noting that cliff erosion is slower where uplift rates are highest and the base of the cliff has been raised to an elevation seldom reached by wave runup. Priest (1999) found that cliffs and bluffs in Lincoln County, Oregon, generally retreated at rates less than 19 cm yr-1 for 1939–1991. In landslide areas, bluff retreat rates were somewhat higher, ranging from 11–50 cm yr-1.
The physical forces driving cliff and bluff erosion include marine processes—primarily wave energy and impact, but also tidal range or sea-level variations—and terrestrial processes, such as rainfall and runoff, groundwater seepage, and mass movements such as landslides and rockfalls. As discussed in Chapter 4 (“Short-Term Sea-Level Rise, Storm Surges, and Surface Waves”), waves may be getting higher (e.g., Figure 6.1). Increased wave heights mean that more wave energy is available to erode the coastline. Rising sea level would exacerbate this effect because waves will break closer to the coastline and will reach the base of the cliff or bluff more frequently, thereby increasing the rate of cliff retreat.
Cliff and bluff retreat is an episodic process whereby large blocks fail suddenly under conditions of heavy rainfall, large waves at times of elevated sea levels or high tides, or earthquakes, followed by periods of little or no failure. In steep, mountainous areas, failure is often through large landslides or rock falls (Figure 6.2), usually driven by excess or prolonged rainfall during the winter months. With very large landslides, such as the Portuguese Bend slide on the Palos Verdes Peninsula, the shoreline may actually be extended seaward for a decade or more before basal wave action removes the protrusion (Orme, 1991). The episodic nature of cliff retreat, combined with the frequent absence of an identifiable edge or reference feature, makes it difficult to quantify or verify cliff erosion rates in mountainous areas over short time intervals, such as a few decades, or to project future erosion rates (Priest, 1999).
FIGURE 6.1 Boiler Bay, Oregon. Some evidence suggests that waves have been increasing in height off the west coast. SOURCE: Courtesy of Erica Harris, Oregon State University.