166 Stewart et al.: QTRA and Risk-based Cost-benefit of Tree Assessment that considers risk of harm, costs of tree removal, benefits of tree amenity, and other costs and benefits can be used to com- pare costs and benefits in a rigorous manner. This type of risk and economic assessment can better inform decision-makers. QUANTIFIED TREE RISK ASSESSMENT The QTRA process was developed and documented by Ellison (2005) and the Quantified Tree Risk Assessment User Manual V3.06 (2012). Risk of harm (ROH) is calculated as (Ellison 2005): [1] Risk of harm = Probability of failure × Target value × Impact potential where harm is defined as serious injury or death; prob- ability of failure is the annual probability that a tree or selected tree part will fail; the target value is the probability that a person, a vehicle, or the property will be impacted; and the impact potential is the probability of harm a falling tree, or part of a tree, can cause to a pedestrian or vehicle. The risk is deemed “unacceptable” if the ROH exceeds 1 in 10,000. Case Study: Laman Street Fig Trees in Newcastle The City of Newcastle in New South Wales, Australia, had 17 mature fig trees (Ficus microcarpa var. hillii (Hill’s weep- ing fig) on both sides of Laman Street in the heart of the city (Figure 1). These trees were estimated to be in excess of 70 years of age. They have been topped on multiple occasions. Over the last few decades, however, topping has ceased and the trees were left unpruned. Although growing in confined openings between the street and sidewalk, the trees were con- sidered to have good health and vitality. Some of the trees had started to partially grow over the adjacent hard surfaces. In June 2007, a severe east coast weather movement caused wind speeds of 124 km/hr and intense rainfall. As a result, gaps appeared between the base of the four trees and the adjacent hard surfaces (Figure 2). Without any further testing, three of these trees were removed. The fourth tree that was thought to have moved was monitored over the following month to check for further movement. This tree was then determined by Newcastle City Council (NCC) to be stable, and was subsequently retained. It is important to be aware that, even if the trees had moved, this movement would not have harmed people or property. In August 2009, NCC engaged a consultant to assess the stability and safety of the remaining 14 trees. The Stage 1 Visual Tree Assessment concluded that the trees be removed within five years (Marsden 2009). In response, NCC sought to have the risk posed by the trees assessed in order to determine an appropriate management strategy. In September 2009, a Quantified Tree Risk Assessment was prepared for Newcastle City Council. It concluded that the risk of harm was 1 in 19.8 per tree per year, and that the probability of tree failure was 1 in 7.5 per tree per year (Table 1). A subsequent report by another arbori- cultural consultant reviewed this first QTRA report and concluded in 2010 that, “On review, I believe that this report provides a reasonable assessment of the quanti- fied risk of harm” (QTRA Newcastle 2010). Both consul- tants were licensed QTRA practitioners. Since the ROH ©2013 International Society of Arboriculture Figure 1. Laman Street looking east, April 2010. Figure 2. The gap observed between one of the trees and the adjacent hard surface (Marsden 2007). exceeded the QTRA suggested ROH threshold of 1 in 10,000, the reports declared the level of risk as “unacceptable.” Based on the level of risk, NCC made a decision to remove the trees in late 2011, and the trees were finally removed in February 2012. These 14 remaining trees survived for over 4.5 years (June 2007 to February 2012) before they were eventually removed. Tree failures on Laman Street were isolated events (i.e., one or two trees may fail but not all trees at the same time) due to the highly variable nature of root conditions and wind environment. In the absence of evidence to the contrary, these trees can be treated as statistically independent. Using QTRA, the formula for determining fatalities in this scenario is [2] E = N × ROH × T where N is the number of trees (N = 14), and T is the time period in years (T = 4.5). Using the QTRA results, the ex- pected (average) number of deaths or serious injuries (E) in four and a half years between the storm and tree re- moval is calculated as follows: 14 × 1/19.8 × 4.5 = 3.2.
July 2013
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
