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

• The soil mantle should be preserved for surface
infiltration BMPs and excavation should be
minimized. Those soils that do not need to be
disturbed for the building program should be left
undisturbed. Macropores can provide a significant
mechanism for water movement in surface
infiltration systems, and the extent of macropores
often decreases with depth. Maximizing the
soil mantle also increases the pollutant removal
capacity and reduces concerns about groundwater
mounding. Therefore, excessive excavation for
the construction of infiltration systems is strongly
discouraged.
• Isolate hot spot areas. Site plans that include
infiltration in hot spots need to be reviewed
carefully. Hot spots are most often associated with
some industrial uses and high traffic – gasoline
stations, vehicle maintenance areas, and high
intensity commercial uses (fast food restaurants,
convenience stores, etc.). Infiltration may occur in
areas of hot spots provided pretreatment is suitable
to address concerns.
• Utilize pretreatment. Pretreatment should be
utilized for most infiltration BMPs, especially for
hot spots and areas that produce high sediment
loading. Pretreatment devices that operate
effectively in conjunction with infiltration include
grass swales, vegetated filter strips, settling
chambers, oil/grit separators, constructed wetlands,
sediment sumps, and water quality inserts.
Selection of pretreatment should be guided by
the pollutants of greatest concern, site by site,
depending upon the nature and extent of the land
development under consideration. Selection of
pretreatment techniques will vary depending
upon whether the pollutants are of a particulate
(sediment, phosphorus, metals, etc.) versus soluble
(nitrogen and others) nature. Types of pretreatment
(i.e., filters) should be matched with the nature of
the pollutants expected to be generated.
• The loading ratio of impervious area to bed
bottom area must be considered. One of the more
common reasons for infiltration system failure is
the design of a system that attempts to infiltrate a
substantial volume of water in a very small area.
Infiltration systems work best when the water is
“spread out”. The loading ratio describes the ratio
of imperious drainage area to infiltration area,
or the ratio of total drainage area to infiltration
area. In general, the following loading ratios are
recommended (some situations, such as highly
permeable soils, may allow for higher loading
ratios):
• Maximum impervious loading ratio of 5:1
relating impervious drainage area to infiltration
area.
• Maximum total loading ratio of 8:1 relating total
drainage area to infiltration area.
• The hydraulic head or depth of water should
be limited. The total effective depth of water
within the infiltration BMP should generally not be
greater than two feet to avoid excessive pressure
and potential sealing of the bed bottom. Typically
the water depth is limited by the loading ratio and
drawdown time and is not an issue.
• Drawdown time must be considered. In general,
infiltration BMPs should be designed so that they
completely empty within a 72-hour period in most
situations (a 48-hour period is preferred).
• All infiltration BMPs should be designed with a
positive overflow that discharges excess volume
in a non-erosive manner, and allows for controlled
discharge during extreme rainfall events or frozen
bed conditions. Infiltration BMPs should never be
closed systems dependent entirely upon infiltration
in all storm frequency situations.
• Geotextiles should be incorporated into the
design as necessary. Infiltration BMPs that are
subject to soil movement into the stone medium or
excessive sediment deposition must be constructed
with suitably permeable non-woven geotextiles to
prevent the movement of fines and sediment into
the infiltration system. The designer is encouraged
to err on the side of caution and use geotextiles as
necessary within the BMP structure.
• Aggregates used in construction should be
washed. In general, bank run material will contain
fines that will wash off and clog the infiltration
surface.
• Infiltration utilizing vegetation. Adequate
soil cover (generally 12 to 18 inches) must be
maintained above the infiltration bed to allow
for a healthy vegetative cover. Vegetation over
infiltration beds can be native grasses, meadow
mix, or other low-growing, dense species
(Appendix C). These plants have longer roots
than traditional grass and will likely benefit from
LID Manual for Michigan – Chapter 7 Page 202