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

lower flows (two-year storm) to drain through the stone,
while allowing higher flows (10-year storm) to drain
through a lower section in the center (thereby reducing
the potential erosion from water flowing around the
sides of the check dam). Flows through a stone check
dam are a function of stone size, flow depth, flow width,
and flow path length through the dam.
Conveyance
It is highly recommended that a flow splitter or diversion
structure be provided to allow larger flows to
bypass this practice as needed. Contributing drainage
areas should be limited to five acres and an overflow
should be provided within the practice to pass the excess
flows to a stabilized water course or storm drain. Weirs
are common overflow systems with media filters and
can control velocities so that they are non-erosive at the
outlet point to prevent downstream erosion.
Figure 7.60
Large Swale with subsurface storage
Source: Hubbell, Roth & Clark, Inc.
Media filters should be equipped with a minimum
eight-inch diameter underdrain in a one-foot gravel
bed. Increasing the size of the underdrain makes freezing
less likely. The porous gravel bed prevents standing
water in the system by promoting drainage. Gravel is
also less susceptible to frost heaving than finer grained
media. It is also highly recommended that a permeable
filter fabric be placed between the underdrain and
gravel layer but not extend laterally from the pipe more
than two feet on either side (Figure 7.59).
Variations
Vegetated swale with infiltration trench
This option includes a six to 24-inch aggregate bed or
trench, wrapped in a nonwoven geotextile (See Infiltration
BMP for further design guidelines). The addition
of an aggregate bed or trench can substantially increase
volume control and water quality performance although
cost is also increased.
Residential grass swale
Source: Pennsylvania Stormwater BMP Manual, 2006
Figure 7.60 shows a regraded area with a series of infiltration
trenches (geotextile fabric, crushed aggregate,
topsoil, and planting mixes). Additional stone energy
dissipaters were installed along the width of the swale.
A combination of plant plugs and seed mixes were then
installed.
Wet swale
Source: Hubbell, Roth & Clark, Inc.
LID Manual for Michigan – Chapter 7 Page 318