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INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE
Green Infrastructure Research: Rain Gardens at EPA's
Edison Environmental Center
Background
Large volumes of stormwater runoff
from urban or paved areas can
pollute receiving waters and cause
flooding and erosion. Rain gardens
allow stormwater to infiltrate to
underlying native soil and eventually
contribute to groundwater. The
resulting reduction in peak flow
rates and volumes protects the
physical and biological integrity of
receiving streams.
Guidance on effective rain garden
design is limited. In federal and state
rain garden manuals, sizing criteria
as a fraction of the impervious area
draining to the rain garden varies
widely, leaving designers with
differing guidance. Sizing criteria is
a critical need given the importance
of avoiding flooding and overflows,
while avoiding unnecessarily
oversized rain gardens. Sizing
considerations takes on added
significance in urban settings where
land area is at a premium.
An additional area of uncertainty in
rain gardens involves high-quality
monitoring data. Relatively few
studies have measured rain garden
performance and infiltration
dynamics with multiple embedded
instruments and replicated
experimental design. Previous
research has shown that rain gardens
must be designed with the capacity
for long-term monitoring, as
retroactively equipping an existing
structure is often impractical.
Research Objectives
EPA scientists are evaluating rain
gardens as part of a long-term
research effort examining
stormwater management practices.
Underground Walls
Engineered Media
Sravel
Underlying Soil
Pipes (under sidewalk)
conveying runoff from roof
Ground Water
Figure 1. Rain garden design at EPA's Edison Environmental Center
EPA's research addresses two
objectives: 1) quantify the
hydrologic performance of rain
gardens and changes with season
and rain garden age, and 2) test
ratios of impervious surface area to
rain garden area in terms of
hydrologic performance.
Approach & Methods
In October 2009, EPA finished
upgrading a major parking lot at its
Region 2 laboratory in Edison, New
Jersey, with green infrastructure
features, including a rain garden
(Figure 1).
The rain garden consists of six
separate cells, hydraulically isolated
with plastic sheeting. The six cells
receive stormwater runoff from an
impervious section of the parking lot
and adjoining sidewalk through curb
cuts at the south end of the parking
lot. Stormwater runoff from the roof
of an adjacent building is collected
from multiple downspouts and
conveyed beneath the sidewalk in a
common manifold. Another pipe
distributes the roof runoff into each
rain garden cell just south of the
curb cuts. The drainage area to each
cell is roughly equal, but the
different sized rain gardens represent
different percentages of their
drainage areas.
The two smallest rain garden cells
are 4.5%, the two medium-sized
cells are 9%, and the two largest
cells are 18% of their drainage areas,
respectively. Each cell size is
duplicated for statistical purposes.
The cells were constructed with 86
U.S. Environmental Protection Agency
Office of Research and Development
June 2016 EPA/600/F-16/130
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centimeters (cm) of engineered
media (97% sand-sized particles)
over 10 cm of gravel.
Each cell is equipped with soil water
content reflectometers (WCRs) to
measure soil moisture, and
thermistors to measure temperature.
These instruments are installed at
two vertical depths at the front and
rear of each cell. A cluster of
piezometers and wells at various
depths is located in the center of
each cell.
All instrumentation contributes to
quantifying the timing of the wetting
front and moisture movement in the
rain garden. Researchers used this
data to evaluate the effectiveness of
the monitoring plan.
Results
Key research results are summarized
in Table 1. The WCRs successfully
measured the timing of the wetting
front in the rain gardens. Spatial and
temporal variability outweighed any
effects of rain garden cell size on
wetting front rates (Stander et al.,
2013).
After three growing seasons, basal
area and height of shrubs were
measured to quantify growth in the
rain gardens. Generally, shrubs
nearer the inlet (source of runoff)
were larger than those farther away.
The monitoring data from the WCRs
supported the hypothesis that
stormwater infiltration concentrated
near the inlet, while areas farther
from the inlet often received direct
rainfall as the only water source.
Therefore, rain garden design should
consider plant placement and species
selection relative to the proximity of
the runoff source (Brown et al.,
2015).
Roof runoff from a 0.64 acre area on
an adjacent building was directed
into the rain gardens as well, which
supplemented the plants' water
needs. Runoff to the rain gardens, in
addition to runoff collected for non-
potable usages at the facility,
contributed towards a diversion of
about 900,000 gallons of rainwater
from the stormwater sewer system
annually (O'Connor and Amin,
2015).
Table 1. Key research results from EPA's Edison rain gardens
Feature Studied
Instrumentation
Rain garden size
Spatial infiltration
of runoff
Vegetation growth
patterns
Runoff collection
Key Results
• Water content reflectometers (WCRs) quantified size
and timing of the wetting front, as well as infiltration
rates in the underlying soil.
• Cell size does have a significant effect on WCR
responses to storm events, but was not an important
driver of system infiltration rates.
• Oversized cells increase the likelihood that vegetation
farthest from the runoff source/inlet will experience-
water deficit stress.
• Infiltration is concentrated at the inlet and does not
necessarily spread evenly throughout the cell.
• Nutrients (nitrogen) concentrate near the inlets.
• Plants farther from the inlet experienced more frequent
water-deficit stress and received limited nutrient inputs.
• Plants with deeper roots are more likely to access
infiltrated water in the cell; those with shallower roots
will depend on direct rainfall.
• About 900,000 gallons of rainwater is being diverted
from the existing storm water sewer system annually.
• Stormwater runoff helps limit the frequency and
duration of water-deficit stress on vegetation.
Impact
The successful application of rain
gardens at the Edison Environmental
Center allows for technology
transfer to other facilities and
municipalities considering adopting
green infrastructure. Results from
this research are helping to inform
design and implementation decisions
at other rain garden sites.
These studies document a method
for defining and quantifying rain
gardens' hydrologic performance.
Quantifying performance allows for
evaluation and comparison of
different rain garden designs, which
enables EPA decision makers to
develop national guidelines on rain
garden design, construction,
maintenance, and monitoring.
References
Brown, R, T. O'Connor, and M.
Borst. (2015). "Divergent vegetation
growth patterns relative to
bioinfiltration size and plant
placement." Journal of Sustainable
Water in the Built Environment,
1(3), 04015001.
O'Connor, T. andM. Amin. (2015).
"Rainwater collection and
management from roofs at the
Edison Environmental Center."
Journal of Sustainable Water in the
Built Environment, 1(1), 04014001.
Stander, E., A. Rowe, M. Borst, and
T. O'Connor. (2013). "Novel use of
time domain reflectometry in
infiltration-based low impact
development practices." Journal of
Irrigation and Drainage
Engineering, 139(8), 625-634.
Additional Information
EPA's Green Infrastructure
Research: epa.gov/water-
research/green-infrastructure-
research
Contact
Michael Borst, EPA Office of
Research and Development
732-321-6631 I borst.mike@epa.gov
U.S. Environmental Protection Agency
Office of Research and Development
June 2016 EPA/600/F-16/130
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