Stormwater Wetland Design Update:
Zones, Vegetation, Soil, and Outlet Guidance
Distributed in furtherance
of the Acts of Congress of
May 8 and June 30, 1914.
North Carolina State Uni¬
versity and North Carolina
A&T State University com¬
mit themselves to positive
action to secure equal
opportunity regardless of
race, color, creed, national
origin, religion, sex, age, or
disability. In addition, the
two Universities welcome
all persons without regard
to sexual orientation. North
Carolina State University,
North Carolina A&T State
University, U.S. Department
of Agriculture, and local
governments cooperating.
Many stormwater wetlands have been
built in North Carolina since 2000, par¬
ticularly in the Neuse, Tar-Pamlico, and
Cape Fear River Basins. After examin¬
ing many of these wetlands, N.C. State
University researchers have revised ex¬
isting design guidelines. Specific design
focus points include:
• a redefinition of internal wetland
zones,
• a revised list of herbaceous plants that
have been found to commonly thrive
in stormwater wetlands,
• a review of a proper growing medium,
and
• the importance of a flexible outlet
structure and its construction.
This fact sheet updates and revises
Designing Storm water Wetlands for
Small Watersheds (AG-588-2) and is a
companion to Stormwater Wetland Con¬
struction Guidance (AG-588-13).
RECENT RESEARCH
Stormwater wetlands reduce pollut¬
ant loads in stormwater runoff and
thus have become preferred stormwa¬
ter management tools. Studies across
North Carolina have revealed that both
stormwater wetlands and wet ponds
trap sediment effectively, but storm¬
water wetlands remove nutrients and
STATE UNIVERSITY
A&T STATE UNIVERSITY
COOPERATIVE
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Helping People Put Knowledge to Work
mitigate temperatures more efficiently
than wet ponds:
• A stormwater wetland studied in
Johnston County reduced nitrogen and
phosphorus concentrations by over
80 percent, well above state-assigned
removal rates.
• Studies conducted in Charlotte found
removal rates of 40 percent for total
nitrogen and 55 percent for total phos¬
phorus.
• A study in the mountains indicated
that a well-vegetated stormwater wet¬
land reduced outflow temperatures by
3 to 5°F more than wet ponds, which
are unshaded and exposed to sunlight.
In areas with high water tables or a reli¬
able base flow, which often are where
stormwater wetlands are sited, a storm¬
water wetland is often the most efficient
practice available for pollution removal.
Figure 1 (page 2) highlights several
wetlands that NCSU researchers have
monitored.
INTERNAL WETLAND ZONES
Figure 2 (page 2) illustrates a storm¬
water wetland design, and Figure 3
(page 3) depicts a cross-section of the
wetland’s internal topography by zone.
The internal topography of a storm¬
water wetland can be divided into five
zones: deep pools, transitions between
deep and shallow water, shallow water,
temporary inundation areas, and the
upper bank that ties the wetland into its
