ties of the cliff materials (e.g., lithology, structural weaknesses, rock strength, weathering) and extrinsic factors impacting the cliffs (e.g., wave energy, offshore bathymetry, rainfall). Although waves are the primary cause of seacliff erosion, the physical properties of the cliff materials in the San Diego study area strongly affect the erosion rates (Benumof and Griggs, 1999; Benumof et al., 2000).
Coastal Hazard Assessments
The U.S. Geological Survey developed an index of coastal vulnerability to sea-level rise in 2000 (Thieler and Hammar-Klose, 2000). The relative vulnerabilities of different coastal environments along the U.S. west coast to long-term sea-level rise were quantified based on variables including coastal geomorphology, regional coastal slope, rate of sea-level rise, wave and tide characteristics, and historical shoreline change rates. The rankings for each of the six variables at any particular location can be averaged to produce an overall coastal vulnerability index from 1 (very low) to 5 (very high; Table 6.2). This index provides a broad overview of how different regions of the west coast are likely to change in response to sea-level rise. Two specific regions (southwestern Washington/northwestern Oregon and San Francisco to Monterey, California) are covered in more detail, with maps delineating the distribution of various risk factors and an overall ranking of risk (Figure 6.13).
Living with the Changing California Coast (Griggs et al., 2005) provides a different approach for assessing coastal hazards in California and includes mile-by-mile maps of the entire coastline. Information on the maps include shoreline environment, erosion rates where published or known, presence and type of armoring, notes or comments on individual coastal areas and specific issues or problems, and a hazard ranking ranging from stable/low risk to hazard/high risk. An example is shown in Figure 6.14.
The high spatial variability portrayed in these maps underscores the difficulty of generalizing the response of coastal cliffs and bluffs, beaches, and dune to sea-level rise along the west coast of the United States.
Estuaries and tidal marshes are valuable ecosystems, providing a variety of services as well as the economic livelihoods of many communities (Mitsch and Gosselink, 2000; MA, 2005). Open waters, mudflats, and marshes offer refuge and forage for wildlife, fishes, and invertebrates. Shallow ponds and seed-producing vegetation provide overwintering habitat for millions of migratory waterfowl. Wetlands help absorb nutrients and reduce loading to the coastal ocean. They also help protect local communities from flooding, either by storing riverine floodwaters or by damping storm surges from the ocean.
Estuaries are bodies of water formed at the coastline where fresh water from rivers and streams flows into the ocean. The largest estuaries along the west coast of the United States include Puget Sound, the Columbia River Estuary, and the San Francisco Bay-Delta. Tidal marshes—herbaceous wetlands frequently or continu-
TABLE 6.2 Ranking of Variables Determining the Coastal Vulnerability Index
|Ranking of Coastal Vulnerability Index|
|Very low||Low||Moderate||High||Very high|
Low cliffs, glacial
Barrier beaches, sand
beaches, salt marsh,
mangrove, coral reefs
|Coastal slope||> 1.9||1.3–1.9||0.9–1.3||0.6–0.9||< 0.6|
|Relative sea-level change (mm yr-1)||< -1.21||-1.21–0.1||0.1–1.24||1.24–1.36||> 1.36|
|Shoreline erosion or accretion (m yr-1)||> 2.0
|-1.1– -2.0||< -2.0
|Mean tide range (m)||> 6.0||4.1–6.0||2.0–4.0||1.0–1.9||< 1.0|
|Mean wave height (m)||< 1.1||1.1–2.0||2.0–2.25||2.25–2.60||> 2.60|
SOURCE: Thieler and Hammar-Klose (2000).