Quantified Tree Risk Assessment (QTRA)
I use a system called QTRA to help ensure a consistent approach in evaluating tree risk. However, that’s the problem: when it comes to trees, risk is never black and white.
Further: there’s a vast range of controversies surrounding tree risk assessment, which can make it tricky for tree owners and land managers to decide on the best line of approach. Examples include:
- No uniform, set frequency for tree inspections or type of inspection required
- Variability in what the courts view as ‘proportionate precautions’ for tree owners to reduce risks
- No UK standard for competencies or knowledge to inspect trees
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It’s all about probability
QTRA balances the risk of harm from falling trees with the benefits from trees. It may seem counterintuitive, but tree condition isn’t the first consideration. Instead, tree managers should first consider the uses of the land on which the trees stand, which in turn informs the process of assessing the trees.
The method moves the management of tree safety away from labelling trees as ‘safe’ or ‘unsafe’. Instead, it quantifies the risk of significant harm from tree failure and allows tree managers to balance safety with tree value and operate to predetermined risk thresholds.
As a result, tree managers can spend less on assessing and managing tree risk, while maximising the benefits of their tree populations. Moreover, if there is a ‘tolerable’ or ‘acceptable’ tree risk, they are in a position to demonstrate that they have acted reasonably and proportionately.
How QTRA works
QTRA applies established and accepted risk-management principles to tree safety management in accordance with ISO 31000:2009, Risk management – Principles and guidelines.
Land-use (people and property) on which trees could fail is first assessed and quantified. This enables tree managers to determine whether and to what degree of rigour a survey or inspection of the trees is required.
Where necessary, the tree or branch is then considered in terms of size (potential impact) and probability of failure. Values derived from assessment of these components are combined to calculate the risk of harm as a probability that can be compared to advisory levels of acceptable risk.
These are the risk thresholds and actions:
| Thresholds | Description | Action |
|---|---|---|
|
Unacceptable Risks will not ordinarily be tolerated |
Control the risk |
|
| 1/1,000 | ||
|
Unacceptable (where imposed on others) Risks will not ordinarily be tolerated |
Control the risk Review the the risk |
|
|
Tolerable (by agreement) Risks may be tolerated if those exposed to the risk accept it or the tree has exceptional value |
Control the risk unless there is broad stakeholder agreement to tolerate it or the tree has exceptional value Review the the risk |
|
| 1/10,000 | ||
|
Tolerable (where imposed on others) Risks are tolerable if ALARP |
Assess costs and benefits of risk control Control the risk only where a significant benefit might be achieved at a reasonable cost Review he risk |
|
| 1/1,000,000 | ||
|
Broadly acceptable Risk is already ALARP |
No action currently required Review the risk |
Source: QTRA