et al. (2002) measured the effect of brush bundles in attenuating waves from boat wakes and found up to a 60 percent reduction in wave energy impacting a delta levee when the bundles were in place, depending on the tides. In a study of small waves in a shallow lake, Lövstedt and Larson (2010) found an average decrease in wave height of 4–5 percent per meter within the first 5–14 m of beds of Phragmites australis. If these results are applicable to tules (Schaenoplectus spp.), which are similar in height, extensive tule restoration could result in substantial attenuation of waves (produced by wind or vessels) within the delta.
The transition from tules to Salicornia virginica dominated marshes in the bay is accompanied by a major change in plant morphology. Salicornia virginica resembles Atriplex portulacoides above ground, which has a lower stem density, height, and diameter than the two Spartina spp. (Feagin et al., 2011). This suggests that Salicornia virginica marshes in the bay may play less of a role on attenuating storm set-up and waves than the reed-like architecture of tule marshes in the delta.
Modeling of the propagation of long waves into the south bay (Letter and Sturm, 2010) suggests that small areas of marsh can ameliorate the effects of storm events on water level. Letter and Sturm (2010) predicted changes in water level at specific locations during simulated storm events, based on the roughness and extent of vegetation cover and other parameters. They found that water levels are lower at the edge of salt ponds fronted by some marsh than they are at the edge of mudflats. For storm tides during the January 1983 El Niño event, which set records for high sea level (see “Changes in Ocean Circulation” in Chapter 4), water elevations on levees not fronted by a small area of marsh were higher than those with marsh between the levee and the intertidal flats. The extent of marshes in the south bay is limited, so whether reductions in water levels in small areas can be extrapolated to larger landscapes will require more detailed modeling of potential future landscape configurations.
Case Study on the Puget Sound
Puget Sound includes more than 8,000 square kilometers of marine waters and nearshore environment, and 4,020 kilometers of shoreline. About 4 million people live in the Puget Sound watershed, and the population is expected to reach 5 million by 2020 and 8 million by 2040 (Puget Sound Regional Council, 2004). Commercial fish and shellfish harvesting in Puget Sound is an important industry for the state.
Tidal marshes and eelgrass beds are among the most important coastal habitats in Puget Sound. Extensive tidal marshes occur at the mouths of rivers that empty into Puget Sound. Eelgrass is found from the intertidal zone to the shallow subtidal zone in central and north Puget Sound. Loss of these habitats has been dramatic. Nearly three-quarters of the original salt marshes and essentially all river delta marshes in urbanized areas of the sound have been destroyed (Gelfenbaum et al., 2006). Eelgrass habitat is almost completely gone in Westcott Bay and several other small embayments (Mumford et al., 2003; Wyllie-Echeverria et al., 2003).
The nearshore environments of Puget Sound are maintained by a complex interplay of biological, geological, and hydrological processes that interact across the terrestrial-marine interface. Many of these processes have been significantly affected by human activities (Bortelson et al., 1980). For example, dikes have altered nearshore sedimentation patterns and eliminated the tidal influence that forms salt-marshes, and dams have reduced the magnitude and frequency of floods, limiting the sediment supply to river deltas. More than 33 percent of shoreline in the Puget Sound region has been modified (Puget Sound Action Team, 2002).
The dramatic nature of these changes and the need to accommodate future population growth without further environmental degradation has led to concerted efforts to improve coastal management and restore ecosystems (e.g., Puget Sound Partnership; Puget Sound Nearshore Ecosystem Restoration Program). Such efforts must factor in the effects of future sea-level rise, which is complicated by the strong gradients in vertical land motion in the area (Figure 6.21). Whether vertical land movements enhance or counteract the effects of regional sea-level rise has important implications for existing coastal habitats, the viability of future restoration, and the potential of these habitats to help mitigate the effects of future storms.