Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[Acer ventura]

Appologies Rob/Acer I have nothing to input here regarding the thread, would just like to clarify something.

Tony, ftr it is the entire site of the City of London run Hampstead Heath (approxixmately 600 acres) that has an estimated 7 million annual footfall (based on electronic counters at strategic entrance points), not just the one gate between the two sites of HH & the Kenwood Estate at the Kenwood gate.

 

Hi David

 

On the contrary, that's a really useful contribution and clarification because it highlights the issue that QTRA is about assessing the 'tree risk' and not looking at 'tree defects'. Target valuation is the most important element of tree risk. Getting this part of the risk equation wrong can mean the risk assessment is as inaccurate as missing a defect-riddled tree of poor vitality with symptoms of incipient failure adjacent to a high value target, or pointlessly pursuing the removal of deadwood in parks.

 

One of the skills that an arborist needs to acquire with QTRA is working out Target values. With roads that's often quite easy because the Department of Transport has average road use statistics for each region and class of road, and the Highways Department of the Local Government Agency often has very accurate data. For pedestrian use, the owner or manager is usually the most informed and the risk assessor the least knowledgeable. Part of the skill of the assessor is teasing the answers out of those that know the site better. This is key element because trees can fail at any time of the day over the year whether people are there or not. You need an annual average occupancy, which includes winter and night time, and it's not uncommon for only the more apparent higher occupation rates to be recollected unless prompted and directed by the assessor.

 

By quantifying the risk assessment, there are sometimes clues as to whether the information you're given about the Target might be unintentionally biased or inaccurate. So if there were 7 million going through Kenwood Gate a year than that would equate to just over 800/hour, 14/minute, or 1 every 4-5 seconds, pedestrians going through the gate, day and night, fair and foul. If the gates are closed at dusk, then these footfall rates would be roughly doubled - not that doubling of a rate over half the time would affect the average occupancy over the whole year.

 

I was once asked to carry out a QTRA at the home of a barrister who had been involved in a high profile tree failure case. He had a very nice house adjacent to a single track country road, which I know from experience is unlikely to figure on Highways monitoring. I asked my client to be conservative about it and, "On average, over a year, how many cars/day do you think come down this lane?" After a bit of thought, his reply was “50”. I said this would be my Target and I would reference them as my source in my report. Initially, they seemed a bit shocked but then upon consideration, realising this was something they would know far more about than I would during my hour on site, they recognised it was a 'proportional' and 'reasonably practicable' solution to assessing the tree risk.

 

Cheers

 

Acer ventura

[RobArb]

Hi David,

 

Quick question. The 1/10000 threshold, is that an exact figure? For example, after addressing the site, tree, targets etc and putting the results into the calculator, the quantitative figure you get, is that adhered to exactly? So if it was slightly over, or slightly under what would the outcome be? Is it black and white or is there still lots of little grey areas?

 

Thanks

Robin

 

(and sorry if this sounds like a daft question)

 

 

 

Sent using Arbtalk Mobile App

[Amelanchier]

... I said this would be my Target and I would reference them as my source in my report. Initially, they seemed a bit shocked but then upon consideration, realising this was something they would know far more about than I would during my hour on site...

 

If you'll excuse the interruption, I think this seemingly small point is actually quite critical to the robustness of a report. I have lost count of the number of statements in reports, applications, objections and the like that claim to define a target in relation to trees without any supporting evidence; "the tree is near a busy footpath". Busy compared with what? Oxford Street at Christmas? Frame the argument for the reader or they will frame it for you.

 

The reasoning behind the evaluation should be laid bare - if it doesn't quite feel right saying "It's a guess because I have no real idea having only been on site for half an hour." then don't worry, that's just your common sense kicking in...

[Acer ventura]

(and sorry if this sounds like a daft question)

 

Hi Robin

 

It's a great question. As I said earlier in the thread, there's no such thing as a daft question.

 

Quick question. The 1/10000 threshold' date=' is that an exact figure?[/quote']

 

Quick answer, yes.

 

For example' date=' after addressing the site, tree, targets etc and putting the results into the calculator, the quantitative figure you get, is that adhered to exactly? So if it was slightly over, or slightly under what would the outcome be? Is it black and white or is there still lots of little grey areas?[/quote']

 

Longer explanation. You'll need a cup, glass, or pint of something. It's easier to do this in the workshop, but bear with me.

 

The thresholds (ie 1/10,000) are points on a risk gradient that inform the tree management decision. The assessor is calculating the risk and giving the manager the RoH: giving them the odds, if you like. It's how you move along that slope and the degree of uncertainty or confidence when you get close to those threshold points that's important.

 

With a QTRA, using the manual calculator, each component, Target (T) x Impact Potential (IP) x Probability of Failure (PoF), are broad ranges, and it is the highest value in that broad range which is inputted to the RoH calculation. If the actual inputs are some distance from the highest value of the range then the RoH calculation will have a safety margin because it will be an over-estimate of the actual risk. This doesn't usually matter unless the RoH is higher than 1/10,000. If the actual inputs are very close to the haighest value of the range, and the RoH is close to the 1/10,000 threshold, then you need to be more confident of the inputs.

 

Another point I should clarify is that the RoH is expressed to 1 significant figure. On the manual calculator there are only 2 combinations of T x IP x PoF that give a RoH of 1/10,000. The next lower RoH is 1/20,000, and the next higher RoH is 1/9,000.

 

To illustrate what I've said with an example using the manual calculator (I've attached a scan of this alignment).

 

T = 30mph road with 325 vehicles/day (Highways Department) = Fits in Range 3

IP = 300mm stem = Fits in Range 2

PoF = Very defective co-dominant leader from old pollard = Fits in Range 3 (1/100 – 1/900)

 

RoH = 1/10,000 (the manual calculator show a Risk Index 10, which is 1,000th the RoH)

 

T = Range 3 has the highest value of 649 vehicles/day at 30mph, which is the one that goes into the risk calculation. However, the actual figure at 325 is, near as dammit, half of that. So you can have a very high level of confidence the end RoH has a considerable safety margin.

 

IP = Range 2 has the highest value of 450mm. The actual diameter of the stem is 300mm. The highest value of the range is significantly greater than the actual IP. If you look at the allometric date in the Practice Note at Page 6, Table 2, you can work out that the 450mm value used in the calculation is around x 2.5 higher than the actual IP for the 300mm stem. Yet again, you can have a high confidence the RoH has a considerable safety margin with this input as well.

 

If your inputs were very close to the highest values of the ranges, say 645 vehicles a day and size part was 450mm, you would want to be very confident of these figures because there is now little safety margin. If you have confidence then a RoH 1/10,000 calculation is regarded as a tolerable level of risk.

 

Does that answer your question?

 

Cheers

 

Acer ventura

 

PS I've just realised I've been calling you Rob sometimes, so my apologies about that.

[RobArb]

Robin/Rob, Its all good (just as long as its not Derek, or Gerald etc (apologies to Derek's and Gerald's etc)).

 

Yes Acer, I now understand the maths you have shown here, so thanks for that. Now please don't take this the wrong way, its harder to put across an expression over an internet forum and i don't want you to mis-interpret my written word due to the lack of inquisitive intonation in my voice.

 

 

What are your thoughts then on the range of "PoH" you have shown here on that there calculator? What i mean is, are the ranges shown not open to interpretation/perception/subjectivity (professional judgement) which then therefore negate the "exactness" of that threshold that has supposed to have been quantified?

 

 

Again, I'm not trying to be funny, just trying to get my head round it:biggrin:

 

And do you prefer David or Acer? (seeing as you asked me)

[Acer ventura]

What are your thoughts then on the range of "PoH" you have shown here on that there calculator? What i mean is, are the ranges shown not open to interpretation/perception/subjectivity (professional judgement) which then therefore negate the "exactness" of that threshold that has supposed to have been quantified?

 

Hi Robin

 

I’m not sure what your question is here. Do you mean RoH or PoF when you say ‘PoH’. Or are you referring to ‘Pooh’, who clearly understands the principles of QTRA, ‘after careful thought’? (see attached).

 

Are you asking a variation of the accusation of ‘implied precision’/‘exactness’ to the subjective probability/professional judgement PoF element of the risk assessment? Or similarly about the ‘implied precision’/‘exactness’ in the RoH?

 

which then therefore negate the "exactness" of that threshold that has supposed to have been quantified?

 

Just to reiterate' date=' the ‘exactness’ of the tolerable level of risk ‘threshold’, the 1/10,000, is not set or ‘quantified’ by QTRA. It’s one of the thresholds of tolerable or acceptable risk that has been established by a myriad of external independent bodies that we’re looking to assess and measure the risk from trees against; which is why we’re quantifying.

 

Rather than carpet bomb your question with an answer to a question I think you’re getting at and ending up going Cambodia on it, could you please have a go at clarifying what you’re asking?

 

And do you prefer David or Acer? (seeing as you asked me)

 

Acer/David. I’m not that fussed. I’ve adopted Acer ventura as a forum name since January 1999 when out of a drunken Christmas catch up with Chris Hastie the UKTC germinated and I made the first posting. It’s borne out of a frustration that I decided to pretend to be a grown up and didn’t have the cahoonas to call my consultancy Acer ventura’s Tree Detective Agency, and it’s become habitual.

 

Cheers

 

Acer ventura

Edited February 8, 2013 by Acer ventura
Attachment failed - was in preview

[RobArb]

Hi Robin

 

I’m not sure what your question is here. Do you mean RoH or PoF when you say ‘PoH’. Or are you referring to ‘Pooh’, who clearly understands the principles of QTRA, ‘after careful thought’? (see attached).

 

Are you asking a variation of the accusation of ‘implied precision’/‘exactness’ to the subjective probability/professional judgement PoF element of the risk assessment?

 

Cheers

 

Acer ventura

 

Yes that's what I meant, PoF. Typo that I didn't check so apologies.

 

Also, I understand what you are saying about the myriad of bodies creating this 1/10000 threshold that QTRA adheres to (and didn't set). But who's to say a tree with an outcome of 1/10050 is safer than a tree with 1/9997...?

 

It's this variability that I can't seen to get past. Life just isn't exact?

 

Cheers David, I liked that attachment and agree with Poohs sentiments:biggrin:

 

Robin

 

 

 

Sent using Arbtalk Mobile App

[Paul Barton]

I love that Pooh attachment, brilliant!

 

(sorry, that's my only comment so far but am reading this thread with interest!).

[Tony Croft aka hamadryad]

I love that Pooh attachment, brilliant!

 

(sorry, that's my only comment so far but am reading this thread with interest!).

 

Pooh and piglet are my faves too! it's a brilliant slide:thumbup1:

[treeseer]

I'm a big Tigger fan myself, but appreciate immensely the effort to get away from the defect-driven approach. The ISA's BMP, for all its matrices and addenda, is still used to justify witch hunts for defects over exaggerated targets here in the US, and in SE Asia.

 

Utter rubbish and vanity; anti-arboricultural to boot. Carry on, QTRA folk!