HomeMy WebLinkAboutRS_pinnacle on lk wa_arborist memo_final
Date: March 15, 2022
To: Peter Lynch, Service Manager – Essex Property Trust
Subject: Hazard Trees
Location: 1400 Lake Washington Blvd (Pinnacle on Lake Washington Apts)
From: Scott Selby, Registered Consulting Arborist #749, Board Certified Master Arborist #PN-
1775B, ISA Tree Risk Assessment Qualified
Peter:
Overview
At your request, I visited the Pinnacle on Lake Washington site March 14th to evaluate trees of concern. A recent
landslide resulted in significant undermining of three Douglas fir (Pseudotsuga menziesii) trees and resulted in burying
the root crowns of two large black cottonwood (Populus trichocarpa) trees and exposing the root crown of one other
large black cottonwood. A very broad swath of soil was released as well as a large intact block of soil.
You had a geotech visit the site to assess the condition of the slope and trees undermined. You indicated the geotech
recommended removal of the fir trees due to the precarious placement at the top of the slope and the potential for
failure of the trees to impact targets (structures or parked cars) at the base of the slope.
During my site visit, I noted the three cottonwood trees mid slope downhill from the fir trees and located in the middle
of the slide area. The demonstrated instability of the slope and the potential for the cottonwood trees to impact the
same targets in the event of failure represents an imminent risk.
Risk Assessment
Tree risk assessment involves qualitative analysis of several factors including the Likelihood of Failure, Likelihood of
Impacting Target, and Consequences of Failure (likelihood of injury or damage). The two tables that follow are from the
International Society of Arboriculture Tree Risk Assessment Form and are the current standard for assessing tree risk.
Matrix 1. Likelihood Matrix Likelihood of
Failure
Likelihood of Impacting Target
Very low Low Medium High
Imminent Unlikely
Somewhat
Likely Likely Very Likely
Probable Unlikely Unlikely
Somewhat
likely Likely
Possible Unlikely Unlikely Unlikely
Somewhat
Likely
Improbable Unlikely Unlikely Unlikely Unlikely
Matrix 2. Risk Rating Matrix
Likelihood of
Failure & Impact
Consequences of Failure
Negligible Minor Significant Severe
Very Likely Low Moderate High Extreme
Likely Low Moderate High High
Somewhat Likely Low Low Moderate Moderate
Unlikely Low Low Low Low
I rate all six trees as follows: the Likelihood of Failure as Probable, the Likelihood of Impacting Target as High and the
Consequences of Failure as Severe = HIGH Risk Rating.
Recommendation
Due to the potential for all six trees to impact targets in the event of failure and the imminent nature of the situation
due to failure of the slope, I recommend all six trees be removed without delay.
Tree Removal Plan
Due to the sever undermining of the fir trees as a result of slope failure, these trees are unsafe to climb and dismantle.
Safety dictates the trees be felled into the adjacent slope with the resulting debris left in place to help stabilize the bank.
Activity along the slide area should be kept to a minimum to avoid destabilizing the slope further. The trees will be cut to
short stumps with the roots left in place.
The cottonwood trees appear safe to climb and will be cut to a short snag height of approximately 10 feet. The removal
of these trees will generate a significant amount of debris which could represent a fire hazard if left in place.
Consequently, the wood will be left in place to help hold the slope, but the limbs will be removed and chipped.
Please let me know if you have any questions about this report.
Scott Selby, Account Manager
SavATree
206.849.4718
sselby@savatree.com
47
slide area
4950
52
Targets
49
50
49
5052