Efforts to control stormwater runoff at the urban campus of the Centers for Disease Control and Prevention led to the novel installation of an underground detention system.
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.
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Photo: CDC
Installation of the underground detention
system on the CDC campus |
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 (Table 1).
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. As
shown in Table 1, 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.
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Photo: StormTech
Underground stormwater
detention saved space on the
densely developed site. |
Isolating
the First Flush
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.
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.