tree risk assessment_mo appa
TRANSCRIPT
Tree Risk Assessment Developing New Standards and BMP’s
Some diagrams and text provided by International Society of Arboriculture
The Importance of Tree Risk Assessment
• Safety Concerns
Tree failures that cause damage and injury do not occur often
The Importance of Tree Risk Assessment
• Safety Concerns
But when they do, the consequences can be severe
The Importance of Tree Risk Assessment
• Legal Concerns
Attorneys love this stuff!!
Our desire and ability to perform recordable tree assessments (inventories) that include risk assessment is increasing.
The number of communities with public tree inventories is dramatically increasing….
• WHY?
The Importance of Tree Risk Assessment
• Trees have value. In fact, value we can now measure this value in economic terms using tools such as i-Tree.
• Developed by the United States Forest Service to document the benefits of “green infrastructure”
• Peer reviewed and studied to document the science behind benefit calculations
• Recognized by the ISA and the scientific community as the foremost calculator of tree benefits
The Importance of Tree Risk Assessment
• It is largely because of this understanding of tree values,and the growing interest in conducting inventories and assessments that the topic of tree risk assessment has gained growing interest as well.
• Tree risk assessments are typically part of the inventory and assessment process.
The Importance of Tree Risk Assessment
• An accurate assessment could mean that we actually remove fewer trees based on perceived risk.
Duty of care – legal obligation that requires an individual to apply reasonable actions when performing tasks that may potentially harm others.
• Tree owners and managers – responsible for ensuring a level of safety for people or property near trees under their care.
• Tree risk assessors (arborists) – responsible for using the generally accepted standard of care.
Legal Side of Tree Risk -
Standard of care – degree of care that a reasonable
person should exercise in performing duty of care.
A measurement used in legal matters to assess whether
an individual acted in a reasonable manner.
Standard of care is defined by:
• applicable standards
• best management practices
• qualifications
• training courses
Legal Side of Tree Risk -
• ANSI A300 (Part 9)-2011 (Tree Risk Assessment)
• ANSI = American National Standards Institute
• A consensus document that unifies and takes authoritative precedence over tree care industry standards.
• Copies available from TCIA
A New Standard Exists…
Best Management Practices• Consensus document
• More than 75 reviewers… 13 countries
• Based largely on ISO 31010 and ANSI A300
• Primary authors:• Tom Smiley• Nelda Matheny• Sharon Lily
• Now available from ISA
• 10th in the BMP series
Risk Management Principles
• It is impossible to maintain trees free of risk;
• some level of risk must be accepted to experience the benefits that trees provide.
Risk Management Principles
• Trees provide a wide variety of benefits to society
• Trees are living organisms and naturally lose branches or fall
• The risk to human safety is extremely low
• Tree owners have a legal duty of care
• Tree owners should take a balanced and proportionate approach to tree safety mgmt.
The National Tree Safety Group (NTSG), United Kingdom
Risk Management Principles
• Everyone’s willingness to accept risk is different
Risk Management Principles
• Some accept high risk but take reasonable steps to identify it and then mitigate
Types of Risk - Failures
• Whole tree failure (uprooting)
• Trunk failure
• Branch failure
Levels of Assessment
The new ANSI Standards and BMP’s describe three levels (types) of assessments
• Level 1 – Limited Visual
• Level 2 – Basic
• Level 3 – Advanced
Level 1: Limited Visual Assessment
Involves a visual assessment of an individual tree or a population of trees near specified targets.
• Conducted from one perspective
• Identifies trees with imminent and/or probable likelihood of failure
• Sometimes not a complete “risk assessment”
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When to Conduct
a Limited Visual
Assessment
• Manage large populations
of trees
• Limited time and resources
• Immediately after storms
• Scheduled routine
assessments
• Tree inventories
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Limited Visual Assessment Process
• Identify location and select trees to be assessed
• Determine route
• Assess tree(s)
• Record locations of concern
• Evaluate risk
• Identify trees that need a higher level of assessment
• Submit report
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Walk-By Limited
Visual Assessment
• Inspects one or more
sides of the tree
• Performed as the
inspector walks past
a tree
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Drive-By Limited
Visual Assessment
• Visual assessment
of one side of a tree
• Performed from a
slow-moving vehicle
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Aerial Patrol Limited
Visual Assessment
• Visual assessment made
from an aircraft
• Overfly of utility rights-of-
way or other large areas
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Level 1: Limited Visual Assessment
Advantage: relatively quick screening tool for
assessing large populations of trees.
Limitation: some conditions may not be visible from
a one-sided inspection. Major problems need to be
obvious given the limited observation time.
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Level 2: Basic Assessment
Detailed visual inspection of a tree and its surrounding site, and a synthesis of the information collected.
• Requires 360° visual inspection around tree
• Detailed visual inspection of buttress roots, trunk, and branches
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When to Conduct a
Basic Assessment
• In response to client’s
request for a tree risk
assessment
• Part of a tree risk
management plan
• Part of a tree inventory
• Sometimes mandated by
law or ordinance
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Basic Assessment Process
• Assess targets
• Assess site conditions
• Assess tree health, structure, and loads
• Record observations
• Analyze data and categorize risk
• Evaluate level of risk
• Develop mitigation options
• Submit report
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Basic Tree Risk
Assessment Form
• Tool used to record and
analyze data collected
during a Level 2 Basic
Assessment
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Optional Basic
Assessment ToolsUsing these tools does not
elevate the assessment to a
Level 3 advanced assessment.
• measuring tools
• binoculars
• magnifying glass
• mallet/hammer
• probe
• trowel/shovel
• compass
• camera
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• Broad-headed mallet
• Tap the tree trunk and
root flare in multiple
places and listen for
tone variations
Sounding
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• Stiff, small-diameter
rod, stick, or wire
• Insert into an open
cavity to estimate the
cavity size
Probing
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Use to conduct shallow
excavations to expose the
root collar or roots near the
soil surface.
Trowel or Shovel
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Level 2: Basic Assessment
Advantage: provides adequate information for most
risk assessments; moderate time requirement.
Limitation: notes only conditions that can be
visually assessed or sounded from a ground-based
inspection at the time of inspection; not all conditions
are present in all seasons.
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Level 3: Advanced Assessment
Used to gather detailed information in conjunction with, or after, a basic assessment.
• Provides detailed information about specific tree parts, defects, targets, or site conditions
• Specialized equipment, data collection, analysis, and/or expertise are usually required
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Level 3: Advanced Assessment
Advantage: can provide additional information that
may make the difference between recommending
tree retention or removal.
Limitation: advance assessment tools can be
expensive and may require extensive training to
use; time requirements highly variable.
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Aerial inspection
(crown inspection) is
the inspection of the
aboveground parts
of a tree.
Aerial Inspection
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• Drives a small-
diameter flat-tipped
spade bit into
the tree
• Records a graphic
profile of the wood’s
resistance
Resistance-
Recording Drill
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Sends sound (stress)
waves through the
wood and measures
the time for the wave
to travel.
Sonic Assessment
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Level 3 - Advanced Assessment
TargetsPeople, property, or activities that could be injured, damaged, or disrupted by a tree failure
Identify Known Targets and Assess Occupancy Rates
TARGET
TYPES:
Static
Moveable
Mobile
Target Zone
Likelihood
• Likelihood of failure
– Defects
– Loads
– Response growth
• Likelihood of impact
– Target occupancy rate
– Target protection factor
Failure occurs when
load exceeds strength
Biomechanics video
Loads on Trees
• Gravity
– Self weight
– Rain, snow, ice
– Other (epiphytes, climbers, rigging)
• Wind
– This force is dynamic – it varies in speed , frequency and direction
Defects and Conditions
• Dead parts
• Broken/hanging branches
• Cracks
• Weakly attached branches and codominant stems
• Decayed or missing wood
• Unusual architecture (lean, unbalanced)
• Loss of root support
Likelihood of Failure
• Improbable – the tree is not likely to fail even in severe weather conditions
• Possible – failure could occur, but it is unlikely during normal weather
• Probable – failure is expected under normal weather conditions
• Imminent – failure has started or is most likely to occur in the near future, even if there is no wind or increased load
Likelihood of Impact
• Very Low – The chance of impacting a target is remote
• Low – It is not likely that the target will be impacted
• Medium – The target may or may not be impacted.
• High – The target is likely to be impacted
• Likelihood of Failure??Within 1 year
When it fails, what is the…..
• Likelihood of Impact??
Target is a child playing in the back yard
Likelihood of Failure AND Likelihood of Impact(Table 1)
When the tree fails and hits the target….
How bad is it going to be?
• Consequences
Target is a child playing in the back yard
Consequences
• Negligible – low-value property damage; personal injury is unlikely
• Minor – low- to moderate-value property damage; personal injury is unlikely
• Significant – moderate- to high-value property damage; people could be injured
• Severe – high-value property damage; one or more people could be injured or killed
Risk Rating Matrix – TABLE 2Results of Table 1
Risk Levels - Recommendations
• Extreme - recommend that mitigation be done as soon as possible
• High risk - recommend mitigation measures be taken
• Moderate - recommend mitigation and/or retaining and monitoring
• Low risk - recommend retaining and monitoring as well as mitigation that does not include removal of the tree
Risk Mitigation
• Move or remove the target
• Prune the tree
• Install structural support
• Modify the site to improve conditions
• Remove the tree
Residual Risk
The risk remaining after mitigation.
If cabling was used to mitigate risk, is there any residual risk?
Risk Reporting
• When tree was assessed, and by whom
• Location or identification of the subject tree
• Inspection method used and details
• Targets and consequences of the failure
• Site factors that were considered
• Details on defects that were quantified
Risk Reporting
• A list of tree conditions and structural defects that were observed
• Risk assessment information and conclusion
• Recommendations for mitigation
• Residual risk information
• Recommendations for reassessment
