Flower Creek Dam
This project received two engineering excellence awards from the American Council of Engineering Companies of Montana (ACEC). In 2017, it received the top honors in the Water Resources category and in 2013 in the Studies, Research, and Consulting Category.
Since 1946, residents of Libby, Montana, had relied on the Upper Flower Creek Reservoir impounded by 55-foot-high Flower Creek Dam for high-quality drinking water. After an inspection in 2009 and a report from the Montana Department of Natural Resources and Conservation in 2013 that both identified a serious deficiency in the concrete at the dam, the City of Libby immediately began considering options to maintain a safe and reliable water supply for the community.
The original Flower Creek Dam was completed in 1946. It was a thin-arch concrete dam, and it was built in the only place in the entire canyon that had enough bedrock to support such a structure. It was 60 feet high and 9 feet thick at the base and 3 feet thick at the crest. In 2010, a concrete core sample was taken from the dam which fell apart upon removal. Morrison-Maierle then looked for competent bedrock and the best location for a new dam.
The solution to this lack of bedrock was to construct a concrete core wall in the glacial till with a rock buttress on the upstream and downstream sides. The team used part of the dismantled old dam to create an upstream cofferdam which eliminated the need for new construction of both an upper and lower bypass solutions. This resulted in major cost savings.
Nestled in the mountains in northwestern Montana, the project location was subject to cold temperatures and inclement weather. In order to work around these obstacles, like to meet the concrete temperature specification (70 F), nearly all of the mass concrete was placed at night. The project also survived two flood events — one slowed the project for a couple of weeks. But the contractor recovered quickly and got back to work.
The new dam and reservoir is the City of Libby’s sole source of raw drinking water at about 200 acre-ft. or 65 million gallons.