of coastal habitats in ameliorating the effects of future storms or tsunamis on the west coast.
The morphodynamic interactions among topography and bathymetry, vegetation, sediment deposition, and turbulent flows are difficult to predict, increasing uncertainties about the extent to which coastal habitats will mitigate the effects of future sea-level rise and storms. A means for reliably determining wave damping by vegetation for engineering studies has not been developed (Augustin et al., 2009). Models that reliably predict coastal morphology (independent of the role of vegetation) over decades and under episodic storm forcing are not widely available. For these reasons, significant tolerance for future coastal habitats, vegetation, and coastal morphology configurations will have to be built into coastal protection systems.
As shown above, the response of marshes to future sea-level rise and storminess along the west coast of the United States depends on local conditions. Marsh restoration is also site specific. Consequently, the committee chose two areas where data on prior restoration are available—the California Bay Delta and the Puget Sound—to explore the potential for marsh restoration given future sea-level rise and the effect of marshes on storm and wave attenuation.
Case Study on the California Bay-Delta
California’s Bay Delta estuary is one of the largest estuaries in the United States. The estuary consists of a series of interconnected bays and channels connecting San Francisco Bay to the Sacramento-San Joaquin River Delta. Salinity increases from the delta to the Golden Gate at the mouth of San Francisco Bay. At times of high river flood, fresh conditions can penetrate into the bay.
The estuary has been modified extensively by anthropogenic activities over the past 150 years (The Bay Institute, 1998; Goals Project, 1999; Brown, 2003). Approximately 80 percent of the tidal wetlands in San Francisco Bay and 95 percent of the tidal wetlands in the Sacramento-San Joaquin Delta have been lost (The Bay Institute, 1998). In the south bay, more than 90 percent of the historic tidal marsh area has been converted to salt ponds, agricultural areas, and urban developments (Foxgrover et al., 2004; Figure 6.20). Many of these areas are protected by an aging collection of levees.
The extensive loss of tidal marsh habitat has prompted calls for marsh restoration in the San Francisco Bay Delta (e.g., Goals Project, 1999; CALFED, 2000; Steere and Schaefer, 2001). Given the large investment required to restore thousands of acres of tidal marsh, it is important to understand the likely role of restored marshes in attenuating storms and waves and whether they will persist under future sea-level rise.
Potential for Marsh Restoration
One of the first steps in marsh restoration is to return the land surface to elevations that can be colonized by marsh vegetation. Many land surfaces within the delta are currently on the order of 3–5 m below water levels. Data from interferometric synthetic aperture radar show that the delta-interior regions are subsiding 3–5 mm yr-1 and that local regions in the delta are subsiding up to 2 cm yr-1 (Brooks et al., 2012). In areas where subsidence exceeds sediment accumulation, it may be necessary to fill low-lying areas to enable colonization. Sedimentation rates are low in much of the delta because fine sediments are slow to settle and waves keep them in suspension (Simenstad et al., 2000). In some shallow areas with nearly 100 years of sedimentation (e.g., Sherman Lake and Big Break), sediment accumulation has not yet been sufficient to allow vegetation to become reestablished.
Where sediment accumulation exceeds subsidence, vegetation colonization may proceed naturally. For example, high vertical accumulation rates of 3 cm yr-1 for 1955–1963 and 4.2 cm yr-1 for 1963–1983 were inferred from 137Cs measurements of marsh cores at Alviso in the south bay (Patrick and DeLaune, 1990). Orr et al. (2003) found accretion rates for restored marshes in San Pablo Bay of 18–70 mm yr-1 for low marsh and 9–10 mm yr-1 for high marsh. At these rates, marsh restoration could progress under all except the committee’s high projections of 2100 sea-level rise. However, high rates of past accretion may not