using ROVs
  • Innovative techniques to manoeuvrer the old and new valves around existing infrastructure inside the diversion tunnel.
  • Construction of a new pipeline (riparian turnout) to discharge required riparian flows during construction period


  • Safe and low risk replacement of the deteriorated valves.
  • Reliability and functionality improvements to the low-level outlet operation
  • Valve replacement completed without disruption to flow of water to wildlife and agriculture industry