Can "nature-based solutions" head off flooding in metro Detroit?
Elisabeth Barrow moved to her subdivision in Troy a decade ago to be near nature. The neighborhood where she lives on Robart Drive is quintessential suburbia – tidy homes and manicured lawns.
But at the end of her street is Turtle Woods – an 80-acre tract of forest, wetlands, trails and wildlife. Named for the rare spotted turtle once found on the site, the parcel offers a rare bit of nature in the middle of a bustling part of Metro Detroit.
“We walk the trails. And my son, when he was younger, he and his friends would go back and play in the woods and come out soaking wet and full of mud and all that good stuff that kids do,” she said.
The local school district owned the parcel for over 50 years, using it as an outdoor classroom. But it decided a few years ago to sell the property. That’s when Six Rivers Land Conservancy and Oakland County Parks got involved. They’re working on acquiring and preserving the land as part of a larger effort to secure natural areas within urban landscapes like this one.
Natural areas serve as a buffer against climate change. They provide “ecological services” – reducing the urban heat island effect, sequestering carbon, holding and infiltrating stormwater into the soil and reducing flooding.
It’s that last item that has the attention of regional planners and utility officials as they contemplate a warmer, wetter future driven by climate change.
The Great Lakes region has already seen about a 14% increase in total precipitation since the 1950s, and experts expect it to increase. A Southeast Michigan Council of Governments (SEMCOG) analysis predicts rainfall volume for a 10-year storm event in Metro Detroit will increase 67% by mid-century and 138% by the end of the century.
That has Chris Bunch, executive director of Six Rivers, thinking about how his organization’s work can contribute to more than just preserving habitat and recreational areas – he’s considering how preserving nature can help with climate resilience.
“How do we start to intervene in some of our flooding issues? Because we have so much impervious surface,” he said. “Where can we start to identify places that have the capacity to do some flood storage?”
Bunch began working with engineer Jamie Burton, a vice president of the engineering firm Hubbell, Roth & Clark, on a way to analyze those ecological benefits for water quality and flood storage. Burton’s analysis found that by preserving the Turtle Woods property instead of turning it into another subdivision, hundreds ofpounds of pollutants like sediment, nitrogen and phosphorus and more than two million gallons of stormwater runoff are prevented from flowing into the Clinton River yearly.
“The best water treatment systems out there are natural systems, wetlands, grasslands, areas that you can store water and actually infiltrate it into the ground,” Burton said. “So we're looking at the potential of taking some of these conservation parcels and incorporating stormwater management.”
Using natural landscapes to help handle flood waters is a complex solution, and it remains to be seen how effective this strategy can be at scale. Land, especially in urban areas, is expensive. And the land needs to be in the right place – in a low-lying area, connected to a creek or a floodplain, or near existing stormwater infrastructure. Regrading land to create more stormwater capacity can damage ecosystems. Stormwater runoff contains pollutants like road salt that can stress plants, and even too much water can harm certain sensitive wetlands and kill vegetation.
Burton and others are weighing all of these issues.
“We have to do it without impacting these parcels. You can't just flood an area. But if it's got capacity to hold water, you could do minor changes to the landscape to hold more,” he said.
“Some of these might be good places to let that water sit, evaporate, infiltrate, or be treated before it releases. But we’re trying to do that without damaging the natural landscape.”
Scaling up green infrastructure
Burton is taking the analysis he developed for Bunch to help other organizations like The Huron-Clinton Metroparks and SEMCOG look proactively for “nature-based” stormwater solutions across the landscape.
A 2020 study showed that the region’s Metroparks system prevents more than million cubic feet of stormwater runoff – the equivalent of 3,120 Olympic-sized swimming pools – from polluting local rivers and lakes yearly.
Director Amy McMillan wanted to see how Metroparks could do even more for climate resiliency. At the same time, Metroparks wanted to increase its benefit for underserved areas of Detroit and Wayne County, where many residents have little access to the parks system in outlying suburban areas.
Those combined interests led McMillan to launch a project to assess vacant parcels in Wayne County for their capacity to store runoff and other ecological benefits. Once Metroparks identifies a list of candidate parcels, it plans to work with local municipalities and other partners like land conservancies to acquire and restore them for stormwater management.
SEMCOG is also developing a list of candidate parcels that could host stormwater storage – weighing things like location, ability to capture and store runoff, land cost and property owner interest. They’re piloting the project in a small subwatershed but hope to assess the entire region eventually.
“Our priority is to identify those nature-based solutions – large areas like wetlands – that can manage the excess runoff from large flood events and map those opportunities across the region,” said Kelly Karll, Southeast Michigan Council of Governments Environment & Infrastructure Group manager. “And then seek funding to construct features where we can redirect runoff from existing impervious areas to these features during times of large storms.”
Part of that work is in partnership with Michigan Technological University researcher Mary Ellen Miller, who is evaluating the flood water storage capacity of wetlands in Metro Detroit. Miller uses topography and water level data to determine which wetlands might have extra capacity to receive stormwater runoff from nearby roads, pavement and rooftops. The data could help utilities and stormwater engineers find places to direct runoff to avoid flooding.
According to Miller, one danger is that urban stormwater carries pollutants like road salts, grease and metals, which can degrade the quality of sensitive wetlands. So, she’s combining her analysis with data about wetland quality.
“So we can look at the [map] and know whether or not it's a pristine wetland with native vegetation or if it's degraded,” Miller said. “Wetlands are the ultimate green infrastructure.”
Green and gray together
Nature-based solutions and green infrastructure are part of the answer, according to Karll. Still, gray infrastructure – pipes, tunnels and underground storage – will always be part of managing flood resiliency in Metro Detroit.
Green infrastructure can include small constructed systems, like rain gardens, bioswales, porous pavement, and green roofs that can hold and slow rainfall during smaller rain events and clean and filter water. It often also provides visual interest and wildlife habitat.
But, those smaller systems cannot manage flood events, according to Karll. For that, we must rely on gray infrastructure.
A proposed 4.5-mile-long stormwater drainage tunnel along I-94 in Detroit is one example of a gray infrastructure project that could build climate resilience. If funded, the project would outlet to MDOT’s I-96 storm sewer and remove 5 miles of freeway and 10 miles of service drive stormwater runoff from the local sewer system.
The project includes extra planned capacity based on SEMCOG’s rainfall predictions and could help to reduce freeway flooding.
“Ideally, green and gray work together, with gray infrastructure conveying stormwater to areas that can hold and manage runoff,” Karll said.
One caveat, Karll noted, is determining what size rainfall event to design size these features to handle. That’s tricky because engineers still haven’t adopted any new design standards to account for future rainfall.
“We can’t just upsize everything because much of the downstream system is undersized – we can’t upsize a road storm sewer that connects downstream to a smaller sewer. So it is a strategic analysis of where and how this can happen,” she said.
One example that creating new green infrastructure is a project to transform one of the largest parking lots in the region – located at Lake St. Clair Metropark – into an asset that can receive and treat stormwater. Phase three of this $1.875 million project will remove two acres of pavement and install a wetland with over 71,000 cubic feet of storage volume, 37 vegetated parking spots, and 55 native trees.
“That's going to manage millions of gallons of stormwater each year,” McMillan said. “Because we have so much property and so much of it is in a natural state, and we manage it in a very, a very deliberate way, we can contribute to stormwater management and water quality throughout the region.”