This page lists some of the fundamental factors you should consider when designing
a bioretention area for a highway, a neighborhood street, or a general roadway.
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 educational materials
that can be used if the bioretention area will require maintenance by homeowners.
Please call the County at (301) 883-5834 for more information.
The success of a bioretention cell 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
if greenspace is limited, depth may be increased up to 1' as long as the
drainage area is 1/4 acre or less
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
urban highway runoff includes solids and significant levels of metals
primarily from traffic activities; vehicle brake, tire and frame wear; fluid
leakage; pavement degradation; and de-icing salts
a shallow bioretention area with a full mulch layer may be used to focus on
metal removal (see
Watershed Benefits...)
Underdrain
recommended where the in-situ soil infiltration rate is less than 1"/hr
(if an underdrain is not being used, soils investigation/geotechnical reports
are required)
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
?
Inflow
special design considerations are necessary to direct the impervious drainage
area's runoff to the bioretention cell
