58 Ellison: Quantified Tree Risk Assessment in Management of Amenity Trees owner, and the person who accepts some degree of risk in return for a benefit, such as a tree owner or visitor to a woodland or forest. Having considered The British Medical Association Guide’s Living with Risk (Henderson 1987) and with particu- lar reference to the conclusion “few people would commit their own resources to reduce an annual risk of death that was already as low as 1/10,000,” Helliwell (1990) suggests that 1/10,000 might be a suitable figure to start with as the limit of acceptable risk. Furthermore, “for members of the public who have a risk imposed on them ‘in the wider interest,’ HSE [Health and Safety Executive] would set this limit at 1/10,000 per annum” (Health and Safety Executive 1996). In the management of trees, a property owner or manager might adopt the 1/10,000 limit of acceptable risk or choose to operate to a higher or lower level. Cost and Benefit Trees confer many benefits, being essential to our well being and generally enhancing our built and natural environments. Removal of all tree hazards would lead to certain impoverish- ment in the quality of human life. Therefore, it is necessary to maintain a balance between the benefits of risk reduction and the costs of that risk reduction, not only financially but also in terms of lost amenity and other tree-related benefits. Value of Statistical Life Value of statistical life is a term used in risk assessment to express the monetary value of an individual life. In the United Kingdom, this value is currently in the region of £750,000 to £1,000,000 (US$1,387,500 to US$1,850,600) (Health and Safety Executive 1995) and is proposed here to correlate the value of damage to property with the value of human life. HAZARD ASSESSMENT For a tree failure hazard to exist, two criteria must be fulfilled. There must be potential for failure of the tree and potential for injury or damage to result. The issue that the tree manager must address is the likelihood, or risk, of a combination of factors resulting in harm, and the likely severity of harm. Most tree defects can be identified and assessed by the skilled inspector, but there is no evaluation methodology currently in general use that enables the inspector to quantify risk in a way that the risks associated with the retention of trees can be compared with a broadly acceptable level of risk. A landowner or manager with responsibility for a diverse tree population, on a site comprising locations as disparate as a boundary with a busy highway, a children’s play area, and a remote woodland walk, must rely on the subjective judgment of the tree inspector, employed at any point in time, when formulating management strategies and allocat- ing budgets. Such subjectivity could result in the implemen- tation of remedial work, perceived by the current tree ©2005 International Society of Arboriculture inspector to be necessary for the abatement of a hazard and possibly resulting in unnecessary cost and degradation of both the amenity and conservation value of a site, without having first established the risk of significant harm arising from the hazard. Probably the most significant recent development in the field of tree hazard evaluation is the methodology proposed by Matheny and Clark (1994). Designed primarily to assist the evaluation of tree failure hazards in urban areas, the system is relatively easy to apply and enables tree inspectors and managers to prioritize remedial action in a structured manner. The guide proposes a system of rating tree failure hazards by assessing and applying a numerical value of 1 to 4 to each of the three components that contribute to a tree failure hazard: (1) failure potential, (2) size of the tree part that could fail, and (3) target rating. The sum of the three equally weighted scores is termed the “hazard rating.” A hazard rating of 12 represents the most severe hazard. The system enables the broad prioritization of tree failure hazards but does not quantify the associated risks. THE PROPOSED SYSTEM Quantified tree risk assessment is not a wholesale departure from current practice, but it quantifies risk within a struc- tured framework and utilizes, but renames, the three components of the tree hazard proposed by Matheny and Clark (1994): (1) probability of failure, (2) impact potential, and (3) target value. The system enables quantification of the independent probabilities of the three components, enabling their product (risk of harm) to be compared with a generally accepted level of risk. The quantified tree risk assessment process might, in areas of very high access, involve the detailed inspection and assessment of every tree, or might, in low-access areas, require only a general overview of trees and targets. To simplify the assessment process, probabilities are presented in ranges in Tables 3, 5, and 6 (e.g., 1/1 to 1/19 and 1/20 to 1/100). A probability of death or serious injury of 1/10,000 is suggested as the limit of acceptable risk to the public at large (Helliwell 1990; Health and Safety Executive 1996). Using the 1/10,000 limit, all risks with a probability exceeding 1/10,000 require remedial action to reduce the risk to an acceptable level, unless the risk is limited to a particular individual or group—such as a tree owner—who may choose to accept a greater or lesser risk. Additionally, the hazard could confer benefits that might be set against the risk of harm. Target Evaluation A target is anything of value that could be harmed in the event of tree failure. Frequent inspection of trees and assessment of associated risks may be essential in areas of high public access or where trees are within striking range of valuable or fragile structures. Conversely, in a location
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