Tree Hazard Risk Assessment: Understanding the Process
A tree risk assessment is a professional evaluation conducted by a certified arborist to determine a tree’s health, its likelihood of failure, and the potential risks it poses. This process helps assess whether a tree presents an immediate threat to people or property and what actions can be taken to mitigate the risk.
Tree Hazard Risk Assessment Calculator
Key Factors in Tree Risk Assessment
Arborists evaluate multiple factors when assessing the risk associated with a tree. Here’s what they look for:
1. Tree History
- Previous branch failures indicate potential weaknesses.
- Presence of pests such as borers and termites may compromise structural integrity.
2. Tree Vigor
- Arborists analyze canopy density, leaf color, and bark condition.
- Signs of significant deadwood or poor healing of past pruning wounds indicate declining health.
3. Species Profile
- Some tree species are naturally more prone to structural failure.
- For example, Eucalyptus nicholii is known for shallow roots and susceptibility to wind damage.
4. Crown Density
- A thinning or dying crown (crown dieback) is an early warning sign of declining health.
- The uppermost branches showing fewer leaves than normal is a key indicator.
5. Potential Targets
- The level of risk depends on the surroundings.
- A tree in an open field poses less risk than one near homes, roads, or playgrounds.
6. Likelihood of Failure
- Based on history, health, and species characteristics, arborists rate failure risk from low to high.
7. Consequences of Failure
- If a failing tree endangers people, homes, or schools, action must be taken.
- Minor risks (e.g., a falling branch damaging a fence) may not warrant drastic action.
8. Mitigation Options
- If the risk is moderate to low, pruning or selective branch removal may suffice.
- If the risk is high, complete tree removal may be necessary to ensure safety.
Do Risk Assessments Include an Arborist Report?
Not always. If an individual requests an assessment for personal reasons, a written report may not be required. However, if a city council mandates the assessment (e.g., for a development application), an official arborist report with a risk evaluation is usually necessary.
In some cases, local authorities may challenge a request for tree removal. If public safety is a concern, the property owner might need to submit a comprehensive arborist report detailing the risk assessment findings.
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The Principles of Tree Failure Risk Assessment
Tree failure risk assessment is the process of evaluating the likelihood of a tree or part of it failing and the potential consequences of such failure. The complexity of this process varies based on the requirements of the tree manager. A simple assessment might have limited value, while an overly complex method can be impractical.
With the introduction of Quantified Tree Risk Assessment (QTRA) in the mid-1990s, the term “tree risk assessment” replaced “hazard assessment.” The distinction between “hazard” and “risk” is crucial:
| Term | Definition |
|---|---|
| Hazard | The inherent potential of a tree to cause harm. |
| Risk | The probability of the hazard causing harm. |
Tree failure risk assessment involves evaluating the mechanical integrity of a tree and considering whether its failure could pose a significant risk to people or property. If risk management resources are to be allocated, it should be for significant risks rather than minor concerns.
The assessment is generally divided into two parts:
- Likelihood of failure: Evaluating the probability of tree failure and the likelihood of a target being present.
- Consequences of failure: Assessing the size of the failed part and the potential impact on the target.
Risk Assessment Methods
Various methods exist for tree risk assessment, many of which use ranking systems. Norris (2007) reviewed fifteen such methods, including QTRA, and found significant variations in how different arborists applied them. One widely used method is the ISA Tree Hazard Evaluation Method developed by Matheny and Clark (1994), which scores tree failure risk based on three components:
| Component | Score 1 | Score 2 | Score 3 | Score 4 |
| Failure Potential | Low | Medium | High | Severe |
| Size of Part | <150mm | 150-450mm | 450-900mm | >900mm |
| Target Rating | Occasional | Intermittent | Frequent | Constant |
The sum of these scores produces a “Hazard Rating” (3 being the lowest risk and 12 the highest). This system allows for comparative ranking within a tree population but is less effective for assessing individual trees.
Quantified Tree Risk Assessment (QTRA)
Developed in 1996, QTRA applies risk management principles to tree safety. It evaluates risk based on three key components:
| Component | Description |
| Target | Assesses the likelihood of people or property being affected by tree failure. |
| Impact Potential | Considers the size and weight of the failing tree part. |
| Probability of Failure | Estimates how likely the tree or part of it is to fail. |
The combined values of these components express the “risk of significant harm.” Unlike traditional methods that classify trees as “safe” or “unsafe,” QTRA quantifies risk to balance tree safety with tree value.
QTRA Target Evaluation
| Target Range | Property Value (Repair Cost) | Pedestrian Frequency | Vehicular Frequency | Probability Ratio |
| 1 | >£50,000 – £1,000,000 | >36 per hour | 26,102 vehicles at 110kph | 1/1 |
| 2 | >£13,888 – £50,000 | 10-36 per hour | 1,305 vehicles at 110kph | 1/20 |
| 3 | >£1,388 – £13,888 | 1-10 per hour | 363 vehicles at 110kph | 1/72 |
| 4 | >£57.87 – £1,388 | 1 per day – 1 per hour | 36 vehicles at 110kph | 1/720 |
| 5 | >£8.60 – £57.87 | 1 per week – 1 per day | 1.5 vehicles at 110kph | 1/17,280 |
| 6 | <£8.60 | ≤ 1 per week | None | 1/120,960 |
The frequency of pedestrian and vehicle presence influences the level of risk assessment required. In areas with low traffic and no valuable property, detailed assessments may not be necessary.
QTRA Impact Potential
| Impact Potential Range | Size of Part Likely to Impact Target | Impact Potential |
| 1 | >450mm diameter | 1/1 |
| 2 | 250-450mm diameter | 1/2 |
| 3 | 100-250mm diameter | 1/8.6 |
| 4 | 25-100mm diameter | 1/82 |
| 5 | 10-25mm diameter | 1/2500 |
The severity of injury or damage increases with the size of the failed part, but past a certain size, additional increases do not proportionally raise the severity of impact.
QTRA Probability of Failure
| Probability of Failure Range | Probability Percentage | Probability Ratio |
| 1 | >10% – 100% | 1/1 |
| 2 | 1% – 10% | 1/10 |
| 3 | 0.1% – 1% | 1/100 |
| 4 | 0.01% – 0.1% | 1/1,000 |
| 5 | ≤ 0.01% | 1/10,000 |
The Effects of Weather on QTRA
Weather conditions can significantly impact tree failure probabilities. For example:
- High winds may increase failure potential but decrease site occupancy.
- “Sudden Branch Drop” in hot weather may increase risk due to higher pedestrian presence.
- A “Weather Factor” can be applied to adjust risk calculations.
Case Study: Goode v City of Burnside (2007)
The case involved a dispute over the removal of two River Red Gum trees. The key issues were:
- The risk of tree failure.
- Potential damage from root activity.
QTRA was applied but met criticism for its perceived subjectivity. The court’s concerns included:
- The assignment of scores being subjective.
- Differences in expert opinions regarding failure likelihood.
- The expression of risk outputs in overly precise numerical terms.
Despite these concerns, QTRA remains a widely used system, providing a structured approach to balancing safety with tree conservation.
