Stormwater Management Standards Manual - Toledo Metropolitan ...

work best in gently sloping areas (5 percent maximum) where the swale is cut into the existing
landscape. See figure 5-1.
Design Guidelines
Route & Length – Open vegetated swales should follow pre-development drainage patterns. Swale
length should be a minimum of 200 feet when possible, to increase the contact time of stormwater.
Longitudinal Slope – The maximum allowable longitudinal slope should be 5 percent. The slope
should provide for sufficient drainage and uniform flow, while preventing excessive velocities. For
slopes greater than 2 percent, a series of check dams or drop structures across swales should be used
to maintain acceptable flows.
Channel Shape – Swales should be designed to promote shallow, low velocity flow (i.e. trapezoidal
channel). This maximizes the channel filtering surface, facilitates sedimentation and infiltration, and
increases the travel time to the discharge point. The side slopes should be gentle enough to maximize
the filtering surface and resist bank erosion, and steep enough to adequately contain anticipated flows.
The side slopes of the channel should not be steeper than 2 Horizontal : 1 Vertical.
Capacity – The swale should be designed to adequately convey the 10-year, 24-hour storm event at
non-erosive velocities.
Flow Velocity – The bottom of the channel should be wide enough to maintain the desired shallow flow,
but narrow enough to maintain sufficient velocity to prevent rills from forming during low flows. The
velocity for a 2-year, 24-hour storm should not exceed 2 feet per second. A series of check dams or
drop structures across swales should be provided where necessary to enhance water quality
performance and reduce velocities.
Channel Stabilization – Slopes and bottoms of swales should be stabilized to prevent erosion. This is
commonly accomplished with an erosion-resistant grass, such as tall fescue. If the grass is planted from
seed, temporary matting is required to stabilize the soil while the grass is established. For swales in
areas with steeper slopes, check dams may need to be constructed within the channel to slow the
velocity.
Third Edition Stormwater Standards Manual September 2008 Page 64