UW computer model reveals vulnerability of Madison's Isthmus during historic rain events

Lee Bergquist | Milwaukee Journal Sentinel

This week’s devastating floods in Madison — and the potential for more rain this weekend and beyond — underscores the vulnerability that record rainfall may pose on a populous strip of land that rests between Lake Mendota and Lake Monona.

Some roadways in the Isthmus are already closed due to flooding, sandbags are being filled if needed and the National Guard is on call. City officials have released a map showing a broad swath of potential flood impacts. Dane County officials said Friday their initial assessment has pegged damage at $108 million.

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Densely urbanized, with many low-lying areas, the Isthmus lies between two slow-draining lakes that flow into the Rock River. That has prompted longstanding concerns about flooding.

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Meanwhile, development has made it more difficult for heavy rains to soak into the ground. Climate change has also meant increased precipitation trends in Wisconsin.

With that in mind, researchers at the University of Wisconsin-Madison used computer modeling in 2014 to approximate the impact of a historic storm in 2008 near Madison, had the rain fallen on the central part of the city instead.

It was not a pretty picture. Huge portions of the Isthmus would be flooded, especially areas near the two lakes and the Yahara River. As the simulation played out, it showed flooding that lasted days, maybe weeks.

One of the researchers, Ken Potter, now a professor emeritus of civil and environmental engineering, said the research should be viewed with the understanding that a simulation today would be better because of the adjustments made from the initial work.

Nevertheless, he said the modeling serves as a cautionary tale: Dane County and other communities should be thinking about improving stormwater management to keep water on land and not spilling into storm sewers and taxing watersheds.

Heavy rains flood streets in Madison and Dane County Katy Brunette and Keith Furman shot video of flooded streets in Dane County. There were reports of 10 to 13 inches of rain overnight.

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Potter says the state took a step backward earlier this year when the Republican-controlled Legislature rolled back regulations that allowed for more development of isolated wetlands, which play key roles on retaining water.

Lawmakers and business interests said changes were needed to allow property owners to use more of their land. Sufficient protections are in place to protect against flooding, they said.

“We haven’t developed a very large percentage of the Mendota watershed yet,” Potter said on Friday. “But we are having a big impact.

“Lake levels are rising. The amount of water going into the lakes is increasing. If we keep developing without controlling that runoff, it could get very, very bad. And if you couple that with climate change possibilities, then we are in real trouble.”

Since 1895, 11 of the 20 years with the most precipitation have occurred since 1970, according to data provided by the Wisconsin State Climatology Office.

Precipitation on average across Wisconsin last year totaled 37.54 inches — the eighth highest in 122 years. Rainfall the year before averaged 39.38 inches. That ranked second over the period, figures from the climatology office show.

Between 1990 and 2010, development in Dane County increased by an average of 2,300 acres per year, or 40 percent, according to a 2017 report of the Dane County Lakes and Watershed Commission and the Capital Area Regional Planning Commission.

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With the help of students, Potter and colleague David S. Liebl, who worked in engineering professional development at UW before his retirement, created computer models using National Oceanic and Atmospheric Administration rainfall radar data from 2008 flooding in the Wisconsin Dells and Baraboo areas.

The 2008 storm was different — more than 12 inches of rain fell over a seven-day period. The National Weather Service estimates a comparable amount fell on parts of Dane County on Monday and Tuesday in a 24-hour period.

The 2008 storm attracted national attention when waters breached Lake Delton and emptied the lake.



“2008 was worse, but this one is pretty bad,” Potter said.