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.