In the last issue of VM Insights I stated that VM requires specific annual funding to maintain equilibrium. Borrowing a term from forestry, it was said that funding must provide for the removal of the annual volume increment (AVI). Funding below what’s needed to remove the AVI results in increased future costs and tree-related service interruptions.
If we are to take this beyond a conceptual understanding, we need to quantify the AVI value. Doing so requires identifying the area or number of each work type and the average maintenance cycle for each. Determining the maintenance cycle requires quantifying tree/brush growth and re-growth rates.
Let me use conditions in my home province of Alberta to illustrate. Within the right of way, the fastest growing trees are various poplar species. The re-growth rate after mowing is 2 meters over the first three years and then 0.5 m/yr thereafter. If the maximum height we can tolerate under the conductors is 5 m, then we can calculate the mechanical maintenance cycle to be nine years.
For herbicide work, the threshold is set by provincial regulation that specifies either a maximum treated height depending on public visibility of the treatment area or more restrictive height limits imposed by herbicide label conditions. Therefore, the herbicide cycle varies with area. For treatment area visible from rural roads the height limit is 2.5 m, which results in a 3-year maintenance cycle. (Are you questioning my math, Sparky? You think it’s four years? Well I’m glad you caught that but in four years, the average height would be 2.5 m, which by the definition of average means half the brush would be greater than 2.5 m in height and, therefore, illegal to spray.) When all the area for the work method is identified and the cycle determined, then the amount of work that needs to be done annually can be calculated. To arrive at the AVI value, the units by work type times the average unit costs are summed.
We need also look at what work lies outside the right of way. This will involve some pruning. Also by far, the majority of hazard tree removal work will originate outside the right of way. From work done examining the extent of natural tree mortality, I believe we as an industry have generally underestimated this aspect of the work. Consequently, I would recommend against using historical spending in the determination of the AVI. Rather, total danger tree exposure should be determined and local annual tree mortality rates applied.
Having worked as a utility forester, I know the question that would arise from my senior managers would be “So if we fund the VM program at the AVI value we will never have a problem with increasing outages and costs going forward?” Sorry folks, but this is where utility foresters must prevaricate and raise the issue of biological variability. As long as average conditions prevail, the answer is yes. However, an extended period of higher than normal moisture or drought or the introduction of a new pest can have a major impact, changing the balance and the AVI for years.
The approach I’ve delineated being based on averages represents the base case. The influence of modifiers such as drought and pests requires subsequent and separate quantification.