NorthWestern Energy hopes to move six towers that support a key 210,000-volt transmission line away from dangers posed by the Clark Fork River, where floodwater pushed three power poles over last spring.
The overhead transmission line is at the north end of Tower Street. It connects the Reserve Street and Target Range substations, and helps support power in the Bitterroot.
On Tuesday, Joe Wulf, a transmission engineer with NorthWestern Energy, presented dramatic drone footage that outlined how extreme river bank scouring and flooding have caused severe structural damage on the north and south sides of the river.
The video starts with the spring floodwater on the north side of the Clark Fork, showing three poles with broken tops. Across the roiling river are the three other poles, two of which are bent over so far that their tops appear just above the floodwater and the lines dipped into the river.
“We have lost (the line) in the past and it’s been out for several months. During this last go-round, they tripped out in May and were restored in August,” Wulf told members of the Missoula Parks and Recreation Board. “We spent over $100,000 to get that line back up. When this happened again, our corporate folks basically said, ‘Do what you need to do to get that line back up and keep it up.’
“It’s an extremely important facility for us.”
The transmission lines are located along an easement that was in place when the city purchased the Tower Street Conservation Area in 2001. Working with the Parks Department’s Morgan Valliant and Elizabeth Erickson, the trio came up with a route that jogs to the east that all three believe will be more protective of the towers. The route crosses the river onto a vacant tract of land the city owns, then hooks back into the existing transmission line within the tree farm on the north side of the river.
Putting the new line in place will be fairly dramatic.
Instead of the current 35-foot-tall lines on wooden towers, the new lines will stretch anywhere from 75 to 95 feet in the air. The six single-pole, tapered structures made of weathered steel will be mounted on 8-to-10-foot diameter concrete shafts, drilled to 20 to 30 feet into the ground.
The concrete steel casings at each structure will extend four feet above ground to protect the towers from floodwater and debris, and the steel towers will bolt to the top of the concrete casings.
“These are similar to a pier on a bridge, to where if the water comes up on them, so be it,” Wulf said. “It will withstand that flow if the water decides to come through there.”
He expects that an un-engineered levee upstream from the towers also will protect them from floodwater and the channelization this spring that almost made an island out of one of the tower sites. A portion of the new route will follow an existing road.
“We’re taking the line directly over the parking lot,” Valliant said. “It’s worth noting there will be trees cut, but we’re trying to minimize it. But we will lose large cottonwoods and there will be these large concrete things with the trail right next to it.”
Erickson is quick to note a couple upsides to the project. The current easement corridor will be reclaimed with high-quality vegetation, with a particular eye toward enhancing raptor habitat. In addition, the new wires will be high enough that some trees and shrubs can grow underneath them.
“There’s a lot of public benefit,” Erickson said. “The conservation committee asked to ensure that Morgan (Valliant) or someone has an opportunity to review the restoration program on both the old line and post-construction of the new line.”
Wulf also offered the services of NorthWestern Energy’s biologist and vegetation management team.
He said the new towers also are designed to withstand Montana’s weather, with both high winds and ice loading taken into consideration.
“We look at wind loading, so if the wires start to dance and get around they don’t get together,” Wulf said. “”We put a half-inch of ice on the conductors, plus the wind load, and make sure the structure doesn’t topple over. We’re designing for an extreme weather situation.”
The Parks and Recreation Board unanimously supported the proposal, which needs the final blessing of the city council before the changes can be made.
“I’ve been very pleased during the last several years with how NorthWestern Energy partnered with parks to restore easements,” said John O’Connor, a board member. “This is a really important area; a lot of people like to be out there.”
Wulf said they hope to do the core sampling of the ground underneath the new towers by the end of November, drill the shafts in December and January, and possibly erect the towers in March.
“I’m in a race right now against the runoff,” Wulf said.