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

such as pathways through woodlands. In summary,
even when used for stormwater management, berms
can be designed to serve multifunctional purposes and
are easily incorporated into the landscape.
Design Considerations
The following general design considerations are for
all BMPs utilizing infiltration. These include: site
conditions and constraints, as well as general design
considerations. Specific design considerations for each
BMP follow these same considerations.
Site conditions and constraints for all
infiltration BMPs
• Depth to seasonal high water table. A four-foot
clearance above the seasonally high water table is
recommended. A two-foot clearance can be used,
but may reduce the performance of the BMP. This
reduces the likelihood that temporary groundwater
mounding will affect the system, and allows sufficient
distance of water movement through the soil to assure
adequate pollutant removal. In special circumstances,
filter media may be employed to remove pollutants if
adequate soil layers do not exist.
• Depth to bedrock. A four-foot minimum depth
to bedrock is recommended to assure adequate
pollutant removal and infiltration. A two-foot depth
can be used, but may reduce the performance of the
BMP. In special circumstances, filter media may
be employed to remove pollutants if adequate soil
mantle does not exist.
• Soil infiltration. Soils underlying infiltration
devices should have infiltration rates between 0.1
and 10 inches per hour, which in most development
programs should result in reasonably sized infiltration
systems. Where soil permeability is extremely low,
infiltration may still be possible, but the surface area
required could be large, and other volume reduction
methods may be warranted. Undisturbed Hydrologic
Soil Groups A, B, and C often fall within this range
and cover most of the state. Type D soils may require
the use of an underdrain.
Soils with rates in excess of six inches per hour
may require an additional soil buffer (such as an
organic layer over the bed bottom) if the Cation
Exchange Capacity (CEC) is less than 10 and
pollutant loading is expected to be significant.
In carbonate soils, excessively rapid drainage
may increase the risk of sinkhole formation, and
some compaction or additional measures may be
appropriate.
• Setbacks. Infiltration BMPs should be sited so that
any risk to groundwater quality is minimized and
they present no threat to sub-surface structures such
as foundations and septic systems. (Table 7.11)
Table 7.11
Setback Distances
Setback from Minimum Distance (feet)
Property Line 10
Building Foundation* 10
Private Well 50
Public Water Supply Well** 50
Septic System Drainfield*** 100
* minimum with slopes directed away from building. 100 feet
upgradient from basement foundations.
** At least 200 feet from Type I or IIa wells, 75 feet from Type IIb
and III wells (MDEQ Safe Drinking Water Act, PA 399)
*** 50 feet for septic systems with a design flow of less than 1,000
gallons per day
General design considerations for all
infiltration BMPs
• Do not infiltrate in compacted fill. Infiltration
in native soil without prior fill or disturbance is
preferred but not always possible. Areas that have
experienced historic disturbance or fill are suitable
for infiltration provided sufficient time has elapsed
and the soil testing indicates the infiltration is
feasible. In disturbed areas it may be necessary
to infiltrate at a depth that is beneath soils that
have previously been compacted by construction
methods or long periods of mowing, often 18
inches or more. If site grading requires placement
of an infiltration BMP on fill, compaction should
be minimal to prevent excess settlement and the
infiltration capacity of the compacted fill should be
measured in the field to ensure the design values
used are valid.
• A level infiltration area (one percent or less
slope) is preferred. Bed bottoms should always be
graded into the existing soil mantle, with terracing
as required to construct flat structures. Sloped
bottoms tend to pool and concentrate water in
small areas, reducing the overall rate of infiltration
and longevity of the BMP. The longitudinal slope
may range only from the preferred zero percent
up to one percent, and that lateral slopes are held
at zero percent. It is highly recommended that the
maximum side slopes for an infiltration practice be
1:3 (V: H).
LID Manual for Michigan – Chapter 7 Page 201