City Commissioners approved a $167,733 contract change order for the first phase of the Missouri River North Bank Stabilization.

The original contract of $581,700 was awarded to Winkler Excavating in August.

The project will stabilize the bank along the north shore of the Missouri River where erosion is threatening both a 36-inch buried sewer line and the Rivers Edge Trail.

Portion of River’s Edge Trail to close for bank stabilization project

Construction began in September and during the excavation process for the revetment, (a facing of impact-resistant material applied to a riverbank, crews found ground water and petroleum contamination seeping into the dig area, according to city staff.

No contamination entered into the active stream during any of the contractor’s activities, according to staff.

Design changes were needed to address the contamination for an increased cost of $167,000, according to staff.

City considering contract for riverbank stabilization project

In 2002, the Cascade Conservation District contracted Land and Water Consulting to prepare the Missouri River Urban Corridor Inventory and Assessment.

“The purposes of the study were to document existing conditions and provide potential restoration and enhancement opportunities for the urban shoreline along the Missouri River as it flows through Great Falls,” according to the staff report.

City receives FEMA grant for riverbank stabilization project

The study identified the area along the north bank from the 9th Street Bridge upstream about1,440 feet as the highest priority for bank stabilization.

The Cascade Conservation District sponsored the development of the preliminary engineering report for the project, published November 2015, which identified three separate areas within proposed project area.

In March 2021, commissioners accepted a Federal Emergency Management Agency grant and voted to move forward with the first phase of the stabilization project, which includes 810 feet of riverbank, beginning at the 9th Street Bridge and extending upstream.

The design called for the revetment to extend below the Missouri’s high-water mark and applied for the Army Corps of Engineers permit. During that permitting process, the Army advised they would only allow gabion baskets, rip-rap and other forms of “hard armor” revetment to extend for 300 feet along the riverbank without requiring mitigation credits, which were estimated between $372,000 and $559,000.

Local groups pursuing FEMA grant to stabilize west bank erosion

That design was unfeasible so Winkler and the city agreed on a modified design, which was approved by the Army.

The total project cost for the original contract was $712,300, which included inspection fees, and was funded with:

• FEMA Hazard Mitigation Grant: $397,749.75
• NorthWestern Energy and the Missouri/Madison River Fund: $73,383.95
• Sanitary Sewer Enterprise Funds $150,182.80
• Parks and Recreation funding: $90,983.50

During excavation to prepare for the placement of a gabion basket wall, Winkler found a large amount of brick concrete asphalt fill near Calumet, according to staff.

When crews removed the top cover, they found a larger than expected fill area and petroleum contamination which was seeping into the dig area. Ground water in the dig area also showed signs of a contaminate plume within the project area, according to staff.

Work stopped until the area could be further assessed and the city, Winkler and Calumet, met to address the issue.

“Calumet staff indicated they were aware there could be possible contamination in this area of the jobsite,” according to city staff.

City staff met again with Calumet staff, Winkler, and the Montana Department of Environmental Quality to figure out how to move forward with the project. The decided another redesign was needed, which modified the revetment design using riprap instead of a gabion wall structure.

Calumet also performed hand borings along the bank line in an effort to determine the extent of the contaminated fill area.

The group also agreed that a clay plug consisting of a 30 percent mix of bentonite should be applied to the contaminated area already exposed, according to staff, to help cap the site and reduce the risk of petroleum contaminates entering the active stream channel.

The revised design also moves the toe protection section of the proposed revetment toward the historic bank line to further prevent exposing more contaminated material and lessen the risk of contamination getting into the river, according to staff.

As of Nov. 1, construction plans had been updated and permits approved, according to staff.

Winkler’s initial proposed cost for the additional work was $217,573.

City staff were able to negotiate a $49,800 credit toward the cost of the proposed change in scope.

This credit reduces the proposed cost of the change order from $217,573 to the $167,733.