Covington, Ky. — Strand Associates Inc,’s Terraced Reforestation Combined Sewer Overflow (CSO) Reduction project earned a 2014 American Council of Engineering Companies (ACEC) national Engineering Excellence Award for its green designs to manage stormwater runoff in Covington, Ky. At the onset of the project, more than 6 million gallons (mg) of stormwater runoff flowed into the local sewer system from a large hillside, located on the right-of-way of Interstate 71/75 in Covington. As a result, 4 mg of CSO streamed into the Ohio River annually. While this hillside propagated the runoff, it also provided the foundation for a unique solution.
With the 120 feet of elevation change, poorly draining soils, and geotechnical instability risks, the hillside seemed a difficult and even improbable site for stormwater reduction implementation. In spite of the challenges, the firm’s project team developed a design that would inevitably reduce the overflow by 93 percent.
To achieve this, Strand designed 12 terraced berms along existing 10-foot interval contours on the hillside. Totaling 4,740 linear feet, these berms contained a 6-foot-deep biofiltration system complete with a ponding zone to collect stormwater, engineered bioretention soil, and underdrains to capture and slowly release runoff, thereby limiting hillside instability. Design of the berms and surrounding areas called for planting 8 acres of native seeding and 300 trees to serve as evapotranspirators by effectively transferring stormwater from the soil to the atmosphere through evaporation or transpiration. The stormwater runoff pools on each terraced berm and, once collected, is funneled to an underdrain. This underdrain slowly releases a portion of the runoff to be filtered through the bioretention soil, which enriches and supports the surrounding vegetation. The remaining runoff then travels via the underdrain to the next berm to repeat the process. By the time the runoff reaches the sewer system, the original load is greatly reduced.
After project completion and final observations, the project removed 5.6 mg of CSO annually at a cost of only $0.17 per gallon. These results serve as a testament to green infrastructure’s ability to successfully and sustainably reduce stormwater runoff and CSO pollution.
The hillside location of the project not only provided a unique design, but also the perfect location to display green infrastructure at its best. The right-of-way of Interstate 71/75 is the most highly trafficked corridor in the region and with this high visibility, travelers and residents alike are reminded of how green infrastructure can sustainably, cost-effectively, and beautifully solve stormwater runoff and other pollution problems.
“The project was the first of its kind to demonstrate how green infrastructure stormwater management techniques can be designed and constructed on sloped topography and in clay soils while still providing cost-effective solutions for CSO control,” said James P. Gibson, Director of Integrated Water Management at Sanitation District No.1 of Northern Kentucky (SD1).
Strand provided critical planning and design services throughout the project. These services included the initial development of project concept plans, stormwater modeling and calculations, detailed design drawings and specifications, and construction observation. Project stakeholders include the design team, SD1, the City of Covington, and the Kentucky Transportation Cabinet.
“The Terraced Reforestation CSO Reduction Project has exceeded our expectations,” said Gibson. “We take great pride in the project.”