City studying inadequate I-35W storm water tunnels

Deep in the ground below I-35W are storm water tunnels 100 feet long and 12 feet in diameter. They carry runoff from West 39th Street through downtown to the Mississippi River, a process that takes about one hour.

I-35W drivers and Southwest residents can see that the tunnels are overcapacity, thanks to the geysers that erupt on the highway during fast, heavy rains, with associated flooding in Southwest neighborhoods such as Kingfield, Lyndale, Whittier and Stevens Square.

The situation is unsafe — and upcoming construction projects on Lake Street, I-35W and the Crosstown Commons are expected to make the situation worse. So, in July, the city and state formed a task force with neighborhood representatives to study possible solutions. The task force will meet twice more this fall.

Project Manager and Engineer Mitchell Sawh explained that the tunnel system was originally built to handle a "two-year" rainfall — the biggest event expected during a two-year period. He said the city wants the tunnels to move enough water for a 10-year event and ensure that a 100-year-event won’t flood homes.

Sawh said solutions could include:

– Adding water-storage containers along I-35W

– Temporarily flooding parks or playing fields

– Tearing out homes for new water-retention ponds

– Having residents incorporate rain gardens on their properties.

– Expanding a storm water tunnel.

Sauh said the most likely scenario, given cost and public sentiment, would be adding storage capacity, temporary flooding parks and expanding a tunnel.

Sawh said getting storm water away from the highway and surrounding neighborhoods could affect Mississippi River water quality. If water travels faster from Southwest to the river, more sediment would be dragged into the river, degrading water quality, Sawh said.

David Saddoris, Kingfield’s representative, attended the first task force meeting and said residents are worried about losing houses and harming water quality. He said he thinks the Crosstown Project is driving the city’s task force study of overflow solutions.

Saddoris said few neighborhood representatives came to the first task force meeting; Sawh said he has contacted many representatives and hopes more will show up at the second meeting in September.

Once the study is complete, the city will narrow the renovation options to three and start whittling them down from there.

Sawh said because city engineers would need coordinate any storm water tunnel renovations with the pending Lake Street, I-35W and Crosstown reconstruction projects, the storm water project’s date and cost have yet to be defined.