Quantitative Analysis of Holocene Shoreline Retreat in Unconsolidated Sediment within the Northern Puget Lowland, WA. Heather Rogers, University of Washington Earth and Space Sciences Department Box Terry Swanson, University of Washington Earth and Space Sciences Department Box 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. 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. 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 (~ 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 post- glacial shoreline position by extending the longitudinal profile of the remnant channel to a point that intersects the original post-glacial terrace elevation (140 ft). 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. 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 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 ( 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 post- glacial 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. Literature Cited Baum, R.L., Chleborad, A.F., and Schuster, R.L., 1998, Land­slides triggered by the winter storms in the Puget Lowland, Washington: United States Geological Survey Open File Report , 16p. Gosse, J.C. and Phillips, F.M Terrestrial in-situcosmogenic nuclides: Theory and application: Quaternary Science Reviews. 20, Haugerud, R High-Resolution Lidar Topography of the Puget Lowland Washington- A Bonanza for Earth Science, GSA Today, Vol. 13, Keuler, R.F., 1988, Map showing coastal erosion, sediment sup­ply, and longshore transport in the Port Townsend 30- by 60-minute quadrangle, Puget Sound Region, Washington: United States Geological Survey Miscellaneous Investiga­tion I-1198-E. Shipman, H. 2004, Coastal Bluffs and Sea Cliffs on Puget Sound, Washington. 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