Impacts of Armoring on Sheltered Shorelines: Puget Sound, Washington

timber-pile bulkheads built to protect residential property from erosion at Ledgewood Beach

Above: Photograph from the cover of the new USGS report shows timber-pile bulkheads built to protect residential property from erosion at Ledgewood Beach, west side of Whidbey Island, Puget Sound. Photograph by Hugh Shipman, Washington Department of Ecology. [larger version]

Science experts from various agencies and universities have compiled a "state of the science" summary of information on the impacts of armoring on sheltered shorelines—such as those of Puget Sound, Washington—to help inform shoreline communities, planners, and agency decision makers. Their findings were recently released in a report published by the U.S. Geological Survey (USGS).

The report—Puget Sound Shorelines and the Impacts of Armoring—Proceedings of a State of the Science Workshop, May 2009, edited by Hugh Shipman (Washington State Department of Ecology), Megan N. Dethier (University of Washington), Guy Gelfenbaum (USGS), Kurt L. Fresh (National Oceanic and Atmospheric Administration), and Richard S. Dinicola (USGS)—addresses the geologic, oceanographic, and biologic responses to armoring and includes a section on management needs, a summary of science needs, and a literature review. The proceedings were published as USGS Scientific Investigations Report 2010-5254, available online at http://pubs.usgs.gov/sir/2010/5254/.

One of the largest estuaries in the United States, Puget Sound has roughly 2,500 miles of sheltered coastline, about one-third of which is armored. An increasing regional population and rising sea level will likely increase the pressure for additional shoreline armoring. Bulkheads, seawalls, and other armoring structures protect shoreline properties from damage and loss due to erosion, but armoring can also affect the nearshore habitat that is so important to restoring and preserving the health of Puget Sound.

The effects of armoring on shorelines are complex, and communities need to have the best available scientific information when facing difficult decisions about regulating shoreline activities and prioritizing restoration projects. To address this need, a scientific workshop was held near Hood Canal, Washington, on May 16-19, 2009, bringing together 38 local and national scientists to review the state of the science regarding the physical and biological impacts of armoring on shorelines—with a special emphasis on sheltered shorelines such as those of Puget Sound. The workshop produced 22 scientific papers, which are compiled in the USGS report.

The summary of the report contains the following conclusions, based on breakout-group discussions during the workshop and findings reported in the scientific papers:

Although armoring alters the shoreline in different ways in different ecosystems around the world, almost every study has demonstrated impacts to some beach feature or function that society regards as valuable. These impacts range from loss of space for recreation on the beach, to a decrease in the number of foraging shorebirds, to erosion of adjacent properties. The benefits accrued by erosion-control structures must be weighed against their negative impacts to public resources and shoreline ecosystems.

Armoring constructed to prevent shoreline or bluff erosion also reduces sediment supply from the bluff onto the beach and into the drift cell (a segment of shoreline in which alongshore movement—or drift—of sediment is confined by natural barriers such as headlands, or manmade barriers such as jetties). Not all armoring, however, is likely to be equally harmful in terms of reducing sediment supply to the shoreline because natural sediment delivery to beaches varies so widely. Specific coastal assessments can suggest types of landforms or locations (for example, position within a drift cell) where armoring will have the least impact on sediment supply.

The lower armoring is placed on the shore, the worse its impacts, particularly armoring built below the highest high-tide line. As sea level rises, even structures originally built high on the beach may encroach farther into the intertidal zone and produce increasingly negative impacts on the shoreline.

Armoring of individual properties is often treated as a benign activity, but the cumulative result of armoring multiple properties may have significant long-term impacts on beaches and drift cells.

As sea level rises, ongoing erosion in areas with armored shorelines will result in the progressive loss of beaches around Puget Sound. This will reduce both the recreational benefits and the ecological functions provided by the beaches.