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

An infiltration trench differs from an infiltration bed in
that it may be constructed in more confined areas. The
designer must still consider the impervious area to infiltration
area loading rate. It can be located beneath or
within roadways or impervious areas (Figure 7.22) and
can also be located down a mild slope by “stepping” the
sections between control structures.
Figure 7.22
Residential rain garden with surface connection
to subsurface infiltration bed under garden.
Water quality inlet collects
and conveys roof runoff to
infiltration trench
Infiltration trench
with continuously
perforated pipe for
distribution with
positive overflow
Infiltration basin
Infiltration basins (Figure 7.23) are shallow, impounded
areas designed to temporarily store and infiltrate stormwater
runoff. The size and shape can vary from one large
basin to multiple, smaller basins throughout a site.
Infiltration basins use the existing soil and native vegetation
to reduce the volume of stormwater runoff by
infiltration and evapotranspiration. Therefore, the use of
Figure 7.23
Schematic of infiltration basin
sediment pretreatment is imperative to prevent clogging
of the infiltration surface area within the basin. Sediment
pretreatment can take the form of a water quality
filtering device, vegetative filter strips, a settling basin,
or a sediment trap. The key to promoting infiltration is
to provide enough surface area for the volume of runoff
to be absorbed within 72 hours.
An engineered overflow structure must be provided
for the larger storms and can be designed for peak rate
attenuation. With the use of a properly designed outlet
structure, infiltration basins can be designed to mitigate
volume and water quality for small frequent storms,
while managing peak rates for large design storms.
Dry well
A dry well (Figure 7.24) is a subsurface storage facility
that temporarily stores and infiltrates stormwater runoff
from rooftops. Roof leaders usually connect directly into
the dry well, which may be either an excavated pit filled
with uniformly graded aggregate wrapped in geotextile
or a prefabricated storage chamber or pipe segment.
For structures without gutters or downspouts, runoff
can be designed to sheet flow off a pitched roof surface
and onto a stabilized ground cover that is then directed
toward a dry well via stormwater pipes or swales.
Dry wells discharge the stored runoff via infiltration
into the surrounding soils. In the event that the dry well
is overwhelmed in an intense storm event, an overflow
mechanism (e.g., surcharge pipe, connection to larger
infiltration area, etc.) will ensure that additional runoff
is safely conveyed downstream.
LID Manual for Michigan – Chapter 7 Page 199