19_01_16_AIA_Alba_25 Kauri St_V1 · MCU18/0165

                                                                                                                                       Consult Arborist

  A PPENDIX 2 – T REE R I SK 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 as guided in the Risk
  Rating.


  A PPENDIX 3 - 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_16_AIA_Alba_25 Kauri St_V1                                                                                                                  Page 19 of 19