Tree Hazard/Risk Assessment

Tree hazard/risk assessment is a proactive system for managing the likelihood of trees causing harm or injury as a result of their failure. It does not address other forms of damage that may be caused by trees, for example, subsidence.

 

There are a number of published methodologies the professional arboriculturist can follow when assessing tree hazard/risk. The following is based on the Southampton Tree Operational Risk Management System or STORMS.

For the purposes of this hazard/risk assessment the following definitions apply:

  • Hazard - the potential of a tree to cause harm or injury,
  • Risk - the likelihood of that harm or injury occurring,
  • Harm - the physical damage that may be caused to property,
  • Injury - the personal injury or mental anguish that may be caused.

Objectives

The objectives of tree hazard/risk inspections are to:

  • identify physically and/or structurally defective trees,
  • assess the hazard associated with the defect,
  • assess the risk associated with the hazard,
  • assess the risk associated with the target,
  • recommend the appropriate remedial action to remove or reduce the likelihood of the event occuring.
  • recommend a time frame in which the remedial action should take place.

The Assessment

STORMS defines the hazard and the associated level of risk based on the following three criteria:

1. Size of Defective Part (Hazard)

The assessment rates the size of the part of the tree most likely to fail. The size of the part reflects the potential of the tree to cause harm or injury.

The defective part is given a point score ranging from 1 to 4, where 4 represents the greatest hazard.

Range of Scores Actual Rating Size of Defective Part Potential Harm or Injury
4   >500mm Major harm or risk of fatal injury
3   100-500mm Minor harm or major injury
2   25-100mm Minor harm or injury
1   Less than 25mm  Fragile objects damaged


2. Likelihood of Failure (Risk)

The assessment rates the likelihood of failure of the physical and/or structural defect within the defined inspection period.

The likelihood of failure is given a point score ranging from 1 to 64, where 64 represents the greatest risk.

Range of Scores Actual Rating Target Frequency and Use
64   Definite/imminent Major failure
3   Probable/soon Major defect
2   Likely Moderate defect
1   Unlikely Minor Defect


3. Target Rating (Target)

The assessment rates the frequency, occupancy and use of the potential target area that would be impacted in the event of a tree failure.

The target rating is given a point score from 1 to 4, where 4 represents the greatest target.

Range of Scores Actual Rating Target Frequency and Use
4   Very high risk Constant and busy use
3   High risk Frequent but not constant use
2   Medium risk Area  Occasional or sporadic use
1   Low-risk Area infrequent, hardly used


The points in each criteria are then multiplied together to obtain an overall hazard/risk rating:

Hazard score x Risk score x Target score = Hazard/Risk Rating

The hazard/risk rating determines the time frame in which the remedial action should take place.

Range of Scores Actual Rating Time Frame for Remedial Works
64+   Emergency 2 hours
48   72 hours
36   72 hours
32   3 months
27   3 months
24   12 months
18   12 months
1-16   No work necessary


A Note About Remedial Action

All tree hazard/risk assessments strive to be objective in nature but are, in part, subjective and rely on the judgment of the tree inspector. Proposed remedial action should consider not only the hazard/risk rating of a tree but also the amenity value.

For example, a tree with a high hazard/risk rating but a low amenity value might be removed with only a minor loss to the overall character of the landscape. On the other hand, a tree with both a high hazard/risk rating and a high amenity value might justify an investment of time, money and effort in order to retain it for as long as possible.

Whilst this proposal adds further to the subjective nature of the assessment, it offers a practical option to retaining notable and prominent trees.

 

Trees to be Assessed

The assessment includes all individual trees with a stem diameter equal to or greater than 150mm measured at 1.5 metres above adjacent ground level. This is based on documented research from the USA that found that most failures occurred in trees with a stem diameter of greater than 150mm.

Trees within groups or woodlands are only assessed if they are within striking distance of a recognised target.