Death by wind-related tree failure – a USA case study

Whilst deaths associated with tree failures do happen, they are generally very rare. In the UK, for example, the HSE reported that the chance of dying because of a falling tree (or tree part) was 1 in 10,000,000 when excluding high wind events (you can access the report here – see page 23). In the US, because of more extreme weather events, mortality rates may be slightly different, and the author of this study seeks to invesitgate exactly how many deaths occurred (during wind events) between 1995-2007, and the type of event that caused the tree to fail.

Before delving into the data however, the author notes that tree species vary in their tolerance to wind gusts. Wood properties and structural morphologies will impact upon durability, and by-and-large (for uprooting and stem snapping, though not branch failure) hardwoods will fare ever so slightly better in wind gusts than softwoods (though all tree species can fail if they are subjected to ten minute gusts of over 67mph). However, the condition of the tree (health), its age, size, and setting (how exposed it is) will also impact upon how likely it is to fail under wind loading.

Back on track, data was trawled from multiple sources (from the USA) to identify all wind events (extratropical cyclones, thunderstorms, tornadoes, tropical cyclones, though also ice and snow accumulations) where there was at least one death. All of the events were then inspected for deaths caused by a tree falling, and all such deaths were recorded. In the information obtained, multiple aspects relating to the deaths were collected. For example, victim age and gender were noted, as was the type of event that caused the death. In addition, the location of the death was noted, including what the victim was in at the time (house, vehicle, outside).

In total, there were 407 tree-related deaths between 1995-2007 (an average of 31 per year) across 41 states. New York had the highest death rate at 30, then Washington at 29 (see the table near the end for a greater breakdown). 62% of victims were male, and the average age was 44. In terms of where the victim was at the time of the tree indicdent, 18% were within their home (mobile or ‘static’), 38% outdoors, and 44% in a vehicle. Below, we look at deaths associated with fallen trees only during different weather events, though the below table and map outline all fallen tree-related deaths and their locations very well.

treedeathwind
A breakdown of the total number of deaths between 1995-2007.
treemapdeathwind
A localtion showing all of the locations where trees caused a death during wind events. The numbers down the side appear to relate to total deaths at each location.

Thunderstorms

53% (165) of all deaths were during thunderstorms, and 96% of these deaths in the eastern half of the USA were due to fallen trees (because the eastern half of the USA suffers more thunderstorms, and has a higher population density and number of trees). 78% of deaths occurred between May and August (when most storms occur). Most deaths (87%) occurred when the victim was not within their home.

Nonconvective winds (extratropical cyclones, gap winds, downslope winds)

A total of 143 (46%) deaths were caused by fallen trees during such winds. 88% of all deaths occurred outside of the home, with 50% being when the victim was in a vehicle. October to April was when 88% of deaths occurred, as this is the time when such extratropical cyclones are most potent.

Tropical cyclones

15 tropical cyclones caused 57 deaths, though not including deaths from Hurricana Katrina in Louisiana and Mississippi. Deaths were observed at similar rates across all victim locations, be it in the home, outside, or in a vehicle, and all deaths occurred within 200 miles of the coastline. September was when 58% of all deaths were recorded, and the range was from July through to October. The table below breaks down the deaths by each cyclone.

cyclonetreedeath
Deaths associated with tropical cyclones between 1995-2007.

Tornadoes

Fallen trees as a result of a tornado caused 28 deaths, accounting for only 4% of all fallen tree-related deaths. 42% of all victims were killed in their own home, which was higher than 32% in a vehicle and 25% outdoors (though not being in a home still leaves an individual with a higher chance of dying, during a tornado event). Deaths as a result of tornadoes occurred all year round, though 32% were in either April or November. Wind speeds of of 70-90mph (or above) are most likely to cause death, during a tornado event.

tornadotreefall
An uprooted tree that, during a tornado event, fell onto a house. Source: NBC News.

Snow and ice

Ice accumulations were the cause of 10 of the 14 deaths within this category, and most deaths happened whilst victims were outdoors. Snow accumulations did however cause death, too. Most deaths were concentrated in the north east and Washington, which correlates with expected weather patterns for the USA.

Final remarks

In light of all of the above data, it is evident that thunderstorms and nonconvective winds will cause most deaths, of which most will be outside of the home. The higher likelihood of being killed within a vehicle is interesting, as it suggests that it may very well be street trees that cause deaths. In a way, this is hardly surprising, as trees within a woodland are likely to not have a target zone, though it does highlight the fact that consistently passing within the target zone of trees during loading events increases an individual’s chance of death at the hands of a falling tree. Staying within the home may very well be the safest thing to do. Or, as the author suggests, get in the car and drive into the middle of an open field, and wait there (unless it’s a tornado event, where one must either seek refuge in a large building or drive to beyond the reach of the tornado).

Despite this, the chance of dying is still only 1.45 in 1,000,000 (on average – some states have higher death rates, as shown in the below table). Of course, in states where there is a higher population density, the risk of death may likely be higher than the national average.

statetreedeath.jpg
How the total number of deaths were split amongst the ‘top’ (meant in the worst way possible!) 15 states, of the 41 where deaths occurred.

We should also remember that this data doesn’t take into account injuries associated with fallen trees, or even indirect deaths caused by trees blocking roads (stopping emergency vehicles), falling on power lines (cutting out electrical power), and so on. Furthermore, the window of investigation spanned only 13 years, which isn’t much in the grand scheme of things. Trees with structural defects may also be more likely to fail during loading conditions, and therefore street trees, or those where there is a target zone beneath, should be inspected more regularly than those where there is not. Granted, a touch of realism is needed, and it must be recognised (again) that the death rate as a result of a fallen tree is incredibly low to negligible.

Source: Schmidlin, T. (2009) Human fatalities from wind-related tree failures in the United States, 1995–2007. Natural Hazards. 50 (1). p13-25.

Feel free to discuss this either here or on Arbtalk.

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Death by wind-related tree failure – a USA case study

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