The future of stormwater management runs through Kansas City
Faced with worsening floods and aging sewers, the city has become a national leader by combining green infrastructure with digital technology.
By Philip Preville
A mere two blocks away from Diane Hershberger’s home, in Kansas City’s historic Marlborough neighborhood, there are two sewer outfalls into the Blue River, a tributary running north past downtown and into the Missouri. Most of the time those outfalls are dry. But during heavy rainfalls, stormwater runoff enters the city’s aging combined sewer system, where it mixes with raw sewage, fills the pipe beyond its capacity, and discharges its overflow through those outfalls and straight into the Blue River.
“Kansas City has close to a hundred outfalls like those ones,” says Hershberger, a retired civil engineer who’s all too familiar with the workings of aging urban infrastructure. “They are a common sight because of all the creeks that run through the city, and it’s not unusual for them to overflow.” As recently as 2010, those outfalls were discharging roughly 7 billion gallons of untreated sewage into city waterways annually. And once that slurry enters the system, it befouls everything in its path: riverbanks, parks, beaches, marine life, and more, including the drinking water treatment plants of downstream municipalities on the Missouri and Mississippi rivers.
Combined sewers are the ultimate out-of-sight, out-of-mind urban issue. The pipes are buried eight feet under; people flush their toilets, rain or shine, and rarely spare a thought about what happens next. On an aggregate scale, however, combined sewer overflows represent a massive problem. According to the Environmental Protection Agency, there are 748 U.S. cities with combined sewer systems, totaling more than 9,000 raw-sewage outfalls, discharging an estimated 850 billion gallons of untreated sewage into U.S. waterways annually. It’s a colossal blight that’s continental in scope, but that can only be solved in small increments — one city at a time.
Kansas City pursued a unique solution that has made it a national leader in stormwater management: a combination of green infrastructure to soak up excess rainfall and digital technology to monitor its flow. It’s called the Smart Sewer Program, and so far it’s worked. These days, even as Kansas City experiences more and more extreme rainfall events than it did 10 years ago, total combined-sewer discharge is declining. The outfalls near Hershberger’s home remain almost always dry, and the green infrastructure projects in her neighborhood have helped spark Marlborough’s revitalization.
It’s a success rooted as much in philosophy as in technology or engineering. After more than a century of treating rainfall as a nuisance to be disposed of as quickly as possible, Kansas City is discovering that the solution lies in the opposite approach: cities have to make friends with water.
Using greenery to capture rainfall
The general rule of underground pipes is that the older the city, the larger its combined sewer system. Most such systems date back to the latter half of the 1800s and are found beneath areas of the city built during those years. (The construction of Kansas City’s combined sewer began prior to the Civil War.) Built as they were designed for 19th-century needs, many combined systems have strained to manage the increased sewage flows resulting from 20th-century urban environments.
Add to that the intensified precipitation due to climate change. Across North America, major rainstorms are increasingly frequent, and when it rains, it pours. Nine of the top 10 years for one-day extreme precipitation events have occurred since 1990, according to the EPA. Climate scientists and engineers alike have come to realize that the “100-year event” — the notion that a particular type of extreme weather occurrence only happens once in a century — has become confusing, since such events happen with increasing frequency.
Kansas City lives this new reality as intensely as any American city. “My rain gauge holds six and a half inches of rainfall, and it overflows multiple times a year,” says John Kevern, a professor of civil engineering at University of Missouri at Kansas City and the inaugural director of its new Center for Urban Stormwater Research. “If you’d told me 10 years ago that my gauge would be topping out regularly, I’d have told you that we all had better start building arks.” Kevern says it’s no longer possible to bury the problem, literally or figuratively. “For a city to dig its streets up and build pipes wide enough to handle those extreme events, and for those pipes to then run at a tiny fraction of their capacity every other day of the year — you just can’t justify that expense, or that disruption to the city.”
The Smart Sewer program was Kansas City’s response to all these problems. In 2010, the city signed a consent decree with the EPA, pledging to eliminate combined sewer overflows and to spend $2.5 billion to upgrade its stormwater infrastructure. In the initial years following the consent decree, Kansas City’s wastewater fees nearly tripled to raise the money — a tremendous and sudden burden on ratepayers, which sent the city in search of cheaper and more innovative solutions. And as Kevern says: “anything you can do upstream to alleviate problems downstream saves lots of money.”
Hence the appeal of green infrastructure: above-ground installations such as rain gardens, bioswales, planted medians, curb bump-outs, and street tree plantings with deep soil cells, among others. These features are designed to gather, retain, and use stormwater — keeping it from overwhelming the sewer system. Many green infrastructure projects take the form of urban parks on the surface, but underground they are a purpose-built mixture of soils, sands, and aggregates, all designed and engineered to soak up rainfall like a sponge. And their impact can be substantial. A planted median in the middle of a city street can reduce the runoff from a typical rainstorm by 10 percent. Add in sidewalk trees and the total stormwater diversion increases to 28 percent. Since that water is taken up by vegetation, it never enters the sewer system, preventing overflows.
Those are the projections, at least. Kansas City wanted to test the effectiveness of green infrastructure in reality, and from an engineering standpoint, the Marlborough neighborhood was the perfect location to do so. “Marlborough had two 40-acre watersheds, each with the same topography, soil quality, and land use,” says Kevern. “It was planned so that one watershed would serve as a test site, and the other as a control site.”
As perfect as Marlborough may have been from a scientific standpoint, the city still needed buy-in from the local community. Long a neighborhood in decline, Marlborough had just formed a new coalition to try and reverse that trend when the city requested a community meeting about its new stormwater pilot, roughly a decade ago. “We were a very disinvested neighborhood at the time,” recalls Hershberger, who was active with the new Marlborough Community Coalition from its outset. Nevertheless, people turned out for the meeting in strong numbers. As Hershberger puts it, “People showed up because they wanted their sidewalks fixed.”
They got their wish. The city’s proposed green infrastructure program included new sidewalks as part of the construction of planted bump-outs. It also included the construction of a rain garden in nearby Arleta Park. The progress of these first installations has been chronicled on the city’s excellently curated Green Infrastructure Story Map, showing the installations at their initial planting and again three years later, when they matured. Once the Arleta test site proved successful, the city built two more: a constructed wetland at 81st and Troost, and a massive, cascading rain garden in Rachel Morado Park.
“The reduction in stormwater flows into the sewer system in Marlborough was outstanding,” says Kevern, who assisted with the project. “We discovered that we can keep substantial amounts of water from going downstream with a few simple projects.”
Knowing when the next storm is coming
You can’t manage what you can’t measure, as the old saying goes. And while the digital revolution has produced a proliferation of data on everything from traffic counts to electricity consumption, stormwater continues to rely on low-tech devices such as Kevern’s rain gauge.
Digital technology is now making inroads into stormwater management, and Kansas City has become an enthusiastic adopter under its Smart Sewer program. Using technology developed by South Bend, Indiana-based Emnet, the city equipped some 300 manhole covers with water depth and flow sensors, conveying real-time data from underground infrastructure and allowing the city to better measure and understand stormwater flow across the city.
But the greater promise of digital technology is its ability to turn passive stormwater infrastructure into active structures that can be managed in real time to alleviate flooding. Detention ponds, for instance, are built to collect and temporarily store excess stormwater, then to release it back into the pipe system through a small drain that constricts and slows drainage. But if these ponds are already full when the next storm arrives, they are at high risk of overflowing, creating a range of problems from flooded streets to increased raw-sewage outfalls.
Kansas City used new cloud-based technology developed by Boston-based OptiRTC to turn its Gardner Avenue detention pond into a piece of intelligent infrastructure. Opti’s technology consisted of a valve controlled by cloud-connected sensors that track the volume of water in the Gardner reservoir as well as local weather forecasts. If the reservoir is full and a storm is on the way, Opti’s algorithms open the valve to drain its water, then close it back up before the next storm hits. “By adding digital technology to existing infrastructure,” says Opti CEO David Rubinstein, “Kansas City took a passive asset and made it into an active asset for managing stormwater flows.” (Sidewalk Labs is an investor in Opti.)
Rubinstein says the Opti system can be used to connect an entire regional network, monitoring water levels in neighboring reservoirs and water flow in the pipes that run between them, so that upstream reservoirs don’t release water until the downstream ones have emptied theirs first. “Sensors are affordable and ubiquitous, to the point where we can now measure far more than we can process,” says UMKC’s Kevern. “The challenge lies in figuring out how to utilize the data effectively.”
Bringing the community together
Having completed its Smart Sewer proof-of-concept for residential areas and digital technology, Kansas City has been moving on to industrial revitalization. In 2015, the city’s Water Services Department turned the parking lot of its Swope Park offices into a testbed for a variety of permeable and porous paving technologies, which allow water to seep through to the ground beneath. This year, in the shadow of I-670 near the old Stockyards, Kansas City has replaced a compacted, impenetrable gravel parking lot with permeable paving stones — the largest permeable brick installation in North America, according to Kevern. The city also targeted another Stockyards-area gravel lot, between Liberty Street and the railway corridor, where the planting of an urban forest is scheduled for completion in October 2020.
Kansas City’s approach is part of a growing body of innovative stormwater solutions in cities around the world. Washington, D.C., is the undisputed North American leader in green roofs, with more than 700,000 square feet of planted rooftops to divert stormwater before it reaches the sewer system. In the U.K., plans are underway to build the largest sustainable drainage system in the country, diverting stormwater from London’s Nine Elms social housing development — the size of 20 soccer fields — through green roofs, bioswales, and rain gardens. In China, Wuhan’s initial claim to fame, prior to the coronavirus, was its status as one of China’s top “sponge cities,” with extensive low-elevation green belts to act as a buffer between the city and the swelling Yangtze River.
But even as green infrastructure projects become more and more extensive, their impact remains local — and these local benefits often go beyond environmental gains. Back in Marlborough, Kansas City’s green infrastructure projects have brought the community closer together in surprising ways. To care for all the new green spaces, the community has set up a workforce development program for green infrastructure maintenance, providing work opportunities for people from disadvantaged communities and issuing them certificates in green infrastructure maintenance after one year on the job.
“I don’t think people here realized the benefits the program would bring,” says Diane Hershberger. “The program doubled our neighborhood green space from 20 to 40 acres. It has transformed some areas into community gathering places.” Marlborough residents thought they wanted new sidewalks, and ended up with much more.
Philip Preville is a writer based in the Toronto area.