Managing
stormwater runoff with detention ponds is like trying to lose
weight by taking diet pills: long-term consequences outweigh
short-term benefits. Natural systems such as wetlands do the
job better, more beautifully, and more responsibly.
By Deborah Snoonian, P.E.
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Case Study
Cottonwood Creek Relocation,
San Marcos, Tex.
Company: Turner
Collie & Braden, Inc., Houston
Developers were required to reestablish a creek displaced
by the expansion of a nearby shopping mall. Cottonwood
Creek receives runoff from not only the mall’s
roof and parking lot, but also from I-35, a major highway,
and from a sand-and gravel-making operation located
not far upstream of the site.
The creek receives runoff from a variety of sources,
then drains into the San Marcos River—one of the
most pristine waterways in Texas hill country. “The
local economy really depends on the tourism generated
by the river—glass-bottomed boats, fishing, and
so forth,” said Georganna Colllins, a wetland scientist
who helped design the project. “We had to make
sure the water leaving Cottonwood Creek was as clean
or cleaner than it was before the mall was expanded.”
The project team decided that the best way to accomplish
this goal was to move Cottonwood Creek—literally
(see photo at right). “We drained the creek and
salvaged all its organisms: aquatic life, vegetation,
even the soil and large trees whose roots grew under
the streambed,” said Collins. If done right, moving
the stream flora and fauna meant less time would be
needed for the wetland to reestablish itself in a new
location.
The project hit a snag when the landscape contractor
walked off the job after a few days, citing the idea
as crazy. Luckily, the project’s general contractor
stepped up to the challenge of relocating the stream.
Within six weeks, plant and animal life had reestablished
itself in the new creek location.
The monitoring of Cottonwood Creek’s water quality
is ongoing. So far, the wetlands are cleaning runoff
to permitted standards.
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Land-based treatment can also reduce the need for conventional
infrastructure. Lance Davis, aia, of RTKL Associates in Washington,
D.C., recommended wetlands and a green roof (a roof planted
with vegetation to reduce runoff) for the upcoming renovation
of the Walter Reed Community Center in Arlington, Va. “It
turns out we won’t have to install pipes or storm sewers,
which is a huge benefit, because it offset the cost of implementing
the green features,” he says. “And those green features
were ones our clients really wanted to incorporate.”
The aesthetic value of these systems can’t be overlooked
either; natural treatment systems can inform building design
in a way that traditional civil infrastructure doesn’t.
“Most people would much rather look at a vegetated pond
than a rip-rap-lined detention basin,” said Jon Calabria,
a landscape architect with North Carolina State University.
Architects can use wetlands as a stimulus for incorporating
design elements that allow users to see, access, and enjoy
outdoor features.
As the population continues to burgeon, open space gets
more and more precious—so using building sites to their
fullest potential has become a more pressing concern for architects.
“Our principle is that every feature of a project, including
the landscape, should serve more than just one purpose,”
says Kevin Pierce, AIA, a principal with Farr Associates,
an architecture and planning firm in Chicago. One of his projects—a
grade school at Prairie Crossing, a self-labeled conservation
community north of Chicago—features wetland treatment
for runoff. “The classrooms are on grade with the wetland,
which is at the edge of the building,” he explains, “so
it’s not only a site amenity—the students can also
walk right out to it for a science lesson.” Double duty,
indeed.
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