This page lists some of the fundamental factors you should consider when
designing a bioretention area for a commercial, industrial or institutional
setting or as an ultra-urban retrofit. 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 is also intended to serve
as a demonstration project. 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
one of the primary pollutant concerns in these ultra-urban areas is metals
from traffic, buildings and rooftops, downspouts, etc.
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
design for overflow is necessary since drainage areas in commercial and
institutional settings are highly impervious
Inflow
special design considerations are necessary to direct the impervious
drainage area's runoff to the bioretention cell
water may be diverted into the bioretention area through the use of an inlet
deflector block, which has ridges to channel the runoff into the cell
in a paved area with no curb, pre-cast car stops can be installed along the
pavement perimeter to protect the bioretention area
parking lot runoff may be captured through the use of vegetated soil/gravel
trenches integrated into the parking area at strategic locations
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
avoid locating the bioretention cell near building areas, well heads, and
septic systems
locate away from traveled areas such as public pathways to avoid compaction
for parking lot islands, a buffer (2' recommended) may be used to minimize
the possibility of drainage seeping under the pavement section and creating
frost heave during winter (alternatively, a geotextile filter fabric curtain
wall along the perimeter of the bioretention island may be used)