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