Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[RobArb]

 

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

 

This is one of the things i don't agree with, buying into a system! Especially if its a system i am not fully sure of. Acer is doing a grand job of delving into the depths of the undergrowth of QTRA, but when statements like this appear, it strikes me as "your ass will only be covered if you suscribe to my method, oh and it'll cost ya too!"

 

 

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.

 

This however, i do agree with, never say never... Rely on what you know and trust but always have a backup, just a shame its not "free".

 

 

 

I think i get the essence of QTRA, but what i have an issue with is going back to the idea that we "need" to quantify it (risk)? We managed well enough before it didn't we?

 

 

 

Acer, would it be possible to run through a hypothetical stand of trees that a fictitious land owner has asked you to give him an assessment of risk on, from start to finish? Or is that too much to ask on a public forum?

[RobArb]

naaaa

 

Thanks for that interesting and thought provoking post:sneaky2:

[JonnyVine]

Hi Rob

 

I failed to "quantify" my use of Acer's terminology, I didn't literally intend that you stump up quite a substantial amount of your hard earned, what I should have said was "using the system, or learning its fundamentals and applying them in some way" sorry.

 

I jollywell hope that I have not come accross as being a paid up member because I'm not, all I wanted to do was to point out that it has its place in tree assessment and whatever the cost of the training etc one should not forget that a huge ammount of time and effort has gone into the concept, however like yourself, there are a quite a number of features that I don't personaly agree with, but if one wants to assess trees for their damage potential then a certain ammount of calculation of the risk involved is unavoidable.

 

My own personal preference is to put the risk into broader categories wherever possible keeping things as simple as possible, hence the Lonsdale statement about the "exaggerated objectivity" by using too many numbers, I see nothing wrong with this and it is also a defencible and accepted system.

 

 

I would however support some form legislation governing tree assessment which could involve buying in to a scheme of some sorts, all too often I read posts on here that can retain a tree and prescribe a management plan just by looking at a photograph! I would love to see some of these so called "arbs" with their sphincters winking while being questioned by that barrister about their stupidity and arrogance.

[WorcsWuss]

 

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.

 

I'm right on the periphery of this, an interested bystander. I assess only more immediate risk using the similar likelihood & severity scoring system in construction, but there's one thing I noticed I agree with which I want to wave a flag over, and that's prediction by probability.

 

A numerical 'acceptable risk level' based on odds is, in my view, practically meaningless.

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.

 

A 1/6 probability of rolling a 6 on a dice means nothing, 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.

 

If a tree has weaknesses, why not assess it properly, just as an engineer assesses my building designs....? No matter how 'roughly' it was done it has to be better that using the same sort of process used to fob off the HSE when working at height? For instance we could surely calculate at what speed and from which direction the wind would be coming from which would most likely cause a given tree to fail when in full leaf surely....? And that would give a REAL set of parameters against which tree management could be carried out....

 

When that young woman was killed in gales recently, had the trees been assessed mathematically and structurally rather than statistically, Kew could have known to close that area until the forecast winds had died down...

 

I accept that you can't do this everywhere for every tree, but surely it would be preferable to have a PROPER set of accurate management tools based on real structural principals rather than the language of bureacrats, for critical trees in public spaces....

 

Anyway, I'm off back to look at pictures of trees and shiny stuff....

Edited February 14, 2013 by WorcsWuss

[Acer ventura]

I think i get the essence of QTRA, but what i have an issue with is going back to the idea that we "need" to quantify it (risk)? We managed well enough before it didn't we?

 

Hi Robin

 

Your first question goes back to the first couple of posts in the thread.

 

Did we really manage it well enough before? I don’t think we did, and I don't think we do now – though things are changing.

 

If we don’t quantify risk then there is a natural and hard-wired tendency to be risk averse, which results in wasted resources where more money and time is spent assessing trees for risk than is necessary. And trees, branches, and associated habitat can be unnecessarily removed because they are regarded as having too high a risk. I think this is mainly down to the difficulty risk managers and risk assessor's have without a tolerable or acceptable level of risk to measure against. Otherwise, how do you work out whether the the tree/branch is safe or not? - Particularly if it could be regarded as defective.

 

A common theme that is apparent amongst those that come out of the other end of QTRA training is they spend less time assessing tree risk than they used to before. They can assess more trees within the same period of time than before. Their assessments also generate a less tree work, and they feel more comfortable about their position if a risk were to be realised.

 

However, those that benefit most from quantifying tree risk are the managers and tree owners.

 

Hopefully, I will be co-presenting a paper with John Flannigan at the AA conference this year that covers part of this. North Somerset Council were one of the first Local Government Agencies to adopt the principles behind QTRA as policy in their Tree Risk Management Plan. The beneficial consequences of this are many fold. Not least because they now spend less money assessing tree risk, less money felling trees, and therefore retain more tree-related benefits. Saved money they can now spend on the likes of establishment and formative pruning.

 

An important and additional spin off is in the event of a risk being realised. If that risk was less than 1/10,000 they are in a very robust position to defend a claim against them, and fend off the HSE possibly serving an Improvement Notice, which could leave them vulnerable to civil claims. Part of what I hope to be covering is a case where this happened in the car park in one of their woodlands.

 

 

Because the risk was 1/400,000 they were able to politely refuse to pay out on claims from the car insurance companies for the four cars that were written off because the risk was tolerable.

 

Acer' date=' would it be possible to run through a hypothetical stand of trees that a fictitious land owner has asked you to give him an assessment of risk on, from start to finish? Or is that too much to ask on a public forum?[/quote']

 

Let’s give it a go. It’s something I hoped Jules would’ve been up for because the tree he talked about sounded like a really good example of why Target valuation is the most important part of tree risk assessment. What town do you live in? I don’t mean that in a weird stalker way, but I’ll have a look through google and bing maps and pick something that will be a good starting point.

 

Cheers

 

Acer ventura

[Acer ventura]

My own personal preference is to put the risk into broader categories wherever possible keeping things as simple as possible, hence the Lonsdale statement about the "exaggerated objectivity" by using too many numbers, I see nothing wrong with this and it is also a defencible and accepted system.

 

Hi Jonny

 

It’s a good preference, and QTRA works in broad ranges. I don’t know whether you’ve had the stamina to go through all the thread but it’s a key element to what we were discussing starting from around post 24. Sometimes we can refine the Target if better and credible information is available from an authorative source because some of those ranges are so broad, but only if otherwise the risk might be unacceptable because we’re using broader ranges and calculating from the highest value.

 

BTW David Lonsdale is happy to refine elements of QTRA beyond the broad ranges. He did this in his ‘Felbrigg’ expert witness report.

 

Cheers

 

Acer ventura

[RobArb]

 

 

Let’s give it a go. It’s something I hoped Jules would’ve been up for because the tree he talked about sounded like a really good example of why Target valuation is the most important part of tree risk assessment. What town do you live in? I don’t mean that in a weird stalker way, but I’ll have a look through google and bing maps and pick something that will be a good starting point.

 

Cheers

 

Acer ventura

 

 

Hi Acer,

 

I live in Burscough in West Lancs, but how about 2 scenarios:biggrin:

 

First one, take a look at this

 

Got one of my marks back from a uni module today. Twas the tree survey and inspections module, now i have never done an "official" report before so this was my first real attempt.

 

 

88%, well chuffed:thumbup:

 

 

If anyone wants a gander, its here. Ignore some of the minor editing, had too make a few changes to create a pdf file.

 

 

PS, this is NOT a legal report, just one i had to put together for college, all real data has been hidden or edited and no claims can be made from it:lol:

 

 

Hopefully if you click the little blue arrow box in the quoted text it should take you to my other thread where you will find said assignment survey for uni.

 

(let me know if it works or not for you)

 

 

Secondly, 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??

 

 

 

 

Thanks for going through this:thumbup:

[daltontrees]

Hi all, exhausted but free after several hard days' survey and tree work I have alittle time to catch up on the debate. Please take my earlier comments in context, I am often leaving the house at 7am, 2 hours before light, measuring and chalking up tree diameters in the dark so that I can start survey at 8.30 or 9 if it is overcast. And back home at 5.30. Or as happened this week we were taking down a large Black Pine that shifted considerably in the wind and rain last week. I was 15 metres up it in the dark trying to redistribute its centre of gravity. I characterised it as urgent and advised the owner to get rid of it despite the expense. You can use it as an example if you want and can question my judgement on that advice. Let me know and I will provide details. I have a a lot of first-hand information about the tree.

My snippiness last posting came from tiredness, lack of time and what I perceived as criticism of my professional judgement. I would prefer to see QTRA justified on its own merits than by trying to find inadequacy in my methods. Acer Ventura is clearly very eloquent and a practiced in rhetoric, and I would not like to see the opportunity for open discussion stifled by fear of attrition.

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.

So back to the Practice Note ("PN") and what I understood from it, and I think this might illuminate things a little for anone still looking in. The PN says much about probablility of target being present at the time when failure is most likely, which I think is QTRA's strongest suit. It says a little about the severity of harm and attaches weighting to it relative to the cost of damage to property. It says relatively little about the assessing the probability of failure. Let's say the three things that combine to produce the overall risk rating are these; target presence, severity of harm/extent of damage to property and likelihood of failure. Where I think people struggle to complete the arithmetic is the last one. It is really difficult to evaluate. 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. Yet, such parameters may be enough to take the overall risk from 1:1,000,000 to 1:100,000 or from 1:100,000 to 1:10,000.

The PN as I see it says little about this, but are there any users that can say whether QTRA is helpful on the matter or whether this is left to the judgement of the inspector and the limitations of budget for sophisticated investigations. Or both?

[daltontrees]

Attached is a pic of the Black Pine being topped out.

[Gary Prentice]

I've been following this thread with great interest. Although I can understand the reasoning and quantifying elements assessed by QTRA, I can't still have a moral dilemma with the entire process.

A couple of years back, I was involved in an application and an appeal to fell a large ash. The tree had a history of dropping limbs and had a large limb over the road way with a diametric crack almost two meters long. The road was a private access to the rear of twenty odd houses.

So, the the odds of hitting someone was low, the risk of harm (death) was high as the limb was 4-500 dia, and the potential of failure was very high.

The appeals inspector stated that it would fail.

The tree officer told me that the odds of failure were lottery winning figures that it would hit someone, just as a neighbour drove past.

"Try explaining to her family just how unlucky she was" was my answer.

I do find it difficult, in that particular situation, to accept a tolerable level of risk. Like the picture of the cars in the car park. If they'd been occupied, would 1:400,000 still been acceptable?