Transcript Wind-resistant urban design

Urban Forest Hurricane
Recovery Program
http://treesandhurricanes.ifas.ufl.edu
Wind-resistant urban design
Dr. Edward F. Gilman and Traci Partin
Could this have been prevented?
Photo credit: Chuck Lippi
The answer is yes!
Wind-resistant urban design
1. Design conflicts that can cause tree failure
2. Design solutions that promote wind
resistance
- existing design situations
- new design/construction
3. How to take action
How often do trees fall down due
to poor design?
What design conflicts cause
trees to fail?
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Large-maturing trees
within 10 feet of a paved
surface, without space
designed for root growth
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Poor soil conditions
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Single specimens
(versus grouping trees)
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Lack of open soil space
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Wrong species selected
for the site
Large roots cut during
construction
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NOT ENOUGH ROOT
SPACE!
Large maturing tree near curb
Photo credits: Brent Marable
Space is needed for roots to spread
roots
• Fine roots can be found well beyond the canopy
of the tree at full maturity
• These roots help anchor the tree under high
winds
Soil depth is necessary for root stability
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Roots need adequate soil depth to
anchor the tree under high winds.
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Soil should be at least 3 feet deep
for large maturing trees
Lack of open soil space
Root flare is interrupted by curb and sidewalk
Open soil space allows root flare
to develop
• The swelling at the base of
the tree (where the large
roots meet the trunk) is
commonly referred to as
the root flare or buttress
• The root flare provides
balance and stability for the
Flare commonly 2.5 to 3.5
massive weight of a tree
times trunk diameter
Roots cut during construction
Step two
Step one
3 x trunk rule
Step three
Photo credit: Andy Kittsley
Poor soil conditions
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Compacted
Shallow soil – rocky;
high water table
Drainage issues
High clay content
Alkaline soil/ Lime rock
- inhibits uptake of
essential nutrients
Little to no organic
matter (i.e. builder’s
sand)
Photo credit: Jim Urban
This is a familiar site
for many who live in
South Florida.
What can be done with
shallow, rocky soil?
Single specimens vs. grouping trees
Larger groupings are less damaged
This design did not
suffer any tree damage
when a hurricane blew
through
Consider the natural setting for a tree
Notice the trees still standing
Selecting the wrong species for the site
Choose small trees for
sites where soil space is
limited.
What design conflicts cause
trees to fail?
•
Large-maturing trees
within 10 feet of a paved
surface, without space
designed for root growth
•
Poor soil conditions
•
Single specimens
(versus grouping trees)
•
Lack of open soil space
•
•
Wrong species selected
for the site
Large roots cut during
construction
•
NOT ENOUGH
ROOT SPACE!
Where do we grow now?
Wind-resistant urban design
1. Design conflicts that can cause tree failure
2. Design solutions that promote wind
resistance
- existing design situations
- new design/construction
3. How to take action
Existing design situations
Problem:
Mature tree with large roots
interfering with hardscape.
Solutions:
• Install different surface
material
• Add fill and re-pour walk
• Bridge over roots
• Re-route walk
CUTTING ROOTS IS
NOT AN OPTION!
Install different surface material:
rock dust
Spread rock dust
Remove slabs
Pack dust tightly
Photo credits:
Final product:
looks attractive
and is no longer a
tripping hazard
Install different surface material:
porous pavers
To protect the root zone
around the existing trees in
this parking space at a
botanical garden, porous
pavers were selected
Add soil then re-pour
over roots
Bridging over roots
Re-routing walk around tree
• When damaged
sidewalks are repaired
they can be re-routed
around the tree trunks
• This can eliminate the
need to prune roots
that caused the walk to
lift
Do not cut roots!
New design situations
How do we make
trees fit?
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Parking lots
Planting islands
Road medians
Sidewalks
Streets
Buildings
Good design happens in two ways…
• Design the right
• Choose the right tree
place to fit the trees for the conditions of
you want
the site
Designing the right place
1. Plant trees in the open soil
space available
2. If this is not possible, direct
roots toward the open soil
3. If there is not open space for
root growth, design
appropriate soil space
4. Consider groupings vs.
individual plantings
“Appropriate” soil space
At least 1000 to 2000
cubic feet of soil for
each healthy, large
maturing tree
Open soil space 3 X
wider than trunk
diameter at maturity
(dbh) to allow root
flare development
Planting strips in successful designs
Structural soil
Small aggregate material (angular rocks ~ 1 in. diameter)
with enough soil to almost fill the space between the rocks.
Roots grow well in the soil between the aggregates.
Illustration credit: Jason Grabosky
Tree growth in structural soil
Photo credit: Jason Grabosky
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These trees were
planted into a strip of
structural soil installed
in this retrofitted
parking lot between the
blue arrows
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They have performed
quite well
Suspended sidewalk
Suspending the sidewalk
avoids issues with soil
compaction so that roots
can spread without
interrupting the hardscape
Suspended sidewalk:
Soil cells
This particular
system provides
structure to support
the hardscape, filling
in the remaining
space with quality
soil.
Pavement
Structure
Soil
Use alternate surface materials
• Materials other than concrete can be used as a
wearing surface:
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crushed granite
gravel
wood decking
brick-in-sand
porous pavers
porous asphalt
• These materials allow oxygen to penetrate the
soil and may encourage root growth
Porous paving surfaces
Porous surfaces are a
good design idea for
areas prone to flooding:
• reduce runoff
• provide aeration to the soil
Designing the right place
1. Plant trees in the open soil
space available
2. If this is not possible, direct
roots toward the open soil
3. If there is not open space for
root growth, design
appropriate soil space
4. Consider groupings vs.
individual plantings
The goal: A healthy urban forest
• Canopy cover
• Diversity of
tree species
grouped together
Plant trees closer for canopy closure
• Trees planted 60-70
feet apart grow large
lower limbs due to
wide spacing, and
require pruning to
allow clearance.
• Group trees 30 ft.
apart to encourage an
upright form.
Grouping vs. individual planting
Larger soil space shared by more trees = healthier trees!
This is a sustainable
design that will last
many years.
In a few years, this
tree will outgrow the
space.
Example of sustainable parking lot design
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Trees are located only in
the buffer strips
surrounding this parking
lot – trees are not in small
islands scattered
throughout parking lot.
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This provides adequate
space for tree roots to
grow.
Remember: small spaces do not
support large trees!
Solutions can happen in two ways…
• Design the right
• Choose the right tree
place to fit the trees for the conditions of
you want
the site
Small trees for small spaces
When planting within 10 ft. of
curbing
Plant small or medium
sized trees (no greater than
about 35 ft. at maximum height)
Wind resistant species
To get a
comprehensive list of
wind-resistant species,
refer to our website:
(http://treesandhurricanes.
ifas.ufl.edu/index.html
Do not over-plant one species
• Restrict one genera or species to < 20% for a
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few years
Develop a list of alternatives for each
commonly planted tree
Examples of alternatives to live oak:
• Swamp chestnut, redbay, trident maple, sugarberry,
ash, sweetgum, american elm, cedar elm, overcup
oak
Wind-resistant urban design
1. Design conflicts that can cause tree failure
2. Design solutions that promote wind
resistance
- existing design situations
- new design/construction
3. How to take action
Where to begin…
1. Get the right people involved
2. Set new building ordinances for the
community
3. Become a Tree City USA
Who should be involved in the
planning process?
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City engineer
Landscape architect
Urban forester/arborists
Builders & developers
Planners
Parks and Recreation
Power companies
Homeowner’s Association
City or county commissioners
What is Tree City USA?
• Program started by The National Arbor Day
Foundation to promote communities that take
care of their trees
• Provides a framework for starting a tree
management program.
More information:
http://www.arborday.org/programs/treeCityUSA.cfm
Contact your local county extension!
• Get information specific to counties across the
state
i.e. tree species that grow well in the area,
local soil conditions, etc.
• Cuts time spent on looking up information in
half!