Geomorphic responses of beaches to shoreline armoring in the northern Salish Sea

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McBride, Aundrea; Dethier, Megan; Beamer, Eric; Salish Sea Ecosystem Conference (2014 : Seattle, Wash.)
Digital content made available by University Archives, Heritage Resources, Western Libraries, Western Washington University.
Terrestrial and Aquatic Ecology
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We quantified geomorphic parameters at 36 armored-unarmored site pairs in Snohomish, Island, Skagit, San Juan, and Whatcom counties. Most sites were in relatively wave-protected waters, except for the exposed side of Whidbey Island. In all cases, both beaches in a pair were within a drift cell and most were on bluff backed beaches that are eroding and contributing sediment to the drift cells. Sites were matched for aspect and nearshore bathymetry. Key parameters that were affected by armoring included elevation of the top of the beach (lower in front of armoring) and width of the beach (narrower when armored). Detailed topographic surveys and sediment grain size data were used to test for a signal of scouring impact in front of armoring, and increase in slope of armored beaches. Detailed grain size analyses, within the context of beach sediment source material, tested for hypothesized coarsening of sediments on armored beaches. Beach-scale comparisons were structured to identify local-scale impacts to the beach ecosystem, but our analyses also compared armoring effects at larger spatial scales by seeking differences in largely-armored vs. largely-unarmored drift cells. The inter-related sediment dynamics within a drift cell build and change habitats, thus impacting shoreline ecology. We compared pocket estuary tidal prism to pocket estuary opening, backshore width and area, and bluff retreat in cells of similar exposure, length, and sediment input. In addition, we have tested for long-term impact by comparing beach elevation and topography from sites where historic survey data are available, as much as possible using data prior to development, e.g. in the 1880s. The drift cell scale analyses are intended to identify the cumulative habitat impacts of shoreline armoring over time.