Evaluating Puget Sound Beach Services for Protection and Restoration

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This project conducted by the Washington State Department of Ecology Coastal Monitoring & Analysis Program (CMAP) will advance ESRP’s understanding of beach processes, the impact of shoreline armor, and relative benefits of restoration measures in the Puget Sound region by comparing beach features between armored and adjacent unarmored shorelines. Existing boat-based LiDAR collected between 2015 and 2021 as part of the Mapping Bluffs and Beaches to Quantify Sediment Supply effort, will be used to create a robust dataset of key physical indicators of beach health extracted along cross-shore profiles from ~25 pairs of armored and unarmored shoreline. The results from analysis are intended to generate guidance for strategic prioritization and planning of recovery projects to maximize ecological lift objectives.

The impacts of shoreline armoring to the nearshore habitat have been observed throughout the Puget Sound region, however an inadequate understanding of the relationship between physical and ecological beach features makes it difficult to prioritize restoration sites and assess the effectiveness of recovery projects. As shoreline restoration and soft-shore protection efforts increase, it is essential to gain a strong understanding of beach characteristics on a natural beach and how they compare to armored beaches in order to make the best use of restoration and protection resources.

This project will use existing high-resolution boat-based lidar collected between 2015 and 2021 from ~25 pairs of armored and adjacent unarmored reaches of shoreline. Beach characteristics including beach width, slope, backshore width, large woody debris area, vegetation coverage, and bluff or armor toe elevation will be extracted from 1-m spaced cross-shore transects and averaged within 10-m alongshore intervals. This high spatial-resolution sampling strategy will ensure robust statistics and allow us to observe alongshore trends that may reveal any edge effects of shoreline armor.

Outcomes from the project are intended to inform management and planning of recovery projects. High-resolution baseline conditions of unarmored shorelines can function as target conditions for restoration and protection measures. Determining the quantitative impacts of armoring based on encroachment within the context of shore type, sediment supply, and drift cell setting, can offer guidance for prioritization in order to achieve maximum ecological lift. These data may also be applied to improve the guidance around soft-shore approaches to shoreline protection such as beach nourishment, gravel berms, and installation of large wood and riparian vegetation.

Site Selection[edit]

Sites were selected using a combination of qualitative and quantitative parameters describing the feasibility and geomorphic setting. The following criteria were considered:

  • ~500-meter stretch of shoreline with roughly 50% armored and 50% unarmored
  • Geographic and geomorphic distribution
  • Drift cells with mapped drift direction and apparent sediment source
  • Availability of high-resolution boat-based lidar data for analysis, with preference given to sites with two or more lidar datasets for temporal analysis

In some instances, more than one site was identified within a drift cell, which may offer insights on beach metric alongshore variability.

Key Physical Indicators[edit]

Physical indicators of ecological function were identified for analysis based on a literature review and the ability to map using existing boat-based lidar. Beach metrics include backshore width, foreshore width, foreshore slope, bluff or armor toe elevation, and relative vertical and horizontal encroachment. Habitat metrics include area of large woody debris, riparian vegetation, and overhanging vegetation. Cross-shore variability and alongshore trends will also be considered while interpreting results.

Interim Update[edit]

Physical indicators were extracted from four selected sites to showcase the methodology and explore initial results. The sites selected include two locations from western Whidbey Island, one site from the eastern shore of Maury Island, and one site from Edgewater Beach near the entrance of Eld Inlet.

While each site had unique results, an initial synthesis has revealed patterns that are seen consistently across sites. In general, armored shorelines had less woody debris coverage and riparian vegetation with a narrower or absent backshore area than their adjacent unarmored shorelines. Measures of beach width below mean high water (MHW) were narrower on armored shorelines particularly when the armor toe was below MHW, however beach width was about the same in armored and unarmored areas at the Ebey’s landing site on Whidbey Island where the armor toe was well above mean higher high water (MHHW).

Correlation results suggest that riparian vegetation cover is related to backshore width. It is possible that these two physical indicators share a positive feedback loop where riparian vegetation can establish on relatively wide and stable backshore areas. Once established, the vegetation may offer stability to the backshore area and even promote sediment accumulation, allowing propagation of vegetation that can further enhance sediment accumulation processes.

Plots revealed some indication of the “edge effect” of armoring where unarmored areas directly updrift and downdrift of armored areas act as a transitional zone because they are impacted by the nearby armor. In our data, this transition appears to occur between 10 and 30 meters alongshore on either side of the armor boundary. In some cases, armor extending seaward of the natural bluff toe location acted as a cross-shore barrier, accumulating large amounts of woody debris directly updrift.

Analysis from these four selected sites was shared at the American Shore and Beach Preservation Association conference in Fall of 2023. The presentation slides can be found here: File:ESRP BeachServices ASBPA 102023.pdf.

In the next phase of this project, Ecology will incorporate results from 12 additional sites for a total of 16 sites that will be analyzed. Results will be compared between sites and interpreted in their geomorphic context. By increasing our sample size, these initial results will be validated and explored in detail to make suggestions for restoration prioritization and design.