Six sites. Five phases. Four years. About $10 million-$15 million. That’s what makes up the Dane County sediment removal project in the Yahara Chain of Lakes aimed at mitigating the risk of flooding.
Dane County Executive Joe Parisi was joined by a select group of local and county officials at Lottes Park in Monona on Thursday, May 21, to announce the beginning of the work. Dredging equipment is in the water and is being prepared for the work that is expected to begin within the next few weeks.
“It’s an important milestone, and we’re really excited to be at this point,” Parisi said.
Severe flooding affected much of the county in the later summer of 2018, which catapulted the county into motion to devise a comprehensive mitigation plan.
“We’re seeing more rain events. We’re seeing rain events of greater intensity and greater duration,” Parisi said. “Two summers ago, we certainly saw the worst of it. We had record rainfall and record flooding that impacted our entire county.”
He noted that Dane County is the fastest growing county in Wisconsin, and while that is good economically, it presents some challenges as well.
“Every year, we lose more and more impervious surface and have less of an ability for the rain that falls to be absorbed,” he said of the water runoff that flows into the lakes. “We can’t get the water moving through the chain of lakes fast enough to stave off flooding.”
It is estimated that more than 8.5 million pounds of sediment enters the Yahara River and lakes each year from urban runoff.
A special task force identified five choke points along the lakes where large amounts of sediment prohibit the steady flow of water. By dredging each of these points, water can more easily flow through the chain of lakes, helping to prevent flooding.
The channel between Lake Monona and Lake Waubesa represents that first point.
The county hopes to remove about 40,000 cubic yards (or more than 3,000 dump truck loads) of sediment in the project’s first phase.
This location between lakes Monona and Waubesa is one of six sites the county will target in five phases to improve water flow, with each phase being carried out as Dane County secures permitting. Dredgit Corp. was awarded the $3.25 million contract for the first phase of the project. The goal is to complete most of the dredging by late summer, pending any changes related to the COVID-19 pandemic.
The project will use hydraulic dredging to remove 2-3 feet of sediment about 50 feet wide and about 1.5 miles long. The material will be pumped bout 3 miles away using a series of booster pumps to land owned by the Madison Metropolitan Sewerage District (MMSD). MMSD is allowing Dane County to utilize its land for the duration of the project. A dewatering basin will be built on this land to collect the sediment.
John Reimer, assistant director, Land & Water Resources Department, said the project should result in lake levels being lowered by about a foot.
“In the end, once these phases are done, the goal is – we did some modeling to compare – if these phases are dredged, how would it be in a 2018 flood year?” he said. “What we found was that these lakes would be lower by about 12 inches.”
The biggest benefit will be when all phases are done Reimer said. Right now, Lake Monona and Lake Waubesa have about a 6-inch water level differential. The Wisconsin Department of Natural Resources would like to see to it at 2 inches, which is about where it will be after the first phase is complete.
The type of sediment removal that will take place during this project is comparable to the county’s Suck the Muck initiative that removes phosphorous-laden sediment from local rivers and streams.
Parisi has included money in his 2020 budget to create a sediment removal crew and purchase the equipment needed for the county to do its own hydraulic sediment removal. This will allow the county to complete the rest of the flood mitigation plan in house.
The 2020 budget also includes $5 million to purchase equipment needed for sediment removal work and the staff to carry out the job. This will add four new positions dedicated to Yahara chain of lakes sediment removal work in the Land and Water Resources Department.
“We’re a little more nimble that way,” Parisi said. “We can better control where wo go, when we go.”
Reimer said the county will continue to use the equipment after the four-year project is complete.
“There will be maintenance dredging, other projects such as county boat launches,” he said.