Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[daltontrees]

Av, is it possible when you have time to outline how the QTRA system factors in or is adapted to deal with a fairly typical situation where the target is exposed to the possibility of a whole tree failing at the base and thus a whole canopy of branches coming down, say 20 metres wide, ranging from 1mm to 600mm diameter? There is arguably a whole spectrum of IPs.

And a second question, again if and when you have time, does QTRA differentiate between the IP of a branch of given diameter falling from the bottom of the canopy at 3 metres and from the top at say 20 metres?

Thanks in anticipation of your clarification.

[Acer ventura]

Av, is it possible when you have time...

 

Hi Jules

 

Apologies for the delay in getting back to you on this, and thanks for your patience. I’ve finally found a bit of time whilst at my stopover in Singapore to reply. However, by way of warning, I’m likely to be even slower to respond over the next week and a half than I have recently because upon arrival in Australia I’m being whisked off by friends for acclimation and elocution lessons in ‘Strine’ over the weekend in an internet free outback and beyond place I believe is called Wolf Creek. Then next week, in Perth, we’ve had to put on an additional QTRA workshop to meet demand. So, along with the VTA and QTRA Update workshops, this means by a week Friday evening, having run 4 one day workshops back to back, the risk that I’ll be completely and utterly mentally ‘fecked’ is between 1/1 – 1/10.

 

I'm still happy to answer questions anyone wants to ask, but the speed of correspondence might be more akin to sending telegrams in the age of steam, over the next few weeks.

 

...to outline how the QTRA system factors in or is adapted to deal with a fairly typical situation where the target is exposed to the possibility of a whole tree failing at the base and thus a whole canopy of branches coming down' date=' say 20 metres wide, ranging from 1mm to 600mm diameter? There is arguably a whole spectrum of IPs[/quote']

 

A good question. It depends on the Target. With human Targets we take the DBH, or diameter of the branch measured just beyond its basal flare, and that is our IP Range. Though the smaller diameter outermost branches may be what hits the person, the momentum and potential force behind even relatively small branches is partially driven by the largest part that fails to which they're attached, and this can generate a considerably greater force than if the last metre of a branch was to have failed alone. With a 600mm DBH tree failing onto a human it is a 1/1 Consequence and the RoH is determined by the Likelihood of them being there, and the Likelihood of the tree falling, no matter where they are in relation to the tree’s potential footprint on the ground. We don’t think it’s reasonably practicable, or proportional, to try and quantify the likelihood that someone who could, say, be the Target of a 15m tall decurrent Birch failing at its root plate and them being at the outermost 1.0m branches of it, which results in only knocking the person’s hat off and giving them a few scratches.

 

With structures/property it’s different because which size part of a falling tree hits the Target does affect the Consequences. We value these Targets as the likely cost of repair to the structure if it was hit. So the IP is already accounted for in the likely extent of damage during the Target valuation. Therefore, with the risk to structures, the IP is set at 1/1 (you can see that IP 1/1 is orange and says this on the manual calculator), otherwise we are double counting because ALL of the Consequences side of the Risk = Likelihood x Consequences equation is eaten up by the Target. If a house were 20m away from a 22m high Oak tree, the amount of damage to it would be minimal, and so we might value it in Range T4. However, if the house was only 10m away, it is likely there would be significantly more damage because it's been hit by a bigger part, and we may value it in Range T3. The extent of damage, our Target, has the influence of Size of Part built into it's assessment. Our RoH would be T Cost of repair x IP set at 1/1 x PoF.

 

And a second question' date=' again if and when you have time, does QTRA differentiate between the IP of a branch of given diameter falling from the bottom of the canopy at 3 metres and from the top at say 20 metres?[/quote']

 

This was looked at during the inception of QTRA. There are so many variables as to how the branch might fall, get hung up, bounce off others, get blown from height, was it in or out of leaf, or how it would land, it was decided it wasn’t reasonably practicable or proportional to try and account for this. We simply have an IP based on an allometric equation.

 

Cheers

 

Acer ventura

[daltontrees]

Av there's no need to apologise, you have brought upon yourself the enormous taks of fielding all manner of questions about QTRA, I have no more right to demand speedy or any answers than you have to demand that we stop asking obvious questions.

Your response regarding property is logical. Regarding moving targets, particularly slow moving ones, your position is clearly stated but personally I try to take account of increased target presence time for wider canopies. The difference between a Lombardy Poplar and a Horse Chestnut of the same DBH for example could be considerable.

As regards height of fall, again I would tend to differentiate when if the shape of the canopy allows me to. A clear fall of a high-up snow-laden Scots Pine limb (close-grown examples of said species can commonly be 20 m high with canopy restricted to last 5m) onto a road I think deserves higher rating of 'IP' than same size limb lower down. But I take your point about how difficult it could be to generalise for all the ifs buts and maybes.

More questions come to mind but you may be pleased to hear that rather than ask I am trying to find the answers from the Guidance Note and this thread, so that I can restrict myself to questions I can't get answered otherwise. The questions I ask are a bit broader than they need to be so as to provide a bit of interest for other readers. Anyway, I can think of better things to do in Singapore than answer questions about QTRA, so your elucidations and our education can continue whenever.

[daltontrees]

And if you wish something to ponder in the unlikely event on not being mentally fecked down-under, here is a wee thing that troubles me about quantification of risk and how QTRA might address the issue.

I am taking as read the 1:1,000,000 and 1:10,000 brackets that HSE has suggested as guidelines. To make it really really clear, this is the guidelines for risk of death. This seems to be ingrained in QTRA calculations.

However, does not the common law and the Occupiers Liability legislation create an obligation to avoid foreseeable risk of injury or damage? I don't want a war on the exact wording of the law, I just want to point out that the HSE figures are about death and the law definitions are about 'injury'. To confuse matters, HSE analyses industry accident figures based on 'serious harm'. There could be as much as a degree of magnitude of risk between death and serious harm and another order of magnitude between serous harm and injury. I have worked on large tree surveys where the instruction has been to satisfy the client's Duty of Care responsibility. Had I used the unacceptable/tolerable/broadly acceptable verdicts of the QTRA system or indeed a home-knitted version of it, I could have been far far off the mark and have failed the client. I am not sure I have a question for QTRA, but do you recognise the differentiation I am making and accept that duty of care obligations may not be immediately compatible with QTRA outputs without some sort of adjustment being made to the threshold? Even if it were just to allow for a tree presenting a 1:100,000 risk of death simultaneously presenting a (say) 1:10,000 risk of injury or serious harm?

As a tag-on I would suggest that QTRA output for damage to property may be in line with duty of care obligations since the damage is defined by cost of repair.

The HSE 2001 document, whilst very thorough, discusses risk without saying risk of what, then jumps straight into guidelines for risk of death. That it seems is a jump that leapfrogs risk of injury or serious harm and leaves no guidance as to what the corresponding thresholds would be for those less serious but higher probability risks.

[Tony Croft aka hamadryad]

And if you wish something to ponder in the unlikely event on not being mentally fecked down-under, here is a wee thing that troubles me about quantification of risk and how QTRA might address the issue.

I am taking as read the 1:1,000,000 and 1:10,000 brackets that HSE has suggested as guidelines. To make it really really clear, this is the guidelines for risk of death. This seems to be ingrained in QTRA calculations.

However, does not the common law and the Occupiers Liability legislation create an obligation to avoid foreseeable risk of injury or damage? I don't want a war on the exact wording of the law, I just want to point out that the HSE figures are about death and the law definitions are about 'injury'. To confuse matters, HSE analyses industry accident figures based on 'serious harm'. There could be as much as a degree of magnitude of risk between death and serious harm and another order of magnitude between serous harm and injury. I have worked on large tree surveys where the instruction has been to satisfy the client's Duty of Care responsibility. Had I used the unacceptable/tolerable/broadly acceptable verdicts of the QTRA system or indeed a home-knitted version of it, I could have been far far off the mark and have failed the client. I am not sure I have a question for QTRA, but do you recognise the differentiation I am making and accept that duty of care obligations may not be immediately compatible with QTRA outputs without some sort of adjustment being made to the threshold? Even if it were just to allow for a tree presenting a 1:100,000 risk of death simultaneously presenting a (say) 1:10,000 risk of injury or serious harm?

As a tag-on I would suggest that QTRA output for damage to property may be in line with duty of care obligations since the damage is defined by cost of repair.

The HSE 2001 document, whilst very thorough, discusses risk without saying risk of what, then jumps straight into guidelines for risk of death. That it seems is a jump that leapfrogs risk of injury or serious harm and leaves no guidance as to what the corresponding thresholds would be for those less serious but higher probability risks.

 

good post!

[Amelanchier]

What can injure one person may kill another, so if you assess against the risk of death do you not already encompass the broadest possible spectrum of 'harms'?

 

Would it be reasonable to expect an assessor to estimate what types of injury would qualify as serious harm? Presumably trees (and those responsible for them) 'take their victims as they find them'?

[daltontrees]

Tony, I don't know who your question is addressed to. I can't answer a question about a question but I can elaborate a bit.

Personally I feel able to distinguish between tree risk of death and lesser risks, from being flattened outright by a tree trunk right down to getting a poke in the eye from a protruding branch. The taking of victims as you find them matter is an interesting and important one in law. Although someone who has for example brittle bones might come off very badly if struck with a branch compared with someone who has normal bones, the assessor I think can only assess trees with reference to the potential for harm to an average physiology which statistically most people are. I wouild be amazed and astounded, impressed even, if QTRA made a distinction between risk on an average street and on a street in front of a sheltered housing complex where pedestrian usage by elderly and infirm is slightly higher than usual. That said, I always take a cautious view when surveying near primary schools where children could not withstand the injury adults could.

Maybe I could illustrate my point with an example which is met with surprising frequency. A tree is set back from a pavement at a distance that means if the whole tree fell only the tips of the top of the crown would reach the pavement and the rest of the tree would land harmlessly in undergrowth. The worst outcome barring freak occurences and particularly fragile targets would be laceration and mental scarring. If the tree failure had been foreseeable and nothing had been done about it the landowner would be negligible and responsiple for recompense to the victim. Would QTRA have out this tree off the scale on the 1:1,000,000 threshold for death and could a landowner be left bemused by his misunderstanding that QTRA had his duty of care covered?

I await comments from our Wolf Creek correspondent.

Edited March 2, 2013 by daltontrees

[Amelanchier]

Tony, I don't know who your question is addressed to.

 

Just thinking out loud really. One of those times where you want to post, start to realise the depth of the response needed and end up being a bit superficial and trite in an effort to be short and sweet!

 

Sent using Arbtalk Mobile App

[daltontrees]

I know what you mean, asking a simple question can be very complicated.

 

And it's not even lining up a criticism of QTRA, I just would like to know these few things so that I and others can delineate the limitations of QTRA so that (in my case) if I ever have a big client who wants recommendations for a system to use I can properly advise on the pros and cons in case the client wants to adopt QTRA. So far it's only doing what I already do and depending on answer to last question I may already be exceeding its capabilities.

[daltontrees]

Mr Av, I see you are back in the country as you have been posting about QTRA on UKTC. I would be interested to a completion of our education about QTRA here when you are fully unpacked and are taking a break from provoking your adversary on UKTC.