and larvae. Lacy and Wyllie-Echeverria (2011) studied the influence of eelgrass (Zostera marina) on near-bed currents, turbulence, and drag in the San Juan archipelago of Puget Sound. Zostera marina grows at water depths less than 5 m relative to mean lower low water along 43 percent of Puget Sound’s shoreline (Berry et al., 2003). Lacy and Wyllie-Echeverria (2011) measured velocity profiles up to 1.5 m above the sea floor over a spring-neap tidal cycle, including measurements above and within the canopy. They found that eelgrass attenuated currents by a minimum of 40 percent, and by more than 70 percent at the most densely vegetated site, with attenuation decreasing with increasing current speed. Even sparse canopies influenced near-bed flow and significantly attenuated currents.

Most Puget Sound shorelines are sheltered, and waves are generated by local winds with little or no energy component from ocean swell. The topographic confines of Puget Sound limit the height of waves (Finlayson, 2006). Large waves (greater than 0.4 m significant wave height) occur only during infrequent wind storms. Consequently, the effect of eelgrass beds, and to some extent coastal marsh vegetation, on wave attenuation can be substantial.

CONCLUSIONS

Sea-level rise and storms along the west coast of the United States have caused significant coastal retreat. Cliff and bluff retreat, caused mainly by wave erosion and terrestrial processes (e.g., landslides, slumps, rockfalls, runoff), ranges from a few centimeters to tens of centimeters or more annually, with weaker rocks and areas of lower topography retreating more than resistant bedrock cliffs and headlands. Cliff retreat is not reversible. Although coastal armoring can buy time, today’s seawalls and revetments will eventually be overwhelmed by sea-level rise and increasing wave heights.

Sand dunes and beaches, which consist primarily of unconsolidated sand, provide little resistance to severe wave attack, especially at times of elevated sea level. Consequently, beaches and barrier spits may grow and shrink several meters or more per year. Because beaches are nearly flat, a small rise in sea level can cause a large retreat of a beach. Where beaches and barrier spits are prevented from migrating by coastal armor or structures, they will eventually be inundated by future sea-level rise.

Rising sea levels and increasing wave heights will exacerbate coastal erosion and shoreline retreat in all geomorphic environments along the west coast. Projections of future cliff and bluff retreat are limited by sparse data in Oregon and Washington and by a high degree of geomorphic variability along the coast. Projections using only historic rates of cliff erosion predict 10–30 meters or more of retreat along the west coast by 2100. An increase in the rate of sea-level rise combined with larger waves could significantly increase these rates. Future retreat of beaches will depend on the rate of sea-level rise and, to a lesser extent, the amount of sediment input and loss.

Some of the coastal damage expected from sea-level rise and storminess may be mitigated in some areas by coastal mudflats and marshes. Mudflats and marshes protect inland areas from inundation and wave damage, but the specific effect depends on local conditions. Some studies have found that certain plants, such as eelgrass, slow water currents. Other studies have found that marsh vegetation with high roughness, stem height, and density—along with coastal topography and bathymetry—reduces wave height and energy. However, this relationship has not been specifically demonstrated for many of the species populating west coast marshes.

West coast tidal marshes can survive sea-level rise by building elevation to keep pace with rising water levels, which requires an adequate supply of sediment and/or organic matter accumulation. They may migrate inland if the area is unobstructed, but unless they maintain elevation under sea-level rise, the area of marsh will be limited by the slope of the land surface and the tidal range. Storms are an important agent for delivering sediment and increasing the elevation of marshes. For the sea-level changes projected by the committee for 2030 and 2050 in central and southern California, frequent storms that increase tidal inundation and promote sediment deposition could allow marshes to survive. In northern California and southern Oregon, fluvial inputs of sediment, which depend on storms and water management practices, also are important for sediment deposition. Entrapment of sediment behind dams makes marshes less able to survive sea-level rise in this area. Coastal areas in Oregon and Washington are projected to have lower rates of sea-level rise, in



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