Civil engineers will be meeting with the Libby City Council next week to discuss plans to replace the deteriorating Flower Creek Dam southwest of town. The concrete arch dam, completed in 1946, is owned by the city and holds the reservoir that supplies Libby with its drinking water.
In 2009, the dam was inspected by Morrison-Maierle, Inc. of Kalispell as part of an operational permit renewal process governed by the Montana Department of Natural Resources and Conservation, which issues dam permits every five years. The inspection found multiple leaks in the face of the dam, where water was seeping through, yet the concrete was deemed strong enough and a five-year renewal was issued. In 2010, officials with Morrison-Maierle undertook another investigation into the dam’s condition, taking 17 core samples of the concrete. Of the cores drilled, only three were stable enough to test and it was found that the strength of the concrete was less than 1,000 pounds per square inch. Normal strength is between 3,000 and 4,000 pounds per square inch.
A report, issued by Morrison-Maierle in September 2011, stated that under normal conditions the dam would be useable for another five years, however if there was a seismic event the dam could be compromised, flooding Flower Creek and Libby.
“There’s no immediate threat of failure under standard load conditions, however the risk of failure will increase over time,” said Ryan Jones, civil engineer with Morrison-Maierle.
The report also stated that in 1995 a membrane liner was installed on the upstream side of the dam in hopes of stopping the leakage, but that is now also leaking.
Engineers recommended that a new gravity dam be built about 85 feet downstream from the current one, a project that Jones estimated would cost somewhere around $7.5 million and take almost two years to construct.
Libby Mayor Doug Roll said after reviewing the situation, the city council agreed last year and decided to move forward with a plan to construct a new dam. But before that can happen, Morrison-Maierle must gather more information about the possible site, a challenging proposition because of the dam’s remoteness and the rugged terrain that surrounds it, which is partly why the dam needs to be replaced.
The completed core tests suggested that the concrete was weak because when it was mixed builders used too much water. Jones hypothesized that it was done so that builders had to haul in less cement. He also said that it was possible the concrete never fully settled before it froze the winter following the dam’s construction.
This year, an access road will be completed from the Flower Lake Road to the dam site. From there engineers will be able to inspect the site and begin the process of designing a new dam.
“Major improvements to access will help us with the geotechnical investigation and construction,” Jones said.
Jones said it will be an expensive project, but there are state and federal grants available to help the city pay for the new dam, a process in which his firm will be heavily involved. He also said that it was “unrealistic” that the city would be able to pay for it entirely with grant money.
Grant money from the state did, however, cover the installment of a new sensor and alert system that will warn emergency officials in Libby and the surrounding area of problems at the dam. Lisa Oedewaldt of the Lincoln County Emergency Management Department said the system was installed late last month. If there is a sudden drop in water in the reservoir, 911 dispatchers will be notified and be able to look at images from the dam to determine if a siren needs to be sounded in the city. If the dam breaks and water begins to cascade down Flower Creek, it would hit city limits in less than 20 minutes, although it’s a scenario Oedewaldt called “unlikely.”
Oedewaldt said the sirens are currently being installed and will be active within a few days. Once that is completed, officials plan on testing them, while simultaneously informing residents about the new system.
“We want to test and educate at the same time as to not worry the public,” she said.
On Jan. 16, Jones and others from Morrison-Maierle will travel to Libby to update the council on the progress that has been made.
“It’s a new year and it’s a good time to meet with the city and say, ‘Here’s what’s being done and here’s what we need to do to keep moving,’” Jones said.
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