Rain gardens, also known as
biorentention cells, are a suitable
and flexible solution for retrofits.
These strips or trenches are engineered natural treatment systems
consisting of a recessed landscape
area with a specialized soil mixture, an aggregate base, an underdrain and appropriate plants that
tolerate a given region’s wet and
dry seasons. They can be used as
parking lot islands, road medians
and edges along buildings and
Because they are designed to
drain within 24 hours, rain gardens
avoid standing water, explains
the Whole Building Design Guide
(WBDG) website ( wbdc.org) of
the National Institute of Building
Sciences. The cost for rain gardens
on commercial, industrial and institutional sites ranges from $10 to
$40 per square foot.
While an appropriate choice of
plantings depends on local weather,
Cooper says the design of rain gardens should strive to have aesthetic
appeal in all seasons.
While the terms rain garden and
bioswale are often interchangeable, vegetated swales are typically
larger, deeper and more linear than
rain gardens. They are designed to
handle specific volumes of runoff
from large impervious areas while
providing variable filtering and
infiltration capabilities. They intercept the first runoff until their soil
is saturated, at which point swales
act like a channel that directs further runoff to bioretention areas.
According to WBDG, the design
and cost of swales is roughly 50
cents per square foot.
Seattle is building large swales
along two city streets to treat and
divert runoff from a 430-acre area
in its Capitol Hill district. The swales
will prevent runoff that would
otherwise flow untreated into the
stormwater collection system and
Lake Union. The project is funded
by a public-private partnership
between Seattle Public Utilities and
developer Vulcan Real Estate.
Stormwater that exceeds the
swales’ capacity is directed to a
GUTTERS ROUTE RAINWATER TO ROCK BEDS and other bioretention features at Hawaii's Leeward Community College. The campus also harvests rainwater for irrigation.
BIOSWALES ALONG TWO CITY STREETS IN SEATTLE treat runoff that would otherwise flow
directly from a 430-acre district into Lake Union.