This page lists some of the fundamental factors you should consider when designing
a rain garden for a higher density housing area. These facts can guide you when
modifying the basic AutoCAD file to suit your particular site. Detailed design
and construction information can be found in The Bioretention
Manual1 available from Prince George's County Department of
Environmental Resources Programs and Planning Division. The County also has
available in bulk form for distribution various materials that can educate the
neighborhood about the use of rain gardens in common areas.
These include the pamphlet Rain Gardens: The Natural Solution and the
booklet How Does your Garden Grow? A Reference Guide to Enhancing your Rain
Garden. Please call Prince George's County at (301) 883-5834 for more information.
The success of a rain garden is extremely dependent on both a designer developing
proper installation specifications and a contractor properly implementing them.
Remember that poor construction techniques can cause the best-designed facility
to fail prematurely. Construction technique and inspection are critical to ensure
proper landscaping, soil mixtures, and grading around the facility, as well as
the use of approved materials. Keep in mind that the plant and soil components
are crucial elements of the facility and are the key to the bioretention cell's
basic function. A plant list of 150 suitable perennials is provided in
The Bioretention Manual and should be used when selecting plants for
mid-Atlantic regions. Soil mixture guidelines are also provided in the manual
and should be followed closely.
maximum 6" recommended for soils with an infiltration rate of at least
2"/hr
maximum 3 to 4" recommended for soils with low infiltration rates or high
hydraulic loadings (combine with a smaller drainage area)
ponding depth may be increased if using sandy soils and underdrains to
increase filtration
any pooled water should be drawn down within 4-6 hours after a storm event
Plants
must be able to tolerate
expected pollutant loadings
highly variable soil moisture conditions
ponding water fluctuations
cannot be an invasive species
the use of native species is recommended
minimum recommended caliper size for trees is 1"
a minimum of 3 species of trees and 3 species of shrubs is recommended to
insure diversity
avoid species that require regular maintenance
Soil
homogeneous mix of
50% construction sand
20-30% topsoil with less than 5% maximum clay content
20-30% organic leaf compost
pH between 5.5 and 6.5
recommended minimum depth of 2 to 2.5 feet without large tree plantings
if shallow rooted plants are used, soil depth may be reduced to 1.5 feet
recommended depth of 4 to 4.5 feet with large trees
soil infiltration rate should exceed 1.5"/hr
Mulch
maximum 2 to 3 inches deep
should be fresh, not aged
apply uniformly, do not pile around the base of trees
do NOT use grass clippings
Groundwater
water table depth below the facility invert at least 2' (or an underdrain may
be used)
Slope
if greater than 15%, technique not recommended or use a weep-garden design
(see The Bioretention Manual)
Pollutant Concerns
design considerations based on pollutants will differ depending on the
location of the rain garden
if sited to treat primarily landscaped areas, the major pollutants of concern
are fertilizers such as nitrogen and phosphorus
for these, the removal efficiency increases with depth suggesting that deeper
cells reaching approximately 2 to 3 feet should be used (see
Watershed Benefits...)
if sited to treat primarily parking lots, rooftops, and sidewalks, the main
pollutants of concern will be metals, solids, and oil & grease
a shallow bioretention area with a full mulch layer can be used to focus on
metal removal (see
Watershed Benefits...)
Underdrain
***required for all rain gardens in residential areas
build with an accessible cleanout well
do not locate within the groundwater zone of saturation
must have a hydraulic capacity greater than the planting soil infiltration
rate
may outfall to a suitable location such as common space area, stream valley,
drainage swale, roadside open-section, or existing enclosed drainage system
Surface Overflow
design for overflow is necessary, especially if the drainage area is highly
impervious
in open areas, the rain garden may be graded so that the overflow discharges
into or near the baseline of a grass swale (the bottom of the rain garden
invert should be a maximum of 3-4" below the swale invert)
Inflow
special design considerations are necessary to direct the impervious drainage
area's runoff to the rain garden
inlet deflector blocks may be used in parking areas
grassed swales and filter strips may be used in open common areas
drainage dispersion techniques may also need to be used for highly impervious
drainage areas
when inflow exceeds 3 cfs the designer should evaluate the potential for
erosion
Location
place the rain garden near the source of the runoff generation
avoid locating the garden near building areas, well heads, and septic systems
locate away from traveled areas such as public pathways to avoid compaction