2.0) Management of the council's trees
2.1 Overview
The council employs two office based staff, namely a tree officer and a tree and landscape officer to lead on all decisions and technical advice in relation to trees. The council have an experienced and competent in house team of tree surgeons who carry out the practical work to trees on the advice of the tree officers.
Arun District Council undertakes inspection of its trees and the information gathered from these inspections is held on a database and plotted on the council’s Geographical Information System (GIS).
The software package used is called Arbortrack, which records inputted data creating an inventory of the council’s trees.
The inspection programme is primarily designed to assess the trees health and condition, whilst also highlighting any work that may be required to ensure the tree stock is managed for the future on parks, public open spaces and Arun housing land.
If a tree is highlighted to be dead, dying, diseased, dangerous or damaging property and is posing an unacceptable risk to public safety, it will be identified for felling. The decision to prescribe work to a tree is calculated on a risk basis. Risk is calculated through the process of a visual tree assessment (Department of Environment, ‘Research for Amenity Trees No.4 The Body Language of Trees’). An evaluation of the tree takes into account many factors including:
- Size
- Species
- Presence of structural decay or defects
- Relationship of any fungal infection relative to species
All these factors are considered in relation to the potential target, the damage that could be caused if the tree were to fail and the likelihood of it doing so. If defects are observed, further detailed examination may be carried out using a range of decay detection equipment by an arboricultural consultant before a decision is taken regarding the trees future management.
2.1.1) Proactive operations outside of dead, dying, diseased or dangerous classification
In conjunction with the inspection programme and in addition to works on trees identified to be dead, dying, diseased or dangerous, the council will maintain a rolling maintenance programme. This will involve a proactive programme of works which aims to reduce potential future risks and issues, for example:
- Vehicle and pedestrian collision
- The removal of identifiable risks
- The removal or pruning of trees where its relationship to a property causes excessive problems
- Obstructing footpaths or driveways by branches or epicormic growth
As part of good arboricultural management the removal of trees will be carried out when the removal will benefit the long-term development of adjacent better quality trees which is a key principle of good woodland and copse management. Furthermore, formative pruning may be carried out following the Arboricultural Officer’s inspections, for example:
- Removal of crossing, weak or competitive branches
- Crown balancing
- Dead wooding
- Crown lifting
- Crown thinning
2.2) Principles of risk based tree management
Risk based tree management takes account of the usage of the site and the value of assets at risk (sometimes referred to as the target value) and the size and condition of the tree population.
From this information risk ratings can be derived to help prioritise inspection regimes and resources.
When assessing the condition of trees the likelihood and consequence of failure need to be assessed along with the likelihood of harm to guide whether action is justified.
It will not always be appropriate to undertake work to trees when significant defects are identified. In many cases the risks can be managed in other ways for example by removing the target.
2.3) A risk based procedure for Arun District Council sites
2.3.1 Tree Risk Rating
It is essential to establish a tree risk rating. For larger sites it may be appropriate to zone the site according to the distribution of trees and/or levels of usage (see Zoning of Sites below).
The Tree Risk Rating relates to the potential for a tree failure to harm an individual or property and is assessed using a simple algorithm taking account of the size and number of trees on or affecting the site.
The physical size of the trees on the site will have a direct bearing on the capacity for tree failure to cause harm to people, property and structures. As size may also relate to age and condition larger trees can be given a proportionately higher weighting.
The assessment should take account of trees adjoining the site that may impact upon users should they fail.
Table 1 - Tree Risk Rating
| Tree Size |
Number of trees <5 (1) |
Number of trees 5-10 (2) |
Number of trees 11-20 (3) |
Number of trees 21-50 (4) |
Number of trees >50 (5) |
|---|---|---|---|---|---|
|
Very large: >1m diameter and/or >20m tall (6) |
(6) Medium |
(12) High |
(18) High |
(24) Very high |
(30) Very high |
|
Large: 50-100cm diameter and/or 10-20m tall (5) |
(5) Medium |
(10) Medium |
(15) High |
(20) High |
(25) Very high |
|
Medium: 15-50cm diameter and/or 3-10m tall (3) |
(3) Low |
(6) Low |
(9) Medium |
(12) High |
(15) High |
NB it may be appropriate to assess the risk for each size class and use the highest risk score
2.3.2 Site Risk Rating
The number of people accessing a site and the length of time they are exposed onsite to the risks from tree failure the greater the risk rating for the site.
As there is limited observed data regarding the use of council sites so a typical measure for gauging levels of risks needs to be adopted: ‘The Frequency of Use Score’. A typical measure of use but will be varied accordingly to ensure that relative site specific risks are reflected.
For larger sites it may be appropriate to zone the site according to levels of use or tree population. This can be done by tree inspectors in conjunction with site managers and reasons recorded.
Using a simple algorithm the Site Risk Rating is assigned accordingly:
Table 2: Frequency of Use Score
|
Asset frequency of use score |
Hierarchy description |
Description |
Examples |
|---|---|---|---|
|
4 |
Very heavily used site |
A site that is in use by a high number of users on a daily basis and throughout the day. Typically 30+ people per hour averaged across a 24hour period |
High footfall parks, used for a wide variety of leisure and recreational pursuits. Often associated with large scale events. Principal offices, leisure centres, school grounds, major roads |
|
3 |
Heavily used site |
A site that is in use by a high number of users on a frequent basis but not throughout the day the day. Typically 10-20 people per hour averaged across a 24hour period |
Large recreation grounds and popular public open spaces used for a variety of leisure and recreational pursuits. May hold a small number of small scale events. Minor roads |
|
2 |
Moderately used site |
A site with moderate numbers of users or sporadic use. Use may vary due to seasonal factors Typically less than 1 person per hour averaged across a 24 hour period |
Local open spaces used for a small number of specific activities. |
|
1 |
Occasionally used site |
A site which is only occasionally used. Typically less than 1 person per day averaged across the year |
Open spaces with little amenity/recreational value, generally small in size |
Notes on Frequency of Use Scores
The risk score reflects both the level and nature of use. Sites where people go and stay (within impact range of the trees) for some time will have inherently higher risks than those passing through.
Where there are additional factors (as outlined below – Site history factors) which may impact on usage/risk the frequency or use score may be adjusted.
Some sites are often large areas with varying degrees of usage across the site. Where appropriate these sites will be zoned to identify the high risk areas (see Zoning of Sites below)
Site specific factors
Whilst the above hierarchy will for most sites predict the likelihood of risk based on use, there may be certain factors particular to the location that can increase (or decrease) the inherent risks on the site.
These can be factors such as:-
- Targets adjoining or within the site such as buildings or roads that increase the risks above those typical of the site type
- Specific conditions noted for individual trees. (It may be appropriate to just target specialist inspections to these trees)
- Seasonal pressures arising from a concentration of holiday accommodation
- The proximity of trees to the main areas of use will influence the likelihood of impact
Where these factors increase the risk from trees on a site an adjustment should be made to the Frequency of use score of the site/zone through the use of the site history risk score (usually by one point). Equally there may be factors of site usage that justify a reduction in the score.
The reason for the site history score should be recorded in the database.
Use of the site for an event or performance (NB The specific event risk assessment should assess the risks posed by the trees as this cannot be taken into account through this procedure).
Site history risk score
|
Site history |
Site history risk score |
|---|---|
|
Issues reported such as described above that suggest a higher risk |
+1 or +2 |
|
Issues that suggest a significantly lower site risk |
-1 or -2 |
|
No issues reported |
0 |
2.3.3 Overall Risk Rating
Using a simple algorithm that takes account of the tree risk rating and site risk rating an ‘overall risk rating can be assigned for the site/zone
Table 3 Overall Risk Rating
| Overall site risk rating |
Tree risk rating Very low (1) |
Tree risk rating Low (2) |
Tree risk rating Medium (3) |
Tree risk rating High (4) |
Tree risk rating Very high (5) |
|---|---|---|---|---|---|
|
(5) High |
(5) Low |
(10) Medium |
(15) Medium |
(20) High |
(25) High |
| (4) |
(4) V. low |
(8) Low |
(12) Medium |
(16) Medium |
(20) High |
|
(3) Medium |
(3) V. low |
(6) Low |
(9) Medium |
(12) Medium |
(15) Medium |
| (2) |
(2) V. low |
(4) V. low |
(6) Low |
(8) Low |
(10) Medium |
|
(1) Low |
(1) V. low |
(2) V. low |
(3) V. low |
(4) V. low |
(5) Low |
2.3.4 Site Inspection Policy
Based upon the resulting site tree risk rating a proportionate and prioritised inspection regime can be put in place:
Table 4 inspection policy
| Site tree risk rating (from table 3) | Inspection priority |
Inspection policy General inspection |
Inspection policy Specialist inspection |
|---|---|---|---|
| 1-4 | Very low | Recorded every 5 years | Reactive only |
| 5-8 | Low | Recorded every 5 years | Reactive only |
| 9-16 | Medium | Recorded every 3 years | Every 3 years |
| >16 | High | Recorded every year | Annually |
Zoning of sites
The same principles may be applied to zones within a site to target resources to the high risk areas on larger or complex sites.
Individual tree risks
Some sites may have few large trees but the condition of individual trees may require more frequent specialist inspection. This will be noted in the database to ensure that these inspections can be programmed. Geographically it will often be sensible to inspect the whole site.
2.3.5 Tree inspections and training requirements
There will be 2 types of inspections:
- Specialist inspections
- General inspections supported by reactive specialist inspections
All inspections will be undertaken by a competent person from the Greenspace Service or an external contractor with the appropriate level of training, as follows:
Specialist inspections
Specialist inspections will be undertaken on a cyclical basis at the frequency identified above. These will be undertaken by arboricultural staff.
Inspections will be detailed condition surveys using the visual tree assessment principles undertaken where suspect trees are identified. The degree of inspection will reflect the level of risk. The details of the inspection and the significant defects identified will be recorded along with recommendations for further action within the corporate tree database (Arbortrack). The minimum qualification of a Professional Tree Inspection Course must be completed, or equivalent.
General inspections
All sites will be inspected by either trained greenspace staff or arboricultural staff. All obvious and significant tree defects that require further action will be recorded and passed to the tree officer for review.
Where the significance of, or solution to, the defect is beyond the scope of the inspector’s knowledge and training, the defect will be reported to the tree officer for further inspection and recommendations. The findings of these inspections will be recorded. The minimum qualification of a Basic Tree Inspection Course must be completed, or equivalent.
Reactive inspections
Concerns about the safety of trees within a site that might affect users or neighbours whether reported by the public, following an incident or referral from other staff will be referred back to the tree officer for further investigation. The recommendations will be recorded within Arbortrack.