Wind gusts and street tree branch failures

From the popularity that was associated with my post on wind-related tree deaths in the USA, I thought I’d build slightly on how wind can create hazards by going over a really interesting article in the book Tree Structure and Mechanics Conference Proceedings: How Trees Stand Up and Fall Down. The article looks at how, over the period of 1992 to 1999 in Rochester, New York, wind gust speed impacted upon the rate of street tree branch failures. Because data relating to such failures was held electronically, and old climatic data is ‘easily’ obtainable, the methodology was very simple and solely involved relating branch failure date to the wind speed in that area at the same time. The study is also not lengthy and was in fact created on the back of this study, but the data contained within is absolutely brilliant and so it’ll certainly be of interest to many reading this.

As a means of setting the scene so to speak, the authors first establish two things: (1) when branches failed, and (2) when wind gusts were deemed significant (in this case, over 40mph for over 5 seconds).

In relation to the first point (see the below graph), it was found that branch failures were most frequent during the months when foliage was present upon the trees (May to October / November) – specifically, July, August and September (even including the 243 failures in strong winds in September 1998) had a very high number of branch failures in comparison to other months. In a way, this is to be expected, as full leaf crowns will drastically increase wind resistance of a crown. Branches will therefore be loaded not only by greater weight from the leaves they are supporting, but the leaves they are supporting will have a large surface area that can readily catch in the wind (which will further increase the loading upon the branch, pushing it closer – or beyond – it’s safety factor). Furthermore, if it’s raining, then the water caught by the leaves will yet further increase the weight the branch will bear.

The number of branch failures, per month, in Rochester. Note that 243 failures occurred during September 1998, as a result of strong winds.

However, when looking at when the strongest wind gusts were, we can observe something peculiar – the frequency of such gusts is almost an inverse of the rate of branch failures across the calendar months (see below). This suggests that it is not the frequency of strong wind gusts that correlates with branch failure, but simply the occurrence of such a wind gust.

The number of wind gusts of 5 seconds or greater, and how strong such gusts were (see legend key).

From here, the authors then look to segment the total number of branch failures into categories relating to wind speed. As somewhat expected, there is an exponential increase that begins at around 40mph, and discernibly starts to rise at 50mph. The lack of 70-79mph data is because no such gusts occurred during the survey period. The data is shown below.

The number of branches failures in Rochester’s street tree population, and at what wind speed the branches were overloaded at.

Pulling apart this data further, the authors sought to compare at what wind speeds branches failed at, during both the period in which foliage was present, and when foliage was not present. Results for the branch failures when foliage was present (shown below) are interesting, in the sense that it appears to be that wind gusts alone are not the cause of failure (as noted by the high spike of branch failures at 43mph). Of course, there is marked correlation between wind gust speed and branch failure rate. The authors in fact suggest that the effect of the observed wind gusts on branches may be exacerbated by simultaneous (or prior) precipitation, drought (where wood rays become very dry), and previous stronger and more sustained gusts. Therefore, calmer winds in ‘right’ conditions may at times more readily cause branch failure than when stronger wind gusts occur and such aforementioned influencing conditions are not evident.

Branch failure frequency during the time when foliage was still present upon the tree, in relation to the wind gusts that caused such failure.

Diverting attention to the leafless period, there seems to be little correlation between branch failure and wind gust speed (as shown below). Perhaps, other climatic faactors (snow, ice, etc), in conjunction with wind loading, will induce failure in such instances; and in times where snowfall or ice accumulation is significant, branches may certainly fail under such extreme loads.

Gusts that induced branch failure during the leafless period.

So what can we gain from this study? Hopefully, a lot. Admittedly, the sample size was limited (only eight years and in one city), though it goes to show how, at least for broadleaved trees, wind gusts during the summer will indeed be more likely to cause structural failure. Without question, we must not ignore other loading factors that may combine with wind, during the growing season (most probably precipitation). During the winter (leafless period), it seems wind gusts alone are far less likely to correlate to any degree with branch failures, and instead wind gusts may be significantly aided, through the presence of snow or ice, if branch failure does occur. Failure may even occur independently of wind gusts.

Source: Luley, C., Pleninger, A., & Sisinni, S. (2002) The effect of wind gusts on branch failures in the city of Rochester, New York, U.S. In Smiley, E. & Coder, K. (eds.). Tree Structure and Mechanics Conference Proceedings: How Trees Stand Up and Fall Down. USA: International Society of Arboriculture.

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