Madison Hydroelectric Plant Powerhouse

Project Location

The Madison Dam and Hydroelectric Plant are located on the Madison River in the southwestern part of Montana. The timber-crib dam was constructed in 1906 to replace a similar 1901 dam and powerhouse on the same site. The dam is 39 feet high and 257 feet long at its crest and is placed into the narrows of the Bear Trap Canyon.

The hydro project is a run-of-the-river configuration. The reservoir it creates, Ennis Lake, is 2.6 miles long and has a maximum storage capacity of 42,053 acre-feet. The powerhouse is constructed of rock masonry with a concrete slab infill. The original turbine and generator units were first commissioned in 1906 and operated until 2019, when Morrison-Maierle began the replacement project.

Retrofitting a Historic Facility

Morrison-Maierle designed a retrofit of the Madison Hydroelectric Plant Powerhouse to replace the four existing horizontal Samson double-runner Francis turbines with four new horizontal Francis turbines. The retrofit included stabilizing the over 100-year-old powerhouse rubble walls, demolishing the existing powerhouse floor, developing construction access and sequencing, and providing new structural foundations and anchorages for the new turbines.

The new turbine and generator units have a transition inlet section that ties to the existing penstock with a new draft tube. Morrison-Maierle also provided a 3-D laser scan of the powerhouse facility to develop an accurate as-built model used to develop construction drawings and layouts. The equipment suppliers also used this model to integrate detailed 3D models of the proposed equipment.

Adjusting for Unexpected Conditions

The Madison project was successfully completed despite many unique challenges and unexpected conditions. The original plans for the powerhouse were minimal and rudimentary, providing very little helpful information on which to base the design. Very early in the demolition stage, the project team discovered that where solid concrete was expected within the powerhouse floor, there was a thin slab over rock and sand fill.

In addition, the cofferdam that was constructed to isolate the powerhouse from the river did not perform as intended due to numerous unforeseen seepage paths and the powerhouse fill. The demolition plan had to be adjusted for water-filled excavations, and oil releases from past spills regularly occurred.

To adjust for the numerous unexpected conditions, Morrison-Maierle worked cooperatively with the owner and contractor to develop a plan to re-sequenced portions of the work and added large-scale use of tremie-placed structural concrete to vastly reduce the time and expense of dewatering the powerhouse.

Numerous impacts to the project were caused by the COVID-19 pandemic, which began shortly after the construction started. Morrison-Maierle supported the construction efforts as the design engineer and was a core member of the multi-national team that completed the project to meet the owner’s expectations.