Landscape – Great Idea! X-LArch III - Department für Raum ...

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research conducted in Portland has shown that by replacing
4% to 7% of the drainage area with rain gardens,
50% to 96% of total runoff can be captured and infiltrated
(Kurtz, 2008). Capturing and infiltrating runoff may also
improve the quality of runoff; rain gardens filter contaminants
contained in street runoff, which will be partially
cleansed when retained.
The design of rain gardens is dependent on the amount
of stormwater runoff generated from the drainage area.
Rain garden capacity should be no less than the amount
of runoff from the drainage area during the design rainfall
event. Currently, the percentage of impervious streets
and driveways in downtown Vancouver’s West End is
similar to the Downtown District, but the area of green
spaces in the West End is five times the size of the area
in the Downtown District. In a 12ha case study area
in the West End, 17% of the site area is found to be
comprised of streets and 28% of pervious green space,
while in a 20ha case study area in the Downtown District
24% of the total site area consists of streets but less than
5% of pervious green space. As a result, different design
strategies for rain gardens should be applied to the West
End and Downtown District.
To calculate the size of rain gardens, this paper selects
a typical residential neighbourhood block in the West
End and calculates street runoff generated from the site
during a 24-hour rainfall event of 25.4mm. In Vancouver,
more than 95% of the daily precipitation throughout a
year is less than 25.4mm (Environment Canada, 2008).
Two scenarios developed on the selected site are shown
as follows in Figure 1.
For the purpose of this paper, rain gardens are assumed
to havea loamy soil with a soil depth of 1,000mm and a
ponded level [2] of 50mm (Greater Vancouver Regional
District, 2005: 44). The potential water storage capacity
of rain gardens is based on the calculation of soil water
balance, the potential runoff during a 24-hour rainfall
event of 25.4mm, the size of drainage area, and the soil
properties in accordance with the Water Balance Model
(Water Balance Model, 2008). The selected site in the
West End is contained by the following 4 streets: Nicola,
Comox, Broughton and Pendrell streets.
event of 25.4mm is 20mm. A total runoff volume of 10m³
will be generated from the street surfaces. This runoff
volume can be retained by rain gardens alone [Fig2], or
in combination with existing green spaces through the
incorporation of grass swales [Fig3].
The cost range of rain gardens varies from $30 to
$400 per square metre, and is dependent on subsoil
conditions, plant selection, curbing, storm drains and
underdrains (City of Chicago, 2009). Compared with
rain gardens, the cost of a grass swale is much lower,
around $5 per square metre (USEPA, 2006). Therefore,
the best case scenario will be determined the size of rain
gardens, soil properties and cost. A cost comparison between
scenario 1 and 2 is calculated in accordance with
soil properties and is shown in Figure 4.
Conclusions
During a 24-hour rainfall event of 25.4mm in downtown
Vancouver, an impervious street area of 500m² requires
a rain garden area of 38m² to retain street runoff. In
the West End, the measurements of the selected site
show that 28% of the total site area consists of pervious
surfaces while 20% is covered by streets and driveways.
Incorporating rain gardens within the existing green
space in the West End through the use of grass swales
could reduce the additional area needed for rain gardens
by 53%. Because the cost of implementing a rain garden
can vary from $30 to $400 per square metre, depending
on the subsoil conditions, the cost saving by reducing the
According to the SCS-CN method, the runoff rate generated
from impervious surfaces during a 24-hour rainfall
Papers