Revision to the Tree-Related Damage Indicators of the Enhanced Fujita Scale

Organized under the direction of the American Society of Civil Engineers (ASCE)/Structural Engineering Institute (SEI)/American Meteorological Society (AMS), the Standard on Wind Speed Estimation committee formed a working group to revise the tree-related Damage Indicators (DIs) in the Enhanced Fujita Scale. This presentation will summarize the proposed changes to the tree-related DIs. In the proposed revision, the previous DIs of hardwood trees (DI 27) and softwood trees (DI 28) have been replaced by a single-tree DI that is partitioned into two sub-DIs based on tree size. Shrubs and small trees are excluded. Trees with diameter of less than 38 cm at a height of 1.4 m are considered intermediate trees and those with diameter greater than this threshold are considered large trees. For lower DoDs, intermediate trees are assigned a slightly higher wind speed than large trees, because the flexibility of small trees makes them somewhat more resistant to broken branches, uprooting or trunk breakage. However, for extreme damage (highest DoDs), the wind speed estimate of intermediate trees is slightly less than large trees. The previous DoD of “trees debarked” has been eliminated in light of the determination that debarking is more likely a result of flying debris than of wind speed per se. Uprooting and snapping (trunk breakage) of trees are retained as distinct DoDs but are assigned the same estimated wind speed. A new higher DoD (DoD 7) is added based on whether trees are bodily removed from the ground and thrown (lofted) to a new location. This proposed single-tree DI considers several influential features of the subject tree or conditions in making an estimate of wind speed, including tree species, soil saturation, rooting depth, ratio of tree height to tree diameter, presence of obvious wounds or decay, presence of a heavy crop of fruit or nuts; and whether a tree is in leaf-on or leaf-off condition. A proposed list of notably strong and weak species is provided in the documentation.

In addition, a newly-proposed multi-tree DI (FOR, for forest) closely follows the multi-tree aspects of the tree DI implemented by Environment Canada in 2013. The FOR DI allows for wind speed estimation across a substantially larger area than the single-tree DI, which is intended to provide point estimates of wind speed, and applies to damage areas larger than 1 hectare. The primary distinction of the FOR DI is its focus on the proportion of trees damaged within the area under examination, where higher proportions of damaged trees correspond with higher DoDs. The FOR DI also includes DoD 7 (bodily removal and lofting). An important influence on the damage experienced by groups of trees is whether they are growing as a natural forest of mixed species, sizes, and spacing, or as a plantation with single species, and uniform size and spacing. Wind speeds are adjusted downward for plantations in comparison to natural forests. Similar to the single-tree DI, wind speeds in the FOR DI are adjusted downward for saturated soils or weak tree species. In the FOR DI, DoDs 3-7 can be readily differentiated using remote imagery. The authors will also share a scalable box method developed by the Northern Tornadoes Project in Canada to estimate the proportion of trees damaged.