HomeMy WebLinkAbout002273035 Geotech AssessmentStratum Group File: 8.17.20c
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PO Box 2546, Bellingham, Washington 98227
Phone: (360) 714-9409
March 21, 2022
Abrie Bester
execusa8@gmail.com
Re: Geology Hazard Assessment
Jefferson County Parcel 002273035
Jefferson County, Washington
This geologic hazard assessment was conducted to assess the risk of landslide and erosion
hazards on the above-mentioned property. It is my understanding that you are planning to build a
home on the property. The Jefferson County Public Records landslide map maps the steep
shoreline bluff area as a ‘high’ slide area. The Jefferson County shoreline stability map maps the
shoreline bluff as ‘unstable old-slides’ and ‘unstable recent-slides’. The County erosion hazard
map maps the shoreline area as an erosion area. The integrated shore type map (Coastal Geologic
Services, 2015) indicates the shoreline bluff is a feeder bluff.
Based on my geologic hazard assessment, it is my opinion that a home or homes can be located
on the property outside of any landslide hazard or shoreline bluff retreat areas for at least 100
years as long as the home is located at least 35 feet back from the top edge of the shoreline bluff.
Furthermore, it is my opinion that a home can be located on the site without increasing the risk
of landslides or erosion on or off the site as long as the recommendations in this report are
followed.
This geologic hazard assessment included a field inspection of the subject property and vicinity,
the coastal bluff, and shoreline reach. The assessment included a review of historic aerial
photographs, available geologic mapping, lidar (light detecting and ranging) imagery, and
climate change literature relevant to the site and our own notes and observations we have made
in the vicinity of the site and at locations with similar geologic conditions.
GENERAL GEOLOGY
The Surficial geologic map of the Port Townsend 30- by 60-minute Quadrangle, Puget Sound
region, Washington (Pessl and others, 1989) indicates that the upland of property is underlain by
Vashon-age glacial till (14,000 to 20,000 years old) and the shoreline bluff is underlain by
glacial advance outwash (Figure 1).
March 21, 2022
Jefferson County Parcel 002273035
Geologic Hazard Assessment
Stratum Group File: 3.1.22
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Figure 1. Clipped and annotated geologic map showing the approximate subject property location. Qvt = Vashon
till; Qva = glacial advance outwash; Qm = marsh; Qb = beach, Qyal = young alluvial sediments and br = bedrock.
My observations are consistent with the above described mapping. The upland area of the site
and the upper 15 to 20 feet of the bluff are underlain by very dense and hard glacial till. Below
the glacial till, the shoreline bluff is underlain by layered strata of silt, sand and gravel units in a
dense condition consistent with glacial advance outwash.
Figure 2. View of shoreline area and lower bluff at the shore reach.
Site
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Jefferson County Parcel 002273035
Geologic Hazard Assessment
Stratum Group File: 3.1.22
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SITE OBSERVATIONS
The property is accessed via a gravel/dirt driveway off of the northeast end of Daisy King Road.
The upland of the site slopes very gently towards the east towards the top of steep shoreline bluff
above the east shore of Discovery Bay. Outside a limited cleared area at the end of the driveway,
the property is forested primarily with Douglas fir and a thick rush undergrowth of that includes
salal and ocean spray.
The shoreline bluff is approximately 120 feet high. The upper half of the bluff slopes down at an
angle of approximately 40 degrees. The slope then becomes more moderate for a height of about
10 feet and then becomes steeper on the lower half of the bluff with near vertical areas except
where soil has accumulated at the base of the bluff.
The lower half of the bluff has silt, sand and gravel layers with silt in a hard compact condition
being the predominant layer. These units are consistent with glacial advance outwash, but an
older pre last glacial period glacial drift is present in the vicinity at the lowermost part of the
bluff. The upper approximately 20 feet of the bluff is underlain by glacial till.
The shore consists of a sand and gravel upper beach with a continuous boulder and cobble shore
area in the tidal zone. The ordinary high water mark is very near the base of the bluff, but the
upper beach does not appear to be inundated on a frequent basis based on accumulation of
sediment on the uppermost beach area from the bluffs above.
The shoreline reach is consistent with the Coastal Map designation as a feeder bluff shore line,
but erosion rates appear to be relatively slow on this shore reach as mature trees are present on
the shoreline bluff in some areas, but there area areas of younger vegetation and bare areas
indicating periodic erosion and shoreline bluff retreat has and likely will continue. Overall
shoreline sediment drift does appear to be from the north to the south at this reach.
GEOLOGIC HAZARD EVALUATION
The shoreline bluff is an eroding bluff and over time the base of the bluff is eroded such that the
slopes above are undermined and shallow slope failures will extend to the top edge of the bluff.
The bluff consists primarily of hard and dense glacial advance outwash and hard glacial till and
hence is fairly resistant to erosion by the moderate waves that periodically reach the base of the
slope.
Fully quantifying the long-term top of bluff retreat at the site is not possible due to the relatively
short historic record and somewhat episodic nature top of bluff retreat. A review of historic aerial
photographs and maps did not reveal any discernable top of bluff retreat, but periodic shallow
surficial slides are evident on portions of the bluff both on and in the near vicinity of the site.
Based on the geology of the site bluff, shoreline conditions, and available historic aerial
photographs, it is my opinion that a reasonable conservative estimate for long-term overall top
edge of bluff retreat for the site has been less than 1 inch per year.
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Consideration of Relative Sea-Level Rise
While past long-tern top of shoreline bluff retreat has been very slow and the long-term rate has
likely been less than 1 inch per year, relative sea level rise projections will likely cause an
increase of erosion and shoreline bluff retreat at this shoreline. Utilizing Lavin and others (2019)
and their use of data from Miller and others (2018), a range of percent chances for relative sea-
level rise projections at the site under different greenhouse gas scenarios is provided below.
The results suggest that there is a 4% to 7% chance of 1 foot relative sea level rise at the site by
2050 and an 83% to 97% chance of a 1 foot rise in relative sea level by 2120. For the longer term
out to 2120 and 2150 there is greater uncertainty primarily due to stability of ice sheets and
variable emission scenarios, but in the shorter term out to 2050 the range of uncertainty has
narrowed regardless of future emission scenarios and estimates of global sea level rise over the
next 30 years are projected to be equal to the past 100 years of global sea-level rise (Sweet and
others, 2022).
The increase in sea level will result in a greater frequency of high water reaching the base of the
slope and higher and hence an increase in erosion rate. The increase in erosion will take place
along the entire shoreline drift sector. The increased erosion will add sediment to the beach;
however, the predominance of fine grained sediment in the bluffs at this shore reach and the
relatively short updrift length to the north will limit the sediment supply to the beach and thus
erosion rates and top of bluff retreat should be expected to increase over time due to the relative
sea level change.
Estimated Long-Term Top of Bluff Retreat
Based on the geology of the site bluff, shoreline conditions, available historic aerial photographs,
and consideration of sea level rise, it is our opinion that a reasonable conservative estimate for
long-term top edge of bluff retreat for the site is 3.0 inches per year. The top of the bluff will not
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Jefferson County Parcel 002273035
Geologic Hazard Assessment
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retreat at an incremental rate of 3.0 inches per year, but will retreat in an episodic manner where
2 to at most 10 feet of the upper bluff will fail and then many years to decades will pass by with
only minimal retreat at the top edge of the bluff.
Using a conservative long-term average retreat rate of 3.0 inches per year, the top edge of the
bluff can be expected to retreat 25 feet over the next 100 years. However, due to the potential for
additional larger (10 feet failures) along the top edge of the bluff, the retreat of the top edge of
the bluff could be an additional 10 feet. Therefore, it is our opinion bluff retreat over the next
100 years may be a maximum of 35 feet.
There is no evidence of ongoing or incipient large scale slope failures on the site and given the
geology of the bluff, none are anticipated to develop on this bluff reach.
CONCLUSIONS AND RECOMMENDATIONS
Based on my geology hazard assessment, it is my opinion that homes can be built on the property
that will not be at risk from landslide or shoreline bluff retreat hazards for at least 100 years as
long as the homes are located at least 35 feet back from the top edge of the shoreline bluff slope.
This distance is approximately the same as the setback from the ordinary high water per the
County shoreline regulations.
Vegetation management between the home sites and the steep slope and on the steep slope for
view purposes is acceptable, but should be done consistent with shoreline habitat requirements. I
recommend that no trees larger than 30 inches in diameter should be cut. Limbing of larger trees
is acceptable.
No yard waste or debris should be disposed of on or at the shoreline bluff slope. Adding
significant soil or debris to the slope could form an unstable mass on the slope.
Stormwater should be managed per the Western Washington Stormwater Manual. Full dispersion
and infiltration can be accomplished if stormwater is spread out through multiple dispersion
locations, properly sized dispersion/infiltration trenches and French drains or the use of splash
blocks from limited roof areas.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions regarding this assessment please contact our office at (360) 714-9409.
Sincerely yours,
Stratum Group
Dan McShane, L.E.G., M.Sc.
Licensed Engineering Geologist