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

Vegetated swales with infiltration trenches are best fitted
for milder sloped swales (< 1 percent) or poorly-drained
soils where the addition of the aggregated bed system can
help to make sure that the maximum allowable ponding
time of 48 hours is not exceeded. Ideally, the subsurface
system should be designed like an infiltration trench
(see Infiltration BMP). The subsurface trench should
be comprised of terraced levels, though sloping trench
Figure 7.61
Slow discharge from porous pavement bed to
vegetated swales
Source: Pennsylvania Stormwater BMP Manual, 2006
bottoms may also be acceptable. The storage capacity of
the infiltration trench may be added to the surface storage
volume to achieve the desired storage.
Grass swale
Grass swales are essentially conventional drainage
ditches. They typically have milder side and longitudinal
slopes than their vegetated counterparts. Grass
swales are usually less expensive than vegetated swales.
However, they provide far less infiltration and pollutant
removal opportunities and should be used only as
pretreatment for other structural BMPs. Grassed swales,
where appropriate, are preferred over catch basins and
pipes because of their ability to increase travel time and
reduce peak flow rates from a site.
Linear wetland swale
Wetland swales occur when the water table is located
very close to the surface, incorporating long, shallow,
permanent pools or marshy conditions that can sustain
wetland vegetation. Like the dry swale, the entire water
quality treatment volume is stored within a series of
cells created by check dams.
Potential Applications
• Residential – Swales can be used along road rights
of ways and for side yard and backyard drainage.
• Commercial/Industrial – Swales can provide site
drainage around a site, within a site and can help
take/slow discharge from other BMPs that outlet to
the swale (Figure 7.61).
• Ultra urban – There may be some opportunity for
swales in ultra urban areas. However, swales are
usually no less than two feet deep. With horizontal
to vertical side slopes between 3:1 to 5:1 horizontal
to vertical, the top width of the swale can prohibit
its use in this setting where space is usually at a
premium.
• Retrofit – Potential application in retrofit situations
will depend strongly on space and topographic
limitations. On sites with little to no slope, swales
may not be the best drainage option. In these areas,
swales may end up not moving water fast enough
or may be prone to long periods of flooding or
inundation in areas meant to be mostly dry.
• Highway/Road – Vegetated swales are an
excellent alternative to curb and gutter systems.
Appropriately sized roadside swales should be able
to handle all the runoff from the roadway and may
also be able to handle runoff from areas outside the
road surface.
Design Considerations
1. Sizing
a. Convey the calculated peak discharge from a 10year
storm event. Calculate the peak discharge
for a 10 year storm event using methods
from Chapter 9. Use Manning’s equation (see
stormwater calculations section) to calculate the
velocity associated with the flow and compare to
Table 7.19.
b. Temporarily store and infiltrate the one-inch
storm event, while providing capacity for up to
the 10-year storm with 12 inches of freeboard.
c. Flows for up to the two-year storm should be
conveyed without causing erosion.
d. Maintain a maximum ponding depth of 18
inches at the end point of the channel, with a
12-inch average maintained throughout.
LID Manual for Michigan – Chapter 7 Page 319