RPA's of reduced trees

[Colin Bashford]

You're damn skippy its arbitrary. The key point in the BS is the bit I highlighted in bold above

 

"...which may change its shape but not reduce its area..."

 

So; take for example the TPO'd x Cupressocyparis leylandii (oh yes!) on a development site I looked at today.

Mature specimen tree in the corner of a raised lawn. Corner is formed by two roads. Lets say a 1m dbh for arguements sake (it was bigger and multistemmed) Developers RPA?

 

12m circle. Shown on the map as including the roads...durrrrr why would the roots opt for road when there is a perfectly good lawn?

 

Well they wouldn't so a more accurate rpa would use the same area of the circle (452m squared) and apply it to the lawn to give a 21 x 21m square. (bye bye to your proposed footed hard surfaced access driveway mate!) But that assumes the roots grow in a square I hear you say. Yep. so lets calculate another way...

 

Its in the corner so maybe its fair to say the roots would have exploited the space in a quarter circle fashion? Seems ok? The guidance says we need the same area so we need to calculate a circle that has four times that area that we want (a quarter remember).

 

4 x 452 = 1808m squared

 

1808 / 3.142 = 575

 

square root of 575 = 24m radius

 

So a quarter circle 24m radius from the tree base out into the lawn. Bloody big but slightly more representative of what the RPA might be. Now apply you knowledge of root morphology. Soil type, species, age obstacles...This particular tree, we might comprimise on a 18-20m radius quarter circle given the vigour and age of the tree and its ability to deal with the loss of area. But don't tell the developer that - he hasn't bothered to do the calculations and I don't work for him!

 

 

Pete -

The 12x modifier is referenced in table 2 of the BS as follows:-

 

"NOTE The 12× multiplier is based on NJUG 10 [9] and published work by Matheny and Clark [10]."

 

Where the refernces are:-

 

[9] NATIONAL JOINT UTILITIES GROUP (NJUG). Guidelines for the planning, installation and

maintenance of utility services in proximity to trees. Publication number 10. NJUG, April 1995.

 

[10] MATHENY, N. and J. CLARK. Trees and development. A technical guide to preserving trees during

land development. 1998. International Society of Arboriculture, Champaign, IL 61826-3129. 184 p.

 

Thank Tony this is one of the best explanations I have heard since we wrote 5837.

 

Colin

[Andy Collins]

What never ceases to amaze me is the fact that developers, (with architects in their employ) fail to observe the basics of 5837. Or the rudiments of the planning consent. In my experience, unless there is an on-site arborist to hand, they will just brush all the above regs under the carpet. RPA's are there to be ignored, in their book, a mere hindrance to the greater scheme of things. It wasnt until I threatened a visit from the Enforcement Officer that they listened. Manual dig was specified for RPA's on site, I found them one morning in there with a 360. This resulted in a visitation of the Enforcement team, and emergency TPO's on all trees. The foreman even stated to me that the developer normally employs an arb consultant to "cover-up" for these intrusions. But as I'm employed by the client, I'm not in his pocket. What these developers dont seem to realise is that these breaches of regs infringe the planning consent, as such if a Stop Order were to be placed on site, they would have to re-apply for the Planning Permission for the whole build on that site, which could be refused as they havent obeyed the rules in the first place.

Working closely with the LPA is a must, get them on your side, keep them informed, including reporting bad practices.

[Paul Barton]

Great thread here - just learning about all this stuff on the Tech cert - good to have some more 'realistic' insight in to the goings on on site.

 

Great example Tony, really helpful.