Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[daltontrees]

OK donme the first bit, albeit without actual; traffic counts and knoiwledge of pedestrian usage, but I get the idea so far.

[RobArb]

Bit of info on the woodland

 

Its used as an amenity/recreational woodland, a lot of dog walkers use it and famillies on walks i have noticed. Vicarage lane on a weekend is always full of parked cars, the pavement adjacent to the woods rarely gets used from passers by but i would be guessing on exact figures, same on traffic, just waiting as Acer:biggrin:

[sloth]

Hi all, just thought I would poke my head in and say hi. I may not have contributed yet, but I am all ears (and eyes) on this thread. Thanks for taking the time to go through all of this, most appreciated.

 

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[daltontrees]

It looks like a well used woodland, the Stretview pic of the bend at Vicars walk shows good paths and signage. I don't suppose the actual figures matter in this exercise though. So far it feels like the risk zoning that I already use.

I see there is more than an order of magnitude of probability between Target Ranges in the QTRA system, but I am parking my concerns until the lesson is over. We can chat about it at playtime.

Edited February 18, 2013 by daltontrees

[Acer ventura]

I can see the value in qtra's output, but I'm just curious as to how it would stand up if, or when, something goes wrong, and someone is killed by a low risk tree? Which it could, because people win the lottery all the time....?

 

Hi Simon

 

Almost everyone who plays the lottery loses it nearly all time. The risk of wining it is so low, it's even less than being killed by a tree, at around at 1/14,000,000.

 

David Lonsdale used QTRA, as the defendant’s expert in Harry Bowen and Others v National Trust, in a failed claim for negligence brought against the National Trust, where one child was killed and three others badly injured. I believe he also used elements of QTRA in relation to Target valuation and tolerable risk in an unreported case, before QTRA was published and registered users were trained.

 

What is the next step if qtra throws up an unacceptable risk?

 

If the risk is unacceptable then there are a number of options. It might be possible to manage the target to reduce the risk. Or some pruning may also reduce the risk. Or if the tree is particularly valuable' date=' a case might be made to manage the risk to within a higher band of tolerability 1/1,000 – 1/10,000. Finally, the most suitable form of risk mitigation might be to fell the tree. What is especially useful with QTRA is that you can work out which component to manage, and by how much, in order to achieve a tolerable level of risk. Moreover, because risk is quantified, if a survey reveals a bunch of trees with an unacceptable level of risk, and the tree owner has a limited budget, it’s really clear which tree has the highest risk and needs dealing with first; and so on.

 

What if the qtra has been done wrong?

 

QTRA was controversially used in the ‘Poll’ case, where both experts ended up misleading the court as to the level of risk from the tree. They put the wrong inputs into QTRA and came out with a much higher level of risk. Unfortunately, as both experts made the mistake together in a joint statement that said the risk from the tree was high, rather than medium, the court was none the wiser.

 

An additional advantage of using QTRA is that you can see whether a component of the risk might have been 'massaged' to get the result the client wants, and challenge it.

 

Cheers

 

Acer ventura

[Acer ventura]

Hi Robin and others

 

Still nothing from Highways, so in the meantime here's how I marked my T values. The vehicular ones are estimates from what I would expect, but I am mindful that things might be different on the ground.

 

QTRA Ruff Wood Walkover.doc

 

Treating this exercise like it was a proper instruction, I contacted West Lancashire Borough Council (WLBC) Countryside Rangers to find out whether they might have some insight into how the wood is managed. Also to find out whether they are given any basic tree hazard recognition training. And out of courtesy to let them know what we’re up to on here, in case they hear about it, stray across it, or have a member of the public express concern. However, I couldn’t get to the person I needed to talk to.

 

It also appears that WLBC don’t have a Tree Risk Management Plan or Policy, like this;

 

http://www.n-somerset.gov.uk/NR/rdonlyres/CCA983EC-0291-4B9B-B2E7-8891E0042D02/0/Mar2012reviewNSCTreeRiskManagementPlanADOPTEDFebruary2011.pdf

 

So, those of you who are having a go with this, how close were you?

 

Any questions about the T values?

 

Based on the T values, any thoughts about where you might expect the highest potential risk to be?

 

Cheers

 

Acer ventura

[daltontrees]

As saui before, without actual usage figures or local knowledge the values can't be ascertained but the relative T values you have allocated seem appropriate to me. I would be focusing on the variations arising from individual trees at road junctions, proximity to residential buildings and pedestrian nodes nd any overhead utilities.

[WorcsWuss]

Thanks for your usual high standard of response David, much appreciated, I 'get it' now.

I'll continue to follow this with interest.

I wonder if such a system could be applied to construction.....

[Acer ventura]

A very helpful lady called Ellie, from Lancashire County Council’s highways, has kindly sent me the usual incredibly accurate traffic data, which these departments regularly capture, from a counting point at Ruff Lane to the west of Vicarage Lane, which confirms the T value is in Range 2.

 

From a virtual stroll down the road, it looks like the trees within falling distance of the Ruff Lane are in IP Range 2. This means for any of them to be regarded as an unacceptable level of risk, the PoF has to be as high as the 1/100 - 1/900 Range 3. For whole tree failure, that means it must be 10,000 times more likely to fail than our notional 1/1,000,000 optimised tree. This is a tree that would be so defective, its symptoms are likely to be readily apparent to a passing arborist from the comfort of their own car, and may even cause a momentary loss of concentration and drift whilst swearing at how ‘fecked’ it is. I’ve aligned the manual calculator to show the 1/4,000 RoH from a 1/100 - 1/900 PoF Range 3.

 

 

Even if some of the trees within falling distance of Ruff Lane exceed IP Range 2, and crossed over to IP Range 1, the PoF would still have to be as high as the 1/100 - 1/900 Range 3, because even if it was as high as 1/1,000 - 1/9,000 Range 4, the RoH is 1/20,000.

 

 

One of the Rangers, Dan, told me the main paths of the site are walked every two weeks by a Ranger, and they are all trained in basic tree hazard recognition. Apparently, the most common form of tree failure is snow-laden upper lateral branches of the Scots Pines. However, the signage at the entrance points doesn’t have a simple 'ring this number if you think there is a dangerous tree' annotation to it. This is an easy to do thing that would be worth considering because it’s in the HSE’s Sector Information Minute ‘Management of the Risk From Falling Trees’. Having this in place would leave any HSE Inspector with at least one less stick with which to potentially beat the council with in the event of a risk being realised.

 

All this detail can be got hold of and sketched out before visting the site.

 

I’ll explain how we would go about the 'Walkover Survey' when visiting the site later.

 

Cheers

 

Acer ventura

[Acer ventura]

Thanks for your usual high standard of response David, much appreciated, I 'get it' now.

I'll continue to follow this with interest.

 

Hi Simon

 

Thanks for that. If the legal cases I cited interest you, there’s a wealth of information at the QTRA Case Law page

 

QTRA Case Law

 

I did draft most of a reply to your bending moment question, but I’m trying to see whether I can edit and speed up a video of a tree ‘mass damping’ to illustrate the point.

 

Cheers

 

Acer ventura