Chin Chute Structure Replacement Project

Chin Reservoir is an off-stream storage reservoir located roughly in the middle of the St. Mary River Irrigation District (SMRID) network approximately 10km southeast of the Town of Coaldale. The Chin reservoir is supplied by the SMRID main canal, which conveys irrigation water from the Milk River Ridge Reservoir, south of the Town of Raymond, a distance of 74 km.

In order to convey the main canal flows from the top of the valley to the base of Chin Coulee, a ~50 m drop, a concrete “chute” structure was constructed in 1951 by the Prairie Farm Rehabilitation Administration (PFRA), a branch of the Federal Government of Canada. The Chin Chute operated as the main inlet for Chin reservoir until the commissioning of the IRRICAN Chin Hydro Facility in 1994.

Figure 1: Birdseye view of Chin Reservoir Appurtenant Infrastructure

The original Chin Chute was built utilizing the designs and construction techniques of the day. The structure has an entrance width of 5.7 m and is 200 m long. A 8.8 m wide by 6.0 m long submerged stilling basin at the base of the structure acts as the mechanism of energy dissipation. It was sized with a maximum flow rate of 65 m³/sec (2150 cfs). Construction photographs from the PFRA archives are provided below.

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Shortly after commissioning, the chute structure experienced a failure of the floor slab. This failure, and a similar slab failure at the upstream Raymond Chute structure, highlighted a deficiency of the original PFRA design. Both structures were promptly repaired and put back into service and continue to be key elements of the SMRIDs infrastructure network for conveying irrigation water and overland drainage intercepted by the main canal.

The Chin Chute Project

As part of the SMRIDs infrastructure capital works program, it was recognized that the existing 70-year-old Chin Chute was at the end of its service life. The structure’ design deficiencies, its overall condition, and flood handling capacity were identified as conditions that necessitated the need to develop a replacement plan for the structure.

The SMRID and the Raymond Irrigation District (RID) own and operate the St. Mary Main Canal downstream of Ridge reservoir. The district’s have mutual interests and share costs per the terms of a recently signed Infrastructure Access Agreement. Further to that partnership, together, the SMRID and RID have pooled the two parties collective Irrigation Rehabilitation Program (IRP) Funding to finance the Chin Chute Replacement Project.

The Lethbridge branch of MPE, an Englobe Company, having successfully executed the 2021 Replacement of SMRIDs Sauder Chute and several other large water control structures in the Southern Alberta region, was award the contract to design and administer the project that would replace the Chin Chute structure.

The new chute structure was designed with the ability to handle a regular normal design flow of 65 m³/s, but with sufficient freeboard capacity to handle up to 115 m³/s (3,800 cfs) in the event of floods that exceed the full bank capacity of the upstream SMRID main canal. The structure would be founded to excavated bedrock. A USBR Type 3 stilling basin was designed to accommodate the expected worse case hydraulic energy dissipation needs.

In Summer 2023, the project was awarded to Dennis Dirtworx Ltd. of Coaldale, Alberta as the general contractor supported by VHL Construction Ltd. of Taber, Alberta, and various other subcontractors. The project was valued at $24 Million.

The project as designed consists of constructing site access roads, a 200,000 m³ isolation earth cofferdam, a structure consisting of 4,000 m³ of structural concrete, and a new conveyance channel to the structure. The existing structure and main canal will be decommissioned upon completion of the project.

The project began in July 2023. The new structure will be put into service for the 2025 irrigation season.