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

Design Considerations
Bioretention is flexible in design and can vary in
complexity according to site conditions and runoff
volume requirements. Design and installation procedures
may vary from very simple for “backyard” rain
gardens to highly engineered bioretention areas in ultraurban
areas.
Infiltration BMPs should be sited so that they minimize
risk to groundwater quality and present no threat to
subsurface structures. Table 7.4 provides recommended
setback distances of bioretetnion areas to various lot
elements.
Table 7.4
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
** 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
The distance from the bottom of the infilration BMP
to the seasonal high groundwater level or bedrock is
recommended to be four feet. Two feet is allowable, but
may reduce the performance of the BMP.
Bioretention is best suited for areas with at least moderate
infiltration rates (more than 0.25 inches per hour)
– see Infiltration BMP. In extreme situations where
permeability is less than 0.25 inches per hour, special
variations may apply, such as using amended subsoils
or underdrains (or using constructed wetlands instead).
The following procedures should be considered when
designing bioretention areas:
1. The flow entrance must be designed to prevent
erosion in the bioretention area. Some alternatives
include flared end sections, erosion control mats,
sheet flow into the facility over grassed areas, rock
at entrance to bioretention area, curb cuts with
grading for sheet flow, and roof leaders with direct
surface connection.
2. A positive overflow system should be designed to
safely convey away excess runoff. The overflow
can be routed to the surface in a non-erosive
manner or to another stormwater system. Some
alternatives include domed risers, inlet structures,
weirs, and berms.
3. Sizing criteria
a. Surface area is dependent upon storage volume
requirements, but should generally not exceed a
maximum loading ratio of 5:1 impervious drainage
area to bioretention area and no more than one
acre drainage area to one bioretention cell.
However, for design purposes, the total volume
of water generated from the contributing drainage
area must be used, not just the impervious portion.
See Infiltration BMP for additional guidance on
loading ratios.
The required bioretention surface area is
determined by taking the volume of runoff to be
controlled according to LID criteria, maintaining
the maximum ponding depth, the loading
rate, and the emptying time. Infiltration and
evapotranspiration are increased by increasing
the surface area of the bioretention area. The total
surface area needed may be divided into multiple
cells. This configuration may be useful to collect
runoff from both the front and back of a building.
b. Surface side slopes should be gradual. For
most areas, maximum 3:1 side slopes are
recommended.
c. The recommended surface ponding depth is six
inches. Up to 18 inches may be used if plant
selection is adjusted to tolerate water depth.
Drain within 24-48 hours.
d. Ponding area should provide sufficient surface
area to meet required storage volume without
exceeding the design ponding depth. The
Preparing bed with planting soil
Source: City of Troy
LID Manual for Michigan – Chapter 7 Page 140