HomeMy WebLinkAboutGeologic and Beach Process Reconnaissance - GEO General - 1/25/2021 HuiVE
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wwooCoastal Solutions, LLC LANNING
JUL o l 2021
615 W. Alder Street
11027 Manitou Beach Drive NE
Bainbridge Island,WA 98110
206.459.7264
January 25,2021
Frank Dahl
22 Dale Ct
Massapequa,NY 11758
Re: Geologic and Beach Processes Reconnaissance
890 Dewatto Beach Drive
Tahuya,WA
Project No. 20077
Dear Mr. Dahl:
This letter-report summarizes Coastal Solutions' observations made during a recent geologic
and coastal processes reconnaissance of your property on the southeastern shore of Hood Canal
in Mason County,Washington. The purpose of our study was to observe and evaluate the
existing shoreline conditions at the site with regard to the proposed bulkhead replacement and
extension project.
Observations
Site Conditions and Topography
The site consists of two adjacent parcels located on the south side of Dewatto Bay on Hood
Canal. The combined 2.97-acre residential property consists of a gently sloping upland to the
south and a steeply sloping beachfront to the north where the house is located.Vegetation
around the house consisted of scattered shrubs and landscaping. The house consisted of a
wood-frame structure with a daylight basement foundation. The northern footing for the house
is located approximately 10 feet from the top of the shoreline bulkhead. According to the
owner the septic system is located above the house to the south.
Total vertical relief between the house foundation and the beach was on the order of 7 feet.
The owner reported the house is likely piling supported with fill material below the floor slab
and below the deck above the bulkhead. We noted landslide activity on the western parcel that
extended from the upper parking area to the beach. This portion of the site is not protected
from shoreline erosion. The site was surrounded by similar private properties to the east and
west,undeveloped forestland to the south,and the tidelands of Hood Canal to the north.
Existing Shoreline Conditions
The existing bulkhead that protected the house consisted of both rocks and logs. The eastern
approximately 45 feet consisted of stacked rocks on the order of 6 to 7 feet high. The western
50 feet consisted of a stacked log bulkhead on the order of 6 feet high. This stacked log
bulkhead begins at the parcel the house is on and extends to the undeveloped western parcel.
The western 40 feet of the western parcel where the landslide occurred was unprotected. The
bulkhead was still protecting the shoreline,but showed signs of settlement,deterioration,and
displacement,which is typical of these types of bulkheads. We noted voids on the order of 4
Frank Dahl
January 25,2021 Project No. 20077
to 5 horizontal feet behind the bulkhead that extended below a broad patio area. Based on the
size of the patio and reports regarding piling support of the house we estimated 20 to 25 feet
(measured horizontally)of fill exists landward of the rocks and logs. The adjacent property to
the east is owned by the Dahls and was protected with an ecology block bulkhead. The
adjacent property to the west is unprotected. The bulkhead at the site was still performing as
intended,but is at the end of its service life and in need of replacement.
Current plans call for the replacement of the existing log bulkhead with a new irregular-faced
rock bulkhead placed within the same footprint as the existing log bulkhead. The new rock
bulkhead will be approximately 6 feet high with a batter on the order of 6H:1 V
(horizontal:vertical) and will extend the entire length of the shoreline along the western parcel
and will butt against the existing rock bulkhead that will remain on the eastern parcel. This
means an extension of approximately 40 feet of rocks onto the western parcel where the
landslide occurred.
Beach and Coastal Conditions
The beach consisted of an approximately 1011:1V sand,gravel,and mud beach-face. Based on
our observations of barnacles and other indications of periodic inundation on the beach,
ordinary high water(OHW)was located at the face of the existing bulkhead. The most recent
data regarding coastal processes published by the Washington State Department of Ecology,
(Ecology) Washington Coastal Atlas indicated that the property lies within a littoral drift cell
that begins approximately 6 miles south at The Great Bend and terminates approximately %z-
mile to the west in Dewatto Bay.
Drift cells contain zones along the shoreline that include erosion areas,transport corridors, and
deposition or accretion areas. We noted subtle signs of littoral sediment deposition on the
south sides and scour/erosion on the north sides of rocks and other obstructions within the
upper intertidal area consistent with the published drift directions. Sediment transport is
largely the result of wind-driven waves striking the shoreline at an angle. Erosion at the
project site consisted of scour in and around the log bulkhead that is allowing piping of soil
and collapse of the stacked log system.
Drainage
We noted that roof runoff for the house was routed below ground,but we could not determine
an outfall at the beach. There was no evidence of surface erosion within the upland area due to
stormwater runoff. We noted no signs of groundwater seepage at the site or on the adjacent
sites during our field visits. It should be noted that groundwater seepage levels and flow
volumes will vary with precipitation,irrigation practices,time of year,and upland land uses
both on and offsite.
Comprehensive drainage analysis,including testing the downspouts during our field visit,was
beyond the scope of services for this phase of the project. The proposed replacement bulkhead
design should include a suitable stormwater outfall structure to capture and route drainage
where necessary to a suitable low-energy discharge location landward of the bulkhead. A
dispersion/energy dissipater placed behind the bulkhead should be placed in such a manner as
not to jeopardize the integrity of the bulkhead or cause erosion at the beach or the toe of the
slope.
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Frank Dahl
January 25,2021 Project No. 20077
Geology
Soil exposures on site were limited due to the fill soils and bulkhead that had buried the former
natural toe of the slope. However,the landslide area reported to have occurred in March of
2019,was on the order of 30 feet wide and 25 feet in height and exposed interbedded sand and
gravel. These sediments are variously mapped as undifferentiated glacial and non-glacial soils.
Our interpretation is a very dense compact sand and gravel derived during the Possession
glaciation of the region.
The Possession advance outwash sand and gravel was deposited during a previous glaciation of
the Puget lowland approximately 60,000 years ago. As the ice sheet advanced south through
the area, silt,sand,and gravel were deposited ahead of the advancing ice sheet by pro-glacial
streams meandering through a broad outwash plain. The advance outwash consists of
interbedded sequences of sand,gravel,and silty sand. Where a prominent silty unit is present
underlying more granular sand,the silt layers can act as aquitards that impede the downward
percolation of groundwater through the more permeable sand. In places this can result in
groundwater seepage at the silty sand/sand contact. The advance outwash deposits have been
overridden and compressed by the advancing ice sheet and display similar low compressibility
and high shear strength characteristics. Despite these strength characteristics,the Possession
outwash is still highly erodible by wave action. The landslide that occurred in March of 2019
followed a high tide wind event that eroded the toe of the slope.
The bulkhead at the site protects fill soils placed landward of the bulkhead and seaward of the
former toe of slope. The fill,where exposed,consisted of loose,moist, silty sand with gravel,
concrete debris,and creosote logs and was likely placed with no engineering oversight or
control. As a result,the fill and debris is highly suspect in terms of its erosion resistance and
strength qualities. Even properly placed and compacted select fill is highly susceptible to
shoreline erosion if not protected from wave action.
Shoreline retreat rates are dependent on factors such as upland land use,precipitation,geologic
conditions, over-water fetch distance and direction,and shoreline protection. However,
adequate protection of the fill soils from rapid erosion is the most important factor that will
affect erosion rates and subsequent property damage at the site if the fill is exposed to wave
action.
Despite its relatively poor condition,the log portion of the bulkhead is still providing toe
protection. A comprehensive natural bank retreat analysis could not be performed due to the
long-term presence of the existing bulkhead and the historically placed fill soils.
Conclusions
There is limited structural integrity of the log bulkhead based on signs of deflection,wood-rot,
and piping of backfill soils. Based on the existing deteriorated condition and the degree of
piping and undercutting observed,the log bulkhead is at high risk of being breached and in
need of replacement. Continued piping will eventually result in continued settlement and
movement of which will directly affect the patio and the upland development on the eastern
lot.
Current plans call for replacing the entire log bulkhead with a new rock bulkhead with an
approximate 30 feet extension to the southwest to protect the exposed landslide area. Erosion
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Frank Dahl
January 25,2021 Project No. 20077
on the unprotected western parcel has resulted in destabilization of the developed eastern
parcel. The bulkhead will abut a prominent cedar tree at the shoreline with a well-developed
root structure.
Replacing the existing log bulkhead with a properly constructed rock bulkhead as proposed
will provide a more predictable erosion control structure(in terms of long-term performance),
which will protect the existing upland features at the site. Replacement will also result in the
removal of concrete and treated wood from the shoreline environment.
Alternatives
Shoreline erosion and stability can be addressed by several methods depending on site
conditions, location,and project goals. Shoreline erosion control methods can range from the
use of planted supplemental vegetation to control surface erosion,constructing soft or bio-
engineered shoreline protection systems,constructing hard surfaced bulkheads or rock
revetments,or the application of a hybrid system that employs both hard and soft armoring
techniques. Replacing an existing erosion control structure however,limits the available
alternatives for the property owner to adequately protect existing improvements on the
property and on adjacent properties.
Supplemental Vegetation
In our opinion,the existing upland vegetation at the site does not contribute to protection of the
property from wave action. Supplemental vegetation will have no mitigating effect on wave-
generated toe erosion and is not a viable option as a replacement for protection at the site.
Rock Bulkhead
Rock bulkheads consist of essentially vertical rock walls constructed at or near the ordinary
high water line. 3,000 to 5,000 pound rocks are keyed into the beach approximately 4 feet,
stacked to a typical height of approximately 5 to 7 feet above the beach surface and backfilled
with free-draining rock ballast. Out of the alternatives discussed in this report,rock bulkheads
are the most effective method to control toe erosion over the anticipated service life of the
structure. The most significant effect of constructing a rock bulkhead is reduced sediment
input to the beach. This is the result of building a structure that is necessarily designed to stop
or significantly slow toe erosion. While the effects of reduced sediment input to the beach can
be mitigated,rock bulkheads result in an alteration of the shoreline as compared to natural
conditions. Where a rock bulkhead replaces an older outdated design such as the bulkhead at
the site,erosion control is maintained while adverse effects to the upper intertidal area are
reduced. A rock bulkhead represents less of an impact to the upper intertidal zone when
constructed to replace a log bulkhead.
Soft Bank Protection
The use of soft bulkhead techniques includes the combined use of beach nourishment and
restrained large woody debris(beach logs)at the site. Beach logs are restrained using cables
and an earth retention system such as dead-man anchors or tieback style earth anchors. The
logs are arranged in such a way as to impede wave energy acting on the toe of the slope.
Imported beach sediments are then placed to embed the logs aiding in energy dissipation. The
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Frank Dahl
January 25,2021 Project No. 20077
beach nourishment also provides sacrificial sediment that is actively recruited by the natural
coastal processes acting on the shore. In areas of diminished littoral transport volume and
areas where sediment recruitment is high,soft bank protection typically results in a net loss of
nourished sediment. As the sediment is eroded over time,additional nourishing sand is placed
to maintain the erosion control function of the system. The amount of beach nourishment is
initially estimated using average bank retreat rates for the project site but is adjusted based on
actual long-term performance.
Soft bank protection is best suited for low wave-energy environments where a broad,relatively
flat back-beach and upland area exists to minimize the angle of the constructed surface and
maximize the wave run-up distance. Soft bank structures are also not well suited for sites that
are flanked by existing hard surface structures such as the northwest side of the project site or
where existing structures such as dock approaches need long-term protection. A breach of a
soft bank system during a storm event would not only jeopardize the project site,but would
affect the integrity of adjacent structures.
The performance of a soft bank system is directly related to the intensity,duration,and
frequency of high-energy storm events. As a dynamic system,they result in the lowest overall
impact to the shoreline environment but provide the least amount of bank protection during
storm events. The topography and aspect of the site is an environment not suited to soft bank
protection.
Mitigation
Beach nourishment volume estimates for the entire 80 feet of bulkhead to be constructed was
not performed due to the extensive historical modification of the shoreline and because no
natural erosion(that could contribute sediment to the beach)is currently occurring behind the
existing log bulkhead. However for estimating purposes,the 30 feet of bulkhead extension
will impede approximately 6 inches of soil per year based on our observations of the
neighboring property. This translates to approximately 2 cubic yards of soil that will no longer
reach the beach each year the bulkhead is in place. The excavated natural sand and gravel that
will be removed as part of the project should be placed on the beach as natural sediment input.
Based on our calculations,the footprint of the bulkhead will generate approximately 53 cubic
yards of soil that may be used for beach nourishment. 53 yards is the equivalent of
approximately 25 years of erosion along the extension portion of the bulkhead.
Recommendations
Based on our field observations,the existing log bulkhead is at the end of its service life and is
in need of replacement. To reduce the probability of acute and severe erosion of the fill
between the slope and bulkhead,and to reduce the impacts of the existing structures to the
existing coastal processes at the site,we recommend replacing the existing bulkhead as
proposed. The new and extended rock bulkhead should be placed at or above the existing
OHW(estimated at the face of the existing bulkhead). Construction of the bulkhead flush to
the existing rock bulkhead to the east and ending at the prominent cedar tree to the west will
minimize scour and erosion that may be occurring. Removal of treated wood and construction
of the new bulkhead within the same footprint will have no adverse impacts to the existing
littoral sediment movement in the area resulting in a no net loss of ecological functions.
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Frank Dahl
January 25, 2021 Project No. 20077
To reduce the probability of erosion to the fill soils behind the proposed bulkhead we
recommend the following:
• Verify that the existing surface water drainage systems including roof runoff from the
house are functioning properly and can still be routed to the beach when the new bulkhead
is constructed. This pipe should be located,protected during construction,and replaced to
its current location. No concentrated flow should be allowed to discharge within the
existing fill soils behind the proposed bulkhead.
• Place sufficient size rocks landward of the OHW line to create a gravity wall system. The
rocks should be placed in a manner to provide a smooth connection to the existing
bulkhead to the northeast and to a suitable stopping point landward of the OHW mark at
the southwest property line.
Plans Review
We have reviewed the proposed plans prepared by Sealevel Bulkhead Builders,titled"New
Rock Bulkhead for Frank Dahl, 890 Dewatto Beach Drive, Tahuya, WA"dated 11/11/20
provided to us on January 13,2021. In our opinion,the plans as proposed are consistent with
the geotechnical opinions and recommendations in this letter-report.
Limitations
This letter-report was prepared based on a limited field reconnaissance and no subsurface
explorations were performed. A more in-depth characterization of slope conditions and
geotechnical analysis,including quantitative slope stability analyses for the subject property or
the adjacent properties was beyond the scope of work for this study. There are inherent risks
associated with construction in the shoreline environment. The longevity and effectiveness of
shoreline armoring is highly dependent on the frequency and severity of wave-generating
storm events that are inherently unpredictable. Other factors that impact the effectiveness of
shoreline armoring are the frequency and magnitude of upland landslide activity and erosion,
and the skill and care used by the contractor during construction of the revetment. The
opinions in this letter report are meant to help the property owner manage issues related to
erosion at the site and do not constitute a legal opinion or a warranty,express or implied. This
letter-report was prepared for the exclusive use of Frank Dahl and his agents with specific
application to the project site.
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Frank Dahl
January 25,2021 Project No.20077
It has been a pleasure to provide these services to you. If you have any questions, please do
not hesitate to call.
Sincerely,
COASTAL SOLUTIONS, LLC
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ngineedng Geologist Z� 32152 \
2220
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ed Ge
l Robert F.Cousins EXPIRES: 02/064t/
Robert F. Cousins,LEG Nina L.Cousins,PE
Principal Geologist Principal Engineer
rob@coastalsolns.com nina@coastalsolns.com
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