HomeMy WebLinkAbout901054001 Geotech ReportStratum Group File: 8.17.20c
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PO Box 2546, Bellingham, Washington 98227
Phone: (360) 714-9409
March 17, 2022
Jennifer Muller
jmuller5366@me.com
Re: Geology Hazard Assessment
Jefferson County Parcels 901054017 and 901054001
Jefferson County, Washington
This geologic hazard assessment was conducted to assess the risk of landslide and erosion
hazards on the above-mentioned property. It my understanding that you are considering building
on the property. This report addresses both tax parcels as they are immediately adjacent and the
conditions on two parcels are such that the conclusions and recommendations are the same for
both parcels. This report is an update to the previous report as Jefferson County indicated they
wanted a discussion of the potential hazard areas to the east.
Based on my geologic hazard assessment, it is my opinion that a home or homes can be located
on the two parcels outside of any landslide hazard or shoreline bluff retreat areas for at least 100
years. Furthermore, it is my opinion that homes 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 properties and vicinity
including the former rail grade on the site, slopes above the rail grade, 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 our own notes and
observations we have made in the vicinity of the site and at locations with similar geologic
conditions.
GENERAL GEOLOGY
The Geologic Map of the Port Townsend South and Part of the Port Townsend North 7.5-minute
Quadrangles, Jefferson County, Washington (Schasse and Slaughter, 2005) indicates that the
property is underlain by Vashon-age glacial till (14,000 to 20,000 years old) with some limited
areas on the eastern part of the properties underlain by glacial advance outwash (Figure 1).
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Geologic Hazard Assessment
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Figure 1. Clipped and annotated geologic map showing the approximate subject property location. Qgt = Vashon
till; Qga = glacial advance outwash.
My observations are generally consistent with the above described mapping. The shoreline bluff
is mostly underlain by poorly sorted mix of sediments consistent with glacial drift sediments that
are in a very hard and dense condition. Areas of the lower bluff appear to be underlain by glacial
advance outwash or subglacial water borne sediment or possibly older sediment from before the
last glacial period. There appears to be an older very hard glacial till at the base of the bluff. The
till at the base of the bluff (Figure 2) has cobbles and gravel that are predominantly Olympic
Mountain sourced which suggests an Olympia-age alpine glacial advance versus ice advancing
into the area from the north. I have observed distinct very hard outwash sediments dominated by
Olympic Mountain derived sediments to the north of Aldema Beach.
The upland area of the property is underlain by glacial drift, but the very upper layer appears to
be at least partially reworked by water as the upper soil has very little silt and clay as seen
elsewhere in the drift. There is faint wave cut terracing visible in lidar bare earth imagery of the
site and drainage valleys to the east of the gentle slope areas of the properties and vicinity that
dead end at the gentle slope area (Figure 3). The terracing and dead ending of valleys is at the
same elevation as recessional glacial delta outwash located to the north at Adelma Beach. During
the late stages of the last glacial period the area was inundated with sea water as the glacial ice
retreated. The thick ice had lowered the local land elevation from isostatic loading. Post
inundation, the land surface rebounded to it current elevation.
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Figure 2. Glacial Drift at the base of the slope on the property.
Figure 3. Lidar bare earth imagery of area. Note valleys to the east that dead end and very faint wave terraces on
gentle upland area.
Jefferson County Potential Geology Hazard Maps
The shoreline bluff on the subject property is mapped by Jefferson County as “feeder bluff. This
designation is consistent with my observations.
Jefferson County Public Land Records Shoreline Slope Stability Map (Figure 4) indicates an
‘unstable recent slide’ is located on the shoreline bluff on parcel 901054001 and that the rest of
the shoreline bluff and an area of the upland above the bluff is indicated as ‘unstable’. The
shoreline stability map also indicates an area as ‘unstable’ in the vicinity of Highway 20 that
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Jefferson County Parcels 901054017 and 9010540010
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partially overlaps the upland area of portions of the two parcels and indicates another area on the
very south edge of parcel 9010540010 as ‘intermediate slope’.
The Jefferson County Public Records Landslide Hazard map (Figure 5) indicates the shoreline
bluff and area of the upland near the bluff as a ‘moderate’ landslide hazard area with one area on
the bluff on parcel 9010540010 a ‘high’ landslide hazard area. A portion of the upland area near
the bluff designated on the map as slight landslide hazard and portions of the lots east of
Highway 20 are also designated on the maps as ‘slight’ landslide hazard areas.
The steep bluff slope is designated in the Records maps as a seismic hazard area.
Figure 4. Screen shot of Jefferson County Shoreline Slope Stability Map
Figure 5. Screen shot of Jefferson County Landslide Hazard map
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The Jefferson County map designations are appeared to be derived from the Washington Coastal
Atlas the Slope Stability Map of the Uncas-Port Ludlow Area, Jefferson County (Hanson, 1976)
and the Slope Stability Map of Northeastern Jefferson County (Gayer, 1976). Gayer (1976) and
Hanson (1976) used a numbering system on their maps such as "Class 3: Areas of Poor Natural
Stability". Gayer (1976) also designated Class 5 areas as areas of recent landslides and Class 4
areas as areas of former landslides. Gayer's Class 3 designation appears to include all areas that
were mapped as being underlain by Vashon advance outwash on the Geologic Map of
Northeastern Jefferson County (Gayer, 1976). I speculate that Gayer (1976) made this
designation based on the potential that the Vashon outwash sediments could become saturated
and destabilized by perched water. Those conditions are not present at the subject property or in
the vicinity.
My geology assessment is consistent with the landslide and shoreline bluff stability mapping that
the steep shoreline bluff is unstable or potentially unstable along its entire length and that a
setback from the top edge of the slope is required due to the proximity to this unstable slope area.
However, it is my opinion that the other areas mapped as unstable or slight landslide hazard
areas are not at risk from landslides.
SITE OBSERVATIONS
The two parcels are primarily located between Highway 20 on the east and Port Discovery on the
west, but also have relatively narrow panhandles that extend to the east and north on the east side
of the highway (Figure 6). The main part of the property west of the highway is accessed via
Keystone Place with access drives to grass covered cleared areas (Figure 7).
Figure 6. Aerial view of properties (Jefferson County GIS).
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Figure 7. Open grass area on southern parcel
Outside of the grass covered areas on the upland area of the parcels, the upland is forested with a
predominantly Douglas fir forest with an understory of mixed vegetation. A former rail grade
with a grassy trail is located west of the cleared areas. The rail grade is between 50 and 70 feet
back from the top edge of the steep shoreline bluff on the west side of the property.
The shoreline bluff is approximately 100 feet high and has slopes between 40 and 60 degrees
with some areas that are steeper. Vegetation on the steep bluff slope varies from areas of bare
exposed soil to stands of red alder to stands of mature Douglas fir and western red cedar. The
slopes on the subject property are tree covered with the exception of portions of the lower 3 to 15
feet of the bluff slope which is covered with a mix of brush, a few small trees and bare areas
(Figures 8 and 9). There is a bare of slope that extends to the top of the slope to the north of the
property with a landslide deposit on the beach (Figure 10). Old rough trails descend the bluff
along narrow paths cut into the slope (Figure 11).
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Figure 8. Bluff slope and beach fronting the subject property.
Figure 9. Slope on southern portion of property
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Figure 10. Relatively recent landslide to the north of the properties.
Figure 11. Trail on the property for beach access
The shore consists of a sand and gravel beach. The ordinary high water mark is very near the
base of the bluff or a few feet waterward of the base of the bluff. As such high water reaches the
base of the slope on a periodic basis during very high tide events or storm surges into Port
Discovery. The shoreline reach is consistent with the Coastal Map designation as a feeder bluff
shore line.
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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 drift 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 (Shipman, MacLennan
and Johannessen, 2014 and Johannessen and MacLennan, 2007). However, 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
including future sea-level rise (Miller, 2018) is 2.0 inches per year. The top of the bluff will not
retreat at an incremental rate of 2 inches per year, but will retreat in an episodic manner where
many years of slow erosion will be punctuated by a small portion of the top of the bluff breaking
loose on the order of 2- to 5-foot wide failures based on observations of some of the old bluff
failures observed on this shore reach as well as at sites with similar geology conditions.
Using a conservative long-term average retreat rate of 2 inches per year, the top edge of the bluff
can be expected to retreat a maximum of 17 feet over the next 100 years. Because the shoreline
bluff meets the criteria for a landslide hazard area per Jefferson County Code 18.22.510(1)(b),
development on the site is subject to a 40 foot “setback” from the “landslide hazard areas” per
Jefferson County Code 18.22.530(8)(b)(ii). It is our opinion that this 40-foot distance is more
than adequate to protect any proposed development from landslides and erosion for at least 100
years.
The type of landslides at the site will be shallow slides on the steep bluff slope. Slides that reach
the top edge of the slope will remove just a few feet of the top edge of the slope when they do
occur.
There is no evidence of ongoing or incipient large scale slope failures on the site and given the
hard dense glacial drift makeup 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 40 feet back from the top edge of the shoreline bluff slope.
In regards to trail maintenance work and vegetation cutting, it is my opinion that this work will
not increased the risk of landslides or erosion on or off the site and will not impact the shoreline
geologic processes. The slope where trees and brush had been cut is a moderate slope and is well
drained and the tree and brush trimming will not destabilize the slope.
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
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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.
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