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Issues of the Environment: Takeaways from Washtenaw County's '500-year, 3-hour storm'

Washtenaw County Water Resources Commissioner Evan Pratt
Washtenaw County
Washtenaw County Water Resources Commissioner Evan Pratt


  • The advantages of living in southeast Michigan in the wake of climate change are many, abundant fresh water, limited wildfire risk, and temperate weather. But, the region isn’t immune. Flooding events that historically occurred every century or so are now spaced by decades--or even less.
  • On August 23, 2023, Eastern Washtenaw received unusually heavy rains, with some areas receiving between 4.8” - 4.9" in roughly 3 hours. NOAA calls this a "500-year, 3-hour storm", which means a 0.2% of happening at any given time. More rain was in the forecast for the 24th, and the Washtenaw County Water Resources (WCWR) office encouraged residents to continue reporting drainage issues. 
  • All WCWR staff worked diligently throughout the County on various drainage concerns and reports of flooding.  Responses were prioritized based on critical criteria such as safety, public health concerns, imminent damage, and impassable road conditions. 
  • All that water has to go somewhere, and the stormwater infrastructure of the past can’t keep up. Water Resources Commissioner Pratt reported that, “None of the pipes are designed for even the 50-year storm. Pipes are usually a 10-year rain, and the amount you get in a rain like this or even a 100-year rain is a huge amount more,” he said. “No doubt there were some full pipes out there.”
  • “We’ve looked at the tables and charts, and just statistically (it was) a pretty unlikely event,” said Water Resources Commissioner Evan Pratt. The localized heavy rains seemed to stretch somewhere between Scio Township all the way west to Canton, which experienced significant flooding, with the rainfall also blocking access to a terminal at the Detroit Metro Airport. Pratt said he’s received reports of deluges from a north-south area stretching from the middle of Salem Township all the way south to Dundee, “a pretty big cell for this type of intense rain,” he said. Staff were busy with triage the day after the storm including a washout on the downstream side of the Tyler Dam, near the Willow Run Airport.
  • Water Resources staff also visited areas where infrastructure upgrades have recently been completed, like Apple Ridge Street in Ypsilanti Township. Pratt reported that flooding was prevented on that street, which used to go underwater during heavy rains and flood area basements. Washtenaw County also has a robust rain gardening program with over 100 rain gardens around the county in private yards and public spaces. WCWR is also ahead of the curve in using bioswales, native area restoration, and stormwater retention planning. (Source: *portions quoted* https://www.mlive.com/news/ann-arbor/2023/08/parts-of-washtenaw-county-get-almost-5-inches-of-rain-in-3-hours-during-500-year-storm.html)


David Fair: This is 89 one WEMU, and we're a couple of weeks removed from the severe storms that drenched Washtenaw County and southeast Michigan. There are lessons to be learned and plans to be made. I'm David Fair, and welcome to this week's edition of Issues of the Environment. Some areas of Washtenaw County received nearly five inches of rain in a really short, three-hour period on Wednesday, August 23rd. That was followed by more thunderstorms and wet weather the next night. We also saw seven tornadoes touched down on that Thursday in Michigan. My guest today--and others referred to the Wednesday night rainfall as a 500-year weather event--Evan Pratt is Washtenaw County Water Resources Commissioner. And thanks for making time for me today, Evan. I appreciate it.

Evan Pratt: Absolutely. It's great to be here, David.

David Fair: I mean, Holy Rain Garden, Batman! That was really something, wasn't it?

Evan Pratt: It sure was. And, you know, on top of it all, I've got a dog that's pretty uncomfortable with those type of events. So, it was not good to be awake most of the night knowing that it was going to be a pretty busy Wednesday and Thursday. That's for sure.

David Fair: So, as you're sitting with a shaking dog, did you recognize it as a rather rare event right off or only in retrospect?

Evan Pratt: Not right off the bat. But, you know, I had texts in my inbox from staff that were already, you know, heading out on service calls. It was pretty light Thursday morning. We had a couple of calls by nine or 10 a.m. I live on the north side of Ann Arbor, which got a pretty good amount of rain, but it was nothing like what the eastern half of our county and the southwestern portion and far southeastern portion of Wayne County got. But based on the texts from the staff, I decided I would be skipping the PFAS seminar in Lansing. I still need to learn a lot more about PFAS, but I just got in the car and they spent six or 7 hours just driving around the eastern part of the county and stopping and talking to a lot of different people along the way. But, you know, getting that visual, David, really helps and having a team that's been through this type of thing before and understands how we need to share information with our decision makers in county administration, the county board, emergency management, you know, talking with Ben Pinette at emergency management in the sheriff's office a couple times during the day and even all the way through yesterday, having kind of a final button-up conversation, it's great to get out there and get eyes on things. Of course, one of the reasons I was out there was because there were a couple of things I really needed to get my eyes on. We had a little washout next to a dam, for example, by the Willow Run Airport. So now, the Tyler Road approach to that area is closed and will probably be closed for, you know, at least in a duration of months. We've got a number of follow-up meetings to have with different folks, David.

David Fair: Well, give me a grade. How well-suited is the stormwater system in Washtenaw County to handle such an immense amount of rainfall in such a short period of time after you've done some assessment?

Evan Pratt: Yeah, absolutely. That's great question. Nobody's storm system in the United States--I don't want to say the world because they don't know everything about the whole world--but nobody's system in the United States can handle what, you know, we're calling, colloquially, a 500-year storm. That's the technical term for that, though, and the National Weather Service fella I was with on Monday reminded me of this. There's a 2/10 of a percent chance of that type of rainfall, that intensity, happening in any given year. So, 2/10 of a percent is kind of what you get if you flip, you know, 500 years upside down. So, it often gets called a 500-year storm. I don't think it really matters, as far as our discussion or what we're sharing with the public. But no system is built to handle that. Most stormwater systems, the pipes are typically built since I'll say, the sixties or seventies, for what we might call a ten-year storm. In most communities, not all, some communities have pipes that were built before those design standards. A ten-year storm, of course, if I use the analogy I did with the 500, means a 10% chance of occurring in a year. So, the idea is it'll stay in pipe if you get a storm of that size. And since the seventies, most of the developments that go in and have a large pond, we call them retention ponds to retain water during the storm, and slowly release it after the storm, so that that water is not contributing to the problems we have. Most of the detention basins are built that what we call a 100-year storm or a 1% chance each year. So, when we get events like this, systems are going to be overwhelmed. The key is you need to make sure the water keeps flowing when you have these big events, so that the amount of time and the severity of the issues are minimized. So, our whole game, David, our whole approach to things, is to do everything we can to reduce the frequency and severity of events, whether they're small, medium and large. We want to make sure and avoid property damage. And going and driving around is about learning where did we have problems. And I'd say, you know, three or four of the places I stopped, I made a mental note and have circled back with staff to say, "Hey, this appears to be a place where we need to show up twice a year whether there's a problem or not to, because it's the folks nearby are going to be more readily impacted by, you know, a medium-sized storm," for example. A long answer.

David Fair: We're talking with Washtenaw County Water Resources Commissioner Evan Pratt on WEMU's Issues of the Environment. And, Evan, we know climate change is going to bring an increase in the frequency of major storm events. It's going to happen again and potentially sooner rather than later. How is the Office of the Water Resources Commissioner planning to further adapt to that inevitability?

Evan Pratt: You know, we've got three prongs on that strategy. I would say, at present, based on how the funding setup works, we're not going to achieve that reliance, but it doesn't mean we're not going to try. But our three prongs really are, number one, to identify any place where we have an opportunity to store or retain more water when it's raining hard. So, for example, in the City of Ann Arbor, we've worked with the city to retrofit some of their large scale stormwater basins. A second prong, you know, after the large scale basins, is on those new developments, We passed design standards now nine years ago requiring the new developments to soak as much water into the ground as they could. And the good news is about 55% of those developments, as we look at it every quarter, it varies a little, but most of those developments are actually able to soak in 90% of the annual rainfall, David. So, when we get the 500-year storm on those sites, water is going to leave the site. But anything a 100-year storm and smaller, no water is going to leave those sites. So, that's pretty neat for us. When I say 90% of the annual rainfall, what I'm also saying is at least, currently, the statistical stuff that we're able to see, the most recent information from about a dozen years ago, suggests that storms of one inch and less are where 90% of the rain falls. So, if we can kind of keep pushing that water into the ground, that helps. So, that's our second prong, the private development side of things. It's also kind of prong 2A of that one. We've got a bunch of sensor nodes out right now in development ponds of various areas. So, currently, we're requiring soaking water into the ground. My predecessor, Janice Bobrin, was progressive and changed the standards a couple of times from the eighties forward. We're actually seeing how do those ponds perform? Are any of them performing in such a way that we can actually divert more water to them and have them doing more work for us? We believe there are situations like that. And are any performing poorly either because of, you know, maybe the design didn't work, you got it built incorrectly, or it's been modified since it was built? So, we're kind of, you know, trying to make sure that what we do have is working and where can we take advantage of what we already have. And then, our third prong is those rain gardens that you've been very helpful at WEMU promoting and sharing. There's over 2000 of them in Washtenaw County now. Many people have gone through our Master Rain Gardener class, and many people have, you know, found other avenues to figure out how to build a rain garden on their house. So, we figure we can store two Michigan Stadiums of rainfall throughout those rain gardens that are distributed all over. So, three prongs to try to deal with climate, but we know we're not going to get ahead of climate. We're just hoping to get behind a little slower. I know that's a kind of a chilling thing, maybe, to let people know, but that's the reality of the situation with how stormwater is currently funded.

David Fair: So, we have very little time left together. Evan. But I do want to ask you a final question. You've detailed in a very professional manner in which how you view and assess significant weather events and overall impacts. What would you like us as laypeople to consider as we reflect on the impacts of the most recent storms?

Evan Pratt: Sure. I think there's a couple--three things. One, as I've kind of suggested, we are going to have storms that do impact people. It's super helpful for us and the communities that people live in to let us know when there is a problem, whether that's during a storm or not during a storm. We have an electronic tool on our website at Washtenaw dot org. You just go to Washtenaw dot org, type "flood reporting." People can upload a photo. It's a real help for us, especially during storm triage, when we have more places to go than than people to get there in 5 minutes, to see a photo and understand if this is a 20-foot by 20-foot puddle in the backyard or if we've got three feet of water on a road, like we did at a place called Apple Ridge in Ypsi Township in 2021. We retrofitted those Apple Ridge stormwater structures, so they wouldn't get debris caught on them, and they were great this last storm. You know, water in basements can be an issue. Make sure that roof water is getting away from your basement, four-foot down spout extensions. You may not want a sump pump, but sometimes, that is a thing that you need and want to have. And you don't want to hook your sump pump into the sanitary sewer. If your sump pump is hooked into the sanitary sewer and you've had basement backups, although it's expensive, you really want to consider unhooking that because that will help you as well as many other people.

David Fair: Hey, I'd like to thank you so much for the time and sharing the information, Evan. I appreciate it. That is Evan Pratt, Washtenaw County Water Resources Commissioner, joining us with reflections on a 500-year weather event with an eye towards becoming more climate adapted. For more information, go to WEMU dot org. I'm David Fair, and this is your community NPR station, 89 one WEMU FM Ypsilanti.

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