Low Impact Development Manual for Michigan - OSEH - University ...

Variations
A bioretention system is a depression in the ground
planted like a garden that provides for the storage and
infiltration of relatively small volumes of stormwater
runoff, often managing stormwater on a lot-by-lot
basis. This use of many small stormwater controls
versus one large detention area promotes the low
impact development goal of decentralized treatment
of stormwater. But, if greater volumes of runoff must
be managed or stored, a bioretention system can be
designed with an expanded subsurface infiltration
bed, or can be increased in size. Typically, the ratio of
impervious area draining to the bioretention area should
not exceed five-to-one, and the total impervious area
draining to a single system should not be more than one
acre. Variations noted relate to performance types, flow
entrance, and positive overflow.
Performance types
Depending on varying site conditions, bioretention
can be designed to allow for 1) complete infiltration,
2) infiltration/filtration, or 3) filtration. These variations
will often determine the need for such design features
as the gravel bed, underdrains, and impervious liners.
Bioretention using complete infiltration occurs in areas
where groundwater recharge is beneficial and the soils
have the permeability necessary to accommodate the
inflow. This type of BMP is often less expensive to
construct because there is no underdrain and the soils on
site are often used.
The most common variation to this type of bioretention
includes a gravel or sand bed underneath the planting bed
and often accompanied by the use of an underdrain. This
allows for additional storage or for areas with low permea-
Signage at Rouge River rain garden
bility to use bioretention as infiltration, as well as, filtration
(Figure 7.6). Some volume reduction will occur through
infiltration, as well as evaporation and transpiration.
Another variation is to use bioretention primarily for
filtration. This is often used in contaminated soils or
hot spot locations using an impervious liner to prevent
infiltration and groundwater contamination. The primary
stormwater function then becomes filtration with some
volume reduction through evaporation and transpiration.
For areas with low permeability, bioretention may achieve
some infiltration while acting as detention with peak rate
control for all storms up to the design storm.
Flow inlet
Pretreatment of runoff should be provided where sediment
or pollutants entering the rain garden may cause
concern or decreased BMP functionality. Soil erosion
control mats, blankets, or rock must be used where runoff
flows from impervious areas enter the rain garden.
Flow inlet: Trench drain
Trench drains can accept runoff from impervious surfaces
and convey it to a rain garden (Figure 7.7). The trench drain
may discharge to the surface of the rain garden or may
connect directly to an aggregate infiltration bed beneath.
Figure 7.7
Trench drain and curb cut connected to
bioretention area
Source: Macomb County Planning and Economic
Development
Educational Signage
Once a bioretention area is established, installing
signage will help the general public and maintenance
crews recognize LID practices which can help promote
sustainable stormwater management. Educational signs
can incorporate LID goals, and maintenance objectives
in addition to the type of LID project being employed.
LID Manual for Michigan – Chapter 7 Page 135