GREEN INFRASTRUCTURE RESEARCH PROGRAM
Providing Research Solutions to Manage
Rain Garden Hydrology
Introduction
Rain gardens are vegetated surface
depressions, often located at low
points in landscapes, designed to
receive stormwater runoff from
parking lots, roofs and roads.
Typically constructed with sandy
soils, the gardens allow stormwater to
infiltrate quickly to underlying native
soil and eventually contribute to
groundwater recharge. Vegetation
and soils within the rain garden
remove stressors in stormwater runoff
through biological and physical
processes such as plant uptake and
sorption to soil particles. Compared
with stormwater release to receiving
waters through conventional storm
drains, infiltrating stormwater through
rain gardens reduces peak flow rates
and volumes with stressor loadings.
This reduction improves the physical
and biological integrity of receiving
streams by reducing stream bank
erosion and negative effects on
aquatic communities.
Background
The National Risk Management
Research Laboratory (NRMRL) is
evaluating rain gardens as part of a
larger collection of long-term
research examining multiple
stormwater management practices.
The U.S. EPA recognizes the
potential of rain gardens as a green
infrastructure management tool to
lessen the effects of peak flows on
Wet-Weather Flow
aquatic resources. While local
governments and individual
homeowners are building many of
these systems, relatively few studies
have quantified rain gardens' ability
to infiltrate stormwater to
groundwater, thereby reducing peak
flows.
Objectives
The Green Infrastructure Research
Program's long-term rain garden
research addresses two objectives to
meet these challenges:
• Quantify the hydrologic
performance of rain gardens
accepting parking lot and roof
runoff and changes with
season and rain garden age.
• Test multiple ratios of
impervious surface area to
rain garden area in terms of
hydrologic performance.
Experimental Approach
Controlled-condition research enables
NRMRL investigators to collect high-
quality information. Collecting data
and performing experiments at field
sites away from the laboratory limits
research due to uncertainties in
weather forecasts, site access, utility
locations, vandalism, and other
logistical issues that collectively add
greatly to the costs and timelines of
research projects.
Using on-site, experimental rain
National Risk Management
Research Laboratory
www.epa.gov/nrmrl
gardens enables NRMRL to collect
high-quality data necessary for
evaluating engineered structures. The
laboratory facilities and space
available at the Edison Environmental
Center also allow for construction and
monitoring of functioning, full-scale
rain gardens, producing data directly
relevant to real world applications
while avoiding unnecessary risks to
people and equipment.
Research Background
Cities and towns across the nation are
building or planning to install rain
gardens to accept and infiltrate
stormwater runoff from parking lots,
roofs, and roads in high-density urban
settings. Although hydrologic
properties such as infiltration rates,
surface ponding depths and duration,
and overflow have been well-
researched at the bench and pilot
scale, few studies have been
conducted in full-scale rain garden
applications. As a result, current
sizing criteria in federal and state rain
garden manuals range between 5%
and 50% of the impervious area
draining to the rain garden (NC Coop.
Ext. Serv., 2005; UW-Extension,
2003; U.S. EPA, 2009), leaving
designers with little clear guidance
when making decisions about rain
garden sizing. This is a critical need
given the importance of avoiding
excessively long periods of flooding
and overflow, particularly during the
more common small- and moderately-
sized storm events. The question of
how large to make a rain garden in a
U.S. Environmental Protection Agency
Office of Research and Development
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given location relative to the
impervious area draining to it takes
on added significance in urban
settings where land is expensive and
highly valued for a variety of uses.
An additional area of uncertainty in
full-scale rain gardens involves the
mechanics of acquiring high-quality
monitoring data. Previous
experiences of EPA researchers and
the wider scientific community have
shown that green stormwater
management practices like rain
gardens and pervious parking lots
must be designed with the capacity
for long-term monitoring, as
retroactively equipping an existing
structure to collect monitoring data is
impractical. In this study replicated
rain garden cells are outfitted with
buried instrumentation to collect
long-term hydrologic data. This data
will be analyzed to evaluate the
effectiveness of the monitoring plan
in terms of the location, number, and
types of instruments employed as well
as the measurement frequency,
storage and analyses techniques.
Current Research
The schematic on the following page
details the design of the rain garden
cells located south of a newly-
constructed green parking lot. The
rain garden consists of six separate
cells that are hydrologically isolated
from each other using 3/8 inch-thick
plastic sheeting installed to a depth of
4 feet (see figure on next page). 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 the adjacent building
is collected from multiple downspouts
and conveyed beneath the sidewalk in
a common 8 inch-diameter pipe. A
dedicated 4 inch-diameter pipe
distributes the roof runoff upward into
each rain garden cell just south of the
curb cuts. The drainage area to all six
cells is roughly equal (12,500 m2), but
because the rain gardens are different
sizes, they represent different
percentages of their drainage areas.
The two smallest cells are 2%, the
two medium-sized cells 4%, and the
two largest cells are 8% of their
drainage areas, respectively. Each
cell size is duplicated for statistical
purposes. All cells are equipped with
soil water content reflectometers and
thermistors (to measure soil moisture
and temperature, respectively) at
multiple depths in the soil profile at
the north and south ends 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
and size of the wetting front in the
rain garden during and following
storm events.
In addition to the rain gardens and
associated pervious pavement parking
lot, NRMRL operates the 20-acre
Urban Watershed Research Facility
that includes stormwater mesocosms,
laboratories, greenhouses, fabrication
space, a pipeline testing facility,
swale and pervious parking lot
performance testing, and storage for
equipment and supplies. This unique
facility is part of the larger 200-acre
Edison Environmental Center
operated by the U.S. EPA Region 2.
This land area allows NRMRL to
undertake research on a scale that
cannot be executed at any other U.S.
EPA facility. Additional rain garden
research at the pilot-scale is ongoing
at the research facility (U.S. EPA,
2008). This work focuses on stressor
removal in rain garden media and
vegetation.
Impacts
The successful application of
bioretention and pervious pavement
systems at the Edison Environmental
Center's pervious pavement parking
lot demonstration site, as determined
by the results of the research and
monitoring effort, will allow for
technology transfer to other federal
facilities and to municipalities
considering adopting green
infrastructure to alleviate CSO
problems. A more complete
understanding of how rain gardens
function will enable the U.S. EPA to
provide national guidelines on rain
garden design, construction,
maintenance, and monitoring which
local organizations can use to reduce
peak flows to receiving waters.
Reducing stormwater peak flows will
help maintain the function and
integrity of aquatic resources. Rain
gardens and other management tools
will help watershed managers assure
that receiving waters meet the
"fishable and swimmable" goals that
Congress outlined in the Clean Water
Act and better assure the continuing
supply of high-quality, potable water
needed for human life.
Contact
Michael Borst
Chemical Engineer
U.S. Environmental Protection Agency
Office of Research and Development
National Risk Management Research
Laboratory
732-321-6631
borst.mike@epa.gov
References
North Carolina Cooperative Extension Service
(2005). Designing Rain Gardens (Bio-
retention areas). AG-588-3.
University of Wisconsin-Extension (2003).
Rain Gardens: A How-To Manual for
Homeowners. UWEX Publication GWQ037.
1-06-03-5M-100-S.
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Urban Watershed Management Research
http ://www.epa. gov/ednnrmrl
U.S. EPA (2008). The Urban Watershed
Research Facility, Edison, New Jersey (PDF)
EPA/600/F-08/005
U.S. EPA (2008). Rain Gardens (PDF)
EPA/600/F-08/005
U.S. EPA (2009).
http://www.epa.gov/nps/toolbox/other/cwc_raingard
enbrochure.pdf
curb
42.7m
curb edit
impervious
asphalt
plastic
sheeting
yellow)
-14.9 m
Roof runoff (from adjacent building)
This schematic shows the rain garden cells (in green) located south of the impervious section and
sidewalk associated with the newly-constructed parking lot. All rain garden cells are hydrologically
isolated from each other; the yellow lines represent the plastic walls which separate the cells. All six
cells receive stormwater runoff (represented by red arrows) from the impervious section of the parking
lot through curb cuts. Stormwater runoff from the adjacent building is conveyed to the six rain garden
cells through an underground pipe manifold system (represented by the dotted blue arrows).
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