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Lower St. Anthony Falls pool drawdown for Bassett Creek Tunnel Inspection

Posted: 1/8/2019

Expiration date: 2/8/2019


Purpose and need

The Bassett Creek Flood Control Project was constructed under the direction of the Corps of Engineers in 1979 and is managed by the city of Minneapolis, located in Hennepin County, Minnesota. The Project includes a 12-foot tunnel, the crown of which is under the normal pool elevation of 750.0 feet, and discharges into the intermediate pool of Upper and Lower St. Anthony Falls Locks and Dams. Operation of the project requires that "a thorough inspection of the inside of the tunnel will be conducted at least once every 5 years, except the portion of the tunnel under water which will be dewatered and inspected at least once every 20 years.” The last inspection of the underwater portion of the tunnel was in 2008. The project manager would now like to inspect the project every ten years. Inspection of the tunnel requires a drawdown of the intermediate pool.

 

Proposed action

To facilitate a periodic inspection of the Bassett Creek Tunnel, the intermediate pool at St. Anthony Falls must be drawn down thirteen feet to an elevation of 737.0 feet. The 2019 drawdown is targeted for February 25 – March 1, but the dates may deviate due to weather conditions. The drawdowns would occur outside the Mississippi River navigation season.

Drawdowns for the inspections would typically last five to seven days, and the pool would be expected to return to the normal operating elevation after approximately ten days.

The drawdown would occur gradually and the rate of pool fall must be kept under 1.5 feet per hour when the pool is outside the normal operating band. Minimizing the rate of fall reduces the potential for sloughing on the river banks and the dike constructed at the site of the former hydropower plant adjacent to Lower St. Anthony Falls Dam.

 

Questions on the project or comments on the Environmental Assessment can be directed to Derek Ingvalson at 651-290-5252 or Derek.S.Ingvalson@usace.army.mil. Please address all formal written correspondence on this project to:

U.S. Army Corps of Engineers, St. Paul District

ATTN: Regional Planning and Environment Division North

180 5th St. E., Suite 700

St. Paul, MN 55101

 

Environmental Assessment intermediate pool Lower St. Anthony Falls Minneapolis locks navigation Upper St. Anthony Falls