Location: Cottage Grove, Minnesota
Client: South Washington Watershed District
The South Washington Watershed District (SWWD) saw a water quality concern in the Grey Cloud Channel, which had become cut off from the Mississippi River. The channel once flowed under a bridge, but the structure (referred to by locals as the “First Crossing”) was replaced long ago with culverts. Flow through the channel was completely cut off during an emergency road raise in 1965. This resulted in the stagnation of water and a decline in water quality, as evidenced by an increase in the number and severity of algal blooms, the degradation of fish habitat, the reduction of diversity in the fish community, and an increase in the number of invasive species.
The restoration of Grey Cloud Channel was identified as a priority project by SWWD and several state and federal agencies. Since 2011, the SWWD and HEI worked together to identify and implement a solution.
HEI worked with the District to prepare a feasibility study to restore flow through the Grey Cloud Channel. HEI evaluated several alternatives based on channel navigability, fish passage, water quality, immediate and long-term maintenance, sediment transport construction complexity, and project cost. A HEC-RAS model for the Mississippi River and the channel was created and used for the analysis.
With the feasibility study complete and with an abundance of information at its disposal, the District ultimately decided to construct a new bridge using a unique precast open bottom design. This alternative will restore connectivity between the Mississippi River and the channel, naturally improving the water quality by allowing it to flow more freely, reconnect fish movement, and restore recreational boat access in the entire channel.
HEI led the preparation of an Environmental Assessment Worksheet, public waters, and US Army Corps of Engineers (USACE) 404/Section 10 permitting; coordinated third-party review of sediment transport modeling; coordinated review by the USACE for impacts to long-term maintenance dredging within the navigation channel; and participated in solicitation of comments from local citizens, cities, and townships through public meetings.
Construction was completed in fall 2017. HEI completed a bridge design analysis, completed final design and plans, and provided construction observation for roadway improvements and bridge construction.
- A detailed report and cost estimates for various alternatives aids the decision-making process between stakeholders.
- A thorough analysis of alternatives for numerous criteria accelerated the acceptance of the project by various agencies.
- Improved water quality throughout the Grey Cloud Channel.
- New navigability and connectivity between the Grey Cloud Channel and the Mississippi River.
- Seamless environmental review and permitting.