LIBBY — The city of Libby must replace Flower Creek Dam within five years if it’s going to continue to provide Libby’s sole source of water, the city’s engineer told the council recently.

Ryan Jones of Morrison-Maierle explained that core sample testing results reveal that the dam, built in 1946, is deteriorating quickly and most likely will not meet standards for relicensing.

“We don’t believe the structure is going to fail tomorrow. However, if we see the same degradation in the concrete over the next five years as we’ve seen in recent history — and we believe we will — we don’t believe the factor of safety in five years on the structure will be acceptable,” he said.

The 55-foot tall dam spans 180 feet and holds back a reservoir on Flower Creek three miles south of Libby. The city arranged for core samples to be taken last fall after its 2009 engineer’s report, which was based on a visual inspection, recommended further testing. The inspection was necessary to renew the city’s operating permit with the Montana Department of Natural Resources and Conservation.

Out of the 17 samples, which measured 2 feet deep and 6 inches in diameter, only three stayed intact and could be tested for strength. Seven of the remaining samples fell into pieces because they were drilled along concrete joints, but seven others crumbled because they were weak.

“Everything is together when the structure is in compression under load,” Jones explained. “So as soon as we cored into the face of the structure and removed those cores, we took them out of compression, and that’s when they crumbled.”

Those samples indicate areas where the dam is too weak to withstand 1,000 pounds per square inch of pressure, Jones said, when typical concrete cores demonstrate strength of 3,000 to 4,000 psi. Still, a portion of the dam that was replaced in 1966 measured more than 6,000 psi. It was difficult to quantify the dam’s overall strength because the concrete varies widely in consistency and quality, but engineers estimate that it’s less than 1,000 psi.

“I believe 560 psi is where we’d be in a major overtopping event,” Jones told the council.

Though unlikely, he said, there is a chance that the dam could collapse within the next five years. It would take a major external event, such as an earthquake with a magnitude 4.0 or greater, he added.

When news of the dam’s test results came in, Vic White, director of Lincoln County Emergency Management Agency, called a meeting with local emergency response agencies.

There has always been a possibility that the dam could collapse, White said, which is why the entities met last fall to perform an exercise to address steps that should be taken, given the unlikely event. But now they plan to meet bi-weekly, he said, to prepare even more meticulously.

The Flower Creek Dam, built in an arch, gains strength from the pressure of the water pushing against it. If the dam cracks, causing water to burst through and reduce pressure, the whole structure will collapse, White said.

“If it goes — it’s going to go,” he said. “It’s not going to be just, ‘We have a crack in the dam and need to do something,’ it’s probably going to go.”

Based on engineers’ best estimations, the water would race north through the canyon at a depth of up to 13 feet, potentially uprooting the dozen or more homes along the creek’s three-mile stretch until the landscape opens up into the city. It would most likely flood the city center in up to 5 feet of water.

“This is all built up so much anymore, the water is going to come down [into town] and form mini-dams from the house that it tears up to cars and the garages and businesses,” White said. “It’ll dam up and it’ll break left, it’ll break right, it’ll go like that all the way down until it eventually reaches the river.”

It would flood most of Libby’s major infrastructure — the elementary and middle schools, the Christian school, homes, railroad tracks, downtown businesses, the hospital, ambulance barn, police station and fire department.

The Montana DNRC probably would require the city to install an early-warning device on the dam, Jones said. According to engineer estimations, though, there wouldn’t be much time to get to higher ground. The water would take an estimated 20 minutes to reach town and 72 minutes to flow into the Kootenai River.

As for people in Flower Creek’s immediate path?

“There’s not going to be much warning time if there is a catastrophic failure,” White said. “You’re probably not looking at an emergency evacuation, you’re looking at rescue and recovery.”

The Libby City Council must decide if it will replace the dam or remove it and find an alternate water source. It’s a complicated decision that will come at a steep cost.

If the city had money in hand and property downstream to build the dam, it would take a minimum of 16 months just to get the project out to bid, Jones estimated. In the very best of circumstances, he said, it would take “well over two years” from now to have a new dam installed.

“By the time you’ve got permitting, design, construction, everything all done, you’re probably pushing five years,” Mayor Doug Roll said.

The city council gave Jones the go-ahead to scout out funding sources for whatever solution the council comes to, though discussion steered toward replacement.

Most groundwater sources under the city are contaminated with creosote and are part of a Superfund site. Roll recalled city officials discussing about 15 years ago the possibility of utilizing a clean aquifer west of town before the water treatment plant was built. They decided against it, he said, because the cost of pumping and piping the water to town was prohibitive.

Building a thin arch structure like the current one would cost around $4.5 million, Jones said, while constructing a roller-compacted concrete structure similar to Libby Dam could come closer to $5.5 million. 

If a downstream site can’t be located for the new dam, the city will be looking at an even larger cost to obtain a temporary alternate water source to use during construction.

Roll and city supervisor Jim Hammons held a conference call with representatives of Montana’s Congressional delegation, as well as state Sen. Chas Vincent, to plead for financial help.

“It’s not in danger of falling down right now, other than an external event, but we need to get people to understand it’s a serious situation,” Roll said.

Most government grants require matched funding, Roll pointed out, a cost that Libby may not be able to afford.

“This may require an appropriation to help us out,” he said.