City Council Meeting Date: November 21, 2023
To: Mayor and City Council
City Administrator
From: David Bennett, Public Works Director/City Engineer
Title
Consider Resolution Approving Ames Mill Dam Removal Feasibility Study Recommendations Option 3 - Long Rock Rapids and Authorizing Phase 2 Exploration.
Body
Action Requested:
The Northfield City Council considers a Resolution Approving Ames Mill Dam Removal Feasibility Study Recommendations Option 3 - Long Rock Rapids and Authorizing Phase 2 Exploration
Summary Report:
As part of the 2020 Riverfront Enhancement Action Plan, the Ames Mill Dam Reconstruction/Removal study will explore the impacts related to the removal/replacement of the Dam to activate the Cannon River to more uses.
The Cannon River is a designated State Water Trail, and the Dam is a barrier to canoeing, kayaking, paddle boarding, and is environmentally derogating to the river ecological health. The dam also has a historical designation that will be analyzed in conjunction with the processes established with State of Minnesota.
The City hired Barr Engineering, an experienced firm with work related to dam removal. The phase 1 study looked at different alternatives and impacts related to the dam removal and identify a preferred alternative based on input from board and commission, public, and regulatory input (DNR, SHPO, Army Core). This feasibility study work took place over the last year.
This feasibility study examines the viability of four options for dam replacement. These options are as follows:
• Option 1: Maintain the dam structure as is.
• Option 2: Remove the dam and replace it with rock rapids at a 3- to 5-percent slope using a compact footprint (Figure 4-1). The rock rapids would consist of 10 boulder weir structures spaced every 25-30 feet apart. This design would improve recreational and ecological benefits and minimize water level changes. The rapids would be around 250 feet long, extending from Ames Mill Park to the Fourth Street Bridge.
• Recommended Option 3: Remove the dam and replace it with rock rapids with a 2-percent slope using an expanded footprint. The rock rapids would consist of 10-15 boulder weir structures spaced every 45-55 feet (Figure 4-2). The larger footprint and flatter slope would allow additional recreational and ecological enhancement opportunities. The rock rapids would be around 550 feet long, extending from the Fifth Street Bridge to the Fourth Street Bridge.
• Option 4: The same as Option 3, except a portion of the dam structure adjacent to Ames Mill would remain as a visible reminder of the historic dam (Figure 4-3).
Stakeholder / Public Input
Stakeholders include ownership (Post/DNR), City Council, City commissions, historical (SHPO), permitting (DNR/USACE), and funding/ecology (DNR) entities, as well as the public. Stakeholder meetings were held between July 27 and October 11, 2023, to inform option selection and design.
Post is generally supportive of options to remove the dam because the dam no longer serves a functional purpose to the company. Post requests that any alternative to remove the dam include detailed engineering analysis of the potential impacts to the Ames Mill building structural stability and mitigation of any issues identified.
Feedback from members of the public was generally supportive of replacing the Ames Mill Dam, with comments focusing on the potential for improved river ecology, aquatic recreation (boating), and public safety (removal of drowning hazard). Some respondents referenced the historic nature of the dam structure and its importance as a symbol of Northfield’s founding as reasons to keep the dam in place.
Recommendation
Based upon the comparison of impacts and other factors associated with the alternatives that would satisfy the project purpose and need, Option 3 is recommended as the alternative that best meets the project purpose and need with the “least remaining harm, after mitigation, to the protected activities, attributes, or features” that qualify the property for NRHP eligibility.
Although the estimated cost of Option 3 is significantly higher than that for Option 2, Option 3 more fully meets the project purpose and need in every category except for cultural resources.
Options 3 and 4 have similar project benefits and costs, but the added design complexity of retaining a portion of the dam structure in Option 4 makes it less desirable for the following reasons:
§ It is a potential safety hazard for recreational users.
§ More-complex flow patterns and reduced river width could impede aquatic organism passage under some flow conditions.
§ It is more complicated to design and construct rapids around the remnant.
§ There is less room to isolate portions of the river to construct in the dry.
§ If the remnant were to fail in a large flood, it could damage the adjacent mill building.
§ It would concentrate flows through the east opening of the Fourth Street Bridge, which could create stability concerns.
§ Cultural resources mitigation permitting may require retaining some remnants of the dam structure even if the City elects to pursue Option 3, but the benefits of doing so can be more fully considered during the detailed design and permitting process.
The recommendation is that the City consider pursuing full removal of the Ames Mill Dam and replacement with an extended rock rapids structure (Option 3).
Next Steps
This the first step in a multi-step process related to removal of the dam. This approval would move the City to step 2 related to ownership transfer and exploring grant funding. Depending on how the ownership transfer and funding come together this could be a 5-10 year process to get to construction. Ownership would not be transferred until funding commitments were secured and clarified.
Alternative Options:
Four options were explored identified above.
Financial Impacts:
2023 - Option 3 Removal of Dam with Long Rock Rapid $10 Million
Tentative Timelines:
2024-2026 Phase 2