In the face of bigger storms, engineers reconsider stormwater solutions

The federal government has taken some action on the stormwater front. The 2022 Bipartisan Infrastructure Law included over $200 billion dollars of investment in water infrastructure, including stormwater. Erickson said that while that’s nothing to sneeze at, most of that is marked for drinking water and sanitary sewer infrastructure. 

“The cost of replacing the stormwater infrastructure alone,” he said, could be “billions and likely trillions of dollars.” 

“There can be thousands of miles of stormwater pipe within a single city,” Erickson continued. “So if you extrapolate that to all of the United States, that’s millions, and perhaps billions, of miles of stormwater pipe that are too small or too old.” 

Gallagher’s presentation focused on the ways stormwater infrastructure prevents or abates flooding. He compared the efficacy of three different methods for stormwater abatement: storage, infiltration and conveyance. 

Conveyance strategies like increasing the size of the pipes, Gallagher said, were effective in preventing flooding in the places they’re implemented in — but come with an important complication. 

“That just moves the problem downstream,” he said. While one city’s flash-flood problem might be fixed by increasing the size of pipes alone, New Orleans might not be pleased with the outcomes of a conveyance-heavy flood abatement strategy employed in Baton Rouge, for example.

Gallagher found that storage-based solutions, like collection ponds, were the most effective of those he studied, and the most cost-efficient. A reservoir or pond with a lot of capacity for more stormwater, he said, can collect water and slowly allow it to evaporate. At their cheapest, these solutions just require digging a hole, though some higher-tech versions can give managers more latitude

Those high-tech solutions, University of Minnesota Professor Emeritus of civil, environmental and geo-engineering John Gulliver noted, include emerging technologies like “smart ponds," which use water level sensors and forecasting software to open and close outlet valves in stormwater ponds. Those are still expensive, he said, but predicted costs would come down as more modest enter the market.

Other technological solutions aren’t so much high-tech as they are burdened by history. The Rapidan Dam, Gulliver said, was once a hydroelectric dam managed by a local power company. Northern States Power Company — one of the many companies that eventually merged into Minneapolis power giant Xcel Energy — gave the dam away to Blue Earth County in 1970 after it was damaged by flooding in 1965. The dam was rebuilt just a few years later, but hasn’t provided electricity since 2019. Construction and operation of hydroelectric dams in general has been on the decline for years as they're forced to compete with other green energy sources and fossil fuels while facing up to a complicated environmental legacy. 

In this week’s floods, Gulliver said, the dam had approached failure and the reservoir had swelled to such heights in part because of debris pushed against it by floodwaters.

“What they could have done to handle the debris, it’s hard to say. There are things you can do to handle debris — if they had a hydropower facility active there, that’s kind of an insurance policy against these things,” he said. “But I don’t know if that would have helped in this situation.” 

Northern States and Xcel, he noted, offloaded their dams to other authorities over the latter half of the 20th century and don’t operate any dams in Minnesota today. 

Hydroelectric dams, smart ponds and updated pipes are large-scale solutions to stormwater issues, but one of Gallagher’s three categories of stormwater management is available to anyone with a front yard: infiltration. That describes absorption of water by the soil, and while many drainage authorities have taken on projects to speed that process up, the humble rain garden is a stormwater solution available to any property owner. 

However, In the face of a 100-year flood, Gallagher found, infiltration systems don’t do much for stormwater management. The classic model of such storms has about 50% of the storm’s water dropping in one hour. “We’re looking into whether we can relax that assumption a little bit, just because that will be beneficial to infiltration practices,” Gallagher said, but the deluges seen in the thickest part of a rainstorm are still too much for a rain garden on its own.

Both Gallagher and Gulliver still emphasized that some infiltration is better than none. The majority of stormwater in many smaller storms, Gulliver said, is currently absorbed by lawns, even though most American lawns are less than ideal for absorption. Kentucky bluegrass and other shallow-rooted plants are bad for permeability; if a concerned citizen wants to flood-proof their lawn, the scientist said, deep roots and aeration are the key. He recommended planting fescue grass, and, should a homeowner still see water pooling in their lawn, looking into the consistency of the soil and breaking up any clay layers that impede permeation. 

Gallagher said he’d personally wish to see more municipalities and other authorities looking into storage-based stormwater solutions. Reservoirs and both “wet” and “dry” holding ponds — so named for their condition when it’s not raining — are good options, and cities tend to favor underground storage solutions, like vaults, since above-ground land comes at a premium. 

Without those adaptations, he warned, cities in Minnesota and elsewhere are on notice that it’s a matter of when, not if, they see flash flooding. “You don’t have to live by a river,” he said, noting that flooding can affect anyone with insufficient stormwater management infrastructure. By way of example, the city of Mankato and its surrounding communities were spared a dam breach, but nevertheless have seen substantial damage from flooding because of the same storms that threatened the Rapidan Dam.

Erickson, meanwhile, noted that while the picture of stormwater management he paints is bleak, there’s reason to be hopeful.

“Obviously it’s a big problem, I’ve painted a picture of doom and gloom, but there are many very capable and very motivated people who are working on this.” 

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