Event Title

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

Presentation Abstract

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.

Session Title

Session S-06H: Puget Sound Shorelines and the Impacts of Armoring: State of the Science 2014

Conference Track

Shorelines

Conference Name

Salish Sea Ecosystem Conference (2014 : Seattle, Wash.)

Document Type

Event

Start Date

1-5-2014 1:30 PM

End Date

1-5-2014 3:00 PM

Location

Room 607

Contributing Repository

Digital content made available by University Archives, Heritage Resources, Western Libraries, Western Washington University.

Rights

This resource is displayed for educational purposes only and may be subject to U.S. and international copyright laws. For more information about rights or obtaining copies of this resource, please contact University Archives, Heritage Resources, Western Libraries, Western Washington University, Bellingham, WA 98225-9103, USA (360-650-7534; heritage.resources@wwu.edu) and refer to the collection name and identifier. Any materials cited must be attributed to the Salish Sea Ecosystem Conference Records, University Archives, Heritage Resources, Western Libraries, Western Washington University.

Type

Text

Language

English

Format

application/pdf

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May 1st, 1:30 PM May 1st, 3:00 PM

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

Room 607

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.