Quantitative Analysis of Holocene Shoreline Retreat in Unconsolidated Sediment within the Northern Puget Lowland, WA.
Abstract
The long-term retreat rate of Puget Sound shorelines derived from unconsolidated
sediment is difficult to quantify because the geomorphic evidence used to establish prior
shoreline positions (i.e., wave-cut platforms) is itself, subsequently eroded by
submarine processes. Qualitative assessment of more recent shoreline retreat can be
inferred by comparing the width of wave-cut platforms lying adjacent to the respective
shorelines, bluff steepness, and vegetation age structure on the bluff slope.
Heather Rogers, University of Washington Earth and Space Sciences Department Box 35-1310
Terry Swanson, University of Washington Earth and Space Sciences Department Box 35-1310
A quantitative assessment of Holocene shoreline retreat on southern Whidbey Island,
WA is being completed using reconstructed longitudinal profiles of relict post-glacial
channels. The relict channels are inferred to have formed almost instantaneously after
deglaciation by groundwater sapping of the glacial sediment. The channels were graded
to the post-glacial marine high-stand (~140-160 feet A.S.L. on south Whidbey Island)
and following vegetation became inactive. Where shoreline retreat has been rapid, the
post-glacial marine terrace has been eroded, but the head of the relict channel is still
preserved. Using ground surveying techniques, inferences based on drumlinoid
topography, and high resolution LIDAR imagery, it is possible to infer the original postglacial shoreline position by extending the longitudinal profile of the remnant channel to
a point that intersects the original post-glacial terrace elevation (140 ft).
Geologic Setting
The Puget Sound and southern Georgia Strait form inland seas that contain over 3400
of Washington State’s 4000 kilometers of marine shoreline (Shipman, 2004).
Shorelines within this inland sea are either composed of unconsolidated sediment,
largely glacial and interglacial alluvial deposits or bedrock. Unconsolidated sediment
bluffs dominate much of the central and southern Puget lowland, where most of the
population lives.
County
Miles of Shoreline
Miles Unstable
Percentage
Island
221
112
57
Jefferson
195
81
42
King
113
66
58
Kitsap
246
50
20
Mason
218
96
44
Pierce
232
72
31
San Juan
376
13
3
Skagit
189
46
24
Snohomish
74
19
26
Thurston
111
50
45
Whatcom
118
36
30
TOTAL
2093
641
31
Motivation
Shoreline erosion rates are much higher for coastlines composed of unconsolidated
sediment because bluff slopes are more easily eroded by wave processes and
susceptible to saturation and cohesion-loss during heavy or long duration precipitation
events. While the erosion rate of a given shoreline within the Puget Sound is largely
controlled by its composition, several other factors play important roles and are
summarized by Shipman (2004): wave environment, bluff environment, and beach
environment. It is the varied effects of these secondary factors that explain the
observed differences in shoreline erosion rate for bluffs of similar composition.
The two counties with the highest percentage of unstable shorelines, Island and King
Counties, are projected to have their respective populations rise 47.26% (Island) and
25.58% (King) between 2000 and 2030. Similar population growth trends are
expected for the other counties lying adjacent to the Puget Sound (Washington State
Department of Transportation, 2006) and will experience similar shoreline growth
pressures. Bluff properties along the Puget Sound are in high demand because of their
view potential and proximity to water. Many homes have been, or will be constructed
in bluff locations that are susceptible to future mass wasting events related to shoreline
erosion. Potential loss of property and human life is a likely consequence of this
growth in the absence of realistic estimates of bluff retreat. It is therefore imperative
that shoreline planners and geotechnical personnel have a quantitative basis to assess
recent and future shoreline retreat in order to best adapt long-term shoreline
management policies.
Methods
The long term retreat rate of the shorelines lying adjacent to the Puget Sound is difficult
to quantify because the geomorphic evidence used to establish previous shoreline
positions (i.e., wave-cut platforms) may itself, subsequently be eroded by submarine
processes. Qualitative assessment of recent shoreline retreat may be determined by
comparing the width of wave-cut platforms lying adjacent to shorelines with variable
retreat rates or observing vegetation age structure trends. LIDAR imagery has recently
become available for many unstable shoreline areas lying within the Puget lowland.
This high-resolution topographic data provides valuable information about bluff
morphology and relict landforms that could not be resolved by previous methods
(Haugerud, 2003). We are using this important new tool to define longitudinal profiles
of relict post-glacial sapping channels that were graded to the marine high-stand (140160 feet A.S.L.) on South Whidbey Island. These relict channels are clearly
identifiable in the field and are observed on the LIDAR image of South Whidbey Island,
shown below (Fig…). The long profiles of many of these relict channels have been
truncated by shoreline erosion and their channel floors now lie at elevations above the
original marine high-stand shoreline. We have begun a detailed survey of all of the
major channels on Skagit Head and Possession Point (Fig…) to define the original postglacial shoreline and provide a base point for long-term erosion that occurred following
eustatic (global) sea-level reaching its present elevation (~6000 years ago).
In order to create a quantitative model for bluff retreat in the northern Puget Lowland,
this project will utilize two independent methods to assess both the long-term shoreline
retreat rate since eustatic sea level rose to near its present level (~6000 years ago), as
well as the retreat rate over more recent time scales, including the last century when sea
level rose ~20 cm (Figs…).
In the first phase of this project we will utilize detailed ground surveys and mapping of
55 reconstructed longitudinal profiles of relict post-glacial channels on South Whidbey
Island that are inferred to have been graded to the original post-glacial shoreline
position. For those channels that have been truncated by shoreline erosion, the
longitudinal profile of the remnant channel will be extended to a point that intersects the
former marine high-stand. A comparative baseline for magnitude of shoreline erosion
can then be ascertained for the entire coastline of South Whidbey Island. Because the
relict channels were incised immediately following deglaciation (~15,200 cal yr ago),
which was independently dated by Caffee and Swanson (2001), the timing of their
incision is coincident. The initiation period for major shoreline erosion would likely
coincide when eustatic sea level reached near its present level. By projecting the long
profile of the truncated channels to their original end point (i.e., marine high-stand
terrace), a quantitative assessment of shoreline retreat can be ascertained for a time
period over the last 6000 years.
The second phase of this project will use cosmogenic isotope (Cl-36 and Be-10) age
dating to determine the exposure age of residual till boulders eroded from the bluff
during its more recent retreat.
Further Research
Cosmogenic isotope (Cl-36 and Be-10) age dating to determine the exposure age of
residual till boulders eroded from the bluff during its more recent retreat. The exposure
age of the residual till boulders, which form a lag deposit on the wave-cut platform,
should constrain the timing of bluff erosion and ultimately the recent shoreline retreat
rate (Fig. 5). It is expected that the exposure ages of the residual boulders should
progressively increase from the modern shoreline to the edge of the existing wave-cut
platform. The cosmogenic isotope dating part of this research project is currently being
tested and may provide a very important data set for long-term shoreline planning.
Stream Profile #3
250
Elevation (feet)
In addition to geomorphic evidence, we are testing whether cosmogenic isotope dating
(Cl-36 and Be-10) can quantify recent erosion rates by dating the exposure age of
residual till boulders eroded by wave processes and deposited as a lag on the wave-cut
platform. Transects are being measured from different type localities exhibiting various
degrees of retreat due to differences in shoreline energy and substrate properties (i.e.,
exposure to prevailing storm track, length of fetch and the bluff substrates resistance to
erosion). The results from this research will have important implications for landslide
hazard assessment, residential planning and long-term erosion-sedimentation rates in the
Puget Lowland particularly with continuing rise in population.
200
150
100
50
0
-2000
-1500
-1000
-500
Distance From Source of the Channel (feet)
0
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//www.wsdot.wa.gov/planning/wtp/
datalibrary/population/countiespopgrowth.html.