Dam construction to begin early 2015
The City of Libby is on target to finally begin construction of a new Flower Creek Dam early next year.
Bids on the project are due back to the city next Wednesday. Work will likely begin in January, beginning by drawing down the water in the reservoir.
The city plans to replace the 68-year old dam, which holds the city’s drinking water, with a new gravity-type dam 85 feet farther downstream. Engineers issued a report in 2011 that said the dam was losing its strength and under normal conditions the dam would be usable for another five years.
The estimated cost to construct the new dam is approximately $8 million and is expected to take one year to complete.
Bringing the project — originally planned for construction last year — to fruition has been an arduous process. Construction was delayed after the U.S. Fish and Wildlife Service rescinded a 2012 comment on the project and asked for a biological assessment of the grizzly bear, Canada lynx and bull trout populations to be completed. The project was finally given the go-ahead this spring.
In July the city received nearly $8 million in federal assistance to help replace the aging dam. The U.S. Department of Agriculture approved a $3.2 million loan and $4.7 million grant for the dam replacement project.
Since securing funding, the city’s engineering firm, Morrison-Maierle, have finalized designs for the project and the city went out for bids last month.
Paul Burnham, of Morrison-Maierle, met with city officials Tuesday to give them an update on the how he construction process will take place.
“The reservoir will be drawn all the way down,” he said. “The system operates most the time as if there is no reservoir anyway so it shouldn’t be a problem. For the last 30 years there’s only been two years where the water flowing through the dam didn’t provide enough to serve the city.”
Beginning in January the water in the 220-acre reservoir behind the dam will be released at a rate of about two feet per day until the reservoir is drawn down completely. The process is expected to take roughly a month and then the contractor will be able to begin dismantling the old dam.
By May a bypass pipe will be in place to divert the water from Flower Creek around the construction site.
“This will allow the contractor to be able to work while water is moving through freely,” he said. “Even through spring runoff, the bypass line will be able to handle the water.”
The new gravity-type dam will be constructed 85 feet downstream on almost the same footprint of the existing dam and will have about the same capacity for water storage. The dam is expected to have outlet facilities for greater control of storage levels and in-stream flows.
The replacement of the dam is critical. Morrison-Maierle, the Kalispell-based engineering firm, in 2010 completed a core sample of the arch dam and found that the concrete strength was less than 1,000 pounds per square inch. Typical concrete cores demonstrate strength of 3,000 to 4,000 psi. A report issued in 2011 said the dam would need to be replaced by 2016.
Included in the environmental report is mitigation measures expected to address impacts associated with construction, including the access road, dam construction and tear down of the existing dam.
Morrison-Maierle’s assessment, which was initially drafted in January 2013, recommended replacement of the dam. The report notes that “given the evidence of initial poor construction practices, significant variability in the condition of the concrete and extremely poor concrete condition, combined with the significant risk to public safety and property” that the dam be replaced.