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size of rain garden through the use of grass swales will
be variable. In well-drained areas, the cost of a rain garden
is low. The additional grass swales in scenario 2 will
cost more than the savings created by reducing the size
of rain garden in scenario 1. Therefore, in well-drained
areas, scenario 1 will be more cost-efficient. However,
in areas with compacted, poorly drained subsoil with low
infiltration rates, it will be more cost-effective to apply
scenario 2, incorporating grass swales to reduce the size
of the rain garden.
According to measurements, 21% (116ha) [1] of downtown
Vancouver (579ha) is covered by streets. If the size
of rain gardens required to absorb the runoff generated
by these streets is 7.6% (38 m2 /500 m2 =3.6%) of the
street area in scenario 1 and 3.6% (18 m2 /500 m2
=3.6%) for scenario 2, a total of 40,000m2 to 88,000m2
of rain gardens will be necessary in downtown Vancouver.
This will cost between one million and thirty-five
million dollars, assuming a square metre cost for rain
gardens of between $30 and $400. Therefore, substantial
cost savings can be achieved if rain gardens are designed
efficiently, incorporating grass swales where conditions
are appropriate. In the West End District, which has
a large area of existing pervious surfaces, there is much
potential for grass swales to be incorporated into rain
gardens, decreasing total size and reducing costs.
Endnotes
[1] Measurements are based on four selected sites (10ha to 20ha
each) in Downtown Vancouver.
[2] Ponded level. Ponded depth is the depth of surface water above
the ground surface.
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