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

Volume reduction
Infiltration BMPs can be used to reduce the volume of
runoff and provide infiltration in accordance with LID
stormwater goals. The volume reduction potential is a
function of the storage provided (surface and subsurface,
if applicable) and the infiltration that will occur.
If a perforated pipe or underdrain is used in the design
that discharges directly to surface water, the volume of
water discharged must be subtracted from the volume
reduction calculation.
Total Volume Reduced = Surface Storage
Volume (if applicable) + Subsurface Volume (if
applicable) + Infiltration Volume
Where,
Surface storage volume (ft3) = Average bed area*
(ft2) x maximum design water depth (ft)
Subsurface storage/Infiltration bed volume (ft3) =
Infiltration area (ft2) x Depth of underdrain material
(ft) x Void ratio of storage material
*Depth is the depth of the water stored during a storm event,
depending on the drainage area, conveyance to the bed, and
outlet control.
Estimated Infiltration Volume (CF) = [Bed
bottom area (SF)] x [Infiltration design rate (in/hr)]
x [Infiltration period* (hr)] / 12 inches/ft.
*Infiltration Period is the time during the storm event when bed
is receiving runoff and capable of infiltration at the design rate
(typically 6 to 12 hours). See worksheet 5 in chapter 9.
Peak rate mitigation
The amount of peak rate control provided by infiltration
practices is dependent on the cumulative runoff volume
removed by all the infiltration practices applied to a
site. Where sufficient infiltration is provided to control
the runoff volume from any size storm, the corresponding
peak runoff rate will also be restored and the peak
runoff rate from larger, less frequent storms will be
reduced. Where possible, reducing peak rate of runoff
through volume control is generally more effective than
fixed rate controls.
Some infiltration BMPs (e.g., infiltration basins) can
manage peak rates better than others (e.g., infiltration
berms). However, all infiltration BMPs provide some
peak rate benefit (e.g., by removing direct connections
from impervious surfaces and increasing time of travel).
See Chapter 9 for more information.
Water quality improvement
Infiltration practices are effective in reducing pollutants
such as total suspended solids, nutrients, metals, oil
and grease. The vegetative surface and the underlying
soils allow pollutant filtration and studies have shown
that pollutants typically are bound to the soils and do
not migrate deeply below the surface (i.e. greater than
30-inches). Infiltration practices should be used as part
Subsurface infiltration at Mid Towne Village at the City of
Grand Rapids, MI
Source: Driesenga & Associates, Inc.
of a treatment train when capturing runoff from stormwater
hot spots, such as industrial parking lots, due
to the increased level of pollutants. Typical ranges of
pollutant reduction efficiencies for infiltration practices
are based on available literature data and listed below:
• TSS – 75 to 90 percent
• TP – 60 to 75 percent
• TN – 55 to 70 percent
• NO – 30 percent
3
Construction Guidelines
The following guidelines apply for all infiltration BMPs.
• Do not compact soil infiltration beds during
construction. Prohibit all heavy equipment from
the infiltration area and absolutely minimize all
other traffic. Equipment should be limited to
vehicles that will cause the least compaction, such
as low ground pressure (maximum four pounds per
square inch) tracked vehicles. Areas for Infiltration
areas should be clearly marked before any site work
begins to avoid soil disturbance and compaction
during construction.
LID Manual for Michigan – Chapter 7 Page 211