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

8. Perforated pipes can also be used as underdrains
where necessary. Underdrains can ultimately
discharge to daylight or to another stormwater
system. They should be accessible for inspection
and maintenance via cleanouts, overflow devices
(Figure 7.38), or other structures.
9. Sediment transport to pervious systems should
be minimized as much as possible to reduce
maintenance requirements and extend the life
of these systems. If roof leaders and area inlets
convey water from adjacent areas to the bed,
then native vegetation, water quality inserts, and/
or sumped inlets should be used to prevent the
conveyance of sediment and debris into the bed.
Areas of impervious pavement draining directly
onto pervious pavements should also be minimized
as they can lead to clogging near the imperviouspervious
boundary.
10. Infiltration areas should be located within the
immediate project area in order to control runoff at
its source. Expected use and traffic demands should
also be considered in pervious pavement placement.
An impervious water stop should be placed along
infiltration bed edges where pervious pavement
meets standard impervious pavements.
Figure 7.37
Slope stepping with berms
Source: Andropogon
Figure 7.38
Example detail of an overflow device from a pervious asphalt system
11. The underlying infiltration bed is typically eight to
36 inches deep and comprised of clean, uniformly
graded aggregate with approximately 40 percent
void space. Local aggregate availability typically
dictates the size of the aggregate used. The critical
requirements are that the aggregate be uniformly
graded, clean washed, and contain a significant void
content. See the Specifications section for commonly
used aggregates. The depth of the bed is a function
of stormwater storage requirements, frost depth
considerations, site grading, and structural needs.
LID Manual for Michigan – Chapter 7 Page 249