RFI - Attachment 3 - Arborcultural Impact Assessment Report · 132003.221108.3

coded risk of harm. The tree owner can then compare 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.

The Quantified Tree Risk Assessment (QTRA) system, as proposed by Ellison in 2004, provides a
structured approach to evaluating the risk associated with trees by combining principles of risk
management with tree safety. Here’s a breakdown of the concepts and methodologies mentioned:

Definitions and Key Concepts

   •   Risk: Defined as the combination of the likelihood of a specific event occurring and the
       severity of its potential consequences.
   •   Hazard: A potential source of risk. Hazards are not risks themselves but can contribute to
       risk if there is a target present.
   •   Target: The presence of people or property that could be affected by the hazard.

QTRA System Overview

The QTRA system evaluates risk by considering:

   1. Target Presence: Identifying if people or property are at risk if a tree or tree parts were to
      fail.
   2. Impact Potential: Assessing the severity of the potential failure, which involves the size
      and nature of the tree and its parts.
   3. Probability of Failure: Estimating the likelihood of the tree or tree parts failing.

The system calculates a figure known as the Risk of Harm (RoH), which quantifies the probability
of death or injury resulting from tree failure. This figure is compared against a threshold to
determine if the risk is acceptable or unacceptable.

Thresholds and Comparisons

   •   Threshold Value: Studies, including those by the British Medical Association (BMA),
       NASA, and others, use a threshold of 1 in 10,000 (0.0001 or 0.01%) to distinguish between
       reasonable and unreasonable risk.
   •   Contextual Risks:
           o Risk of death from a road accident in Australia: 1 in 15,000 (2009) to 1 in 19,000
              (2013).
           o Risk of death from falling down stairs: 1 in 310,000.
           o Risk of death from falling out of bed: 1 in 1.1 million.

Public Perception and QTRA’s Role

   •   Public Perception: Trees may seem riskier due to their size and appearance, or because of
       emotional and subjective views. This perception can be influenced by the tree’s age or
       significance.



    Tree Report: (AIA-27 Bayview NH-120226)                           Report Number:    (001)

    Prepared by (Sam Carr – Specialist Arbor Services Pty Ltd)                       Page 14 of 26