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

Subsurface infiltration bed
Source: Driesenga & Associates, Inc.
• Perforated pipes along the bottom of the bed
can be used to evenly distribute runoff over the
entire bed bottom. Continuously perforated pipes
should connect structures (such as cleanouts and
inlet boxes). Pipes should lay flat along the bed
bottom to provide for uniform distribution of
water. Depending on size, these pipes may provide
additional storage volume.
• Cleanouts or inlets should be installed at a few
locations within the bed at appropriate intervals to
allow access to the perforated piping network and
storage media.
• Grading of adjacent contributing areas should
be mildly sloped between one percent and three
percent to facilitate drainage.
• In areas with poorly-draining soils, subsurface
infiltration areas may be designed to slowly
discharge to adjacent wetlands or bioretention
areas.
• The subsurface bed and overflow may be designed
and evaluated in the same manner as a detention
basin to demonstrate the mitigation of peak flow
rates. In this manner, detention basins may be
eliminated or significantly reduced in size.
• During construction, the excavated bed may serve
as a temporary sediment basin or trap, which can
reduce overall site disturbance. The bed should be
excavated to at least one foot above the final bed
bottom elevation for use as a temporary sediment
trap or basin. Following construction and site
stabilization, sediment should be removed and final
grades established.
Incorporating a Safety
Factor into Infiltration
BMP Design
For the purposes of site suitability, areas with tested
soil infiltration rates as low as 0.1 inches per hour may
be used for infiltration BMPs. However, in the design
of these BMPs and the sizing of the BMP, the designer
should incorporate a safety factor. Safety factors between
1 (no adjustment) and 10 have been used in the
design of stormwater infiltration systems, with a factor
of two being used in most cases. Therefore a measured
infiltration rate of 0.5 inches per hour should generally
be considered as a rate of 0.25 inches per hour in
design. See the Soil Infiltration Testing Protocol in Appendix
E for guidance on performing infiltration tests.
Modeling Infiltration
Systems
As discussed in Chapter 9 of this manual, infiltration
systems can be modeled similarly to traditional detention
basins. The marked difference with modeling
infiltration systems is the inclusion of the infiltration
rate, which can be considered as another outlet. For
modeling purposes, it is sometimes useful to develop
infiltration rates that vary (based on the infiltration
area provided as the system fills with runoff) for inclusion
in the stage-storage-discharge table.
LID Manual for Michigan – Chapter 7 Page 208