TREE RISK ASSESSMENTS (TRA)
A tree risk assessment (TRA) is a detailed analysis of the risk posed by a single tree or group of trees within a specific site. Tree risk assessments are often carried out at schools, universities, holiday resorts, parks, and other public spaces where trees have the potential to cause harm to the persons who occupy that space. There are three components to calculating the risk posed by a tree:
- Target: The persons, property, or asset at risk and the likelihood of that target being present/impacted in the event of partial or whole tree failure.
- Size: The size of the tree or branch most likely to impact the target. The size of the failure determines the likely force exerted upon impact.
- Probability: The likelihood of a failure occurring within the assessment period based on the structure and condition of the tree.
The first stage of a tree risk assessment comprises a visual tree assessment of all trees within the site. The visual tree assessment comprises a walkover risk assessment in accordance with the Quantified Tree Risk Assessment (QTRA) method. Where the walkover risk assessment identifies conditions or defects that present an increased risk of tree failure, the identified tree will be assessed using the full QTRA method and a stage one Visual Tree Assessment (VTA) as formulated by Mattheck & Breloer (1994).
Trees identified as posing an increased risk of harm are spatial located, tagged, and allocated an identification number. Following the site inspection, a comprehensive report is prepared to detail the overall structure of each identified tree, the subsequent likelihood of tree failure, and the potential risk of harm posed by that individual tree. General information for each individual tree will include:
- Identification: Allocated number and botanical name.
- Dimensions: Tree height, canopy spread, and trunk diameter at breast height (DBH)
- Condition: Tree health, structure, age class, and any significant/visible defects.
- Risk: Risk of harm, risk rating, and mitigation actions.
Our tree risk assessment includes management/mitigation options to reduce or eliminate the risk posed by each of the identified trees and a tree location plan prepared using AutoCAD or ArcGIS drafting software. The tree location plan displays the location of each identified tree, their allocated number, and risk rating. A photo of each identified tree is provided and included in the report to allow for identification and verification purposes.