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

12. Proper pervious pavement applications are resistant
to freeze-thaw problems because of their permeable
and open-graded components (the pavement surface
should not be saturated and the base has a high void
content which allows for expansion). In somewhat
frost susceptible soils, it may be necessary to
increase the minimum bed depth to 14-22 inches
(depending on loading and specific soil conditions).
In extremely susceptible soils, the bed and/or
improved soils can be placed down to the full frost
depth (Smith, 2006).
13. While most pervious pavement installations
are underlain by an aggregate bed, alternative
subsurface storage products may also be used.
These include a variety of proprietary plastic units
that contain much greater storage capacity than
aggregate, at an increased cost.
Stormwater Functions
and Calculations
Infiltration area
The infiltration area is defined as the plan area of the
storage reservoir under the pervious pavement. The
minimum infiltration area should be based on the
following equation:
Minimum infiltration area = Contributing impervious
area (including pervious pavement) / 5*
*May be increased depending on soil infiltration capacity
(where soils are Type A or rapidly draining).
Volume reduction
Pervious pavements with infiltration provide an excellent
means of capturing and infiltrating runoff. The
storage bed below the pavement provides runoff
volume storage during storm events, while the undisturbed
subgrade allows infiltration of runoff into the
underlying soil mantle. The total volume reduction can
be estimated by summing the storage and infiltration
volumes described below.
Storage volume = Depth* (FT) x Area (SF) x Void
space (i.e., 0.40 for aggregate)
*Depth is the depth of the water stored during a storm
event, depending on the drainage area, conveyance to
the bed, and outlet control.
Infiltration volume = Bed bottom area (SF) x Infiltration
design rate (in/hr) x Infiltration period* (hr) x (1/12)
*Infiltration period is the time when bed is receiving
runoff and capable of infiltrating at the design rate. Not
to exceed 72 hours.
Peak rate mitigation
Properly designed pervious pavement systems provide
effective management of peak rates. The infiltration
bed below the pavement acts as a storage reservoir
during large storm events, even while runoff exfiltrates
through the soil mantle through the process of infiltration.
Outlet structures can be designed to manage peak
rates with the use of weir and orifice controls and carefully
designed systems may be able to manage peak
rates for storms up to and including the 100-year storm.
For additional information relating to peak rate modeling
and routing, refer to Chapter 9, LID Stormwater
Calculations and Methodology.
Water quality improvement
Pervious pavement systems are effective in reducing
pollutants such as total suspended solids, metals, and
oil and grease. Both the pervious pavement surface and
the underlying soils below the infiltration bed allow
pollutant filtration.
When pervious pavement systems are designed to
capture and infiltrate runoff volumes from small storm
events, they provide very high pollutant reductions
because there is little if any discharge of runoff carrying
the highest pollutant loads. Pervious pavement systems
require pretreatment of TSS when adjacent areas drain
to them, resulting in a high reduction of TSS and other
particulates. However, pervious pavement systems will
provide limited treatment of dissolved pollutants, such
as nitrates. Typical ranges of pollutant reduction efficiencies
for pervious pavements are listed as follows:
• TSS* – 65-100%
• TP – 30-90%
• NO – 30%
3
*Pretreatment for TSS is recommended if adjacent areas
drain to pervious pavement
Construction Guidelines
1. Follow the Recommendations for Materials that are
specific to porous asphalt and porous concrete in
Appendix D.
2. Due to the nature of construction sites, pervious
pavement and other infiltration measures should
be installed toward the end of the construction
LID Manual for Michigan – Chapter 7 Page 250