T&D World Magazine

Storm Damage and Climate Change

I intend to go further into Why Asset Management Leads to More Tree-caused Interruptions, but sometimes events emerge that are just too irresistible and invite providing the pin to the balloon.

In the aftermath of Hurricane Sandy, I’ve seen numerous articles where concern about the influence of global warming has been raised. The public perception seems to be that we are experiencing more frequent and violent storms, and there are those who say we should expect this trend to continue.

I have yet to see data that convinces me that storms are more frequent and more violent. If we plot wind speeds experienced, they do not fall outside the historical range of variability around the mean. On the other hand, I don’t dispute that storms are causing increasingly more damage and more costly damage. However, I believe there are other explanations than climate change. Certainly, through the effect of inflation, storms damage costs escalate. You cannot replace the house your father built in 1960 for $50,000 for anything near that amount. Further, if you think back a hundred years then it’s easy to see that we have built more houses, offices and other structures and facilities. What occupied this space before? If it was farms, a tornado resulted in crop losses rather than the destruction of tens upon tens of houses. In other words we’ve put up a lot of valuable infrastructure, which under adverse weather conditions becomes storm interceptors.

Ten years ago, I was working on a project for National Grid Transmission. Tom Sullivan, who was the manager at the time, wanted a better understanding of the risk of tree-conductor conflicts arising from beyond the maintained right-of -way. He had observed that very few transmission outages arose from within the right of way but his perception was that frequency of off-right-of-way outage incidents was increasing. He was concerned that this would get worse in time as many of these outages were caused by emergent white pine. When I asked why now, what had changed, he put forward a very plausible explanation: one that I think affects many if not all utilities but is not generally being considered. Tom told me if we pictured the service territory 100 years ago we would see most of it had been cleared, either to harvest the timber and/or for agriculture. Since then these small farms had been abandoned and eventually the land reverted to forest – forest that was now 80 to over a 100 years old.

One of the things we examined in the project was the percentage of white pine in the forest population. White pine, being a dominant species, would eventually grow above, emergent to the general canopy. If there was a large number of white pines that had not yet grown above the deciduous canopy, then indeed, over time there would be more pine tops acting as sails. Due to continuing growth, the wind loading on these trees would increase, making them more susceptible to failure.

There are a couple of more points that need to be appreciated. Consider a tree of such height that only on a fall at exactly 90° to the line would it make contact. That is, the line has a limited exposure to the tree. However, as the tree grows taller, there is an increasing arc of exposure or an increased probability of a line contact on tree failure. Further, as the trees grow, the distance between the conductor and the point where we find that tree requiring a fall at 90° to the line increases. The depth of the utility forest increases and the impact is that the line is exposed to more trees that could intercept it on failure.

The last point has to do with trees as generating systems. They continue to grow and as they do, the amount of energy it takes to sustain the expanding mass also increases. With the passage of time the availability of water, light and nutrients becomes critical and if these are limiting the energy available falls below what is needed to sustain the mass. While trees may grow for hundreds and even thousands of years, if we consider the trees that arise on an acre of land, most of them will die before attaining 150 years of age. In fact, I believe based on forestry stand data for both even and uneven age stands, we could say most die before attaining 20 years of age.

I think these facts, a tree population facing increased stress loading, an increasing number of trees capable of striking a line on failure, the increased arc of exposure or probability of line contact on failure and natural tree mortality applied to that taller, heavier tree population explains why we are seeing increasing levels of storm damage to electrical systems. We don’t need to speculate about climate change. Doing so distracts from researching things, such as the changing risk trees pose to the electrical system, which could provide immediate tangible benefits.

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