Posted on September 2, 2021
Being on board Dane County’s “Dragon Dredge” is like riding on a slow moving, floating vacuum cleaner.
It doesn’t go fast or far and it sucks up dirt. Sometimes a rock gets stuck in it.
Dane County purchased the $650,000 Ellicott Dredge in March as part of a five phase, multiyear and multimillion dollar effort to decrease the risk of flooding along the Yahara chain of lakes. The dredge removes built-up dirt and clay, clearing bottlenecks in the Yahara River that link the lakes.
The 42-foot-long hydraulic dredge digs through years of compacted sediment exacerbated by urban runoff. Covering 400 cubic yards in a day, the bright red teeth on the cutter head fight their way through gummy clay. The pump sucks up rocks and, occasionally, a spare log.
“You’re at the mercy of the Yahara,” said Ryan Brockner, dredge laborer.
The idea to dredge the river to improve water flow — a unique strategy in Wisconsin — was borne out of the historic rainfall in August 2018 that hammered some areas with more than a foot of water and killed one Madison resident. The deluge caused over $154 million in damage and led to historically high lake levels.
Because of the muck, the channels connecting lakes Mendota, Monona, Waubesa and Kegonsa didn’t have the capacity to push water through the chain of lakes fast enough to keep up with rainfall.
“Water has nowhere to go but up,” said John Reimer, assistant director of the county’s Land & Water Resources Department. Flood waters rise and storm sewers back up, causing flash flooding.
Water comes into the Yahara chain of lakes faster than it goes out — taking two inches of rain over two weeks to leave the system. With the Dragon Dredge removing several feet of sediment, water will have more room and should drain through the system in about half the time.
It should also help Reimer manage lake levels and adhere to water levels set by the state Department of Natural Resources. The DNR has the statutory responsibility to regulate certain dams and create specific management requirements, which are included in the operating order issued to the owner of the dam. In this case, that owner is Dane County.
Reimer said the goal is to minimize the frequency of flooding. Dane County has seen flooding about every 10 years, he said, with major events in 2000, 2008 and 2018.
“We (hope to) go from 10 years and extend that out to 20 years … and mitigate those impacts so they’re not so frequent,” Reimer said.
Dane County Executive Joe Parisi said the county isn’t required by state statute to do this work. Though it involves technical skills and is expensive, he said it’s necessary for Dane County to tackle.
“We do this because someone needed to step up and take care of this because it was challenging, and it really dovetails in with a lot of our key lakes work, too,” Parisi said. “It’s really the only way we’re going to be able to prevent some of the future type of flooding that we saw in this last big rain event.”
Lakes like bathtubs
Lakes Mendota, Monona, Waubesa, Kegonsa and Wingra form a chain of lakes connected by the Yahara River, eventually streaming into the Rock River north of Janesville.
Madison is closely linked to its five lakes. The area’s first residents built villages and farmed around the lakes. Today, the city of 269,840 — the fastest growing in the state of Wisconsin, according to the 2020 census — builds and expands its borders around the bodies of water.
A chain of lakes “is unusual but not unique,” said Emily Stanley, a University of Wisconsin-Madison professor in the Department of Integrative Biology and Center for Limnology. But a chain of lakes in an urban setting with a watershed that’s dominated by agriculture is uncommon, she said.
Stanley described the lake system like a series of bathtubs with one small outlet. Sediment that used to be topsoil accumulates in low spots in the basin and clogs the drain.
“The physical structure of the lakes and the connecting channels is not helpful,” Stanley said. “They have narrow exit points and they are harvesting water from big watersheds.”
The area’s flat landscape also makes it harder for water to move through the system, Reimer said, and aquatic vegetation can impede movement, too.
The county estimates that more than 8.5 million pounds of sediment enter the Yahara chain of lakes every year because of urban runoff. When all of that dirt gets stuck in the choke points between lakes, the flow of water is impeded.
Parisi said that number makes the project’s goals more “tangible.”
