Janesville49.4°

Delavan Lake dredging project well under way

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Catherine W. Idzerda
September 19, 2011
— Scotland gave us the Loch Ness monster.

Hollywood gave us the creature from the Black Lagoon.


Delavan Township’s watery nuisance is less glamorous but more real: sediment build-up.


Earlier this year, the town board approved a $1.46 million contract with JND Thomas, a California company, for dredging that will remove sediment from an inlet leading into the northern end of Delavan Lake.


If the phrases “dredging project” and “sediment build-up” make you want to turn immediately to the comics page, consider this: The lake’s health is crucial to the economy’s health.


Keeping that in mind, here are the top five things you need to know about the inlet dredging project.


1. The basics. JND Thomas is dredging a 3,000-foot navigation channel in the inlet. The channel will be dredged to a maximum depth of 5 feet or hard bottom, whichever comes first.


The area closest to the Highway 50 bridge will be dredged to about 6 feet to help extend the project’s life.


About 45,000 cubic yards of sediment will be removed from the inlet, drained of water in a process called “dewatering” and taken to a landfill approved to handle such waste.


Much of the water will be returned to the lake.


2. The history. The inlet was partially dredged in 1989. By 2008, the sediment had returned and was between 2 and 5 feet deep.


Here’s the problem: The watershed for Delavan Lake is about 26,000 acres, according to a Wisconsin Department of Natural Resources environmental assessment of the project.


All of the fertilizer, topsoil and construction runoff from those 26,000 acres ends up in the lake.


“The lake has a much larger watershed than Geneva Lake,” said John Olson, town administrator.


3. Why it matters. If your local economy depends on a particular attraction, you had better keep it in working order.


“The inlet dredging has a much larger scope,” Olson said. “It’s about overall lake health.”


The inlet is part of a 210-acre wetland that serves as a filter for the watershed, keeping sediment and fertilizer—in particular, phosphorous—out of the lake.


If that filter fails, it sets off a series of ecological dominoes that will eventually destroy the lake’s quality.


First, sediment and excess phosphorous end up in the lake. Phosphorous promotes algae growth. Algae blocks the sunlight, causing aquatic plants to die. The plants’ decomposition process uses oxygen. The only fish that thrive in deoxygenated water are carp.


And when a significant part of a community’s economic engine is filled with carp and murky sediment, it’s bound to fail.


4. Project progress.


As of Sept. 9, JND Thomas has completed 78 percent of the work, said Peter Berrini of HDR Engineering, the company hired to oversee the project.


Berrini keeps the community updated with detailed progress reports on the town’s website, www.townofdelavan.com.


So far, JND Thomas has removed more than 35,375 yards of sediment from the inlet, and that includes about 26,000 pounds of phosphorus, according to Berrini’s report.


5. What’s next.


The project is expected to be finished about Nov. 1, town officials said..


Dredging the inlet isn’t a permanent fix, Olson said.


“It will probably have to be done in another 10 to 15 years,” he said.


The Delavan Lake Watershed Initiative Network and the Delavan Lake Improvement Association have been working on a variety of projects designed to reduce the amount of runoff due to soil erosion and other issues.



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