Boulevard Tree Failures During Wind Loading Events

Abstract

Wind loading events vary in their intensity and degree of damage inflicted on urban infrastructure, both green and gray. Damage to urban trees can begin with wind speeds as low as 25 miles per hour, especially when those trees harbor defects that predispose them to structural failures. The tree damage triangle integrates the three main factors that influence tree failures during wind loading events, namely the site characteristics, the (wind) loading event and any defects of the trees in question. The degree of damage that trees experience is generally a function of these factors overlapping each other. For instance, when the potential damage from wind loading events is exacerbated by poor tree architecture and compromised site conditions, the likelihood of significant damage is realized. Two studies on the damage to urban trees and the predictability of damage are reviewed; one study is a longterm gathering of wind loading events and accompanying damage to trees while the other is a case study of one storm in one city on one day. Both studies revealed critical pre-existing conditions that left trees vulnerable to whole tree losses: large trees in limited boulevard widths and severed roots as a result of sidewalk repair.

Predictors of tropical cyclone-induced urban tree failure: an international scoping review

“…In a study of windstorms in Minnesota, USA, Johnson et al (2019) observed that after construction activities, root failure was less likely in urban street or boulevard trees within larger planting volumes, though this effect was not observed in trees not exposed to construction.…”

“…It is also necessary to examine the interactions between root architecture and soil or site characteristics which are unique to the urban environment, such as engineered soils and planting spaces confined by pavement. Johnson et al (2019) observed that sidewalk replacement activities and boulevard widening influence tree failure during storms with high winds in Minnesota, US. In their recommendations for choosing urban tree species to plant in tropical cyclone prone cities, Zhang et al (2021) advocate for considering root growth characteristics in addition to aboveground qualities.…”

The Tripping Point–Minimum Planting Widths for Small-Stature Trees in Dense Urban Developments

“…Root and pavement conflicts can be similarly damaging to the trees involved. The replacement or repair of paved surfaces near trees can sever or injure roots, reducing tree health [11][12][13] and undermining overall stability in the face of storm events [14]. To reduce root and pavement conflicts, researchers have investigated how planting widths relate to sidewalk damage [9,15] and created allometric models to predict trunk flare diameter (TFD; i.e., the diameter of the enlarged area at the base of the tree where the trunk connects to the main structural roots) based on tree species and stem diameter [16,17].…”

Vulnerability of Street Trees in Upper Midwest Cities to Climate Change

“…The urban heat island effect combined with rising global temperatures can create conditions beneficial to insect pests, which may also be able to better take advantage of already-stressed urban trees (Tubby and Webber, 2010;Long et al, 2019). The increases in storm severity combined with growing space limitations has caused increased mortality of street trees (Johnson et al, 2019). Substantial crown dieback due to increases in drought and heatwave severity (Zhang and Brack, 2021) and reduced tree canopy increases the urban heat-related illness of residents, exacerbating existing inequalities in urban canopy distribution (Jung et al, 2021).…”