The city and state will implement Plan A next week in a combined effort to address the beach rock that is clogging drain pipes and causing flooding and periodic lane closures on Interstate 35 through downtown Duluth.
"We all agree this is a great first-time-around attempt at fixing this," said Tom Johnson, a senior engineer with Duluth public works and utilities. "If this doesn't work we'll keep exploring options."
Three major drain pipes that run under the freeway between Second and Third avenues east and empty into Lake Superior have become increasingly clogged and problematic.
Engineers with the Minnesota Department of Transportation and city met this week in the aftermath of a lane closure on Monday, when flooding closed the off-ramp from southbound I-35 to Lake Avenue and Fifth Avenue West. On Thursday, city workers pumped still-remaining water out of the drains.
The pipes drain into Lake Superior with outlets positioned about at either end of the mosaic tile wall along the Lakewalk. Over time, wave action has filled those pipes with beach rock to the point they've become clogged, backing up at the catch basins along the freeway during periods of heavy rainfall - particularly in the spring.
Beginning next week, a hydro-vacuum truck used by the city to clean sanitary and storm sewers will be parked on the Lakewalk and used in an attempt to reduce the buildup of rock and other material.
Additionally, MnDOT will fabricate new outlet protection structures that will replace failing screens on the ends of the three pipes. The new systems will feature heavy steel in a louvered-baffle system designed to allow water out, knock down wave energy and reduce the likelihood of rocks riding waves into the pipes.
An inspection earlier this week found that the pipes are filled with rock in some places, but appear to be less clogged than the original estimates of being half to three-quarters full.
"There isn't as much as we thought," said Perry Collins, a MnDOT operations engineer based in Duluth. "We're going to try low-cost options first."
The segment of Lakewalk will be closed during the daytime hours beginning Monday for up to two weeks. Johnson said he expected work would start Wednesday, when the weather is expected to be warmer - something the hydro-vacuum truck requires.
If the vacuum mechanism that uses water and suction to pull out sediment does not do the trick, Plan B might include sending laborers into the large pipes to dislodge the material.
"These storms can move some pretty big rocks," Johnson said. "It's not your typical clean-out of a pipe. The debris and rock is heavier and harder to move, but I feel pretty confident this is going to work."
Lakewalk users will be rerouted up into Lake Place Park to bypass the work. Crews will work during the day, reopening the Lakewalk for evening hours and weekend access.
The hydro-vacuum truck features 500 feet of hose, and the pipes are about 170 feet in length. MnDOT already had a project tentatively planned for 2021 that would correct the issue, but the problem has disrupted that timeline by shutting down sections of I-35 multiple times in the past two years - most notably when it forced plow trucks to push water off the freeway during a March 2016 storm.
Johnson said the combination of winter rain while the ground remains frozen is serving to exacerbate the issue.
"We're having to deal with rain events year-round," he said. "It makes it interesting and exciting for those of us having to brainstorm solutions."