How to Prevent CSOs? Capture Stormwater Before It Hits the Combined Sewers.

Green stormwater infrastructure projects help KC Water make progress toward goals for reducing CSO frequency and volume

How to Prevent CSOs? Capture Stormwater Before It Hits the Combined Sewers.

KC Water uses green infrastructure, including rain gardens planted in the parkways between the sidewalk and street, to help attenuate stormwater flows.

KC Water has turned to landscaping to help cope with combined sewer overflows.

The water utility that serves Kansas City, Missouri, operates under a U.S. EPA consent decree to reduce combined sewer overflows and separate sewer overflows.

The decree has been amended three times, most recently in 2021. While the original decree called for building a system of tunnels to retain stormwater, the latest iteration gives the utility freedom to use alternative control measures that are potentially less expensive, including green infrastructure.

“Working with the EPA, we have implemented an adaptive management plan,” says Brian Hess, head of KC Water’s Smart Sewer Division. “That allows us to look at different means of capturing stormwater before it enters and overwhelms our combined sewer system. So basically, with the green infrastructure, we attenuate those flows.”

Green infrastructure alone typically doesn’t attenuate peak flows for the less-frequent, high-intensity, 10-, 25- and 100-year storms. Instead, it is used to manage stormwater from routine events that occur every year. Pilot programs with green infrastructure have shown potential to help keep flows within the pipes’ capacity and mitigate CSOs.

Spaces large and small

Green infrastructure can include bushes, grasses, trees and other landscaping designed to hold water and allow it to infiltrate the ground, or to hold water temporarily so it enters the system slowly after peak flows have passed.

The infrastructure can comprise a small space, like a rain garden, or large area such as a community park. Utility engineers use a hydraulic model of the sewer system to determine where to deploy the green resources.

“We model the impacts of certain types of projects and determine what benefits we can gain from each,” says Hess. “We look at a basin and do a plan that helps us understand what areas we can focus on and what we can expect from implementing green infrastructure or other control measures.”

The newer parts of the KC Water system have separate sanitary and storm sewers, but the older parts, some of which date to the 1850s, have combined sewers, which include 120 relief points called diversion structures, designed to overflow in wet weather. “Our objective is to minimize the number of times they overflow, or to eliminate overflows if possible,” says Hess.

The critical metric

KC Water calculates the effectiveness of green infrastructure using the metric of percent capture: the percentage of wet-weather flow that the combined sewer system captures and sends to the treatment plants.

In 2010, the percent capture was calculated at 45%, according to Srini Vallabhaneni, a former KC Water Smart Sewer officer. “The remainder was going untreated into the waterways, and that amounted to 6.38 billion gallons a year,” Vallabhaneni says. “That’s where the journey really started with the consent decree.”

Ten years later, the percent capture was 62%, but that still meant 4.38 billion gallons a year are overflowing. “Our target is capturing 85% of wet-weather flow by 2040,” says Vallabhaneni.

“Our journey continues, and we are now on the scale toward the next milestone, which is 66% capture by the end of 2024.” The next milestones are 74% by 2030 and 77% by 2035.

Green works

KC Water has a multiple-solution toolbox to reach its milestones. Sometimes a solution is to separate an area’s combined sewer. Sometimes it’s putting in bigger pipes to capture more of the water and then add treatment plant capacity to handle the additional volume. Sometimes it has been pump station improvements.

But for managing some parts of the stormwater system, such as highly developed areas with extensive parking lots and other impervious surfaces, green infrastructure is the preferred solution. “At the source-control level, green infrastructure works well,” Vallabhaneni says.

KC Water installs monitoring devices downstream of large projects to measure their impact. For example, the Middle Blue River pilot project, a green infrastructure installation that includes bioretention, rain gardens, permeable pavement and underground detention, the peak flow was reduced by 76% in a 1.4-inch rain event and flow volume was reduced by 36% from the project area.  

“Once the projects are built, it is written in the consent decree that we do post-construction monitoring of the facilities,” Vallabhaneni says. “We build on the data to validate the hydraulic model further and then confirm that the projects we’ve built are providing the capture needed to tie in with the milestones that we have.”

Smaller bites

The impact of small green infrastructure projects such as curbside bumpouts with rain gardens can’t be measured individually. Many of them are tested to verify that they are performing as intended. but collectively their impact shows up in the percent capture calculations. Kansas City has 230 green infrastructure sites, maintained by city crews, contractors and partner organizations, such as nonprofit agencies.

In late 2022, KC Water had 13 greener infrastructure projects in various stages of development. Some of the larger ones, including the Middle Blue River project, have monitoring devices downstream.

“A number of projects are under design or have just recently been implemented,” Hess says. “We are actively flow-monitoring those areas to be able to include them into our hydraulic model, so we can get a picture of before and after. It’s best to say we’re in the infancy of trying to figure out how effective those solutions are.”


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