Engineers suggest steps to stop bank erosion near trail

A Helena engineering firm presented a report Monday outlining steps to address a 1,440-foot portion of the north bank of the Missouri River where ongoing bank erosion eventually could threaten the River’s Edge Trail and a city sewer main.

WWC Engineering representatives Shawn Higley and Robbie Neihart discussed what’s causing the bank to slough off into the river and best alternatives to mitigate and repair the damage, with an estimated price tag of nearly $800,000.

Concerned group members said they would study the proposal and seek state and federal grants and matching funds to pay for the work, which might be phased in.

Higley said a major 2002 study of the full river core through Great Falls identified this particular area as having major erosion problems. It runs from near the 9th Street Bridge upstream to the city waste treatment plant. Based on aerial photos, he estimated another 15 to 30 feet of bank have sloughed off into the river since then, but said the pedestrian trail and buried sewer line are still 30 to 60 feet from the river bank’s edge.

Last year, River’s Edge Trail officials approached the quasi-governmental Cascade Conservation District saying they were concerned that a key portion of the recreational trail and a sewer line running underneath it could catastrophically slough off into the river.

The Cascade Conservation District obtained a $75,000 grant for the study, conducted by WWC Engineering. Other stakeholders involved with the discussion included the city of Great Falls, Recreational Trails Inc., Calumet Montana Refining and NorthWestern Energy.

“The bank is calving off into the river at periodic intervals and depositing sediment into the river,” the engineering report says. “Large cracks have been observed along the upper bank of this stretch, which suggests periodic bank saturation. The bank is so severely eroded that it has been transformed into a vertical face that is very unstable.”

Higley said the strong erosion is caused primarily by strong waves, driven by winds, hitting the bank, which is made up of unstable soils, including loose fill deposited there by various building projects. In addition part of soil is saturated with water from over-irrigation of the land, he said.

In general, the engineering report recommended a combination of rock riprap, covered with willow and other plants, placed at the bottom of the bank to protect it from waves, with the middle of the banks shaped out to lessen the severe bank angle, with a lot of native grasses and shrubs planted to keep the new soil in place.

The engineers split the 1,440-foot stretch into three “reaches,” or spans, based on the severity of the problem and the recommended solutions.

Reach 3, 350 feet to the 9th Street Bridge, was the highest priority, with the steepest, most vertical banks of from 25 to 30 feet in height. Those banks are also closest to the recreational trail and sewer line.

In contrast, Reach 1, 410 feet near the city sewage treatment had banks of just 8 to 10 feet. Reach 2, 680 feet in between the other two, has 12 to 20 feet banks.

Higley estimated the whole project could be done in one six- to nine-month construction season if the group is able to put together the $800,000 in grants and matching funds. If the project is phased in, he recommended doing the section closest to the bridge first because it has the highest erosion and is closest to the recreational trail and sewer line. Because of its taller, severe banks the mitigation work in that portion will cost about $400,000.

Bruce Pollington, a Recreational Trails Inc. board member, said the group would like the trail replaced when the heavy construction work around it is completed, with at least the asphalt surface it has now if not concrete. The engineers said their cost estimates included replacing the trail with gravel and letting other groups provide funding to improve it further.

Pollington said the West Bank Park portion of the trail is part of a key 3.1-mile loop that’s used to promote in-city recreation.

Dana Leach, Calumet refining operations vice president, said his company is very interested in participating financially in the bank stabilization project. Bank erosion will impact the trail and sewer line first, but could eventually hit refinery property, he said.

City Park and Recreation Director Joe Petrella did not commit any city funding, but said the city is keenly interested in preserving the shoreline and the popular recreational trail. City Public Works Director Jim Rearden said there is “no immediate danger” of the sewer line sloughing off the bank, but that measures should be taken soon to prevent such erosion from happening eventually.

Cascade Conservation District board member John Chase said his agency is in charge of conserving soil and water, and large chunks of sediment dropping into the river can harm water quality and fish habitat.

Reached by phone, trail planner Doug Wicks said he is pleased that a number of partners are coming together to solve a problem that none could solve by themselves.