Polygon RPA

[kevinjohnsonmbe]

Just seeking a view from learned arb talkers. RPA, under normal circumstances, defined by a circle from the centre of a tree. There is allowance, subject to justification, to apply a polygon rather than a circle.

 

If we take that as a given as detailed in 5837, and if we acknowledge that a square is a form of polygon, can we therefore have a square RPA (if the ground conditions / circumstances require)??

 

I've purposely avoided trying to provide example of the types of conditions that might demand it so that we don't get tied down in detail / counter point etc. it's more of an academic question than a practical one because roots wouldn't normally grow in squares (unless, perhaps potted or constrained but there I go getting into the practical rather than staying in the theoretical!)

 

Any thoughts??

[Jesse]

Just my view, Polygon, circle, or box, and the new 5837 does not really account for much, lets understand the growth and development of the tree first, if its worthy of retention ??? Root morphology and function and development in relation to a specific given site first,and the changes that mite happen, then lets look at soil type, then lets look at ground level conditions, loamy now compaction then, then consider root severance, historical and what the developer intends to do. In short, trees no longer are really a constraint .

[ScottF]

It states in the standard that it can be drawn as a circle or a square, and this has been the cause of a great deal of pointless debate in planning meetings and appeals over the years. As you can imagine if you superimpose a square and a circle or square of equal value the square variously ducks inside the line of the circle or sticks out beyond it. I've seen many consultants use the square calculation in order to give them a few perceived centimetres of RPA clearance to squeeze in a building. In my view this approach is not only disingenuous and missing the entire point (outlined well above by Jesse) of why we undertake these assessments in the first place. That's not to mention the point that if you rotate the circle by 45 degrees this daft perceived dodge is negated.

 

My approach when we draw these notional RPAs (as we must, unfortunately) is to draw them as circles (since if we're guessing where the RPA is, a fair assessment is that it's equidistant from the stem), trim where there are hard boundaries (walls, buildings but not always surfaces) and aim to "make up" the area trimmed off. You can end up with any sort of strange shape for an RPA if it's strongly constrained. Just think of a tree growing in a narrow central reservation- I've seen these felled and dug up on sites and the roots can run in long narrow extents for massive distances.

 

The RPA calculation and area plotting is already a bit of a fudge, if we're honest. Trying to "fudge a fudge" seems a bit pointless to me, but many try.

[Paul Barton]

I think the key point here is "modifications to the shape of the RPA should reflect a soundly based arboricultural assessment of the likely root distribution" (BS5837 - 4.6.2)

 

 

My approach when we draw these notional RPAs (as we must, unfortunately) is to draw them as circles (since if we're guessing where the RPA is, a fair assessment is that it's equidistant from the stem), trim where there are hard boundaries (walls, buildings but not always surfaces) and aim to "make up" the area trimmed off.

 

Same here Scott. I trim RPAs where they intersect with buildings and walls etc but have found road surfaces more problematic. Recently a tree officer insisted that I showed RPAs as having no roots at all under an adjacent road - therefore enlarging the RPA on the opposite side where there was unsurfaced ground. Fair enough you might think, but that doesn't explain how street trees survive surrounded by tarmac!

 

I am inclined to slightly offset RPAs away from roads - to show that although some roots may be present they are likely to be fewer.

 

Generally I show the altered shapes as circles trimmed on one site - my CAD program automatically enlarges the remaining circular shape to maintain the overall area. It saves a lot of time adjusting a polygon to achieve the right area.

 

You can end up with any sort of strange shape for an RPA if it's strongly constrained. Just think of a tree growing in a narrow central reservation- I've seen these felled and dug up on sites and the roots can run in long narrow extents for massive distances.

 

Very true - have you ever submitted a plan showing a really oddly shaped RPA? I have a site at the moment where the trees grow in a narrow grass verge with tarmac on one side and a compacted track on the other side. In my opinion the RPA is likely to be a long thin oblong which includes the whole verge - which would be handy for my client but I expect the tree officer would query this!

[kevinjohnsonmbe]

Brilliant, thanks all! All really valid points, relevant and useful. A great help.

[Amelanchier]

I do the same as Scott and Paul. I think most important thing to do when adjusting the shape is to justify yourself - it only takes a paragraph or an annotation. On the flip side, in the event that I use the circular model I feel obliged to justify that also.

 

As far as weird polygons go, I have an ongoing site with a row of Pops between a river, some buildings and bridges. It took most of a day trudging around with a probe, crowbar and an auger just to work out where the viable rooting areas are and the resulting RPAs are as far from circular as you were ever likely to see. Most were vague asymmetric 'S' and 'T' shapes and one even had to be argued down in area - it just wasn't physically possible for it to follow the model given the reality of the site.

 

Recently we had the opportunity to put a hand dug root investigation trench in across the limit of one of the more speculative sections of one of these RPAs to inform the feasibility of a trench footing. Given the distance and convoluted path to the trench from the nearest Pop, I was expecting a number of shallow small diameter (<10mm) roots just under the hard surface but instead we found a single 28mm root disappearing off outside the RPA... I couldn't justify reducing the other extents (they were too obvious) so I just increased the provision for that tree.

 

A model is just a model.

[kevinjohnsonmbe]

I do the same as Scott and Paul. I think most important thing to do when adjusting the shape is to justify yourself - it only takes a paragraph or an annotation. On the flip side, in the event that I use the circular model I feel obliged to justify that also.

 

As far as weird polygons go, I have an ongoing site with a row of Pops between a river, some buildings and bridges. It took most of a day trudging around with a probe, crowbar and an auger just to work out where the viable rooting areas are and the resulting RPAs are as far from circular as you were ever likely to see. Most were vague asymmetric 'S' and 'T' shapes and one even had to be argued down in area - it just wasn't physically possible for it to follow the model given the reality of the site.

 

Recently we had the opportunity to put a hand dug root investigation trench in across the limit of one of the more speculative sections of one of these RPAs to inform the feasibility of a trench footing. Given the distance and convoluted path to the trench from the nearest Pop, I was expecting a number of shallow small diameter (<10mm) roots just under the hard surface but instead we found a single 28mm root disappearing off outside the RPA... I couldn't justify reducing the other extents (they were too obvious) so I just increased the provision for that tree.

 

A model is just a model.

 

Thank you Tony. All understood!

[Paul Barton]

I did a plan for a site today that has lots of large diameter trees in a very tight space, between retaining walls on two sides and a building on the other. I could not plot any RPAs large enough to accommodate the area calculated according to the BS! ...yet they don't fall over..!

 

Reminded me how limited our estimations - and representations on a 2D plan are.

[kevinjohnsonmbe]

I did a plan for a site today that has lots of large diameter trees in a very tight space, between retaining walls on two sides and a building on the other. I could not plot any RPAs large enough to accommodate the area calculated according to the BS! ...yet they don't fall over..!

 

Reminded me how limited our estimations - and representations on a 2D plan are.

 

It's a thought provoking 'flip' on the text book standard Paul. What would you make of it, my first thought was - the trees have compromised / adapted to suit their available situation but would they have grown with more vitality / vigour if they'd had more available space, light, nutrients? I guess the key difference between your situation and an attempt at an argument to reduce an RPA for an otherwise uncluttered tree would be that the trees in your example have grown into available space rather than an existing tree having it's RPA reduced by a proposed development.

[ScottF]

The standard encourages us to think of roots as occupying areas when they actually occupy volumes.

 

Paul, in your constrained rooting area example I wonder if the trees would still be vertical on a shallow substrate where they couldn't make up their root protection volume by going down?

 

Dunno.