Questioning tree risk calculation


I sent an email this morning to treelogic asking them about their(31 page) QTRA assessment of Laman Street. I may not receive a reply but they seem to be a good organisation, if their website and downloadable resources are any guide, so I hope I do.

Their risk assessment of the trees in Laman Street has been made available on council’s website as one of the key documents.

The appendices to their report are fantastic, where they put tree risk into the correct perspective and point out the obvious which is that tree risk needs to be taken into account with other factors such as the value of trees to the community. The other paper they provide is Mike Ellison’s explanation of QTRA as an assessment tool.

My email to them was about the figures they used to come up with the scary 1 in 19.8 risk rating that some councillors are rightly terrified of and council officers use to justify the removal of the trees. This is the email:

I’m a Newcastle resident keen to save the remaining figs in Laman Street that Dean Simonsen assessed for our council.Your firm was the first place I rang when I heard that our council planned to remove the trees and a kind gentleman told me about ground-penetratng radar and made me aware that as you had done work in Laman Street for our council you couldn’t help me.

The council have at last made public the QTRA assessment done by Mr Simonsen on the trees and council will be voting again in May, after a 5 month reprieve, to keep or remove the trees.A community workshop is happening at the end of this week for interested residents.

I have some questions about the calculation of the risk the trees pose and would be grateful if you’d have a look at them. I work in health and my only knowledge of trees, like art, is that I know what I like.

On page 5 in ‘people’ the time that cars will spend near the trees is calculated; I assume this figure is not relating to a storm or other weather event when the figure would be almost zero (?). (I stupidly tried to get in my car and drive to my mother’s when that storm started because there was a blackout but the roads were basically undrivable so I went inside sopping wet having got nowhere.)

The probability of failure in 2007, the Pasha Bulker storm year when 2 trees failed, is given as 1 in 7.5. Can I ask with respect why this figure also doesn’t approach zero since the trees still standing didn’t fall over in that storm or any other? Our city was told it was a 1 in 100 year event.

I think your website and your appendices to that document are just  fantastic but don’t seem to be read by councillors or council officers. They are simply terrified by that 1 in 19.8 figure and read no further and they don’t  seem to take any other considerations into account.

I know QTRA users are proud of its mathematical and scientific basis and so  I wonder if you think there’s any merit in my QTRA-uneducated alternative calculation?

It presumably makes no difference to your work here in Newcastle  but GBG did a ground-penetrating radar investigation of the street for council in December and found apparent radial roots around ten of the fourteen trees. This report three months later is yet to be released by council. I acquired a copy via FOI which was the only way council would agree to release it.

I appreciate the work you do and recognise that you are not in any way obliged to reply to me. I’m just trying to do my best to get council to look at preservation measures as well as risk. Yours sincerely, Caity Raschke                 Home


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One Response to “Questioning tree risk calculation”

  1. Ali Says:

    Well put!

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