Huge storage tanks offer relief from 35W flooding

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MINNEAPOLIS — By the time it's finished, you won't know it's there, but motorists should reap the benefits in terms of fewer floods on Interstate 35W in south Minneapolis.

MnDOT's 35W storm drainage project is the first of its kind in this state, and an effort to divert stormwater runoff into massive storage tanks along the northbound lanes of the interstate between 40th Street and 42nd Street.

Across the past four years, as traffic whizzed by, contractors have built six huge underground tanks with a combined capacity of 4.5 million gallons of water. According to Tim Nelson, MnDOT's project engineer, the tanks will hold onto the excess water until a storm ends and then pump it into the city's storm sewer system which leads to the Mississippi River.

"This project and these tanks are not going to completely prevent flooding on 35W, just reduce how often it does flood and provide more safety to the traveling public and the residents," Nelson told KARE.

Each tank is 85 feet deep with an exterior diameter of 53 feet and an interior diameter of 42 feet, according to Nelson. Contractors removed 35,000 cubic yards of dirt and rock and poured 25,000 cubic yards of concrete. That's roughly 2,500 cement mixer loads.

When water starts to back up on the interstate's storm drainage pipes, it will be showtime for the underground tanks.

"When the pipe backs up a little bit it will go over a weir pipe — essentially a half pipe. That water will go over the top of that and come crashing down at the first tank on the far south there and follow the slope here to the north."

He said all six tanks will fill at the same time and the water will be pumped out as soon as the city storm sewer system can handle it.

Two construction contractors, Nicholson and Kraemer, have been working on the $72 million project since 2019, and are now putting on the finishing touches. Nelson says they've done a great job dealing with the water table in the area and working in a narrow strip of land between the interstate and a retaining wall.

"Our contractor, Kraemer/Nicholson Joint Venture, has been excellent to work with and has been able to attack all these complications to build a nice concrete structure."

The storm drainage project is the last major element of the $239 million 35W/I-94 Downtown to Crosstown project, which opened to traffic a year ago.

Once the project ends, lane restrictions along that stretch of northbound 35W will be removed. MNDOT maintenance crews will still have access to the storage tanks from the highway using a special access lane.

The tanks have been described as "reverse silos" but will be locked up tight to prevent urban sewer explorers from gaining access.