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

Description and Function
Filter strips (Figure 7.48) are gently sloping areas that
combine a grass strip and dense vegetation to filter, slow,
and infiltrate sheet flowing stormwater. Filter strips are
best used to treat runoff from roads and highways, roof
downspouts, small parking lots, and other impervious
surfaces. They are generally not recommended as
stand-alone features, but as pretreatment systems for
other BMPs, such as infiltration trenches or bioretention
areas. Therefore, filter strips generally should be
combined with other BMPs as part of a treatment train
so that water quality and quantity benefits are sufficient
to meet recommended site design criteria.
Maintaining a dense growth pattern that includes turfforming
grasses and vegetation on a filter strip is critical
for maximizing pollutant removal efficiency and erosion
prevention.
The grass portion of the filter strip provides a pretreatment
of the stormwater before it reaches the densely vegetated,
or wooded area. In addition, a stone drop can be located
at the edge of the impervious surface to prevent sediment
from depositing at this critical entry point.
In addition to a stone drop, a pervious berm can reduce
runoff velocity and increase volume reduction by
providing a temporary, shallow ponded area for the
runoff. The berm should have a height of not more than
six to 12 inches and be constructed of sand, gravel, and
sandy loam to encourage growth of a vegetative cover.
Figure 7.48
Diagram showing elements of a vegetated filter strip
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An outlet pipe(s) or an overflow weir may be provided
and sized to ensure that the area drains within 24 hours
or to allow larger storm events to pass. The berm must
be erosion resistant under the full range of storm events.
Likewise, the ponded area should be planted with vegetation
that is resistant to frequent inundation.
Filter strips are primarily designed to reduce total
suspended solids (TSS) levels. However, pollutants
such as hydrocarbons, heavy metals, and nutrients may
also be reduced. Pollutant removal mechanisms include
sedimentation, filtration, absorption, infiltration, biological
uptake, and microbial activity. Depending on soil
properties, vegetative cover type, slope, and length of
the filter strip, a reduction in runoff volume may also be
achieved by infiltration.
Applications
Vegetated filter strips can be used in a wide variety of
applications from residential/commercial developments
to industrial sites and even transportation projects where
the required space is available. Lack of available space
limits use in ultra urban areas and some redevelopment
projects.
Design Considerations
1. The design of vegetated filter strips is determined
by existing drainage area conditions including
drainage area size, length, and slope. In addition,
the filter strip soil group, proposed cover type, and
LID Manual for Michigan – Chapter 7 Page 291