Rethinking the Green Approach

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    Strand Associates, Inc. earned a 2014 National American Council of Engineering Companies Engineering Excellence Award for the 2013 Terraced Reforestation Combined Sewer Overflow (CSO) Reduction project. Photo: Strand Associates, Inc.

    Sustainable practices require engineers to expertly cultivate green infrastructure in the surrounding environment to adapt to, enrich, and enhance the local landscape. However, when the environment is a hillside with 120 feet of elevation change, it’s not always easy to stay green. Nevertheless, for Strand Associates, Inc.’s 2013 Terraced Reforestation Combined Sewer Overflow (CSO) Reduction project, going green to reduce stormwater runoff was the best and most cost-effective option.

    The Sanitation District No. 1 of Northern Kentucky (SD1) is under a consent decree to address CSOs, with an emphasis on the use of sustainable green infrastructure solutions. Strand provided planning and design services for the terraced reforestation project in Covington, Ky., to assist SD1 in reducing overflow volume in the Willow Run sewer system.

    Each year, SD1 faced more than 6 million gallons of stormwater runoff that flowed into the local sewer system from a large hillside, located in the right-of-way of Interstate 71/75 in Covington, the most visible transportation corridor in the Northern Kentucky region. The local sewer system was not designed to handle the substantial overflow and, as a result, approximately 4.2 million gallons of remaining CSO streamed into the Ohio River annually, presenting a potential health hazard as many residents routinely use the Ohio River for recreation.

    The propagating hillside presented unique and almost insurmountable design challenges. However, these challenges sowed the seeds for an unprecedented green solution: terraced earthen berms.

    A series of 12 terraced clay berms at existing 10-foot-interval contours help retain stormwater in 6-foot-deep biofilitration systems. Image: Human Nature

    The principles associated with green infrastructure — stormwater capture and treatment at its source — were employed for the terraced reforestation project. The steep topography and other site challenges and constraints made this hillside seem like an impossible location to implement more common types of green infrastructure. Therefore, the project team developed a green infrastructure concept and design to construct a series of large, interconnected terraced berms along the hillside to store, treat, infiltrate, and slowly release the captured stormwater runoff.

    “Due to the steep topography of the site, there were very few stormwater management alternatives that could have been implemented at this location,” said Chris Rust, Strand project manager and lead design engineer. “The terraced reforestation allowed for the most efficient capture and treatment of tributary stormwater runoff along the entire length of the hillside, as well as from the upstream drainage area.”

    With more than 120 feet of elevation change and clayey soils typically ill-suited for common green infrastructure, it seemed almost impossible to implement the only solution available to the team. However, it was not only the hillside that presented substantial roadblocks to green design; modeling the method and benefits of possible green infrastructure was also a challenge.

    Terraced berms run for 4,740 feet down the hillside and stretch nearly a mile in length. Image: Human Nature

    Standard stormwater models are limited in their capability to model the project’s terraced berm layout, and for the modeling to provide accurate and detailed stormwater calculations, it needed to blend hydraulics and hydrology with soil science to retain the integrity of the hillside. This newly blended model provided essential data concerning the effects of the CSO reduction method on the hillside. Strand provided summary data to SD1’s CSO modeling team to accurately model the potential benefits of terraced berms.

    “The development of the erosion and sediment control plan presented challenges while trying to maintain the steep, disturbed hillsides and concurrently protecting highly susceptible bioretention soils and underdrain installations from silty clay runoff,” Rust said.

    Without succinct modeling, the project team could have implemented a design that caused more harm to the community through increased pollutants entering the Ohio River.

    More than 300 newly planted trees and 8 acres of native seeding serve as evapotranspirators, effectively transferring stormwater from the soil to the atmosphere. Photo: Human Nature

    While there are multiple common methods of green infrastructure, such as bioswales and rain gardens, terraced earthen berms constructed on a hill with this magnitude of elevation change was certainly an unprecedented solution.

    Using a series of 12 terraced clay berms at existing 10-foot-interval contours, complete with a 6-foot-deep biofilitration system, the team turned the hillside’s topography into an advantage. The terraced berms run for 4,740 feet down the hillside and stretch nearly a mile in length. The berms included more than 300 newly planted trees and 8 acres of native seeding to serve as evapotranspirators by effectively transferring stormwater from the soil to the atmosphere through evaporation or transpiration. This strategic layout of the berms maximized the attenuation of peak flow rates during rainfall events, thus providing a significant benefit for SD1 through downstream CSO volume reductions.

    The berms support a treatment-train configuration hydraulically connected via an elaborate underdrain system. During smaller, typical-year rainfall events, stormwater runoff is initially ponded on the surface of each terraced berm and, once collected, is filtered through a specifically mixed bioretention soil that enriches and supports the surrounding vegetation. Flows through the bioretention soil are ultimately collected in an underdrain system that slowly releases a portion of the runoff to the next downstream berm to repeat the process. By the time the runoff reaches the sewer system, the stormwater runoff is significantly reduced and the water quality is substantially improved.

    The terraced reforestation project has transformed the local environment from a plain, mowed-grass hillside to a beautiful landscape covered with blossoming vegetation. Additionally, with the reduction in CSO volume discharging to the Ohio River, the project has improved the quality of life and health of the surrounding community. Finally, the project serves as an education tool and essential lesson that while going green may seem improbable, it is never impossible.

    “This project included innovative engineering concepts, design, and construction of terraced berms intended to intercept and reduce stormwater runoff upstream of SD1’s largest combined sewer overflow, thus providing a water quality benefit for our community,” said James Gibson, director of Integrated Water management at SD1. “Not only does this project provide a CSO reduction benefit to SD1, it also serves as a sustainable gateway for Northern Kentucky along Interstate 71/75.”

    The Terraced Reforestation for Sustainable CSO Control project showcases how green infrastructure can be more cost effective than traditional gray infrastructure solutions, while also providing additional benefits and a community amenity. The collaborative team effort between all project stakeholders, including the design team, SD1, City of Covington, and Kentucky Transportation Cabinet, was instrumental to the success of the project. With more than 140,000 vehicles passing the hillside each day, the project serves as a sustainable legacy that will inspire future generations of engineers to rethink green.

    “The high visibility of the project site will provide a constant reminder for future generations of how stormwater runoff can be managed sustainably and cost-effectively to improve water quality,” Rust said. “The project serves as a national model of how green infrastructure can be adapted in challenging settings, which can encourage a more widespread implementation of green infrastructure throughout the nation.”

    Katelyn Vitek is a writer with Strand Associates, Inc. in Madison, Wis. She can be contacted at katie.vitek@strand.com.