Waverley Borough Council Committee System - Committee Document
Meeting of the Executive held on 04/10/2004
TREE RISK MANAGEMENT GUIDE
WAVERLEY BOROUGH COUNCIL
TREE RISK MANAGEMENT GUIDE
Waverley Borough Council has responsibility for land that is accessible to the public or which in many areas is adjacent to public rights of way. The Council manages parks and common land for recreation, amenity and conservation. It also manages land for Housing purposes and a range of other sites containing trees and woodlands. While trees have many values:- social, environmental and economic, they may, if suffering from certain mechanical defects, represent a hazard in areas where people and property are present. It is therefore important for relevant site managers to be aware of tree-related hazards.
This Risk Management Guide sets out the responsibilities of Departments and managers for identifying ownership of land with trees and zoning sites in respect of different levels of access and risk and also identifies those responsible for assessing the risk of hazards from trees. In addition, the guide explains the risk zoning methodology, the inspection and recording system and sets out an inspection frequency. The guide deals with risks associated with tree failure only and attempts to clarify to which land these responsibilities apply.
The approach set out follows the principle of the Council’s established guidance on general risk assessment and management and takes account of the Health and Safety at Work etc Act 1974 – Management of Health and Safety at Work Regulations 1999 – Occupier Liability Acts (1957 and 1984) and relevant case law.
Further guidance was found in a range of publications and sources amongst which are:-
English Nature 2000 : Veteran Trees, A Guide to Risk and Responsibility.
Lonsdale, D. 1999. Principles of Tree Hazard Assessment and Management (Research for Amenity Trees No. 7).
Forestry Commission 2000 : Hazards from Trees, A General Guide.
Mynors, C. 2002 : The Law of Trees, Forests and Hedgerows.
National Trust 2001 : Inspection of Trees; Trees and Woodlands Instruction 1.
Advice from Treework Environmental Practice (Specialists in Tree Hazard Assessment) May 2004.
This document is mainly for internal purposes; for Members’ information and endorsement, for officer guidance and execution and for tenants and lessees’ information and clarification, as it may be for other interested parties.
In the unfortunate situation of a claim or case being made against the Council as a result of tree failure, this document, together with survey records, should assist in defending the Authority’s position and approach in respect of management of hazards posed by its tree stock.
The feasibility and reasonableness of various aspects of this Tree Risk Management Guide and its workings will need review and adjustment as and when required. An annual reassessment is also programmed.
Background and Objectives
The Waverley landscape is highly valued and is characterised by extensive tree cover. Much of the land controlled by Waverley Borough Council (WBC) contains many trees in the form of more ornamental plantings in parks and other amenity areas, old wood-banks and copses, more extensive woodlands, both naturally generated over time and planted ones, semi-wooded heath areas, shelter-belts adjacent to sports grounds etc, often densely wooded roadside strips of common land and individual trees within grounds of properties including tenanted houses. The nature of land use is such that access by people is locally high. The management of trees should therefore embrace a number of objectives which, for example, may relate to amenity, wildlife conservation, heathland restoration, shelter and the control of hazards. In the case of risk management, it is necessary to take steps to identify trees which represent a significant risk to people or property and to deal with them accordingly. This should, however, be done in a way which minimises the loss of value for people and wildlife.
To this end, a number of objectives relevant to risk management are listed as follows:-
to manage and minimise risks to people or property;
to avoid the unnecessary removal or disfigurement of amenity trees or of trees with high wildlife value;
to conserve habitats that are provided by trees, including those that are old and decaying.
Principles of Tree Risk Assessment
Nothing is without risk
We are at risk every day in our own homes, travelling to work and in the workplace. We expect to take risks, and the law requires only that we should be guarded from risks that are unreasonable. Absolute safety or the eradication of all risk is not expected and arguably is neither possible nor desirable. In the context of tree management, such an approach could result in the loss of all tree-associated amenities. By controlling risks from tree hazards, owners are meeting natural and ethical duties for the safety of others. They are also meeting the requirements of insurers and of the law.
The importance of assessing risks
Whether trees are managed for landscape, habitat, commercial or multi-purpose objectives, the legal obligation to ensure the reasonable safety of others remains the same. The law recognises that there is a balance to be struck between the risks and benefits of trees. WBC, as owner and occupier of land, is required to consider the level of risk associated with a tree and whether it is reasonable to protect against that risk. The duty is to identify apparent sources of danger and to make land safe, so far as is reasonably practicable. Liability is determined on the basis of whether a danger posed by a tree could have been foreseen and whether reasonable remedies could have been undertaken, which would have reduced the risks to an acceptable level.
To meet legal requirements, it is crucial that WBC manages risks and can be seen to do so and is able to provide evidence that this has been done. To manage risk effectively, the hazard must first be identified and ranked according to severity, then prioritised for action.
No tree is entirely safe, given the possibility that an exceptionally strong wind could damage or uproot even a mechanically ‘perfect’ specimen. It is therefore usually accepted that hazards are only recognisable from distinct defects or from other failure-prone characteristics of the tree or of the site. The assessment of risk is based on:
the value of whatever is judged to be at risk, and the likelihood of its being harmed in the event of mechanical failure in the tree, as estimated by:
- what is at risk – people, buildings, vehicles etc. (i.e. Target);
- the probability of impact, based on duration of occupation – for example, in relation to a permanent structure or a given number of people using a path during a given period of time;
(these considerations are clearly linked to the location of the tree, which is a key factor in deciding whether inspection is required in the first place);
target characteristics: e.g. high speed traffic – elderly or very young people frequently present etc;
the magnitude of the hazard, as estimated from the size (diameter) and height of the part of the tree most likely to fail;
the probability of failure, based on the type, position and severity of the defect concerned, the species or cultivar of tree and the nature of the site.
A practical five-step procedure for risk assessment works as follows:-
2. Decide who/what may be harmed and how.
3. Evaluate risk and decide whether works are required – action recommendations and check completion.
4. Record findings.
5. Review assessment and revise if necessary.
The above process should be auditable in all parts.
Hazard: The potential to cause harm to people or property.
Risk: The level of likelihood/probability that actual harm will be caused by a tree (whole or in part).
Harm: Refers to personal injury or damage to property.
Target: Is that which may be harmed.
Target Zone: Zone or area identified and classified according to target value and/or frequency of use to give a notional range of risk from high to low.
Tree risk assessment: Method to establish the probability that harm or damage might result from a particular tree hazard within a stated period. It considers the likelihood of tree failure occurring when a target is present within the falling distance of the tree. The assessment can be ranked as a level of likelihood (see Appendix A for two examples) and provides the means to prioritise action to manage risk.
Tree Survey: Ground level inspections of trees in various target zones to allow assessment of risk posed and, where relevant, recommending works to address risk – recording information. Depending on circumstances, a single survey record could relate to a single tree or to a larger number of similar trees within a group/area.
Tree: For the purpose of WBC tree surveying, a tree is a (generally single stemmed) woody plant of minimally 15 cm diameter at breast height (DBH) and minimally 5 m height. Trees of lesser stature can be included if, in the opinion of the inspector, they pose an obvious risk.
Responsibilities – Levels of Expertise and Lines of Communication
The ongoing responsibility for Tree Hazard Management in Waverley is a corporate one. For practical reasons, however, the Environment and Leisure Department has been given the task to take the lead and supply a service to other landholding departments.
The approach set out in this document has been the subject of extensive consultation, including with the Legal Section, and will only become “live” when ratified by Councillors.
All staff with responsibility for land management in WBC will be responsible for identifying their sites and with the help of the Tree and Woodlands Officer (E & L), to subsequently allocate zones to those sites which reflect targets, frequency of use and level of risk.
In view of the extent of tree cover on WBC land, it is not feasible for the Tree and Woodlands Officer, hereinafter called the Tree Officer, to inspect all WBC trees on a regular basis. Since other staff exist who have sufficient knowledge and competence to carry out inspections on their own sites in the medium to low-risk zones (Competence Level 1), the Tree Officer will be able to concentrate on inspecting trees in high-risk zones (Competence Level 2) and be available to assist the other staff in situations requiring a higher level of specialist input. The approach taken is in line with current risk management practice and considered reasonable in a legal context.
The other officers in question are the four Countryside Rangers and the Parks and Landscape Officer (all in E & L). In order to promote modern tree hazard assessment practice and also to achieve a greater level of consistency between inspectors, training for these officers has been organised. Competence (a legal requirement under H & S legislation and consistency will be a matter which is regularly reviewed.
Where the responsibility for land lies with non Environment and Leisure Departments, as in the case of Housing or Corporate Property, but management is taken care of under the Grounds Maintenance Contract, then the tree hazard management will be dealt with by the Tree Officer and the Parks and Landscape Officer. Depending on workload, the Tree Officer is likely to have to assist the Parks and Landscape Officer in medium-risk zones. Due to the demand on the latter officer’s time in other areas, adjustments to this arrangement may need to be made as required. Other WBC sites which are not managed by Countryside Rangers and are not subject to the Grounds Maintenance Contract will be inspected by the Tree Officer in all zone levels.
Notwithstanding the above, responsibilities will remain with all relevant site managers, including Housing through the Housing Services Manager and Corporate Property through the Property Management Officer to inform the Tree Officer where circumstances have changed (or are about to change) through sales, leases or change in target rating due to alteration of use or access. In addition, there is a need for assistance after severe weather events (see section on Timing of Tree Inspections) and awareness and vigilance in hazard prevention (see relevant section).
Which Trees to Inspect
Subject to specific exceptions, the Council will only take responsibility for trees on land where it has a strict occupier’s liability. There are a few minor exceptions to that approach, the details of which are set out in the Committee Report which introduces this Guide.
The need for a particular tree or group of trees to be inspected depends on the usage of the area within their potential falling distance. Inspection is unquestionably necessary within zones where people, or high-value items of property, are continuously or frequently present close to trees which are capable of being hazardous. Clearly, however, there are remote areas where tree failures are very unlikely to cause injury or damage, even though the risk of such an outcome cannot be entirely disregarded. Even at a more heavily used site, it could be that the risk is currently very low by virtue of the size and species of the trees present. There cannot, therefore, be a hard and fast distinction between sites or part of sites where inspection is essential and where it is entirely unnecessary. The key consideration is foreseeability; if it can be reasonably foreseen that anyone could be at risk, the occupier has a duty of care to reduce that risk within reason.
Target zones classify areas according to target value and frequency of use and provide a notional range of risk from high to low. If a site has a significant number of trees with a variety of targets, the use of target zoning can aid decisions on the nature and priority of inspections. Zone classifications are not absolute values that can be compared from one site to another, but provide information to help determine the need for, and priority of, inspection relevant to a particular site. As the nature of site usage may change, it will be necessary to review these zones periodically.
Level and Frequency of Inspection
It is sufficient initially to look for external signs that may indicate that a hazard exists. If no significant hazard is revealed, further action is not generally required until the next inspection. If evidence of a hazard is found, on trees in medium or low-risk zones, more detailed investigation by the Tree Officer would be advisable where:-
the full extent of the suspected hazard is not clear from external examination;
the tree is of high value (e.g. for amenity or wildlife) and there is reason to believe that it cannot be made safe without significantly lessening its value.
Although it is recognised that ivy and other climbers do not actually kill trees and have wildlife benefits, the presence of such plants can make inspections very difficult and time consuming. Ivy also, when grown into the crown of trees, adds to the “sail” and weight and, in that way, makes a tree potentially more hazardous. It is proposed therefore, where trees in high-risk zones have significant ivy (or other climber) growth, to have them severed at the tree base to reduce the risk of wind-blow/branch breakage and improve inspector “access”. Site managers may decide to take similar action on key trees in other risk zones.
A general principle to be observed is that, in areas where people or property could be at risk from tree failure, routine inspections should be carried out frequently enough to detect any hazards that may have recently developed. Hazards from large old trees sometimes develop quite rapidly, for which reason an annual inspection is generally advisable where such trees occur on high-usage sites. Basic inspections, by way of a “drive by” or “walkover” exercise particularly in high risk zones, should also be made as soon as practicable after any exceptionally severe weather event that might have caused damage to trees. Also, on the basis of expert advice, it may be necessary to make detailed inspections of particular trees at prescribed intervals if they have been found to show signs of progressive deterioration in their condition e.g. if a tree in a medium risk zone, normally requiring inspection every two years, displays signs of decline or decay which are not bad enough to justify removal, then a re-inspection on an annual basis will be justified.
Most of the issues discussed in the previous sections are brought together in the following chart:-
Target and Inspection Level Chart
Colouring on Map
Examples of Target Area
Trees in or adjacent to:
Level of Inspection
Frequency of Assessment
Inspector Level of Competence (minimal)
Remote or inaccessible areas with no or minimal public use and no high value targets
Occasional review of status/ requirement
1 & 2
Woodland: - Open parkland, heathland, fields, paths and bridleways with occasional use and no other high value or vulnerable targets
Infrequent assessment, e.g. every three to five years depending on particular circumstances
1 & 2
Well frequented open woodland areas abutting residential areas; open space, parks and recreation grounds, gardens, minor roads and country lanes, footpaths and bridleways with moderate use, occasionally used buildings and car parks, access routes to other properties, sports pitches
Every two years
1 & 2
Well used areas (most playgrounds, busy car parks and frequently occupied buildings), railways, fast roads, occupied property
Annual tree inspections
Level of Inspection Explained
Informal: Trees are assessed in the normal course of visits through general observation of health and condition and identification of obvious structural weakness or failure.
Routine: Trees are assessed by means of scheduled visual assessment from ground level looking for obvious external signs of mechanical defects which may lead to failure; identification of specific trees requiring more detailed assessment.
Detailed: Assessed by means of scheduled systematic visual assessment initially from ground level as above; where appropriate, further detailed investigation of potential structural weakness may be needed involving aerial inspections, soil and root condition or other procedures for assessing the nature of decay, wood quality or internal trunk condition.
For practical reasons, where sites are largely medium risk, for example, but do have a couple of minor stretches of high-risk adjacent to, say, a couple of dwellings, it is up to the site manager to decide whether it makes sense to join these high-risk stretches together for the Tree Officer to inspect in a continuous line whilst the rest of the site is done by the site manager. However, if the high-risk stretches only contain low-risk trees or ones which are healthy and easy to monitor on an annual basis (by exception) by the site manager, he/she may decide on that basis to mark the entire area as medium risk. Such decisions must be clearly recorded.
Timing of Tree Inspections
The best time to find and identify fungal fruiting bodies which may affect trees is generally in early autumn. Summer inspections are considered better to assess leaf condition and density and winter is best to have a more unobstructed view of the higher trunk and branches and their connections.
It will be clear from this that, taking account of the numbers of trees requiring regular inspection on WBC land, it is impractical to carry out two or more inspections in a given year, nor is this common practice in similar situations elsewhere.
It is therefore proposed to continue to carry out inspections on a year round basis and only, where an inspector is unsure about a tree’s condition, to schedule only that tree for re-inspection in a particular season depending on the nature of concern.
If, between formal inspections, a “natural” event such as exceptionally severe weather (e.g. the 1987 gales) or fire takes place with the potential to have caused trees to have become unsafe, then all relevant site managers, including Housing and others, will carry out a walk-over/drive-by inspection of affected sites, starting in the high-risk zones and working downwards. This should happen, within reason, as soon as practically possible and a record kept of date, site findings and actions. The Tree Officer will determine whether an event was severe enough to justify such inspection(s) and alert relevant staff.
In the case of sites without direct manager involvement, such as several of the larger Housing woodland sites and the Corporate Property sites and most of the non Countryside parks and amenity areas, this work will be shared between the Tree and Woodland Officer and the Parks and Landscape Officer (both E & L).
For tenanted Housing properties in these circumstances, reliance will be on tenants to report concerns which will initially be checked by Housing Management Officers and actioned or subsequently dealt with by the above two officers.
Recording and Marking
Inspection records should include inspection dates, name of inspector, weather conditions and presence of factors obscuring potential defects (such as ivy growth). The areas of trees inspected alongside roads and footpaths should be clearly marked on accompanying maps or otherwise defined. It is important to record inspections, even if only briefly, to be able to demonstrate that this element of duty of care has been fulfilled. Instructions to carry out work to trees, dates of completion, together with any amendments to tree inspectors’ recommendations, should also be recorded.
Records provide the basis for safety management reviews and can, over time, build a valuable historical record of site-specific tree failure or non-failure patterns. Once hazards have been assessed or work completed, re-inspection times should be assigned or reviewed and then recorded.
A paper copy of the record sheet being used is attached as Appendix B. Electronic recording is being progressed and the use of GPS is being investigated, particularly for accurately locating trees in rural areas.
Copies of site plans, with risk zones marked on them, will be held by all relevant site managers. All WBC site plans, with zones, will be kept centrally by the Tree Officer. It is planned to have risk zones marked on a separate layer of Map-Info (GIS) at the earliest opportunity. It is often impractical, nor justified, to plot all trees on a map base and the same applies to physical tagging of individual trees, particularly away from built-up areas.
The plotting or location description should be sufficient to find the tree(s) again for re-inspection or for a contractor to carry out works. The information gathered about the tree(s) and recommendations for works (if any) should be basic but sufficient for the purpose of the exercise.
Tagging may be appropriate in certain circumstances, particularly where individual trees need to be clearly distinguished from others nearby and because of certain important features (such as particular defects, wildlife relationship or historic connection). This should be the exception rather than the rule and Latchenbacher or similar multi-pronged plastic tags will be used. This may also apply to trees in medium or low-risk areas but whose condition requires them to be inspected on an annual basis. Otherwise, marking with spray paint should suffice for works or detailed inspection.
Informal Inspections in Low-Risk Zones
Record date, site and particular reference to path or boundary inspected, possibly linked to plan. Rough description of types/numbers of trees assessed and specifically listing trees requiring works, details of work instructions and date of works completed.
Routine Inspections in Medium-Risk Zones
Inspecting all trees, using standard inspection sheet, plot, record and mark all trees which require works and those which may have defects but for which work is not deemed necessary (with justification why not). General description of other trees inspected with reference to species, age range and numbers. Details of work instruction and date of works completed.
Detailed Inspections in High-Risk Zones
Using standard inspection sheet/hand-held computer, plotting, recording and marking all trees. Only where dense groups of trees are present of similar age/size and condition range, particularly in rural areas, can these be marked as groups on the map and referred to collectively, although each tree will require inspection. In these circumstances, for practical reasons, it is not proposed to individually plot trees under 45cm DBH. If no works are required in general in such groups, but individual trees are present within the group which are different (age, condition or work requirement), then these will be plotted and recorded individually and separately.
Records of all programmed inspections carried out, and confirmation that recommended works have been actioned, are to be copied to the Tree Officer and kept as a central record. Records of “drive-by” observations after very severe weather are to be treated in the same way (see “Timing of Inspections”).
On sites which contain high-risk zones, as well as medium and/or low-risk zones, the Inspectors responsible for the different zones will liaise prior to carrying out inspections to decide on precise boundaries between zones to avoid duplication where this may be an issue.
Reducing the Risk
The risk that a tree poses to people and property can sometimes be reduced by modifying the usage of the immediate surroundings, for example, by moving a path or car-parking spaces or altering a mowing regime. In other cases, the risk can be reduced by tree surgery. Branches weakened by decay or cracks may be pruned and trees with defective main stems or root systems may be made safer by crown reduction. Excessive movement in some types of weak structure can be restrained by bracing or propping. However, in severe cases on high-usage sites, felling may be the only reasonable option. In a woodland area, where the individual tree concerned is known to be of no special value for amenity or wildlife, felling will generally be regarded as a more realistic option than costly arboricultural procedures which are suited mainly to sites where trees are managed on a more individual basis.
When inspections are carried out and defects found, recommendations for works (or re-inspections) shall always follow and should take account of the above and modern arboricultural practice. The Inspector should ensure that recommended works are actually carried out without unreasonable delay.
The budget set aside for Tree Risk Management is to be strictly used for just that and, in the main, should address the more immediate concerns. Tree work paid out of this budget will not cover works to address issues such as nuisance (leaf fall, overshadowing, pavement disruption, subsidence etc), clearance of sight-lines, crown-lifting for access, other pruning for amenity purposes or woodland management type operations. Such works should be paid for out of general revenue budgets.
Having embarked on a pro-active risk management approach, the Council cannot reasonably be expected to “catch-up” on many decades of limited programmed activity in this respect, within a very short time-span.
Considering the extent of WBC tree cover, it will be important to make the budget go as far as possible by prioritising and pacing the survey work. However, it is equally important to ensure that Inspector output, in terms of identification of hazards, is matched by actual treework to address those hazards. If identified hazards are not dealt with within a reasonable period due to the budget having run out, the Council would still be negligent if harm occurred as a consequence.
In the light of the above, it is crucial for the budget to be planned properly and monitored regularly. Tree Risk Management, particularly in the Waverley context, is very much analogous to “painting the Forth Bridge” but it is hoped that, after three to four years “getting on top”, budget requirements will reduce and stabilise.
Trees and Wildlife
It is widely appreciated that trees are vital for many forms of wildlife, although the importance of habitats which develop in dead and decaying wood has only recently begun to come to public attention. The wildlife value of trees can often be identified only with difficulty, so that specialist help may be required, especially with invertebrates, fungi, mosses and lichens. It is important that trees, especially ‘veteran specimens’ should not be felled unnecessarily. This may happen if decisions are made on the basis only of initial observations, such as the presence of fungal fruit bodies, rather than a detailed risk assessment. If there is a need to carry out other kinds of remedial work on veteran trees or other individuals of acknowledged wildlife value, it should be done with great care. However, there is a need to emphasise that the wildlife value of a tree does not lessen the need for safety inspections and for remedial action if such action is found to be necessary. The choice of appropriate remedial action, such as judicious pruning, altering the mowing regime around trees or diversion of access routes can, however, often allow a tree to be retained with its associated habitats intact.
In the same way, the failure potential of deadwood varies according to the type of tree and fungal activity. For example, deadwood in oak may be less likely to break than end-loaded live branches and can remain stable for decades. Deadwood in beech, however, is more likely to fail than in oak; knowledge of the attributes of specific species is therefore important. Where deadwood is found in a tree and considered hazardous, treatment options may be similar to that for live wood. Its significance needs informed assessment.
Tree Hazard Prevention
Tree loss can have detrimental impact on the environment in general and on landscape value and amenities. In addition, where trees have to be pruned or removed for safety reasons, there are financial costs.
These costs collectively can be very significant and it makes sense therefore that, where possible, damage inflicted on trees through people’s actions, is avoided or kept to a minimum.
Poor pruning practices, using trees as supporting structures for sheds, fences, washing-lines, tree-houses etc, buttress and root damage due to construction of buildings, accesses, car parking, patios and walls, drain and services installation, landscaping (soil level changes) and compaction, can all have significant immediate or long-term implications in respect of tree health and stability, particularly where they affect older, larger trees.
In case of below ground activities, within a relatively short time after the event, it will be very difficult for anyone inspecting trees to be able to tell that anything has taken place and even more difficult to assess the impact on the tree’s health and stability.
Although the responsibility for tree inspections may rest with a limited number of Council staff, avoidance of damage and thus hazard prevention, remains the responsibility of all staff involved with site/asset management, those responsible for design and construction of new schemes and responsible for contractor appointment and supervision. Responsibilities lie in the same way with housing tenants and Housing Management staff. Anyone responsible in this way should, where relevant, obtain specialist arboricultural advice. When time permits and, depending on circumstances, this may be sought from the Tree and Landscape Officer in the Planning Department or the Tree Officer in the Environment and Leisure Department. In certain situations, it will only be possible to receive external advice and the officers mentioned above will be able to give relevant Consultant details but should still be informed of details of schemes where Council owned trees are involved.