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Engineered soil media occupies the remaining excavated space above the underdrain system, leaving room
for the desired amount of surface ponding. The area is then mulched and planted with shrubs, perennials,
grasses, and small trees. When shrubs and flowers are used as the plant material, a 2 to 3 inch layer of
mulch is used on top of the media. The mulch acts as a pretreatment device to protect the underlying media
and helps to retain some water in the media for the health of the plant.
Generally runoff is ponded to a maximum depth of approximately 12-24 inches and then gradually filters
through the engineered soil media, where it is retained in the porous soils, utilized by plants,
evapotranspired, and either infiltrated into the underlying soils (when applicable), or drained into an
underdrain or underground storage system over a period of hours. The Bioretention system should be
designed so that surface ponding does not exceed 24 hours.
The layout of a Bioretention system should be determined based on site constraints such as location of
utilities, underlying soil conditions, existing vegetation and drainage patters. The plant selection and layout
should consider aesthetics, maintenance, native versus non-native, invasive species, and regional
landscaping practices. It should be noted that ideal plants for a typical Bioretention system are drought and
inundation tolerant.
An important design factor to consider when applying Bioretention to development sites is the scale at
which it will be applied. Typical system scales are:
• Bioretention Cells or Rain Gardens are small, distributed practices designed to treat runoff from
small areas. Inflow is typically sheet flow, or can be concentrated flow with energy dissipation.
• Bioretention Basins are larger systems treating parking lots and/or commercial rooftops, or other
large areas, usually in commercial or institutional areas. Inflow can be either sheet flow or
concentrated flow. Bioretention basins may also be distributed throughout a residential subdivision
for instance, but in this case they should be located in common area or within drainage easements,
to treat a combination of roadway and lot runoff.
• Urban Bioretention structures include systems such as Tree Filters, Curb Extensions, and Planter
Box Filters.
1.5.2 Engineered Soil Media
The infiltration rate of Engineered Soil Media in a bioretention facility must be designed to treat the first 1”
of runoff volume from the drainage area it serves. In order to minimize maintenance needs and insure
performance, the Design Infiltration Rate (inches per hour) must account for potential future degradation
and therefore, must provide a minimum safety factor of 2.
The Engineered Soil Media shall be placed and graded using low ground-contact pressure equipment or by
excavators and/or backhoes operating on the ground adjacent to the bioretention facility. No heavy
equipment shall be used within the perimeter of the bioretention facility before, during, or after placement
of the media. The Engineered Soil Media shall be placed in horizontal layers not to exceed 12 inches for the
entire area of the bioretention facility. It shall be compacted by saturating the entire area of the
bioretention facility after each lift is placed until water flows from the underdrain. Water for saturation
shall be applied by spraying or sprinkling. An appropriate sediment control device shall be used to treat
any sediment-laden water discharged from the underdrain. Final grading of the Engineered Soil Media shall
be performed after a 24-hour settling period.