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How dam safe are we?

Water is being released from Como lake through the dam’s control gate situated near the center and bottom of the dam, letting enough early snowmelt to go down stream without flooding anyone. With an estimated 70,000 acre feet of water still sitting in the mountain snow pack, there is plenty left to fill the lake to its 38,945 acre-foot capacity for the irrigators and for the late season fishery flows in the Bitterroot River. Michael Howell photo.

Dam safety tested at Lake Como

By Michael Howell

Lake Como dam safety procedures and protocols were put to the test last week by a team from the Northwest Pacific Region of the Bureau of Reclamation, out of Boise, Idaho. The dam is owned and operated by the Bitter Root Irrigation District (BRID) but is regulated by the Bureau of Reclamation for safety purposes. Emergency managers from the bureau annually review, update, and implement emergency action plans (EAPs) for dams in the region. These EAPs are specifically developed for each facility and are tailored for initiating appropriate actions for different situations that could be encountered.
Last Wednesday and Thursday, bureau officials staged some functional exercises at Como Dam simulating realistic, potential emergency scenarios designed to test the installation’s emergency response procedures as well as that of the broader community. The exercise included a mock notification to local county emergency services concerning the simulated disaster.
For BRID Manager John Crowley, simulation on the first day began with notification that the lake was full and half a foot above the spillway. This was followed up with a notice from the National Weather Service predicting 50 degree temperatures and two to three inches of rain on a heavy snowpack.
As all hands showed up on deck to deal with that situation, the totally unexpected occurred, an earthquake measuring 4.7 on the Richter Scale! The water release gate was binding and two cracks appeared on each side of the dam.
What to do? Follow protocol. The actions required have been spelled out in advance and that is what the EAP is all about.
Construction of the dam began in 1906 and was completed in 1910. It was rehabilitated on its crest and upstream face by the Bureau of Reclamation in 1954. The semi hydraulic earthfill dam at the end of a natural lake is 70 feet high with a crest length of 2,550 feet and contains 1,114,000 cubic yards of earth and rock.
Between 1992 and 1994, major modifications to Como Dam were performed to mitigate concerns associated with seepage and piping, liquification during a large seismic event, and overtopping during large floods. During these modifications, the spillway crest was raised to elevation 4246.0 feet increasing the active reservoir capacity from 35,100 acre-feet to 38,500 acre feet. The State of Montana paid for this work and obtained 3,000 acre-feet of capacity for storing water to use in enhancing minimum streamflows in the Bitterroot River.
Following the two-day exercise, Crowley and the Bureau of Reclamation officials hosted a tour of the dam including an examination of the safety features that have been installed over the years.
Suzanne Marinelli, Emergency Management Program Coordinator for the bureau’s Pacific Northwest Region, said the agency was not there to test individuals, but to test the system of procedures and protocols that would be invoked in case of emergency.
“There is no good or bad test,” she said. “If something goes wrong or doesn’t function the way it should, we correct it, and that’s good.” She said that Como Dam was in good shape and was structurally sound.
Standing on top of the dam at the spillway, BRID manager John Crowley noted that the three and a half foot tall concrete wall running along the crest of the dam was not meant to hold back the lake. He said it was installed to keep wave activity from eroding the crest of the dam.
The lake levels are adjusted using a drain channel that runs under the center of the dam. The lake holds a maximum of 38,495 acre feet of water measuring about 46 feet in depth at the dam. According to Crowley, the highest elevation ever reached at the dam was 48.3 feet with 2.3 feet going over the spillway. The Probable Maximum Flow based on size of the reservoir and the catchment basin that might ever happen is estimated at 58.5 feet. But officials state that this is based on conditions that may occur only once in 50,000 years or so.
Crowley keeps a close eye on those water levels. The BRID website contains a link to the bureau’s Hydromet site where the amounts of water in acre feet, lake elevation and temperature are recorded and displayed every fifteen minutes.
“If I see something is changing, someone gets up here and does a visual inspection,” said Crowley. “The area around the dam contains five large monitoring wells that are big enough to allow a person to climb down fifty feet inside the dam wall to check the condition of the “bank’s water storage.” Water is always moving under and through the dam. But the water in Como Lake is extremely clear and any sign of sediment can be easily detected. Aside from the five large wells, there are about 19 smaller tubes set into the ground in the area of the dam where turbidity can also be checked.
“This is an earthen dam,” said Crowley. “If something goes wrong we will see it in the water.”
Crowley works hard to adjust flows at the dam to allow as much water to go down as possible in the early spring before catching what he needs for irrigators. As of last Friday, he had already released a large amount of water, bringing the lake down significantly.
“My lake holds about 38,495 acre feet and with snowpack at about 104% of normal in the area, there is an estimated 70,000 acre feet of water sitting up there in the snowpack right now,” he said. “That’s way more than enough to fill the lake before the snow is gone.”
Despite all the precautions, however, Dallas Erickson, who has formed a group called S.O.S., believes it is not enough. He is calling for the installation of an early warning system that would provide immediate notification in the event of a catastrophic failure.
The old EAP for Como Dam included an “inundation” map that, according to Erickson, showed that a total collapse would lead to a wall of water about 40 feet high, potentially wiping out the county’s infrastructure all the way to Corvallis. He said the hospital would be gone. He said that people should know this.
According to Marinelli, the inundation map for Como Dam is being updated, but was not yet available to the public due to security concerns.
Erickson said that if the potential impacts of that failure were being studied, the study should be public.
“All the players and all the public need to be involved,” said Erickson. “We need to know what will happen and get it out in the open.”

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