QTRA... V.T.A..... Or Lantra Pro tree inspection

[arb culture]

Pretty bloomin close though Tony.

 

Bear with me and you'll see why I ask these geeky questions, there is something I really want to know.

 

[arb culture]

To the client, HSE, law courts, the family of the people affected, the local authority who decides on whether to provide an extra lollypop person or to prune loads of trees.

 

How do QTRA users suggest the threshold is set, assuming the client is a non arb with no specialist knowledge of risk management?

[Amelanchier]

Pretty bloomin close though Tony.

 

Bear with me and you'll see why I ask these geeky questions, there is something I really want to know.

 

 

Ok then. I'll bite - hook, line, sinker and copy of Angling Times.

 

With QTRA we generally use a 1 / 10,000 threshold. Although this is a guide and the threshold should be discussed with the client.

 

IIRC its based upon the HSE's ALARP principle and other funky documentation that I can't currently recall, essentially that there is little practical gain in trying to reduce risks to above 1 /10,000.

Edited January 21, 2010 by Amelanchier
sp

[Amelanchier]

Just for wider understanding. From the ISA Journal of Arboriculture Vol 31:Number 2 March 2005 by Mike Ellison.

 

"Acceptable Risk. We are constantly exposed to and accept or reject risks of varying degrees. For example, if we desire the convenience of electric lighting, we must accept that, having implemented control measures such as insulation, there is a low risk of electrocution; this is an everyday risk taken and accepted by millions of people. When evaluating tree-failure hazards, two types of risk must be considered. We must consider the person upon whom a risk is imposed as with the neighbour of a tree owner, and the person who accepts some degree of risk in return for a benefit, such as a tree owner or visitor to a woodland or forest. Having considered The British Medical Association Guide "Living with Risk" (Henderson 1987) and with particular reference to the conclusion "few people would commit their own resources to reduce an annual risk of death that was already as low as 1/10,000", Helliwell (1990) suggests that 1/10,000 might be a suitable figure to start with as the limit of acceptable risk. Furthermore, "For members of the public who have a risk imposed on them 'in the wider interest' HSE (Health and Safety Executive) would set this limit at 1/10,000 per annum" (Health and Safety Executive 1996). In the management of trees, a property owner or manager might adopt the 1/10,000 limit of acceptable risk or choose to operate to a higher or lower level."

[arb culture]

Nice one, thanks Tony:001_smile:

 

Firstly, let's get the higher and lower risk sorted out. A 1/2 risk is higher than a 1/10 risk (ie 0.5 is greater than 0.1) therefore a 1/10,000 risk is greater than a 1/1,000,000 risk.

 

Now, if you rely on the same sources as the authors of QTRA (the 'living with risk' book I mention earlier) the 1/10,000 threshold is just one of two thresholds. The other being 1/1,000,000.

 

According to this research on which QTRA is based, any risk below 1/1,000,000 is not worth bothering with and anything between 1/1,000,000 and 1/10,000 should be reduced as far as is reasonable. Anything greater than 1/10,000 should immediately be dealt with irrespective of reasonableness.

 

So I understand why trees with a greater than 1/10,000 risk are considered dangerous under the QTRA system. However, I don't understand why trees between 1/10,000 and 1/1,000,000 are deemed acceptable. This seems contrary to the research on which QTRA is based.

 

My main query though is that I don't understand how QTRA deals with a situation where there are lots of trees. How is the total risk from all trees on an estate or a highways network worked out?

[Bundle 2]

Other threads discuss 1/10,000

It starts here...

1:10,000

 

And can be followed on from here...

UKTC Archive :: tree-care.info

Edited January 21, 2010 by Bundle 2

[arb culture]

Thanks Tim, but perhaps you could just let us know how you as a qtra user would calculate the total risk from many trees.

 

I've trawled through most of the links, but I couldn't find an answer.

[Amelanchier]

According to this research on which QTRA is based, any risk below 1/1,000,000 is not worth bothering with and anything between 1/1,000,000 and 1/10,000 should be reduced as far as is reasonable. Anything greater than 1/10,000 should immediately be dealt with irrespective of reasonableness.

 

So I understand why trees with a greater than 1/10,000 risk are considered dangerous under the QTRA system. However, I don't understand why trees between 1/10,000 and 1/1,000,000 are deemed acceptable. This seems contrary to the research on which QTRA is based.

 

My main query though is that I don't understand how QTRA deals with a situation where there are lots of trees. How is the total risk from all trees on an estate or a highways network worked out?

 

Fine questions. The first I'd suggest is only answerable by Mr Ellison. I ain't got a clue! Very interesting though - I'd guess he'd point the finger at the HSE and Helliwell?

 

The second question is bigger, and as I'm an infrequent user i've only ever used the system on up to 600 significant trees on any one site. In that instance, I broadly categorised target zones based on sampling pedestrian / vehicular usage. That allowed me to systematically prioritise works for the client - I didn't bother working beyond the basic calculations (the wheel of fate hama!).

 

I don't think we need to have an calculation for total risk, outside of hypothetical circumstances its rare that any one target is threatened by more than say, 10 -15 trees at a given moment. Static targets don't meet a lot of trees and mobile targets stay under a tree for perhaps 10 seconds at most?

 

Really with QTRA should be looking at this the other way round - rather than how likely is the failure; how likely is the target being in the wrong place at the wrong time? And because a target is not likely to be exposed beyond their own little 'risk bubble' the effects of tree population matter less...

 

You can see I'm not a full time advocate right!

[Amelanchier]

Thanks Tim, but perhaps you could just let us know how you as a qtra user would calculate the total risk from many trees.

 

I've trawled through most of the links, but I couldn't find an answer.

 

I'll try to find a comprehensive example in the QTRA forum and post it here.

[arb culture]

If I was in charge of many trees (ie a local authority parks department) I would want to know total risk from trees to allow me to make budget decisions.

 

Ie should I spend more on preventing deaths from people drowning in the ponds, or on people tripping over uneven paths, or on undertaking tree work?

 

A quantified system should allow this to be straightforward. eg if the total risks from trees is more than 1/10,000 then I must deal with that. If the total risk is between 1/10,000 and 1/1,000,000 then I have to do what is reasonable. If the total risks from the uneven paths is greater than the total risks from trees then it would be reasonable to prioritise the work to the paths etc...

 

Anyway, it's time for wine and rubbish tv I think.