Detention system braves epic Atlanta floods

    Construction of a new baseball stadium and parking lot created a large impervious area that required an onsite detention system to hold and release stormwater to meet Gwinnett County standards and prevent downstream flooding.

    In September 2009, Atlanta and its surrounding counties experienced the deluge of a lifetime. The Chattahoochee River — the largest river in metro Atlanta — reached the 500-year flood level. Many areas of Atlanta experienced storm events in the 100-year to 200-year flood range. An Atlanta TV station reported rainfall totals ranging from 9 inches to 20 inches in some areas, producing stormwater problems on a massive scale and providing an extreme test for any storm drain system in the metro-Atlanta area.

    Gwinnett Braves stadium, Atlanta


    Planners and Engineers Collaborative
    A.L. Grading

    Product application

    Corrugated steel pipes, manufactured by Southeast Culvert Inc., were used in an underground stormwater detention system serving a new baseball stadium and parking lot.

    One of the areas impacted by 100-year to 200-year floods was Gwinnett County, current home of the Gwinnett Braves, an AAA farm team affiliate of the Atlanta Braves. The team relocated to a new facility from their previous home in Richmond, Va., in early 2009, which required construction of a new stadium and a large parking lot. Creation of so much impervious acreage required the site to hold and release a massive volume of water to meet Gwinnett County standards and prevent downstream flooding. This was accomplished with an underground corrugated steel pipe (CSP) detention system consisting of more than 1 mile of 12-foot-diameter pipe for a total storage volume of nearly 5 million gallons.

    A post-rain event visual inspection revealed that the high water mark came within 10 inches of the top of the 144-inch-diameter system — not much room to spare! According to Pam Little, a civil engineer at the time with Planners and Engineers Collaborative, the original design was intended to handle runoff from future development of adjacent properties in addition to the Gwinnett Braves stadium facility.

    “We ran four different future development scenarios and designed a system that worked for all four possibilities,” Little said. What was unknown at the time was that the 650,000-cubic-foot volume capacity would be tested much sooner than expected. The foresight of designing for future development allowed the extra capacity needed to contain the unexpected volume from a 200-year flood event.

    Difficult budget, aggressive timeline
    The stadium was constructed as a design-build project, so three detention alternatives were evaluated, including an open-air pond, an underground concrete vault, and an underground CSP system. Original conditions for the stadium property included wetlands in the form of two spring-fed farm ponds, a live stream flowing through the property, and stormwater runoff from a nearby strip shopping center. The Gwinnett Braves facility was a high-profile project with difficult budget constraints and a schedule to go from ground breaking to opening game within nine months, including six months during the fall and winter. The highest priority for the project was getting the parking lot to grade and graveled so stadium construction could progress. Because of the need to maximize parking space area, along with other design considerations, the above-ground detention option was eliminated, leaving the two underground options.

    No precast structures were available that could handle the stormwater storage volume needed on the project, so a concrete detention vault would have to be poured in place. This option would have required extensive structural engineering, a lengthy curing process, and an extraordinary price tag to achieve the required volume at the design depths of the stadium project. Because of its ability to meet both budget and time constraints, advantages of the CSP underground detention system became obvious.

    The design was for two separate CSP systems totaling 5,800 linear feet of 12-foot-diameter pipe, which was produced by Southeast Culvert Inc., located in Winder, Ga. The system was manufactured according to applicable American Association of State Highway and Transportation Officials (AASHTO) and ASTM specifications using a 1-inch by 5-inch corrugation profile. The two detention systems required more then 1.4 million pounds of American-made aluminum-coated steel (aluminized type 2), which is designed to provide more than a century of service life in this application, according to the Corrugated Steel Pipe Design Manual published by the National Corrugated Steel Pipe Association (NCSPA). The detention systems were installed with cover heights varying from 7 feet to more than 22 feet in some areas, well under the recommended maximum cover depth of 40 feet.

    Two pipe-laying crews with two large excavators installed two detention systems comprising a total of 1 mile of 144-inch-diameter corrugated steel pipe. Tees, elbows, and special riser fabrications were all identified with markings for easy installation.

    Another important aspect of the project was the water quality standards required to protect the wetland and live stream interacting with the site’s runoff water. Runoff from the developed site was treated with a combination of environmental treatment systems and vortex separation devices used to remove suspended solids before the storm drain entered the detention systems. An outlet control system was installed in a large precast box with an internal weir wall designed to detain and release the site runoff in a controlled manner. The weir orifices were protected from obstruction with debris by a system of two perforated CSP half rounds. The half rounds were bolted to the weir wall as a two-layered shell, with No. 57 stone filling the void space between the half rounds.

    The last hurdle to overcome in the design of the site was how to handle the spring water wetlands and live stream flowing through the property. Little accomplished this by rerouting the live stream through an aluminized CSP around the detention system and by designing a system of French drains to deal with the spring water flows. To maintain a natural volume of flow, both the live stream and the spring waters were tied back into the storm system on the downstream side of the outlet control structure, providing minimal disruption of the natural flow of wetland waters across the property.

    Imperative to meeting the tight time schedule was the execution of actual construction, which was the responsibility of Josh Tighe, project manager for A.L. Grading of Sugar Hill, Ga. With such a huge amount of large-diameter pipe on the project, logistics became an important part of the construction process. In coordination with the CSP manufacturer, plans were made to create material storage space on the site so that approximately 30 percent of the 144-inch pipe material could be delivered to the job before installation began, giving a crucial head start to logistical efforts.

    Preparation for installing the underground detention included earth excavation, blasting of rock, piping of the live stream, retaining wall construction, and spreading of a 2-foot bedding of No. 57 stone to support the detention systems. Installing the two detention systems kept two pipe-laying crews with two large excavators and multiple 40-ton dump trucks busy nonstop. The tees, elbows, and special riser fabrications were all identified with markings for easy installation. No. 57 stone was used to backfill the detention systems and was placed by a conveyor system that acted similar to a concrete pump, dropping the stone with precision and allowing the backfill to progress evenly from bottom to top, providing an immediately compacted envelope around the 144-inch pipe, which resulted in no deflection issues as well as time and labor savings. Because a detention system of this magnitude can be manufactured in only a matter of a few weeks, coupled with some crucial coordination of logistical challenges, the entire system was installed rapidly with only a few weather-related delays.

    “The time constraint played a huge factor in the choice of materials for this design-build project and corrugated steel pipe allowed the winter-time construction to be completed dead on targeted time,” Tighe said.

    The CSP system provided for the Gwinnett Braves stadium was put through an extreme test and performed above and beyond expectations. This project is just one example of what a steel detention system can provide — cost reduction, time savings, design flexibility, and excellent service even under extreme storm events.

    A conveyor system placed No. 57 stone to backfill the detention systems, dropping the stone with precision and allowing the backfill to progress evenly from bottom to top, which provided an immediately compacted envelope around the pipe.

    Wallace Johnson is vice president of Southeast Culvert Inc. He also is on NCSPA’s board of directors and chairman of the Product Promotion Committee.