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

Additional design considerations for
infiltration trenches
• The infiltration trench (Figure 7.27) is typically
comprised of a section of uniformly graded
aggregate, such as AASHTO No. 3, which ranges
one to two inches in gradation. Depending on local
aggregate availability, both larger and smaller size
aggregate may be used. The critical requirements
are that the aggregate be uniformly-graded, cleanwashed,
and contain at least 40 percent void space.
The depth of the trench is a function of stormwater
storage requirements, frost depth considerations,
and site grading.
• Water quality inlets or catch basins with sumps are
required for all surface inlets to prevent clogging
of the infiltration trench with sediment and debris.
Parking lot and street runoff must be treated by
vegetated filter strips, bioretention, or water quality
inlets capable of removing oil and grease and similar
pollutants. Untreated parking lot and road runoff
should never be directly discharged underground.
• Cleanouts, observation wells, or inlets must be
installed at both ends of the infiltration trench
and at appropriate intervals to allow access to the
perforated pipe.
• When designed as part of a storm sewer system, a
continuously perforated pipe that extends the length
of the trench and has a positive flow connection
may be include to allow high flows to be conveyed
through the infiltration trench. Depending on size,
these pipes may provide additional storage volume.
Figure 7.28
Schematic of subsurface infiltration bed cross section
• Trees may be planted over the infiltration trench
provided that adequate soil media is provided above
the trench (a minimum of three feet).
• While most infiltration trenches areas consist of
an aggregate storage bed, alternative subsurface
storage products may also be employed. These
include a variety of proprietary, interlocking plastic
units that contain much greater storage capacity
than aggregate, at an increased cost.
Additional design considerations for
subsurface infiltration beds
• The infiltration bed must be wrapped in nonwoven
geotextile filter fabric to prevent migration of the
subsoils into the stone voids. (Bottom, top, and
sides).
• The subsurface infiltration bed (Figure 7.28) is
typically comprised of a 12 to 36-inch section of
aggregate, such as AASHTO No.3, which ranges
from one to two inches in gradation. Depending
on local aggregate availability, both larger and
smaller size aggregate has been used. The critical
requirements are that the aggregate be uniformlygraded,
clean-washed, and contain at least 40
percent void space. The depth of the bed is a
function of stormwater storage requirements, frost
depth considerations, and site grading. Infiltration
beds are typically sized to mitigate the increased
runoff volume from a two-year design storm.
• A water quality inlet or catch basin with sump is
required for all surface inlets to avoid standing
water for periods greater than 72 hours.
LID Manual for Michigan – Chapter 7 Page 207