City considering contract for riverbank stabilization project
City Commissioners will consider during their Aug. 2 meeting awarding a $581,700 contract to Winkley Excavating for the first phase of the Missouri River north bank stabilization project.
The project is a partnership among the City of Great Falls, Cascade County, Cascade County Conservation District, Recreational Trails, Inc. and NorthWestern Energy.
The group identified a 1,440-foot stretch of bank line along the north side of the Missouri River where significant erosion is occurring, according to the staff report.
The project is located immediately west of the 9th Street bridge on the north shore of the river and is adjacent to the city’s wastewater treatment plant and Calumet Montana Refining. It’s along the River’s Edge Trail.
“Large portions of the bank line have sloughed off into the river creating an unstable condition and a safety hazard. The bank is so severely eroded that it has transformed into a vertical face that is very unstable,” according to the staff report.
The height of the bank within the proposed reach is anywhere from seven to 20 feet in vertical height.
The erosion is threatening the Rivers Edge Trail and a 36-inch buried sewer line which are located approximately 30-60 feet from the edge of the bank.
“Providing bank stabilization measures along this stretch of reservoir will provide protection for the pedestrian path and sewer line; eliminate erosion and safety hazards; and reduce sediment deposition into the river,” according to the staff report.
Stabilization is required to protect public infrastructure in the area that includes a 30-inch diameter sewer main that serves Black Eagle and it’s 1,087 residents, according to the group’s letter of intent for the grant. Calumet also has infrastructure along the bank and the River’s Edge Trail runs along the edge.
If the bank were to erode further, it could cause a break in the sewer main, disrupting service in the area and sewage could spill into the Missouri River.
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.
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.
The overall estimated cost for all three areas is $792,700.
The group decided to split the project into multiple phases to request grant funding for the project.
This phase of the project will repair and reduce erosion along an 813-foot stretch of the north bank line of the Missouri River to help preserve the trail, city sewer line and conserve soil, according to the staff report.
“The stabilization will utilize a combination of riprap, gabion baskets, vegetative armoring, and blank slope reduction to prevent further erosion. This phase of the project has been split into two reaches with a different bank stabilization typical section for each depending on the current streambank offsets from the adjacent pedestrian trail,” according to the staff report.
The first section will use riprap to two feet above the ordinary highwater mark and native soil and willow cuttings will fill the voids in riprap. The vegetation will allow for long-term bank stabilization.
The other section will use a series of stacked gabion baskets. The steeper treatment is needed due to the close proximity to the trail and buried sewer line, according to the staff report.
The city received three bids for the project ranging from $581,700.00 to $859,171.01. Winkler Excavating submitted the low bid.
The project has been selected and prioritized in the city public works capital improvement program and was budgeted in the sanitary sewer utility enterprise fund.
The city had budgeted $530,333 for the project and received a $397,749.75 FEMA Hazard Mitigation Grant.
The required local match amount for that grant is split between a grant from the NorthWestern Energy and the Missouri/Madison River fund in the amount of $73,383.95. The remaining match funds of $59,199.30 is programmed in the public works capital improvement plan using sanitary sewer enterprise funds.
The project budget allocated $130,600 for engineering fees and construction inspection. The remaining $399,733.00 is allocated for construction.
The low bid for the project left a difference of $181,967, which will be split between the sanitary sewer enterprise fund and Park and Recreation funds, according to the city staff report.