Lake Bronson Dam replacement project
By: Janet Gronert of SRF Consulting, in cooperation with Jerilyn Swenson, State Aid Bridge Program Engineer and Dave Conkel, State Aid Bridge Engineer
Project history and progress
Picture showing existing bridge 7498 taken sometime in 1937.
The Minnesota Department of Natural Resources (MnDNR) is currently halfway through its largest ever project to replace the aging Lake Bronson Dam at Lake Bronson State Park in Kittson County.
The original dam itself was constructed in 1937 was recently categorized as a high-hazard dam, with risks to the downstream community. CSAH 28 passed over the dam with a single lane bridge built integral to the 85 year old dam. The existing spillway was undersized and unable to pass the required flood flows. The dam had a history of seepage that led to internal erosion of the foundation, and potential risks for failure.
Barr Engineering Co., Freese and Nichols, and SRF Consulting Group were selected to design the replacement which included a new labyrinth spillway sized to double the hydraulic capacity, and a 1400 linear foot cutoff wall 85 feet deep, a new county bridge and roadway approach that spans the spillway and meets current design standards.
Artist's rendering of the Lake Bronson dam after future reconstruction. (Photo credit: Barr Engineering/SRF Consulting)
Labyrinth spillway design
During large rain events, the smaller capacity of the historic dam would cause Lake Bronson water levels to rise, affecting park property and stressing the structure and embankments. Spillway capacity is doubled with the labyrinth geometry. The new zig-zag shaped weir provides an increase in crest length over the limited spillway width, which increases the flow capacity from the upstream reservoir. The weir requires less free board in the upstream reservoir and will allow more stability in the lake levels. The use of a weir eliminates the need for manual operation of traditional gates, which often happened during poor weather or winter storms,and at any time, day or night.
Seepage cutoff wall
The project reduces detrimental water infiltration into and through the embankments with an 80 foot by 1400 linear foot seepage cutoff wall between the current roadway and the lake. Contractors excavated a trench, mixing bentonite and cement with the soil to make a solid wall designed to prevent seepage. Seepage through the embankments and under or around the spillway and labyrinth will be eliminated.
3-D view of cutoff wall upstream of labyrinth and spillway. (Photo credit: Barr Engineering)
The cutoff wall was constructed by DeWind One Pass Trenching utilizing its one-pass equipment. DeWind installs a large cutting boom and then cuts into the soils to the planned depth. Once depth is reached, they add bentonite, cement, water, and other additives to achieve the type of wall and properties desired. The Lake Bronson wall is designed to extend into a lower clay till layer with depths between 70 and 85 feet.
DeWind One-Pass Trencher with Cutting Boom (Photo credit: Barr Engineering)
CSAH 28 and Bridge 35541
The bridge spans 125 feet over the spillway, just downstream of the labyrinth and cutoff wall. Traditional parapet style abutments are supported by a thickened portion of the spillway slab and are integral with the spillway walls upstream and downstream from the bridge. The superstructure uses typical MN54 prestressed girders. When complete, the bridge and roadway meet current bridge standards with a bike and pedestrian trail on the east side of the bridge facing the lake. The exterior of the bridge parapets provides a relief that mimics some of the details of the original dam and barriers. Some grade raise was required for CSAH 28 to account for the longer span and deeper superstructure. Reconstruction includes 1700 linear feet of CSAH 28 and the adjacent lakeside bike trail.
Picture showing new 54 inch prestressed concrete beams for Bridge 35541 spanning the Spillway walls during construction. The Labyrinth Weir is shown upstream of the bridge. (Photo credit: SRF Consulting Group)
Ownership and funding
The MnDNR manages and operates the dam which is owned by the State of Minnesota. The MnDNR is responsible for the design and reconstruction of the new structure, which is in its second and final year of major construction.
Ames Construction bid $28.5 million for the project. It is funded through bonding appropriations of $20 million in 2020, and $4 million in 2023. CSAH 28 owner, Kittson County, received a $2 million grant from the State Park Road Account to build the bridge and road adjacent to the lake and over the dam. The remaining funding came from MnDNR operating budget funds.
Timeline
Construction started in October of 2024 with a 15 foot lake drawdown. Most of the cutoff wall and spillway construction work occurred through the summer of 2025, with some continuation into 2026. The contractor finished setting girders on the coldest day of 2025 and plans for a bridge deck pour in early May 2026. The project is expected to be substantially complete by December 1, 2026. Seeding, planting, cleaning the site up and getting it ready to be a park again will be finalized by July 2027.
Summary
The Lake Bronson Dam replacement project is a major MnDNR effort to address safety issues with the aging 1937 structure. With a new labyrinth spillway and deep seepage cutoff wall, the project significantly improves hydraulic capacity and long-term stability. The reconstruction of CSAH 28 and bridge replacement brings the facility up to current standards. Improved trails, parking and fishing pier access will be fully usable in late 2027.