Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Edited by
M. A. Hampton and G. B. Griggs

U.S. Geological Survey
Professional Paper 1693
2004


   The Ocean Studies Board of the National Research Council recently reviewed the U.S. Geological Survey's Coastal and Marine Geology (USGS-CMG) program. One of the Board's primary recommendations was that CMG prepare comprehensive assessments of the nation's coastal and marine regions, drawing on expertise not only from within the USGS, but also from outside agencies and academic institutions. In response to that recommendation, this report assesses the status and trends of coastal cliffs along the shorelines of the conterminous United States and the Great Lakes. By "status" is meant the present distribution and character of coastal cliffs, as well as their current relevance to social issues such as coastal development. By "trends" is meant the changes in status caused by both geological forces and human activities.

   Coastal cliffs are steep escarpments at the coastline. They commonly form during times of rising sea level, such as the present, as the shoreline advances landward and erodes the elevated landmass. Coastal cliffs are a common landform, particularly on the west, northeast, and Great Lakes coasts of the United States, as well as within large estuaries. The land adjacent to coastal cliffs has been heavily developed along much of the coast, particularly in urban areas where the natural instability and progressive retreat of the cliffs pose a threat to life and property. Coastal land is permanently lost when coastal cliffs collapse and retreat landward, which is an important national issue in coastal planning, management, and engineering.

   The content of this report was derived from the personal expertise of the authors and from the extensive scientific literature concerned with coastal cliffs. As a report to the Nation, it is intended for a broad audience. Both topical and regional aspects are presented. It is important to recognize that the emphasis of this report is on the geology of coastal cliffs; engineering, land-use, and regulatory issues are addressed only where there is a clear link to the geologic nature of coastal cliffs. The editors appreciate the thorough and careful review of the entire manuscript by Alan Trenhaile and Laura Moore. Their editing, comments, and questions greatly improved the content and clarity of the final report.

Introduction

By Monty A. Hampton, Gary B. Griggs, Tuncer B. Edil, Donald E. Guy, Joseph T. Kelley, Paul D. Komar, David M. Mickelson, and Hugh M. Shipman

2004. Introduction. Pages 1-4 in M. A. Hampton and G. B. Griggs, eds. Formation, evolution, and stability of coastal cliffs--status and trends. U.S. Geological Survey Professional Paper 1693.

[NOTE: The Table and Figures are not included here, but captions appear at the end of this document.]


     The term “coastal cliff” refers to a steeply sloping surface where elevated land meets the shoreline. Coastal cliffs are a geomorphic feature of first-order significance, occurring along about 80 percent of the world’s shorelines (Emery and Kuhn, 1982). Like virtually all landforms, modern coastal cliffs are a “work in progress,” continually acted upon by a broad assortment of offshore (marine or lacustrine) and terrestrial processes that cause them to change form and location through time. An important consequence is that coastal cliffs “retreat” (that is, move landward), and the adjacent coastal land is permanently removed as they do so. Retreat can be slow and persistent, but on many occasions it is rapid and episodic.
     Coastal cliff is a general term that refers to steep slopes along the shorelines of both the oceans (where they are commonly called “sea cliffs”) and lakes (where they are commonly called “lake bluffs”). The term “bluff” also can refer to escarpments eroded into unlithified material, such as glacial till, along the shore of either an ocean or a lake. Often, the terms “cliff” and “bluff” are used interchangeably.
     Coastal cliffs typically originate by marine or lacustrine erosional processes, particularly as the shoreline transgresses landward with a rise of water level. However, some initiate as scarps of large landslides or faults (see, for example, Moore and others, 1989; Kershaw and Guo, 2001) or by glacial erosion (Shipman, this volume). Although their ultimate origin is special, these types of features are here included as coastal cliffs, because in many respects they evolve similarly to other coastal cliffs. Unless otherwise mentioned, however, the following discussions are implicitly about coastal cliffs that originate by marine or lacustrine erosional processes.
     The definition of coastal cliffs given above establishes no bounds on the constituent materials, height, or inclination of the eroded surface. In practice, the bounds are established by utility. Erosional processes can carve a cliff face into any geologic material with adequate relief—slowly into hard rocks such as unweathered granite, rapidly into soft sedimentary rocks such as a sandstone, and even more rapidly into unlithified material such as glacial till (Sunamura, 1983). A practical lower bound of bluff or cliff height is a few meters, below which there are few hazard concerns, but above which the serious engineering and land-use issues associated with coastal-cliff retreat become important. Some coastal cliffs are more than 100 m high. Typical inclination of surfaces that are recognized as true coastal cliffs ranges from about 40° to 90°, but it can be as low as 20° in soft sediment such as clay. In some places, overhanging rock faces can exist.
     The terrain landward of a coastal cliff can be steep, rugged, and mountainous at one extreme, as along the Big Sur coast of central California, or relatively flat as is common along much of the urban coasts of California, New England, and along the Great Lakes. Problems related to coastal-cliff retreat exist within both types of terrain. The flat terraces and gently sloping plains in urbanized coastal areas in particular have attracted development, because the flat surfaces provide nearly ready-made building sites, and the elevated position can provide magnificent coastal vistas (fig. 1). Cliff retreat has caused damage to structures in many of these places (fig. 2). A common problem along mountain-backed coastal cliffs, which typically are sparsely developed, is damage to or loss of coastal roadways as the coastal cliff retreats (fig. 3).
     There are many social as well as scientific issues that emerge from the present understanding of coastal cliffs in the United States, and coastal-cliff retreat is an important national issue. Houses, commercial buildings, roads, and other infrastructure located along a coastal cliff, either on the elevated crest or at the base, have been damaged or destroyed when cliffs collapsed. The loss of typically high-value coastal property has an economic impact because it reduces local property-tax revenues and effects Federal disaster relief and insurance programs. For local governments, the loss of public roads and sewer and water lines on coastal cliffs has a burdensome economic impact. Coastal-cliff retreat also can have an impact in relatively unpopulated areas. For instance, cliff retreat in coastal parks causes financial loss to the tourist industry through loss of access, as well as loss of camping and picnicking sites, and in some places, loss of historically significant sites. Arresting the retreat of a coastal cliff is costly, and many attempts have failed (fig. 4). Furthermore, some coastal-cliff stabilization projects have contributed to beach erosion by cutting off an important source of sand and gravel that nourishes the downdrift beaches. Various studies have documented the extent of the U.S. coastlines that are undergoing erosion (USACE, 1971; Habel and Armstrong, 1978; Griggs and Savoy, 1985; Pope and others, 1999; Komar, 1997; Terich, 1987; Kelley and others, 1989; Carter and others, 1987; McCormick and others, 1984); a reported 86 percent of the shoreline of California, for example (Griggs, 1999). Because of the desirability of living directly on the coast, which in many regions means living on a cliff above an eroding coastline, there are significant short- and long-term risks associated with the population migration to, and more intense development of, those areas. Coastal erosion has become an increasingly publicized regional and national issue that is going to affect the Nation for many decades. Globally, more than a billion people live near the coast (Nicholls and Small, 2002; Small and others, 2000), and many of those reside only a few meters above sea level or behind an encroaching hazard, the edge of the coastal cliff.
     Present engineering and regulatory attempts to mitigate the problems associated with coastal-cliff retreat are clearly inadequate, because land, buildings, infrastructure, and lives continue to be lost. There is lively controversy regarding the best approach to a resolution of these problems. “Hard” engineering solutions, such as constructing revetments or seawalls; “soft” solutions, such as replenishing or nourishing protective beaches; “regulatory” solutions, such as establishing effective setback distances; and “passive” solutions that advocate relinquishing threatened land to the advancing sea, all have their vocal constituencies as well as firm opposition. The vast majority of the public, however, does not appreciate the problem of coastal-cliff erosion as well as it does the issue of beach erosion.
     Beaches and coastal cliffs are intimately linked. The release of sand and gravel during coastal-cliff erosion is a significant coastal management issue, because the sediment becomes part of the littoral system and contributes to the sediment budget of the beaches (see, for example, studies by Osborne and others, 1989; Everts, 1991; Best and Griggs 1991; Galster and Schwartz, 1990; Diener, 2000; Mickelson and others, 2002; Runyan and Griggs, 2002; Runyan and Griggs, 2003). Halting coastal-cliff erosion by installing seawalls to protect coastal property might reduce the supply of sand, which thereby reduces the size of the asthetically pleasing beach. Conversely, wide beaches dissipate wave energy, providing natural protection for the cliff. Therefore, if the sediment supply to the beaches is reduced significantly, the beach becomes narrower and the cliff can be subjected to greater wave energy. Installation of groins to create or maintain a beach along one section of coast, unless enough sand is placed on the updrift side immediately following construction so bypassing occurs, can temporarily deprive the down-drift beaches of natural nourishment, causing them to deteriorate and exposing the adjacent cliffs to direct wave attack (fig. 5). Beaches are the Nation’s most popular tourist destination, so their protection and maintenance are important economically (Houston, 2002).
     Efforts to protect coastal cliffs by armoring them with seawalls and revetments have direct and indirect effects on beaches that are clearly evident along many coastlines. For example, much of the U.S. shoreline of Lake Erie is protected, and beaches are narrow or absent along its coastal bluffs. By contrast, the much less developed Lake Superior shoreline of Wisconsin and Upper Michigan, where protective structures are uncommon, has abundant sand and gravel supplied to the beach. In Maine, eroding bluffs of glacial-marine sediment are a major source of mud to tidal flats and salt marshes. When bluffs are stabilized, the sediment supply to the adjacent tidal flat or marsh is interrupted and the environment becomes dominated by erosional processes. As mud from the tidal flat is exported offshore, the salt marsh-tidal flat boundary becomes a steep peat scarp and the marsh begins to erode. In time, by lowering the elevation of the original tidal flat, it becomes narrower and the salt-marsh buffer disappears. The narrower flat and reduced or eliminated marsh buffer ultimately subject engineering structures to damaging waves that necessitate maintenance or structural modification. In California, approximately 10 percent of the entire 1,760 km of coastline has now been armored (Runyan and Griggs, 2002). In the heavily developed southern California area, the extent of armoring is even greater. Thirty-four percent of the combined shorelines of Ventura, Los Angeles, Orange, and San Diego Counties has now been armored. These seawalls and revetments affect the coastline and coastal cliffs in several ways (Griggs, 1999), including (1) protection of the cliff or bluff from wave erosion, thereby cutting off any sand previously supplied to the beach, (2) loss of beach due to the placement of the structure on the beach sand, with a revetment taking up far more beach area than a seawall, and (3) gradual loss of the beach fronting the seawall or revetment as sea level continues to rise against a shoreline that has now been fixed (termed “passive erosion,” see Griggs, 1999). Permits for the construction of new seawalls that cut off the sand contribution from eroding bluffs are now required by the California Coastal Commission to be accompanied by a nourishment program to replace the sand that would have been eroded from the bluff, or the financial equivalent. However, investigation of the magnitude of this sand source in two of California’s littoral cells (Santa Barbara and Oceanside) indicates that the cliffs only contribute about 0.5 percent and 12 percent, respectively, of the littoral sand budget (Runyan and Griggs, 2002).
     The study of processes, especially the acquisition of quantitative data, on shorelines bordered by coastal cliffs is hindered by (1) the slow rates of change, (2) the difficulty of measuring energy exerted on the coast by the high energy/low frequency storms during which much cliff retreat occurs, (3) the exposed and often dangerous environments for wave measurement and submarine exploration, (4) the lack of access to privately owned, precipitous, or heavily vegetated cliffs, (5) poor research funding, and (6) the small number of active researchers in this area. Even if the nature of contemporary erosive processes were completely understood, it would remain difficult to explain the morphology of coasts that often retain the vestiges of antecedent geological conditions quite different from those of today (Griggs and Trenhaile, 1994).
     The large portion of the United States coastline that consists of cliffs or bluffs is not adequately reflected in the modern process-oriented coastal literature, where most emphasis is placed on beaches and other systems that respond rapidly to changing environmental conditions. However, books by Trenhaile (1987) and Sunamura (1992) do consider coastal cliffs in detail. Despite physical and chemical analyses, geochronometric dating, physical and mathematical modeling, and careful measurement of erosion rates, geologists often can only speculate about the development and modification of cliffed coasts (Griggs and Trenhaile, 1994). Nevertheless, geological input is crucial in order to resolve the large-scale social and economic issues associated with a constantly retreating cliffed shoreline over the thousands of miles of developed United States coastline. Geologists face multiple challenges of (1) understanding the fundamental processes and factors that govern coastal-cliff erosion, (2) documenting and quantifying the spatial and temporal variation of retreat rates, and (3) providing this information in a usable format to coastal engineers, planners, and managers, as well as to the general public.
     The published geologic reports covering field, experimental, and theoretical studies in aggregate demonstrate the diversity and complexity of coastal cliffs worldwide. Those publications are cited liberally in this report in an attempt to convey a comprehensive understanding of the geologic nature of coastal cliffs, even though the focus of the report is the cliffs along the shores of the United States, including the Great Lakes. Generalizations about coastal cliffs are difficult, and forecasting the timing and rate of retreat is particularly problematic. This report synthesizes the current knowledge of the status and trends of U.S. coastal cliffs.

References Cited

Best, T.C., and Griggs, G.B., 1991, A sediment budget for the Santa Cruz littoral cell, California, in Osborne, R.H., ed., From shoreline to abyss; contributions in marine geology in honor of Francis Parker Shepard: Society of Economic Paleontologists and Mineralogists Special Publication no. 46, p. 35-50.

Carter, C.H., Neal, W.J., Haras, W.S., and Pilkey, O.H., 1987, Living with the Lake Erie shore: Durham, North Carolina, Duke University Press, 255 p.

Diener, B.G., 2000, Sand contribution from bluff recession between Point Conception and Santa Barbara, California: Shore and Beach, v. 68, no. 2, p. 7-14.

Emery, K.O., and Kuhn, G.G., 1982, Sea cliffs; their processes, profiles, and classification: Geological Society of America Bulletin, v. 93, p. 644-654.

Everts, C.H., 1991, Seacliff retreat and coarse sediment yields in southern California; in Coastal Sediments ’91: Specialty Conference on Quantitative Approaches to Coastal Sediment Processes, Seattle, Washington, 25-27 June 1991, Proceedings, p. 1586-1598.

Galster, R.W., and Schwartz, M. L., 1990, Ediz Hook—a case history of coastal erosion and rehabilitation, in Schwartz, M.L., and Bird, E.C.F., eds., Artificial beaches: Journal of Coastal Research Special Issue, v. 6, p. 103-113.

Griggs, G.B., 1999, The protection of California’s coast; past, present and future: Shore and Beach, v. 67, no. 1, p. 18-28.

Griggs, G.B., and Savoy, L., 1985, Living with the California coast: Durham, North Carolina, Duke University Press, 393 p.

Griggs, G.B., and Trenhaile, A.S., 1994, Coastal cliffs and platforms, in Carter, R.W.G., and Woodroffe, C.D., eds., Coastal evolution; late Quaternary shoreline morphodynamics: Cambridge, Cambridge University Press, p. 425-450.

Habel, J.S., and Armstrong, G.A., 1978, Assessment and atlas of shoreline erosion along the California coast: State of California, Dept. of Navigation and Ocean Development, 277p.

Houston, J.R., 2002, The economic value of beaches—a 2002 update: Shore and Beach, v. 70, no.1, p. 9-12.

Kelley, J. T., Kelley, A.R., and Pilkey, O.H., Sr., 1989, Living with the Maine coast: Durham, North Carolina, Duke University Press, 174 p.

Kershaw, S., and Guo, L., 2001, Marine notches in coastal cliffs; indicators of relative sea-level change, Perachora Peninsula, central Greece: Marine Geology, v. 179, p. 213-228.

Komar, P.D., 1997, Erosion of a massive artificial “landslide” on the California coast: Shore and Beach, v. 65, no. 4, p. 8-14.

McCormick, L.R., Pilkey, O.H., Jr., Neal, W.J., and Pilkey, O.H., Sr., 1984, Living with Long Island’s south shore: Durham, North Carolina, Duke University Press, 157 p.

Mickelson, D. M., Brown, E. A., Edil, T. B., Meadows, G. A., Guy, D. E., Liebenthal, D. L., and Fuller, J. A., 2002, Comparison of sediment budgets of bluff/beach/nearshore environments near Two Rivers, Wisconsin, on Lake Michigan, and at Painesville, Ohio, on Lake Erie: Geological Society of America Abstracts with Programs, v. 34, no. 2, p. A-12.

Moore, J.G., Clague, D.A., Holcomb, R.T., Lipman, P.W., Normark, W.R., and Torresan, M.E., 1989, Prodigious submarine landslides on the Hawaiian Ridge: Journal of Geophysical Research, v. 94, no. B12, p. 17,465-484.

Nicholls, R.J., and Small, C., 2002, Improved estimates of coastal population and exposure to hazards released: Eos, Transactions, American Geophysical Union, v. 83, no. 28, p. 301,305.

Osborne, R.H., Fogarty, T.M., and Kuhn, G.G., 1989, A quantitative comparison of coarse-grained sediment yield from contributing cliffs and associated rivers; southern Orange and San Diego Counties, California: Geological Society of America, Cordilleran Section, Abstracts with Programs, v. 21, no. 5, p. 126.

Pope, J., Stewart, C.J., Dolan, R., Peatross, J., and Thompson, C.L., 1999, The Great Lakes shoreline type, erosion, and accretion [Unpublished map]: Vicksburg, Mississippi, U.S. Army Corps of Engineers, 1 sheet with text, 1:2,000,000.

Runyan, K.B., and Griggs, G.B., 2002, Contributions from coastal cliff erosion to the littoral budget, in Coyne, M., and Sterrett, K., eds., California Beach Restoration Study: Sacramento, California, Department of Boating and Waterways & State Coastal Conservancy, p. 8.1-8.51.

Runyan, K.B., and Griggs, G.B., 2003, The effects of armoring sea cliffs on the natural sand supply to the beaches of California: Journal of Coastal Research, v. 19, no. 2, p. 336-347.

Small, C., Gornitz, V., and Cohen, J.E., 2000, Coastal hazards and the distribution of human population: Environmental Geoscience, v. 7, p. 3-12.

Sunamura, T., 1983, Processes of seacliff and platform erosion, in Komar, P.D., ed., CRC handbook of coastal processes and erosion: Boca Raton, Florida, CRC Press, p. 233-265.

Sunamura, T., 1992, Geomorphology of rock coasts: New York, John Wiley and Sons, 302 p.

Terich, T. A., 1987, Living with the shore of Puget Sound and the Georgia Strait: Durham, North Carolina, Duke University Press, 165 p.

Trenhaile, A.S., 1987, The geomorphology of rock coasts: Oxford, Oxford University Press, 384 p.

USACE (United States Army Corps of Engineers), 1971, National Shoreline Study—California Regional Inventory: San Francisco, California, Corps of Engineers, District Office, 105 p.

Figure 1. This coastal cliff in Daly City, California, is about 150 m high. As evidenced by the large landslide near the center of the photograph, the cliff is unstable, posing a threat to the nearby densely developed area. The San Andreas Fault is a short distance offshore.

Figure 2. Rapid retreat of this sea cliff in Pacifica, California, caused damage to these houses, which later were declared unsafe and demolished. Compare with the cover photo of the same area, taken about 2-1/2 months previously, before the arrival of the 1997-98 El Ni–o storms.

Figure 3. Movement of this large landslide on the Big Sur coast of central California is related to erosion of the coastal cliff at its base, plus other factors such as ground water. Occasional movement of large slides such as this one results in frequent damage to and associated closure of California state Highway 1, which generally follows the coast, as shown here.

Figure 4. Failure of this steep bluff in glaciofluvial and glacial sediment in Puget Sound, Washington, occurred despite a stabilization attempt. The seawall was built to prevent toe erosion the year prior to failure of the slope.

Figure 5. South of Milwaukee, Wisconsin, on Lake Michigan, groins protect the bluff in the distance, but serve to enhance erosion of the bluff in the foreground.