Major dig takes aim at stormwater from Woodbury, east metro
Imagine trying to bury short cylinders end to end underground across a backyard.
Now try doing something similar — albeit on a much larger scale — with sections of concrete pipe at depths of roughly 40 feet across a full mile of sometimes rocky soil.
That’s where the water goes after it runs off roofs, down the driveways and into the storm sewer.
Building a 6-foot diameter stormwater pipe to prepare Woodbury and Cottage Grove for monstrous storms is part of the work that’s creating a mess of dusty roads and traffic closures in the southern part of Woodbury going into Cottage Grove.
In conjunction with improvements this summer and fall to 70th Street and Keats Avenue and the addition of a roundabout to that intersection, crews are installing the large pipe to move overflowing stormwater from the northern portion of the South Washington Watershed District — including parts of Lake Elmo, Woodbury and Oakdale — south through Cottage Grove’s East Ravine area along Keats to a drainage field near the Mississippi River.
From a basin northwest of 70th and Keats to a point southeast of that intersection, roughly 5,000 feet of pipe is being laid eight feet at a time.
The $8 million watershed district-led project is one piece of the entire stormwater system that will give cities and homeowners peace of mind in case a “100-year” storm dumps more rain sooner than previously anticipated.
New data from the National Oceanic and Atmospheric Administration (NOAA) regarding the changing climate related to storm intensity reveals that Woodbury is seeing about a 20 percent increase in the amount of rainfall it would expect to see in a 100-year event.
Drainage basins serving the area will hold stormwater that accumulates if 6.3 inches of rain falls in a 24-hour period. New predictions say more severe rainfall of 7.5 inches triggers the need for the new overflow stormwater pipe.
Woodbury has significant factors built into the existing, original system already, but after a big storm in October 2005 that caused flooding in some parts of the city, engineers have been inspecting the entire system to avoid another headache.
“That was a really good test of our system,” Woodbury Public Works Director Klayton Eckles said. “We did find some problem areas and the council was very responsive in terms of putting additional resources toward modeling, engineering and studying the problem.”
The whole region of central Washington County is unique — it doesn’t have a stream, creek or any formal drainage to the Mississippi River. Water is supposed to go from the high ground of Lake Elmo all the way down to Cottage Grove through a series of basins along the way without flooding any homes.
In the pre-settlement days, those lakes didn’t have outlets so they’d naturally flood or dry up depending on the year.
“Before homes were here, if these lakes flooded, nobody cared,” Eckles said. “Now obviously we need to make sure that the water can flow out of these areas.”
Currently, water from Wilmes and Colby lakes go to Bailey Lake and onto a lift station before it could even get to Cottage Grove.
“The concern is that if and when we fill that basin up it would overflow into Cottage Grove,” Eckles said. “The stream isn’t set up to take this water that comes all the way from Lake Elmo. Cottage Grove isn’t set up to handle that as development occurs.”
The new system is designed to serve a roughly 24 square-mile drainage area. Only 20 percent of that area is developed but both cities predict future housing growth that comes with hard surfaces and more water runoff.
The large pipe requires the heft of large construction equipment to move tons of sandy soil and buried rock but also the finesse of hand tools to move small amounts of dirt as pipe sections are jacked into place and sealed with just a large rubber seal.
Precision and patience are critical.
Once the pipe is buried, crews must carefully refill the hole that was dug to lay the pipe. That can only be done 8 inches at a time. One layer of fill is packed before another 8 inches of loose soil is dumped into the hole and packed.
“It’s a slow-moving project,” David Thompson, resident project representative of HDR Construction, told Cottage Grove officials during a recent tour of the project. “It takes days and days to put the dirt back in the hole.”
The project is being completed in multiple phases. Crews will finish the $3.1 million Phase 1 this construction season. The subsequent four phases will be done in the coming years and completed by 2019, watershed district Administrator Matt Moore said, extending the stormwater system south toward the Mississippi River. It will pass through Cottage Grove Ravine Regional Park as overland flow, beneath Highway 61 in a pipe and across 3M-Cottage Grove property as overland flow, Moore said.
“What we’re really building it for and planning it for is that rare event,” Moore said.
The section of pipe being installed this year will be temporarily capped while work continues on the next phases of the project.
The mile of pipe is expected to be completed by October, but Keats Avenue and 70th Street will not be fully reopened until November, said Cory Slagle, Washington County’s engineering and construction manager.
The road and stormwater projects were coordinated so that as the pipe and other underground utilities are buried, road crews can follow behind and pave the realigned 70th Street.
In response to the 2005 storm, Woodbury inspected 3,000 locations of storm pipes and drainage paths and found 20 to have flooding issues.
Problems have been fixed. The city will finish the final phase of inspections and retrofit projects next year.
“Because we did have that event, we are probably quite a bit more prepared than a lot of our neighbors,” Eckles said. “We experienced some problems and we’ve taken all these proactive steps to try to make sure it doesn’t happen again.”
Eckles said Woodbury officials feel confident the city has a robust, well-designed system that’s prepared for even more than NOAA’s predictions.
“Even with this revelation that we’re going to be experiencing larger, more severe storm events on a more frequent basis, we think we’re ready for that,” he said.