Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[RobArb]

 

A quick calculation, with chalk on the pavement, suggests that if there is someone beneath the tree for only 1 minute a year the risk is unacceptable at 1:1,000,000 and on the ALARP principle with my estimation of the average at-risk time throughout the year, the risk cannot be rendered acceptable except by felling the tree within 3 months.

 

Any chance of sharing this calculation?

 

I just can't work out in my head that if i stand underneath that tree for 1 minute, i have a higher chance risk of becoming a cropper..?

 

1 in a million chance, i quite fancy them odds:thumbup:

[Acer ventura]

Not only am I to carry out a VTA of the trees but I have to assess the risk relative to the 1: 10,000 and 1:1,000,000 thresholds and in the band between I must specify and prioritise the treeworks necessary to achieve the ALARP criteria.

 

Hi Jules

 

How are you working out whether a tolerable risk is ALARP?

 

I don't use any black box system

 

You are the black box with this system.

 

The other day I found myself faced with a medium sized tree with high probability of failure (it definitely won't be there in 5 years) and will kill someone outright if they are unfortunate enough to be underneath when it fails. The usage of the path is low but I know that in summer there will be people passing and lingering frequently. A quick calculation' date=' with chalk on the pavement, suggests that if there is someone beneath the tree for only 1 minute a year the risk is unacceptable at 1:1,000,000 and on the ALARP principle with my estimation of the average at-risk time throughout the year, the risk cannot be rendered acceptable except by felling the tree within 3 months.[/quote']

 

No pressure, and I quite understand if you don't want to under the full glare of the spotlights of the forum, but as someone who's interested in quantifying risk would you be up for going through this risk assessment with me in this thread, and I'll give the QTRA approach to it. It strikes me it could be an interesting and useful exercise given where we are in the thread.

 

Cheers

 

Acer ventura

[RobArb]

 

Sent using Arbtalk Mobile App

[daltontrees]

Just a brief note. I can't share this specific calculation with you because it would be a breach of client confidentiality in terms of my contract. However, I would be willing when time allows to reproduce calculations for a hypothetical tree in the same situation.

 

Rob, if you stand under such a tree for 5 years you will come a cropper. If a rota is drawn up whereby you have a 1 minute slot in those 5 years where you have to stand under the tree, the risk to you is still unacceptable because you may get the minute when it alls over.

[RobArb]

 

Rob, if you stand under such a tree for 5 years you will come a cropper. If a rota is drawn up whereby you have a 1 minute slot in those 5 years where you have to stand under the tree, the risk to you is still unacceptable because you may get the minute when it alls over.

 

Maybe I'm just risk averse then when it comes to my own personal welfare as I also may not, get the minute it falls over.

 

I think this is why I'm struggling to get my head round all this. I know we live in a litigious society, and I hate the fact that this is the case.

 

I understand duty of care and negligence but just feel common sense has taken a vacation...!

 

Maybe once I'm working in an environment where I am faced with other peoples personal welfare and possibility of harm rather than just my own, I may "get it"?

 

 

PS, I'd still stand under that tree, and if it did fail, I'd just move out the way... but that's just me:biggrin:

 

 

Sent using Arbtalk Mobile App

[Acer ventura]

Just a brief note. I can't share this specific calculation with you because it would be a breach of client confidentiality in terms of my contract. However, I would be willing when time allows to reproduce calculations for a hypothetical tree in the same situation.

 

Hi Jules

 

Okay then but it’s a shame. I was rather hoping by breaking this particular tree risk assessment down into easy to digest bite size nibbles, we might be able to address some of the points from your messages and apply other ones that have been raised in the thread.

 

Nonetheless, there’s a couple of rich seams I think we could selectively mine without going all open cast on it. These nuggets regularly crops up in the QTRA workshops as well.

 

Rob' date=' if you stand under such a tree for 5 years you will come a cropper. [/quote']

 

I would strongly recommend not ‘predicting’ anything. Prediction really should be left to prophets, soothsayers, and the Mystic Megs of this world. Though it would be reasonable to say the chances of throwing a 6 with a dice are 1/6. It’s not reasonable to say it will be a 6. To use an analogy often used in the workshops, a doctor would not predict when you would die. Aside from the banana skin of prediction, no matter how confident you are in an outcome, why fashion such a blunt and brutal cudgel for someone to potentially beat you with? A considerable part of this thread to date has been about risk and probability, in tandem with the importance of embracing uncertainty and expressing inexactness when quantifying risk.

 

If a rota is drawn up whereby you have a 1 minute slot in those 5 years where you have to stand under the tree' date=' the risk to you is still unacceptable because you may get the minute when it alls over.[/quote']

 

And you may not get the minute when it falls over; as Pooh might say. Or even if you were under the tree, when it falls over the tree doesn’t fall over in your direction.

 

Sorry, but I’m struggling with your reasoning here. We’ll have to wait until we have the hypothetical tree drawn for us, but from what you’ve described so far, in order to agree with what you seem to be saying about the level of risk, I would be expecting a considerably higher Target value than what has been described. I would also be expecting a tree that was so mechanically compromised it could be pulled over with a rope.

 

Which brings us onto the Target value and accounting for summer lingering and passers-by in good weather conditions that you referred to when assessing it. Do you think you might not be leaning over onto the averse side of tolerability of risk fence here? It sounds like you’re calculating ‘what might happen’ rather than ‘what is likely to happen’. I’ll explain why I ask this. During the field exercises on the QTRA workshops the first question that would be raised is of course about the Target. And the subsequent question that would be directed to the land owner/manager in these circumstances, to repeat post 11, is how many pedestrians an hour/day would pass the tree when the wind gusts to gale force 8 (39–46 mph 34–40 knot)? Say, you use gale force 7 as your point of reference to allow for gusts - "Effort needed to walk against the wind" - where the wind turns Piglet’s ears into banners like this.

Because these are the weather conditions against which QTRA would consider the occupation of a Target.

 

Cheers

 

Acer ventura

[Acer ventura]

I would contend nevertheless that is it possible to quantify risk and to exercise professlional judgements as risk aware rather than risk averse (my client demand nothing more or less than that) without buying in to a formal system.

 

Hi Jules

 

I think there’s two fundamental issues here.

 

Firstly. In pursuit of the published 1/10,000 and 1/1,000,000 thresholds, sure, you can have a SWAG (Scientific Wild Ass Guess) at pedestrian Targets knowing there are 86,400 seconds in a day and 8,760 hours in a year with some basic maths, which is great. But how would you go about quantifying the road use, or putting a value on a structure like a parked car relative to human occupancy? And what about the other components of a quantified risk. How are you quantifying the Impact Potential (Size of Part), and the Probability of Failure? Notwithstanding the ALARP calculation.

 

The second one being. As well-intentioned as your efforts to quantify risk are, how do you think your personal take on quantification would stand up in court to the forensic scrutiny and cross-examination from a barrister in the unfortunate event of a risk being realised? One considerable advantage of buying into a system is not just the pedigree and credibility of the system, but if you follow the guidelines of a tried and tested system, then as a risk assessor you enjoy a substantial amount of protection, as does the tree owner. The tree owner, more importantly, is generally the duty holder who holds the liability that would be especially vulnerable to any claim for compensation in such an event; not least because their pockets are likely to be deeper.

 

Cheers

 

Acer ventura

[daltontrees]

Sorry, I scarcely have time to eat and sleep these days, the Forum is a luxury. However, byu now I feel as if I am defending myself for jocular commetns directed at RobArb whom I know would like help to get his head around the quantification of risk. I don't have a problem with it and my methods are more sophisticated than you seem to think. I'd rather not be the cadaver that is dissected for this particular subject, and we may just have to disagree about whether a tree-owners duty of care can be discharged without the use of QTRA. I and my clients are/am satsfied with the results of my black box and I shan't have to explain in court why someone didn't get hurt. I am sure the courts will find QTRA reassuring but the law doesn't say you have to use it, just what you need to achieve.

[JonnyVine]

Hi all

Should not any potential tree inspector want to use all and every available method to evaluate risk? Trying to prove something that has not yet happened, or does not have a single "fixed" value rather than a set of possible values is difficult, hence the legal term "acceptable level" of risk.

As with any Axiom or subjective randomeness we should not rely too heavily on numerical values, as David Lonsdale put it " Numerical scores should not be allowed to lend an exaggerated air of objectivity to a procedure that is inherently subjective in many respects".

 

I also agree that what Acer has already pointed out, that "buying into a system and following its guidelines brings with it a certain prtection", candidates that have attended the PTI course will be able to recall the "Bolam" case!

 

If there are certain aspects of a particular system that do not appeal to an individual for whatever reason, Then he should at least look to using it as a backup of crossreference, it will broaden the knowledge if nothing else.

[Acer ventura]

However, byu now I feel as if I am defending myself for jocular commetns directed at RobArb whom I know would like help to get his head around the quantification of risk. I don't have a problem with it and my methods are more sophisticated than you seem to think. I'd rather not be the cadaver that is dissected for this particular subject

 

Hi Jules

 

I’m very sorry you hear you feel that way. This is a discussion forum, and you did ask me what I thought about DIY tree risk quantification, and then embellished the notion with an example of how you go about it when assessing human targets. You also indicated you would be willing to discuss the risk assessment of this scenario but with a hypothetical tree using your method, which I still know little about. I have tried to answer you and discuss the issues you’ve raised in a polite and neutral manner and keep the subject matter about the facts.

 

On this thread I have been happy to respond to any criticism levelled at how QTRA goes about risk assessment. Dissection is a good thing. The way I figure it, if QTRA has merit then it will stand up to open scrutiny. If the wisdom of the crowds highlight areas where it could be improved, and their point is well founded, then it would be unwise not to take these on-board and improve the system. One of the driving forces to how QTRA has evolved is the feedback and opinions from the hundreds of registered users from around the world both during the training and when they start to apply it. And yes, even from discussions on forums.

 

Cheers

 

Acer ventura