Landau final 7316 164th St SW_tm.pdf14 LANDAU
ASSOCIATES
TECHNICAL MEMORANDUM ENVIRONMENTAL I GEOTECHNICAL I NATURAL RESOURCES
TO: Michael Clugston, Associate Planner, ACIP
ity of Edmonds Development Services Department
FROM:olin Turnbull, L.E.G. and Dennis R. Stettler, P.E.
DATE: Iay 12, 2011
RE: GEOTECHNICAL PEER REVIEW
CRITICAL AREA VIOLATION
#7316164TH STREET SW
EDMONDS, WASHINGTON
PLN -2010-0033
INTRODUCTION
This technical memorandum provides our geotechnical peer review of technical documents and
plans submitted to the City of Edmonds (City) regarding a post -event tree cutting permit (File No.
PLN -2010-0033) located at #7316 164th Street SW in Edmonds, Washington. According to the City, the
owners of #7316 164th Street SW (Michael Kerr and Diana Clay) cut down 20 trees on the western slope
of their property without a permit during the spring of 2010. This slope had been previously identified as
part of the North Edmonds Earth Subsidence and Landslide Hazard Area as defined in Edmonds
Community Development Code (ECDC) Chapter 19.10. Unauthorized critical area alterations are
regulated according to ECDC Section 23.40.240. The violator subsequently applied for a permit
(PLN -2010-0033) which contains a restoration plan for the mitigation of the unauthorized clearing.
The purpose of this geotechnical peer review was to review portions of the submittal package and
assess its compliance with City alteration and permit requirements as contained in ECDC Chapters 18.45,
19.10, 23.40, and 23.80. This geotechnical peer review was accomplished in accordance with Task Order
No. 11-01 of Landau Associates' On -Call Geotechnical and Environmental Engineering Services
Agreement with the City.
We have received the following information forwarded by the City for review:
® photographs of the subject site, undated (post tree -cutting).
® Geotechnical Evaluation Report, Tree Cutting on West Slope, prepared by Robinson Noble,
dated May 10, 2010.
® Cost Estimate, Removal and Replacement of Downed Trees, prepared by Burris Design
undated, received by the City on June 28, 2010.
o Applicant's Tree Cutting Cover Letter, undated, received by the City on June 28, 2010.
130 2nd Avenue South • Edmonds, WA 98020 9 (425) 778-0907 o fax (425) 778-6409 • www.landauinc.com
® Internal City Memorandum, PLN20100033 Tree Cutting Permit Comments from the Public
Works & Engineering Department, dated November 1, 2010.
® Site Plan Drawing, 164'x' Street SW Right of Way Drainage Repairs, prepared by R. W. Beck,
with details added for location of cut trees, undated.
• Clay/Kerr- Mitigation Plans for Tree Cutting Activity at 7316 164'x' Street SW, prepared by
Burris Design, dated March 18, 2011.
o Clay/Kerr Critical Areas Report for Erosion Hazard at 7316 164`x' Street STV, prepared by
Burris Design, dated March 18, 2011.
® Clay/Kerr Critical Areas Report for Landslide Hazard at 7316164`x' Street STV, prepared by
Burris Design, dated March 18, 2011.
® Supplemental Information in Response to Review Comments, prepared by Burris Design,
dated March 18, 2011.
® Clay/Kerr Critical Areas Report for Stream at 7316 164`x' Street SW, prepared by Burris
Design, dated March 18, 2011.
• Supplemental Report, Response to Review Comments, prepared by Robinson Noble, dated
March 21, 2011.
The project is located within the designated North Edmonds Earth Subsidence and Landslide
Hazard Area (ESLHA) and is therefore subject to the provisions of ECDC 19.10 related to the ESLHA
and ECDC 23.80 related to Geologically Hazardous Areas. Trees within critical areas and critical area
buffers are generally protected because trees play a critical role in preventing slope instability and
erosion, in protecting water quality, and providing habitat. In particular, ECDC 18.45.040 notes that no
tree(s) or ground cover shall be removed from a native growth protection easement or environmentally
sensitive site unless that plot plan and other submitted materials can demonstrate that the removal will
enhance the easement area. Allowed activities and exemptions for protection of trees are detailed in
ECDC Sections 23.40.220 and 23.40.230 and include: 1) hazard trees, and 2) operation, maintenance, or
repair activities conducted by the City.
GEOTECHNICAL DOCUMENT REVIEW
We reviewed the submitted geotechnical documents associated with the subject property to assess
their consistency with the provisions of ECDC 18.45, 19.10, 23.40 and ECDC 23.80. The geotechnical
report does not reference or address the specific requirements of ECDC 19.10 or ECDC 23.80. We
identified a number of inconsistencies between the geotechnical report and City provisions that should be
noted and addressed:
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LANDAU ASSOCIATES
The 2010 Geotechnical Evaluation Report re -states an earlier 2003 recommendation that
"selective pruning and cutting of trees is appropriate to adequately manage and improve the
stability of the slope."
ECDC 18.45.040 notes that this is an environmentally sensitive site and predevelopment
vegetation shall not be disturbed or removed. Any future maintenance plan of this steep
slope area requires permit application prior to any action.
The 1996 Geotechnical Engineering Report, included as an appendix to the 2010
geotechnical report, states that the "site is stable with respect to deep-seated failure. "
The 1996 geotechnical project report, or any of the geotechnical documents submitted to the
City, do not reference any of the comprehensive landslide hazard area studies of the area,
including the 2007 Landau Associates report, as required by ECDC 19.10. The implication
of the discussion in the 1996 Geotechnical Engineering Report seems to be that no large-
scale movements have occurred in historical times, and that is not consistent with the
available historical information. Significant landslides occurred in this area in 1947 and
1955-56 that would be considered by most people to be large-scale. We further note that in
1996 a landslide occurred along a portion of the subject slope, after which, the City installed
an aboveground storm drainage system secured on the slope by pipe anchors to help stabilize
the slope. This drainage system is within a 40 -ft wide utility easement along the north side of
164"' Street SW. The 2010 geotechnical project report delineates the 1996 slide on Figure 1,
but does not mention its existence in the report.
We recommend that the most recent geotechnical document (Robinson Noble, March 21
2011) be revised or supplemented to reference the Landau Associates Summary Report as
detailed in ECDC 19.10 030 (H) to address the specific City requirements for geotechnical
reports as contained in the City's "Geotechnical Report Guidelines " document for projects
in the North Edmonds ESLHA.
MITIGATION PLAN REVIEW
We reviewed the submitted mitigation plan documents associated with the subject property to
assess their consistency with the provisions of ECDC 18.45, 19.10, 23.40 and ECDC 23.80.
The Bun -us Design Group Mitigation Plan (Plan), details the removal of trees that were cut
but left in place. The proposed removal methodology of tree trunks comprises "using a
choker set up at the end of the drive" which "has been approved by Robinson Noble,
Geotechnical Engineers who designed the slope." The City documents the presence of
"multiple ground storm drainage lines that run across the slope in the vicinity of the cut trees.
Pipes all appear to be of HDPE material and are secured on the steep slope with pipe
anchors."
TV(-, note that the Plan is generally in accordance with the ECDC however, we recommend
that the Mitigation Plan should identift the exact number of "cleaned trunks" that are
proposed to be removed. In total Sheet L-1 of the Plan appears to indicate 19 felled trees, 13
of which are greater than six inches in diameter and 6 are less than six inches in diameter.
We recommend that the trees greater than 6 inches be def ned per ECDC 1.9.10.030(G) as a
"significant tree" i.e. "a tree with a trunk diameter of six inches or greater measured four
feet from the ground. "
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We note the presence and concern of the City with regard to the above ground storm
drainage lines. Based on the Figure 1 of the Plan, these lines are contained within the 1996
"approx. edge of slide " area and that 13 of the 19 felled trees are mapped north of this area.
It is our opinion that the ren7oval of these felled trees could be facilitated effectively by
pulling the cut signifr.cant tree trunks away from the 1996 slide zone, i.e., northeast to the top
of the slope to avoid damage to existing lines. According to the Plan, we understand that the
"upper part of the slope is the most highly stabilized" and as such a southeast choker pull
towards 164`x' Street SW is more desirable, however, this proposed action pathway presents
the greatest risk to the existing storm drainage lines. Based on review of the 2010 Tree
Replacement Plant, Sheet L-1, we further note that 6 of the 19 felled trees are mapped south
of the drainage lines. It is our opinion that these trees could be removed from the slope using
southeast choker pull towards 164th Street SIV. We recommend that the Plan include
inspection and verification of the existing storm drainage lines before and after the tree
removal portion of the Plan has been completed.
We further recommend that, per ECDC 23.40.130 (C), a detailed site diagram be referred to
and accompany the Mitigation Plan. An updated diagram of the 2010 Tree Replacement
Plant, Sheet L-1 (submitted as part of an earlier cost estimate) could facilitate this
requirement. The Plan Sheet should also address the ECDC 23.40.220 (C7b iv) provision
that states "replacement trees shall be species that are native and indigenous to the site and
a minimum of one inch in diameter at breast height (dbh) for deciduous trees and a minimum
of six feet in height for evergreen trees as measured fr^orn the top of the root ball. " Currently
the 2010 Plan Sheet notes a nrinirnum height of 3 ft.
USE OF THIS TECHNICAL MEMORANDUM
This technical memorandum has been prepared for use by the City of Edmonds in evaluating the
adequacy of construction documents submitted for #7316 164th Street SW. The focus of this review was
the geotechnical aspects of the documents. The purpose of the review was to assess the adequacy of the
documents for compliance with City requirements contained in ECDC Chapters 18.45, 19.10, 23.40 and
23.80, and conformance with conventionally accepted geotechnical engineering practices. This
geotechnical peer review by Landau Associates does not lessen the requirements for the applicant's
geotechnical consultant and other design professionals to prepare an appropriate design for the site
conditions.
We appreciate this opportunity to be of service to the City. Please contact us if you have any
questions, or if we may be of further service.
CBT/DRS/rgm
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