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Tree Hazard Evaluation and Works Scheduling

The purpose of this page is to advise the number of customer requests that come in quarterly for tree maintenance and removal on public land, and to advise on how we assess and prioritise these requests. 

Emergency responses and works generated from storm events have an impact on our current schedule as these are given immediate attention. 

 

The following table lists the current status of Council's Public Tree Works Schedule July-Sept 2020 . 

Works Schedule tasks carried over from previous quarter (April-June 2020) New tasks listed  (July-Sept 2020) Tasks completed (July-Sept 2020)
 809  104  108

Note - A single customer request can include works for multiple trees .  Council's Arborists also respond to a high volume of Customer Requests which are not recorded in this table.

Total outstanding (30 Sept 2020) Priority 1 Priority 2 Priority 3 - 6
797 11 343 443

 

Priority Rating Definition
 1 Event is imminent or very likely to occur within 12 months
 2 Event is likely to occur within 12 months
 3/4 Event is unlikely to occur within 12 months
 5/6 Either recorded for monitoring (annual inspection) or may not be recorded dependent on determination of the initial inspection

 

 How we assess and prioritise public tree works

 We use two assessment models:

  • Council’s Risk Matrix
  • A Photographic Guide to the Evaluation of Hazards Trees in Urban Area (Matheny & Clark, 1994).

 Risks are assessed as follows:-

Risk Score   Definition   Priority 
 1  Very High  1
 2  High  2
 3/4  Moderate  3/4
5/6  Low  5/6

 A Risk Score between 1 to 6 indicates the likelihood and severity of an event occurring (branch or tree failure) within 12 months following the date of inspection.

Hazard Rating

A Visual Tree Assessment evaluating the current health, condition and structure of a tree, helps to distinguish ‘apparent’ hazardous trees from those that are ‘really’ hazardous. These are conducted by Council’s qualified arborists.

Assessment criteria for Hazard Scores:    

  • Failure potential
  • Size of part most likely to fail
  • Frequency of occupancy

The above elements are evaluated and each receives an individual score between 1 to 4. When each of their individual scores are added together, the resulting score can range between 3 and 12.  

Hazard Score
Definition
1-4 Low
5-8 Moderate
9-12 High

The primary driver for scheduling and prioritising works is the Risk Score, with the Hazard Score used as supplementary information, however consideration of both scores determines the Priority Rating. 

By using these systems Council is able to determine a Priority Rating on the current status and position of a particular tree compared to others within the work schedule. 

 

Example of how works are prioritised

Tree Risk score Hazard score Works would be prioritised in this order
Example A 2 7 4th
Example B 1 10 1st
Example C 2 10 3rd
Example D 2 12 2nd
Example E 3 7 6th
Example F 2 6 5th

Examples:  A tree displaying lower Risk Score / High Hazard Score is generally actioned first, compared to a High Risk Score / Low Hazard Score tree. 

The work schedule is a live document and can change frequently, depending on new inspection requests, emergency works and resourcing.  As a result it is difficult to provide guarantees on when works will be actioned.

Tree works that have not been completed are re-inspected annually to determine if there are any changes in Risk / Hazard scores.

 

 

This page was last updated on: 27 October 2020