City of Atlanta Department of Watershed Management’s Innovative Use of Permeable Roadways Solves Flooding
By Cory Rayburn
The City of Atlanta has embraced the use of green infrastructure on multiple fronts as a means of water quality improvement, flood reduction, and combined sewer capacity relief. Early adoption of runoff reduction standards for private development has led to over 5,500 sites installing green infrastructure practices such as bioretention, permeable pavement, and infiltration systems.
On the planning level, the Department of Watershed Management’s (DWM) Green Infrastructure Strategic Action Plan has provided a roadmap for integrating green infrastructure strategies into decision-making processes. Environmental Impact Bonds (EIB) have been incorporated into the city’s funding efforts for implementing both natural and engineered green infrastructure within historically impacted watersheds. Much of the city’s recent success in green infrastructure implementation is rooted in decisions made in 2012 that led to one of the largest regional green infrastructure projects in North America.
Mid-summer 2012 brought intense storms to the historic neighborhoods in southeast Atlanta that are served by the city’s combined sewer system. Much of downtown Atlanta, including a major intersection of interstates, Georgia State Stadium (formerly Turner Field), and a sea of parking lots, drains down through these residential areas. In July 2012, back-to-back rain events (10-year and 25-year storms) wreaked havoc on the neighborhood of Peoplestown. These rain events caused major surface flooding and combined sewer surcharges in low areas. Historic maps from the late 1800s show the area that was most impacted by the flooding was located where two streams used to converge. Trunk lines, the largest measuring 12 feet by 12 feet, now convey these historic base flows, sewage, and stormwater during rain events. However, in July 2012, the system could not adequately convey the volume of flows from these heavy rains and resulted in dozens of homes being damaged by flood waters.
City leadership responded to the impacted citizens immediately and committed resources to develop innovative solutions that would reduce the chances of future flooding.
The city developed the Southeast Atlanta Green Infrastructure Initiative, a combination of both gray and green solutions to provide sewer capacity relief and localized flood reduction. This strategy focused on developing projects to mimic the natural hydrology in this now built-out watershed.
Phase 1 involved constructing eight small-to-medium-scale projects that utilized common green infrastructure practices such as bioretention areas and stormwater planters. DWM staff and design consultants focused primarily on publicly-owned land in parks and within the right of way (ROW) to avoid any lengthy acquisition processes. These projects were constructed quickly to show the community the city’s commitment to solving the issues at hand; however, collectively, they provided limited relief to the overall problem.
Phase 2 of the initiative required more time to plan and design; however, it resulted in the largest known retrofit of existing streets using permeable interlocking concrete pavers (PICP). All in all, over 570,000 square feet of pavers coupled with 32 stormwater planters were installed across the southeast neighborhoods of Peoplestown, Mechanicsville, and Summerhill. Collectively, these green streets provide over four million gallons of combined sewer capacity relief along four miles of existing roadways. Completed in September 2016, these permeable roadways have proven to be an effective way of taking the pressure off the combined sewers by reducing the volumes, velocities, and rates of stormwater runoff at a critical junction in the system. The gray component of Phase 2 involved constructing a 5.9-million-gallon concrete vault to add capacity to one of the larger trunk lines that serves downtown Atlanta.
The permeable paver roadways were designed to detain the 25-year, 4-hour storm event (3.68 inches), mirroring the type of quick and heavy rainfall that caused the detrimental flooding in 2012. Traditionally, paver systems are sized for the first one-inch of rainfall (i.e., first flush) and roadways are generally avoided; however, the magnitude of the issue prompted the city to maximize the design within the given space.
Due to the limited public property in the basin upgradient of the flooding, the city was forced to look at the public ROW as a potential solution to managing stormwater. DWM engineers modeled the hydrologic and hydraulic flows within the basin and selected roadways that would best address these areas. Major arterial corridors were avoided due to the speed limits that paver roadway systems can adequately support. Locations of existing combined sewer trunk lines, topography, historic drainage complaints, and community input were also used to make these selections.
The construction of the roadways involved three main phases: demolition/excavation, stone aggregate placement, and PICP installation. It was quickly determined that the excavation phase would be the most difficult due to the number and condition of the utilities encountered. Some of the utilities and service connections required excavation using hand tools to prevent damage and many had to be repaired or replaced all together. The city allocated $1 million for utility work in the contract, but by the end of the project, nearly $3 million was spent.
Based on the hydrologic analysis and volume needed to provide significant capacity relief to the combined sewer system, most of the roadways were excavated to a depth of 44 inches. Impermeable liners were installed vertically on the edge of the roadways to prevent lateral seepage onto private property and intermittently across the roadways to create underground check dams on steeper streets. The subsoils at the bottom of the excavation were typically scarified to promote infiltration, and geotextile fabric was only used when poor soils were encountered.
Public involvement during construction was paramount and required a combined effort of the communication teams from both the city and the contractor, Southeastern Site Development. Due to the nature of the work, whole blocks needed to be closed for weeks at a time, directly impacting day-to-day activities of residents. Early on, the contractor committed resources and staff to ensure these impacts would be minimized. Prior to roads being closed, the contractor proactively engaged affected homeowners regarding the imminent work and identified any special needs during construction. City staff also assisted by coordinating messaging, hosting community meetings, and posting notifications on DWM’s social media platforms.
All in all, the paver roadway project took 19 months to complete. The contract included a three-year maintenance period starting in September 2016, during which the contractor was required to make any repairs, address potholes, replace cracked pavers, and conduct regular street sweeping.
The project has shown to be a cost-effective means of solving regional drainage issues in historic neighborhoods. When comparing the cost per gallon of capacity relief provided in Phase 2, the green and gray solutions were comparable; however, green infrastructure provides an aesthetically pleasing solution for the community, results in more tree canopy and green space (stormwater planters) and can manage surface runoff in built-out neighborhoods with limited public space. Gray solutions, such as vaults, have a real and direct benefit to aging infrastructure systems within the city; however, they can be difficult to construct and generally do not result in the same types of social, economic, and environmental benefits that green infrastructure can provide.
With the nearly $40 million spent on Phases 1 and 2, localized flooding still occurs at the known choke points of the system but modeling and recent rain events show a reduction in surface flooding based on the projects installed to date. The city is looking to move forward with Phase 3 of the initiative, which will include an additional vault for capacity relief and a series of regional stormwater ponds to manage surface flooding.
The Southeast Atlanta Green Infrastructure Initiative has helped to establish the city’s commitment to resolving age-old infrastructure challenges in innovative and sustainable ways. This regional approach to managing stormwater highlights Mayor Keisha Lance Bottoms’ commitment “to advancing green infrastructure that improves the quality of life and resilience of Atlanta’s communities.” WW