Washington Utility Dovetails Data Collection With Strong Community Engagement

Washington Utility Dovetails Data Collection With Strong Community Engagement

Thornton Creek winds through a forested area within the study area near Meadowbrook Pond off of 39th Avenue Northeast in Seattle.

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In an effort to explore I&I remediation as an alternative to expensive sewer upgrades, King County, Washington, is embarking on a dual-purpose public outreach and data collection program.

The Thornton Creek Basin Sewer Study and Upgrade Project — run by the King County Wastewater Treatment Division — will ensure the 1.2-mile Thornton Creek sewer trunk that serves 9.6 square miles of North Seattle is able to handle its flow during heavy rains and prevent overflows. Whether that is accomplished by I&I remediation, a sewer upgrade or a combination of both remains to be seen.

Since 2005, three overflows have occurred in the Thornton Creek trunk. In addition, portable flowmeters recorded surcharged water levels in 2010 and 2012, upstream of the Thornton Creek trunk junction structure. Overflows and surcharge conditions indicated a need for additional downstream capacity to convey flows.

To kick off the project, King County completed its first community survey last summer, which served two purposes for the utility, according to Marie Fiore, strategic communications coordinator. The first purpose is to engage and educate the public about I&I problems and potential solutions, and the second is to collect data on problem areas where there might be flooding and/or limited stormwater infrastructure that would lead to the installation of basement drains, sump pumps and downspouts connected to the local sewer system.

“We had a tremendous turnout of over 700 responses to the survey,” she says. “We hope to use the survey responses, along with other data collected, to estimate the amount of flow that may be entering the local sewer system through inflow.”

The next phases of the project include field inspections from the fall of 2021 through winter of 2022 (flow monitoring and inspecting sewer pipes, maintenance hole covers and drainage connections); identification of I&I reduction alternatives in the spring of 2022; a second community survey slated for the summer of 2022; and the final identification of preferred I&I reduction methods in the winter of 2023. Together, the study phases aim to help King County identify a sustainable and cost-effective alternative to a major capital investment to the regional sewer system.

The utility is in the beginning phase of identifying I&I sources for the Thornton Creek basin, and additional data is still being collected from local sewer agencies. “Survey responses point to a combination of private property connections to the local sewer system and infiltration due to an aging 50-plus-year-old sewer system servicing the neighborhoods,” says Fiore. “King County is planning a sanitary sewer evaluation study of select local public sewer pipes to better understand the primary sources of I&I, using CCTV, smoke and dye testing to identify sources of inflow.”

Community engagement

As the project continues, King County officials plan to work closely with the community to learn about its needs and priorities for sewer system improvements, offering email updates on project progress. Meanwhile, teams of workers will contact residents in areas determined to have significant I&I to learn more about the sewer in the area, drainage conditions and potential private-side issues. 

“The response we’ve received from our engagement efforts has been very encouraging,” says Fiore. “There is a real interest in the work that we are doing, and we want to communicate that we’re working hard to meet regulations, improve resiliency and upgrade vital infrastructure as efficiently as possible. The community expects us to protect public health and the environment. We hold their trust dear to us and we take our mission seriously.”

King County is also making headway with social marketing campaigns to educate the public about lateral pipe maintenance and the ramifications of inflow and infiltration. “A lot of homeowners don’t know or plan for side sewer maintenance, and even more don’t realize that a broken or cracked sewer pipe can add so much additional wastewater strain to the system. Public outreach, an open line of communication and education efforts can go a long way to mitigating I&I issues.”

Moving forward

While it’s still in the early stages of the two-year effort, the project has identified flow reduction goals for King County’s 30-year planning horizon. “If it looks like I&I reduction alone won’t meet this goal, we will explore other options like achieving flow reduction goals for a shorter timeframe, and/or looking at alternatives that would combine upsizing of the trunk line with the most limited capacity and I&I reduction projects. The costs and benefits of I&I alternative flow reduction scenarios will later be compared to a traditional pipe capacity increase project alternative.”

Fiore says similar programs could be a good approach for utilities where there isn’t much population growth expected and I&I makes up a significant amount of system flow. “Initial analysis indicated that I&I accounted for over 85% of the 20-year peak flow in 2020. This said, achieving enough I&I reduction to reduce peak storm flows can be difficult, and anticipating how much future I&I projects can reduce these peak flows into the regional sewer system involves a greater amount of risk than a traditional sewer capacity increase project.”

For any utilities looking at a similar program, Fiore emphasized establishing a standard, or methodology, to measure I&I. “We use a 20-year recurrence developed from calibrated hydrologic basin models. Monitor several years of flows and rain gauge data to determine baseline existing I&I, and for comparison with post-rehabilitation monitoring.” 


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