How A 30-Year Rain Event Became a 100-Year Flood

Lake Monona, Madison, WI. Photo: A. Hinterthuer

In case you’ve somehow missed the news, let’s just say that it’s been wet in Wisconsin over the last week. Really, really wet

Here in Madison we’re still holding our breath, filling up sandbags and hoping no new major rain event pushes water-levels even higher. Already, our lakes have hit near-record water levels, causing floods in low lying areas (like our basement!) and prompting beach closures and slow-no-wake zones for the entire chain of lakes.

But, says, Daniel Wright, an assistant professor in the department of Civil and Environmental Engineering here at UW-Madison, it could have been so much worse. 

Wright recently wrote a report that says the August 20th storm “clearly demonstrated Madison’s vulnerability to extreme rainfall.”

The report is excellent and you should read the whole thing here, but we’ll highlight the main points below. 

In Madison, At Least, It Could Have Been Much Worse

First, some basics. All creeks, rivers and lakes have watersheds. The Yahara watershed is 281 square miles of farmland, urban development and a little bit of wetlands that survived the land-use change to farms and cities. Any rain that falls on those 281-square miles and starts flowing downhill and downstream eventually ends up in Lakes Mendota, Monona, Waubesa and Kegonsa. 

So, obviously, a lot of rain raises the levels of our lakes. But, says Wright, while the amount of rain that fell in places like Cross Plains and Black Earth on August 20th was historically massive, the amount that fell in the Yahara watershed wasn’t. 

Left map: National Weather Service radar and citizen rain gauge observations for Aug. 20, 2018 storm. Right map: “Worst Case Scenario,” if the storm had occurred 15 miles to the northeast, directly over the upper Yahara watershed. Radar data courtesy of Dr. Bong-Chul Seo at the University of Iowa.

The image above highlights the part of the Yahara watershed that is upstream (i.e. flows into) Lake Monona and the “rain map” of the August 20th event. You can see that, while the western edge of the Yahara watershed got a ton of rain, most of the watershed received between 3 and 6 inches. While that’s still a lot of water, it’s not historic. As Wright explains in his paper:

“The rainfall that fell inside the 281 square mile watershed works out to about 4.5 inches, equivalent to 33,272 Olympic swimming pools of water. This is a lot of water to be sure, but if the storm had occurred 15 miles to the northeast—as is shown in the “shifted” rainfall map on the right —it would have dropped 8 inches of rainfall across the watershed, almost double what actually occurred. Given the flooding we’ve seen over the past week, it is reasonable to assume that this “worst case scenario” would have been absolutely catastrophic for lakefronts and other low-lying parts of Madison.”

Non-Historic Rain, But Historic Flood?

Lake Monona is on the verge of an unprecedented flood, despite missing the worst of the rain. Photo: A. Hinterthuer 

Thankfully we didn’t get the worst of the rain. By Wright’s estimation (using software designed by the Hydroclimate Extremes Research Group), the amount of rain the Yahara watershed got last Monday fell somewhere in between a 20 to 50 year recurrence interval.  He settled on 30 years as a best guess, which means, Wright writes, “over the course of a 30-year mortgage, a homeowner will, on average, experience one storm similar in magnitude to the August 20th storm.” 

So why, then, did that rain produce a flooding event we’d only expect to see once over the course of three or four 30-year mortgages? 

Well, for starters, Lake Mendota. 

We don’t like pointing fingers at the lake that started it all for limnology here in North America, but, well, its water levels are 5 feet higher than their natural levels thanks to the Tenney Lock and Dam. 

The problem is, people like living on lakes. And, once they’re there, they also like having access to the lakes, which means a pier and a boat and maybe a little beach or swimming area and, most important, consistent water levels. 

But consistently high water levels in Lake Mendota come with a cost. When we get a big rain, the lake doesn’t have the capacity to take that extra load in. It’s already so high that the only thing city officials can do is open the lock and dam and let the excess run downstream. 

Roaring river
Photo: Amber Arnold, Wisconsin State Journal 

Back during the 2008 flood, says Eric Booth, a research scientist in the Hydroecology Lab at UW-Madison, calls were renewed to lower Lake Mendota’s levels so that the water from big rain events in the upper Yahara watershed could be held back and more slowly released downstream.

“People were interested in using Mendota more as a flood control reservoir,” Booth says, “but push back came from shoreline homeowners and people in places like Westport who wanted their docks accessible all summer.” Over the last couple of decades, Booth says, “it’s been a back and forth between those that want to draw Mendota [water levels] down and those that want it consistent. We might see renewed calls depending on how much worse things get this year, but it’s an old argument that quiets down between huge flood flood events. My old advisor called it the ‘flood memory half life.'” [EDITOR’S NOTE: Booth is not implicating all shoreline homeowners – depending on where a property is situated, high water levels are most definitely NOT desired, and many advocate for lower levels]

In addition to Lake Mendota’s artificially high water levels, is the fact that a lot of the rain we got on August 20th fell in the city and surrounding suburbs, where infrastructure like roads and parking lots and roofs keep water from infiltrating into the soil and simply shove it quickly downstream. 

Stormwater runoff in Arlington, Virginia.
Stormwater runoff in Arlington, VA. Photo: Tyrone Turner. 

“It is likely that the impacts would have been less severe if the rain had been concentrated in the northern portion of the watershed in Columbia county,” Wright’s paper says. “This is because the largely agricultural landscape there can soak up more rainfall than the urban and suburban areas in and around Madison.”

In fact, the Wisconsin State Journal highlighted these very problems in an excellent article on August 5th, just two-weeks before the big storm. It’s important that we continue this conversation because storms like the one we saw last week – or the folks in western Wisconsin saw last night – are becoming more common.

And, while storms like that cause floods, it’s our decisions here on land that make them historic.

You can download a copy of Wright’s report here. 

10 thoughts on “How A 30-Year Rain Event Became a 100-Year Flood”

  1. Good article!
    However, it points the finger of blame at me and other lakefront property owners as the political constituency for high lake levels.
    I, and most riparians I know, do NOT like high lake levels. It damages our shore banks and makes it awkward or impossible to use our waterfronts. I would rather have low water than high water.
    I have argued for 25 years with my county reps to lower the target level for Lake Mendota by at least 8 inches. I made the point in 2008 after the Lake Delton washout that it COULD happen here.
    There HAS been progress in improving permeability of drainage basin nodes to reduce runoff but much more is needed.
    The party most strongly opposing my position has been the von Rutenberg family and their “Betty Lou Cruise” line business. Other deep draft boat owners and some fishing groups also favor high water.
    I’m glad the conversation has finally been rejoined but lament that it took an entirely forseeable disaster event to provoke it. Unfortunately, past experience convinces me that the present conversation won’t succeed in effecting real change unless ALOT of people chime in.
    AND, these events WILL continue…

    1. Hi Neil,
      Sorry if shoreline homeowners like yourself got caught in the net of blame! I should’ve said “some” shoreline homeowners as your preference for water levels completely depends upon the topography of your particular place on the lake. Here’s hoping that the discussion continues, though as Eric Booth says in the article, there is a “flood memory half life” we have to fight against!

      1. Greetings Adam
        I’m sure that topography is a big part of it, but not all.
        My family live on a lot 12 feet above the nominal lake level and our hazard is limited to the inconvenience of being unable to use our dock and hoists when the lake goes over 11 feet above datum and having to repair minor erosion after each event. This is very minor compared to our friends on Middleton Beach Road and Harbor Court and others who have direct flooding threats to their houses.
        And yet, I seem to be more passionate about lake level policy that most. This puzzles me; but I suppose having lived on or very near lakes Mendota and Monona for 60 years, I have a longer view of the folly of the current policies about lake levels. I have seen the gross erosion of shorelines along lake Mendota first hand and have seen the degradation of the adjoining wetlands from the air and I have no patience for those who sacrifice these and other factors for their economic expediency.
        As I said, I hope that the silver lining of this event will be a renewed activism in the community for restoring the lake level policy to more reasonable values and for improving the tools which the County authorities have to manage the levels; (like clearing out the downstream Yahara chanels BEFORE a major event forces them to act). Improving permeability of the drainage basin and adopting a more proactive stance about opening the taps ahead of a major event would also help.

  2. “how’s it going, Middleton?”
    People lost their homes due to flood damage. Show a bit of decency. UW Limnology is better than that.

    1. We’re truly sorry that our attempt at keeping the blog’s tone casual ended up sounding flippant. We are, of course, horrified that these extreme events have such terrible impacts and will remember as much in future writings on disasters like these. Thanks for reading and for the feedback.

  3. that’s about right, benefit a few so that the majority may suffer. They should have to pay damages to everyone in the flooded areas

  4. Warner Marsh was filled in more than 40 years ago to build a park.
    Leave the wet lands to be wet and do their job.
    Don’t build homes on marshes like Tiedemans Pond and Cherokee Marsh.

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