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

Flow-through
The flow-through planter box (Figure 7.44) is completely
contained and drains to a stormwater system. These
planters are designed with an impervious bottom or
are placed on an impervious surface. Pollutant reduction
is achieved as the water filters through the soil/
growing medium. Flow control is obtained by ponding
runoff above the soil and in a gravel layer beneath
it. In most storm events, runoff flows through the soil
into the gravel layer and is slowly discharged via the
perforated pipe. In more extreme events, inflow may
exceed the capacity of the soil and some runoff may
be discharged through surface overflow. This type of
planter can be used adjacent to a building if the planter
box and/or building is adequately waterproofed to allow
for saturated soil and temporary ponded runoff next to
the building.
Flow-through planter boxes should be designed to
retain water for no more than three to four hours after
an average storm event. Recommended vegetation
includes native rushes, reeds, sedges, irises, dogwoods,
and currants. The minimum planter width is typically
18 inches with no minimum length or required shape.
Planters should be designed to receive less than 15,000
square-feet of impervious area runoff.
Potential Applications
Planter boxes can be used in urbanized areas of high
pollutant loads. They are especially applicable where
there is limited area for construction of other BMPs.
Planter boxes may be used as a pretreatment BMP for
other BMPs such as wet ponds or infiltration systems.
Areas that would benefit from using a planter box
include:
• Parking garage
• Office building
• Residential building
• Other building use (commercial, light industrial,
institutional, etc.)
• Transportation facilities
• Urban streetscapes
Figure 7.44
Schematic of Flow-through Planter Box
Overflow*
set 2’ below top
of planter
Growing
Medium
Gravel
3/8”-5/8”
or approved equiv
Perforated Pipe
to run length of
planter
Building
Downspout or
other conveyance
system
Splash Rocks /
Block
Filter Fabric
Waterproof
Building
(as needed)
Source: City of Portland, Bureau of Environmental Services
Example of Flow-through Planter Box
Source: City of Portland, OR Bureau of Environmental
Services
LID Manual for Michigan – Chapter 7 Page 261
1’2”
1’3”
1’2”
Pipe to disposal Point
bottom or side out
options
* Water reservoir depth may be reduced
if planter surface is increased.
Foundation Drains
(as required)