Located in a highly developed urban setting within Atlanta, GA, the Roybal Campus of the Centers for Disease Control and Prevention (CDC) contains large areas of impervious surfaces that require stormwater management and detention. Constructed during the 1950s before modern requirements for controlling surface water runoff, the CDC had only minimal stormwater retention. Under a long-term master plan to redevelop and upgrade its existing facilities and infrastructure, the CDC initiated a program to modernize the existing onsite stormwater systems.

The Roybal Campus is divided into two drainage basin sections known as east campus and west campus. Working with Dekalb County, GA, the CDC sought to ensure that stormwater originating on the 13.9-acre east campus is managed in the most environmentally responsible manner possible. Unlike many suburban projects where land is less expensive and space is readily available, the urban campus is essentially landlocked by surrounding developments. With space at such a premium, the setting afforded little room for a detention pond or other surface structures.

Instead, the tight quarters necessitated the use of an underground stormwater detention facility, part of which is located beneath a roadway. In addition to providing storage, the system was designed to provide a water-quality benefit by filtering the first flush of stormwater entering the unit.

The challenge of determining an appropriate detention system for the east campus was unique in that myriad criteria had to be evaluated to select a detention system to meet the needs of the CDC while satisfying local stormwater management requirements. Factors such as a densely developed site, extensive underground utility infrastructure, security requirements, water-quality considerations, and maintenance concerns all had to be evaluated during the selection process. The use of a versatile underground detention system offered an elegant solution that perfectly addressed the needs of the CDC, exemplifying the fact that in urban areas the highest value often results from flexibility, awareness of life-cycle costs, and reduced environmental impacts.

Evaluating Existing Conditions
As noted, the Roybal Campus had only minimal stormwater management because it was constructed well before the local county (DeKalb) government established regulations for controlling stormwater in 1975. In fact, it was not until 2000 that redevelopment of the west campus included the installation of a 1-acre detention pond for controlling stormwater on the west basin.

The east campus, however, continued with its existing stormwater systems until its planned redevelopment began in 2004. Following the redevelopment, the east campus basin would comprise approximately 10.6 acres of impervious area and 3.3 acres of pervious area, which accepts no offsite drainage.

Runoff from the east campus is discharged by means of three pipes: a 24-inch-diameter reinforced-concrete pipe, a 30-inch-diameter reinforced-concrete pipe, and a 42-inch-diameter corrugated metal pipe. Runoff from the east campus eventually enters Peavine Creek, an urban stream that is prone to flooding and flows past extensive residential development downstream.

In accordance with the provisions of Section 303(d) of the Clean Water Act, the Georgia Environmental Protection Division (EPD) has included Peavine Creek on its list of waters not meeting designated water-quality standards. According to the EPD, Peavine Creek is unable to support its designated use of recreation because of fecal coliform contamination. Future assessments of the stream are expected to recognize sediment as another contaminant contributing to the impairment of the stream.

Meeting New Requirements
In 2003, the CDC retained the consulting engineering firm PBS&J to serve as project engineer for the renovation of the east campus. The renovation had to comply with various design criteria stipulated by DeKalb County. A key requirement dictated that the peak release rate of stormwater not exceed 90% of the peak stormwater runoff rate from the area in its natural state for all storms from two-year through 100-year events. In other words, the rate at which runoff exited the east campus could not exceed the rate at which runoff would occur if the area were still in a natural, undeveloped state.

However, hydrologic studies indicated that providing detention equivalent to a “natural,” predeveloped site would prove infeasible. After several pre-design meetings with Dekalb County representatives, project participants agreed that even though the existing site is densely developed, predeveloped conditions would be considered comparable to 0.5-acre residential lots. Even with this design change, the amount of controlled runoff leaving the site following construction of the detention facility was expected to be significantly less than compared to that leaving the existing developed site.

Hydraulic analysis of the site was conducted using the Hydroflow Hydrographs software package from Intelisolve for accumulating and routing flows through storage. The Soil Conservation Service (SCS) curve number method was used to estimate the approximate amount of runoff that could be expected to occur at the site as a result of rainfall. This method generated unit hydrographs for the two-, five-, 10-, 25-, 50-, and 100-year storm events based on 24-hour rainfall runoff data for the Atlanta area. Water-quality measures were designed in accordance with the Georgia Stormwater Management Manual. Manning’s equation was used to size stormwater piping for the 100-year storm event. Finally, the StormCAD software package was used to develop hydrologic computations for the completed site.

The time of concentration for the predeveloped drainage basin is 28.8 minutes. As noted above, the existing site was deemed comparable to 0.5-acre residential lots, with a runoff curve number equal to 70. The peak runoff quantities for the predeveloped conditions were calculated and used as a benchmark.

Meanwhile, the time of concentration for the developed drainage basin is 13.8 minutes, and the site was assigned a weighted curve number of 91. These factors were used to calculate the peak runoff quantities for the developed conditions. Although development of the east campus increases peak runoff to the detention system, a specially designed outlet structure ensures that drainage from the system complies with DeKalb County’s requirement that post-development flows discharged from the site not exceed 90% of the predeveloped peak flow rate. Flows discharged from the detention system are reduced to within one-tenth of the total allowed to leave the site.

Rethinking the Right Approach
Initially, the project called for constructing a basement-level detention vault beneath the newly designed Transshipment Building, which is intended for storing and processing materials for delivery campus wide. Covering the entire footprint of the Transshipment Building, the vault would have been approximately 9 feet deep with a storage capacity of roughly 120,000 cubic feet. Structural support columns were to have been interspersed within the vault to support the main level of the Transshipment Building above. A hatch large enough to allow a small loader to be lowered into the vault for purposes of maintenance and cleaning would have afforded the only access.

However, this solution would have created a variety of safety and maintenance concerns for the building. To address these concerns, PBS&J, the CDC, project architect TVS Design, and the Turner Construction Company–the construction manager for the project–undertook an extensive value engineering process to determine the most cost-effective and maintainable system for managing stormwater on the east campus.

Several alternatives were reviewed during these sessions, including underground pipes, vaults, box culverts, and a subsurface stormwater management system comprising perforated plastic chambers with stone bedding manufactured by StormTech LLC. Although the StormTech system proved to be a cost-effective alternative, the design team was not familiar with the product and initially hesitated to recommend the system for use at the CDC. However, after substantial review and discussion with the CDC and StormTech representatives, the project team agreed that the StormTech system was not only cost-effective but also provided long-term reliability, maintenance accessibility, and installation flexibility. For these reasons, the StormTech system was selected.

Accommodating Tight Conditions
The StormTech system used for the project consists of two interconnected beds of open-bottom chambers, each of which is more than 7 feet long, 2.5 feet tall, and 51 inches wide. The chambers rest on and are surrounded by stone aggregate. Together with the aggregate, each chamber provides 193.2 cubic feet of storage. Other basic components of the system include inlet manifolds, bypass structures, “Isolator Rows” of chambers wrapped in fabric, and underdrain piping. Fabric to protect against scouring was placed at chamber inlets, and each Isolator Row has an inspection port to monitor sediment accumulation.

The length and width of the system is variable to accommodate tight conditions at the site. Bed #1, which has a stone base with a depth of slightly more than 7 feet, is 130 feet at its longest point and 42 feet at its widest. Bed #2, which has a stone base with a depth of slightly more than 9 feet, is 213 feet at its longest point and 105 feet at its widest. The irregular shape of the beds was dictated by the presence of structures and other utilities. One benefit of the system is its flexibility: Rows of chamber units may begin and end as necessary to accommodate site constraints.

For the CDC site, the tight constraints meant that the system footprint had to be small, yet be able to hold a large volume of water relative to the system footprint. For this reason, the depth of the base stone is greater than usual. Typical installations in Georgia commonly employ 6 to 12 inches of base stone. As noted, this installation boasts a stone base of slightly more than 7 to 9 feet of stone, depending on the location. Stone used in StormTech systems typically has a porosity of 40%. All told, the entire system has a storage capacity of 110,877 cubic feet.

The system diverts the first flush of stormwater that typically contains the most pollutants into the Isolator Rows. Bed #1 includes one Isolator Row that consists of 17 chambers, while bed #2 has seven Isolator Rows totaling 86 chambers. These rows are outfitted with a woven geotextile fabric placed between the stone base and the individual chambers. Meanwhile, a nonwoven fabric surrounds the chambers themselves. As stormwater exits through the open bottom and perforated sides of the chambers, these materials remove total suspended solids and total phosphorous. In this way, sediment is captured in the Isolator Rows, preventing its accumulation in adjacent chambers and stone base, simplifying maintenance procedures, and reducing the cost to remove sediment from the system.

Isolating the First Flush

During smaller storm events, all incoming flows likely will enter the Isolator Rows. During larger events, heavier flows that typically contain less contamination than the first flush will crest a weir located in the inlet immediately upstream of an Isolator Row, discharge to a manifold, and enter the rest of the chamber system. Isolator Rows may be sized in terms of flow rate or volume. For this installation, the Isolator Rows were designed to treat the water-quality flow rate entering the system. Based on a third-party evaluation of the trapping efficiency of the Isolator Row system, together with the water-quality flow rate calculations per the Georgia Stormwater Management Manual, the system was estimated to remove more than 95% of sediment.

Although the StormTech system can be designed to facilitate infiltration, this approach was not incorporated for the CDC system because of the limited infiltration rates of the surrounding soils. Instead, drainage of the system is accomplished by means of a system of underdrain pipes located at the bottom of the stone base beneath both beds. The pipes connect directly to an outlet control structure.

Controlling Flows at the Outlet
This structure consists of an 8-by-11-foot concrete box that was specially designed to regulate the flow of stormwater from the site. The outlet structure has an intermediate weir wall containing circular orifices of differing sizes located at varying elevations. After passing through the StormTech system, stormwater backs up behind the weir wall. In keeping with the local stormwater requirements, the weir releases flows through the orifices at a rate not to exceed 90% of the site’s pre-developed flow rate.

The detention system provides the necessary volume to enable flows to back up within it while the stormwater is released at the reduced rate. However, each of the four inlet manholes serving the system connect to a bypass line that runs directly to the outlet control structure, enabling water to flow straight to the outlet if the system is full. In the event that the detention system fills up, the top of the weir wall in the outlet control structure will act as an emergency spillway. In this way, the outlet control structure prevents stormwater from backing up on the site if the detention system is full.

The detention system meets current design criteria regarding the quality and quantity of stormwater discharges, with no negative impact to Peavine Creek. Meanwhile, because the east campus is undergoing a phased process of redevelopment, PBS&J designed the system to control the quantity and quality of stormwater runoff from the site during, as well as after, the various phases of construction on the east campus.

Proceeding Smoothly With Construction
Construction began in March 2007. Excavation of the site, placement of stone, and installation of the StormTech chambers proceeded smoothly and was completed in approximately 1.5 months. Roughly 218,000 cubic feet of stone were included as part of the installation.

The detention system is located near one of the main entrances to the east campus. Following the completion of the detention system, Turner Construction built a roadway above approximately 90% of the underground storage system. For this reason, the StormTech chambers were designed in accordance with ASTM International’s Standard Specification for Polypropylene (PP) Corrugated Wall Stormwater Collection Chambers (F2418-05) and Section 12.12 of the Load and Resistance Factor Design Bridge Design Specifications published by the American Association of State and Highway Transportation Organization (AASHTO).

Typically, the intended service life of a subsurface storm drainage system ranges from 20 to 100 years. The limiting criterion for service life is generally long-term structural stability. For a design to be safe, structural safety factors must be demonstrated for the entire service life of the project to account for uncertainties in loading, installation, and material performance. For example, the AASHTO design procedures mandate load factors of 1.75 for live loads, in addition to factors to account for impact effects and the presence of multiple vehicles, and 1.95 for earth loads on buried culverts. For dead load design, the thermoplastic product must be able to withstand the continuous dead load and remain stable after 50 years or more under sustained load.

To date, the finished system has performed as expected and without incident. Construction of the detention system cost approximately $1.5 million, including the cost of the units themselves. By comparison, the basement-level detention vault that had been pursued originally was estimated to cost $1.3 million. However, after accounting for long-term maintenance costs and health concerns, the project team concluded that the StormTech system ultimately was more cost-effective. Moreover, the CDC desired to avoid having ponded water beneath its Transshipment Building and appreciated the additional water-quality benefit afforded by the StormTech system.

Assessing Operations and Maintenance
As for operation and maintenance requirements associated with the underground detention system, the Isolator Rows require periodic cleanouts by means of vacuum trucks. To facilitate such cleaning, each Isolator Row is connected directly to an access structure. When cleaning out Isolator Rows, maintenance crews feed a self-propelled jet head through the access structure and down a row. In this way, the jet scours sediment that has accumulated on the fabric of the Isolator Row and flushes it toward the access structure. After the jet nozzle has passed down the entire row, the vacuum hose is used to remove sediment from where it has accumulated in the access structure.

Sediment levels within a row may be monitored by means of the access structure or inspection ports. The amount of time between cleanings depends on the length of the row, local rainfall, and site conditions. Generally, longer rows or larger capacities afford longer intervals between maintenance. Depending on local water-quality regulations or requirements, StormTech recommends removing sediment as soon as 1 to 3 inches have accumulated in a row.

Increasing in Popularity
Subsurface underground detention systems similar to the one installed at the CDC campus increasingly are being used to manage stormwater in suburban and urban areas. Installing such systems underground saves valuable land that can be used for parking spaces or other needs. In addition, the flexibility of the system facilitates installation around building footprints and underground utility infrastructure, making it a valuable option for use in densely developed areas.

Certain environmental benefits also accrue from the practice. For example, storing stormwater underground helps conserve water by avoiding evaporation that otherwise would occur aboveground. In appropriate soil types, underground detention can promote groundwater recharge. Furthermore, subsurface storage helps to eliminate potential breeding areas from disease-carrying mosquitoes that pose a health hazard and may require expensive spraying to prevent outbreaks. Finally, ponds must be secured by means of fencing and other measures to prevent accidental drownings and–in the case of this particular installation–comply with stringent CDC security standards. For these reasons, underground detention systems such as the one installed at the CDC east campus can be expected to continue to gain in popularity as a means for managing stormwater in urban areas.

About the Author

Greg Rosser, Donald P. Johnson, Kathy McCabe, and Ed Pisowicz

Greg Rosser, P.E., is a project officer for the Centers for Disease Control and Prevention, in Atlanta, GA. Donald P. Johnson, CCM, CPESC, is a construction project manager, and Kathy McCabe, P.E., is a senior civil engineer, in PBS&J’s Atlanta office. Ed Pisowicz is regional product manager, south region, in the Mableton, GA, office of StormTech LLC.

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