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

• Though roofs are generally not a significant source
of runoff pollution, they can still be a source of
particulates and organic matter, as well as sediment
and debris during construction. Measures such
as roof gutter guards, roof leader clean-outs with
sump, or an intermediate sump box can provide
pretreatment for dry wells by minimizing the amount
of sediment and other particulates that enter it.
Additional Design Considerations for
Infiltration Basins
• Infiltration basins are typically used for drainage
areas of five to 50 acres with land slopes that are
less than 20 percent.
• A six-inch layer of sand must be placed on the
bottom of an infiltration basin (Figure 7.26). This
sand layer can intercept silt, sediment, and debris
that could otherwise clog the top layer of the soil
below the basin.
• An infiltration basin does not normally have a
structural outlet to discharge runoff from the
stormwater quality design storm. Instead, outflow
from an infiltration basin is through the surrounding
soil. An infiltration basin may also be combined
with an extended detention basin to provide
additional runoff storage for both stormwater
quality and quantity management. A structural
outlet or emergency spillway is provided for storms
that exceed the design of the infiltration basin.
Figure 7.27
Infiltration trench cross section
• The berms surrounding the basin should be
compacted earth with a slope of not less than 3:1,
and a top width of at least two feet.
• The overflow from the infiltration basin must be
properly designed for anticipated flows. Large
infiltration basins may require multiple outlet
control devices to effectively allow for overflow
water during the larger storms. Emergency overflow
systems can be constructed to direct large storm
overflows.
• The sediment pre-treatment structure should be
designed to provide for access and maintenance.
• In some cases, basins may be constructed where
impermeable soils on the surface are removed and
where more permeable underlying soils then are
used for the basin bottom. Care should be taken
in the excavation process to make sure that soil
compaction does not occur.
• The inlets into the basin should have erosion
protection.
• Use of a backup underdrain or low-flow orifice
may be considered in the event that the water in the
basin does not drain within 72 hours.
LID Manual for Michigan – Chapter 7 Page 206