Quantitative Analysis of Holocene Shoreline Retreat in Unconsolidated Sediment within the northern Puget Lowland, WA.

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. Prior to the establishment of vegetation on Whidbey Island, 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 became inactive when vegetated. 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 substrate 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 increasing population and pressures on shoreline development.