or migrate under elevated sea levels and large storm waves. Erosion or landward migration of sand spits or barrier bars will occur more frequently with sea-level rise (Pilkey and Davis, 1987).
Back-beach barriers can slow or halt the natural inland migration of beaches because of rising sea level. Where a seawall, revetment, or structure exists, the shoreline cannot advance landward and the beach is progressively inundated (Figure 6.6). This process, known as coastal squeeze or passive erosion, has been documented in a number of locations along the west coast. Similarly, barrier spits that have been developed and then protected with revetments cannot migrate with sea-level rise (Figure 6.7). Depending on the rate of sea-level rise, all west coast beaches with hardened or constrained back beach edges will gradually be inundated.
Only a few studies have quantified rates of change along the sandy shoreline of the U.S. west coast. Kaminsky et al. (1999) found widely varying rates of change for the sandy shoreline of Pacific County, Washington, ranging from +0.8 to +14.2 m yr-1 for 1870–1926, -13.6 to +8.8 m yr-1 for 1926–1950, and -7.0 to +4.2 m yr-1 for 1950–1995. Sand spits eroded or accreted, depending on sand supply, wave energy, and relative sea level. Coastal land change along the sandy shoreline of California was assessed as part of the U.S. Geological Survey’s National Assessment of Shoreline Change program (Hapke et al., 2006). Maps, aerial photographs, and, more recently, lidar (light detection and ranging) were used to determine both long-term (1800s to 1998–2002) and short-term (1950s—1970s to 1998–2002) rates of shoreline or beach change. More than 16,000 transects revealed that the shoreline eroded 0.2 ± 0.4 m yr-1 over the short term. The average rate of long-term change was 0.2 ± 0.1 m yr-1, an accretional trend, although 40 percent of the transects showed net erosion. This net accretional trend was attributed to the large volumes of sediment that were added to the system from large rivers and to the impact of coastal engineering and beach nourishment projects (Hapke et al., 2006). A similar assessment effort is planned for the Oregon and Washington coasts.
Cooper (1958, 1967) mapped and described the coastal dunes of Washington, Oregon, and California, and found that extensive coastal sand dunes accumulate when the following conditions are met: (1) a large supply of fine-grained sand, (2) a barrier such as a headland to trap littoral drift and accumulate sand, (3) a low-relief area landward of the beach where sand can accumulate, and (4) a dominant or persistent onshore wind. Large dune fields are best preserved in areas that have undergone either net subsidence or limited uplift during the Quaternary (Orme, 1992). Dunes back about 45 percent of the Oregon coast and 31 percent of the
FIGURE 6.6 (Left) Passive erosion in front of a revetment, illustrating the loss of beach where the structure restricts the shoreline from migrating landward. The beach continues to migrate inland on either side of the revetment. (Right) Recovery of the beach following removal of the revetment and bluff top structure. SOURCE: Copyright 2002–2012 Kenneth & Gabrielle Adelman, California Coastal Records Project, <www.Californiacoastline.org>.