Urban spaces have evolved through the decades to reflect the needs of the people who live there. Through time, we buried wetlands to build roads and paved stream corridors and other natural spaces to drive water efficiently to the nearest exit. We built the infrastructure necessary to move wastewater and imposed uniformity and order with parking lots, buildings and other hardscapes. For decades, our engineering held up masterfully until the intensity of recent storm events overwhelmed even the most well-built and sophisticated systems.
For all the benefits we derive from streets and sidewalks, impervious surfaces very efficiently funnel stormwater along with a mix of trash, nutrients and pollutants downstream to low-lying spaces where it collects, floods buildings and basements, and brings about extensive damage. In cities that border the Great Lakes, combined sewer overflow events (CSOs), in which sewer systems are overwhelmed by a massive influx of stormwater, are a recurring problem. Some cities—Milwaukee is one example—have achieved a notable reduction in these events through a combination of measures that includes green infrastructure (GI).
As flooding in inner cities grows more frequent and the limits of gray infrastructure become apparent, we bring wetlands and other natural hydrologic systems back into the mix. Of course, even as GI practices rise to the forefront as the new line of defense against urban flooding, some raise questions on the long-term efficacy of these practices; and these are valid concerns. Many communities would benefit from additional research on the long-term viability of these solutions, not just to encourage implementation of these measures, but to validate GI as a worthy and cost-effective urban investment.
Where the Wetlands Were
The city of Milwaukee covers 96.1 square miles of land area, nearly 45 percent of which consists of impervious surface area (bit.ly/MilwaukeeGI). Fortunately, this last value is declining. The city has worked aggressively in recent years to fortify its combined sewer systems with a deep tunnel. In addition, the city’s ambitious Green Infrastructure Plan (bit.ly/MilwaukeeGIPlan) outlines several innovative priorities, including an express “depaving” objective to remove large swathes of pavement and add approximately 36 million gallons of additional stormwater management capacity via 143 acres of GI by 2030.
Milwaukee identified high-priority stormwater project areas by creating a baseline inventory, researching the region’s historic hydrology, targeting neighborhoods that experience recurring flooding, articulating infiltration objectives and implementing a blend of GI strategies—from rain barrels and bioswales to green roofs, permeable pavement and rain gardens—that could achieve specific goals.
As of 2023, Milwaukee has 231 bioretention basins, more than 13 acres of porous pavement and more than 100 green roofs. The city has planted 395 trees and put in place 52 cisterns. According to Milwaukee’s records, existing GI practices have diverted more than 14 million gallons of stormwater from the city’s combined storm sewers. Notably, the frequency of CSO events in Milwaukee has plummeted from an all-time high of 60 discharge events in 1992 to two to three events in 2023.
Blending Green and Gray Infrastructure in Buffalo
Urban areas come with complicated site conditions that may include existing utility lines, established corridors for pedestrians and cars, and other limitations. Nonetheless, the city of Buffalo, N.Y., tackled a major commercial thoroughfare, Niagara Street, through a combination of complementary GI practices (raincheckbuffalo.org).
For this one project, the city removed 2.1 acres of impervious surfaces, planted nearly 260 rain gardens, designed 46 bumpouts (vegetated curb enlargements) and planted 574 trees. Existing green spaces along the boulevards were expanded, and an underground cistern—virtually undetectable to pedestrians—traps additional stormwater. Through this project, a sizable 31 acres of densely developed urban space was retrofitted for enhanced stormwater management, and roughly 613,000 gallons were diverted from the city’s sewer systems.
Other stormwater projects boast similar statistics: Work on the Ohio Street project in an industrial district of Buffalo, for example, removed 3.2 acres of impervious roads and sidewalks, expanded multi-use corridors with porous asphalt and added 260 trees.
Short on Time—and Long-Term Data?
Many believe GI strategies are clear winners with multiple benefits in terms of managing urban stormwater issues, and I generally agree. GI brings natural hydrologic processes back within city limits where—if properly designed—they can reduce peak flows and total storm volumes while improving overall livability and quality of life. We’re literally peeling back the practices of the past, replacing impervious surfaces with living systems in a grand reversal of how we used to build: “pave it and forget it.” But are there limits to what these stormwater management practices can do?
A recent U.S. Geological Survey assessment (bit.ly/USGS-GI-GreatLakes, 2022) reviewed multiple studies analyzing the efficacy of GI practices in the Great Lakes region. Of all GI practices used, the most common include bioretention ponds or cells such as rain gardens. Of the studies documenting the performance of these strategies, most demonstrated significant—up to 80 to 90 percent—reductions in peak storm flow and volume. These performance metrics are nothing to sniff at—even if the word “garden” isn’t a word we historically associate with an engineering solution.
But the study’s authors point out that although most studies establish the significant benefits of GI, these same studies are primarily local in scale and short in term; performance through time and at different landscape scales—as well as under changing conditions—isn’t well established. Knowing the conditions under which GI practices may fail, whether due to maintenance failures, poor vegetation choices or considerations of regional and/or changing climatic conditions are as important as the design specifications for standard gray infrastructure.
While properly designed GI has multiple benefits—environmental, social and economic—further research that details the basis for long-term viability is equally important, particularly in the years ahead as we continue to fortify our cities with the innovative solutions GI represents.