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

Stone check dams
Source: Road Commission for Oakland County
e. The maximum ponding time should be 24
hours. It is critical that swale vegetation not be
submerged during smaller storms, because it
could cause the vegetation to bend over with
the flow. This leads to reduced roughness of
the swale, higher flow velocities, and reduced
contact filtering opportunities.
f. Bottom widths typically range from two to eight
feet. The maximum bottom width to depth ratio
for a trapezoidal swale should be 12:1.
2. Longitudinal slopes between one and six percent
are recommended.
3. Swale side slopes are best within a range of 3:1 to
5:1 and should never be greater than 2:1 for ease of
maintenance and side inflow from sheet flow.
4. Check dams
a. Recommended for vegetated swales with
longitudinal slopes greater than three percent
or when additional detention or infiltration is
desired.
b. Should be constructed to a height of six to 18
inches and regularly spaced.
c. Should be keyed into the bottom and sides of
the swale, usually at least one to two feet on all
sides. The height of the key on both sides should
exceed the water surface elevation of the 10-year
event by at least six inches.
d. The middle of the check dam crest should be
below the sides of the check dam to help focus
flow over the check dam and away from the
channel sides.
5. Maximum drainage area is five acres.
6. Soil testing is required when infiltration is planned
(Appendix E).
7. Runoff can be directed as concentrated flows or
as lateral sheet flow drainage. Both are acceptable
provided sufficient stabilization or energy
dissipation is included. If flow is to be directed into
a swale via curb cuts, provide a two- to three-inch
drop at the interface of pavement and swale. Curb
cuts should be at least 12 inches wide to prevent
clogging and should be spaced appropriately to
minimize the number of cuts but maximize area
drained.
8. Soil should be at least 12 inches of loamy or sand
with an infiltration rate of at least 0.5 inches per
hour.
9. Inundation time is 24 hours. Rototill and replant
swale if draw down time is more than 24 hours.
10. Prior to establishment of vegetation, a swale is
particularly vulnerable to scour and erosion and
therefore its seed bed must be protected with
temporary erosion control, such as straw matting,
straw-coconut matting, compost blankets, or
fiberglass roving. Most vendors will provide
information about the Manning’s ‘n’ value and will
specify the maximum permissible velocity. It is
critical that the selected erosion control measure is
adequate to prevent scour (see calculation section
for more information on Manning’s equation).
LID Manual for Michigan – Chapter 7 Page 320