Why 4 Inches of Rain Caused so Much Flooding in Wisconsin

A lot of rain – but maybe not as much as you think – fell Aug. 20 on the shallow, 281-square-mile basin of land that drains into Madison’s lakes Mendota and Monona.

Most of the 12- to 15-inch deluge that drenched parts of western Dane County flowed away from Madison toward the Wisconsin River.

Only about 4 inches fell across the lakes’ watershed, according to the National Weather Service. Every year, there’s a 1-in-10 chance of a similar rain.

But that 4 inches swelled Madison’s lakes enough to flood streets and homes, and to prompt worries of a dam break that could have been devastating for the densely populated Isthmus and areas around Lake Monona, the Wisconsin State Journal reported.

How could 4 inches of rain cause so much trouble? The answer lies in a decades-old dispute – now being revived – about how much water Lake Mendota should hold and how much higher than the Isthmus the big lake’s water level should be maintained.

The Tenney dam holds Lake Mendota about 5 feet higher than Lake Monona on the downstream side of the Isthmus under a state order that sets minimum and maximum levels for all of the Yahara lakes.

Before a single drop of rain fell on Aug. 20, Lake Mendota was nearly a foot higher than the official maximum. For most of the summer, in fact, the lake was 8 to 9 inches higher than it should have been in part because underwater plants and delays at a small dam hindered efforts to move excess water downstream.

But high, slow moving water is nothing new. The lakes have been above their maximum levels more and more frequently in recent years.

Lake Monona was already 12 inches over its state maximum level when an Aug. 20 rain swelled Madison’s lakes enough to cause serious flooding. Lake Monona was nearly 3 feet over the maximum Friday as city workers arranged oversize sandbags and pumps near the Monona Terrace convention center.

Holding lake levels down is difficult for two major reasons: Wetter weather related to climate change, and more than a century of construction on filled wetlands and other soil that could otherwise absorb rainwater and keep it from running straight into the lakes, county officials say.

An analysis released last week by one UW-Madison scientist suggests another significant driver of the current flood was how high Lake Mendota was when the rain started.

Flash floods following the Aug. 20 rain damaged hundreds of vehicles and homes and pushed Lake Monona to its highest level in history, requiring more than 100,000 sandbags to protect property.

Nearly two weeks later, several Madison streets remain submerged, and it will take weeks, or maybe longer, before the water recedes and the lakes and storm sewers can safely absorb a heavy, prolonged rain.

Starting near Dane County’s northern border, the Yahara River gathers snowmelt and rainwater and carries it south on a slow meander. It courses first through Lake Mendota, then through the Tenney dam floodgates, and across the Isthmus. After that, it runs though lakes Monona, Waubesa and Kegonsa before emptying into the Rock River near Janesville.

How high to keep Lake Mendota’s normal level has been the source of a persistent policy conflict.

“The issue of the level of the lake and where you start has been a political football for years,” said Ken Potter, a UW-Madison professor emeritus who has been at the forefront of research locally on flooding risk.

People who live on Lake Mendota and owners of large boats like the lake high. It helps them avoid the costs of extending boat piers farther from shore, and expensive keel and propeller repairs from running aground in shallow water.

But around Lake Monona and the other downstream lakes, property owners say they suffer flooding and erosion because too much water is let out too quickly from Lake Mendota after a heavy rain to ensure the bigger lake doesn’t rise so high that it threatens the dam.

They say a lower Lake Mendota could absorb more rain without risking the dam, so water could be released more slowly and flow downstream without damaging their land.

And conservationists say unnaturally high water levels have eliminated shallows needed by some fish and destroyed wetlands that previously acted as a sponge following heavy rains.

There have been several unsuccessful pushes to lower Lake Mendota’s level by a few inches or all the way down to where it stood before the first dam was built to power a mill in 1847.

Potter said he hopes the recent flooding will prompt a renewed public discussion of how the lakes are managed.

Such talks have happened before, but since then engineers have built computer systems to predict how water depths and shorelines in each lake would be affected by possible changes in management goals under myriad weather conditions.

“It’s something we have to do,” Potter said. “These are societal questions about how we make trade-offs on things like lake levels, and how society balances competing concerns about things like flooding and boating.”

Another flood-related conflict is between developers and conservationists in local governments who have sought standards for building and landscaping that would allow more rain to infiltrate the soil instead of running quickly off roofs and pavement into the lakes, carrying pollutants and increasing flood risk.

Developers say the standards would unfairly increase their costs and make new homes more expensive. A better solution would be to pool taxes, fees and other money to pay for regional basins that would collect rainwater and allow it to flow into soil, said Robert Procter, spokesman for the Realtors Association of South Central Wisconsin.

Last year, a panel of consulting engineers and others recommended the county require that new developments not increase runoff at all. They proposed mechanisms aimed at addressing the concerns about costs, but state business groups persuaded Gov. Scott Walker and lawmakers this year to forbid higher standards.

The Wisconsin Realtors Association’s Tom Larson said the group would support stormwater controls that didn’t add to costs of real estate.

As Wisconsin’s climate grows wetter and more land is developed, the lakes have exceeded state maximum levels more frequently, said John Reimer, a civil engineer for Dane County who manages flows through the Tenney dam and two smaller ones downstream.

In 1979, the state Department of Natural Resources ordered a summer minimum and maximum level for each lake at the request of Madison and Dane County, department spokeswoman Raechelle Belli said.

But for at least the last 11 years, the DNR hasn’t taken any action about violations of the orders for Lake Mendota, Belli said.

“We treat the Yahara chain of lakes as a system, not as individual lakes,” Belli said. “As such, the levels will fluctuate frequently depending on the situation.”

Reimer said he’s unaware of DNR action on violations of the orders for any of the lakes.

County Executive Joe Parisi said there should be discussion about lake levels.

“Given recent events and our changing climate, I think the state should revisit, with input from stakeholders, the current guidelines to determine the proper levels moving forward,” Parisi said.

The DNR said it can’t consider changes until someone files a formal petition. No petitions are pending.

A few years ago, Potter and others analyzed what would have happened if a heavy rainstorm that dumped 14 inches of rain on Sauk County in 2008 had hit Lake Mendota. The analysis found the lake could have topped the dam by 3 feet, sending water rushing downstream at five times the normal rate. It would have overwhelmed the Yahara River and inundated much of the Isthmus.

Last week, another UW-Madison civil engineering professor analyzed radar and rain measurements from the Aug. 20 storm which, according to at least one unofficial measurement, dumped even more rain on western Dane County.

If the heavier rain that fell 15 miles west of the Madison lakes had instead hit the lakes’ drainage basin, it “would have been absolutely catastrophic for lakefronts and low-lying parts of Madison,” Daniel Wright said in the report.

Wright estimated that 4.5 inches actually fell in the lakes’ basin – about half of what fell to the west. Artificially high lake levels, developed land and increasing rainfall linked to global warming make Madison vulnerable, he said.

“Madison needs to address its vulnerability to extreme rainfall if it wants to prepare for the future,” Wright said in the report.

Reimer, the county dam manager, said 2018 started well from a flood prevention standpoint. Winter rain had swept snowmelt through the lakes system and away by April, leaving Lake Mendota within the state minimum and maximum.

But by mid-June, heavy rain flooded land around Lake Kegonsa, the southernmost lake.

Reimer said unusually heavy growths of aquatic plants in the Yahara River downstream of Lake Kegonsa were slowing the river’s flow so much that it was difficult to bring water levels down.

The county obtained an emergency permit from the state and harvested that stretch of river for the first time, he said. As many as eight floating harvesters worked there for two weeks.

But there was another sticking point. New personnel were running Stoughton’s dam downstream from Lake Kegonsa, and for a time they interpreted state rules in a way that prevented a fast enough release of water, Reimer said.

It took a month and a half to lower Kegonsa to its state maximum level. During that time, Reimer had to use restraint in releasing water from Lake Mendota or risk worsening problems downstream.

The big lake rose 8 to 9 inches above its state maximum, and it never came back down – not even after Lake Kegonsa had finally been lowered in late July in the midst of six to eight weeks of dry weather leading up to the late-August rain and flooding.

The day before the Aug. 20 rainstorm, lakes Mendota, Monona and Waubesa were all about a foot higher than their state maximums.

The rain came. The lakes rose. And it became clear that once again the river was moving too slowly, Reimer said.

For five days starting on Aug. 22, eight harvesters worked 12 hours a day in the river between Lake Waubesa and Lake Kegonsa. Two cutters worked downstream of Kegonsa.

The flow out of the lake system doubled, and lake levels began to stabilize.

Weed-cutting between Lake Waubesa and Lake Kegonsa is supposed to be the top priority because of its importance in flood prevention. But harvesters are also needed to carve channels through lake weeds for boats, Reimer said.

Usually two or three rigs keep the river moving downstream from Lake Waubesa, but this year they couldn’t keep up, Reimer said.

Unnaturally thick vegetation has long plagued the lakes, which are over-fertilized by phosphorus that runs off the land, especially farm fields. Reimer said he’s concerned this summer’s abundant underwater crop could become the new normal.

“This may be the future,” Reimer said.