REVIEWED PLN_RESUB1 BLD2022-1025+Geotechnical_Report+9.25.2022_11.26.22_PM+3127648BLD2022-1025
Duffy Residence
23511 74t" Avenue West (Tax Account #27043200200100)
Edmonds, Washington
Brian Duffy RESUB
c/o The Leren Company Inc.
23511 74t" Avenue West Sep 26 2022
Edmonds, WA 98026 CITY OFEDMONDS
DEVELOPMENT SERVICES
DEPARTMENT
Attention: Brian Duffy and Tom Leren
September 20, 2022
Project #21029
Subject: Geologic Hazards Assessment and Recommendations
Mr. Duffy and Mr. Leren:
Reviewed by
City of Edmonds ;
Planning Division
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The purpose of this letter is to address the geologic hazard areas associated with the subject
property. It is our understanding that the proposed project consists of substantial but not
complete demolition of the existing single-family residence and construction of a new single-
family residence on the foundation remnants. For the sake of brevity, this letter will assume
that readers are highly familiar with the subject property, the proposed construction, and the
referenced critical areas determination (CAD) (City of Edmonds, June 26, 2017).
As indicated in the referenced CAD, there are mapped landslide hazard areas, erosion hazard
areas, and seismic hazard areas on the subject property. We have reviewed the proposed site
plan, the referenced critical areas determination, the available geologic and topographic
information, and we observed the site conditions on April 14, 2021 and on September 15, 2022.
The following subsections will summarize our conclusions with respect to the potential geologic
hazards.
Landslide Hazard Areas
During our site visits we did not observe any signs past or recent slope movement, accelerated
erosion, or adverse groundwater or seepage on the subject property. The observed slopes and
topographic features, such as rockeries, that "trigger" the slope related critical areas mapping
were obviously created by previous legal grading activities associated with construction of the
existing home, the driveway, and construction of 74t" Avenue West. Based on these factors and
in consideration of the intent of the geologically hazardous areas code, it is our opinion that the
subject property should not be encumbered by any landslide hazard area development
standards.
Erosion Hazard Areas
The ECDC defines Erosion hazard areas are defined as soils mapped with "moderate" to "very
severe" erosion hazard, as determined by the USDA-NRCS soil mapping. In this situation, due to
the past development and the current vegetation and landscaping of the property, the "erosion
hazard areas" mapping is irrelevant and the potential for severe or accelerated erosion does
not exist. It is our opinion that all erosion and specifically potential sediment transport to Lake
Ballinger can be mitigated by implementing temporary erosion and sediment control best
management practices (BMPs) during construction. The following is a list of BMPs that should
be implemented during construction:
• Rock -lined construction entrances
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Duffy Residence September 20, 2022
23511 74th Avenue West (Tax Account #27043200200100) Project #21029
Edmonds, Washington
• Prevention and maintenance of vehicle sediment track -out
• Temporary soil cover, as appropriate
• Perimeter controls such as silt fencing, straw wattles, or compost berms, as appropriate
• Establish permanent vegetation soon after grading is completed
The final landscaping of the project will be like the present conditions with a grass covered
lawn. In other area, ground cover vegetation or mulch will provide final soil stabilization for the
project. As previously stated, permanent vegetation or soil stabilization should be completed
as soon as possible during the construction process.
Seismic Hazard Areas
The eastern portion of the property has a mapped seismic hazard area. This is due to the
geologic unit near the lake shore being mapped as beach sediments. In this case, the flatter
area to the east is beach sediments, but the home footprint is on the descending slope from the
west and is underlain by Vashon advance outwash sediments. The glacially consolidated
advance outwash sediments do not have properties that exacerbate seismic risk. The mapped
seismic hazard area does not impact the design of the structure and no special mitigation is
required.
Summary
It is our opinion that the mapped potential landslide hazard areas and erosion hazard areas do
not exist, and the mapped seismic hazard area does not impact the proposed development
since the building footprint and foundations are founded on dense, advance outwash soils. As
such, the proposed development should not be encumbered by any geologic hazard
development standards (ECDC 23.80.060 and 23.80.070).
Statement of Risk
ECDC 23.80.060 sets forth some specific language with respect to alterations of geologically
hazardous areas. Though the some of the mapped hazard areas do not exist, to address the
intent of this code section, we provide the following statement with respect to the proposed
development:
It is the opinion of Andrew L. Glandon (State of Washington Licensed Engineering
Geologist #2689) that the proposed development will not increase the threat of the
geological hazard to adjacent properties beyond predevelopment conditions, will not
adversely impact other critical areas, are designed so that the hazard to the project is
eliminated or mitigated to a level equal to or less than predevelopment conditions;
and are certified as safe as designed and under anticipated conditions.
Recommendations
The following recommendation is appropriate for the site topography/conditions:
Structure Setback from Driveway/74th Avenue West Slope
We recommend a 10-foot building setback from the toe of the slope descending from 74th
Avenue West to the existing driveway. This is not due to a perceived lack of stability or
expected failure, but merely consistent with the standard of practice with respect to slopes of
SOUTH FORK GEOSCIENCES, PLLC Page 2
PO Box 1 275
NORTH BEND, WA 98045
425-890-4858 1 INFO@SFGEO.COM
Duffy Residence September 20, 2022
23511 74t" Avenue West (Tax Account #27043200200100) Project #21029
Edmonds, Washington
this vertical height and to facilitate access around the structure. Based on our review, the
current design is in accordance with this recommendation.
Closure
We have enjoyed working with you on this project and are confident that the information
provided in this letter will aid in the design and completion of your project. If there are any
questions a letter, please feel free to contact us.
0
as
Sincer e" ��9,�
I Andrew L. Glandon
An d r ;zzv--
Engineering Geologist / Owner
South Fork Geosciences, PLLC
References:
"Critical Area Determination (CRA20170076)", Michele Q. Szafran, Planner, City of Edmonds,
June 26, 2017
"Site Plan Sheet A0, Duffy Residence", Puget Sound Planning, PSP Project No. 2021-30, August
3, 2022
Washington State Geologic Information Portal
Web Soil Survey - Home (usda.gov)
Edmonds City Code and Community Development Code (codepublishing.com)
SOUTH FORK GEOSCIENCES, PLLC Page 3
PO Box 1 275
NORTH BEND, WA 98045
425-890-4858 1 INFO@SFGEO.COM