Cliff Erosion and Bluff Retreat Along the California Coast
Griggs, Gary B
Coast of California Caught Between Increasing Numbers of People and Ongoing Process of Shoreline Retreat
California is on the leading edge of a large tectonic plate (the North American Plate) that has been colliding with the Pacific Plate to the west for millions of years. This collision and the plate interaction have produced California’s dynamic coastline. Surface processes such as waves, rainfall and runoff, landslides, rockfalls and other mass movements have also shaped the large-scale coastal landforms such as the mountains, uplifted marine terraces and sea cliffs. In addition, sea level along the coast has changed continuously throughout geologic time in response to constantly changing global climate. As a result, the present position of the coastline is only a temporary one. While the changes are not rapid, the evidence is clear that the sea level has been rising for the past 18,000 years, and will continue to rise into the foreseeable future.
The period of global warming and ice melting that began at the end of the last ice age, and the accompanying sea level rise, flooded the continental shelves that surround the continents. Along the margin of California, this sea level rise advanced the shoreline 10 to 20 kilometers landward along most of the state, but as much as 45 kilometers off of San Francisco, where the continental shelf is considerably wider. Wave erosion pushed back the continental edge forming coastal cliffs as the sea advanced inland into areas of higher relief. Throughout the period of major sea level rise (approximately 5,000 to 18,000 years ago), most of the California coastline was retreating landward at average rates of about 50 to 150 centimeters annually.
As sea level rise slowed over the past several thousand years, the erosion rate declined and began to approach the rates of sea cliff retreat witnessed today, which is closer to approximately 10 to 30 centimeters per year in most places in the state of California.
Distribution of Sea Cliffs
The shoreline of California is extremely diverse, from the steep coastal mountains along the Big Sur and Mendocino coasts to the broad coastal plain and wide beaches of Los Angeles County. The high-relief, steep cliffs, or coastal mountains, are located predominantly in northern California, from Del Norte County to Mendocino County, at the Marin Headlands just north of the Golden Gate Bridge, in the Pacifica to Montara area of San Mateo County, along the Big Sur coast of Monterey and San Luis Obispo counties, and in southern California along the coastal flank of the Santa Monica Mountains and Point Loma in San Diego County. These steep, rugged stretches of coast typically consist of rock that is very hard and resistant to erosion. It is these rock types that form many of the resistant headlands or points along the state’s shoreline, such as Trinidad Head, Bodega Head, Point Reyes, Point Pinos and Point Sur.
Elevated marine terraces are characteristic features of collision coasts, such as California, where uplift is taking place. These benches, which typically resemble a flight of stairs, are normally a kilometer or so in width and may ascend to elevations of several hundred meters above the present sea level. Each terrace consists of a nearly horizontal or gently seaward-dipping erosional platform backed by a steep or degraded relict sea cliff along its landward margin. These sequences of uplifted marine terraces are the geologic record, or strip chart, of wave-cut platforms, formed in the near-shore zone at repeated sea-level highstands of the past several hundred thousand years, superimposed on a tectonically rising coastline.
Much of the coastline of San Diego, Orange, Santa Barbara, San Luis Obispo, Santa Cruz and San Mateo counties is characterized by low bluffs or cliffs cut into uplifted marine terraces, typically consisting of younger sedimentary rocks. The number of terraces exposed along the coast of California range from one in Santa Barbara to as many as 13 on the Palos Verdes Peninsula of Los Angeles County. The existence and distribution of these flat, nearly horizontal benches adjacent to the shoreline have allowed California’s extensive coastal development to take place. ease-of-access and construction has made development easy in oceanfront communities situated on these widespread terraces. Unfortunately, however, the relatively weak, often well-bedded sedimentary rocks that lent themselves to wave erosion and the formation of wave-cut terraces in the geologic past are also the same materials exposed in the coastal cliffs today. These features are very susceptible to wave erosion, and the continued breakdown and retreat of the cliffs as sea level rises has produced a dilemma of increasing magnitude for many California coastal communities.
Sea Cliff Erosion and Failure
While the long-term rates of cliff retreat are a function of the rate of sea level rise, there are significant local or regional differences in the rate of sea cliff or bluff erosion. These erosion rates and resulting cliff geomorphology vary as a function of both the resistance of the materials making up the cliffs and the physical forces acting to wear away the cliffs. The hardness or degree of consolidation of the cliff rock; the presence, spacing, continuity and orientation of internal weaknesses such as joints; and the presence of groundwater all work to directly affect the resistance of the material to both slope failure and wave action. The wave energy reaching any particular stretch of cliffs, the presence or absence of a protective beach, the tidal range or sea level fluctuation, frequency of El Nino events or damaging storms, climate patterns including rainfall and runoff, as well as groundwater flow also influence the rate and scale of sea cliff retreat.
sections of coast consisting of hard crystalline rock usually erode at very slow rates. At some locations on the Monterey Peninsula, for example, which consist of granite, virtually no change was detected between photos taken over a 50-year time span. In striking contrast, erosion can be far more rapid (up to 30 centimeters or more per year, on average) where the cliffs consist of weaker sedimentary rocks such as shale, siltstone or sandstone. A few miles north of the Monterey Peninsula, along the shoreline of southern Monterey Bay, the coastline consists of unconsolidated dune sand that retreats at rates of one to two meters each year. Cliffs consisting of sedimentary rocks, which are characteristic of much of San Diego, Santa Barbara, Santa Cruz and San Mateo counties, for example, often retreat in a linear fashion, producing relatively straight coastlines.
Coastal bluff failure in California during strong seismic shaking represents a significant, but little appreciated, coastal hazard, primarily due to the infrequent nature of large earthquakes. The potential for earthquakes that can affect coastal cliffs is high along the entire length of the state’s coastline, considering no part of the coast of California is more than 25 kilometers from an active fault. Future earthquakes, particular/y in southern California, can be expected to produce widespread failure of the heavily populated cliffs and bluffs of areas like San Diego County.
Documenting Erosion Rates
Sea cliff erosion is an episodic process with most of the major incidents of cliff retreat occurring in the event of simultaneous high tides and large storm waves. Waves can then reach high enough up on the shoreline to attack those areas that are less frequently inundated, and the material that has been weakened progressively through weathering can be dislodged and removed. The sequence of processes may include beach scour followed by direct wave attack, and undercutting of the base of the cliff followed by collapse of the overlying unsupported material. Terrestrial processes such as landslides or rock falls triggered independently of a wave attack may also be extremely important, and even dominant in areas where a seawall, revetment or a wide sandy beach protects the base of the bluff.
Although the term “average annual cliff erosion rate” is often used, in reality, it is a simple comparison of the position of the cliff edge at different points in time (whether from historic maps, aerial photographs or actual field measurements), and dividing by the total number of years between these data points to derive an average annual erosion rate. Cliffs or bluffs may remain superficially unchanged for years and then, due to the right combination of bluff saturation, tidal level, wave attack and/or seismic shaking, several meters may fail instantaneously. Averaging this loss over the time interval between major storms produces an average rate that may vary from centimeters per year along resistant granitic coasts to a meter or more per year in weak sedimentary rock or unconsolidated materials.
Because of the time involved, as well as the equipment and aerial photographic or map data base needed to accurately measure long-term sea cliff erosion rates, there have been few comprehensive studies, and relatively few data are readily available. A statewide coastal hazards study completed a decade ago determined that the most frequently cited data needed by planning or permitting staff was that of shoreline or bluff erosion rates. Yet, there are still very limited cliff erosion data easily available for most of the state’s coastline.
Human Occupancy and Response
Although the length of California’s coastline has not changed significantly in historic times, the state’s population continues to increase. At the time of the last significantly damaging El Niño in 1982-1983, the state’s population stood at 24.8 million. By 2004, the population had increased 45 percent to approximately 36 million. Eighty percent of these people live within 50 kilometers of the shoreline, and over four million people live within five kilometers of the water’s edge. Coastal communities from one end of the state to the other have lost entire oceanfront streets, utility lines and homes through cliff erosion over the last century. New developments on eroding or unstable bluff tops continue to be proposed, and small and older weekend beach cottages are still being extensively remodeled or torn down and replaced by larger new homes.
As populations worldwide migrated to the coastline, the impacts of their development began to alter both coastal landforms and cliff stability. Development continues to occur on cliff-tops, on the faces of the cliffs themselves, as well as on the fronting beach. There are many densely populated coastal areas where very little of the natural cliffs can be seen, as they have been completely armored. As the urbanization of the California coast has intensified, and the state has experienced increased coastal storm damage and property loss in recent years, the extent of shoreline protected by armor incrementally increased, from 42.4 kilometers (2.4 percent of the entire coastline) of armor (seawalls or revetments) in 1971, to 180 kilometers (10 percent of the coastline) in 2002. This is even higher for the more developed coastline of southern California; one-third of the entire combined coastlines of Ventura, Los Angeles, Orange and San Diego counties is now armored.
Coincident with the increase in seawall construction has been an increased concern with the potential impacts of these structures on the shoreline, including visual or aesthetic impacts, restriction of beach access, loss of beach through placement of revetments, loss of beach sand that would have been provided by the continued erosion of the bluffs and the impacts of seawalls on the erosion of the adjacent shoreline. The fundamental issue is often to what degree private oceanfront property owners should be allowed to protect their homes and land at the expense of the general public who use the coast.
Conclusions
The coast of California is caught between the increasing numbers of people who want to live as close as they can to the shoreline, and the natural processes such as sea level rise, El Niño events and wave attacks, which all lead to cliff and bluff erosion and coastline retreat. Cliff erosion is a process that has been going on for 18,000 years and will continue into the foreseeable future, threatening California’s oceanfront development. Seawalls and revetments continue to be the most common approaches proposed to control coastal erosion, but both the effectiveness and impacts of these protective structures are beginning to restrict additional armor emplacement, /si/
For more information, e-mail oceanbiz@sea-technology.com.
By Gary B. Griggs
Director
and
Kiki Patsch
Student
Institute of Marine Sciences
Department of Earth Sciences
University of California
Santa Cruz, California
Gary Griggs is a professor of Earth sciences and the director of the Institute of Marine Sciences at the University of California, Santa Cruz. He has been studying hazards and erosion processes along the California coastline for over 30 years and, along with Kiki Patsch and Lauret Savoy, is author of “Living with the California Coast. ”
Kiki Patsch received her undergraduate degree in environmental science at the University of Virginia. She is currently a Ph.D. candidate at the University of California, Santa Cruz, in the Earth sciences department. Her interests lie primarily in coastal hazards and shoreline change. She is currently working on the development of sediment budgets for the littoral cells of California.
Copyright Compass Publications, Inc. Sep 2004
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