Overview of two pilot projects of the Parking Forest, an assembly of common materials used to increase tree canopy and manage stormwater in parking lots without losing parking spaces.

1. The Parking Forest:
Linear Tree Wells Using Structural Soil
503.334.8634
www.greengirlpdx.com
greengirl@greengirlpdx.com
a certified women business enterprise
Sustainability for all the places between the buildings

2. Clean Water Services
Healthy Streams Plan
Main Street Tigard

3. Effective Impervious Area
within the Tualatin Basin UGB
8%
Source: Clean Water Services Healthy Streams Plan

4. 50%
evaporation
100% XX” average
annual rainfall
Why do we need more trees in the urban fabric?
Water Balance BEFORE Development
25% groundwater
(deep infiltration)
0.5% runoff

5. reduced
evaporation
2%
evaporation
98% runoff
No
infiltration
Reduced
infiltration
100% average
annual rainfall
Water Balance AFTER Development
Example: EVERYWHERE

6. Water Balance Runoff Compared
0.5% runoff
98%
runoff

7. Hydromodification
from a Watershed Perspective
Please! Keep your
runoff volume to
yourself!
• Additional volumes
result in exceeding predeveloped rates that
scour stream banks.
• Additional durations of
flow impact habitat
further.

8. What’s wrong with the trees we’ve got?

9. What’s wrong with the trees we’ve got?!
Photo Source: gratefulmommy.com

10. What about all those lovely
tree-lined streets in Portland?

11. Trees Need Soil to Grow
Graph Source: USDA Forest Service Center for Urban Forest Research
Slide courtesy of Todd Prager ,Todd Prager & Associates

12. Trees Need Soil to Grow
5 ft Crown
40 cu ft Soil
20 ft Crown
450 cu ft Soil
40 ft Crown
2000+ cu ft Soil
Photos and observations courtesy of Todd Prager, Todd Prager & Associates

13. The Parking Forest
An Assembly of Common Materials

14. The Parking Forest
A Close Up
Crushed
aggregate
Tree
Re-paved area
Pavement
outside of
Parking Forest
Wheel stop
Undisturbed native
clayey soil
Amended native
clayey soil
Structural
Structural
Soil
Soil (dry)

15. Two Sites
Redevelopment
Retrofit
Watershed base map courtesy of Tualatin River Watershed Council

16. THPRD Sunset Swim Center
Redevelopment with Pervious Concrete

17. THPRD Sunset Swim Center in Plan
Redevelopment with Pervious Concrete

18. Structural Soil
Our “Secret Weapon”
Source: Cornell University – CU Structural Soil - http://www.hort.cornell.edu

19. How do we know it works?
Olympia Results 8 Years Later
Control Trees Planted in Compacted Native Soil
Trees Planted in Structural Soil

20. Assembly Procedure

21. Assembly Procedure

22. Assembly Procedure

23. Assembly Procedure

24. Assembly Procedure

25. Assembly Procedure

26. Assembly Procedure

27. Assembly Procedure

28. Why these materials?
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Structural soil
Amended Native Soil
Trees
Wheel stops
Crush rock (in landscape area)

29. Why these materials?
Structural Soil
• Structural soil versus structural supports

30. A Second Way of Providing Tree Roots with
Adequate Soil Volume Under Pavement
. Photo courtesy of Jim Labbe

31. AAnother Way of Providing Tree Roots with
Second Way of Providing Tree Roots with
Adequate Soil Volume Under Pavement
Adequate Soil Volume Under Pavement
. Photo courtesy of Jim Labbe

32. A Second Way of Providing Tree Roots with
Adequate Soil Volume Under Pavement
Jim Labbe in Nijmegen Netherlands.
Photo courtesy of Jim Labbe

33. A Third Way of Providing Tree Roots with
Adequate Soil Volume Under Pavement
Photo probably ripped off the Silva Cell website by Brian Wegener

34. Why these materials?
Proprietary mix versus mixed by contractor

35. Mixing Structural Soil
Photo Source: www.greelysand.com/cu-soil.html

36. Olympia Soil Recipe
4 cubic yards Crushed Rock ¾”-1¼”
+ 1 cubic yard Loam/organic topsoil
+ 1 pound Soil Binder “Stabilizer”
+ 46 gallons water__________
Makes 4.6 cubic yards Structural Soil

37. Structural Soil Costs - Olympia
“The price to supply, deliver and install the structural soil material per
cubic yard ranged from $19 to $85 with an average bid of $42, the median
price was $39.50.
The city paid $24 per cubic yard for the structural soil material to the low
bidder.
This compare to a price quote we received for the supply of CU structural
soil at $65 per cubic yard plus delivery. The source was over an hour’s drive
away and delivery charges were $85 per hr with a truck and trailer hauling 22
cubic yard. The expected cost delivered was $72 per cubic yard.”
Excerpted from “Structural Soil Demonstration Project”
http://olympiawa.gov/~/media/Files/CPD/Urban%20Forestry/Forms/StructuralSoil.as
hx
Source: City of Olympia -Structural Soil Demonstration Project

38. Why these materials?
Amended native soil
Amended native soil versus faster draining “3-way” mix

39. Why these materials?
Trees
Trees versus only shrubs &/or groundcover:
• Interception & evaporation
• Evapotranspiration
• Heat island effect

40. Why these materials?
Wheel stops
Wheel stops versus curbs

41. Why these materials?
Crushed rock
¼”x 10 crushed rock versus rounded rock or pavers

42. Siting Criteria
• Site is suitable for infiltration
• Non-expansive soils
• Soils should infiltrate at least 0.2 inches/hour*. Clay soils
are OK!
*Based on 2ft of structural soil, 25% void ratio, facility empty in 30 hours to be
ready for the next Type IA storm

43. PCC Sylvania Lot 10
Drainage Area
Parking Lot
1919 sf
Sidewalk
754 sf
<50% Grass HSG C
615 sf
Woods, Good HSG C
2815 sf
6103 sf
6133 sf

44. PCC Sylvania Lot 10
in Plan

45. The Parking Forest Retrofit
at PCC Sylvania Lot 10
Ignore the piles
of dirt. They’re
gone.

46. Design Differences at PCC Sylvania Lot 10
Installed an impervious liner
adjacent to impervious
asphalt
Stakeholders
chose a native
conifer
Left most of
the parking lot
unmodified/
impervious

47. Water quality and quantity monitoring

48. Design Differences at PCC Sylvania Lot 10
Straight headin parking
Just trees
Donated “old school”
solid bottom wheel
stops = not ideal

49. Erosive forces
are an issue at PCC

50. Non-attentive forces
are an issue at PCC

51. Thank You!
Partners Who Donated In-Kind Services/Products
Alder Geotechnical Services

52. Siting Criteria at PCC
• Site is suitable for infiltration
• Non-expansive soils
• Soils should infiltrate at least 0.2 inches/hour*. Clay soils
are OK!
*Based on 2ft of structural soil, 25% void ratio, facility empty in 30 hours to be
ready for the next Type IA storm

53. More Information
www.parkingforest.org
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