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Cost Considerations
Cost at our projects was influenced by a number of factors:
  • We bought a proprietary blend of structural soil, which was more expensive than if we'd blended the recipe created by the City of Olympia, WA.
  • THPRD Sunset Swim Center was a redevelopment with contractors already at the site with heavy equipment. This also made it difficult to isolate what exactly the costs associated with our project were.
  • PCC Sylvania Lot 10 was a retrofit project, so mobilization costs were higher.
Costs (from 2013) are summarized as follows:
  THPRD Sunset Swim Center (Redevelopment with Pervious Concrete) PCC Sylvania Lot 10 (Retrofit with Conventional Asphalt)
Area of trenches [sf]
2,400
400
Construction & permit costs
~$21,000
$9,447
Cost per square foot of facility
$105
$23.62
# of trees
15
3
Cost per tree
$1,400
$3,149
Cost per square foot of area managed for stormwater
$105.00
$1.55
Cost per square foot of impervious area managed for stormwater
N/A. No impervious area drains to this trench.
$3.53
Cost benefits of trees (from the iTree Design webapp)
$1,179
$523
Cost benefits of trees per tree
$79/tree
$174/tree

What do these costs mean? The cost per square foot to build the Parking Forest at the Sunset Swim Center was much more expensive, but we really don't know why. It should have been less expensive, since equipment and contractors were already mobilized. If we look at this simply from an urban forestry perspective, the cost per tree is very expensive, but if we look at this from the perspective of increasing urban canopy AND managing stormwater, then the cost becomes competitive with other more conventional green infrastructure approaches.

Conclusion: Despite the fact that construction costs were higher at the Sunset Swim Center, we would expect that the cost for this BMP would be higher anyway. The Sunset Swim Center only manages rainfall, since the trees are completely surrounded by pervious concrete, which does not generate runoff. The PCC Sylvania site is a cost-effective retrofit project that is competitive with other bioretention approaches that might be retrofitted into a parking lot, such as a rain garden or a stormwater planter, which can range from 0.45/sf to $4.50/sf area managed (per the University of Connecticut Stormwater Center rain garden cost calculator and my own experience in the Portland metro region).

Creative Commons LicenseParking Forest Website by Green Girl LDS is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. We believe the term "Parking Forest" was coined originally by Brian Wegener of the Tualatin Riverkeepers in an email on June 29, 2013.
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