About QTRA

What is QTRa?

The Quantified Tree Risk Assessment Method

Tree safety management is a matter of limiting the risk of harm from tree failure while maintaining the benefits conferred by trees. Although it may seem counterintuitive, the condition of trees should not be the first consideration. Instead, tree managers should consider first the usage of the land on which the trees stand, and in turn, this will inform the process of assessing the trees.

The Quantified Tree Risk Assessment (QTRA) method, developed by Mike Ellison at Cheshire Woodlands, applies established and accepted risk management principles to tree safety.  The method moves the management of tree safety away from labelling trees as either ‘safe’ or ‘unsafe’ and thereby away from requiring definitive judgements from either tree assessors or tree managers. Instead, QTRA quantifies the risk of harm from tree failure in a way that enables tree managers to balance safety with tree values and operate to predetermined limits of tolerable or acceptable risk.

By quantifying the risk from tree failure, QTRA enables a tree owner to manage the risk in accordance with widely applied and internationally recognised levels of risk tolerance. QTRA further provides a decision-making framework which considers the balance between the benefits provided by trees, levels of risk they pose, and costs of risk management.

QTRa Advisory Risk thresholds

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 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 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 costs

Reviews the risk

1/1 000 000

Broadly Acceptable

Risk is already ALARP

No action currently required

Review the risk

Using QTRA, the land-use (Target) upon which trees could fail is assessed first.  By valuing the Target first, the tree owner and the risk assessor are able to determine whether or not, and to what degree of rigour, a survey or inspection of the trees is required.  Where necessary, the tree is then evaluated in terms of its size and probability of failure.  Ranges of value for Target, Size, and Probability of Failure are entered into a QTRA calculator which generates a traffic light colour-coded risk of harm.  The tree owner can then compared the risk to advisory levels for risk tolerance.


By taking a QTRA approach, tree owners commonly find they spend substantially less resources on assessing and managing tree risk than they did previously, whilst maximising the many benefits their trees provide.  Moreover, in the event of a ‘tolerable’ or ‘acceptable’  risk being realised, they are in a position to demonstrate the risk has been managed reasonably and proportionately.

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The Quantified Tree Risk Assessment method

Duty of care

Property owners and managers have a duty (under English law) to ensure, insofar as is reasonably practicable, that people and property are not exposed to unreasonable levels of risk from the mechanical failure of their trees. To achieve this, prudent owners and managers employ arboriculturists to advise on health, mechanical integrity and management of trees.

Without Quantified Tree Risk Assessment, a landowner or manager, perhaps having responsibility for a diverse tree population, is likely to rely on the judgement of the tree assessor employed at any point in time.  But repeated assessment by different individuals can lead to the degradation of the tree resource as each, mindful of their exposure to future litigation, attempts to minimise the risks from tree failure.

Acceptable Risk

Tree safety management should not seek to minimise the risk of falling trees, but should balance the benefits of risk reduction with the associated costs in terms of both lost tree value and financial expenditure and maintain risks and benefits at a reasonable level.

If absolute safety from tree failure were achievable, society would almost certainly find the cost unacceptable in terms of tree losses. In this regard, the concept of ‘reasonable practicability’ is a central tenet of English law, which is evident throughout the English Health and Safety legislation and guidance (e.g. Health and Safety at Work Act 1974), and in judgments of the higher courts in relation to tree failure. This concept is central to the Quantified Tree Risk Assessment method.

Structure Assessment

Quantified Tree Risk Assessment provides a framework for the assessment of the three components of tree-failure risk – ‘Target’ (land-use), ‘Size’ and ‘Probability of Failure’.  By first assessing the value or usage of targets upon which trees might fail, tree owners and site managers can establish whether or not and at what degree of rigour tree assessments are required. By assessing the probabilities of the three components and calculating their product, it is possible for the skilled tree assessor to quantify the risk of significant harm from tree failure in a way that enables owners and managers to make balanced management decisions.

The use of quantification in the assessment of tree-failure risk enables property owners and managers to operate, insofar as reasonably practicable, to a predetermined level of acceptable risk. By using quantifiable values we can, with training, assess the risk from tree-failure with sufficient accuracy that property owners and managers are able to compare the risk of significant harm from tree failure with levels of tolerable or acceptable risk. Using the Quantified Tree Risk Assessment method, it is possible, not only to identify unacceptable risks, but also to identify the components of the risk, which when adjusted will most effectively reduce the risk in the most cost efficient or appropriate manner.

Reduced Levels of Remedial Action

Application of Quantified Tree Risk Assessment in both urban and rural situations over many years indicates that when using the method, risk reduction measures are frequently considerably lower than assumed or perceived prior to applying the method.

A walk through woodland and other recreational areas after a moderate storm will often reveal paths and tracks littered with dead and recently living branches. Often the nature of a defect is such that the probability of failure is greater during windy weather, whilst the probability of the site being occupied during high winds is considerably reduced, having the compound effect of substantially reducing the risk of harm from tree-failure.  People may venture beneath trees during high winds either in the pursuit of recreation, thus voluntarily contributing to their increased exposure to harm from tree failure, or out of necessity such as en route from home to a workplace. Even in the latter example, weather conditions may be so extreme that the risk of harm from the failure of not only trees but the collapse of buildings and other storm related hazards is such that to venture out at all would be foolhardy.  Quantified Tree Risk Assessment includes a facility for considering reduced levels of access to recreational areas during high winds.

Prioritising Assessments

When confronted with the management of a large tree population, of perhaps a municipality or country estate, the scale of the task often daunts the manager. Where to start in the face of limited resources?

Taking an overview of the whole tree population and the targets that trees interface with, will provide tree managers with priorities for tree risk assessment.  Targets can be broadly zoned in the six Quantified Tree Risk Assessment target ranges.  Tree cover can be zoned by age, mechanical integrity and other attributes.  The interface of high value targets and older trees usually presents the greatest risk of harm and provides an obvious starting point.

Using Quantified Tree Risk Assessment target ranges, forward planning can be informed to design out many potentially high-risk situations.  For example, if a glass cafeteria is constructed in an open area rather than adjacent to trees, a potentially high-risk situation may be avoided.  The positioning of car parking, bus stops, outdoor seating and play areas are elements of design that can affect tree safety management.

Quantified Tree Risk Assessment informs tree safety management by providing a framework within which trees can be assessed at all levels of detail, from an overview of a municipality to the detailed appraisal of a single tree.  The method moves away from predictive assessment (we do not need to state that a tree is either safe or unsafe, or that it will or will not fail) and provides protection for both the tree inspector and manager.

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