Approved Document OPW18-0319 · OPW18/0319

                                                                                                              Consult Arborist

A PPENDIX 1 - T REE R ISK A SSESSM ENT M ETHODOLOGY
The methodology of rating risk used in this report is based on the following International Society of Arboriculture’s Tree Risk
Assessment and is calculated through the use of qualitative risk matrix tables.

Initially, the Likelihood Matrix is used to estimate how likely a failure will be to impact a target. This is calculated by categorising
how likely failure is to occur and combining that with how likely that failure is to impact a target, as per Table 1.

                                                              Matrix 1 – Likelihood Matrix
                                                                         Likelihood of Impacting Target
Likelihood of Failure
                                             Very low                     Low                   Medium                 High
       Imminent                              Unlikely               Somewhat Likely               Likely             Very likely
       Probable                              Unlikely                    Unlikely           Somewhat Likely            Likely
        Possible                             Unlikely                    Unlikely               Unlikely           Somewhat Likely
      Improbable                             Unlikely                    Unlikely               Unlikely              Unlikely
Table 1: Matrix 1 – Level likelihood of impact to a target.


Once the Likelihood of Failure & Impact level is determined a final level of Risk Rating can be calculated by combining the
Likelihood with the Consequences of Failure for each identified risk. This ascertains an overall Risk Rating for the tree, determined
in Table 2.

                                                              Matrix 2 – Risk Rating Matrix
Likelihood of Failure                                                       Consequences of Failure
      & Impact                              Negligible                    Minor              Significant                Severe
     Very Likely                              Low                       Moderate                 High                  Extreme
        Likely                                Low                       Moderate                 High                    High
  Somewhat Likely                             Low                          Low                Moderate                 Moderate
       Unlikely                               Low                          Low                   Low                     Low
Table 2: Matrix 2 – Level of Risk

Where multiple risks are noted these tables will be used for each risk and the highest level will be taken as the overall risk rating
for the tree. In this scenario, multiple mitigation options may be offered to the tree manager to return the risk to an acceptable
level within a reasonable timeframe.


A PPENDIX 2 - D EFINITION OF ISA T ERMS
Likelihood of Failure
Improbable: The tree or branch is not likely to fail during normal weather conditions and may not fail in many severe weather
               conditions within the specified time frame.
Possible:      Failure could occur, but it is unlikely during normal weather conditions within the specified time frame.
Probable:      Failure may be expected under normal weather conditions within the specified time frame.
Imminent:      Failure has started or is most likely to occur in the near future, even if there is no significant wind or increased load.
               This is an infrequent occurrence for a risk assessor to encounter and may require immediate action to protect
               people from harm.
Likelihood of impacting a target
Very low:      The likely hood of the failed tree or part impacting the specified target is remote. Occupancy rates are rare with
               no protection or occasional with moderate protection.
Low:           It is not likely that the failure will impact the target. The occupancy rates are frequent with partial protection or
               constant with significant protection.
Medium:        The failure is as likely as not to impact the target. The occupancy rates frequent with single side exposure or
               constant with partial protection.
High:          The failure is most likely to impact the target. Occupation is constant with no protection.
Occupancy rates
Constant:      24 hours a day, 7 days a week.
Frequent:      A large portion of the day/week.
Occasional: The zone is occupied by people or targets infrequently or irregularly.
Rare:          Not commonly used.
Consequences of Failure
Negligible:    Low-value property damage or disruption that can be replaced or repaired, no personal injury.
Minor:         Low to moderate property damage, small disruptions to traffic or infrastructure, or very minor injury to persons.
Significant:   Property damage of moderate to high value, considerable disruption, or personal injury.
Severe:        Serious personal injury or death, damage to high-value property or disruption of important activities.
Levels of risk
Extreme:       Failure is imminent with a high likelihood of impacting the target, and the consequences are severe.
High:          Failure is very likely or likely with significant consequences, or failure is likely with severe consequences.
Moderate:      Failure is very likely or likely with minor consequences, or failure is somewhat likely with significant or severe
               consequences.
Low:           Failure is unlikely with negligible consequences or failure is somewhat likely with minor consequences.




19_01_24_TRA_RACV Resort _Noosa_V3                                                                                   Page 31 of 32