Stormwater Stalwarts
The EPA is getting tough on stormwater
erosion and contaminants. Here’s a look
at how precast concrete can successfully meet
their requirements.
By Kent Sipes
Stormwater treatment used
to be just a matter of channeling excess water
safely into a lake or river. And channeling
the water “safely” only meant making
sure the water volume didn’t damage life
or property on its way to the destination. Once
it reached the lake or river, it was someone
else’s concern. New EPA guidelines have
changed all that.
When stormwater runs over
a hard, non-porous surface like a parking lot
or a street, it washes oil, silt and trash from
the surface. Unless the stormwater is filtered,
these pollutants may enter the water supply.
In the past, this was not as big a problem as
it is today, because most stormwater was filtered
by running into and through several feet of
soil. However, an increase in paved surfaces
has created the need for man-made filtration
of stormwater.
New EPA guidelines require
larger cities and counties to take steps to
improve stormwater quality prior to discharge.
These steps include public education about stormwater
runoff, street sweeping programs to remove trash,
regulation of construction sites to minimize
erosion, and the production of a “stormwater
atlas” that details all components of
a stormwater collection and discharge system.
The target result is an 85 percent reduction
in contaminants that would have otherwise entered
the water system.
Here is a look at two
stormwater treatment projects that used precast
concrete components to solve design and installation
challenges. Both projects adapted antiquated
stormwater treatment methods to meet modern
standards.
Largo, Fla.
The stormwater runoff
from a 90-acre basin in Largo, Fla., was a major
concern to the city’s Stormwater Program,
especially since the site also contained a 25-acre
apartment complex. The previous system used
in the basin merely drained the water, along
with runoff containing hydrocarbons and trash,
into the 8th Street discharge point. Since Largo
is part of the EPA’s NPDES (National Pollution
Discharge Elimination System) Phase 1 program,
requiring the city to take proactive measures
to improve water quality, the basin was a natural
target for a retrofit.
Mike Sepessy, stormwater
program manager for the City of Largo, says
he faced several challenges in this project.
“First, since this was a municipal project,
we were operating under a very tight budget.
Using precast components helped to greatly reduce
our cost,” he says.
“Second, we needed
a system that would incorporate two input pipes
and a diversion box,” he continues. “Since
precast components could be custom-designed
for our needs, this was not a problem.”
Finally, since there was such a large drainage
basin and lots of upstream flow, he needed to
use a large stormwater separator system. “Again,
custom-designed precast components provided
our solution.”
Sepessy adds that the lower
cost and increased flexibility of precast concrete
components compared to other types of construction
helped to solve problems with other above- and
below-ground water. “Since the water table
here in Florida is only inches below the ground
surface, and because excess water on the surface
is a constant problem, we needed to use well-pointing
equipment and mud pumps. Because we used precast
concrete pipes, boxes and walls, we were able
to add this other equipment in a cost-effective
manner.”
Chris Landt of CDS Technologies,
the developer of the project, says, “Since
the weir box we needed for the 8th Street Outfall
project was so large (10 feet by 8 feet by 8
feet), it helped that the precaster shipped
it to us in two pieces with a horizontal seam.”
He says this made installation of the box a
lot simpler.
“For stormwater
treatment systems, it’s just a given that
precast is the first choice,” adds Landt.
“It lasts longer than anything else and
it’s very strong.” In fact, precast
concrete can be designed to provide various
strengths, depending on the need, and its strength
gradually increases over time.
Atlanta
Grant Park, home of the
Atlanta Zoo, had a problem with debris washing
off its two parking lots into the storm drains.
Overloaded storm drains became clogged with
leaves and trash, thus excess rainfall had nowhere
to go. Water collected in low spots, giving
the parking lots a flooded appearance.
Erosion was also a problem
in the park. Because of insufficient drainage,
recreational fields flooded and washed silt
into the stormwater system. Further, the old
stormwater collection system used 3-inch clay
pipes that were totally inadequate.
The City of Atlanta and contractor
Site Engineering Inc. decided to build a retention
pond large enough to hold rainfall from a 100-year
storm event. They also installed two large precast
concrete systems to separate stormwater from
trash and oil washing off the lots. The reinforced
concrete pipes now carry the filtered runoff
to the retention pond.
David Hess, Project Manager
for Site Engineering Inc., says, “We were
under severe time constraints, because the zoo’s
peak attendance occurs between April and September.”
His company completed the first lot’s
drainage prior to April and began work on the
second lot in September. “The use of precast
components assisted in completing the critical
path in time.” Hess adds that the work
was not hampered by the very wet spring, since
the installation of precast components isn’t
greatly affected by the weather.
Clint Voyles, foreman for
Site Engineering, says precast concrete even
improved the safety of the job. “Because
precast can be immediately installed, the ditches
don’t have to be open for as long as with
other types of construction.” Since precast
concrete pipes don’t require backfill
material that’s as costly as the material
required for other types of piping, additional
cost savings were realized on the project.
Voyles also noted that design
changes occurring during the construction phase
would have created much more of a problem if
precast hadn’t been used. “There
were changes made to grades, and the Grant Park
Conservancy Organization asked us to save as
many of their trees as possible,” he says.
“We had been
scheduled to remove 30 to 40 of the large, old-growth
trees in the park, but we were able to alter
our design and removed only five of the trees,”
says Hess. The smaller footprint typical of
precast components (compared to other types
of construction) helped with the design as well.
Stormwater treatment will
continue to be a concern for both established
cities and growing population centers, and EPA
regulations on water quality will continue to
impact both public and private sectors. Precast
concrete provides a wealth of flexible, cost-effective
solutions for stormwater treatment.
Project Profile
Project Name:
8th St. Outfall
Owner: City
of Largo, Fla.
General Contractor:
Site Engineering
Inc., Atlanta
System Developer:
CDS Technologies,
Canton, Ga.
Precast Manufacturer: Atlantic Precast, Florida
Division, Sarasota, Fla.*
*Atlantic Precast,
Florida Division is a certified plant under
NPCA’s Quality Assurance/Plant Certification
program.
Project Name:
Grant Park
Owner:
City of Atlanta
General Contractor:
Site Engineering
Inc., Atlanta
System Developer:
CDS Technologies,
Canton, Ga.
Precast Manufacturer:
Atlantic Precast,
Georgia Division, Douglasville, Ga.*
*Atlantic
Precast, Georgia Division is a certified plant
under NPCA’s Quality Assurance/Plant Certification
program.
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