Climate change is likely to bring more extreme rainfall and flooding to Michigan. So, flood risk in the next 100 years will probably look very different than in the last. But, much of our infrastructure, like culverts, bridges, and storm drains, is still being designed and built based on the floods of the past.
There’s a culvert in the southern part of the Huron-Manistee National Forest.
The Michigan Department of Transportation built it last year to replace an older, rusted-out model. It funnels a small creek under a busy state highway, M-20.
On a warm day in July, the creek is just a trickle. The culvert looks like it could hold a lot more water.
In fact, it’s big enough to handle a flow rate of 255 cubic feet per second. That’s equal to about 2,000 gallons per second. So, how did MDOT arrive at that size?
The 50-year flood
When engineers build a culvert, bridge, or storm drain, they consider how long the structure is supposed to last and the largest flow it’s likely to see in that time period.
Engineers can calculate, for example, the “50-year flood” by looking at decades of historical flow data, if it’s available.
The 50-year flood is the flow that has a 2% chance of happening in any given year. You’d expect to see it about once every 50 years, based on the flows of the past.
If historical flow data isn’t available, you can still estimate flow by looking at rainfall.
For example, MDOT sized this particular culvert by using values from the 50-year rainstorm in the area.
And, the 50-year rainstorm is determined in much the same way as the 50-year flood: by looking at historical data.
"We are not climate scientists"
Kristin Schuster, an engineer with MDOT, confirms that her agency is not incorporating climate change when they size culverts, bridges or storm drains.
“We're using our existing datasets and the existing information to calculate flow rates,” she says.
There’s no guarantee the 50-year flood will stay the same size. And yet, we’re still designing structures as if it will.
“You know, we have not tackled that issue to determine if we need to change those numbers,” says Schuster.
The Michigan Department of Environment, Great Lakes and Energy, or DEGLE, is responsible for calculating design flows on bigger watersheds.
Susi Greiner is an environmental engineer there, and she says they aren’t accounting for climate change in their calculations, either.
“We are not climate scientists,” she says. “We are engineers. And what we have to work with is the data that exists now.”
No good method
Gabriele Villarini is an associate professor of civil and environmental engineering at the University of Iowa. He says there just isn’t a good method yet for adjusting design flows based on climate change projections.
“How do we design, how do we take into account these changes now that the historical record is not necessarily representative of future conditions,” he says.
It’s difficult to figure out how climate change has already impacted floods, and consequently, how it will impact them in the future. This is because long-term, human-caused climate change isn’t the only factor. There are short-term climate cycles, too, and changes to the landscape.
Right now, Villarini is working with the U.S. Army Corps of Engineers to incorporate climate change projections into some of their designs.
He says he feels a sense of urgency, but that this kind of research takes time.
“It's a wide problem, it's a big problem, it's a recognized problem, but it's also not an easy one,” says Villarini. “And so it's one that we need to tackle with care, given what the potential repercussions are.... Develop something that has been vetted, verified and then put in place.”
Susi Greiner of DEGLE wants to stress that even though they aren’t including climate change in their flow calculations, the state requires structures to be designed on the large end – often to withstand the 100-year flood.
“So even if you're getting more intense storms more frequently, the odds are good that you're still gonna have built the structure to withstand those storms,” she says.
But, climate change is likely to keep shifting the odds on storms. And that means infrastructure will need to keep up, or risk being overwhelmed.