Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[Acer ventura]

I'm right on the periphery of this, an interested bystander.

 

Hi WorcsWuss

 

Interested bystander's points of view are very welcome.

 

If a tree is structurally compromised, simply assessing the 'probability' of failure is something of a fallacy. Tree hazards could actually be far more accurately 'quantified' in the same way that buildings are structurally designed. Afterall, it is for this very reason that we don't look at a floor beam when designing a building and decide on it's suitability based on a subjective opinion on how likely it is to fail and how many people will be crushed to death when it does.

 

Unfortunately, trees aren’t like floor beams though. In terms of risk assessment it would be brilliant, because they could actually be far more accurately ‘quantified’ in the same way that buildings are structurally designed. The trees might not be quite so interesting though.

 

Trees are both anisotropic and generating systems growing a new layer of wood every year. They have the capacity to self-optimise and preferentially lay down more of the new wood, with improved structural quality, where there is increased mechanical loading, and lay down less wood where there is reduced mechanical loading. It’s their ability to do this that can give them a body language we can begin to interpret with Visual Tree Assessment. Additionally, their ability to resist decay is partially down to their vitality, which not only determines how quickly they can lay down new wood in response to increased movement from decay, but affects their capacity to maintain functional wood, which is generally an inhospitable environment to many decay fungi. All of which are some of the reasons why PoF is a judgement call within a broad range, and a reason why strength-loss formulae are so limited. It's the component we are most uncertain about with present knowledge.

 

A 1/6 probability of rolling a 6 on a dice means nothing' date=' other than how many 6's are on a dice... you could roll it 6 times and get a 6 every time, or 6000 times and get no 6 at all.[/quote']

 

I’m not sure of your point here. You could roll a dice 6 times and get a 6 each time, but the probability of that happening is (1/6)^6, or, 1/6 x 1/6 x 1/6 x 1/6 x 1/6 x 1/6 = 1/46,656. Why does that mean nothing?

 

Cheers

 

Acer ventura

[WorcsWuss]

Thanks for taking the time to read my ramblings David!

I take your point, but what is your opinion of this example?

A VTA of a veteran tree at the main gates of a busy public city park has determined that a tree has a structural weakness, for instance a cavity just above a union. This damaged limb projects 5m at 15 degrees and in full leaf presents 6m2 of surface area to the prevailing wind. The limb has a diameter at the union of 12 inches but the cavity has taken out 30% of the section. The remaining wood is healthy primary growth green oak. (This is where I'll have to get a bit more vague, I'm no engineer!) a south westerly wind of 30mph will exert a force of 1.5kn/m2 on the sail end of the limb, giving a force of 1.5x6x5=45kn on the union. A healthy 12" diameter piece of oak is capable of sustaining a force of 80kn (I've no idea, structural engineers have tables for many green wood strengths) but we've lost 30% of this capacity, so the remaining timber is capable of sustaining 56kn, therefore, structural analysis has determined that in these conditions the limb is safe.

 

If the wind speed doubles, the limb will fail. But if the wind speed doubles and also comes from the south east, the force applied to it will be minimal and the limb will not fail..... So a set of easily defined management parameters - with a clearly defined danger zone - can be established

 

It's just a question really more than a suggestion, I'm not qualified to claim I know what I'm talking about (I design by eye and then get my engineer to prove it for me!), it's just curiosity!

 

And ref the dice, I was only making the point that it's statistical rather than factual, and I don't like statistics or probability. I like fact and justification....but that's why I'm not a gambling man...

 

Simon

[Acer ventura]

As there’s suddenly been an increase in traffic on this thread and questions raised, I thought I should point out that I could take some time to reply, and I may soon have to abandon it for a month if it’s not run its course by then. Like you all, I have proper work to do but with some particularly pressing deadlines because at the end of the month I’m running a QTRA workshop in London, and then heading off to Australia to run QTRA, VTA, and QTRA Update workshops over there over 3 consecutive weeks. Besides that, once over there I’m led to believe those ice cold beers won’t drink themselves.

 

In the meantime, I’m going to try and reply to messages in the chronology they were posted. So if you’ve asked something and you’re wondering why I haven’t replied yet, I’m not ignoring you.

 

Cheers

 

Acer ventura

[Acer ventura]

Goode Judgment

 

I've had a quite a few comments about this off-forum since coming on to Arbtalk, others have raised it with me in the past, and Robin led with it in his QTRA thread before Christmas. It seems that some are claiming the Goode Judgment is a legal silver bullet to the big bad QTRA werewolf. In my first post in this thread I attached a reply from Mike Ellison that covers some serious concerns about the reasoning behind the judgment, and raised the remarkable contradiction with the Commissioner criticising the implied precision in the QTRA calculation, and then adapting and misusing the QTRA calculation to claim the risk was unacceptable. I would urge anyone who's had the Goode Judgment waived at them to have a look at Mike's comments.

 

What I want to briefly do here is dispel what appear to be some common misconceptions around it that have been brought to my attention, and make a quick additional point about subjectivity in the judgment.

 

Though the words 'court' and 'judgment' are used, it's worth noting it's the judgment of a 'Commissioner'. This is an important clue to its status because Goode is not a High Court case. The best analogy is that it's an Australian equivalent of a 'planning appeal' resulting from a refusal to fell a protected tree. It does not set any legal precedent.

 

I have already dealt with the implied precision issue, and expanded on subjectivity v objectivity early in this thread. The comment about the 'subjectivity' in QTRA is a theme that I'm told recurs when Goode is raised. If you read the judgment you'll see appellant's expert's (Mr Nicolle) risk assessment is that he looked at the trees, their branches, and garden beneath it and concluded the risk is 'moderate', and that a 'moderate' risk is an unacceptable risk. In terms of a risk assessment, and a determination of whether a risk is tolerable or acceptable, I'm not sure it's possible to be any more subjective. Nonetheless, this is the evidence Commissioner Hodgson preferred. Notwithstanding the entirely subjective nature of Mr Nicolle’s risk assessment that he was able to convince the Commissioner a moderate risk is an unacceptable risk is rather concerning.

 

Cheers

 

Acer ventura

[Acer ventura]

Hi Acer,

 

I live in Burscough in West Lancs,

 

Is that near Mordor?

 

First one, take a look at this

 

Congratulations with your assignment score, you square.

 

I think it would be best to go for your woodland. There’s a potential can of worms in your assignment that it would be better not open and have wriggle all over this thread. As an aside, was it your lecturer who suggested using the Matheny & Clark Hazard Evaluation Form and Rating?

 

for a stand of trees, Google "Ruff Wood" Ormskirk and Google street view "Ruff Lane", would it be possible to use the row of trees that are adjacent to Ruff Lane from Ruff Woods itself??

 

Shall we start here

 

Ruff Lane, Ormskirk - Google Maps

 

Bing Maps - Ruff Lane, Ormskirk

 

Cheers

 

Acer ventura

[RobArb]

Is that near Mordor?

 

 

 

Congratulations with your assignment score, you square.

 

I think it would be best to go for your woodland. There’s a potential can of worms in your assignment that it would be better not open and have wriggle all over this thread. As an aside, was it your lecturer who suggested using the Matheny & Clark Hazard Evaluation Form and Rating?

 

 

 

Shall we start here

 

Ruff Lane, Ormskirk - Google Maps

 

Bing Maps - Ruff Lane, Ormskirk

 

Cheers

 

Acer ventura

 

Haha mordor:lol: I ain't that much out in the sticks:biggrin:

 

Yes, Ruff Woods, that's the one staying on the corner of ruff lane and vicarage lane.

 

Also, the oak in my report, why the can of worms? I don't mind an answer away from here if your up for it:biggrin:

 

I also appreciate your busy too, so if it's too much, ney bother

 

Sent using Arbtalk Mobil

[Acer ventura]

I would prefer to see QTRA justified on its own merits than by trying to find inadequacy in my methods…..

Moving on, I have now just read the Practice Note. It doesn't really show the nuts and bolts of QTRA but what it does tell me is that the methods I use are similar. I do not try to put across the financial savings on tree work and the tree benefits of retention that might come from refinements of our version of risk assessment. The client invariably makes the view on that clear. We advise then we take instruction then we act entirely within that instruction.

 

Hi Jules

 

Sorry to hear you had a bad day. I’m very happy to keep the thread on topic with QTRA Q&A and for you to keep your methods covered up. However, if you’re going to expose your methods, like you did earlier, and as you are again above, and I would like some clarification on them, think there’s a shortcoming in them, or even that they’re a good idea, then I think it only reasonable and fair that I comment on them.

 

So, on this occasion I'll not seek further clarification on what looks an unusual take on ALARP principles.

 

I have now just read the Practice Note. It doesn't really show the nuts and bolts of QTRA

 

I beg to differ. I think the PN clearly shows the nuts and bolts. It’s 8 pages long and shows what’s in QTRA and how it’s put together. Its purpose is to inform the client of what the QTRA assessor is doing and why' date=' and we recommend the PN is included in the appendix of any QTRA reports. What it doesn’t do, and isn’t supposed to, is tell the reader how to use QTRA. If we go back to the car analogy, the PN is a very detailed but extremely dull brochure. The 34 page User Manual, supporting PowerPoint presentation, private Registered User’s Discussion Forum, the day’s training, along with the recommended but optional VTA day to expand further on the PoF element, are what will help the assessor drive QTRA.

 

A weak fork, an extensive cavity, an already lifted root plate, no matter how good our invasive and non-invasive investigation toolbox it is very very difficult (I would go as far as saying impossible) to pin down, even to differentiate between say a 1:5 an a 1:50 probability.

 

Just to clarify, you can’t get a 1/5 and 1/50 PoF using QTRA.

 

Cheers

 

Acer ventura

[JonnyVine]

Rob

Blistering report, no stone unturned, I felt the assessor was a little stingy with the percentages? Worth a tad more in my Est as the only thing I picked up on was 2.6, underground roots and services.

 

Acer

Sorry I had not looked at the whole thread, only a superficial glance which has got me into trouble before and I found myself arguing my way out!

For me human life represents max target value and I always work from this assumption, does QTRA have a value order? Also would QTRA take into consideration that a motorcar travelling at 60 mph would susstain more serious damage in the unfortunate event of a collision with a falling tree or branch than it would at 30 mph, but actual contact for the faster car would be less probable?

 

You are doing an excellent job of defending your product by the way, impressive stuff and I would like to take a closer look at QTRA but can not even find the PN, quite useless realy.

[daltontrees]

To Acer ventura

Comments noted, I wasn't having a bad day just a busy couple of ones in a row.

As you are the sort opf person who clearly tries to use languiage precisely, a term such as 'nuts and bolts' was always going to be wooly. I suppose what I meant and which you have more or less confirmed is that the PN really just says that there are nuts and bolts but doesn't let us see them. I suspect the people opn the Forum will reserve their position on QTRA until tehy have the nuts and bolts in their hands to poke and pull at and satisfy themselves that they are solid.

You are now getting asked a lot of questions and I am not for now going to trouble you with any more. I would just reiterate that the example in Kelvinside is effectively confidential but that I am quite happy to describe exactly the Black Pine situation (without stating what I then advised and why) and if you are willing to illustrate what advice your approach would have resulted in I will simultaneously do the same. The spoecific case doesn't matter, it is more for onlookers and me to understand how your system works in practice and what a more rudimentary system does by comparison. I do not want conflict, only advancement of understanding. It can go in a new thread if need be, this one is bifurcating like an Elm branch.

[JonnyVine]

To Acer ventura

I suspect the people opn the Forum will reserve their position on QTRA until tehy have the nuts and bolts in their hands to poke and pull at and satisfy themselves that they are solid.

.

 

Perhaps this may be a little on the extreme side for those on here that are not that way inclined Jules