HomeMy WebLinkAboutRS_ArboristReport_v1Tree failure and incident inspection notes
• City tree failed at root plate 10/12/2022 around 7am
• Unique tree inventory database ID: TRE-24595
• Black Cottonwood - Populus trichocarpa
• 44” DBH, height ~145-155’ feet based on Range Finder laser measurements of adjacent trees
• Last inventory database update April 15, 2020
• Not yet inspected this year but trees here are in the annual inspection list based on size
category for cottonwood species
• Adjacent leaning tree inspected earlier in 2022 (still standing)
• Site is part of Liberty Park location on riverbank, unmaintained area
• Open grown / unrestricted site, 50% location rating
• 50% Tree condition rating, minor structural defects, few signs of poor health
• No risk rating noted, tree appeared to be fairly healthy, little deadwood in upper canopy
• No obvious root rot, girdling roots or basal decay pockets
• Entire root plate pulled out of river bank, soil very dry, suspect soil shear strength compromised
as a result
• No injuries, 2 vehicles damaged
• Weather data shows windspeed measurement at Seatac of 13 mph from the NNE at 6:53 on
10/12/2022 which, although not even close to being a strong breeze usually capable of toppling
trees, would be consistent with the direction the tree fell
Observations:
The leaner in right side of the photo was inspected recently but did not raise concerns as the likely direction of fall
posed little risk. The cottonwood tree that failed pulled completely out of the riverbank with the root plate. There
were no signs of basal decay or root rot from what could be seen, but the soil was visibly very dry and the shear
strength of that soil was probably compromised as a result. The fine roots appeared to be healthy . Ironically, from
the look of the upper canopy the tree appeared to be generally healthy and had little deadwood. On the other
hand, while inspecting adjacent trees as we were there, it is clear that some of them will require substantial risk
mitigation work in the near future. Trees along the river often exhibit basal damage due to impacts from floatin g
debris during high water events and this can be seen at the base of adjacent trees still standing. This side of the
trunk of the fallen tree is not visible, but even if there was basal wounding it would not have impacted the condition
of the root plate in the riverbank behind the tree. Sometime the river side of the tree root plate can be undercut by
erosion at high water, but again, it is unlikely that this would have impacted the condition of the root plate behind
the tree up the riverbank.
As for next steps, out of an abundance of caution no parking should be allowed along that wall next to the river at
200 Mill until the remaining trees can be addressed. That won’t likely take place until early next year at this stage.
The remaining tree trunk that is laying across the river is not impeding water flow for now and given that we are in
the midst of the salmon run the trunk will be left as is. We do not want to disturb anything for now and the salmon
are already using the shade for habitat and schooling beneath it. At a later date we can discuss whether the log
should be craned out, cut and floated at high water, or pulled tight parallel to the bank from which it fell as
‘natural’ woody debris in the river.