U.S. Department of the Interior

U.S. Geological Survey

Formation, Evolution, and Stability of

Coastal Cliffs–Status and Trends

Professional Paper 1693
Formation, Evolution, and Stability of

Coastal Cliffs–Status and Trends

Monty A. Hampton and Gary B. Griggs, Editors

Cliffs are a common feature along U.S. coastlines. Understanding the

geology of coastal cliffs is essential to addressing the impact of landward

cliff retreat in developed areas.

Professional Paper 1693

 

U.S. Department of the Interior

U.S. Geological Survey 
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

United States Government Printing Office: 2004

For additional copies please contact:

USGS Information Services

Box 25286

Denver, CO 80225

This report and any updates to it are available at

http://pubs.usgs.gov/pp/pp1693/

Additional USGS publications can be found at

http://geology.usgs.gov/products.html 

For more information about the USGS and its products:

Telephone: 1–888–ASK–USGS (1-888-275-8747)

World Wide Web: http://www.usgs.gov/

Text edited by Peter H. Stauffer and James W. Hendley II 

Layout and design by Stephen L. Scott 

Manuscript approved for publication, March 18, 2004

FRONT COVER

Photograph taken just before the 1997-98 El Niño storms, showing the northward view along the approximately 30-m tall sea cliff at North 

Explanade beach in Pacifica, California. The soft cliff shows signs of erosion, and a rip-rap sea wall is being constructed at the cliff base 

to protect houses along the cliff edge. The sea wall was not completed before the storms, and the cliff retreated more than 10 m (see later 

photograph on page 1). Most of the houses along the cliff were condemned and razed after the storm season.

Cataloging-in-Publication data are on file with the Library of Congress
Preface

 The Ocean Studies Board of the National Research Council recently reviewed the U.S. 

Geological Survey’s Coastal and Marine Geology (USGS-CMG) program (National Research 

Council, 1999). 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, landuse, 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.

Reference

National Research Council, 1999, Science for decisionmaking; Coastal and Marine Geology at 

the U.S. Geological Survey: Washington, D.C., National Academy Press, 113 p.

 iii
Contents

Preface--------------------------------------------------------------------------------------------------- iii 

 By Monty A. Hampton and Gary B. Griggs

Contributors to this Professional Paper--------------------------------------------------------------- vii

Introduction---------------------------------------------------------------------------------------------- 1 

 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

Topical Issues

Processes That Govern the Formation and Evolution of Coastal Cliffs--------------------------- 7

 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

The Measurement and Interpretation of Coastal Cliff and Bluff Retreat------------------------- 39

 By Cheryl J. Hapke

Regional Issues

California’s Coastal Cliffs and Bluffs------------------------------------------------------------------- 53 

 By Gary B. Griggs and Kiki B. Patsch

Oregon’s Coastal Cliffs: Processes and Erosion Impacts------------------------------------------- 65

 By Paul D. Komar

Coastal Bluffs and Sea Cliffs On Puget Sound, Washington--------------------------------------- 81

 By Hugh M. Shipman

Coastal Bluffs of New England------------------------------------------------------------------------- 95

 By Joseph T. Kelley 

Erosion of Coastal Bluffs In the Great Lakes--------------------------------------------------------- 107

 By David M. Mickelson, Tuncer B. Edil, and Donald E. Guy

 v
vii

Contributors to this Professional Paper

Edil, Tuncer B.

 Geological Engineering Program

 University of Wisconsin

 Madison, WI 53706

Griggs, Gary B.

 Institute of Marine Sciences

 Department of Earth Sciences

 University of California

 Santa Cruz, CA 95064

Guy, Donald E.

 Ohio Department of Natural Resources

 Division of Geological Survey

 Sandusky, OH 44870

Hampton, Monty A.

 U.S. Geological Survey

 Menlo Park, CA 94025

Hapke, Cheryl J.

 U.S. Geological Survey Pacific Science Center

 Santa Cruz, California 95064

Kelley, Joseph T.

 Department of Global Sciences

 University of Maine

 Orono, ME 04469

Komar, Paul D.

 College of Oceanic and Atmospheric Sciences

 Oregon State University

 Corvallis, OR 97331

Mickelson, David M.

 Geological Engineering Program

 University of Wisconsin

 Madison, WI 53706

Patsch, Kiki B.

 Institute of Marine Sciences

 Department of Earth Sciences

 University of California

 Santa Cruz, CA 95064

Shipman, Hugh M.

 Washington State Department of Ecology

 Bellevue, WA 98008
1

The term “coastal cliff ” refers to a steeply sloping surface 

where elevated land meets the shoreline. Coastal cliffs are a 

geomorphic feature of fi rst-order signifi cance, 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 unlithifi ed 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 defi nition 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 

unlithifi ed 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 fl at 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 fl at terraces and 

gently sloping plains in urbanized coastal areas in particular 

have attracted development, because the fl at surfaces provide 

nearly ready-made building sites, and the elevated position 

can provide magnifi cent coastal vistas (fi g. 1). Cliff retreat 

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 Pacifi ca, 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 Niño 

storms.

 U.S. Geological Survey Professional Paper 1693

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

has caused damage to structures in many of these places (fi g. 

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 (fi g. 3).

There are many social as well as scientifi c 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 propertytax 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 fi nancial 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 signifi cant sites. Arresting the retreat of a coastal cliff is costly, and many attempts 

have failed (fi g. 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 signifi cant shortand 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 fi rm 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 signifi -

cant 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 reducFigure 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 glaciofl uvial 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.
3

es 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 signifi cantly, 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 (fi g. 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 fl ats and salt marshes. When 

bluffs are stabilized, the sediment supply to the adjacent tidal 

fl at or marsh is interrupted and the environment becomes 

dominated by erosional processes. As mud from the tidal fl at is 

exported offshore, the salt marsh-tidal fl at boundary becomes 

a steep peat scarp and the marsh begins to erode. In time, by 

lowering the elevation of the original tidal fl at, it becomes narrower and the salt-marsh buffer disappears. The narrower fl at 

and reduced or eliminated marsh buffer ultimately subject engineering structures to damaging waves that necessitate maintenance or structural modifi cation. 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 

fi xed (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 fi nancial 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 diffi culty 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 

diffi cult 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 refl ected 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 modifi cation 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 fi eld, 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 

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.

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

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 diffi cult, 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, 

profi les, and classifi cation: 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., Artifi cial 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 artifi cial “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 

Offi ce, 105 p.
 U.S. Geological Survey Professional Paper 1693 5

TOPICAL ISSUES
6 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends
7

Setting and General Features

For many people, their image of the coastline is a rocky 

sea cliff being pounded by waves. This spectacular scenery, 

which characterizes so many of the world’s coasts, is primarily a result of the abrupt meeting of storm waves and elevated 

land. Although most of Earth’s coastal cliffs are nearly vertical 

and composed of bedrock, there is a wide range of cliff types, 

depending on material, morphology, and relief. They range 

from the low, several-meter-high bluffs cut into glacial till on 

Long Island, New York, to the 500-m-high, nearly vertical 

cliffs cut into lava fl ows on the north shore of Molokai in the 

Hawaiian Islands (fi g. 1). Material variations within or among 

coastal cliffs often are refl ected in morphologic variations 

(fi gs. 2-4).

On global and regional scales, coastal morphology often 

correlates closely with tectonic setting (Inman and Nordstrom, 

1971). Major geomorphic contrasts exist between the subdued 

coastlines typical of the passive continental margins of the east 

and gulf coasts of the United States and the rugged coastlines 

that dominate active or collisional plate boundaries along the 

west coast.

Many exposed coastlines along the passive continental 

margins of the United States consist of a low-relief coastal 

plain bordered offshore by a wide continental shelf. There are 

no rocky cliffs and few bluffs along these generally stable and 

low-elevation coastlines. Instead, they are characterized by 

Processes that Govern the Formation and Evolution 

of Coastal Cliffs

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

Figure 1. Steep cliff in basalt, more than 300 m high, on the northern 

coast of Molokai, Hawaii.

Figure 2. This vertical coastal cliff of basalt in the San Juan Islands 

of northern Puget Sound, Washington, has a narrow erosional ramp at 

the base, but no beach. Because of the strength of the rock and low 

wave energy, the erosion rate is negligible, with the exception of rare 

block falls.

Figure 3. A moderately inclined, high bluff of sandy sediment in 

Puget Sound, Washington. Erosion rates are consistently at a relatively high rate here because the sediment is weak and because the 

exposure and orientation result in rapid longshore redistribution of 

sand, preventing buildup of a wide, protective beach.

 U.S. Geological Survey Professional Paper 1693
8 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

depositional landforms such as wide sandy beaches and dunes, 

as well as offshore barrier islands or bars (fi g. 5). Extensive 

reaches of eroding bluffs occur only along the inland shores of 

estuaries, such as the Calvert Cliffs of Chesapeake Bay (Wilcox and others, 1998; Ward and others, 1989).

In marked contrast to the Atlantic coast of the United 

States, the Pacifi c coast is an active plate-boundary margin, 

where two of the Earth’s lithospheric plates are colliding or 

were colliding in the relatively recent past. Even a casual comparison of typical segments of this coastline immediately reveals striking differences in coastal landforms and geological 

history. In addition to seismicity and volcanism, active margins typically are characterized by narrow continental shelves, 

coastal mountains, and commonly by uplifted marine terraces 

(fi g. 6). Erosional landforms, such as steep coastal cliffs and 

rocky headlands, produce a very different coastline from that 

on typical trailing-edge coasts with depositional landforms. 

They also pose very different issues for human occupation of 

the coastal zone.

Along parts of the U.S. west coast, resistant coastal cliffs 

are carved into uplifted bedrock such as crystalline granite or 

lithifi ed sedimentary rock. Tectonic uplift along the coasts of 

California and Oregon has preserved older, degraded wave-cut 

platforms (uplifted or emergent marine terraces) with their abandoned coastal cliffs, at present elevations of up to several hundred meters above sea level (Bradley and Griggs, 1976) (fi g. 7).

Despite the importance of tectonic setting, it should be 

noted that rugged coasts with rocky cliffs are not restricted to 

collisional margins. There are high cliffs, for example, around 

the British Isles, as well as in northern France, southeastern 

Australia, and eastern Canada. All of these coasts would be 

appropriately termed passive margins. In the United States, 

Figure 4. This 60-m-high bluff in Washington in glacial sediment 

shows the role of material composition in defi ning bluff shape. The 

upper till unit (in shade) fails in vertical slabs and does not support 

vegetation, whereas the central sandier outwash unit rests near the 

angle of repose, with substantial revegetation between erosional 

events. The lower, steep and unvegetated glacial unit is subject to 

wave action when storms and high tides coincide and when beach 

volume is reduced.

Figure 5. Depositional coastal landforms along the Outer Banks of 

North Carolina.

Figure 6. Steep coastal cliff with rocky headlands, sea caves, and 

sea stacks with a pocket beach, San Mateo County, California.

Figure 7. A series of uplifted marine terraces along the San Mateo 

County coast, central California.
9

formerly glaciated coasts in New England, although on a passive margin, possess sea cliffs carved into abundant elevated 

exposures of hard, erosion-resistant igneous and metamorphic 

rocks that are punctuated with outcrops of softer till, outwash, 

and muddy glacial-marine sediment (Kelley and others, 1989; 

Kelley and Dickson, 2000).

Close examination of nearly any section of a cliffed coast 

reveals features related to alongshore variations in lithology, 

stratigraphy, and structure that have infl uenced erosion. Cliffs 

in more resistant rocks generally stand seaward as headlands, or persist offshore as sea stacks or islands, whereas the 

weaker or softer materials occupy a more landward position. 

Embayed coastal cliffs can occur at locations of structural 

weakness such as faults and joints, as well as intrinsically 

weak rock types such as typical mudstones and glacial till. 

These geologic differences produce a wide variety of coastal 

landforms and confi gurations under the same environmental 

conditions. Rocks such as massive granite commonly erode 

in a relatively uniform fashion because of their homogeneity 

in composition and strength. In contrast, layered sedimentary 

rocks display heterogeneity between layers that respond differently to weathering and erosional processes. Differential 

erosion of alternating sandstone and shale beds, for example, 

produces an irregular cliff or shoreline with the typically more 

resistant sandstone forming ledges or protrusions between the 

softer and weaker shale beds. Much of the coastline of Oregon 

is rugged because it is dominated by bold volcanic headlands 

with intervening embayments eroded into weaker sedimentary 

rocks.

To summarize, the primary requirement for the existence of coastal cliffs is elevated land, irrespective of material 

type. Consequently, cliffs are most typical along the coasts 

of tectonically active margins, but they also exist along vast 

stretches of inactive margins where previous tectonic or sedimentary (for example, glacial) activity has raised the land surface. Spatial differences in geological material and structure, 

augmented by differences in geologic processes (discussed 

below), cause differences in steepness of cliffs and ruggedness 

of the coastline.

Formation and Evolution of 

Coastal Cliffs 

The majority of cliffs along today’s coastlines are relatively young geologic features, having formed after the most 

recent ice age—the Wisconsinan stage of the Pleistocene 

epoch—or during earlier Pleistocene transgressions (see, for 

example, Minard, 1971). About 21,000 years ago, the climate 

was considerably cooler and the Earth was in the waning 

stages of a period of extensive glaciation (Peltier, 1999). About 

44 million km3

 of sea water was locked up on the continents 

as ice sheets and glaciers that covered large areas of the Earth. 

Removal of this water from the oceans caused a worldwide 

drop in sea level of about 120 m. Consequently, the shoreline 

along the coast of California at that time was 10 to 25 km west 

of the present one, and that of New York was more than 100 

km farther to the east. 

The ice sheets and glaciers melted as the climate warmed 

after the Wisconsinan stage; meltwater fl owed into the ocean 

and sea level rose globally. This worldwide process, related 

to the total amount of water in the oceans, is termed eustatic 

sea-level change. Sea level rose at an average rate of nearly 1 

cm/yr between 21,000 and about 5,000 years ago (Fairbanks, 

1989). From that time until the present, the rate has slowed, 

although sea level has continued to rise at about 2 mm/yr for 

the past century (Cabanes and others, 2001). Waves breaking 

along the transgressing shoreline eroded elevated land in front 

of them, forming a gently inclined platform just below sea 

level, with an abrupt step—a sea cliff—at its leading edge. A 

similar process occurred as the Great Lakes basins fi lled with 

water during glacial melting.

These global events were imposed on a variety of regional 

geologic frameworks and processes, resulting in a substantial 

diversity of cliffed coastlines. For example, when sea level was 

low during the ice age, tectonic forces uplifted large regions of 

the U.S. west coast, exposing resistant bedrock to cliff-forming 

processes during the subsequent sea-level rise. In contrast, glaciers and aggrading streams deposited sediment along many 

parts of the shorelines of New York, Massachusetts, Puget 

Sound, and the Great Lakes. Coastal cliffs carved into glacial 

till or alluvium typically consist of relatively weak gravelly, 

sandy, and muddy sediment that is particularly susceptible 

to erosion. As the ice retreated across the Maine coast about 

14,000 years ago, the ocean fl ooded the coastal lowlands, 

mantling them with glacial-marine mud (Dorion and others, 

2001). Later, the land rebounded upward, and relative sea level 

fell locally to a lowstand about 60 m below present. Relative 

sea level has risen since then, with consequent erosion of sea 

cliffs, as a result of land stability and eustatically rising ocean 

waters (Barnhardt and others, 1997).

Although it is tempting to ascribe the formation of coastal 

cliffs entirely to marine or lacustrine erosional processes, 

subaerial processes can be equally or more important (see, for 

example, Nott, 1990). The shape of a coastal cliff, in particular its steepness, can be related to the interplay of marine and 

terrestrial processes. Emery and Kuhn (1982) proposed that 

the dominant process of coastal-cliff erosion (marine/lacustrine versus subaerial) and the state of activity (active versus 

abandoned) are recognizable from a cliff profi le. A steep, 

sharp-crested, unvegetated profi le with sparse debris at the 

cliff base indicates an actively retreating coastal cliff dominated by marine erosion (for example, undercutting by waves), 

whereas a convex to sigmoidal profi le with a rounded crest and 

an accumulation of talus at the base indicates an inactive or 

abandoned coastal cliff dominated by subaerial processes (surface runoff and erosion, landslides) (fi gs. 8-10). Alternating 

dominance of marine and terrestrial processes during glacialinterglacial cycles produces composite cliff profi les, consisting 

of both steep, wave-eroded sections and convex, terrestrially 

eroded sections at different vertical levels (Trenhaile, 1987, p. 

178-187; Griggs and Trenhaile, 1994). It is the combination 

 Processes that Govern the Formation and Evolution of Coastal Cliffs
10 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

of subaerial and marine processes, as well as the nature of the 

constituent materials, that create distinctive coastal cliffs and 

bluffs, and whether or not the cliff assumes a vertical free face 

depends upon the relative importance of these two different 

factors (Pethick, 1984).

Erosion of coastal cliffs can be envisioned as a 4-step 

sequence: (1) detachment of grains, slabs, or blocks from 

the cliff face, (2) transport down the inclined surface of the 

coastal cliff, (3) deposition at the base of the coastal cliff, 

and (4) removal of debris by marine or lacustrine processes 

(Lee, 1997). This concept can be extended to defi ne a cyclical, 

episodic model of coastal-cliff evolution (see, for example, 

Hutchinson, 1973; Quigley and others, 1977; Everts, 1991; 

van Rijn, 1998). Waves erode the base of a coastal cliff, undercutting and oversteepening it. This destabilizes the overlying 

Figure 8. Undercut cliff (A) and adjacent cliff failure caused by 

undercutting (B) at Capitola, California. Note that the cliff-face 

morphology is determined by near-vertical jointing.

Figure 9. This nearly vertical coastal cliff in the Solana Beach area 

of southern California is fronted by a narrow beach and is under frequent attack by waves, which remove talus and erode the cliff base.

Figure 10. Coastal bluff with a convex profi le in the northern Monterey Bay area of California, fronted by a wide protective beach. The 

convex profi le exists because of a large talus build-up on the cliffs 

and refl ects a dominance of terrestrial processes over wave erosion.
11

slope, causing it to collapse. The resulting talus accumulation, 

which temporarily protects the cliff base, is then attacked by 

waves. Meanwhile, subaerial erosion decreases the slope of the 

coastal cliff. Once waves have attacked the talus and eroded 

it away, cliff undercutting resumes and the cycle repeats. This 

cycle typically repeats at time scales of years to decades (see, 

for example, Quigley and others, 1977; Brunsden and Jones, 

1980; Shuisky and Schwartz, 1988).

As an aid to conceptualizing the mechanics of coastal-cliff 

evolution, the factors that affect it can be placed into two categories: the “exposure” to geologic and anthropogenic forces, 

and the “susceptibility” of the material to these forces. Exposure refers to the magnitude and frequency of the destructive 

forces, both marine and nonmarine, that act on coastal materials. It is a function of wave climate and rainfall, for example. 

Susceptibility refers to the passive resistance that coastal-cliff 

materials present to the destructive forces, which mainly 

depends on the material strength, both the small-scale intact 

strength of the unaltered rock and the larger scale rock-mass 

strength of deformed and weathered rock. In some situations, 

exposure-related or “extrinsic” variables exert the strongest 

infl uence on cliff evolution; in others it is the susceptibility-related or “intrinsic” ones (Benumof and others, 2000).

Processes

The preceding section mentions some geological processes that play a role in the formation and evolution of coastal 

cliffs. Water-level rise and wave action are fundamental in 

that they operate from the inception through the mature stages 

of coastal cliff evolution. In the absence of these two marine/lacustrine processes, a coastal cliff is considered to be 

abandoned and generally degrades. During the active stages, 

several additional processes—mostly terrestrial—can infl uence cliff evolution. Most of these processes have a complex 

variety of effects. Their importance varies with cliff lithology 

and structure, or they may operate only at particular times or 

locations. Consequently, coastal cliffs display a wide range of 

form and stability. Detailed investigation and in-depth understanding of geologic processes typically is required in order to 

decipher and predict the behavior of a particular coastal cliff. 

Water-Level Change

Changes in water level (sea level or lake level) can change 

the frequency and duration of wave contact with coastal rocks, 

a primary control of cliff development. In the oceans, sea level 

varies across a broad range of temporal scales. Over a long 

time scale, net global warming since the end of the Wisconsinan stage has caused extensive melting of glaciers, which 

in turn has added a large volume of water to the oceans (~44 

million km3

), resulting in a global (eustatic) rise in sea level. 

The warming also has caused thermal expansion of sea water, 

again raising eustatic sea level. Persistent tectonic forces (for 

example, those associated with mountain building and largescale movements of the Earth’s crust) raise or lower some 

sections of coastal land, causing a local change in relative sea 

level (the net difference between eustatic sea-level change and 

tectonic land-elevation change). Similarly, melting of glaciers 

at high latitudes unloads the crust, causing a type of continental uplift called “glacial rebound.” Over shorter time scales, 

sea level varies with the tides and with the passage of large 

storms or hurricanes. It also varies with heating and cooling 

caused by annual climate variation and with more occasional 

events such as El Niños.

Sea-level change is of fi rst-order importance in the evolution of modern sea cliffs, and it might become even more 

important in the future if the pessimistic predictions of global 

warming prove to be correct. Historical records indicate a 

eustatic sea-level rise of a 1 or 2 mm/yr over the past century 

(Emery and Aubrey, 1991), whereas more recent satellite 

measurements demonstrate a higher rate of 3.2 mm/yr (Cabanes and others, 2001). Although these magnitudes may not 

seem particularly large, they can result in rates of horizontal 

shoreline transgression that are many times larger, particularly 

along gently inclined coastal plains, so the rise is an important 

factor in the long-term fl ooding and erosion of many coasts. 

For example, over the past 18,000 years on the low-relief sections of the U.S. east coast, while sea level rose about 130 m, 

the shoreline retreated 130 km in places. Therefore, the ratio 

of shoreline retreat to sea-level rise averaged over this interval 

was 1000:1, or 1 m of retreat took place for each 1 mm of sealevel rise. Furthermore, many scientists believe eustatic sealevel rise will accelerate as a result of global warming (Houghton and others, 2001).

The local change in the level of the sea during approximately the past 100 years is best documented in tide-gauge 

records. A progressive rise in sea level is obvious at most 

tide-gauge sites. Examples of sea-level change derived from 

tide-gauge records of as much as 80 years duration are shown 

in fi gure 11. The curve from New York City is typical of those 

for much of the east coast of the United States, documenting 

an average long-term rise of approximately 3 mm/yr. Roughly 

half that rate of increase can be attributed to the global eustatic 

rise in sea level, the other half being due to subsidence of the 

land that occurs in much of that region.

The sea-level curves in fi gure 11 illustrate the extreme 

spatial variation in relative sea-level change along U.S. coasts, 

a variation that results from major tectonic activity and associated land-elevation change. The large apparent sea-level 

rise at Galveston, Texas, refl ects subsidence of the coast there, 

superimposed on the eustatic rise. The net apparent drop of 

sea level at Juneau, Alaska, results from the extremely high 

rates of glacial rebound combined with tectonic uplift, which 

outpace the eustatic rise. The tide-gauge data from Astoria, Oregon, do not show a net long-term trend of either increasing or 

decreasing mean sea level, only large variations from year to 

year. This lack of an upward trend in the Astoria data is caused 

by the tectonic uplift of the Pacifi c Northwest due to the eastward movement and descent (subduction) of the Juan de Fuca 

 Processes that Govern the Formation and Evolution of Coastal Cliffs
12 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Plate’s ocean crust beneath the North American Plate’s continental crust. The rate of land-level rise is variable along the 

coast, but the tide-gauge record from Astoria indicates that at 

this specifi c site coastal uplift must be occurring at effectively 

the same rate as global sea-level rise. 

A common feature of many sea-level curves from the U.S. 

west coast is the large degree of variation from year to year. In 

particular, exceptionally high annual sea levels occurred during 1982-83 and 1997-98. Both of these periods included major El Niño events, as did most of the other years with higher 

annual sea levels. The studies by Flick and Cayan (1984) and 

Flick (1998) demonstrated the importance of El Niños in elevating annually averaged sea levels along the California coast. 

Monthly averaged sea level can be 50 to 60 cm higher during 

an El Niño compared with the long-term average (Allan and 

Komar, 2002; Ryan and Noble, 2002; Komar and others, 2000; 

Komar, 1986).

The elevated water levels that occur along the U.S. west 

coast during El Niño events are accompanied by short-term 

transgression of the sea. This transgression, together with 

the increased severity of the wave climate, is an extremely 

important agent in the unusual extent of erosion that occurs 

along the west coast during an El Niño (Griggs and Johnson, 

1983; Griggs and Savoy, 1985; Komar, 1986, 1998b; Storlazzi 

and Griggs, 1998). The coastal damage in southern and central California during the 1982-83 El Niño was exceptionally 

large, in part because of the coincidence of a high storm surge 

and the highest tides in 4 years (Flick, 1998). Seven large wave 

or storm events occurred during the fi rst three months of 1983, 

when most coastal erosion took place, and the arrivals of these 

large waves coincided with times of very high tides, thereby 

concentrating more wave action directly on the shoreline and 

cliffs.

The cycle of the tides above and below the mean level of 

the sea is an important aspect of coastal-cliff erosion, and to 

the coast in general, even in the absence of storms. Typically, 

at least where beaches fl ank the cliffs, waves are able to attack the cliffs only at times of high tide, and the more extreme 

the elevation of the tide, the greater the potential for erosion. 

Therefore, of particular interest are the elevations reached by 

the highest tides—the predicted spring tides produced each 

month by the alignment of the Earth, Moon, and Sun.

Unusually high spring tides occur when the Moon also 

happens to be at perigee in its orbit—that is, closest to the 

Earth—while simultaneously being in line with the Earth and 

Sun. Such predicted tides are termed “perigean spring tides.” 

This combined occurrence of perigee and the alignment of 

the Earth, Moon, and Sun adds roughly 40 percent to the total 

range of the tide, signifi cantly affecting water levels (the percent enhancement actually depends on the coastal location). 

The erosion of coastal cliffs is more likely at times of perigean 

spring tides, because the high tide levels place the water’s edge 

closer to the cliffs behind the beaches.

When tidal elevations are measured, they generally are 

found to differ, sometimes signifi cantly so, from the predicted 

levels that are based solely on the gravitational forces of the 

Moon and Sun. This difference can be attributed to a variety of 

atmospheric and oceanic processes. Of interest to sea-cliff erosion are those processes that signifi cantly elevate water levels. 

The most dramatic example is the occurrence of a storm surge, 

resulting from strong winds blowing toward the coast that 

force water against the shore. Another contributing factor is 

the low atmospheric pressure of the storm, as occurs beneath 

the eye of a hurricane, which in effect “sucks” the water surface upward so it achieves a higher elevation directly beneath 

the storm.

Along the shores of the Great Lakes, long-term, annual, and storm-induced rises in lake level increase the 

frequency and duration of wave contact with coastal bluffs. 

The major infl uence on lake-level change is the variation of 

Figure 11. Annually averaged relative sea levels as determined from 

tide-gauge records at various coastal sites (thin jagged lines). Heavier 

lines represent smoothed trends. The results illustrate the effects 

of a slow global rise in the water level within the oceans, plus local 

changes in the levels of the land. After Hicks (1972).

  

	

	




		



		



		












 !"#!!$%

"#! &
!$%'
13

regional precipitation that produces 7- to 25-year cycles of 

water-level change of a meter or two, which signifi cantly affects lake-bluff erosion (Quigley and others, 1977; Quigley 

and Di Nardo, 1980; Quigley and Zeman, 1980; Lawrence, 

1994; Cox and others, 2000; Brown, 2000; Brown and others, 2001). Changes in lake outfl ow, and in the amount of 

evaporation, also cause measurable variations in lake level. 

Moreover, lake level undergoes annual cycles of tens of centimeters because of seasonal temperature differences, and it 

experiences daily variations caused by tides and storm surge. 

Being small bodies of water, the Great Lakes have tides that 

amount to only a few centimeters in amplitude. However, 

seiching (oscillatory waves moving back and forth across a 

confi ned body of water) is important, governed by the natural 

period of oscillation of the lake—the period is on the order of 

one day or somewhat less, and amplitudes can involve tens of 

centimeters, so the effect is much like tides. Storm surge also 

can be signifi cant on the Great Lakes, because of their shallow water depths. For example, a water-level change due to a 

storm surge measured at Toledo, Ohio, on Lake Erie was on 

the order of 1.5 m (Dewberry and Davis, 1995). A pattern of 

seiching also was associated with that storm. 

Bluff-base erosion and subsequent recession of the bluff 

crest typically occur more rapidly during rising or high stages 

of lake level, and the cyclic nature of lake-level variation correlates with a cyclicity in the volume of sediment removed 

from the bluffs (Lawrence, 1994; Quigley and Di Nardo, 1980; 

Cox and others, 2000). However, the erosion and recession 

typically are preceded by downcutting of the nearshore lake 

fl oor during periods of low lake level (Nairn, 1997). Downcutting of nearshore cohesive deposits results in a lowering of the 

nearshore profi le. If there is insuffi cient sand to build up the 

nearshore profi le during rising or high lake levels, a typical 

case for many areas of the Great Lakes, then larger waves will 

be able to reach closer to shore before breaking, thereby contributing to more rapid erosion.

For Lake Erie and Lake Michigan, there may be a threshold lake level above which erosion proceeds more quickly. 

A study of wave erosion at the bluff toe at fi ve sites on Lake 

Erie found that wave erosion increased whenever the combination of lake level and storm surge produced a storm lake level 

exceeding 1.3 m above chart datum (Carter and Guy, 1988). 

Similar anecdotal observations have been made for the Illinois 

lakeshore of Lake Michigan.

Evidence for the types of coastal impacts that can be 

expected along the ocean shores as a result of a major rise in 

water level is provided by the erosion that has occurred in the 

Great Lakes when lake level is unusually high (fi g. 12). To illustrate this point, consider a documented situation at Harbor 

Beach, Michigan. Figure 13 shows the data for water levels 

there, as reported by Hallermeier (1996). There are marked 

fl uctuations, with the amplitude of change occurring within 

a decade being on the order of 1 to 1.5 m. Broadly speaking, 

there has been an overall decrease in lake level from 1860, 

when measurements began, to a low in the 1940s, followed by 

a net upward trend.

These fl uctuations in large part refl ect the amount of 

precipitation within the watersheds of the lakes and the losses 

of water due to evaporation and outfl ow, the latter occurring 

through the St. Lawrence seaway and by water diversion into 

the Chicago River. These components are graphed in fi gure 

14, derived from the review by Changnon (1997). The paral-















	






 













  !      



Figure 12. Recession rate of the lake-bluff crest at three 

sites on Lake Michigan near Manitowoc, Wisconsin, compared to lake-level height. Slightly negative recession rates 

are interpreted to be zero recession. Negative rates are 

within the estimated error (0.2 to 0.5 m/yr for these sites). 

From Brown (2000).

Figure 13. Measurements of water levels in Lake Michigan. 

There are large fl uctuations, with the amplitude of change 

being on the order of 1 to 1.5 meters and occurring within a 

decade. From Hallermeier (1996).

Harbor Beach, Michigan

WATER ELEVATION, IN FEET (NGVD)

582

581

580

579

578

577

1860 1880 1900 1920 1940 1960

YEAR

1980

 Processes that Govern the Formation and Evolution of Coastal Cliffs
14 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

lel between the lake level and the precipitation record is apparent, but with a lag of two to three years in the response of 

the lake. Water discharge in the St. Lawrence responds to the 

lake levels, with some human control, while diversion into the 

Chicago River is completely controlled, with the decrease in 

post-1940 volumes having been regulated by a Supreme Court 

decree.

Waves

From a process perspective, wave action distinguishes 

coastal cliffs from inland cliffs, whose morphologies are the 

product solely of terrestrial processes (such as surface runoff, 

groundwater seepage, and slope failure). When waves reach 

the base of a cliff, they can erode the cliff material directly 

or they can erode loose material that has collected at the cliff 

base (Edil and Vallejo, 1980). Either case tends to destabilize 

the cliff and ultimately induce failure of the overlying material. 

An understanding of the role of waves in coastal-cliff erosion 

requires knowledge of (1) the deep-water wave climate, (2) 

how the wave climate responds to changes in storm intensities 

and global climate regimes such as El Niños and the decadalscale variations in the forces that create hurricanes, (3) how 

the deep-water energies of waves are modifi ed by shoaling 

(including refraction and bottom friction) before they reach 

the coast, (4) the dissipation of wave energy by breaking in the 

nearshore and the transformation of wave motions into swash 

runup on the beach that might front a coastal cliff, (5) the 

forces that waves impart to a cliff, and (6) the resistance that 

cliff materials present to these forces.

The primary evidence that waves have eroded a cliff is 

the presence of an undercut notch along the cliff’s base (fi g. 

15), which can initiate the collapse of the overlying rock or 

unlithifi ed sediment because of oversteepending and removal 

of support (Wilcox and others, 1998; Carter and Guy, 1988; 

Vallejo and Degroot, 1988; Edil and Vallejo, 1980; Edil and 

Haas, 1980). However, as discussed elsewhere, not all notches 

are wave-cut.

Further evidence of wave impact is in the form of irregularly bounded recesses in the cliff face (fi g. 16) from which 

blocks have been quarried by pounding waves. An erosional 

scarp at the seaward edge of cliff-base talus accumulations 

implies that waves have approached the cliff, and that erosion 

99

94

89

84

79

74

ANNUAL

PRECIPITATION, 

IN CENTIMETERS

1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

Precipitation of Lakes Michigan - Huron

(average for 5-year period)

Levels of Lakes Michigan - Huron

LAKE

HEIGHT,

IN METERS177

176

175

DIVERSION AT CHICAGO,

IN CENTIMETERS

295

205

145

85

0

Changes in outlet of Lakes

Michigan - Huron at Detroit

1 4 5

7

9

2 3

8

10

6 1, 2, and 3 = Changes in Detroit river

4 = Canal deepened

5 and 6 = Grosse Point channel/canal changes

7, 8, 9, and 10 = Changes in St. Clair River/canal

Diversion to

I&M Canal

Main

channel

opened

Diversion at Chicago

Supreme

Court

decree

Special

drought

release

YEAR

Figure 14. Historic variations in water levels in Lakes Michigan and Huron, which respond with a lag of two to three years to the precipitation within their watersheds. The other factor in controlling lake levels is water losses due to evaporation and exit discharges into the 

Chicago River and St. Lawrence Seaway. After Changnon (1997).
15

will probably commence once the talus has been removed. 

The presence of caves, tunnels, arches, or embayments attests 

to continued wave attack along structural weaknesses such as 

joints or faults (fi g. 17). Some bluffs in cohesive sediment can 

be smoothed by waves.

The strongest wave impacts are episodic and so is the 

associated cliff retreat (fi g. 18). This is largely related to the 

wave climate. In temperate regions, waves tend to be most 

powerful and the protective beaches narrowest during the winter storm season, so most wave erosion occurs during those 

months. However, in the Great Lakes region and other highFigure 15. Wave-cut notch, approximately 3 m high, at the base of 

a sea cliff eroded into interbedded sandstone and shale at Cabrillo 

National Monument, San Diego County, California. The erosion is 

enhanced by the abrasive effect of the coarse beach sediment that 

is entrained within the waves. Notice the sea cave at the left side, 

where a steep fault cuts the strata.

Figure 16. This photo, taken after large El Niño storm waves battered 

the sea cliff at Vallejo Beach in central California, shows irregularly 

bounded depressions that are typical when waves quarry blocks from 

a sea cliff. Cliff is about 2 meters tall. Figure 17. Wave-eroded arch along the Oregon coast.

latitude areas, ice can limit wave generation by winter storm 

winds as well as armor the shore from wave attack, so bluff 

retreat actually can be diminished during the winter (see, for 

example, Forbes and Taylor, 1994).

Storm seasons range in magnitude from year to year, and 

years of relative quiescence and stability can be punctuated by 

years of rapid retreat and severe coastal damage. A prime example is the occurrence of El Niño events that strongly affect 

the U.S. west-coast wave climate and resulting coastal-erosion 

impacts (Griggs and Johnson, 1983; Komar, 1986, 1998b; 

Storlazzi and Griggs, 1998; Seymour, 1996, 1998). Similarly, 

decadal-scale changes in wave climate can produce parallel 

trends in the extent of coastal-cliff erosion. Analyses of wavebuoy measurements collected during the past 25 to 30 years in 

the eastern North Pacifi c show that deep-water wave heights 

and periods have both increased during that span of time (Allan and Komar, 2000; Graham and Diaz, 2001). The highest 

rate of increase occurred off the coast of Washington, where 

the winter (October through March) average deep-water signifi cant wave height increased by 2 m in 25 years. The increase 

was slightly less offshore of Oregon, smaller still off the coast 

of northern California, and off southern California there was 

no statistically signifi cant decadal-scale increase. This latitude 

dependence is largely controlled by the strengths and paths of 

storms arriving from the North Pacifi c that impact the westcoast shoreline.

A similar increase in wave height has occurred in the 

eastern North Atlantic, documented by wave measurements 

off Land’s End at the southwestern tip of England (Carter and 

Draper, 1988). This decadal-scale increase has been related to 

trends in atmospheric pressure in the North Atlantic (Bacon 

and Carter, 1991), the difference in pressure between the Iceland Low and Azores High. However, this increase in North 

Atlantic wave conditions does not appear in buoy measurements off the northeast coast of the United States or along the 

coast of eastern Canada (Jonathan Allan, oral communication, 

2002).

 Processes that Govern the Formation and Evolution of Coastal Cliffs
16 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Long-duration winter storms known as “nor’easters” are 

the prime agent of severe coastal erosion along the northeast U.S. coast, particularly those that occur at times of high 

astronomical tides and large storm set up, the sum of which 

is known as the “storm tide.” The magnitude of the effect 

has shown an increase with sea-level rise (Zhang and others, 

2001). By inference, sea-cliff erosion is similarly affected. 

There is no long-term trend in the frequency or strength of 

nor’easters, but hurricanes are a different matter. Goldberg and 

others (2001) report multidecadal periods of relatively high 

and low levels of major hurricane occurrence, with the current 

period of high-level activity having begun in 1994. Furthermore, Gray (1984) documented an inverse relation between 

Atlantic seasonal hurricane frequency and moderate to strong 

El Niños.

The deep-water wave climate affects nearshore processes 

such as wave-breaker height and swash-runup elevation that 

have a direct role in erosion of coastal cliffs. A number of 

studies have demonstrated that wave runup level depends on 

both deep-water wave height and period, and also on the slope 

of the beach (see review in Komar, 1998a). For example, based 

on the runup measurements by Holman (1986) at the Field Research Facility of the U.S. Army Corps of Engineers in Duck, 

North Carolina, and their own runup measurements on the 

coast of Oregon, Ruggiero and others (2001) clearly demonstrated that a change in the deep-water wave height and period, 

whether seasonal or decadal in scale, results in a corresponding change in the runup elevation on beaches.

This connection has been demonstrated by Komar and 

Allan (2002) in their analysis of wave-dependent nearshore 

processes, including the wave runup. An example analysis is 

presented in fi gure 19 (lower) for the decadal increase in wave 

runup on Washington beaches. This progressive increase in 

Figure 18. Examples of episodic cliff retreat at three sites along 

the coastline of northern Monterey Bay between the cities of Santa 

Cruz and Capitola, California.

Figure 19. Decadal-scale increases of deep-water signifi cant wave heights and calculated R2% runup levels on 

Washington beaches. Signifi cant wave heights and runup 

levels are averaged for the winter months (October through 

March) each year.

220

210

200

190

180

26th Avenue

1920 1940 1960 1980 2000

1920 1940 1960 1980 2000

1920 1940 1960 1980 2000

YEAR

20

18

16

14

12

10

35th Avenue

60

50

40

30

20

Moran

DISTANCE FROM REFERENCE TO BLUFF EDGE, IN METERS

Washington (#46005)

Washington (#46005)

Y = 0.032x - 60.188

 R = 0.48 2

SIGNIFICANT WAVE HEIGHT, H, IN METERS

1976 1980 1984 1988 1992 1996 2000

YEAR

1976 1980 1984 1988 1992 1996 2000

YEAR

4.0

3.5

3.0

2.5

2.0

Y = 0.010x - 17.744

 R = 0.49 2

WAVE RUNUP, R , IN METERS 2%

1.70

1.80

1.60

1.50

1.40

1.30
17

the average runup has resulted in a horizontal “transgression” 

of the mean shoreline by about 8 m during the past 25 years, 

greater than the transgression from the relative rise in sea 

level along the Washington coast during that period. The shift 

in the deep-water wave climate and associated nearshore processes is of obvious signifi cance to the Washington coast, resulting in increased erosion of coastal cliffs and dune-backed 

shores. In similar analyses for the coast of California, it was 

found that El Niño events account for the most extreme runup 

elevations, decadal-scale trends not being a factor (Komar and 

Allan, 2002).

The nearshore wave climate in relation to the deep-water 

wave conditions can be affected by wave shoaling, including 

wave refraction and the loss of energy due to bottom friction 

exerted on the waves. The effects of wave refraction in controlling the extent of sea-cliff erosion are illustrated by the 

long-term cliff retreat that has undermined the city of Dunwich on the North Sea coast of England (Robinson, 1980). 

In the 13th century, Dunwich was one of England’s major 

cities and the location of a commercially important harbor. 

Over the centuries, erosion of the glacial-till bluffs has progressively undermined the city to such an extent that today it 

is only a small village of a few houses. Shoals are common 

in the North Sea, formed by strong tidal currents, and one 

shoal—Sizewell Bank—is present directly offshore from 

Dunwich. Robinson (1980) demonstrated that wave refraction 

over Sizewell Bank was for several centuries important to the 

erosion of Dunwich, because the shoal acted like a lens to focus the energy of the waves on its shoreline. This focusing of 

wave energy intensifi ed the storm-related nearshore processes 

on the Dunwich shore, including the size of breaking waves, 

the swash-runup elevation, and the storm surge that elevated 

tides. Robinson also demonstrated that the erosion has diminished during the last fi fty years, concluding that this reduction 

has resulted from the slow northward migration of Sizewell 

Bank, shifting the focus of wave energy by refraction away 

from Dunwich.

The amount of wave-induced erosion is a function of the 

energy expended against the cliff by the waves, through the 

compressional force of impact and the tractive force of uprush 

(Trenhaile, 1987; Sunamura, 1992). Waves impart the most 

energy in the form of shock pressure if they collide directly 

with the cliff just as they break. Compressed air within cavities in the cliff can expand explosively as the wave recedes, 

hydraulically quarrying blocks from the cliff face (fi g. 16). 

However, waves more commonly reach a cliff as swash on a 

fronting beach, having already broken offshore. Erosion then 

occurs mainly by the tractive force of the wave as it washes 

up the cliff face, particularly if coarse, abrasive sediment has 

been entrained from the beach (Robinson, 1977; Kamphuis, 

1987; Nairn, 1997). Elsewhere, in the absence of a beach or 

exposed platform, some cliffs are in constant contact with 

the water. In this case the water level oscillates up and down 

the cliff face, and a notch can be slowly eroded just above the 

mean water level. Of course, many cliffs experience different 

types of contact with waves as conditions change.

The response of a cliff to the erosive forces of waves, and 

its feedback response that modifi es the wave’s erosive action, 

are simply but elegantly illustrated by the laboratory wavebasin experiments undertaken by Sunamura and summarized 

in his book, “Geomorphology of Rocky Coasts” (1992). 

Sunamura constructed artifi cial cliffs composed of loosely cemented sand having the consistency of a natural sandstone that 

is moderately resistant to wave attack. In one series of experiments there initially was no fronting beach, so the cliff was under the direct attack of waves. The cliff retreated as the waves 

cut away its base (fi g. 20), forming a notch centered at the 

still-water level and extending a few centimeters up the cliff 

face. The cliff recession released a supply of sand that progressively accumulated at the foot of the cliff to form a beach. This 

initially produced a higher rate of cliff erosion because at this 

stage the waves used the released sand as a “blasting” agent (a 

process technically termed “corrasion”). At a later stage, when 

more sand had accumulated, the wider beach caused the waves 

to break offshore, providing protection to the cliff and reducing the rate of cliff erosion. Ultimately, the beach grew to such 

an extent that it no longer allowed the waves to reach the cliff, 

and the erosion ceased altogether.

To a degree, this sequence happens on natural beaches, 

but here the process is more complex because of the presence 

of tides that continuously alter water levels, the occurrences 

of storms that periodically generate larger waves that can still 

reach the cliff, and the fact that in most places littoral drift 

moves the sand generated from cliff erosion downcoast. Generally, in the natural situation the beach develops enough to 

provide protection to the cliff for much of the year, and it is 

only under a combination of high tides and storm waves that 

runup reaches the cliff. Therefore, erosion of the cliff usually 

becomes highly episodic.

In his extensive series of laboratory experiments, Sunamura constructed artifi cial cliffs having a range of strengths, 

formed by varying the proportions of fi ne sand and cement. 

As expected, the greater the strength of the artifi cial cliff, the 

lower the rate of erosion. The experiments also documented 

the existence of a critical wave height and associated impact 

force on the cliff face that is required to initiate cliff erosion, 

the value depending on the strength of the cliff material. On 

Figure 20. The laboratory experiment of Sunamura producing the 

erosion of an artifi cial cliff composed of sand and cement, with the 

erosion forming a notch. The development of a fronting beach from 

the sand released by the eroding cliff altered the attack of the waves 

by making them break away from the cliff. After Sunamura (1992).

cement / sand

sea cliff

0

0

5

5

10

10 cm

109

100 80 60 hrs

40 30

20 10

10 20

100 hrs 109

sand beach

still water level

CENTIMETERS

 Processes that Govern the Formation and Evolution of Coastal Cliffs
18 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

the basis of his experimental results, Sunamura (1992) developed a mathematical model that relates the rate of cliff retreat 

to the forces of wave impact compared with the resistance 

strength of the rock. By assuming that the force of erosion is 

proportional to the height of the assailing waves, and using the 

compressive strength of the rock as a measure of its resistance, 

Sunamura derived an equation for the rate of cliff retreat as a 

function of these parameters:

 Retreat rate = k(C + ln qgH/Sc),

where k and C are constants, ρ is the density of water, g is the 

acceleration of gravity, H is wave height at the cliff base, and 

Sc is the compressive strength of the cliff-forming material. 

The mathematical relationship was in part substantiated by 

his laboratory experiments.Although the laboratory experiments of Sunamura were greatly simplifi ed compared with 

the multitude of factors that are important to the erosion of 

coastal cliffs in nature, fi eld investigations have confi rmed 

many similarities to his results. For example, Quigley and others (1977) documented a positive correlation between wave 

power and erosion rate along a section of Lake Erie shoreline 

backed by glacial clay deposits whose properties vary over a 

limited range. This is an instance where the spatial response 

of the bluffs refl ects the spatial variation in the exposure to 

geologic forces (wave power). In contrast, Benumof and others 

(2000) found an inverse correlation between cliff retreat rate 

and wave power within the Oceanside littoral cell in California 

(fi g. 21), where material properties vary over a wider range; 

the rate is highest where the cliff-forming material is weakest. 

They concluded that wave power is secondary to lithology and 

material strength in explaining the variability in rate of erosion 

and overall retreat of these coastal cliffs. In this instance, the 

spatial variability is primarily a refl ection of variation in the 

susceptibility (material strength).

Sunamura’s experiments demonstrate that cliff retreat rate 

is a function of both wave forces and material properties. Field 

studies demonstrate that either factor can dominate in specifi c 

situations. Nevertheless, without some degree of wave attack, 

retreat rates of coastal cliffs eventually would diminish to insignifi cant levels, irrespective of material properties.

The details of wave-erosion processes were considered in 

a study on the Yorkshire coast of northeast England by Robinson (1977). At locations where there is no fronting beach, 

cliff erosion depended on the hydraulic force of the waves, 

which produces the sporadic quarrying of small blocks of the 

cliff material and a more continuous micro-quarrying of shale 

fragments. Robinson found that where there is a beach at the 

foot of the cliff, the action of sediment blasting (corrasion) 

increases the rate of erosion 15 to 20 times higher than where 

there is no beach.

Implicit in the preceding discussion is that waves must 

actually come in contact with a coastal cliff in order to erode 

it. A reasonable corollary is that the duration of contact is 

important. Ruggiero and others (2001; see also Shih and others, 1994) determined the amount of time that runup elevation 

equals or exceeds the elevation of the cliff base along dissipative beaches in Oregon. Using historical wave, tide, and 

beach morphology records, the amount of cliff erosion at three 

different sites correlated reasonably well with a calculated estimate of the hours of wave impact per year. 

Terrestrial Water: Surface Runoff and 

Ground Water

It is well recognized that fresh water, at the ground surface or beneath it, has a major infl uence on the geomorphic 

evolution of the land surface in general (Leopold, and others, 

1995; Higgins and Coates, 1990; White, 1988). This statement 

also applies specifi cally to cliffs, both inland and at the coast. 

In a more restricted sense, it even applies to beaches (Urish, 

1989) and to the submarine environment (Robb, 1990). 

Surface Water 

Surface runoff and rain impact can sculpt broad areas of a 

cliff face because of the tractive force that water exerts on the 

erodible surface, as well as its softening effect. Sheet-fl ow ero-



	



	


		 	














 ! 
"!
!
#$

 %&  %"

! %
 #
' 

, ,*+ ) )*+ (

-

.-

,-

)-

(-

+-

Figure 21. Relation between sea-cliff erosion rate and mean 

wave power at sites in San Diego County, California. Note that 

the exponential decay curve is strongly leveraged by the Carlsbad site. Modifi ed from Benumof and others (2000).
19

sion of cliff faces emphasizes the details of bedding structures 

in sedimentary deposits, and it can erode vertical fl utes and 

impart a badlands topography (see, for example, Norris, 1990) 

(fi gs. 22 and 23). Fresh-water sheet fl ow generally causes only 

a small amount of coastal-cliff retreat, but concentrated runoff can carve gullies that deeply indent the cliff (fi g. 24). The 

amount of cliff erosion caused by sheet wash typically is overshadowed by other processes such as slumping, landsliding, 

or rock falls, but the magnitude of cliff-face relief can reach 

meters between episodes of gravitational slope failure (see, for 

example, Sterrett, 1980). Smooth unvegetated sections of cliff 

face adjacent to sculpted or vegetated ones can signify locations of recent slope failure (fi g. 25).

Gullies form where surface water is concentrated in channelized fl ow; point-source ground-water seepage high on a 

coastal cliff can have a similar effect. Gullies can be deep and 

wide, seriously interrupting the continuity of a coastal cliff, if 

the source fl ow is sustained or of high discharge (fi g. 24).

Ground Water

Ground water has both physical and chemical effects that 

can infl uence coastal-cliff stability. If ground-water discharge 

is strong enough, it can dislodge grains from a cliff face, a process referred to as “seepage erosion” or “sapping” (Higgins, 

1982; Howard and McLane, 1988). Concentrated discharge 

around plant roots and animal burrows causes cave-like “piping” (Zaslavsky and Kassif, 1965; Jones, 1990). Ground-water 

fl ow also can be concentrated along structural discontinuities 

such as joints, and erosional widening of the joints can decrease the outcrop-scale (rock-mass) strength, destabilizing a 

coastal cliff (Benumof and Griggs, 1999) (fi g. 26). Chemically, 

ground water can dissolve unstable grains or the chemical cements that give sedimentary rock its strength, once again leadFigure 22. Western Lake Michigan bluff face in mid summer 

shows the effects of sheetwash and the development of gullies. 

These shallow gullies form every year on clayey till where solifl uction and shallow slides occur in winter. From Sterrett (1980).

Figure 23. Deep gullies on the eastern Lake Michigan shoreline are 

primarily initiated by sapping of ground water or capture of surface 

runoff. In this case surface runoff diverted by development rapidly 

caused substantial incision.

Figure 24. These large gullies at County Beach in San Mateo County, 

California, were carved by a combination of seepage erosion and 

concentrated surface runoff. Cliff is ~15 m high.

 Processes that Govern the Formation and Evolution of Coastal Cliffs
20 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

ing to weakening or destabilization of a coastal cliff (Turner, 

1980). Solution effects are particularly prominent in coastal 

cliffs composed of carbonate rocks (Norris and Back, 1990).

Ground-water level varies seasonally and generally is 

highest after periods of prolonged rainfall and snowmelt. The 

change in level can be large; for example, Sterrett and Edil 

(1982) measured as much as 13 m of water-level change over 

the period of a year in glacial bluffs along Lake Michigan. Stability analysis indicated a signifi cant decrease in slope stability 

at some locations, because of the weight and pore-pressure effects of the ground water, but seepage erosion at the cliff base, 

which destabilizes a cliff, was the most important consequence 

of high ground-water levels. Landscape irrigation and clifftop septic systems have been documented to add as much as 

150 cm to the groundwater level, which can infl uence coastal 

cliff stability (Kuhn and Shepard, 1984). Many instances of 

coastal-cliff collapse have been correlated with measured high 

levels of ground water due to either of these factors (see, for 

example, Turner, 1981; Sterrett and Edil, 1982).

Dissolution of coastal cliffs composed of carbonate sediment tends to be concentrated near the underground contact of 

fresh and saline waters, at places where ground water more or 

less continuously emerges from a cliff, typically forming a notch 

or “nip” at the cliff base (Norris and Back, 1990), although biological and biochemical processes also can play a role (see, for 

example, Trenhaile, 1987, p. 258). Ground-water seepage in terrigenous sedimentary deposits also can be localized at the cliff 

base, because of the presence of an impermeable rock or soil 

layer at that level (fi g. 27). The consequent undercutting related 

to ground-water seepage (sapping) can resemble wave erosion, 

but the cause and remediation are different: subsurface drainage 

control for sapping versus toe protection such as a seawall for 

wave erosion (Palmer, 1973). Note that inland cliffs can exhibit 

similar cliff-base sapping (Bryan, 1928).

Ground-water seepage and erosion can occur above the 

cliff base where downward percolation is retarded by impermeable horizons (Norris and Back, 1990; Rulon and others, 

1985; Rulon and Freeze, 1985; Sterrett and Edil, 1982), so 

several lines of seepage might appear on a particular coastal 

cliff. Seepage at the contact between the relatively permeable 

marine-terrace or glacial-sand/fractured-clay deposits that are 

widespread at the top of coastal cliffs and the less permeable 

underlying bedrock is particularly common. Concentrated 

Figure 25. Signs of recent sea-cliff failures in northern San Mateo 

County, California. Cliffs are about 10 m high. A, The relatively smooth 

section of cliff face (right side of photo) adjacent to a roughened 

section (left side of photo) implies recent slope failure, although other 

evidence such as fallen blocks has been removed. B, An unambiguous example of recent failure.

Figure 26. Block failure along joint sets accelerated by seepage in 

sedimentary rocks of northern Monterey Bay, California. Seepage 

imparts a dark shade to the cliff, visible at the left side of the photo.
21

ground-water seepage along fractures in competent rock probably enhances the formation and growth of sea caves, although 

the primary cause of such features is believed to be wave erosion along faults and joints.

Wide valleys carved into cliffs, termed “scallops” or “theater-headed valleys,” are formed by concentrated ground-water 

fl ow (Laity and Malin, 1985; Bryan and Price, 1980). These 

valleys characteristically have a rounded, bowl-shaped head 

and a V-shaped cross section. The basic process of their formation involves concentration of ground-water fl ow (along fractures, for example) that leads to localized sapping. A feedback 

mechanism then begins, whereby sapping leads to valley formation, which in turn leads to further concentration of groundwater fl ow, which leads to accelerated erosion of the valley.

Weathering

Coastal cliffs are exposed to a severe weathering environment if they undergo repeated wetting by salt spray or surface 

runoff, interspersed with periods of drying and heating. The 

weathering process is accentuated if the surface material is 

fractured. Further, cliff-forming material may have undergone 

weathering beneath the original ground surface, before cliff 

formation. Weathering typically weakens a cliff and makes it 

more susceptible to erosion. Weathering effects normally are 

overshadowed by wave erosion or slope failure, but they can 

be dominant in certain circumstances.

If fresh surface water is the primary weathering agent, 

repeated wetting-drying cycles can degrade the outer few 

centimeters of the cliff face, particularly in the presence of 

expandable clay minerals that cause surface fi ssuring that promotes further infi ltration, as well as slaking and the formation 

of prismatic blocks (Quigley and others, 1977; Hampton and 

Dingler, 1998). Infi ltration can soften the sediment and induce 

thin slides or fl ows, whereas intact blocks fall in response 

to the pull of gravity. Hutchinson (1973) remarked that it is 

impractical to control weathering directly, so stabilization of 

weathered cliffs should be approached by other means, such as 

toe stabilization or drainage.

Where salt spray persistently wets a cliff, it is not the 

chemical corrosive effects of salt that are important, but 

rather the pressures within voids as salt crystallizes or when 

it is heated (Bryan and Stephens, 1993; Johannssen and others, 1982; Wellman and Wilson, 1965). These pressures can 

mechanically disintegrate the cliff face, producing a weak, 

crumbled layer.

Johannssen and others (1982) termed the crystallization 

effect “salt-crystallization weathering” and the heating effect 

“salt-expansion weathering” in their study of coastal cliffs in 

the temperate climate of the Oregon coast. Note that drying, 

rather than just exposure to salt water, is critical. Johannssen 

and others (1982) supported this contention with an example 

in which an exposed sandstone bedding plane was eroded to a 

rough surface within the spray zone, whereas it was smoother 

below in the intertidal zone (not enough drying) and also 

above (not enough wetting). They also measured greater retreat 

rates along south-facing coastal cliffs that are exposed to the 

sun, even where protected by waves, compared to north-facing, sun-shielded cliffs. Where the salt on the cliff was washed 

away by fresh-water runoff, retreat also was slow. Bryan and 

Stephens (1993) noted that the shore platform seaward of the 

coastal cliff in Hanauma Bay, Hawaii, is widest where the cliff 

receives the most intense daily heating and therefore, by implication, experiences the most intense salt weathering. 

Ice and Cold Climate

The formation of ice, either in front of a coastal cliff or 

within it, can affect cliff recession both directly and indirectly. 

Ice-related processes dominate coastal-cliff recession in many 

northern regions of the United States that have prolonged 

intervals of below-freezing winter temperatures, particularly 

around the Great Lakes. Ice processes have surprisingly varied and complex effects on coastal cliffs, and they can either 

hinder or promote cliff erosion. For a general discussion of 

coastal ice effects, see Forbes and Taylor (1994) and Chen and 

Leidersdorf (1988).

Offshore and coastal ice develops in front of a cliff fi rst 

when swash run-up, wave spray, and interstitial water freeze 

the adjacent beach, followed by freezing of the lake or ocean 

surface (Davis, 1973). Offshore ice indirectly serves to reduce 

cliff recession, because it inhibits wave formation, and both 

offshore and beach ice dampen the impact of incoming waves. 

The net effect is to reduce or eliminate the wave energy exerted on a beach or cliff during winter (McCann, 1973). This is 

in contrast to the situation in temperate regions, where waves 

typically are largest and most destructive during winter.

Ice push, resulting either from thermal expansion or wind 

forcing, can move coastal ice directly into contact with a bluff. 

Most reports about the effects of ice push refer to the formation of sedimentary features on the beach (Hume and Schalk, 

Figure 27. Ground-water discharge at the base of a coastal cliff in 

northern San Mateo County, California, that is underlain by impermeable rock has eroded a low notch, which undercuts and potentially 

destabilizes the cliff. Cliff is about 3 m high.

 Processes that Govern the Formation and Evolution of Coastal Cliffs
22 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

1964). Nevertheless, some studies of ice-push/bluff interaction 

have been conducted, but from the point of view of ice dynamics rather than bluff erosion (see Forbes and Taylor, 1994). The 

reported inland incursion of ice push into the forested area 

beyond a lake in interior Canada (Pyökäri, 1981) and as much 

as 185 m inland on Somerset Island, N.W.T. (Taylor, 1978) suggests that this process can impact coastal bluffs if they are present (Kovacs and Sodhi, 1988). Dionne (1979) describes a low 

lake bluff in Quebec that is armored by ice-pushed boulders, 

which is likely to protect the bluff against erosion by waves 

(see also Kovacs and Sodhi, 1988). We know of no studies that 

explicitly report ice push as an erosional agent of coastal cliffs.

Coastal ice can incorporate and remove sediment from a 

beach and thereby have a delayed and indirect impact on cliff 

erosion. Ice incorporates beach sediment during and after its 

formation, and it carries entrained sediment along and away 

from the nearshore zone during breakup. The degraded beach 

henceforth can expose a bluff to wave erosion during ice-free 

times. This has been documented to be an important process 

along the Great Lakes shorelines, and it affects the rate of 

coastal erosion and bluff recession (Barnes and others, 1994; 

Kempema and others, 2001). Ice that resides either on the 

beach (an ice foot) or in the lake (brash ice, slush, frazile ice, 

anchor ice) incorporates sediment that is eventually moved 

alongshore and offshore during breakup. Sediment also can be 

added to the coastal ice complex by wind.

The volume of sediment moved varies, depending on the 

amount of sediment contained in the ice and the wave and current pattern when breakup occurs (Miner and Powell, 1981). 

Studies on Lake Erie found that relatively small amounts of 

sediment were moved in some years (on the order of 1 percent 

of total sediment moved in the beach/nearshore system) to 

about 10 percent in other years (Barnes, and others, 1996). In 

southern Lake Michigan, the amount of sediment in ice averages about 136 kg/m of coast, but is highly variable (Barnes 

and others, 1994). Barnes and others (1993) estimated that 

about 250 x 103

 t/yr are required to supply the observed icerafted sediment concentrations. This is equal to about 0.83 

t/yr/m of shoreline in southern Lake Michigan, which is about 

the same as the amount of sand being supplied annually by 

bluff erosion in the region (Barnes, and others, 1994).

Freezing of interstitial water to produce ice within a 

coastal cliff can have a direct impact on cliff recession, particularly when the ice melts. For example, solar-induced melting of near-surface interstitial ice and the inability of the melt 

water to infi ltrate the underlying frozen soil can lead to high 

water content and reduced strength of a thin surfi cial layer of 

bluff material (Vallejo and Edil, 1981). As an example, Sterrett 

(1980) documented along the Lake Michigan shoreline that, 

except in areas of large deep-seated slumps, most sediment is 

removed from coastal bluffs in the spring, largely by shallow 

slides over still-frozen ground. Many of the till and lacustrine 

deposits that make up the eroding bluffs along the Great Lakes 

are at least weakly overconsolidated (Edil and Mickelson, 

1995). When water within the sediment freezes, the pore spaces expand. Even if thawing takes place slowly and the sediment does not immediately become fl uid and slide downslope, 

the pore volume probably does not return to its original condition, which reduces the soil’s strength.

A nearly opposite situation is described by Chase and 

others (1999) in a study of lake bluffs along southeastern 

Lake Michigan. They describe a situation whereby the frozen 

bluff face impounds perched ground water within interlayered 

glacial sand and clay. The resulting increase of pore pressure 

behind the frozen bluff face induces landslides during times 

of sub-zero temperature in late fall, winter, and early spring. 

Displacements are small to nil during the summer and early 

fall, when ground water is not impounded and pore pressures 

are lower.

Lastly, frost weathering can cause growth of cracks in 

cliff material, which promotes disintegration. Walder and 

Hallet (1985, 1986) concluded that crack growth occurs as a 

result of the pressure associated with thermodynamically controlled water migration toward freezing centers. The process 

is most effective at subfreezing temperatures. They dispute 

the notion, promoted in many studies, that volumetric expansion during the water-ice transition within sealed cracks is the 

cause of frost cracking. The thermodynamically controlled 

process seems likely to occur in coastal cliffs, but we are unaware of documented examples. 

Slope Instability, Slope Failure, and Coastal 

Landslides

Slope instability, slope failure, and landslides are a subject 

of broad scope and complexity that transcends their occurrence along the coast (see, for example, Turner and Schuster, 

1996). However, coastal landslides are common and typically 

have a coastal cliff associated with them. At one extreme, 

small blocks less than a cubic meter in size can detach and fall 

from an existing cliff face. At the other extreme, failure of an 

entire coastal mountainside with a cliff at its base can displace 

several thousand cubic meters of material into the surf zone. 

Mass movements and landslides are major factors in coastalcliff retreat.

Slope instability is a condition that initiates a process 

called slope failure that creates an object called a landslide. 

However, it is common for the terms to be used interchangeably. Landslides can have many forms: a fall (detachment and 

free descent), a topple (forward rotation and tumbling), a slide 

or slump (shear deformation along a single or a few planar or 

curved rupture surfaces), and a spread (movement of competent beds atop water-bearing layers of sediment) (Cruden and 

Varnes, 1996). Note that sliding in the usual sense occurs in 

only a few of these forms. Related gravity-driven fl ow processes involve shear along innumerable planes within a fl uidlike mass. For example, a debris avalanche consists of many 

rapidly sliding, falling, and (or) toppling blocks. A debris fl ow 

is similar but occurs in wet, typically muddy sediment (fi g. 

28). Creep refers to the slow downslope movement of blocks 

of unlithifi ed material. Often the only evidence of creep is the 
23

bent trunks of trees that attempt to maintain a vertical attitude 

as the block they rest on slowly descends a slope (fi g. 29).

The geology of a coastal cliff determines its susceptibility to slope failure. A lithologic hierarchy from hard, massive 

crystalline rocks such as granite, through lithifi ed and stratifi ed rocks such as sandstone-shale sequences, to weak deposits 

such as Quaternary alluvium or glacial till is related to increasing degrees of landslide susceptibility. Geologic structure 

also plays a role: joints and faults can serve as locations of 

high susceptibility to slope failure in otherwise stable rock 

(Benumof and Griggs, 1999; Griggs and Trenhaile, 1994). 

Orientation of bedding or fractures, particularly where they dip 

seaward, is a particularly important factor (Moon and Healy, 

1994; Barton, 1973). For example, the orientation, shape, and 

slope of the sea cliffs around northern Monterey Bay in California are in large part directly related to joint orientation and 

spacing (fi g. 30). Spatial variation in lithology, bedding and 

fracture orientation, and fracture density commonly correlates 

with spatial variation of landslide type and cliff stability overall (Davies and others, 1998), and maps or tabulations of these 

features can form the basis of a susceptibility analysis (Benumof and Griggs, 1999).

Ground-water fl ow and fl uctuation are signifi cant factors that affect slope-failure susceptibility in many ways. 

Ground-water saturation can decrease the frictional strength 

through an increase in pore-water pressure and an associated 

decrease in effective stress between grains (Terzaghi, 1936; 

Mitchell, 1976; Ritter, 1986). Ground-water wetting can reduce cohesional strength by dissolving chemical cement (for 

example, calcium carbonate or salt) or by softening binding 

clays (Barden and others, 1973; Ritter, 1986; Houston and 

others, 1988; Hampton, 2002). Ground-water fl ow generates 

dynamic forces that decrease strength and can lead to gravitational failure (Iverson and Major, 1986), as can the repeated 

loads imparted on saturated sediment by earthquakes or waves 

(Sangrey and others, 1978; Seed and Rahman, 1978; Ashford 

and Sitar, 1994, 1997). Seepage forces also can cause grainby-grain erosion (Howard and McLane, 1988). Ground water 

increases the bulk density of sediment, thereby increasing the 

gravitational driving force, and a rise in ground-water level reduces the maximum stable slope (Edil and Vallejo, 1980). Engineered ground-water drainage can be an effective approach 

Figure 28. Shallow debris fl ows on this coastal cliff in Rio del Mar, 

California were initiated by sustained high intensity rainfall. Note the 

damaged houses.

Figure 29. An eroding bluff of glacial-marine sediment, Rockland, 

Maine. The bluff is about 13 m high and composed of muddy marine 

sediment deposited about 13,000 years ago. As a large, detached 

block of material in the left and center of the photo (beneath arrows) 

creeps downslope, trees on its surface are bent and twisted by the 

movement.

Figure 30. Rock falls along a seaward dipping joint set in Tertiary 

sedimentary rocks at Capitola, California. Another example is shown 

in fi gure 8.

 Processes that Govern the Formation and Evolution of Coastal Cliffs
24 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

to coastal-cliff stabilization (Bryan and Price, 1980; Turner, 

1980; Sterrett and Edil, 1982; Hutchinson and others, 1985). 

Note that in partially saturated sediment, liquid surface tension 

at grain contacts can actually increase the strength and stability of the material (Towner and Childs, 1972; Fredlund and 

Rahardo, 1993)—build a sand castle to appreciate this effect!

Large coastal landslides have been studied extensively 

along the coast of the British Isles, California, Oregon, and the 

Great Lakes. By “large” (relative to the size of the coastal cliff) 

we mean landslides that encompass the entire height of a coastal cliff; some extend inland a considerable distance (fi g. 31). 

They also typically extend tens to hundreds of meters alongshore. Activity of large landslides can be separated into prefailure movements (small precursory displacement and offset), 

failure (the main phase of movement, usually abrupt and with 

maximum displacement), and reactivation (renewed movement 

after a period of quiescence and typically a result of degredation of material properties or exposure to more hostile environmental conditions) (Lee, 1997). Wave-induced steepening (that 

is, coastal-cliff formation and retreat) at the toe of the landslide 

and high ground-water levels commonly are invoked as the 

principal triggering mechanisms of large landslides (Moore and 

others, 1998; Hutchinson and others, 1985; Edil and Vallejo, 

1980; Quigley and Zeman, 1980; Hutchinson, 1969).

Large coastal landslides can be reactivated from time to 

time, particularly as waves erode the toe or as ground-water 

levels rise as a result of high-intensity or prolonged rainfall 

(Hutchinson and others, 1985; Hutchinson, 1969). An important aspect of reactivation is that the highly deformed and 

remolded material along an old failure surface can have a 

reduced strength (that is, the residual strength) that decreases 

stability with respect to the original situation (see, for example, Hutchinson, 1969). However, this effect might be counteracted by the stabilizing effects of a reduced surface slope of 

the displaced mass.

Large landslides have a curved rupture surface (forming a 

slump), unless a properly oriented plane of weakness, such as 

seaward-dipping bedding or joints, controls failure (forming 

a translational slide). Furthermore, large landslides can have 

a zoned morphology: a steep rear scarp, a less steep displaced 

slide mass (“undercliff ” in British terminology; fi g. 32), and 

a steep frontal coastal cliff. Distinct processes and secondary 

landslide types occur in each zone (Hutchinson and others, 

1985). Rock falls and topples cascade from the rear scarp, 

as do slides and falls from the actively eroding coastal cliff. 

The old slide mass that forms the undercliff can reactivate in 

response to long-term or seasonal processes, such as rainfall, 

accompanied by surfi cial fl ows and shallow slides. Sometimes 

the reactivation of the old slide is caused by the increased load 

of material that falls from the rear scarp or by the decrease 

of the buttressing force that accompanies wave erosion of the 

frontal coastal cliff. Remediation and stabilization of large 

coastal landslides can be very diffi cult and expensive (fi g. 33). 

Furthermore, environmental issues related to dumping of large 

volumes of landslide sediment into the nearshore zone can 

constrain remediation options (see, for example, Komar, 1997; 

Hapke and Griggs, 2002). The typical approach is to remove 

material from the head of the slide and place it as a buttress at 

the toe, supplemented by ground-water drainage and surfacewater diversion (Works, 1983; Orr, 1984).

Many coastal cliffs, even tall ones, do not have large landslides associated with them. The question arises as to what 

controls the geographic distribution of large landslides. The 

Figure 31. Large-scale coastal cliff landslide south of San Francisco, 

California.

Figure 32. A large rotational slump in sedimentary rocks in the Half 

Moon Bay area of central California, showing a clearly developed 

rear scarp and undercliff (displaced landslide mass) with a cliff 

eroded at the seaward edge.
25

largest landslides involve bedrock and exist along rugged and 

elevated coastal land, such as mountainous or glacial terrain. A 

deep, weak stratum that extends far inland, such as beneath the 

Portugese Bend landslide in southern California (Ehlig, 1992), 

can set the stage for a large landslide. Along the northern New 

England coast, large landslides always involve bluffs of glacial-marine sediment more than about 5 m thick (Berry and 

others, 1996). Three-dimensional modeling techniques, such 

as the one presented by Reid and others (2000), show promise 

for relating failure potential to hillslope topography.

Moderate-scale landslides encompass a large percentage 

of the height of a coastal cliff but have relatively restricted inland and lateral extent. Many of these occur as topples at local 

promontories, points of land, or cliff-gully intersections (fi g. 

25). These places seem to fail because the slope is not supported at its sides, but we do not know of an analytical treatment of 

this situation. Moderate-scale landslides are typical on Great 

Lakes shorelines with cliff heights greater than about 20 m.

Small failures, less than the total cliff height, are common. 

They occur in the form of small rotational slumps, block falls, 

or topples that typically involve a few cubic meters of material, or less (fi g. 34) (Hampton, 2002; Hampton and Dingler, 

1998). Some small rotational slumps occur in stratigraphically 

inhomogeneous cliffs at levels of perched ground water (Chase 

and others, 1999). Many small failures occur at places where 

erosion or previous slides have oversteepened the slope and 

thereby decreased the resistance to failure. Intersecting joints 

can bound prismatic blocks that fall from a cliff (wedge failures, fi g. 8B), as can shrinkage fractures (Hampton and Dingler, 1998; Benumof and Griggs, 1999). Small slides typically 

are not isolated; instead, multiple slides tend to affect broad 

areas of a cliff face with similar properties (fi g. 34).

Many steep coastal cliffs in sedimentary deposits experience stress-release jointing, which causes sequential detachment of relatively small blocks that typically are less than a 

meter thick (fi g. 35). Stress-release jointing is a response to 

horizontal tensile stresses that exist naturally within the mid to 

upper part of a vertical cliff and the ground behind it. Models 

Figure 33. Reconstruction of California State Highway 1 along the 

Big Sur coast after a major coastal landslide in 1983. Total cost: 

$7,500,000.

Figure 34. The relatively large failure scar on this coastal cliff (left 

foreground), rather than being formed by a single landslide, developed 

gradually as a consequence of many small block falls, the remains of 

which constitute the debris apron. The other debris aprons in the background formed similarly. This sea cliff in northern San Mateo County, 

California, retreats mainly as a consequence of small block falls.

Figure 35. Stress-release jointing (beneath arrows) can occur on 

nearly vertical coastal cliffs. It is a consequence of the release of 

confi ning pressure in a horizontal direction as a coastal cliff retreats, 

with consequent expansion of the rock near the cliff face.

 Processes that Govern the Formation and Evolution of Coastal Cliffs
26 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

indicate that the tensile stresses abruptly decrease as the coastal cliff departs from vertical (Sitar and Clough, 1983), lessening the likelihood of stress-release fractures. This conclusion 

is confi rmed by fi eld observations of real coastal cliffs (Hampton, 2002). Persistence of small-scale failure and stress-release 

jointing is the primary mechanism of retreat along some coastal cliffs (Hampton, 2002). It also abets failure of cliffs that are 

undercut by waves (Wilcox and others, 1998).

Analytical Approaches for Predicting 

Coastal Slope Instability

Coastal landslides vary greatly in size and form—from 

large, deep, rotational landslides that displace an entire coastal 

mountain slope with a cliff at its base, to shallow planar slides 

that cover most or all of a cliff face, to small rotational slumps 

at one or several levels on a bluff, to very small block falls that 

remove only a few cubic decimeters of material. Each type tends 

to be associated with a particular set of causitive processes, material types, geological structures, and slope geometries that can 

change through time and space. Therefore, engineering analysis 

and prediction of coastal landslides can be a complex exercise; 

both probabilistic and deterministic engineering approaches 

have been developed (for example, Edil and Vallejo, 1977; Edil 

and Haas, 1980; Hutchinson and others, 1998; Walkden and 

others, 2000). Either approach must be preceded by careful geologic study.

A slope, such as a coastal cliff or a portion of one, becomes 

unstable when the downslope component of force associated 

with the mass of the rock body becomes equal to the resisting force along a potential rupture surface. This force relation, 

termed “limit equilibrium,” is the basis of most engineering 

slope-stability analyses (for example, Duncan, 1996). Limit 

equilibrium can be infl uenced by many of the geologic features 

and processes discussed throughout this report, including slope 

declivity and height, material properties such as strength and 

unit weight, the position of the ground-water table, seepage 

forces, freeze-thaw processes, weathering, and seismic accelerations (see, for example, Edil and Vallejo, 1980).

Coastal slopes commonly are analyzed by the effective 

stress method, in which pore-water pressure is taken into account explicitly, and either effective or drained strength parameters are used. The effective strength parameters (the effective 

internal friction angle and the effective cohesion) relate 

shear strength of the material to effective normal stresses 

(gravity-induced normal stress along the rupture surface minus 

the pore-water pressure). Pore water in these analyses is seeping ground water and its pressure is equal to pressure head 

times the unit weight of water at a given point on the failure 

surface. Pore pressures may be obtained from fi eld hydrogeological investigations and analysis of ground-water fl ow. In 

addition to strength parameters, unit weights, and pore-water 

pressures, the geometry of the slope has to be considered.

Predictions of slope failure generally are made under conditions of uncertainty, because future events that may trigger 

failure, such as intense or sustained rainfall, earthquakes, and, 

in the case of coastal cliffs, wave erosion, cannot be reliably 

forecast. Uncertainty also arises because of insuffi cient information about, and insuffi cient ways of assessing the effects of, 

lithology, material properties, ground-water and seepage conditions, and changes in geometry due to wave and surface erosion. 

Finally, there is often uncertainty and incomplete understanding about the mechanisms of slope failure or landsliding. The 

concepts of hazard and risk assessment and decisionmaking 

as applied to landslide management have been described (for 

example, Wu and others 1996), and these concepts can also be 

applied to coastal cliffs.

Analytical models that predict slope instability (for example, limit equilibrium models) typically use a performance 

function, that is, a factor of safety. Factor of safety is the ratio 

of the maximum shearing resistance (strength) along a potential 

failure surface to the downslope component of force associated 

with the mass of the rock or soil body above the potential failure 

surface. When the factor of safety is equal to 1, imminent slope 

failure is implied, and values progressively greater than 1 indicate an increasing margin of safety against failure.

Landslide models that use principles of static equilibrium 

have as their input data carefully considered parameters that 

are likely to operate in a particular slope. Deterministic slopestability analysis uses a single set of slope parameters based on 

best estimates of material strength, ground-water level, planes 

of weakness, and other factors, and these result in a single factor of safety. Alternatively, a probability distribution of the factor of safety can be derived using the probability distributions 

of the input parameters. The probability distribution of safetyfactor values then can be used to calculate the failure probability. This approach is referred to as probabilistic slope-stability 

analysis, and it has been broadly applied to coastal slopes (Edil 

and Schultz, 1983; Bosscher and others, 1988; Chapman and 

others, 2002).

Because of the diffi culties of defi ning the probability distribution of the controlling factors, explicit use of the probabilistic 

approach has been limited. Available applications typically incorporate the spatial randomness of controlling factors and not the 

temporal randomness of triggering mechanisms. Therefore, these 

analyses defi ne the current probability of failure and do not incorporate critical environmental and climatic changes that might 

take place in the future.

Analytical models of slope stability, whether used in a deterministic or a probabilistic analysis, have some typical features. 

The most widely used models have been developed for landslides and use the limit-equilibrium approach, which is based on 

an evaluation of static equilibrium of forces or moments operating on a potential sliding mass, such as gravity and the resistive 

forces along the rupture surface, yielding a factor of safety. The 

analytical models are most commonly two-dimensional, that is, 

they evaluate the equilibrium of a slice of material along a vertical profi le aligned normal to the slope. Thus, lateral forces are 

not considered. In most cases, the analysis of a unit length of 

the slope provides a reasonably accurate value of the factor of 

safety. As the geometry varies along the shoreline, a number of 
27

representative shore-perpendicular profi les are considered for 

analysis in search of the one that is most likely to initiate the sliding. There are also three-dimensional analysis methods for slope 

stability , such as CLARA-W (Hungr and others, 1989), and 

their use is warranted where the assumption of uniformity in the 

transverse or alongshore direction cannot be made.

One of the challenging aspects of conducting a slope-stability analysis, whether deterministic or probabilistic, is that 

the shape and location of a rupture surface and the most likely 

mode of failure are not known a priori. Typically, several modes 

of failure and locations of the failure surface are modeled until 

a minimum safety factor and (or) a signifi cantly large failure 

mass are identifi ed (Edil and Haas, 1980). On slopes of unconsolidated material, such as till, lake, or marine sediment, where 

slopes typically are much less steep than rock slopes, the most 

common failure modes are translational and rotational slides. 

Translational sliding takes place along shallow (compared to 

length) planes parallel to the slope face (“infi nite slope slide”), 

or it could be along a plane that is not parallel to the slope face 

(“block slide”) or along multiple planes involving more than one 

block (“wedge slide”) as shown in fi gure 36.

Translational slides are analyzed using limit equilibrium 

between the driving gravity force and the shear resistance force 

along a potential failure surface. Infi nite slope slides occur in 

granular clean materials like sand and gravel (fi g. 37). They 

also occur in fi ne-grained materials such as silt and clay, especially where there is seepage parallel to the slope face or 

emerging from the slope and where cohesion is suffi ciently low 

as a result, for instance, of weathering. Translational slides of 

infi nite-slope type are analyzed using limit equilibrium along a 

potential failure plane parallel to the slope face. The resulting 

equation for safety factor is a simple equation that can be solved 

by hand calculation or on a spreadsheet and therefore can be 

used with ease. The depth to failure plane can be established 

by trial and error as corresponding to the lowest (most critical) 

safety factor.

	

	













	
	

	












		


	
	 
	

 





	


Figure 36. Various idealized failure modes for analyzing landslides.

Figure 37. Failing bluff on the south shore of Lake Superior in Michigan, showing shallow slide of tree-and-root mat down sand slope that 

is at the angle of repose.

Block slides are typical in rock slopes and in situations 

where there is a plane of weakness such as a joint or interface 

with a weak material. Wedge slides are similar to block slides, 

except that the sliding surface may consist of several planes, resulting in motion of several blocks that interact with each other. 

Because the sliding surfaces can be anticipated on the basis of 

geological investigation without requiring signifi cant trial-anderror effort to locate them, block slides and wedge slides can 

also be analyzed using hand calculations or spreadsheets of 

equations based on force equilibrium along the potential failure 

surface or surfaces. A trial-and-error method can be used in 

those cases where the failure surface cannot be fully anticipated 

on the basis of site information to determine the sliding planes 

that give the lowest (most critical) safety factor.

Rotational slides, or slumps, have a curved rupture surface 

(fi g. 38). Slumping masses have a center of rotation above the 

Figure 38. Slump on west shore of Lake Michigan, showing characteristic backward rotation of surface of block (above and to right of 

people).

 Processes that Govern the Formation and Evolution of Coastal Cliffs
28 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

slide mass. Rotational slides are analyzed using limit equilibrium by dividing the slide mass into vertical slices normal to 

the direction of sliding. This is known as the method of slices. 

This method allows incorporation of variations in slope geometry, properties of materials at the base of slices, and pore-water pressures along the sliding surface. A widely used method 

of analysis for circular rupture failures is given by Bishop 

(1955). There are computer codes that allow application of 

these methods of rotational slide analysis to numerous random 

trial surfaces in a systematic manner until the critical sliding 

surface is identifi ed. Such programs also provide block and 

wedge analysis options.

Earthquakes

The west coast of the United States is “earthquake country,” and seismically induced failure of coastal cliffs has been 

documented for several earthquakes, particularly in California: 

the 1865 earthquake in the Santa Cruz Mountains (Plant and 

Griggs, 1990), the 1906 and 1957 San Francisco earthquakes 

(Youd and Hoose, 1978; Bonilla, 1959; Lawson, 1908), the 

1989 Loma Prieta earthquake (Plant and Griggs, 1990; Sitar, 

1990), the 1992 Petrolia earthquakes (Ashford and Sitar, 1994), 

and the 1994 Northridge earthquake (Ashford and Sitar, 1994).

Griggs and Scholar (1997) compiled records of historical 

earthquakes and seismically induced coastal-cliff failure along 

the U.S. west coast and stressed that this entire coastline is 

tectonically active and should be considered subject to strong 

seismic shaking. They point out that seismically induced 

coastal-cliff failure took place as far as 22 km from the epicenter of the 1994 Northridge earthquake, as far as 80 km from 

the epicenter of the 1989 Loma Prieta quake, as far as 150 km 

from the epicenter of the 1906 San Francisco event, and as 

far as 200 km from the great 1964 Alaska earthquake. These 

failures suggest that future earthquakes pose major risks for 

development on the cliff top, as well as for private and public 

development and beach use at the base of the cliffs.

Plant and Griggs (1990) studied the collapse of coastal 

cliffs in northern Monterey Bay after the Loma Prieta earthquake (fi g. 39). Most coastal-cliff failures occurred along 

promontories and in jointed rock. Not only were some houses 

along the cliff edge damaged by landslides, but some near the 

base of the cliff were struck by falling debris. Notably, the 

earthquake occurred at the end of the dry season after two 

years of drought conditions; damage might have been much 

greater during a wet period (Sitar, 1990).

Nicolas Sitar and his colleagues have conducted the most 

in-depth studies of earthquake effects on coastal cliffs, including fi eld observation, laboratory testing, and numerical modeling (Sitar and Clough, 1983; Sitar, 1990, 1991; Ashford and 

Sitar, 1994, 1997; and Ashford and others, 1997). These studies 

focus on steep, weakly lithifi ed coastal cliffs that are common 

in California, but the results apply to steep slopes in general. 

Laboratory tests show that these materials weaken as the number of loading cycles increases (related to the duration of the 

earthquake) by 10 to 15 percent compared to the static strength, 

implying a greater chance of failure during long-duration 

earthquakes. Observations and models indicate brittle material 

behavior, so sudden block falls and slab-type slides can be expected. There is little sign of incremental permanent deformation. The steepest coastal cliffs fail by tension in the upper parts 

of the slope, followed by toppling and perhaps accompanied by 

the formation of tension cracks behind the cliff edge (fi g. 40). 

Failure of moderate slopes (<70°) is likely to be in the form of 

translational slides subparallel to the slope surface. Slides on 

both steep and moderate slopes generally are shallow (2-5 m) 

and tend to originate near the cliff edge. The cliff material can 

either disintegrate or separate into intact blocks.

Seismic accelerations at the crest of a coastal cliff can be 

affected by cliff topography (height, inclination), but they are 

most strongly affected by the natural frequency of the site relaFigure 39. Shallow bluff failures that originated near the top of a 

moderately sloped sea cliff in northern Monterey Bay following the 

1989, magnitude 6.9 Loma Prieta earthquake.

Figure 40. A shallow cliff failure on the upper part of a steep sea cliff 

in the San Francisco area of California, caused by seismic shaking 

during the 1989 Loma Prieta earthquake.
29

tive to the predominant seismic frequency. A rule of thumb: 

adding 50 percent to the free-fi eld motion gives a reasonable 

estimate of acceleration at the cliff edge. The response of a 

coastal cliff to seismic accelerations depends on many factors, including the trend of the cliff relative to the approach 

direction of the seismic waves and discontinuities in the trend 

(for example, at points of land), in addition to the other factors already mentioned. Consequently, landslides tend to be 

spatially concentrated. Sitar and Clough (1983) concluded that 

slope angle, maximum seismic acceleration, and the ratio of 

the natural period of the deposit to that of the earthquake shaking are the most important parameters that control stresses and 

accelerations in steep coastal cliffs. Ashford and Sitar (1994) 

outline an engineering approach for evaluating seismic stability of steep, weakly consolidated coastal cliffs.

Ocean waves also can impart seismic energy to coastal 

cliffs. Preliminary results of seismic monitoring of a coastal 

cliff by Adams and Anderson (2000) indicate that the shaking 

intensity of wave impact is related to tides, deep-water wave 

height, and approach direction, with no single variable always 

dominating. The importance of wave-related shaking to coastal-cliff stability are unknown. 

Presentation and Assessment 

of Geologic Information for 

Management and Engineering 

Applications

Coastal managers and engineers who address issues of 

coastal-cliff stability require site- or region-specifi c geologic 

information in order to properly evaluate or mitigate risks for 

a particular project. In this context, there is no standard protocol for presenting the relevant information or assessing the 

degree of risk of coastal-cliff erosion hazards. The available 

geologic information often is qualitative, leading to subjective 

assessments of hazard potential. However, as more quantitative measurements of physical properties or rates of change are 

employed, geologic analysis merges with deterministic engineering analysis.

Strip maps are a common way of portraying coastal geology, landforms, and other relevant features. Symbolic representations of categories or magnitudes of physical attributes 

can be plotted as continuous strips parallel to the coastline 

(fi gs. 41-43). Several attributes can be presented in adjacent 

strips. Examples of attributes are beach width, shore morphology, wave energy, cliff material, and cliff height and slope 

(see, for example, USACE, 1971; Habel and Armstrong, 1978; 

Quigley and Zeman, 1980; Griggs and Savoy, 1985; Flick, 

1994; SEWRP, 1997). Accessory information, such as the location of engineering structures (seawalls, for example) or spot 

values of cliff retreat rate, might also be displayed. The hazard 

potential based on the mapped attributes typically is assessed 

subjectively, and hierarchical categories are defi ned and plotted (for example, high, medium, or low hazard potential).

Lee (1997) presented another approach to coastal mapping based on the concept of the “cliff behavioral unit” (CBU), 

which uses commonality of landforms along a given stretch 

of coast, from the nearshore to the cliff top, as an indicator of 

uniform geological and environmental conditions. A particular 

CBU is presumed to behave similarly throughout its extent, 

and the relative signifi cance of various geologic processes 

that determine cliff evolution will differ between units. Once 

these units are mapped for a length of coast, their geologic 

and geotechnical characteristics, as well as an evaluation of 

the hazard potential, are presented in tabular form in order to 

guide engineering and management actions or decisions. The 

Figure 41. A strip map for a portion of the California coast that 

combines shoreline environment with a hazard ranking (from Griggs 

and Savoy, 1985). These maps also depict type of shoreline armor if 

present, and include coastal erosion rates, if known, which are very 

useful to planning and regulatory agencies.

 Processes that Govern the Formation and Evolution of Coastal Cliffs
30 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

method is effi cient for covering large areas if the CBUs can 

be determined from aerial photographs and geologic maps.

A technique of hazard zonation presented by Grainger 

and Kalaugher (1988) ranks the landslide hazard at both the 

top and base of a cliff and displays the rankings of contiguous cliff sections on coastal maps. The cliff-top ranking is 

obtained from an estimate of probable retreat distance over 

the next 5-10 years and also the next 100 years, based on the 

historical record and on subjective geological evaluation of the 

current state of the cliff top. The cliff-base ranking is based on 

the yearly likelihood of damaging rock falls or slumps. The 

evaluations are made using oblique aerial photographs supplemented by site investigations.

Hutchinson and others (1998), using CBUs as the unit 

of analysis, devised probabilistic methods to predict cliff 

failure and retreat rates based on event-tree and Monte Carlo 

techniques, respectively. An event tree uses estimated probabilities, over a particular time period, of the possible initiating events of cliff failure (for example, threshold antecedent 

rainfall, large storm waves) and the conditional probabilities 

of consequent system response (for example, shallow slide 

versus large rotational failure) and outcomes (for example, 

damage to coastal homes or a seawall) to derive the probability 

of a consequence (for example, renewed cliff erosion due to 

seawall failure). They suggest that setback zoning can be based 

on their Monte Carlo method, using historical cliff retreat data 

to estimate the future retreat distance that is virtually certain to 

occur over a given time interval, the retreat distance that has a 

50 percent probability over that same interval, and the distance 

that has a 10 percent probability. The most conservative setback would be the distance based on the 10-percent probability. Other, more recent developments of probablistic methods of 

forecasting coastal cliff retreat are presented by Lee and others 

(2001) and Hall and others (2002). 

In a comprehensive investigation of coastal hazards policies and practices along the coastline of California (Griggs 

and others, 1992), local government agencies were queried 

regarding how they determined setback distances for coastal 

cliff development. Of the fi fty-six city and county government 

planning departments contacted, 13 percent rely on erosionFigure 42. A hazard strip map from Assessment and Atlas of Shoreline Erosion along the California Coast (Habel and Armstrong, 1978) that 

uses six different categories for shoreline condition. In this example, by using the U.S. Geological Survey topographic base maps reproduced at ~half scale, all of the details of topography and development can be included, making them easy to use.
31

rate information to establish setbacks and also specify a lifespan of the structures; 41 percent use a baseline-stability-age 

formula to determine structure setback, a method that defi nes 

the appropriate siting distance from the cliff edge such that the 

structure will remain standing for a specifi c time period; and 

46 percent prescribe a standard or fi xed setback distance, ranging from 10 to 320 feet.

Moving toward a system based more on material properties, Benumof and Griggs (1999) developed a semiquantitative 

stability rating system for coastal cliffs in San Diego County, 

California (table 1). The rating system’s components, which 

infl uence the rock mass strength (the strength of large volumes 

of rock that might contain fractures, stratifi cation, or other 

discontinuities) include intact rock strength (spot values determined with a Schmidt hammer, which measures strength as a 

function of the rebound distance of a steel hammer after it collides against a rock surface), ground-water outfl ow, degree of 

weathering, and joint properties (spacing, orientation, width, 

and continuity). Numerical rating values were subjectively 

estimated for some components and quantitatively measured 

for others. The sum of the component ratings, termed the total 

stability rating, was used as a proxy of rock mass strength for 

several coastal cliffs in San Diego County. High-resolution 

measurements of retreat rates, derived from a 62-year time 

series of aerial photographs, correlated most strongly (r

2

=0.76) 

with the values for intact rock strength among the individual 

components, and only slightly more strongly (r

2

=0.81) with the 

total stability rating (fi g. 44). Wave power, calculated at both 

the 10-meter water depth and the plunge point, showed a weak 

but signifi cant inverse correlation with cliff retreat rate (Benumof and others, 2000). On the basis of this study and ones in 

other areas, Benumof and Griggs (1999) speculated that material properties, as summarized by the stability rating, are the 

primary control on coastal-cliff erosion. They speculate that 

components other than intact strength might correlate strongest with retreat rate in other areas.

In a study of sea cliffs in southern Italy, Budetta and others (2000) also found a strong correlation between cliff retreat 

rate and material strength, in this case a rock-mass compressive 

strength that takes into account the strength reduction due to the 

presence of joints. They emphasize that the correlation only has 

local signifi cance, based on a particular set of material properties and wave conditions. Application to other areas would require the measurement of local strengths and retreat rates.

Mickelson and others (1991) used a combination of geological, geotechnical, and probabilistic methods to assess the 

failure potential of bluffs along Wisconsin’s southern Lake 

Michigan shoreline and particularly to identify sections susceptible to large rotational failures. Geologic mapping and 

sampling led to subjective judgement of failure potential at 

104 sites along 50 km of shoreline. A subsequent geotechnical slope-stability analysis used single values of slope height 

and declivity plus a range of Monte Carlo-generated values 

of the position of geologic contacts, geotechnical properties, 

and ground-water elevations. The values were randomly chosen from within the range of measured values over a reach of 

shoreline, whose extent was determined on the basis of similar 

geology. Repeated calculations for 2,000 possible combinations yielded factor of safety values for 100 potential failure 

surfaces at each site. The percentage of values less than 1.0 

classifi ed the reach as either stable, marginally stable, or unstable. Engineered stabilization measures were suggested for 

each marginally stable or unstable reach.

The above examples, though not exhaustive, span a 

range of approaches from strongly geological (strip maps) to 

strongly geotechnical (deterministic slope-stability analysis). 

They evaluate either the failure (landslide) potential or the 

controls on retreat rate, both of which have coastal engineering and management applications. The slope-stability analyses 

that underlie deterministic estimation of failure potential are 

a traditional element of geotechnical engineering. Gravitational failure of unstable slopes is often the primary cause of 

the landward advance of the cliff top, and the resulting slides, 

falls, and fl ows are the strongest threat to lives, stuctures, and 

infrastructure at the cliff base. However, slope-stability analyses do not directly address temporal aspects of cliff retreat on 

Figure 43. A combined shoreline-risk-assessment and shorelineprotection strip map along a portion of the California coast (Flick, 

1994). This map used U.S. Geological Survey topographic sheets 

as the base, reproduced at full (1 in = 2,000 ft) scale, providing great 

detail. The risk conditions on the original maps were reproduced in 

color, which makes recognition of various risk conditions even easier.

 Processes that Govern the Formation and Evolution of Coastal Cliffs
32 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

which setback regulations are based. Retreat rates are more 

valuable for this application. Although the controls on past 

retreat rates are becoming better understood as more research 

on coastal cliffs is carried out, extrapolating to the future is 

another matter. This was, nevertheless, the objective for the 

nationwide FEMA study recently completed (Crowell and 

Leatherman, 1998), in which 60-year erosion hazard zones 

were delineated on the basis of historical shoreline erosion 

rates.

To illustrate the diffi culty of forecasting the amount of 

cliff retreat over some time interval, Kuhn and Shepard (1984) 

noted that sea-cliff erosion in southern California was minor 

during the three decades from 1940 to 1970, when North 

Pacifi c storms were of relatively low intensity. This was also 

a period when much of southern California’s coastal development took place. After that time, retreat rate increased in 

response to a resumption of stronger storms and more frequent 

El Niño events, and damage consequently increased. Setback 

distances established from conditions during the low-stormintensity period of 1940-70 would be inadequate for the later 

period, which has proven to be the case. A similar development/storm frequency/coastal damage history has been recognized for the central coast of California (Storlazzi and Griggs, 

2000; Griggs, 1999). Although present and past coastal-cliff 

positions can be determined precisely using modern photogrammetric techniques (Moore, 2000; Hapke, this volume), 

extrapolations into the future are problematic because of the 

inherent uncertainties in predicting several key factors: future 

Table 1. Semiquantitative stability rating system developed by Benumof and Griggs (1999) for coastal cliffs in San Diego County, 

California.

[Numbers are assigned ratings (r values) for each parameter, with the numerical total rating indicating overall stability of a sea cliff.]

Parameter Very Strong Strong Moderate Weak Very Weak Unconsolidated

Intact rock

strength

(type-N 

Schmidt 

hammer)

25+

r.20

25-20

r.18

20-15

r.14

15-10

r.10

10-0

r.5

Weathering Unweathered

r.10

Slightly

r.9

Moderately

r.7

Highly

r.5

Completely

r.3

Joint

Spacing 

>3 m

r.30

3-1 m

r.28

1-0.3 m

r.21

300-50 mm

r.15

<50 mm

r.8

“infinite”

r.5.5

Joint

Orientation

Very favorable, 

steep dips into 

slope, cross 

joints interlock

r.20

Favorable, 

moderate dips 

into slope

r.18

Fair, horizontal 

dips, or nearly 

vertical (hard 

rock only)

r.14

Unfavorable, 

moderate dips 

out of slope

r.9

Very 

unfavorable, 

steep dips out 

of slope

r.5

Exteremely 

unfavorable

r.3

Joint width <0.1 mm

r.7

0.1-1 mm

r.6

1-5 mm

r.5

5-20 mm

r.4

> 20 mm

r.2

Unconsolidated

r.1

Joint continuity None. 

Continuous or 

well cemented

r.7

Few. 

Continuous 

or partially 

cemented

r.6

Continuous, no 

infill

r.5

Continuous, 

thin infill

r.4

Continuous, 

thick infill

r.1

Continuous

r.0.5

Groundwater 

outflow

None

r.6

Trace, isolated 

dripping water

r.5

Slight, wet cliff 

face with drips, 

point-source 

seeps

r.4

Moderate, 

point-source 

seeps with 

flowing water

r.3

Great

r.1

Total rating 100-91 90-71 70-51 50-26 <26
33

storm frequency, duration, and strength; the simultaneous occurrence of high tides and large waves; rates of sea-level rise; 

and the timing of El Niño events and earthquakes.

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39

Introduction

A variety of techniques for measuring the retreat of coastal 

cliffs and bluffs in the United States have been developed and 

utilized over the past century. Some of the earliest documentation of cliff or bluff recession is in the Great Lakes region, 

where fi eld survey methods were used as far back as the late 

nineteenth century (Andrews, 1870; Chamberlain, 1877, Leverett, 1899). Field surveys of the bluffs along the northeastern 

U.S. Cape Cod coast were also conducted in the late 1800’s by 

Marindin (1889). More traditional methods of measuring recession, such as fi eld surveying, profi ling, and standard aerial 

photographic techniques, are slowly being supplemented, and in 

some cases replaced, with state-of-the-art approaches, such as 

digital photogrammetry and light detection and ranging (lidar), 

as these newer technologies become more readily available and 

affordable.

Commonly, the techniques applied to the measurement of 

coastal cliff and bluff retreat have come from techniques developed to measure shoreline change along low-relief and linear 

coasts, where erosion or accretion is documented by measuring 

the change in the horizontal position of a line on the beach, such 

as the wet/dry line (Dolan and others, 1980; Anders and Byrnes, 

1991). However, along rocky or bluffed coasts, the coastline 

proxy is more adequately defi ned by the geomorphology of a 

particular area rather than a linear datum. In regions of elevated 

marine terraces, the recession of the top edge of the cliff may 

best describe trends in shoreline change. Along very steep 

coastal slopes, the feature that best captures coastline change 

may be the base of the slope, the fi rst signifi cant slope break, 

or some other geomorphic feature specifi c to a particular geographic location. In addition to the diffi culties associated with 

identifying the best feature to measure, there are problems associated with delineating the chosen feature. Examples include 

vegetation obscuring the top edge of a cliff, rounding of the cliff 

edge due to weathering or overwash processes, rock or rubble 

obscuring the base of the cliff, and the lack of continuity of a 

distinct feature. Because of the complexities associated with 

identifying and measuring the desired geomorphic feature along 

cliffed or bluffed coastlines, techniques developed for shoreline 

change measurement on low-relief coasts may not be readily applicable.

In addition to measurement and identifi cation errors and 

ambiguities associated with accurately measuring long-term 

cliff or bluff erosion, there are diffi culties in interpretation of the 

data and understanding what the data mean and how it can be 

applied both for process studies and community planning. For 

instance, it is frequently cited that cliff retreat is both spatially 

and temporally episodic, but there have been very few studies 

that actually quantify this episodicity.

This paper fi rst provides a broad review of traditional 

techniques used to measure coastal cliff and bluff retreat in the 

United States and then describes some of the modern state-ofthe-art techniques currently being developed to overcome the 

limitations of earlier techniques. A discussion of the usefulness 

of the various techniques for adequately describing the evolution 

of cliffed or bluffed coastlines, as well as errors to expect from 

the various methods, is also presented. Finally, the implications 

of the spatially and temporally episodic nature of cliff and bluff 

retreat are discussed in the context of long-term erosion rate 

analyses and how these data frequently do not accurately represent coastal cliff evolution nor predict areas of future erosion 

hazard.

Field Methods

As previously mentioned, the oldest published record of 

bluff-retreat measurement was by Andrews (1870), who measured the retreat of the bluffs along the Lake Michigan coast of 

Wisconsin and Michigan by fi eld survey methods. Soon thereafter, Chamberlain (1877) and Leverett (1899) also conducted 

fi eld line-surveys of bluff erosion along the Great Lakes coastlines. In roughly the same period, Marindin (1889) measured 

bluff retreat along the Cape Cod coast by similar fi eld survey 

techniques. Buckler and Winters (1981) describe data collected 

by the U.S. Government General Land Offi ce in 1829 and 1855 

along the bluffs of the coastlines of Michigan and Wisconsin, 

although these data were never formally published. The fi eld 

surveys mentioned above utilized the standard technique of 

surveying lines from a fi xed position (such as a road, house, or 

tree) to the bluff edge (fi g. 1). In this method, lines are surveyed 

either perpendicular to the cliff edge, or sited along the edge of 

a structure to the cliff edge (fi g. 1). More recently, Miller and 

Aubrey (1981) in Cape Cod, and Buckler and Winters (1981) 

along Lake Michigan, to name a few, surveyed the top cliff edge 

as measured from a fi xed position. Miller and Aubrey (1981) 

extended their transects to include profi les of the cliff face. 

The Measurement and Interpretation of Coastal Cliff 

and Bluff Retreat

By Cheryl J. Hapke

 U.S. Geological Survey Professional Paper 1693
40 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Vaughn (1932) used a variation of the line-survey technique 

to acquire repeated measurements of the cliff base in southern 

California. The measurements were made from fi xed points on 

the shore platform. Another fi eld technique for measuring cliff 

base recession employed in southern California by Lee and others (1976) involved pounding nails in a horizontal position into 

the vertical cliff face, and returning periodically to measure recession based on exposure of the nail. Nail or stake techniques 

such as these are clearly designed to measure shorter-term surfi cial erosion rates as a larger mass movement would remove all 

the markers in one occurrence. One of the more innovative fi eld 

techniques of measuring cliff recession rates was developed by 

Emery (1941) who measured the depth of inscribed graffi ti on 

the face of coastal cliffs in southern California. Emery documented that the lifespan of a 3-mm-deep inscription varied from 

6 to 11 years, and thus he was able to establish a rate of surfi cial 

retreat of the cliffs.

Field survey techniques of measuring coastal cliff or bluff 

retreat are generally quite precise, and have become more so as 

surveying techniques and data have improved. Today, line surveys can be conducted using global positioning system (GPS) 

data, which can be as precise as a millimeter or two. While fi eld 

surveying may provide the most accurate data on coastal cliff or 

bluff retreat, it is limited by the time and expense of mobilizing 

a fi eld crew, and by spatial limitations of the data. Even if a series of profi les or survey lines are measured throughout an area, 

it is very diffi cult to get high spatial coverage over a long section 

of coast. In most cases, surveys are repeated for a season or two 

but it is rare that such data collection lasts beyond several years. 

Thus, the data are usually temporally limited as well.

Historical Maps

Historical maps have been used in several studies to 

measure long-term recession rates of coastal cliffs or bluffs 

(Gelinas and Quigley, 1973; U.S. Army Corp of Engineers, 

1985). In many cases, the oldest paper maps available of 

the coast are National Oceanographic Service topographic 

sheets (t-sheets) that are land-surveyed maps of the coastal 

zone and have a surveyed shoreline, as well as other topographic data. The oldest published t-sheets are from the 

mid-1800’s, and coverage exists for most of the coastal 

United States from the 1930’s to the 1970’s. The U.S. Army 

Corp of Engineers (1985) measured cliff erosion based on 

1852 t-sheets and 1982 aerial photography of the southern 

California coast. As a result of diffi culties identifying and 

defi ning the cliff base, the erosion estimates have an accuracy +12 m. Much of this error comes from the lack of detailed information as to what exactly was surveyed in terms 

of the top edge and base of coastal cliffs and bluffs on the 

historical maps (fi g. 2). Hannan (1975) used U.S. Geological Survey (USGS) topographic maps from 1912 and 1966 

along with fi eld measurement to obtain cliff retreat measurements of 5.8 to 16.4 m for the cliffs in La Jolla in southern 

California. Much of this range of measurements, however, 

falls within the error typically associated with USGS topographic maps (approximately 12 m), and as a result these 

data do not record any actual change. Although USGS topographic maps contain elevation information in the form of 

contour lines, a contour line rarely represents the top edge 

or base of a cliff and therefore they are only appropriate for 

measuring changes greater than the contour interval of the 

map. Griggs and Savoy (1985) used both historical maps 

and uncorrected aerial photographs to obtain erosion rates 

1

2

3 4

5

Figure 1. A series of transect lines (1-5) are typical of a ground 

survey of coastal cliff or bluff retreat. In general, lines are measured 

from a permanent object and are either measured perpendicular to 

the cliff edge (lines 2, 3, and 4) or are sited along a straight edge such 

as the side of a building (lines 1 and 5). Measurements are repeated 

in a time series to obtain rates for cliff or bluff retreat.

Surveyed 

shoreline

Base of cliff (?)

Top edge

of cliff (?)

Figure 2.—A National Oceanographic Survey topographic sheet (tsheet) that was surveyed from 1853-1874 (1:10,000 scale) shows a portion of the inner Monterey Bay, California. The cliffs here are about 

30 m high. The high water line was surveyed (dotted line), but there is 

uncertainty as to whether the top or bottom of the cliffs were actually 

surveyed or simply sketched in by the surveying team.
41

for much of the California coast; in their study, the same 

methods were used to determine erosion rates for both linear 

beaches and cliff edges. Cottonaro (1975) used an innovative approach by obtaining the original survey of a plot of 

land above coastal cliffs near Santa Barbara, California, 

that included the cliff edge as a property boundary. He resurveyed the cliff edge and was able to use these data along 

with the surveyors map to measure cliff retreat. This method provides accurate measurements but is limited spatially 

as a function of the original surveys. Overall, historical 

maps (t-sheets and older topographic maps) provide the oldest data available for coastal change measurements; they are 

readily available, cover large stretches of coastline and are 

inexpensive. However, it is cautioned that the errors associated with the use of historical maps may be quite high and 

include 8 to 15 m positional errors (Shalowitz, 1964; Ellis, 

1978) in addition to errors associated with the determination 

of the actual cliff edge, which may result in an additional 12 

m error (U.S. Army Corp of Engineers, 1985).

Ground and Uncorrected Aerial 

Photography

Shepard and Grant (1947) estimated coastal cliff retreat 

in southern California by using ground photographs repeatedly taken from the same location and comparing these with 

historical photographs obtained from local residents and 

various other sources. This technique allowed for identifi cation of large-scale changes such as arch or sea cave collapse, 

but did not provide precise measurements of coastal cliff 

erosion rates. Emery and Kuhn (1982) refi ned the ground 

photograph method using data originally collected by Emery 

in the 1940’s. They revisited the same locations and were 

able to measure cliff face retreat rates at La Jolla, California 

of 0.03 to 33 cm/yr, although they did not provide any error 

estimates. 

Aerial photographs provide the best complete record of 

coastal change available to researchers. The earliest aerial 

photographs are from the 1920’s and photographs are available from a wide variety of sources including Federal agencies (for example, USGS, U.S. National Archives) as well as 

state and local governments. Numerous researchers have used 

aerial photography to measure the change in cliff edge position using uncorrected (U.S. Army Corp of Engineers, 1946; 

Gelinas and Quigley, 1973; Bokuniewicz and Tanski, 1980; 

Buckler, 1981; Griggs and Savoy, 1985; Griggs and Johnson, 

1979; Guy, 1999), partially corrected (Thorton and others, 

1987; Griggs, 1994) and fully corrected (Moore and others, 

1999; Priest, 1999; Hapke and Richmond, 2002) aerial photography to derive their measurements of coastal cliff or bluff 

retreat. Although aerial photographs provide a long record of 

coastal change, the photographs themselves have inherent distortions and displacements (table 1), which, if not corrected 

for, can introduce signifi cant error into any measurements 

made from the photography.

Characteristics of Aerial 

Photography

Distortions and displacements in aerial photography stem 

from internal parameters related to the camera system and 

from parameters external to the camera system, including the 

position of the camera (and hence the aircraft) and the relief 

of the terrain being imaged. The internal parameters are those 

that relate the geometry of the photograph (image space) to 

the geometry of the camera system. The geometries of the 

camera and photograph are related by the fi ducial marks on 

the photograph, the calibrated focal length, and the distortion 

characteristics of the camera lens. The distortions resulting 

from the camera system are caused primarily by lens distortion and fi lm deformation. All camera lenses have distortions 

and optical defects that affect the representation of objects on 

fi lm. Lenses typically used today for aerial photography have 

as much as 0.110 mm radial distortion (Slama, 1980). Greater 

amounts of lens distortion are more common in historical 

photographs taken prior to World War II, at which time much 

effort was put into the collection of accurate photography and 

improved lens quality. This is a particularly important source 

of error to consider when using historical aerial photography 

(see table 1). Displacements caused by radial distortion are 

smallest in the center of the photographs, and thus making 

measurements in the center of the photograph will reduce the 

error due to radial distortion.

Film deformation can occur during data collection or 

during subsequent processing. During an aerial survey, fi lm 

buckling can occur as a result of irregularities in temperature, 

humidity, or fi lm spool tension in the camera (Slama, 1980). 

Additional fi lm deformation can be introduced during development of the original negatives as well as each time prints and 

diapositives (transparencies) are made from the original negatives. The end result of these deformations is a photograph that 

is no longer accurate with respect to the actual geometric relationship between the fi ducial marks and image points in the 

photo. Additional distortions to the fi lm depend upon the age 

and type of material (glass, fi lm, or paper). Standard diapositive fi lm is generally stable within 0.005 mm (Slama, 1980). 

Photographic paper prints (contact prints), however, are far 

Table 1. Potential error sources in uncorrected aerial photography, 

1:12,000 scale.

 The Measurement and Interpretation of Coastal Cliff and Bluff Retreat

Error Source Error at photo

scale (mm)

Error at ground

scale (m)

Radial distortion

 modern photography

 historical photography

0.110

0.4

1.3

4.8

Film deformation

 diapositive film

 contact prints

0.005

1-2

0.06

12-24

Tilt displacement

(1o tilt, 10 cm from photo center) 0.68 8.2

Relief displacement

(30-m-high cliff, 4 cm from photocenter) 0.66 7.9
42 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

less stable and can change in size from 1 to 3 percent during processing alone (Slama, 1980; Wolf and Dewitt, 2000). 

Thieler and Danforth (1994) found 1-2 mm of shrinkage or 

expansion in contact prints due to age and paper quality. For 

aerial photographs at a scale of 1:12,000, this can result in errors of 12 to 24 m (table 1).

The parameters external to the camera system that can 

cause points on fi lm to be displaced from their true position 

are primarily related to tilt displacement and relief displacement. Atmospheric displacement can also occur, although it 

is only of concern in high-altitude aerial photography which is 

not commonly used for measuring coastal cliff erosion. 

Tilt displacement occurs as a result of changes in the attitude of the aerial camera during the collection of photography. 

The aircraft carrying the camera can easily deviate from being 

exactly level; the result is a difference in scale across the photograph (see Leatherman, 1983; Moore, 2000). Some degree 

of tilt is always present in an aerial photograph, and can produce signifi cant errors (10 to 20 m), even with a tilt as small as 

1 to 2o

 (table 1) (Anders and Byrnes, 1991).

Relief displacement is caused by changes in ground elevation or objects (such as buildings) within a photo such that objects that are closer to the camera are larger (at a larger scale) 

than those farther away. The result is a shift in the position of 

an object relative to the elevation of the object above a datum 

(for example, mean sea level). Relief distortion is a function 

of the height of an object, the distance of the object from the 

center of the photograph, and the fl ying height (and thus scale) 

of the photography (Slama, 1980; Wolf and Dewitt, 2000). For 

example, the relief displacement of the edge of a 30-m-high 

cliff located 4 cm (in image space) from the center of a photograph on a 1:12,000 scale photograph is 7.9 m (table 1). Although relief displacement may be negligible along low relief 

coasts, it can be a signifi cant source of error when measuring 

change along coastal cliffs and bluffs, and must be accounted 

for either by elimination with photogrammmetric processing 

or incorporation in an error analysis.

Rectified Aerial Photography

The science of modern photogrammetry has been used 

for years to remove the inherent distortions and displacements 

in aerial photographs in order to make accurate measurements 

from the corrected photographs. The fi rst airplane fl ight to 

collect aerial photographs for mapping was in 1913 (Wolf and 

Dewitt, 2000). The use of photogrammetry to produce accurate maps escalated during World War II, and advancements 

in instrumentation and technologies have continued at a rapid 

pace ever since.

Although full orthorectifi cation processing of aerial photography is required to remove all distortions and displacements from aerial photography, the time and cost of such processing can overwhelm a project. Numerous researchers have 

instead developed methods to partially rectify photography in 

coastal cliff and bluff erosion studies, which lessens the errors associated with the photography but does not completely 

remove them.

Leatherman (1983) developed a single-frame resection 

technique called “metric mapping” that removes radial distortion and tilt displacement from aerial photography but does 

not account for relief displacement. Although metric mapping 

was applied primarily to low-relief coasts, extensive mapping 

of coastal bluffs in Massachusetts was conducted using this 

method (Leatherman, 1983). Griggs and Johnson (1979), 

Thorton and others (1987), and Griggs (1994) used comparators to assess coastal cliff retreat in central California. Thorton 

and others (1987) employed a stereocomparator to minimize 

tilt and relief displacement on aerial photographs of the cliffs 

in Monterey Bay. This method incorporates surveyed ground 

control points and stereo visualization to accurately adjust 

for scale differences within a single photograph in the area of 

interest on the photograph. The result is accurate but requires 

a large number of ground control points per stereo pair, thus 

requiring a substantial amount of fi eld work. Griggs and 

Johnson (1979) and Griggs (1994) utilized a monocomparator to measure a time series of coastal cliff retreat in Santa 

Cruz. A monocomparator allows a user to measure the distance between two points on maps or photographs of different 

scales. This technique requires some fi eld data from which 

the maps or photographs are scaled, but it does not adjust the 

photographs for any relief or tilt displacement, nor does it accommodate for fi lm or radial distortions. This technique can 

be precise, although not highly accurate, for measuring coastal 

cliff retreat. It can, however, quickly provide information on 

relative change. Comparators are no longer widely used for 

photo-interpretation applications. 

Digital Stereo Photogrammetry, Lidar, 

Digital Terrain Models, 

and GIS

Over the past several decades digital photogrammetry, 

lidar and geographical information systems (GIS) techniques 

have found widespread application among coastal researchers. 

Digital photogrammetry is currently the most accurate method 

of determining coastal cliff and bluff retreat from aerial photographs, but it is also relatively expensive and time consuming. 

Lidar is increasingly being used to document coastal change 

(Sallenger and others, 1999), but has not yet been widely applied to coastal cliff and bluff erosion studies. Sallenger and 

others (2002) measured coastal cliff retreat from profi les extracted from lidar data, and correlated the cliff retreat to beach 

elevation. Like digital photogrammetry, lidar is capable of providing accurate topographic information and has an advantage 

in that data are easily collected over large portions of the coast, 

but it is still quite expensive. Both digital photogrammetry 

and lidar provide data that can be used to produce 3-dimensional digital terrain models (DTMs) of the coast. These models can then be incorporated into GIS to perform any number 

of spatial and temporal analyses.
43

One of the characteristics that currently makes digital stereo photogrammetry a good option for coastal cliff and bluff 

erosion studies is that the 3-dimensional models can be viewed 

in stereo while the cliff or bluff is being digitized (with some 

but not all software packages). Using this technique, there is 

little ambiguity as to the exact location of the edge of the cliff, 

as the topographic break can easily be seen (Hapke and Richmond, 2002). Moore and others (1999) and Priest (1999) were 

the fi rst to apply fully rectifed orthophotographs in cliff retreat 

analyses. However, in both studies, the cliff edge was digitized on 2-dimensional orthophotographs. In both cases stereo 

models were referred to when there was ambiguity as to the 

exact location of the cliff edge. Test studies by the author have 

shown that ambiguities as to where the cliff edge is on a 2-dimensional orthophotograph result in placement errors ranging 

from 2.5 to 6.3 m. In general, the most common placement 

of the cliff edge on 2-dimensional imagery is based on tonal 

contrasts between exposed cliff and vegetation. However, on 

fi eld inspection, it is common to fi nd vegetation growing over 

the edge of the cliff (fi g. 3A) or where a portion of cliff has not 

eroded in the recent past the entire cliff face may be vegetated 

(fi g. 3B). Lidar data, which form a network of XYZ points, 

also may present a problem with depicting the exact topographic break of a cliff or bluff edge. In this case, if the edge 

falls between survey points, the exact location of the cliff will 

not be known, and the resulting model of the cliff morphology 

may not refl ect the true cliff (fi g. 4). Although a closer spacing of points would reduce this error, tighter point networks 

result in increasingly large fi le sizes that are often diffi cult to 

impossible for a standard offi ce computer system to handle.

Digital photogrammetry requires that aerial photographs 

fi rst be converted to digital format, which requires a highresolution photogrammetric-quality scanner. The precision 

scanning assures that the spatial relationship of objects on the 

original fi lm is preserved in the digital conversion. Traditional 

desktop and graphic arts scanners do not offer this level of 

precision and if used for scanning aerial photographs may introduce additional nonsystematic errors to those described in 

detail in the previous section. In order to assess these potential 

errors, Hapke and others (2000) designed a study to quantify 

the error associated with using a nonphotogrammetric scanner, 

as well as using paper contact prints versus dispositive fi lm in 

a digital photogrammetric workfl ow. A grid of photo-identifi -

able points was constructed on an orthophotomosaic with an 

assessed root mean square (RMS) positional error of less than 

1 m. Images from the various combinations of media and 

scanner sources were processed in a full stereo photogrammetry workfl ow, and the photo-identifi able points were located 

and their positions were measured against the position on 

the original orthophotomosaic. The resulting offsets of these 

points are shown in table 2, which clearly shows the large error 

associated with using contact prints for data analysis. Of importance in analyses of coastal cliffs and bluffs, elevation data 

Figure 3. Photographs of vegetated coastal cliffs. A, Thick vegetation growing well over the top edge of a coastal cliff near Natural 

Bridges State Beach, California; if the cliff edge were being digitized 

in two dimensions on an aerial photograph, it would be very diffi cult 

to pick out the edge. B, Heavy vegetation covers most of this slope at 

Seacliff State Beach, California. Although adjacent areas have been 

recently active, the cliff edge on this slope would be diffi cult to isolate 

on a two-dimensional aerial photograph.

 The Measurement and Interpretation of Coastal Cliff and Bluff Retreat
44 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

(Z) derived from photographs processed using any of these 

scanner or media types had signifi cant error, which should be 

considered when mapping the 3D evolution of coastal cliffs.

Full stereo-orthorectifi cation processing requires the generation of a DTM (from either stereo aerial photography or 

lidar data) that defi nes topography within the stereo overlap 

region of images using a series of points and is required if the 

displacement from terrain relief is to be removed from the orthorectifi ed images. Two formats of DTMs may be generated 

from stereo images: grid or TIN (triangulated irregular network). The grid is a regularly spaced network of points where 

the network spacing must remain constant. Grids are not recommended when modeling areas of rapid topographic changes 

(such as coastal cliffs or bluffs), since the spacing of the grid 

points may miss the actual edge (fi g. 4). In a TIN model, 

points can be irregularly spaced. This is advantageous in areas 

where a greater density of points is desired to better defi ne the 

topography, or points can be deleted in problematic areas (for 

example, on vegetation and in water). Another advantage of 

TIN models is the ability to add breaklines, which allows for 

accurate defi nition of subtle topographic changes. A breakline 

is a manually entered line composed of a series of points that 

are incorporated into the DTM. Breaklines can only be added 

to the model while viewing in stereo, as the operator must be 

able to identify the elevation change in order to correctly place 

the line. Breaklines are crucial to accurately defi ning the topographic signal of narrow or sharp features such as cliff edges 

in the surface model. 

Three-dimensional data such as DTMs, whether derived 

from photogrammetry or lidar, can be incorporated into GIS to 

perform a variety of analyses, including measurement of cliff 

or bluff retreat rates, spatial and temporal distribution of slope 

failure, and calculating area and volume. Hapke and Richmond (2002) used digital photogrammetry and GIS to quantify 

not only the linear extents and landward retreat of coastal 

cliff failure associated with the 1989 Loma Prieta earthquake 

versus the 1997-98 El Niño, but were also able to characterize 

the types of slope failures and the geologic units involved. In 

another study utilizing photogrammetry and GIS, Hapke and 

Griggs (2002) processed historical and recent aerial photography using digital stereo photogrammetry to produce DTMs 

of areas prone to coastal landslides along the Big Sur coast in 

California. The historical DTM is subtracted from the modern 

DTM in a GIS to calculate volume losses to the littoral system 

over a 52-year period. GIS technology is also used to plot the 

spatial distribution of the volume losses and gains; this can 

provide information on slope processes and can be correlated 

to other spatial data such as lithology and geologic structures.

Interpretation of Coastal Cliff 

Erosion Data

Planners for coastal zone management frequently rely on 

long-term average erosion rate data when making decisions regarding the use and development of coastal areas. While these 

data may aid in comparing relative erosion trends in a regional 

sense along cliffed coastlines, they provide little local information on specifi c hazard zones. This is due to the fact that 

coastal cliff and bluff retreat is both spatially and temporally 

episodic, at a range of scales.

Probably the most dominant infl uences on the temporal 

distribution of coastal cliff retreat are related to weather variations, such as increased storm intensity and frequency, climate 

variations such as El Niños on the U.S. west coast, and fl uctuations of water levels due to variations in precipitation or to 

long-term sea-level rise. Spatial distributions are more closely 

tied to lithologic variations, proximity to active fault zones, 

and anthropogenic changes related to land use, irrigation, and 

construction practices.

On a short (seasonal) time scale, waves generated by 

storms will remove natural protection (for example, blocks and 

debris-fan material) from the base of a coastal cliff or bluff 

and may also temporarily remove the buffer of a sandy beach 

as well. Once the natural protection is removed, waves will 

gradually begin to erode basal notches, although only in weaker lithologies will this notching potentially be deep enough 

to result in collapse of overlying material in a single season. 

Increased pore pressures from rainfall infi ltration during a 

rainy season may also lead to cliff or bluff failure, and surface 

run-off during storms removes loose weathered material and 

Table 2. Maximum offsets of test grid points for different scanner 

Scanner/media type Max. X

offset (m)

Max. Y

offset (m)

Max. Z

offset (m)

Graphic arts/diapositive 1.75 2.00 6.10

Graphic arts/contact print 5.47 1.60 6.64

Desktop/diapositive 1.97 1.20 4.55

Desktop/contact print 10.28 9.40 8.50

Figure 4. A typical cliff or bluff surface is shown with a profi le that 

would result from an interpolated grid surface derived from photogrammetry or light detection and ranging (lidar) data. Without the 

ability to add points to defi ne the exact topographic breaks of the 

cliff edge and base (as with a triangulated irregular network, or TIN), 

the resulting surface does not accurately represent the true ground 

surface and the position of the cliff edge on the interpolated surface 

may not be properly positioned.
45

may cause rapid gullying in poorly lithifi ed materials. The 

short-term impacts of these seasonal processes are spatially 

localized, but they occur across vast time scales, and are 

thus extremely hard to predict both spatially and temporally. 

Detailed fi eld mapping such as measuring locations and 

depths of notches, conducting local tests of material strength 

variations, and mapping topography may assist in predicting 

where a particular section of cliff is likely to fail in the future, but would not enable a good temporal prediction. 

Although seasonal storms gradually lead to erosion of 

coastal cliff or bluffs, failure or retreat is accelerated when 

storm frequency and intensity increase such as during El 

Nino years and during longer climatic fl uctuations such the 

40-year cycle of quiescience with short (6-10 year) periods 

of more intense storm activity documented by Kuhn and 

Shepard (1980) along the Pacifi c coast. Several researchers 

have documented that storm intensity (Graham and Diaz, 

2001) including storm wave heights and periods (Allen and 

Komar, 2000) in the North Pacifi c have been increasing over 

the past 50 years. This could result in more energy for waves 

to erode along longer reaches of coastline, as those cliffs that 

are currently not impacted by wave energy could become 

inundated at base level by storm waves. In addition to increased storm activity, seismic shaking from earthquakes can 

lead to both instantaneous cliff retreat and accelerated retreat 

in the years immediately following an earthquake from weakening of the cliff-forming material (Plant and Griggs, 1990a 

and 1990b; Hapke and Richmond, 2002). 

Damaging El Niños (on the West Coast) and other extreme storm events (for example powerful hurricanes or 

nor’easters on the East Coast) occur on time scales of once 

every decade or two, and large earthquakes may lead to 

widespread cliff failure along cliffed coastlines in tectonically active regions perhaps once per century. Although 

many authors refer to the association of extreme events and 

episodic coastal cliff or bluff retreat (Kuhn and Shepard, 

1979; Sunamura, 1980; Griggs, 1994), there has been little 

quantifi cation and assessment of the spatial distribution of 

cliff failures during extreme events. 

Recent detailed analyses of coastal cliff erosion along 

three individual sections of cliffed coastline in the northern 

Monterey Bay, California (Hapke and Richmond, 2002), 

show that the increase in storm intensity over the course of 

one strong El Niño, or the seismic shaking associated with a 

large earthquake, can account for as much as half of the total 

average long-term retreat of coastal cliffs. In this study the 

spatial and temporal distribution of cliff retreat are quantifi ed 

for the decade immediately following the 1989 Loma Prieta 

earthquake (magnitude 6.9) and during the 1997-98 El Niño. 

The short-term, event-driven retreat of the coastal cliffs is 

compared to the long-term signal in fi gure 5 for three sections of coast within the late-Miocene to Pliocene Purisima 

Formation, a poorly indurated sandstone and siltstone unit 

forming the coastal cliffs in this portion of the Monterey Bay. 

These cliffs average about 25 m in height and are capped by 

several meters of unlithifi ed marine terrace deposits.

The total amount of long-term retreat for a 41-year time 

period from 1953 to 1994 (Moore and others, 1999) is shown 

in in fi gure 5 in light gray for the three sections of coastal 

cliffs, with the postearthquake decade and El Niño storm 

retreat superimposed on the long-term. For Seabright Beach 

(top graph), the long-term retreat is uniformly distributed 

along the cliff section, very similar to the pattern of cliff failure along that section during the short-term. In the two areas 

where the cliff did retreat in the short-term, the amount of 

retreat makes up more than half of the total long-term retreat. 

This suggests that the long-term retreat shown could have 

occurred during two large-scale events, such as the 1982-83 

El Niño and (or) the 1989 Lona Prieta earthquake. Furthermore, during a period of climatic and tectonic quiescence, 

this section of coast may be fairly stable, and the long-term 

rates misleading in terms of what to expect in the future.

The long-term versus short-term retreat for another section of coast, Depot Hill, is shown in the center plot of fi gure 

5. Along this section of coast, both the long- and short-term 

retreat occur nonuniformly; the locations where the highest 

retreat is measured in the long-term did not retreat in the 

decade following the Loma Prieta earthquake, nor did the 

cliffs retreat in these particular locations during the 1997-98 

El Niño. It appears that for this section of coast, the longterm rates are poor indicators of short-term erosion, and that 

“erosion hotspots” shift spatially through time. This shifting 

would be expected if one portion of the cliff section undergoes successive failures (making it a hotspot) but eventually 

reaches an equilibrium (or quasiequilibrium) profi le. In 

areas along the Depot Hill cliff section where retreat occurred over all time periods, the decadal and El Niño retreats 

combined make up nearly half of the long-term signal, again 

supporting the concept that the long-term retreat can be attributed to several large events. The infl uence of faulting on 

the long-term cliff retreat along Depot Hill is also explored, 

with the faults shown as small inverted triangles along the 

top of the graph. The relationship between the faults and 

locations of cliff failure is not consistent. Although a concentration of faults does occur in the area of highest retreat (610 

– 630 m), other areas of high retreat occur where there are no 

faults (~500 m).

The long-term retreat at Seacliff State Beach (bottom 

plot, fi gure 5) shows a fairly uniform distribution with no 

apparent hotspots. In the short-term, of the three study sections of cliffed coastline, Seacliff State Beach experienced 

the largest amount of cliff retreat during the 1997-98 El 

Niño. This is most likely attributed to the weakness of the 

cliff forming material in this location where numerous debris 

fl ows are initiated during high rainfall events (Hapke and 

Richmond, 2002). The one stretch of cliffs that did not fail 

during any of the time periods (620 – 790 m) is the only location where there is no development on the top of the cliffs, 

suggesting that increased runoff and/or lawn irrigation may 

be playing a signifi cant role in the retreat of the cliffs. The 

long-term record again does not seem to be a consistent indicator of where the cliff is prone to failure in the short-term.

 The Measurement and Interpretation of Coastal Cliff and Bluff Retreat
46 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends Cliff retreat (m)

0

2

4

6

8

10

12

14

Decade

El Niño

Seabright Long-term

Along-cliff distance (m)

Cliff retreat (m)

0

2

4

6

8

10

12

14

0 200 400 600 800 1000

Seacliff State Beach

Long-term

El Niño

Decade

Cliff retreat (m)

0

2

4

6

8

10

12

14

Long-term

El Niño

Decade

Depot Hill

Figure 5. Long-versus short-term cliff retreat for three study sections of coastal cliffs in Santa 

Cruz, Calif. In areas where retreat occurs over both time periods the short-term retreat makes 

up nearly half the long-term retreat amounts, suggesting that large-scale events such as 

earthquakes and El Niños are responsible for much of the long-term signal. Locations of faults 

mapped along the Depot Hill section are shown by inverted triangles along the top of the plot.
47

Figure 6A shows a schematic plot of the relationship between average long-term coastal cliff retreat and the episodic 

short-term pattern of retreat for a particular section of coastline, based on data from Hapke and Richmond (2002). Sunamura (1980) constructed a similar plot (fi g. 6B), but it shows a 

dramatically different pattern in which retreat from the shortterm curve greatly exceeds the the long-term average retreat. 

Sunamura bases each short-term retreat episode on a single, 

Figure 6. Two schematic plots show temporal cliff retreat patterns. A, Based on data derived from 

the quantifi cation of short-term cliff erosion along the central California coast, the amount of retreat 

predicted by the episodic short-term curve is consistent with the amount of retreat predicted by the 

long-term average curve. B, In contrast to the plot based on actual data shown in A, Sunamura’s (1980) 

conceptual model of long- versus short-term temporal retreat patterns shows that the long-term average 

is a poor predictor of the amounts of episodic retreat.

1 10 100

A

1

10

100

long-term average retreat (20 cm/yr)

3 m episodic retreat

Time (years)

Retreat (m)

Recession distance

0

Time τ

X

average erosion rate

short-term erosion rate

(after Sunamura, 1980)

B

 The Measurement and Interpretation of Coastal Cliff and Bluff Retreat
48 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

documented retreat event in which 12 m of bluff failed during a hurricane on Long Island, New York. This single-event 

retreat is then schematically applied to a series of episodic 

failures through time.

In contrast, a log-log plot of data derived from the quantifi cation of short-term retreat shows a striking consistency 

with the amount of retreat that is predicted by the long-term 

average. In this plot, the long-term curve is based on an average long-term retreat of 20 cm/yr (based on data from Moore 

and others, 1999), and is projected to 500 years. The shortterm retreat curve is based on data from Hapke and Richmond (2002) that shows for this portion of cliffed coastline 

in Monterey Bay, California, the episodic retreat during both 

the 1997-98 El Niño and the 1989 Loma Prieta earthquake 

is approximately 3 m per event. If the 3-m retreat is applied 

episodically every 15 years (average length of time between 

damaging El Niños), as well as once every 100 years (approximate large earthquake occurrence), the long-term prediction of 

retreat is quite good. However, in the short-term, it is diffi cult 

to know where in the stair-step pattern a particular stretch of 

coast is temporally located, and the graph also implies that the 

portion of coast repeatedly fails in the same location, which 

(as discussed above) does not seem to be consistent with shortterm data. Moore and Griggs (2002) applied a statistical approach to attempt to predict the spatial distribution of coastal 

cliff retreat by assuming that those areas that did experience 

failure over their 40-year measurement period would be the 

least likely to fail over the next 40 years. While this technique provides a useful way of determining locations where a 

particular stretch of cliff is unlikely to fail over a certain time 

period, it does little to predict the spatial distributions in the 

short-term.

Conclusions

Coastal cliff and bluff retreat continues to be a source of 

concern for land owners and community planners. Detailed 

and accurate measurements of coastal cliff and bluff erosion 

are crucial not only for planning and management purposes, 

Technique Advantages Disadvantages

Ground Surveys Very Accurate

Easily repeatable

Poor temporal and spatial coverage

Time consuming (and therefore

expensive)

Historical maps Inexpensive

Widely available

Very long temporal coverage ( 1850ʼs –

1979ʼs )

Good spatial coverage

Low accuracy

Ambiguous cliff/bluff edge position

Aerial photographs

Unrectified

Rectified

 Partially

 Fully

Inexpensive

Widely available

Good temporal coverage ( 1920ʼs –

present)

Good spatial coverage

Widely available

Good temporal coverage ( 1920ʼs –

present)

Good spatial coverage

Improved accuracy over unrectified

Widely available

Good temporal coverage ( 1920ʼs –

present)

Good spacial coverage

Very high accuracy

Cliff/bluff edge can be digitized in 3D

Low accuracy

Ambiguous cliff/bluff position in 2D

Am biguous cliff/bluff position in 2D

Hardware/software for processing m ay

be expensive

Processing time consuming

Required software expensive

Lidar Good spacial coverage

Very high accuracy

Expensive

Poor temp oral coverage

Cliff edge may not be captured in data

Table 3. Advantages and disadvantages of measurement techniques commonly used in coastal cliff and bluff 

erosion studies.
49

but also to understand the processes of slope failure and the 

factors that drive failure in any given area. A variety of techniques have been developed to calculate cliff and bluff retreat 

rates, including repeated ground surveys, determining the cliff 

edge position on historical maps and comparing this with recent aerial photography, and state-of-the-art techniques using 

digital photogrammetry, lidar, and GIS. Each technique has 

advantages and disadvantages over others (table 3). Researchers need to assess what spatial coverage and accuracy are 

required for a particular project and choose a technique that is 

appropriate. A full error analysis is essential with any quantifi cation of coastal cliff or bluff retreat, and at present this has 

been largely ignored in the published literature. A substantial 

amount of the published data on coastal cliff and bluff retreat 

present values that are more accurate than the error for a particular method. Standard methods of calculating and presenting error should be developed and utilized by the coastal cliff 

and bluff research community. 

Regardless of the method used to calculate coastal cliff 

or bluff retreat rates, the interpretation of cliff retreat data 

poses an additional challenge for researchers and planners. 

Typically, long-term average erosion rates are derived for a 

particular stretch of coastline and the erosion pattern is used to 

determine erosion hotspots and (or) hazard zones. However, 

quantifi cation of short-term cliff retreat amounts and their spatial distribution suggest that areas identifi ed as hotspots in the 

long-term record are not always good predictors of future retreat because the zones of rapid erosion shift spatially depending on the current equilibrium state of the particular section of 

cliff.

Cliff retreat amounts determined by averaging over the 

long-term appear to be consistent with total retreat that occurs episodically during extreme events. Therefore, long-term 

average erosion rates are valid for determination of how much 

the cliff will retreat and this is very useful for planning and 

management purposes. However, the long-term average retreat 

patterns do not provide information on precisely where and 

when any given section of cliff will fail in the short-term, and 

thus short-term hazard prediction using currently implemented 

techniques is diffi cult. 

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San Diego: Benton Engineering, Project No. 75-9-18F6, 

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Hapke, C.J., and Griggs, G.B., 2002, Long-term volumetric 

sediment contribution from landslides—Big Sur coastline, 

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Conference Proceedings, p. 652-663.

Hapke, C.J., and Richmond, B.M., 2002, The impact of climatic 

and seismic events on the short-term evolution of seacliffs 

based on 3-D mapping—Northern Monterey Bay, California: Marine Geology, v. 187, no. 3-4, p. 259-278.

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erosion related to unusual storms in southern California: 

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cliff erosion in San Diego County, California: in CRC 

Handbook of Coastal Processes and Erosion, p. 267-284.

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techniques: Shore and Beach, v. 51, p. 28-33.

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erosion: ASCE National Water Resources and Ocean 

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erosion hotspots along the central shores of the Monterey 

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608 p. 
 U.S. Geological Survey Professional Paper 1693 51

REGIONAL ISSUES
52 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends
53

Introduction

California is on the leading edge of a large tectonic plate 

(the North American Plate) that has been colliding with the 

Pacifi c Plate to the west for many millions of years. This collision and the subsequent plate interaction have produced 

California’s unique and dynamic landscape. Surface processes 

such as waves, rainfall and runoff, and 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 shoreline is only a temporary one. Although the 

changes are not rapid, the evidence is clear that eustatic sea 

level has been rising for the past 18,000 years and will continue to rise into the foreseeable future. This should raise serious 

concerns about our increasingly intensive development of the 

coastline, not just in California, but worldwide. For this report, 

the shoreline is defi ned as the intersection of the sea with the 

shore or beach; it migrates with changes of the tide or water 

level. Coastline will be used to denote the general boundary 

between the land and sea.

About 18,000 years ago, the climate was considerably 

cooler, and the Earth was in the waning stages of a period of 

extensive glaciation. About 45 million km3

 of seawater was 

locked up on the continents as icecaps and glaciers that covered large areas of the Earth’s surface. The removal of this 

seawater from the oceans led to a worldwide drop in sea level 

of about 130 m. The shoreline along the coast of California at 

that time was 10 to 20 km offshore to the west. As the climate 

warmed, the ice caps began to melt and the glaciers retreated. 

The melt water fl owed into the ocean and sea level rose globally at average rates of nearly a centimeter a year until about 

5,000 years ago. From that time until the present, the rate has 

slowed, although sea level has continued to rise at about 2 mm/

year for the past century.

The period of global warming and ice melting that began at the end of the last ice age, and the accompanying sea 

level rise, fl ooded the continental shelves that surround the 

continents. Along the shoreline of California this sea level 

rise advanced the shoreline 10 to 20 km landward, with waves 

eroding back the landscape and forming coastal cliffs as the 

sea advanced into higher areas. Throughout the period of accelerated sea-level rise (~18,000 to 5,000 years ago) most of 

the California shoreline was retreating landward at average 

rates of about 0.6 to 1.8 m annually. As sea-level rise slowed, 

the erosion rate declined and began to approach present rates 

of sea-cliff retreat, closer to 10 to 30 cm/year in most places in 

the state (Griggs and Savoy, 1985).

There are as many social as scientifi c issues that emerge 

from the present understanding and status of coastal cliffs in 

the United States. Although various studies (United States 

Army Corps of Engineers, 1971; Habel and Armstrong, 1978; 

Griggs and Savoy, 1985; Griggs and others, 1992) have documented the extent of the coastline of California that is undergoing erosion, practically speaking, the entire unprotected 

coastline of the United States has been eroding over the past 

18,000 years and will continue to migrate landward as long 

as sea level continues to rise. Although sandy shorelines or 

beaches may erode and then accrete seasonally, or respond 

at least temporarily to additional sand input, coastal cliffs 

or bluffs only migrate in one direction, resulting in a loss of 

coastal land. For at least the next century, the trend will most 

likely be for cliffs to continue to erode under the infl uence of 

both marine and subaerial processes.

Because of the desirability of living directly on the coast, 

which for many areas of the Pacifi c and Great Lakes coastlines, 

in particular, means living on a coastal cliff or bluff, there are 

signifi cant short- and long-term risks associated with population migration to and more intense development of these areas 

(fi g. 1). Shoreline or coastal erosion has become an increasingly publicized regional and national issue that will be signifi cant 

California’s Coastal Cliffs and Bluffs

By Gary B. Griggs and Kiki B. Patsch

Figure 1. Developed sea cliffs in Northern Monterey Bay showing 

“riprap” at base of cliff and landslides from the 1989 Loma Prieta earthquake.

 U.S. Geological Survey Professional Paper 1693
54 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

for many decades to come. Both in this country and globally, 

millions of people now live within a meter of sea level or within a few tens of meters of the edge of the coastal sea cliff. The 

elevation of sea level or the position of the shoreline will continue to fl uctuate or migrate in response to climate changes as 

it has throughout the nearly 4-billion-year history of the oceans.

We face the multiple challenges of (1) understanding the 

fundamental processes and factors that govern coastal cliff formation, erosion or failure, (2) the need to document and quantify how these rates of retreat vary spatially and temporally, 

and then (3) dealing with the large-scale social and economic 

issues of how to best deal with a constantly retreating coastline, cliffed in many places, over the 1,760-km length of the 

California coast.

Geographic Distribution of Sea Cliffs 

in California

The coastline of California is extremely diverse, from the 

steep coastal mountains along the Big Sur and Mendocino 

coasts (fi g. 2) to the broad coastal plain and wide beaches of 

Figure 2. Location map for California.

Figure 3. Steep, high cliffs south of San Francisco.

Los Angeles County. One logical breakdown is to categorize 

the coastline as consisting either of (1) steep coastal mountains 

and sea cliffs with hundreds of meters of relief (fi g. 3), (2) 

uplifted marine terraces and sea cliffs a few meters to perhaps 

100 meters in height (fi g. 4), and (3) coastal lowlands with 

beaches and sand dunes (fi g. 5). The great majority (1,267 km 

or 72 percent) of the California coast consists of actively erod-
55

Figure 4. Cliffs eroded into uplifted marine terraces in Santa 

Cruz County.

Figure 5. Coastal lowlands, Orange County.

ing sea cliffs that would include 1 and 2 above. Of this 1,267 

km, 1,038 km, or 59 percent of the entire coast, consists of 

lower relief cliffs or bluffs typically eroded into uplifted marine terraces, and 229 km, or 13 percent of the coast, consists 

of steep cliffs or mountains (fi g. 6). This report focuses on 

the cliffed portions of the coastline, whether a few meters or 

hundreds of meters in height, what we know and don’t know, 

Del Norte

Humboldt

Mendocino

Sonoma

Marin

San Francisco

San Mateo

Santa Clara

Monterey

San Luis Obispo

Santa 

BarbaraVentura

Los 

Angeles

Orange

San

Diego

Figure 6. Distribution of cliffs along California’s coast.

 California’s Coastal Cliffs and Bluffs

how these coastal cliffs and the processes acting on them have 

affected human occupancy of the coast, and how we have responded to coastal cliff erosion.

The high relief, steep cliffs or coastal mountains are located predominantly in northern California from Del Norte 

County to Mendocino County (fi g. 2), at the Marin Headlands just north of the Golden Gate (fi g. 7), in the Pacifi ca 

to Montara area of San Mateo County (fi gs. 2, 3), and along 
56 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

the Big Sur coast of Monterey (fi g. 8) and San Luis Obispo 

Counties. High relief, steep cliffed outcrops and headlands 

can be found along several areas of the southern California 

coastline as well; the Santa Monica Mountains and Point 

Loma in San Diego County are two examples. These steep, 

high relief stretches of coast typically consist of older and 

more resistant rock types such as the Franciscan Formation, 

as well as granitic and volcanic rocks. In general, these rocks 

tend to be much harder and more resistant to erosion, and it 

is these rock types that form many of the resistant headlands 

or points along the state’s coastline. For example, along the 

northern California coast, Point St. George, Trinidad Head, 

and Point Delgada are all Franciscan Formation outcrops; 

Bodega Head, Point Reyes, Montara Point, Point Pinos, and 

Point Cypress (fi g. 2) are all granite. Proceeding southward, 

Point Sur, Point San Martin, Piedras Blancas, and Point San 

Luis (fi g. 2) are all Franciscan Formation outcrops. These are 

resistant rock types that erode very slowly, but in the case of 

some units of the Franciscan, may also be subject to largescale landsliding or slumping as is common along the Big 

Sur coast (fi g. 8).

Elevated marine terraces are characteristic features of 

collision coasts where uplift is taking place, and the coastline 

of California has many excellent examples of these features. 

These terraces, which typically resemble a fl ight of stairs, are 

commonly less than a kilometer in width and may ascend to 

elevations of several hundred meters above 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 (fi g. 4). On the basis of 

modern nearshore process observations, the shoreline angle, 

or the intersection of the relict platform and the relict sea cliff, 

provides a good approximation of the location and elevation of 

the abandoned or former shoreline, and hence the position of a 

relative sea-level highstand (Lajoie, 1986). 

Over the past 25 years, a consensus has developed that a 

sequence of uplifted Pleistocene marine terraces is the geologic and geomorphic record of repeated glacio-eustatic sea-level 

highstands superimposed on a rising shoreline. Thus, a rising 

shoreline is a continuous strip chart on which relatively brief 

sea-level highstands were successively recorded as erosional or 

depositional landforms (Lajoie, 1986). While earlier studies 

of these uplifted marine terraces focused on surface morphology and the sedimentary deposits overlying these abrasional 

platforms, more recent work has concentrated on the signifi -

cance of the terrace sequences as tools to help unravel the recent tectonic history of the associated coastlines. 

Much of the coastline of San Diego, Orange, Santa Barbara, San Luis Obispo, Santa Cruz, and San Mateo Counties 

(fi g. 2) is characterized by low bluffs or cliffs cut into uplifted 

marine terraces, typically consisting of Tertiary sedimentary 

rocks (fi g. 6). In a few places such as Half Moon Bay and 

Pacifi ca in San Mateo County, terraces consist of alluvial 

deposits that aggraded during sea-level low stands. Along the 

northern California coast, portions of the Sonoma, Mendocino, Humboldt, and Del Norte County coasts (fi g. 2) are also 

eroded into marine terraces. The number of terraces exposed 

along the coast of California ranges from 1 in Santa Barbara 

to as many as 13 on the Palos Verdes Peninsula of Los Angeles 

County. The existence and distribution of these fl at, nearly 

horizontal marine terraces, adjacent to the shoreline, have allowed California’s extensive and intensive coastal development 

to take place (fi g. 9). Ease of access and construction, as well 

as stability, has facilitated development in oceanfront communities situated on these widespread terraces. Unfortunately, 

however, the relatively weak, often well-bedded sedimentary 

rocks that lend themselves to wave erosion and the formation 

of wave cut terraces are also the same materials exposed in the 

coastal cliffs and bluffs today. These materials are very sus- Figure 8. Massive landslide on the steep Big Sur coast of 

central California

Figure 7. Large-scale slump and coastal cliffs in the Franciscan Formation of the Marin Headlands, north of the Golden 

Gate.
57

ceptible to erosion by waves as well as by subaerial processes, 

and the continued breakdown and retreat of the cliffs as sealevel rises has produced a dilemma of increasing magnitude 

for California coastal communities. Most sea-cliff exposures 

eroded into marine terraces consist of an underlying sedimentary bedrock unit and an overlying sequence of unconsolidated 

and weaker marine terrace deposits (fi g. 10). 

Figure 11. The irregular, granitic sea cliffs of the Monterey Peninsula.

 California’s Coastal Cliffs and Bluffs

Figure 10. Eroding coastal bluffs in northern Monterey Bay exposing 

both mudstone bedrock at beach level and overlying sandy terrace 

deposits.

Figure 9. Intensive coastal cliff development in Solana Beach, 

located in southern San Diego County.

Sea-Cliff Erosion and Failure

Although the overall long-term statewide rates of coastal 

erosion or retreat in California are a function of the rate of sealevel rise, there are signifi cant local or regional differences in 

the rate of sea-cliff erosion. The reasons for these differences 

and the processes of sea-cliff erosion in California have been 

speculated on since the late 1940’s (Shepard and Grant, 1947; 

Krumbein, 1947). These rates and resulting cliff geomorphology vary as a function of both the resistance of the materials 

making up the cliffs to erosion (intrinsic factors; Benumof and 

Griggs, 1999) and the physical forces acting to wear away the 

cliffs (extrinsic factors; Benumof and others, 2000), as well as 

the dominance of either marine or subaerial processes (Emery 

and Kuhn, 1982). The hardness or degree of consolidation 

of the cliff rock, the presence of internal weaknesses such as 

joints or faults, and the presence of groundwater, all 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 fl uctuation, frequency of El Niño events 

or damaging storms, as well as the climate, including rainfall 

and runoff, as well as groundwater fl ow, also infl uence the rate 

and scale of sea-cliff retreat.

Sections of coast consisting of unweathered crystalline 

rock, such as the granite of the Monterey Peninsula, usually 

erode at very slow rates. At some locations on the Monterey 

Peninsula, for example, virtually no change was detected between photos taken over a 60- to 70-year span (Griggs and Savoy, 1985). Between these generally resistant areas, however, 

erosion rates can vary considerably. Waves attack the weaker 

zones over time, the fractures and joints for example, to form 

inlets and coves (fi g. 11). The more resistant rock is left behind as points, headlands, and sea stacks. 

In striking contrast, erosion can be far more rapid (as 

much as 30 cm or more per year, on average) where the bluffs 

consist of weaker sedimentary rocks, such as shale, siltstone, 
58 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

or sandstone, or unconsolidated materials, such as alluvium, 

dune sand, or marine-terrace deposits. In these areas, which 

are characteristic of much of Santa Barbara, Santa Cruz, and 

San Mateo Counties, for example, the cliffs often retreat in a 

more linear fashion, producing relatively straight coastlines 

(fi g. 12). Lithologic, stratigraphic, and structural weaknesses 

or differences are the key factors affecting erosion rates in sedimentary rocks. Where the coastal bluff consists of relatively 

homogeneous sedimentary rocks, the shape of the coastline, 

the sea-cliff morphology and the average rates of erosion are 

often similar alongshore.

Cliff erosion, as mentioned earlier, is due not only to 

waves undercutting the base of the cliff, but also to rockfalls, 

landsliding, and slumping higher on the cliff face. The orientation and spacing of joints in the sandstones, siltstones, and 

mudstones that make up the cliffs surrounding northern Monterey Bay are the dominant factors affecting cliff retreat in this 

area (fi g. 13; Griggs and Johnson, 1979). 

Cliff failure in California during strong seismic shaking 

represents a signifi cant but little appreciated coastal hazard, 

Figure 12. Linear cliffed coastline along northern Monterey Bay.

Figure 13. Episodic coastal bluff failure in Capitola, Santa Cruz 

County.

primarily due to the infrequent nature of large earthquakes 

(fi g. 14). The potential for earthquakes that can affect coastal 

bluffs is high along the entire length of the State’s coastline 

(Griggs and Scholar, 1997; Plant and Griggs, 1990 a, b). No 

part of the coastline of California is more than 25 km from an 

active fault (Jennings, 1975), and many areas are considerably 

closer. 

The Eroding Coast of California—Historical 

Perceptions

A 1971 Corps of Engineers regional inventory of the 

California shoreline classifi ed only 14.2 percent of the coast 

as “noneroding” while 7 percent (123 km) was classifi ed as 

“critical erosion” (defi ned as areas where structures and (or) 

utilities were threatened), with the remainder designated as 

“noncritical erosion” (United States Army Corps of Engineers, 

1971). A subsequent investigation by the California Department of Navigation and Ocean Development (Habel and 

Armstrong, 1978) defi ned the erosion problem somewhat differently. Approximately 160 km (9 percent) of the coast was 

delineated as eroding with existing development threatened, 

and an additional 480 km (27.3 percent) of the coast was classifi ed as eroding at a rate fast enough that future development 

would eventually be threatened. Thus a total of 640 km (36.3 

percent) of the California shoreline was considered threatened 

as a result of high erosion rates. 

A subsequent inventory of hazardous coastal environments expands the scale of the problem areas again. In 1985, 

16 coastal geologists participated in the preparation of a statewide inventory of shoreline conditions. They classifi ed 504 km 

(28.6 percent) of the coastline as high risk and an additional 

648 km (36.8 percent) as requiring caution (Griggs and Savoy, 

1985). These data indicate that two-thirds of the California 

shoreline constitutes a signifi cant coastal hazard (fi g. 15). 

Documenting Sea-Cliff Erosion Rates

It has been long recognized by coastal researchers working with sea-cliff evolution and erosion that cliff or bluff 

retreat is usually an episodic process (fi g. 13). Most of the 

major episodes of cliff erosion occur during the simultaneous occurrence of high tides and large storm waves, typically 

with heavy rainfall (Kuhn and Shepard, 1984). At these times, 

waves can 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 cliff or bluff attack, 

undercutting of the base of the cliff followed by collapse of the 

overlying unsupported material, or simply hydraulic quarrying 

of blocks or rocks that were stable during conditions of lower 

wave energy. Terrestrial processes, such as landsliding, slumping, or rock falls, triggered independently of wave attack, 
59

Figure 15. Hazard zones along the California coast with a blowup of the Santa 

Barbara area (Modifi ed from Griggs and Savoy, 1985).

Figure 14. Seismically induced bluff failure north of Pacifi ca 

in Daly City, 1989.

may also be extremely important, and even dominant in areas 

where the base of the bluff is protected by a seawall, revetment, or a wide sandy beach (for example, see Norris, 1990). 

Although coastal geologists often use or report average 

annual cliff-retreat or erosion rates, in reality we are simply 

comparing the position of the cliff edge at different points in 

time (whether from historic maps or aerial photographs, or 

actual fi eld measurements) and dividing by the total number 

of years between these data points to derive an average erosion rate. Cliffs or bluffs may remain superfi cially unchanged 

for years, and then as a result of 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 sedimentary 

rock or unconsolidated sediments.

Most studies of sea-cliff erosion have relied on measurements from a temporal sequence of historic vertical stereo 

aerial photographs or maps, as these typically provide the database of long duration. In populated coastal areas of the United 

 California’s Coastal Cliffs and Bluffs
60 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

States, for example, the aerial photographic record may extend 

back 60 or 70 years. A span this long will include both representative storm and calmer weather conditions, as well as wet 

and dry periods that can span a few decades. Thus, the range 

in actual erosion rates derived from sequential measurements 

of the location of the cliff edge, relative to some baseline or 

benchmark (a road or structure, for example) over the time 

span of the photos, can produce a reasonably accurate picture 

of the pattern of cliff retreat. 

The extent of the database used (aerial photographs, maps, 

or ground measurements), the resolution of this database (for 

example, aerial photo scale and clarity), the skill or experience of the investigator, the time span between individual 

photograph or map sets, and the methodology or techniques 

used (Moore, 2000), are all important factors that affect the 

reliability of calculated cliff erosion rates. Wide variations can 

result depending on the length of the historical record used or 

the particular segment of coast analyzed as a result of factors 

such as long-term climatic or storm-frequency variations or 

Figure 16. Erodable bluffs consisting of unconsolidated marine terrace deposits and soil in Pacifi ca; the site of previous mobile home 

pads that had to be abandoned because of sea-cliff retreat during the 

1983 El Niño.

Figure 17. Published erosion rates along the coast of California, with a blowup of the 

Santa Cruz area.
61

Figure 18. Apartments in Capitola that had to be demolished as a 

result of being undercut by cliff erosion. 

alongshore differences in the geological materials and their 

resistance or susceptibility to erosion. The long-term hazards 

of constructing on geologically active coastal cliffs where 

long-term erosion rates have not been carefully evaluated or 

where average erosion rates from some nearby areas have been 

extrapolated to the site can be very costly (fi g. 16). Given recent gains in the understanding of longer-term climatic periods 

and the impacts of El Niño events on the shoreline, short-term 

records or erosion data (for example, less than 25 or 30 years) 

should be used with caution as they may not be representative 

of long-term trends. 

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 

surprisingly few comprehensive studies, and relatively few 

data are readily available (fi g. 17; Griggs and Savoy, 1985). 

“Living with the California Coast” (Griggs and Savoy, 1985) 

included regional input from a group of coastal geologists 

in California, and maps included in that volume summarize 

the site specifi c cliff-erosion rates known at that time. More 

recently, Moore and others (1998) completed shoreline erosion studies for San Diego County and the developed portion 

of Santa Cruz County as part of a nationwide study funded 

by the Federal Emergency Management Agency (FEMA) that 

documented average coastal retreat rates over the past 50 to 60 

years.

In a 1992 statewide coastal hazards study (Griggs and others, 1992) it was determined from interviews with local government planning staff that the most frequently cited data need 

was that of shoreline/bluff erosion rates. Nearly half of the 

respondents indicated a need for such information. Yet, there 

are still very few additional published or easily accessible cliff 

erosion rates beyond those previously published for Santa 

Cruz and San Diego Counties. There have been a number of 

local, site specifi c studies for individual parcels where documentation of cliff erosion rates were required as a condition 

for construction or protection permits, but there has been no 

attempt to consolidate or compile these for broader application. Although qualitative information on coastal-bluff retreat 

is readily available (for example, old photographs, eroded 

roads, exposed storm drains, and similar structures), accurate 

rates of shoreline erosion are more diffi cult to come by. Yet it 

is these long-term rates that are critical in establishing safe setback lines for any proposed oceanfront construction. 

Human Occupancy of the 

California Coastline and 

Cliff-Erosion Hot Spots

 California has approximately 1,760 km of shoreline; 

this length has not changed signifi cantly in historic times, but 

the population that uses and develops the coast continues to 

increase. At the time of the last major damaging El Niño in 

1982-83, the State’s population stood at 24.8 million people. 

By the time of the arrival of the 1997 El Niño event, the State’s 

population had increased 29 percent to 32 million, and by 

2003 it had reached 35.5 million. Eighty percent of these people live within 50 km of the coastline and more than 4 million 

live within 5 km of the water’s edge. The buildable coastline 

itself has continued to fi ll in with houses and other structures.

As a result of topography, climate, access, availability 

of water to some degree, and therefore historical development patterns, the coastal population is unevenly distributed 

throughout California’s coastal counties. If divided up evenly, 

residents in rural Humboldt County would have about 2.5 m 

of shoreline each, whereas residents of suburban Los Angeles 

County would have about a centimeter each. Overall, each resident of the State would have about 5 cm of shoreline if it were 

accessible, but this is not the case for much of the rugged and 

inaccessible central and northern coast. In addition, the shoreline must be shared with nearly 100 million visitors each year. 

To make matters worse, the population of the State is projected 

to reach 50 million by the year 2020. This is cause for concern 

as we look at coastal hazards and cliff erosion.

Eroding bluffs and cliffs represent California’s most extensive coastal hazard. Because of California’s location in an active geologic setting, uplift of the coastline has produced many 

square kilometers of easily developed, nearly level marine 

terraces. From Humboldt County in the north to San Diego 

County in the south, these fl at benches have been developed 

with houses, condominiums, apartments, restaurants and hotels. 

In most locations, this development has encroached right to the 

cliff or bluff edge, where views of the ocean are unobstructed 

 California’s Coastal Cliffs and Bluffs
62 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

and property values are the highest, but where the risks to 

structures of continuing bluff retreat are the greatest. 

Coastal communities from one end of California to the 

other have lost entire oceanfront streets, utility lines, lots-ofrecord, and homes through the ongoing process of cliff erosion over the last century (fi gs. 1, 18). New developments 

are still being proposed on eroding or unstable bluff tops and 

small, older weekend beachfront cottages are still being torn 

down and replaced by larger new homes. When the California 

Coastal Act was passed in 1972, coastal hazard issues were not 

as obvious as they have become since 1978, when there was 

a major climactic shift. During the last two decades, winterstorm wave attack has been more severe along the coast than it 

had been in the previous three decades (Storlazzi and Griggs, 

2000). Although statewide guidelines were established in the 

Coastal Act for determining the stability of coastal bluffs and 

potential development sites, there is no statewide policy establishing safe setback distances from cliff or bluff edges. As a 

result, some local governments use a predetermined, fi xed setback; this varies from as little as 3 to as much as 100 meters. 

Others employ a cliff retreat rate applicable over a specifi c 

time period or structural lifespan, most commonly a 50-year 

period.

Each area of California’s coastline has its problem areas 

or erosional hot spots that have been repeatedly damaged and 

which regularly are featured in news stories (Moore and others, 1998). Gleason Beach, along the Sonoma County coast, 

is an area where a group of cliff-top homes was built in an 

otherwise very rural and undeveloped area. The underlying 

geology consists of the heterogeneous Franciscan Formation, 

which forms both resistant headlands and very erodible, slide 

prone slopes. Houses have been undermined, collapsed, and 

destroyed, and many others have been left in precarious positions 25 m above the waves (fi g. 19).

At Pacifi ca, in coastal San Mateo County, a row of oceanfront homes was built on an uplifted marine terrace consisting 

of poorly consolidated Tertiary marine sediments, terrace deposits, and eolian deposits. During the 1982-83 El Niño, these 

homes were threatened but undamaged. A protective rock revetment was subsequently built at the base of the bluff. When 

the 1997-98 El Niño high tides and storm waves struck, the 

revetment was destroyed. The waves attacked the unprotected 

bluff and groundwater weakened the cliff sediments—erosion 

proceeded until most of the oceanfront houses were undercut 

or threatened to the point where they had to be destroyed.

In the Isla Vista area of Santa Barbara County (fi g. 2), a 

number of apartment buildings were built on a coastal bluff 

cut into weak sedimentary rocks in the 1960’s and 1970’s to 

accommodate the rapidly growing student population of a University campus. The beaches appear to have narrowed in recent 

years from sand supply reduction, and a number of dwellings 

are now being undermined with structural collapse a very real 

threat.

In the Torrey Pines area of San Diego County (fi g. 2), 

cliffs exceeding 90 m in height have been eroded into Eocene 

sandstone and shale. Subaerial mass wasting is the dominant 

failure mechanism in this area and many landslides have occurred. In 1982, a 175-m-long section of the cliffs failed and 

approximately 1.4 million m3

 of material was deposited on the 

beach (Vanderhurst and others, 1982). The Torrey Pines area 

is devoid of any coastal armoring and the top edge of the cliff 

has been retreating at average rates of a few to more than 50 

cm/yr.

Human Responses to Coastal 

Cliff Erosion

As the development of the southern and central California 

coast has intensifi ed, and the State has experienced a recent 

(beginning in 1978) era of increased coastal storm damage 

and property loss; the extent of shoreline protected by armor 

has incrementally increased. In a 1971 Corps of Engineers 

statewide shoreline inventory, 42.4 km of shoreline (2.4 percent) was listed as protected by some sort of armor (exclusive 

of breakwaters and groins). Only 7 years later, in 1978, the 

California Department of Navigation and Ocean Development 

(now the Department of Boating and Waterways) determined 

that 100 km or 5.7 percent of the State’s shoreline had been 

protected by engineering structures. 

In “Living with the California Coast” (Griggs and Savoy, 

1985) coastal hazards were analyzed kilometer by kilometer 

along the State’s shoreline. Their maps indicate that approximately 136 km or 7.7 percent of the coast was armored by 

seawalls or revetments, and another 32 km was protected by 

breakwaters, for a total of 168 km of armor by 1985 (9.5 percent of the entire 1,760 km of shoreline). This is a four-fold 

increase in the length of shoreline protected by seawalls in just 

14 years. In a subsequent study analyzing the State’s coastal 

hazard policies and practices, Griggs and others (1992), using 

fi rst-hand interviews of local government planners, reported 

that a total of 208 km or 11.8 percent of the coast was now 

protected by some form of hard, engineering structure. This 

Figure 19. Ongoing cliff erosion in the Franciscan Formation of 

Northern California threatens cliff-top development.
63

study looked at the extent of armor by city and county, and, 

expectedly, the heavily populated and developed central and 

southern portions of the State’s coast had been protected to 

a far greater degree. Seventy-seven percent of the 14.4 km 

shoreline of northern Monterey Bay had been armored, 77% 

of the 28.8 km shoreline between Carpenteria and Ventura had 

been protected, and 86 percent of the 12.8 km shoreline from 

Oceanside to Carlsbad was protected (fi g. 2). In the extreme, 

virtually the entire 12.8 km reach of shoreline from Dana Point 

to San Clemente had already been armored. Subsequent investigation, however, indicated that some of the armor totals provided by local governments included interior protection within 

harbors and river mouth jetties, such that the totals overestimate open coast protection.

The most recent compilation of coastal armor in California was completed in 2002 (Runyan and Griggs, 2002b), 

and was based on earlier data augmented by oblique video 

photography of the more developed and armored southern 

and central coast fl own in 1998. This most recent summary 

indicates that about 180 km or 10.2 percent of the State’s 

shoreline has been armored, and that 172 km of this represents seawalls and revetments, whereas the remaining 8 km 

are breakwaters and similar offshore structures. Of the 10.2 

percent of the State’s armored shoreline, 57 percent lines 

coastal lowlands and dunes and the remaining 43 percent is 

protecting sea cliffs. 

Human Activity

As populations worldwide have migrated to the shoreline, 

either seasonally or permanently, the impacts of those humans 

and their development have begun to alter both coastal landforms and cliff stability. Buildings, utilities, and coastal protection structures have been built 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 little of the 

natural cliffs can any longer be seen, as they have been completely armored with protective materials.

Heavy construction on bluff tops as well as exotic landscaping and the required irrigation or watering required to 

support that vegetation has added to the normal average annual precipitation along the shoreline of southern California 

(Griggs and Savoy, 1985). The net result is to increase the 

pore pressures in the cliff materials, thereby decreasing their 

strength and accelerating the cliff failure process. In addition, 

the runoff from the impervious surfaces accompanying urbanization of coastal bluff top areas has typically been directed 

into culverts or drains which have focused runoff on the bluff 

face, thereby increasing local cliff erosion or stability. 

Coastal engineering structures have had several effects on 

the development and stability of sea cliffs. Large structures, 

such as breakwaters and jetties, typically induce up-coast 

impoundment of littoral drift, thereby widening the beach 

and protecting the cliffs from direct wave attack. As a consequence, however, the down coast beaches are initially starved, 

and this beach loss leads to an increase in exposure to wave 

attack and results in accelerated cliff erosion (Wiegel, 1964; 

Griggs and Johnson, 1976). Local protection structures such 

as seawalls and revetments are constructed to control waveinduced cliff retreat and as such can temporarily halt cliff erosion and stabilize coastal cliffs or bluffs. The type of structure 

utilized and its height, depth, lateral extent, and durability are 

important factors in determining the effectiveness of these 

shoreline erosion control devices (Fulton-Bennett and Griggs, 

1986). 

New concerns have arisen in recent years in California 

about the impacts of seawalls and revetments (Griggs, 1999). 

These include visual or aesthetic impacts, potential loss of 

beach access, loss of beach through placement of revetments, 

as well as the loss of beach sand that would have been provided by the continued erosion of the bluffs. In a recent study by 

Runyan and Griggs (2002a,b), it was determined that armoring 

has reduced the natural sand supplied from sea-cliff erosion 

by approximately 20 percent in both the Oceanside and Santa 

Barbara littoral cells. It should be noted that the overall contribution of sand from sea-cliff erosion to the sediment budget 

in the Oceanside and Santa Barbara littoral cells is minor, 12 

and 0.5 percent respectively, and thus, coastal armoring does 

not signifi cantly reduce the sand supplied to the beaches in 

these cells. 

Conclusions

Seventy-two percent of California’s 1,760 km of coastline 

consists either of high steep cliffs or lower bluffs eroded into 

nearly horizontal marine terraces. Sea-level rise over the past 

18,000 years has led to continuous coastal retreat of the shoreline with cliffs and bluffs being cut into the coastal landscape. 

Cliff erosion takes place through a combination of marine and 

terrestrial processes and is dependent upon the properties of 

the cliff-forming materials (rock type, structural weaknesses, 

presence of groundwater, for example), as well as physical 

forces such as wave energy, tidal range, degree of protection 

offered by a beach, climate, and frequency and magnitude of 

severe storm events. In general, the softer sedimentary rocks 

making up the coastal bluffs erode at average rates of 10 to 30 

cm/year whereas the harder igneous and metamorphic rocks, 

which often make up the resistant headlands, erode at slower 

rates. Cliff erosion rates can be measured using sequential 

aerial photographs or historic surveys or maps and are necessary for making wise coastal land use decisions and establishing setbacks for coastal construction. The wide, fl at uplifted 

marine terraces, which are characteristic of much of California’s coast, are easily developed and have been the sites where 

most of California’s coastal communities have evolved. The 

low bluffs fronting these terraces, however, typically are easily 

eroded and coastal land loss has become a key issue along the 

State’s shoreline. Cliff-erosion hot spots are widespread and 

well recognized along the State’s shoreline (Moore and Griggs, 

2002) and public and private property damage during the last 

 California’s Coastal Cliffs and Bluffs
64 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

two major El Niño events (1982-83 and 1997-98) was high. 

Historical approaches to dealing with the ongoing erosion 

of the State’s cliffs and bluffs have focused on seawalls and 

revetments, but concerns with the effectiveness and impacts 

of these structures have begun to restrict additional armor emplacement.

References

Benumof, B.T. and Griggs, G.B., 1999, The relationship between 

seacliff erosion rates, cliff material properties, and physical 

processes: Shore and Beach, v. 67, no. 4, p. 29-41.

Benumof, B.T., Storlazzi, C.D., Seymour, R.J., and Griggs, G.B., 

2000, The relationship between incident wave energy and 

seacliff erosion rates, San Diego County, California: Journal of Coastal Research, v. 16, no. 4, p. 1162-1178.

Emery, K.O., and Kuhn, G.G., 1982, Sea cliffs—their processes, 

profi les, and classifi cation: Geological Society of America 

Bulletin, v. 93, p. 644-654.

Fulton-Bennett, K.W., and Griggs, G.B., 1986, Coastal protection structures and their effectiveness: Joint Publication 

of California State Department of Boating and Waterways 

and Institute of Marine Sciences, University of California, 

Santa Cruz, 48 p. 

Griggs, G.B., 1999, The protection of California’s coast; past, 

present and future: Shore and Beach v. 67, p.18-28.

Griggs, G.B., and Johnson, R.E., 1976, The effects of the Santa 

Cruz Small Craft Harbor on coastal processes in northern 

Monterey Bay, California: Environmental Geology, v.1, 

p. 229-312.

Griggs, G.B., and Johnson. R.E., 1979, Coastline erosion, Santa 

Cruz County: California Geology, v. 32, no. 4. p. 67-76.

Griggs, G.B., and Savoy, L.E., 1985, Living with the California 

Coast: Durham, North Carolina, Duke University Press, 

393 p.

Griggs, G.B., and Scholar, D., 1997, Coastal erosion caused by 

earthquake-induced slope failure: Shore and Beach, v. 65, 

no.4, p. 2-7.

Griggs, G.B., Pepper, J.E., and Jordan, M.E., 1992, California’s 

coastal hazards; a critical assessment of existing land-use 

policies and practices: Berkeley, California, Special Publication of California Policy Seminar Program, 224 p.

Habel, J.S., and Armstrong, G.A., 1978, Assessment and atlas of 

shoreline erosion along the California coast: Sacramento, 

California, State of California, Department of Navigation 

and Ocean Development.

Jennings, C.S.,1975, Fault map of California: California Division 

of Mines and Geology, 1 sheet .

Krumbein, W.C., 1947, Shore processes and beach characteristics: U.S. Army Corps of Engineers, Technical Memorandum No. 3, 35 p.

Kuhn, G.G., and Shepard, F.P., 1984, Sea cliffs, beaches and 

coastal valleys of San Diego County: Berkeley, California, 

University of California Press, 193 p.

Lajoie, K.R., 1986, Coastal tectonics, in Active tectonics: Washington, D.C., National Academy of Sciences, p. 95-124.

Moore, L.J., 2000, Shoreline mapping techniques: Journal of 

Coastal Research, v.16, no.1, p. 111-124.

Moore, L.J., and Griggs, G.B., 2002, Long-term cliff retreat 

and erosion hotspots along the central shores of the Monterey Bay National Marine Sanctuary: Marine Geology, 

Special Issue on Seafl oor Geology and Natural Environment of the Monterey Bay National Marine Sanctuary, 

p. 265-284.

Moore, L.J., Benumof, B.K., and Griggs, G.B., 1998, Coastal 

erosion hazards in Santa Cruz and San Diego counties, 

California: Journal of Coastal Research, Special Issue no. 

28, p. 121-139.

Norris, R.M., 1990, Sea cliff erosion—a major dilemma: Geotimes, v. 11, p. 16-17.

 Plant, N.G., and Griggs, G.B., 1990a, Coastal landslides caused 

by the October 17, 1989 earthquake, Santa Cruz County, 

California: California Geology, v.43, p.75-84.

Plant, N.G., and Griggs, G.B., 1990b, Coastal landslides and the 

Loma Prieta earthquake: Earth Science, v. 43, p. 12-17.

Runyan, K.B., and Griggs, G.B., 2002a, Chapter 8—Contributions from coastal cliff erosion to the littoral budget, 

in Coyne, M., and Sterrett, K., eds., California Beach 

Restoration Study: Sacramento, California, California 

Department of Boating and Waterways and State Coastal 

Conservancy.

Runyan, K.B., and Griggs, G.B., 2002b, The effects of armoring 

sea-cliffs on the natural sand supply to beaches of California: Journal of Coastal Reseach, v. 19, no. 2, p. 336-347.

Shepard, F.P. and Grant, U.S., IV, 1947, Wave erosion along the 

southern California coast: Geological Society of America 

Bulletin, v. 58, p. 919-926.

Storlazzi, C.D. and Griggs, G.B., 2000, The infl uence of El 

Nino-Southern oscillation (ENSO) events on the evolution 

of California’s shoreline: Geological Society of America 

Bulletin, v. 112, no. 2, p. 236-249.

U.S. Army Corps of Engineers (USACOE), San Francisco District, 1971, National Shoreline Study—California Regional 

Inventory: San Francisco, California, Corps of Engineers 

District Offi ce, 105 p.

Vanderhurst, M.L., McCarthy, R.J., and Hannan, D.L.,1982, 

Black’s Beach landslide, in Geologic studies in San Diego: 

San Diego, California, San Diego Association of Geologists, p. 46-56.

Wiegel, R.L., 1964, Oceanographical engineering: Englewood 

Cliffs, New Jersey, Prentice-Hall, 532 p.
65

Introduction

Much of the Oregon coast (fi g. 1) is dominated by steep 

cliffs that plunge directly into the sea. The southern one-third 

of the coast especially has this character, as does the scenic 

stretch of shore along Cape Perpetua on the mid-Oregon coast 

north of Florence. Further to the north, the coast consists of an 

interplay between pronounced rocky headlands composed of 

resistant basalt, separating longer stretches of beach that are 

backed by less resistant sea cliffs composed variously of Tertiary mudstones and siltstones or loosely cemented Pleistocene 

sandstones that are readily eroded when attacked by waves. 

The Tertiary rocks were originally deposited in relatively deep 

water on the sea fl oor, but were then pushed up and added to 

the continent during tectonic subduction of the ocean crust. 

The Pleistocene sandstones are marine terrace deposits that 

have been uplifted during plate subduction, having originally 

formed tens to hundreds of thousands of years ago as beaches 

and dunes. A typical sea cliff consists of Tertiary rocks at its 

base, which are moderately resistant to wave attack and erosion by subaerial processes such as rainfall and ground-water 

seepage, topped by a layer of Pleistocene terrace sandstone. 

Commonly, the units of hardened sediments within the Tertiary 

rocks, having originally been deposited as horizontal layers on 

the sea fl oor, now dip with marked slopes, contrasting with the 

nearly horizontal layers of beach and dune sands within the 

Pleistocene deposits. In areas where the Tertiary mudstones 

dip steeply in the seaward direction, their instability has led to 

the development of large-scale landslides.

Many of Oregon’s coastal communities are situated on the 

nearly level marine terraces (fi g. 2), where their seaward edges 

are being cut away by sea-cliff erosion and landsliding (fi g. 3). 

Communities, such as Cannon Beach, Lincoln City, Gleneden 

Beach just south of Lincoln City, and Newport, have suffered 

as sea cliffs retreated when attacked by the waves of winter 

storms. A number of State parks have been affected, with the 

loss of picnic grounds and camping facilities. Stretches of 

Highway 101 have similarly suffered from cliff erosion and 

landsliding, costing taxpayers millions of dollars in repairs. In 

Oregon’s Coastal Cliffs:

Processes and Erosion Impacts

By Paul D. Komar

Figure 1. The geography of the Oregon coast.

Figure 2. Sea-cliff erosion into the marine terrace at Lincoln City, 

threatening homes that had been constructed close to the cliff edge.

 U.S. Geological Survey Professional Paper 1693
66 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

total, sea-cliff retreat and landsliding affect hundreds of kilometers of the Oregon coast.

The management of Oregon’s rocky shores to reduce society’s losses to cliff erosion and landsliding is diffi cult because 

of the extreme variability of the coast’s geology and geomorphology, with a wide range of rock types. There is also a range 

of susceptibilities to wave erosion, with cliffs varying from 

being constantly attacked by waves to sites where the bluff is 

fronted by a wide sandy beach so waves reach it only once in a 

decade or longer, and the resulting erosion is episodic.

The objective of this chapter is to summarize what is 

known about sea-cliff erosion on the Oregon coast. In order 

to understand the variability of cliff-erosion impacts along the 

coast, it is necessary to have an appreciation of the underlying 

importance of the tectonic setting of the Pacifi c Northwest, its 

associated geologic evolution, and how the tectonic processes 

result in land-elevation changes whose rates can exceed the 

global rise of sea level. Within that tectonic framework are 

numerous local factors and processes that affect rates of seacliff erosion and can give rise to large-scale landsliding. These 

processes have been the focus of research undertaken in recent 

years, and the products of those studies will be reviewed. Ultimately of interest is the use of that research to develop methodologies that can be employed to assess natural hazards along 

the rocky shores of Oregon, leading to a rational basis for the 

establishment of hazard zones or setback distances for their 

safer development.

Tectonic Setting and Geology

The tectonics and geology of the Oregon coast are controlled by its location within a zone of convergence and 

collision of three of the Earth’s tectonic plates (fi g. 4), the 

oceanic Juan de Fuca and Gorda Plates, and the continental 

North American Plate. New ocean crust is being formed at the 

Juan de Fuca and Gorda Ridges, offset by the Blanco Fracture Zone, a giant fault in the ocean fl oor that has been the 

source of minor earthquakes felt along the coast. The newly 

formed ocean crust at the ridges is carried eastward toward 

the continent, the North American Plate, resulting in their collision. The rate of convergence between the ocean crust and 

land mass is about 2.5 cm per year, much less than occurs in 

other plate convergence zones along the margin of the Pacifi c 

Ocean.

The collision of the Juan de Fuca and Gorda Plates with 

the continental North American Plate results in the subduction 

of the denser ocean plates, which slide beneath the less dense 

continental plate. Such subduction occurs along much of the 

Pacifi c Ocean margin, producing submarine trenches offshore 

and volcanoes inland from the coast as a result of the melting of the ocean crust as it descends into the Earth’s mantle, 

the main features of the so-called “Ring of Fire” around the 

Pacifi c rim. Generally associated with this subduction of the 

oceanic plates is the generation of major earthquakes, caused 

by the scraping of the descending ocean plate against the unFigure 3. Erosion of the sea cliff at Gleneden Beach south of Lincoln 

City. Waves were able to attack the cliff because a rip current had 

locally cut an embayment into the beach.

Figure 4. The tectonic setting of the Pacifi c Northwest, with the collision and subduction of the oceanic Juan de Fuca and Gorda Plates 

beneath the continental North American Plate. The triangles represent volcanoes of the Cascade Mountains, formed by the melting of 

the subducting plates. (From Komar, 1997.)
67

derside of the continental plate. This is best seen on the coasts 

of South America and Alaska, where the resulting earthquakes 

are frequent and have achieved magnitudes of 8 and 9, the 

most extreme on Earth.

There has not been a subduction earthquake in the Pacifi c 

Northwest since its settlement by Euro-Americans in the midnineteenth century, and for a time it was thought that we had 

escaped this major hazard because of the slow convergence 

rate of the plates and perhaps because sediment accumulation 

on the ocean fl oor “lubricates” the subduction. However, in recent years evidence has come to light demonstrating that such 

earthquakes have occurred in the prehistoric past at intervals 

of hundreds of years. The principal evidence initially came 

from investigations of estuarine marsh sediments buried by 

sand layers, deposits which suggest that portions of the coast 

had abruptly subsided at the time of an earthquake, followed 

by an extreme tsunami that swept over the area to deposit the 

sand (Atwater, 1987; Atwater and Yamaguchi, 1991; Atwater 

and Hemphill-Haley, 1997; Darienzo and Peterson, 1990; 

Darienzo and others, 1994). On the basis of the numbers of 

such layers found in various bays and estuaries along the coast 

and the carbon-14 dates of those layers, it has been established 

that catastrophic earthquakes have occurred repeatedly during the past 7,000 years, at intervals ranging from 300 to 600 

years. Dating of the buried marshes indicated that the most 

recent earthquake occurred approximately 300 years ago. The 

timing of that event was fi rmly established by Satake and others (1996) on the basis of the historic occurrence of a large 

tsunami that destroyed a number of villages along the east 

coast of Japan. They developed a computer model to simulate 

the movement of the tsunami as the waves crossed the Pacifi c 

Ocean, pinpointing their generation by an earthquake in the 

Pacifi c Northwest during the evening of January 26, 1700. 

Furthermore, on the basis of the sizes of the tsunami waves 

that reached Japan, Satake and others (1996) concluded that 

the earthquake must have been approximately magnitude 9.

The evidence is conclusive that major subduction earthquakes have occurred in the Pacifi c Northwest. The lack of 

a subduction earthquake during historic times suggests that 

the ocean plates and the North American Plate are temporarily locked together and accumulating energy. The longer the 

plates remain locked, the greater the amount of stored energy 

and the more catastrophic the resulting earthquake when the 

plates fi nally do break apart along the subduction zone. Given 

the frequency of occurrence of earthquakes in the past, documented by the buried marshes along the coast, there is a strong 

possibility that another major subduction earthquake will occur during the next 100 years.

Sediments accumulate on the ocean plates after their formation at the spreading ridges, the thickness increasing with 

time. The Juan de Fuca and Gorda Plates have unusually thick 

accumulations because of their close proximity to the continent where rivers and coastal erosion deliver mud and silt to 

the ocean. Most of this sediment on the plates is scraped off 

during subduction and is added to the continental mass. This is 

the origin of the Tertiary mudstones and siltstones forming sea 

cliffs along the Oregon coast. Nearly all of western Oregon has 

been created by continental accretion of ocean sediments and 

a series of volcanic seamounts and islands, or entire blocks of 

the sea fl oor. The oldest rocks found in western Oregon date 

back to the Paleocene and Eocene geologic periods, about 

60 to 40 million years ago. These rocks are ocean basalts, 

much like those being formed today by volcanic activity at the 

spreading ridges. During the Miocene, 35 to 30 million years 

ago, volcanic activity generated the immense fl ows of the Columbia River Basalts. At the same time, volcanic activity recurred to the west, somehow connected with the generation of 

the Columbia River Basalts because the rocks are almost exactly the same. These Miocene volcanic rocks are particularly 

important to the modern coastal morphology because they are 

resistant to wave attack and form many of the major headlands 

along the Oregon coast; Yaquina Head, Cape Foulweather, and 

Cape Lookout are examples (fi g. 5).

About 5 million years ago during the Pliocene the Coast 

Range mountains progressively emerged and western Oregon 

came into existence. On emergence from the sea, the rain, 

rivers, and ocean waves went to work to erode what formerly 

had been deep-sea crustal rocks and volcanoes and ocean sediments now hardened into rock. These processes have etched 

out the land, cutting away the weaker rocks and leaving behind 

the more resistant rocks to form the peaks of the Coast Range 

and headlands along the coast. The modern morphology of the 

coast is the product of the erosion that has taken place during 

the past 5 million years.

As a result of this tectonic setting and geologic history, 

the rocks found in sea cliffs along the coast vary from highly 

resistant volcanic basalts to weaker mudstones and siltstones 

that originally were deposited in the ocean basin. The differenFigure 5. The division of the northern Oregon coast into a series of 

littoral cells that contain stretches of sandy beaches, isolated by large 

rocky headlands. The left of the fi gure shows the northernmost coast, 

continuing southward on the right. (From Komar, 1997.)

 Oregon’s Coastal Cliffs: Processes and Erosion Impacts
68 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

tial rates of erosion and long-term retreat of these contrasting 

rock types has yielded the irregular outline of the coast, with 

headlands composed of basalt separating embayments formed 

by the more rapid retreat of the less resistant mudstones and 

siltstones. These embayments are the principal sites of beach 

sand accumulation, each constituting what is termed a littoral 

cell (fi g. 5), basically a stretch of sandy beach that for the most 

part is isolated by the large rocky headlands that prevent the 

exchange of beach sand with adjacent cells. The individual 

littoral cells contain different quantities of sand, usually apparent in the widths of their beaches. This depends on the local 

budget of sediments (Komar, 1998), in particular the presence 

and importance of sand sources such as rivers and sea-cliff 

erosion which varies widely from cell to cell, with some cells 

having virtually no modern-day sources. The sand found there 

is “relict,” having reached the cell thousands of years ago with 

the rise in sea level at the end of the last ice age (Clemens and 

Komar, 1988). As will be discussed in a later section, these 

variable quantities of sand within the littoral cells and the 

widths of their beaches result in markedly contrasting rates of 

sea-cliff erosion as governed by the capacity of the fronting 

beach to protect the cliff from wave attack.

As well as controlling the geology and geomorphology of 

the Oregon coast, the tectonic processes of plate subduction 

also produce land-elevation changes that are an important factor in the erosion of the coast. Of particular signifi cance is how 

the land-elevation changes compare with the long-term change 

in global (eustatic) sea level produced by the melting of glaciers and thermal expansion of the ocean’s water. Important 

to the erosion of a specifi c coastal site is the relative sea-level 

change, which accounts for both the eustatic sea-level rise and 

the changing elevation of the land at that site.

As discussed above, the initial evidence for past subduction earthquakes came from studies of buried marsh deposits 

in estuaries, which also supported the conclusion that much of 

the Pacifi c Northwest coast abruptly subsides at times of earthquakes, often by 1 to 2 m. However, measurements of landelevation changes spanning several decades reveal that many 

areas of the coast are now rising, in some places, at rates that 

exceed the global rise in sea level. These ups and downs of the 

coast are interpreted in terms of the accumulation of subduction strain between earthquake events, causing the slow rise of 

the land while the plates are locked together, and the release 

of that strain at the time of the earthquake, which results in the 

abrupt subsidence of the land.

The primary evidence for land-elevation increases during 

historic times has come from bench marks; those installed by 

the government to be used as reference points by land surveyors. Every few years, the government resurveys their locations 

and elevations, and changes over the decades provide direct 

measurements of long-term variations in land elevations. Such 

analyses have been undertaken for the Pacifi c Northwest coast 

by Vincent (1989) and Mitchell and others (1994). Komar and 

Shih (1993) compared the results with changing sea levels so 

as to make them relevant to associated coastal erosion patterns. The results for the repeated surveys along a north-south 

line extending the full length of the Oregon coast are graphed 

in fi gure 6. The bench-mark data have been coupled with measurements of relative sea-level changes determined from tide 

gauges at Astoria in the Columbia River and Crescent City in 

northern California, so the vertical axis is for the rate of landelevation change compared with the global (eustatic) rise in 

sea level. The graph demonstrates that, south of Florence, the 

coast is rising faster than the eustatic rise in sea level. On the 

basis of the levels of uplifted marine terraces, it is believed 

that this rate of rise reaches a maximum in the area of Cape 

Blanco between Bandon and Gold Beach. Along the northern half of the coast the eustatic rise in sea level exceeds the 

land-elevation increase (fi g. 6), with the net submergence rates 

being on the order of 1 to 2 mm per year (10 to 20 cm per century). These submergence rates are comparable to those found 

in assessments of the global eustatic rise in sea level, based on 

analyses of tide-gauge records throughout the world, implying 

that there must be minimal on-going land elevation change 

along the northern Oregon coast. Although a submergence rate 

of 10 to 20 cm per century can be expected to be a factor in 

the erosion of the north coast, these rates are substantially less 

than the 40 to 60 cm per century rates common along the East 

and Gulf coasts of the United States, areas that are subsiding, 

so relative sea-level change has had less of an impact on the 

Oregon coast.

The effects of land subsidence at the time of the 1700 

earthquake and its subsequent uplift are evident in the geomorphology of the coast. This has been considered by Komar 

and others (1991), specifi cally for Bandon on the southern 

Oregon coast, and more generally by Komar and Shih (1993) 

in their investigation to understand the along-coast variability 

in the extent of sea-cliff erosion. At Bandon, a high sea cliff 

composed mainly of mudstone (fi g. 7) clearly was cut back by 

wave erosion in the distant past, with the numerous offshore 

sea stacks providing evidence that this erosion was rapid and 

extensive. At present, wave attack of the cliff is minimal, so 

it is now covered with vegetation and there has been little 

erosion during the past century other than from subaerial processes, such as ground-water seepage. Our conclusion was 

that massive cliff erosion occurred following the last major 

Figure 6. The rate of the vertical land movement compared to the 

rising global (eustatic) sea level, based on tide-gauge measurements 

at Crescent City and Astoria, with data between derived from elevation changes of surveyed bench marks. (From Vincent, 1989, modifi ed 

by Komar and Shih, 1993.)
69

subduction earthquake 300 years ago, when the land subsided 

relative to the sea, promoting direct wave attack of the cliff. 

The subsequent rise of the land compared with the sea, evident 

in fi gure 6, has slowly elevated the cliff, so that with time, the 

wave swash has been less able to reach its base to produce additional erosion (Komar and others, 1991). A similar history 

can be seen in the cliff geomorphology along much of the 

Oregon coast south of Florence and also to the north of Cannon Beach at the northern most stretch of the Oregon coast, 

areas where coastal uplift exceeds the rise in sea level. From 

Florence north to Cannon Beach, where the land is being submerged by the rising sea (fi g. 6), active sea-cliff erosion continues, affecting coastal properties; this impact is illustrated 

by fi gures 2 and 3. Komar and Shih (1993) concluded that this 

tectonic control on land-elevation changes and local rates of 

relative sea-level rise is a fi rst-order factor in governing rates 

of sea-cliff erosion along the Oregon coast, but that secondorder factors, such as the volumes of sand on the beaches 

within the individual littoral cells, also exert a strong control 

on the local cliff erosion.

Factors Important to Localized Sea-Cliff 

Erosion

Most of the research undertaken to investigate occurrences and processes of sea-cliff erosion on the Oregon coast 

focused on the series of littoral cells in fi gure 5 along the 

northern half of the coast. This is the most developed stretch 

of coast, so the erosion impacts have been a signifi cant management issue, a prime inducement to undertaking research 

there. Beyond that, each littoral cell exists as a nearly isolated 

pocket beach bounded by headlands, containing variable quantities of beach sand and experiencing different degrees of wave 

attack and cliff erosion. This variability provides a natural 

“laboratory” in which to investigate the relative roles of wave 

attack versus subaerial processes in the erosion and resulting 

morphology of the sea cliffs.

The erosion of sea cliffs is often viewed as a result of 

waves attacking and undermining the cliff, which in turn triggers landsliding or sloughing of its upper unstable portion. 

This view is oversimplifi ed in that a number of processes can 

be involved, and various responses of the cliff are possible. 

In addition to the energy of the waves and their runup levels, 

other ocean processes include tides and the mean level of the 

sea, processes that combine to determine elevations of the 

water against the cliff and hence the position of wave attack. 

The extent of the fronting beach controls the degree to which 

it acts as a buffer between the waves and cliff. Relevant to 

its buffering ability may be the presence of a seaward-fl owing rip current that hollows out an embayment into the shore 

so waves are better able to reach the cliff; an example of the 

resulting erosion is shown in fi gure 3. In that example, the 

cliff erosion was limited to four or fi ve lots and two houses, 

the longshore extent of the rip embayment. There can also be 

a variety of subaerial processes that contribute to the cliff erosion, being most signifi cant during the long periods between 

episodes of direct wave attack. On the Oregon coast, the principal subaerial processes are rain wash down the cliff face and 

extensive ground-water seepage. In this natural “laboratory” 

provided by the Oregon coast, from cell to cell, there is a wide 

range in the relative roles of ocean versus subaerial processes 

in sea-cliff erosion, a range that assists in their investigation.

Ocean Processes and Sea-Cliff Erosion

Research documenting the roles of ocean processes in 

property erosion, whether the properties are atop sea cliffs or 

within foredunes, has centered on the model depicted in fi gure 

8 (Shih and others, 1994; Ruggiero and others, 1996, 2001). 

Of interest are the multiple processes that combine to produce 

a total water level that may reach the toe of the sea cliff or 

foredune to produce property erosion. As depicted in fi gure 8, 

the model in essence involves the summation of the predicted 

tide, the various atmospheric and oceanic processes that elFigure 7. The sea cliff at Bandon on the southern Oregon coast, 

where the high rate of land elevation increase exceeds the global rise 

in sea level, reducing the wave attack of the cliff.

Figure 8. Model for the evaluation of the total water level due to the 

summation of the measured tide and wave runup, compared with the 

elevation of the toe of the sea cliff.

 Oregon’s Coastal Cliffs: Processes and Erosion Impacts
70 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

evate measured tides above predicted levels, and fi nally the 

addition of the wave runup on the sloping beach in front of the 

cliff. Although simple in concept, the challenge is to obtain an 

adequate understanding of the various processes that contribute to the total water level, and how these sometimes independent processes can be combined to predict the potentially most 

extreme (for example, 100-year) combination that might occur 

on the Oregon coast, the event that would result in the greatest 

extent of cliff erosion and loss of property.

On the Oregon coast the measured tides can be 1 to 2 m 

above predicted elevations. Part of this difference occurs at 

times of major storms when high winds and low atmospheric 

pressures generate a storm surge. There has been little focused 

research into the generation of storm surges on the coast of the 

Pacifi c Northwest. In analyses of fl ood levels for the Federal 

Emergency Management Agency (FEMA), Dorratcague and 

others (1977) employed a numerical computer model to predict peak storm surge heights, with comparisons to measured 

heights recorded on tide gauges. There was reasonable agreement between predicted and measured levels, which ranged 

from about 0.5 to 1.4 m. Although not specifi cally stated, it 

is likely that the storm-surge height was taken as the difference between the measured and predicted tidal elevations, not 

recognizing that other processes contribute to this difference. 

The storms analyzed by Dorratcague and others (1977) also 

represented the more extreme events during their analyzed 

time period, 1955 to 1975. Allan and Komar (2002) examined 

the characteristics of a series of unusually severe storms that 

occurred during the El Niño winter of 1997–98 and La Niña 

of 1998–99, each of which generated deep-water signifi cant 

wave heights greater than 10 m that, before 1997, had been 

projected to represent the 100-year storm. The most extreme in 

this series of storms in terms of high-wind speeds and generated waves occurred on March 2–3, 1999. If the storm surge 

is defi ned as the measured tidal elevation minus the predicted 

tide, then that storm generated a surge of 1.76 m. However, 

the monthly mean water level was already elevated by 0.2 

m, so the measured tides were higher by that amount even 

without the occurrence of the storm surge. Subtracting that 

amount yields a 1.6-m level that can be directly related to the 

conditions of the storm. Similar analyses by Allan and Komar 

(2002) of the other major storms during the 1997–99 period 

yielded values ranging from 0.15 to 0.35 m for the storm 

surge. These few results indicate that storm surges are small 

compared with those experienced on the U.S. East and Gulf 

Coasts, particularly during hurricanes, with maximum values during the most severe Pacifi c Northwest storms perhaps 

reaching 2.0 m but more commonly being less than 1.0 m. 

Although lower than experienced on other coasts, storm surges 

on the Oregon coast still play an important role in elevating 

mean-water levels so that superimposed waves are better able 

to attack sea cliffs and shore-front properties.

Aside from occurrences of storm surges that generally last 

for only for a few hours to a couple of days, measured tides on 

the Oregon coast typically differ from predicted values by tens 

of centimeters. This difference results from varying water temperatures of the coastal ocean, which alter the water’s density, 

and the geostrophic effects of ocean currents. These water-level factors important to coastal erosion have been documented 

by calculations of monthly mean water levels, determined by 

averaging the measured tides over the span of a month (Huyer 

and others, 1983; Komar and others, 2000; Allan and Komar, 

2002). Figure 9 is the result from Allan and Komar (2002) of 

such an analysis of the tide-gauge record from Yaquina Bay 

at Newport. The curve for the complete 33-year record shows 

that water levels tend to be lowest during the summer, a result 

of coastal upwelling that produces cold, dense water in the 

summer, depressing the mean level of the sea along the coast. 

In the winter, the water is warmer because of the absence of 

upwelling, and its thermal expansion contributes to the elevated water levels. The coastal currents also play a role, with 

the northward direction of the currents affecting the cross-current geostrophic slope of the water’s surface produced by the 

Earth’s rotation, raising water levels to the right of the current 

along the Oregon coast; the stronger the current, the greater 

the rise in the water level.

Included in fi gure 9 are monthly averages for the 1982–83 

and 1997–98 major El Niños, the results demonstrating the occurrence of unusually high mean water levels. This is attributed to the offshore water being abnormally warm during El Niños and the geostrophic effects of stronger northward fl owing 

currents. In contrast, the results in fi gure 9 for the 1998–99 La 

Niña show that the mean water levels nearly returned to their 

long-term averages. The somewhat higher levels in February 

and March 1999 were produced by storm surges of the series 

of extreme storms, rather than by the longer-term processes 

like those associated with El Niños (Allan and Komar, 2002).

The results in fi gure 9 demonstrate that, during an El 

Niño, the monthly mean water levels are elevated by about 0.5 

m; that is, measured tides throughout the winter will be that 

much higher than predicted. On a beach with a slope of 1-in25 (0.04), typical of the Oregon coast, this enhanced water 

level shifts the mean shoreline landward by 12.5 m, increasing 

Figure 9. Monthly mean water levels derived from tide-gauge measurements in Yaquina Bay (Newport), showing the enhanced water 

levels during the 1982-83 and 1997-98 El Niños. (From Allan and Komar, 

2002.)
71

the probability that the swash of storm-generated waves will 

reach sea cliffs backing the beaches, resulting in their erosion. 

If a 1-m-high storm surge is superimposed on this already 

elevated monthly water level, the total enhanced water level 

 of 1.5 m shifts the shoreline landward by 38 m, a substantial 

portion of the widths of most Oregon beaches.

A number of processes therefore determine mean water 

elevations and, thus, measured tides on the Oregon coast, in 

part controlled by climate events such as El Niños. All of these 

processes must be accounted for in the total-water level model, 

depicted in fi gure 8, used to assess occurrences of sea-cliff 

erosion. The other contributing factor is the wave climate, 

more specifi cally the runup levels of those waves when they 

swash up on Oregon beaches. The Pacifi c Northwest is noted 

for the severity of its wave climate, with major winter storms 

generating waves having deep-water signifi cant wave heights 

(the average of the highest one-third of the waves) in excess 

of 7 to 10 m. Wave data from offshore buoys were analyzed by 

Tillotson and Komar (1997) using conventional procedures to 

establish the wave climate. However, soon after the completion 

of that study a series of exceptionally severe storms occurred, 

well above previous experience. On the basis of wave measurements collected through 1996, Tillotson and Komar (1997) and 

Ruggiero and others (1996) had projected that the 100-year 

storm would generate a deep-water signifi cant wave height 

of approximately 10 m. During the El Niño winter of 1997-

98, two storms produced waves that reached or exceeded that 

projected value, and the winter of 1998-99 saw four additional 

events exceeding 10 m, including the most severe storm on 

March 2-3, 1999, that generated deep-water signifi cant wave 

heights of 14 m. Including those recent storms, the projected 

100-year deep-water signifi cant wave height is now placed at 

15.0 m (Allan and Komar, 2001).

The deep-water wave climate affects a range of nearshore 

processes, including those that have a direct role in coastal 

cliff erosion such as wave breaker heights and runup elevations 

of wave-induced swash at the shore (Komar and Allan, 2002). 

A number of studies have demonstrated that wave runup levels depend on both deep-water wave heights and periods, and 

also on the slope of the beach (see review in Komar, 1998). 

This was also found in a fi eld study on the Oregon coast, 

where wave-runup elevations were measured under a range of 

wave conditions, with wave heights and periods measured by 

offshore buoys, and on different beaches in order to include 

a range of beach slopes (Ruggiero and others, 1996, 2001). 

Those measurements served as the basis to establish an equation that can now be used to calculate swash runup elevations 

(its vertical component) from the storm wave heights and periods measured by buoys in deep water offshore.

The model in fi gure 8 adds the water-level factors controlled by the various atmospheric and oceanic processes to 

determine the total water level. The water levels of these individual processes, and thus the total water elevation, vary from 

hour to hour during a storm, change with the season, and are 

affected by climate events such as an El Niño. As depicted in 

fi gure 8, ultimately of importance are occurrences when the 

addition of the elevation of the measured tide ( ET ) and the 

vertical component of the wave runup ( R ) exceeds the elevation of the toe of the sea cliff ( EJ ), its junction with the fronting beach. Accordingly, of interest is a comparison between 

the total water elevation (ET + R ) and the elevation of the 

beach-cliff junction ( EJ ), erosion of the cliff occurring only 

when ET + R > EJ.

This model was used by Allan and Komar (2002) to examine the hour-by-hour variations in water levels on Pacifi c 

Northwest beaches in the major storms that occurred during 

the 1997-98 El Niño and 1998-99 La Niña winters. Figure 

10 shows the analysis for the March 2-3, 1999, storm, the 

most extreme in the series. Included are separate analyses for 

the Washington and Oregon coasts. The upper-most graphs 

include the wave breaker heights and R runup levels, both calculated from deep-water wave heights and periods measured 

by offshore buoys, and assuming a beach slope of 0.04 for the 

calculation of the runup. In terms of the wave conditions it 

is seen that the storm peaked on March 3, 1999. The second 

pair of graphs contains the predicted and measured tides, respectively measured in Yaquina Bay on the mid-Oregon coast 

and in Willapa Bay on the southern Washington coast. The 

difference between the predicted and measured tides largely 

refl ects the generation of the storm surge, which was substantially greater on the Washington coast due to the storm center 

having crossed the shore of central Washington. The fi nal pair 

of graphs in fi gure 10 give the hour-to-hour total water levels, 

ET + R, the combination that is important to property erosion. 

The results illustrate that the combination yielding extreme 

water levels lasted for only a few hours, limiting the duration 

of swash attack of foredunes and sea cliffs. Allan and Komar 

(2002) confi rmed the results of the analysis by comparing 

the calculated total water levels with the elevations of eroded 

dunes that were cut back by the storm. This testing of the model provided support for its use in the management of the coast, 

particularly its application to analyze extreme events such as 

the 100-year storm to assess the potential extent of dune erosion and impacts to sea cliffs.

A similar analysis was undertaken by Ruggiero and others (1996, 2001), adding the measured tides and wave runup 

levels calculated from measured deep-water wave heights and 

periods, but spanning the entire 15-year period from 1981 to 

1996 to derive long-term assessments of total water elevations. 

Although one can evaluate extreme-value projections independently for measured tides, deep-water wave heights and wave 

runup elevations (Komar and others, 2002), thereby obtaining 100-year projections for each process, an interpretation 

is required as to how they are added to yield an extreme total 

water level that by itself represents a 100-year occurrence. The 

calculation of the 15-year time series of total water levels by 

Ruggiero and others (1996, 2001) proceeds directly to projections of its extreme elevations, including its 100-year level. 

Another useful product of this analysis is the graph in fi gure 11 

that predicts the average number of hours of wave impacts per 

year at an elevation , which could be the toe of a sea cliff, its 

junction elevation with the fronting beach as depicted in fi gure 

 Oregon’s Coastal Cliffs: Processes and Erosion Impacts
72 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

8. As expected, the lower the elevation , the greater the number of impact hours per year. The values for wave impacts are 

long-term averages and could range widely from year to year 

depending on the number and intensities of storms or the occurrence of an El Niño.

Ruggiero and others (1996, 2001) tested this analysis approach in a portion of the 20-km-long Newport Littoral Cell on 

the central Oregon coast (fi g. 5), comparing the model assessed 

hours of wave impacts per year to measurements by Oregon’s 

Department of Geology and Mineral Industries of long-term 

cliff recession rates derived from aerial photographs and house 

to cliff-edge surveys. This stretch of sea cliff analyzed by Ruggiero and others is uniformly composed of a medium resistant 

siltstone having a low development of joints (fi g. 12A), so differences in cliff resistance along its length were not a signifi -

cant factor in the comparison between erosion rates and the values of . At the Lost Creek sea-cliff site in fi gure 12A, the curve 

of fi gure 11 predicts that on average there are 90 wave impact 

hours per year, a fairly high value for the Oregon coast, so one 

would expect that this site experiences persistent erosion problems, which is the case. The stretch of sea cliff along the length 

of the littoral cell has a range of toe elevation values from about 

2.6 to 3.8 m relative to the NGVD29 elevation datum, and it is 

seen in the graph of fi gure 12B that there is a linear relationship 

between the average rate of cliff erosion and the value of ; sites 

having the lowest toe elevations have retreated at an average 

rate of about 0.28 m/yr, whereas those with the highest elevations have retreated at about 0.10 m/yr.

Hb

R

Hb

R

ET ET

ET R ET R

R

R

Figure 10. Analyses of the water-level factors during the major storm that occurred on March 2-3, 1999, which produced signifi cant erosion 

along the Washington (right) and Oregon (left) coasts. (From Allan and Komar, 2002.)
73

rate of increase for wave heights off the Washington coast, with 

only a slightly lower rate of increase off the Oregon coast. The 

increase was still smaller offshore from northern California, 

and negligible in central to southern California and in the Gulf 

of Alaska. The fi ndings of increasing wave heights at mid latitudes in the North Pacifi c have been supported by the study of 

Graham and Diaz (2001) of the storm systems, demonstrating 

that the frequencies and intensities of major storms have increased progressively since 1948.

This decadal increase in deep-water wave heights and 

periods off the Pacifi c Northwest has obvious implications to 

coastal erosion impacts, and can be expected to have been a 

signifi cant factor in the increased erosion of foredunes and 

sea cliffs experienced in recent decades. This connection has 

been demonstrated by the analyses of Komar and Allan (2002) 

in the establishment of wave-dependent nearshore-process 

climates, including the wave runup. This is shown in fi gure 13 

for the decadal increases in deep-water wave heights and wave 

runup levels on Pacifi c Northwest beaches, having used the 

runup equation of Ruggiero and others (2001) to calculate the 

swash runup. This progressive increase in the average runup 

has resulted in a horizontal transgression of the mean shoreline 

by about 8 m during the past 25 years, greater than the transgression from the relative rise in sea level along the Northwest 

coast. This shift in the deep-water wave climate and associated 

nearshore processes such as the swash runup is of obvious 

signifi cance to coastal impacts, resulting in the increased erosion of coastal cliffs and dune-backed shores. Its existence and 

possible climate controls must therefore be recognized in the 

management of the Oregon coast.

The Morphodynamics of Oregon Beaches 

and Their Buffer Protection of Sea Cliffs 

From Wave Attack

The beach acts as a buffer between the waves and coastal 

properties in foredunes or atop sea cliffs. In simple terms, 

the volume of sand on the beach and the resulting width of 

Figure 11. Return periods and average wave-impact hours per year 

calculated from 15-year records of measured tides and waves analyzed according to the model in fi gure 8, with the results depending on 

the beach-cliff junction elevation. (After Ruggiero and others, 2001.)

Figure 12. A, The sea cliff at Lost Creek in the Newport Littoral Cell, 

where wave erosion has cut back the Tertiary mudstone. B, The rate 

of sea-cliff retreat along the Newport Littoral Cell versus the average 

hours of wave impact per year assessed from the graph in fi gure 11, 

depending on the local elevation of the cliff toe. (From Ruggiero and 

others, 2001.)

Any assessment of the overall wave climate, as undertaken 

by Tillotson and Komar (1997) with a projection of the 100-

year event as a deep-water signifi cant wave height of 10 m, or 

the graph in fi gure 11 of wave impact hours per year developed 

by Ruggiero and others (1996, 2001), is affected by the recent 

discovery that wave conditions off the Pacifi c Northwest coast 

have been increasing during the past three decades, presumably 

due to a shift in global climate. The seemingly abrupt increase 

in storm severity and generated waves since 1996 induced 

a re-examination of the wave climate of the Oregon coast, a 

study that was then expanded to cover the entire eastern North 

Pacifi c (Allan and Komar, 2000, 2001). This was accomplished 

through analyses of wave data collected during the previous 20 

to 25 years by six deep-water buoys extending from the Gulf of 

Alaska in the north to Point Arguello in south-central California. It was found that there have been progressive increases in 

wave heights and periods at mid-latitudes, reaching a maximum 

 Oregon’s Coastal Cliffs: Processes and Erosion Impacts

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

the dry berm are the main factors in providing protection. 

Also important is the dynamic response of the beach to the 

changing ocean conditions, the beach’s morphodynamics as 

originally defi ned by Wright and Short (1983). The dynamic 

response of the beach depends on both the wave conditions 

and coarseness of its sediment. With an increasing coarseness 

of the sand, the beach tends to steepen, and this increase in 

slope results in higher levels of wave runup and the potential 

for property erosion. More generally, with an increase in grain 

size the morphodynamics classifi cation refl ects the progressive 

change in the beach from dissipative to intermediate to refl ective, a change that depends on the wave conditions as well as 

sediment grain size (Wright and Short, 1983). Within this classifi cation, nearly all beaches on the Oregon coast are fully dissipative, signifying that they have low slopes so waves arriving 

from deep water initially steepen and break well offshore, and 

then cross a wide surf zone as turbulent bores. During a storm, 

the higher waves break still further from the shore and have 

a wider surf zone to cross, enhancing the dissipation of their 

energy.

The morphodynamics of the beach and how this affects 

the buffer protection it offers to coastal properties is illustrated 

by the Lincoln City Littoral Cell (fi g. 14). This cell has been of 

particular interest because of its extensive development, with 

homes and condominiums lining the cliff edge over most of 

its length (fi gs. 2, 3) and with Siletz Spit having experienced 

frequent problems with erosion and property losses (Komar, 

1997). In addition, an unusual feature of this cell advances 

its scientifi c interest — there is a marked longshore variation 

in the coarseness of its sand, and this produces systematic 

longshore changes in the beach morphology, in the nearshore 

processes, and in the resulting factors important to cliff erosion (Shih and Komar, 1994). The beaches toward the central 

to south part of this cell have the coarsest sand, including those 

fronting Siletz Spit and the sea cliffs of Gleneden Beach south 

of the Spit (fi g. 14). Sand sizes decrease somewhat toward the 

south, but particularly along the northern half of the cell, with 

the sand being fi nest in the Roads End area of Lincoln City at 

the far north end of this cell. This systematic longshore variation in grain sizes of the beach sand within this littoral cell 

was found to result from the addition of coarse sand and gravel 

to the beach by sea-cliff erosion in the area of Gleneden Beach 

and then its subsequent dispersal by waves and currents along 

the length of shore. 

The effect of this longshore variation in grain sizes on 

the beach morphology is signifi cant, with the coarse-grained 

beach at Gleneden Beach being relatively steep and intermediate to refl ective in the morphodynamics classifi cation, while 

the beaches along Lincoln City and at Roads End are low in 

slope and fully dissipative. During a storm with high waves, 

the coarser grained beaches fronting Gleneden Beach and SiH

R

Figure 13. Decadal increases in deep-water wave heights 

and swash runup elevations on a beach of slope 1-in-25 (0.04), 

representative of the Oregon coast. (After Komar and Allan, 2002.)

Figure 14. The Lincoln City Littoral Cell and its longshore variation 

in median grain sizes of the beach sand caused by the addition of 

coarse sand at Gleneden Beach from sea-cliff erosion. (From Komar 

and Shih, 1993.)
75

letz Spit respond much faster and with larger profi le changes 

than do the fi ne-grained beaches, which hardly change at all. 

This pattern of contrasting profi le responses is further documented by the seasonal range in profi le elevations shown in 

fi gure 15, derived from the study of Shih and Komar (1994). 

Furthermore, we found that the development of rip-current 

embayments is extremely important on the coarse-sand beaches, and these embayments have been signifi cant in the attack of 

sea cliffs at Gleneden Beach (fi g. 3). Rip-current embayments 

on the fi ne-sand beaches of north Lincoln City are broader in 

their longshore extents, but do not cut as deeply into the beach. 

Cliff retreat in north Lincoln City depends mainly on subaerial 

processes, with the storm waves acting mainly to remove the 

accumulated talus rather than cutting into the cliff base. The 

much larger profi le shifts and development of rip-current embayments at Gleneden Beach make that beach a poor buffer 

between the waves and sea cliff, so cliff erosion has been signifi cantly greater than to the north where the cliffs are fronted 

by low sloping, dissipative beaches. This enhanced cliff erosion at Gleneden Beach contributes additional coarse sand to 

the beach, maintaining its dynamic response to storms and the 

capacity for the ocean processes to further attack the cliff.

When one considers the other littoral cells along the 

Oregon coast (fi g. 5), the total volume of sand on the beach 

becomes important to the degree of sea-cliff erosion, in addition to the beach’s morphodynamic response at times of 

severe storms. This cell-to-cell variability was analyzed by 

Ruggiero and others (2001) through evaluations of the wave 

impact hours per year using the graph in fi gure 11, the value 

depending on the beach-cliff junction elevation, which refl ects 

the buffering ability of the fronting beach. These coast-wide 

comparisons substantiated that land-elevation changes relative 

to sea level as given in fi gure 6 are important in governing the 

wave impact hours and resulting degrees of sea-cliff erosion, 

as are the buffering capacities of the fronting beaches within 

the individual littoral cells.

Cliff Resistance to Erosion

Of obvious importance to the degree of cliff erosion is the 

inherent resistance of the cliff material to wave attack. This 

factor is well illustrated on the Oregon coast because of the 

extreme variability of rocks exposed in its cliffs, ranging from 

highly resistant basalts to relatively weak Pleistocene terrace 

sandstones, with the Tertiary mudstones and siltstones being 

intermediate but in themselves highly variable depending on 

which rock formation is exposed in the cliff. As discussed earlier, this variability in rock resistance accounts for the irregular 

outline of the Oregon coast, with the resistant basalt forming headlands while the less resistant rocks are eroded into 

embayments. On a coast-wide scale this produces an inverse 

relationship between the degrees of erosion and wave impact 

hours per year, with the headlands having the highest impact 

hours per year but the lowest erosion rates, a trend found quantitatively by Benumof and Griggs (1999) for the coast of San 

Diego County in southern California. The variability of rock 

resistance has similarly governed the long-term evolution of 

the Oregon coast, while at the same time there is a relationship 

between wave impact hours and erosion rates for specifi c rock 

formations as shown in fi gure 12 for the Newport Littoral Cell. 

However, the empirical correlation established by fi gure 12 is 

suitable only for the rock formation exposed in that particular 

cell and generally cannot be applied to other rock formations 

along the coast.

To date, no research has been undertaken along the Oregon coast to systematically and quantitatively investigate 

rock resistance as a factor in governing the rates of sea-cliff 

erosion. In his series of laboratory simulations of cliff erosion 

by waves, Sunamura (1992) began to establish models relating 

cliff erosion rates to the rock resistance, evaluated by strengthof-material tests (for example, the compressive strength of the 

rock). In a comprehensive study of sea-cliff erosion in southFigure 15. Seasonal changes in profi les along the length of the Lincoln City Littoral Cell, refl ecting the morphodynamic responses of the 

beaches depending on their grain sizes. (From Shih and Komar, 1994.)

 Oregon’s Coastal Cliffs: Processes and Erosion Impacts
76 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

ern California, Benumof and Griggs (1999) demonstrated 

through empirical correlations that cliff-erosion rates are governed not only by the inherent resistance of the rock as measured by the compressive strength of an intact rock sample, but 

also by the degree of faulting, fracturing, and bedding within 

the cliff as a whole. Using analysis procedures to quantify all 

of these factors governing the cliff resistance, they were able 

to establish signifi cant correlations with measured cliff recession rates. Research such as this is needed on the Oregon coast 

if the correlation seen in fi gure 12 for the Newport Littoral 

Cell is to be expanded in its coast-wide application.

Thus far the only research completed on the Oregon coast 

to investigate the structural control of rocks on their erosion by 

waves was that undertaken by Byrne (1963) at Cape Perpetua, 

and it focused on a shore platform rather than a sea cliff. The 

principal set of joints found in the platform is oriented in a 

northwest-southeast direction. Although the greatest wave energy is from the southwest, erosion is still predominantly along 

this joint direction, producing a series of surge channels, crevices, caves, and blow-holes. In general, the irregularities and 

small bays and inlets in the headlands are governed by joints 

and fault-controlled erosion or by dikes and layering within 

the ancient lava.

Subaerial Processes of Sea-Cliff Erosion

The composition of the cliff is also important in so far as 

its ability to accumulate as talus when it is eroded, providing 

partial protection from continued wave attack. If fi ne grained 

the loosened material is carried away, which is generally the 

case for the eroded mudstone and siltstone unless it contains 

resistant layers that break off as individual blocks and remain 

at the cliff toe. The Pleistocene terrace sands are better able to 

accumulate as cliff-base talus, and when the talus is eroded by 

waves it becomes a source of sand for the beach.

The accumulation of talus generally begins soon after 

an episode of wave attack and erosion of the cliff. The waves 

remove the talus that had accumulated since the previous erosion episode, which may have been several years to decades 

earlier. The erosion generally leaves a nearly vertical cliff, so 

during the subsequent week or two there may be active sloughing of the terrace sand and collapse of the mudstone, with 

the rapid development of a new talus accumulation. This can 

involve minor slumps of the cliff, effectively a vertical drop of 

masses of intact sandstone and mudstone. Talus accumulation 

may also result from a host of subaerial processes that become 

important to the recession of the cliff during the long periods 

of time between episodes of wave attack. In many areas of the 

Oregon coast where wave attack is infrequent, the cliff retreat 

is produced mainly by the subaerial processes.

It is apparent that on the Oregon coast the most important 

subaerial processes that play roles in sea-cliff erosion are the 

direct fall of rain on the cliff face and the seepage of ground 

water, both being at a maximum during the high-precipitation 

months of the winter. As noted earlier, many sea cliffs have 

composite compositions with the less resistant and more porous Pleistocene terrace sandstones overlying the resistant and 

nearly impermeable Tertiary mudstones and siltstones. This 

layering concentrates the ground-water seepage at the base 

of the Pleistocene sandstone, the top of the mudstone, cutting 

back the sandstone by sapping while producing rills down the 

surface of the mudstone and continuing across whatever talus 

has accumulated.

The extent of talus accumulation and its degree of vegetation cover provide evidence for how recently the site has 

experienced wave attack. The absence of talus implies a recent 

episode of wave erosion, that is if the cliff materials are suitable for the development of talus. Where the fronting beach 

is narrow, wave erosion may occur each winter so that only 

minor talus accumulations are found during the summer. Such 

areas are generally those that have the highest wave impact 

hours per year and the maximum rates of cliff recession. In 

other areas the wave attack is so infrequent that the talus may 

accumulate over several years or even decades, permitting the 

development of a vegetation cover including the growth of 

small trees. In the extreme, vegetation may completely cover 

the bluff, both the cliff face and accumulated talus as seen at 

Bandon in fi gure 7. This vegetation cover can be important in 

reducing erosion in that it protects the cliff from the attack of 

winter rains and may also resist sapping and rill formation by 

ground water. The amount of talus accumulation and degree 

of vegetation cover are observed to vary with a regular pattern 

along the coast, following the trend of relative sea-level rise 

graphed in fi gure 6 as the fi rst-order control, with the buffering 

capacities of beaches within individual littoral cells representing a second-order factor (Komar and Shih, 1993).

One unusual form of subaerial sea-cliff erosion results 

from people carving graffi ti or even caves into the exposed 

cliff (fi g. 16). Although this may seem insignifi cant, on the 

Oregon coast where natural cliff recession rates are generally 

small, such human impacts can be the primary cause of cliff 

erosion. A general observation is that rates of sea-cliff retreat 

are enhanced in parks such as Roads End State Park shown in 

fi gure 16, the park providing greater access to the public, and 

with the human-induced erosion gradually diminishing with 

along-shore distance from the park.

Cliffs Instabilities and Landslides

Large-scale landslides are common along the Oregon 

coast, and their occurrence has damaged homes, parks, and 

highways. The largest are found on headlands, or more precisely, within the loose debris shed from the basaltic headlands 

that has accumulated along their fl anks. An example is the 

huge landslide on Cascade Head (fi g. 17), that abruptly gave 

way in 1934 (North and Byrne, 1965). Wave action has cut 

away its toe, forming a high cliff in the debris. Such massive 

landslides associated with headlands have affected a few private homes, but in particular park lands such as Ecola State 

Park on Tillamook Head immediately north of Cannon Beach. 
77

The large landslides that cross the park become active every 

few years, disrupting access roads and other facilities (Schlicker and others, 1961; Byrne, 1963). A number of large inactive 

landslides are found along the coast, which are believed to 

have formed at the time of the last subduction earthquake in 

1700. Although largely inactive since that extreme tectonic 

event, a few landslides have experienced renewed movement 

in recent decades when the forest cover was removed by commercial logging.

Sea cliffs cut into the marine terraces found along the 

coast are particularly susceptible to mass movement on various 

scales, with the occasional formation of large landslides. This 

susceptibility is due in large part to the muddy consistency 

of the cliffs that are composed of Tertiary mudstones. Landsliding is found to be most active where those deposits dip 

markedly in the seaward direction. It has been estimated that 

seaward-dipping rocks are present along more than half of the 

northern Oregon coast as a result of its tectonic history (Byrne, 1964; North and Byrne, 1965).

The importance of the cliff composition and slope of its 

layers to the inception of landslides is illustrated by occurrences of mass movement in the Newport and Beverly Beach 

Littoral Cells. The sea cliffs there consist mainly of Tertiary 

mudstones that locally dip seaward at 30° in the Nye Beach 

area of Newport. This is the site of the infamous Jump-Off 

Joe landslide that has been destructive to coastal properties 

(Sayre and Komar, 1988; Komar, 1997). Its initial movement 

began during the winter of 1942-43, affecting about 15 acres 

and 15 houses. Several houses were immediately destroyed 

by the ground movement, but a few remained intact on the 

down-drop block of the landslide as seen in the photograph 

of fi gure 18 taken in 1961; they eventually succumbed to toe 

erosion produced by waves. Jump-Off Joe was recognized as 

the best example on the Oregon coast of instabilities leading 

to landsliding, and it also had the highest rate of cliff recession found anywhere on the coast, so it was a surprise when 

an attempt was made to develop the site in 1980. Initially the 

developer planned to build condominiums on the down-drop 

landslide block, but was prevented from doing so when the 

State rejected their request to construct a sea wall along the toe 

of the slide to prevent its further erosion. Instead, the developer constructed the condos on the small remnant of marine 

terrace to the immediate north of the Jump-Off Joe landslide 

(fi g. 19), beyond which was a second and older landslide of 

comparable size. As the condominiums approached completion, slippage in this remnant terrace undermined the condo’s 

foundation, leading to its destruction (Sayre and Komar, 1988; 

Komar, 1997).

Landsliding is also important in the Beverly Beach Littoral Cell, to the north of Yaquina Head which separates that 

cell from the Newport Littoral Cell. The landsliding there has 

been less catastrophic compared with that in the Newport cell, 

in part because the layers within the mudstones have lower 

seaward dip angles. Even so, mass movement has been a signifi cant management problem. Figure 20 shows the Stratford 

Estates development that was to include a number of homes, 

Figure 16. Graffi ti and caves carved into the sea cliff at Roads End 

State Park in north Lincoln City.

Figure 17. A large landslide on Cascade Head north of Lincoln City, 

which initially moved in 1934. (Photo by John V. Byrne.)

Figure 18. The 1942-43 landslide at Jump-Off Joe in Newport as 

photographed in 1961, with surviving houses on the slide block still 

inhabited. (Photo from the Lincoln County Historical Society, Newport.)

 Oregon’s Coastal Cliffs: Processes and Erosion Impacts
78 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

from newspaper accounts, there being a close parallel with 

the monthly variations in rainfall. This is not surprising since 

rainfall and the amount of ground water are primary agents in 

the generation of landslides. However, there is also a parallel 

increase in the ocean wave activity, so there may be some contribution by waves undercutting cliffs, increasing their instability during the winter months.

Summary and Discussion

The objective of this chapter has been to summarize what 

is known about sea-cliff erosion on the Oregon coast. This 

is an important issue in that the erosion of the State’s rocky 

shores affect private homes, parks, and infrastructure such 

as coastal highways. In particular, many of Oregon’s coastal 

communities are situated on the nearly level ground of marine 

terraces, which are ideal for development but whose seaward 

edges are being cut away by the attack of winter storms. Occasionally the eroded cliffs become unstable, leading to largescale landsliding like that at Jump-Off Joe, abruptly bringing 

destruction to people’s homes. Sea-cliff retreat and its associated landsliding have impacted hundreds of kilometers of the 

Oregon coast, costing the State and its citizens millions of 

dollars.

The management of Oregon’s rocky shores to reduce 

such losses is diffi cult due to the extreme variability of the 

coast, there being a wide range of rock types having different 

susceptibilities to wave erosion and contrasting degrees of 

wave attack depending on rates of land-elevation change relative to the global increase in sea level, and due to local factors 

such as the extent of the fronting beach that buffers the cliff 

from the erosive processes of the sea. Because of these variations along the coast, generally each site has to be inspected 

individually to document its past history of cliff retreat and its 

present susceptibility under today’s environmental conditions. 

This clearly demands a major effort directed towards the satisfactory management of Oregon’s rocky shores. To date this 

management has seen mixed results, with policies and procedures varying from community to community. In some areas 

setback lines have been established, but with their placement 

being subjective rather than rationally based on what is known 

about the processes and factors that govern the erosion of sea 

cliffs. The resulting erosion has led to a proliferation of shoreprotection structures, seawalls and revetments, particularly in 

the Lincoln City Littoral Cell (Good, 1994). This proliferation 

is of concern in that cliff erosion of the Pleistocene terrace 

sands represents the primary, and sometimes the sole source 

of new sand to the beach, a source that is cut off by the structures. 

Although our understanding of the processes and factors 

important to sea-cliff erosion along the Oregon coast remains 

incomplete, the most important components are reasonably 

well understood and can be used to more rationally guide the 

management of Oregon’s rocky shores for the safer development of properties atop sea cliffs.

Figure 19. The condominiums built in 1981 on the remnant terrace 

immediately north of the Jump-Off Joe landslide in Newport. (From 

Sayre and Komar, 1988.)

Figure 20. The disruption of streets and sewers in the Stratford 

Estates development north of Yaquina Head (fi g. 5), resulting from a 

slowly moving landslide. (From Komar, 1997.) 

but streets and sewers placed close to the cliff edge were soon 

destroyed by ground movement. Instead, the site became a 

recreational vehicle park with the facilities located back from 

the cliff edge and beyond the area of instability. Highway 101 

is also in close proximity to the cliff edge along much of the 

Beverly Beach cell, and has similarly been affected by mass 

movement, requiring its repair each spring after movement 

during the winter. A study is underway to determine if the 

highway can be protected from further erosion and to reduce 

the mass movement, or whether the highway should be relocated to an inland position to escape these problems.

In spite of landsliding being a signifi cant management 

problem on the Oregon coast, there has been minimal scientifi c and engineering study of its cause. It is clear that the 

initiation and movement of landslides occurs mainly during 

the wet winter months. This was shown by the study of Byrne 

(1963) of the numbers of landslides per month as compiled 
79

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a transgressing sea: Journal of Sedimentary Petrology, v. 

56, p. 15-22.

Darienzo, M.E., and Peterson, C.D., 1990, Episodic tectonic 

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central Cascadia margin: Tectonics, v. 9, p. 1-22.

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Good, J.W.,1994, Shore protection policy and practices in Oregon; 

an evaluation of implementation success: Coastal Management, v. 22, p. 325-352.

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Niño: Coastal Oceanography and Climatology News, v. 

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 Oregon’s Coastal Cliffs: Processes and Erosion Impacts
80 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

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81

Introduction

Puget Sound and Georgia Strait form an inland sea that 

straddles the U.S.-Canadian border and that lies within a 

north-south trending depression separated from the Pacifi c 

Ocean by the Olympic Mountains and Vancouver Island. Of 

Washington State’s approximately 4,000 kilometers of marine 

shoreline, more than 3,400 occur within this inland sea (fi g. 

1). Coastal bluffs are the most common landform encountered on this shoreline.

Rapid population growth in the region has greatly increased development along the shoreline. With the low-lying 

river deltas already developed as ports and cities by early in 

the past century, much of the current pressure is taking the 

form of residential construction along the bluffs. Unfortunately, much of this development occurs with little awareness 

of the risks associated with erosion and landsliding, the costs 

of successfully mitigating the bluff hazards, or the role of the 

bluffs in maintaining both the geological and biological integrity of Puget Sound’s beaches and ecology.

Previous work on the geology of Puget Sound bluffs appears in broader discussions of coastal geomorphological 

processes (Downing, 1983; Terich, 1987) and Puget Sound 

oceanography and geology (Burns, 1985), in descriptions of 

landslide hazards (Thorsen, 1989; Gerstel and others, 1997; 

Shipman, 2001), or studies and maps of regional geology 

(Easterbrook, 1994; Washington Department of Ecology, 

1978-80). The purpose of this chapter is to review current 

knowledge of the distribution and character of coastal bluffs 

on Puget Sound and the processes that shape them.

Geologic Setting

Western Washington lies on the tectonically active 

western margin of North America. Subduction of the Juan 

de Fuca Plate under the continent has resulted in the formation of the Cascade volcanoes and regional deformation 

that causes uplift of the ocean coastline and subsidence of 

the Puget Lowland. The late Pleistocene sediments of the 

Puget Lowland are underlain by a complex series of faultcontrolled bedrock basins.The Puget Lowland has been 

repeatedly occupied by glaciers that have advanced from the 

north, the most recent of which was the Puget Lobe during 

the Vashon advance between 15,000 and 13,000 years ago 

(Booth, 1994). The ice extended south of Olympia in the 

Puget Sound and a separate lobe extended westward along 

the Strait of Juan de Fuca. The surfi cial geology of the Puget 

Lowland largely refl ects the infl uence of this last glacial advance.

Holocene sea level history has differed regionally due 

to large variations in the rates and magnitude of isostatic 

rebound in the early Holocene and due to gradual tectonic 

tilting (Shipman, 1990). Currently, the southern part of Puget 

Sound is submerging as much as 3 mm/yr, whereas the northern Puget Lowland remains relatively stable with respect to 

global sea level.

Puget Sound consists of a complex network of deep 

linear basins (more than 200 m in places) and its coastline 

is characterized by a narrow shore platform. The mean tidal 

range increases from 2 m near Port Angeles to 4 m at Olympia. Beaches are composed primarily of gravel, though variability is high, refl ecting differences in sediment sources and 

complex redistribution of sediment by waves and longshore 

Coastal Bluffs and Sea Cliffs on Puget Sound, 

Washington

By Hugh Shipman

Figure 1. Map of Western Washington, showing Puget Sound, the 

Georgia Strait, and the Strait of Juan de Fuca. The greatest urban 

development occurs in the Tacoma-Seattle-Everett corridor.

 U.S. Geological Survey Professional Paper 1693
82 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

currents. The highly convoluted shoreline results in rapid 

changes in shoreline orientation and in the compartmentalizing 

of longshore drift (Schwartz and others, 1989).

Extent and Distribution of Bluffs and Sea 

Cliffs in Washington

Coastal bluffs occur throughout the Puget Lowland. No 

systematic geomorphologic classifi cation of the shoreline is 

available that would allow a reliable calculation of the length 

of shoreline characterized by coastal bluffs. The best estimate 

of the distribution of coastal bluffs comes from mapping of 

slope stability in the 1970s (table 1), but these fi gures emphasize unstable slopes and readily identifi able landslides and under-represent the total extent of coastal bluffs (and completely 

ignore rocky shores and bedrock sea cliffs). In northern 

portions of the Puget Lowland (Skagit, San Juan, and Whatcom Counties), the relative proportion of coastal bluffs along 

the shoreline decreases due to the greater extent of bedrock 

shores, the larger proportion of depositional beaches and spits, 

and the presence of several large river deltas.

Formation of Coastal Bluffs

Puget Sound’s coastal bluffs are relatively recent geologic 

features, having formed only since glaciers retreated from the 

region 14,000 years ago. In fact, bluffs are believed to have 

largely developed only after sea level reached its current levels about 5,000 years ago and the modern shoreline began to 

evolve. The widespread occurrence of bluffs on Puget Sound 

is a direct consequence of the shaping of the landscape by the 

last glaciation, which deposited an extensive blanket of poorly 

consolidated sediment across the region at elevations above 

modern sea level and which left a complex system of deep 

channels that has resulted in a very long, convoluted shoreline.

Booth (1994) observed that the overall elevation of the 

upland surface within the Puget Lowland was established by 

the deposition of a broad outwash plain in front of the advancing ice. This surface was subsequently modifi ed by the 

passage of the glacier, which left a relatively thin, but highly 

irregular layer of till and recessional deposits on the outwash 

surface. Post-glacial erosion and redeposition, by both fl uvial 

and hillslope processes, further modifi ed this landscape, but 

in general, the 100-150 m elevation of much of the Lowland 

still refl ects the original outwash surface.

Whereas the deposition of sediments above modern sea 

level set the stage for the formation of the coastal bluffs, the 

length of the shoreline and the extensiveness of the bluffs is 

related to the reach of marine waters far into the Puget Lowland by a complex network of deep troughs. Most of these 

troughs are believed to have been formed as subglacial meltwater channels (Booth, 1994). These interconnected, northsouth trending basins dominate the modern landscape.

County Miles of

Shorelines

Miles

Unstable

Percentage

Island 221 112 51%

Jefferson 195 81 42

King 113 66 58

Kitsap 246 50 20

Mason 218 96 44

Pierce 232 72 31

San Juan 376 13

Skagit 189 46 24

Snohomish 19 26

Thurston 111 50 45

Whatcom 118 36 30

TOTAL 2093 641 31%

Modified from Downing, 1983. Data from Washington Department of Ecology, 1978-1980.

74

3

Table 1. Miles of shoreline mapped as unstable in the Coastal Zone 

Atlas of Washington (Washington Department of Ecology, 1978-1980). 

(Figures do not include Clallam County along the Strait of Juan de 

Fuca).

In the northern part of the Puget Lowland, this simple 

picture of an outwash plain dissected by deep meltwater channels becomes more complicated. Complex isostatic rebound 

and sea level history, widespread deposition of glacial marine 

drift, and the abundance of bedrock terrain left a more variable landscape than farther south. In addition, the expansion 

of several large deltas at the base of rivers draining the Cascades has modifi ed large portions of the northeastern coastline of Puget Sound.

Rocky shorelines are common in many portions of the 

northern sound where bedrock is exposed at the coast. Steep 

cliffs are not unusual, but these features are rarely actively 

eroding sea cliffs (fi g. 2). Rather, they represent glacially 

scoured knobs and hills of moderate relief that have experienced little marine erosion due to their resistant lithologies 

and the modest wave energy of the sound. Marine erosion 

may have removed a veneer of glacial sediment, but the resisFigure 2. Basalt sea cliff in the San Juan Islands of northern Puget 

Sound. The base of the cliff is marked by a narrow erosional ramp. 

Erosion rates here are negligible, with the exception of rare block falls.
83

tant bedrock has undergone little change, except possibly the 

formation of a narrow ramp on higher energy shorelines or in 

more erodible lithologies.

Composition of Coastal Bluffs

Coastal bluffs along Puget Sound have eroded into a sequence of late Pleistocene glacial and interglacial sediments, 

most of it consisting of glacial drift deposited during the latest 

(Vashon) advance. Where pre-Vashon units are exposed at sea 

level, they are typically interglacial sediments or in some locations, drift from earlier glaciations (Easterbrook, 1986, 1994). 

The Vashon-age drift commonly consists of older lakebed silts 

and clays deposited in pro-glacial lakes (the Lawton Clay in 

central Puget Sound), a thick package of advance outwash 

sands and gravels (locally referred to as Esperance sand), and 

a capping glacial till (Vashon Till). In some locations, the 

till is overlain by glacial marine drift, recessional outwash, or 

post-glacial lake sediments. 

Although the Vashon-age geologic units are widely distributed within the Puget Lowland, they exhibit signifi cant 

spatial variability in thickness, elevation, and composition. 

This heterogeneity leads to rapid lateral variation in geologic 

characteristics such as hydrology, mass-wasting, and erodibility, and therefore the character of the bluffs can change 

over distances as short as hundreds of meters. This spatial 

variability, the diffi culty in distinguishing Vashon-age deposits 

from earlier glacial sediments, and limited exposures due to 

colluvial cover and heavy vegetation, makes detailed mapping 

of geology diffi cult. Inferences about stratigraphy, even where 

adjacent outcrops are relatively close, are often inaccurate.

Glacial till is usually highly resistant to erosion and typically forms steeper cliffs and slopes. Glacial marine drift 

resembles till in its poorly-sorted character and its tendency to 

form steep faces but was not compacted by overriding ice and 

is generally less resistant to erosion than the till. Vashon-age 

advance outwash deposits and pre-Vashon fl uvial sediments 

show modest consolidation but vary in their resistance to erosion and slope-forming properties depending on texture, hydrology, and other factors. Recessional gravels that have not 

been overridden by ice are typically very poorly consolidated, 

erode quickly, and often form angle-of-repose slopes.

Morphology of Coastal Bluffs

The height and shape of bluffs on Puget Sound can vary 

greatly due to differences in upland relief, geologic composition 

and stratigraphy, hydrology, orientation and exposure, erosion 

rates, mass-wasting mechanisms, and vegetation (fi g. 3). Many 

of these factors are interrelated and can change rapidly along 

the shoreline, leading to diverse bluff morphologies along fairly 

short reaches.

Bluff heights range from less than a few meters to over 100 

m, depending on the elevation of the upland surface into which 

the shoreline has advanced. Low banks and bluffs occur where 

relief is low or where shoreline retreat has only cut into the lowest portion of a more gradual slope. Higher bluffs generally occur where substantial retreat has occurred in areas of high relief.

Bluff Profi le

Bluff profi les refl ect a complex combination of lithology, 

toe erosion, and upslope mass-wasting. The simplest bluffs 

are those dominated by a single lithology and a single erosional process. High bluffs of glacial outwash gravel on the west 

side of Whidbey Island and at Cattle Point on San Juan Island 

form remarkably uniform slopes at the angle of repose of the 

unconsolidated material (fi g. 3A). In contrast, bluffs consisting solely of glacial till or marine drift may form near-vertical 

banks (fi g. 3C).

Most bluffs, however, are cut through a sequence of sedimentary units, each with distinct slope-forming properties. 

This can lead to complex bluff profi les containing both steep 

and gradual segments (fi g. 4), depending on the lithologic, 

hydrologic, and mechanical properties of individual units. 

Poorly consolidated sands and gravels become slope-forming 

units, whereas glacial till and lacustrine silts and clays are often cliff-forming.

The presence of distinct stratigraphic elements also impacts hydrologic characteristics that infl uence mass-wasting 

mechanisms, leading to more complex profi les. For example, 

many high bluffs on Puget Sound are marked by a mid-slope 

bench that forms at the contact between permeable advance 

outwash deposits and underlying impermeable lakebed clays. 

Saturation along this contact drives upslope failures that result in more rapid retreat of the top of the slope than the base, 

causing the bench to widen. These benches, which can vary 

from a few meters to 100 m in width, may exhibit highly irregular topography as a result of their origin in landslides from 

the upper cliff (fi g. 5).

Erosion Processes on Coastal Bluffs

The general model of bluff recession involves a cyclic 

process by which wave action removes material at the toe 

of the slope creating an unstable bluff profi le that eventually 

leads to mass-wasting and the delivery of new material to 

the base of the slope. On Puget Sound, this process is 

complex—adjacent segments of the shoreline may be at 

different stages in the cycle, the mechanisms of erosion and 

mass-wasting may differ over short distances, and the time 

scales and frequencies which control toe erosion may be 

different than those that control slope processes. Regardless, 

erosion mechanisms can be divided into those that are best 

distinguished as related to wave action and toe erosion and 

those that are related to hillslope processes. The former affect 

the long-term retreat of the bluff, whereas the latter affect the 

shape of the bluff.

Coastal Bluffs and Sea Cliffs on Puget Sound, Washington
84 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Wave-Induced Erosion

Waves can directly erode either in-place geologic materials exposed at the toe of the slope or they can erode colluvial 

materials delivered to the beach by mass-wasting. Although 

some wave-induced erosion appears to involve direct mechanical plucking or abrasion of blocks of material, most sedimentary units appear to erode as a result of repeated wetting and 

disaggregation of more coherent materials until waves can 

simply wash away the granular detritus. Some direct erosion 

has been attributed to battering by fl oating logs, which are 

abundant on Puget Sound beaches. In many situations, however, woody debris is believed to actually protect the toe from 

wave attack.

The width and height of the beach and berm control the 

frequency with which the toe of the bluff can be directly attacked by waves. Most bluffs on Puget Sound rise behind 

narrow sand and gravel beaches (fi g. 6). Berm width depends 

primarily on sediment availability, whereas berm height 

depends on tide range, wave exposure, and sediment type. 

Berm crests typically form about one-half meter above Mean 

Higher High Water (MHHW). For waves to directly attack 

the bluff toe, water levels must either exceed the height of the 

berm, which requires storm waves to coincide with unusually 

high tides, or the berm itself must be eroded away.

Hillslope Processes

Raveling.—Poorly consolidated deposits of glacial outwash sands and gravels may erode primarily through raveling 

of the bluff face (fi g. 3A). Failures tend to be progressive, 

beginning with undercutting at the toe by wave action, then 

gradually expanding upslope. Raveling tends to occur in areas where loose sediments are eroding rapidly enough so that 

vegetation cannot become established or in areas that for other 

Figure 3. Examples of coastal bluffs from different parts of Puget Sound. A, High bluffs composed entirely of poorly consolidated recessional 

outwash gravels. B, 100-m high bluff in Tacoma consisting primarily of advance glacial outwash. Vegetation establishes rapidly after periodic 

failures. C, 15-m bluff in southern Puget Sound. Compact glacial sediments form near-vertical face; vegetation occurs along top of bluff and 

on colluvial material at toe of slope. D, Upper portion of these 40-m bluffs in northern Puget Sound are gradually sloped and heavily vegetated, 

whereas lower slopes are steeper and more exposed. 
85

reasons cannot support woody vegetation that would bind soils 

into larger coherent units.

Soil creep.—Soil creep is commonly observed on steep, 

vegetated slopes where discrete slope failures have not occurred. Creep is a slow process and slopes prone to more rapid failures do not tend to remain intact long enough for creep 

phenomena to become signifi cant. Long-term creep can lead to 

the gradual buildup of living roots and woody material at the 

toe of the slope which appears capable of providing signifi cant 

stability to the toe of some slopes.

Block failure.—Coherent geologic units, such as glacial 

till and glacial marine drift, tend to fail as blocks on near-vertical slopes. When basal support is lost, through direct undermining by waves or by erosion of underlying units, failures occur along joints or tension cracks that form parallel to the bluff 

face. Block failures are typically a meter or less in thickness, 

although faces with greater relief seem capable of generating 

thicker failures. Failures often expose planar root mats that 

extend many meters in depth, but the degree to which roots 

and water exacerbate fracture development or simply take advantage of their presence is unknown.

Hydrology. — Both surface runoff and seepage can modify 

bluffs, although the Puget Sound region’s heavy vegetation 

generally limits signifi cant surface erosion to situations where 

runoff has been concentrated by human actions or to locations 

where vegetation has been removed from easily eroded soils, 

such as on a recent landslide. Surface erosion can range from 

the development of small rills on slopes to deep gullies and 

ravines. Groundwater seepage along distinct stratigraphic horizons can result in sapping of sandy soils and the undercutting of 

overlying units. Finally, although freezing temperatures are not 

common along the sound, extended freezes can lead to substantial ice buildup at seepage zones and there is evidence that this 

can precipitate failures either by increasing pore pressures behind the bluff face or by simply loading an already steep slope.

Figure 4. High bluffs near Port Townsend illustrate role of distinct 

stratigraphic units in defi ning bluff profi le. Upper unit of glacial 

till fails in vertical slabs and does not support vegetation. Central 

sandier outwash unit is at angle of repose, with substantial revegetation between erosional events. Lower glacial unit is subject to wave 

action when storms and high tides coincide and when beach volume 

is reduced.

Figure 5. High-resolution image of topography obtained with 

LIDAR (Light Detection and Ranging) along complex coastal 

bluff in central Puget Sound. Mid-slope bench occurs along the 

contact between underlying glacial clays and overlying sandy 

outwash, where hydrologic conditions lead to instability. In some 

locations, the bench itself has been affected by deep-seated 

sliding. (Puget Sound LIDAR Consortium)

Figure 6. Typical mixed sand and gravel beach at base of high 

coastal bluff on Puget Sound. Berm, which consists of sandy material 

overlying a coarse gravel core, protects slope toe from wave action 

except when storm waves coincide with unusually high tides.

Coastal Bluffs and Sea Cliffs on Puget Sound, Washington
86 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Coastal Slope Stability

Most bluff retreat on Puget Sound occurs through landsliding. Slope failures can range from shallow slides a few 

meters across to reactivations of existing, deep-seated landslides many hundreds of meters in size (Thorsen, 1987; Shipman, 2001).

Shallow Landslides

Most landslides that occur on Puget Sound’s bluffs consist 

of shallow landslides and debris avalanches (fi g. 7). Shallow 

landsliding is pervasive along many shoreline reaches, although any one site may slide only once every 30 or 40 years. 

These landslides typically involve only a thin layer (<1-2 m) of 

soil and associated vegetation. Some extend the entire height 

of the bluff, but others only affect a portion of the slope. Shallow failures may occur as small slumps, debris avalanches, or 

as topples of overlying glacial till. Single slides may occur by 

several mechanisms — for example, a block failure of glacial 

till high on a bluff may develop into a debris avalanche as it 

moves downslope.

Slides are usually triggered by heavy rainfall over a period of hours to days (Coe and others, 2000). They are easily 

caused by drainage failures or modest redirection of surface 

drainage. Heavy rainfalls during the winter of 1996-97 led to 

widespread shallow landsliding throughout much of central 

Puget Sound (Baum and others, 1998; Shipman, 2001).

Large Slumps and Landslides

Puget Sound is subject to occasional, much larger, landslides that may involve many tens of thousands of cubic feet 

of material (fi g. 8). These slides are much less common than 

Figure 7. Shallow landslide extending entire height of bluff on Puget 

Sound. Landslide was likely triggered by saturated conditions at contact between lakebed clays and overlying outwash sand (note dark 

band above bottom portion of bluff).

Figure 8. Large landslide north of Seattle that occurred in January 

1997, following heavy rains. Note distinct mid-slope bench to the right 

of the landslide, marking the contact at the base of the glacial outwash. The landslide, which pushed a train into the Sound, involved a 

deeper failure in the underlying clay units. The toe of slope had been 

protected by the railroad grade at beach level for approximately 100 

years and was not involved in the slide.

Figure 9. Large, prehistoric landslide on Whidbey Island. Portions of 

this slide periodically reactivate during wet weather. The toe of this 

landslide occurs slightly below beach level in this area. The landslide 

extends almost 2 km along the shoreline. 

shallower slides, but would be devastating if they occurred in a 

developed area. Understanding of the geologic conditions that 

give rise to these large slides is poor, but such slides seem to 

be associated with higher bluffs and have been triggered both 

by elevated groundwater levels (Arndt, 1999) and by seismic 

activity (Chleborad, 1994).

Prehistoric Landslide Complexes

The Puget Sound shoreline contains many large prehistoric landslides, portions of which may reactivate during particularly wet periods. These slides, which typically consist of 
87

a complex of individual slide blocks, may reach several hundred meters inland and extend more than a kilometer along the 

shoreline (fi g. 9). Movement is often less than a few meters 

during any particular event and may only affect a small area 

of the larger slide complex, though in some cases deep-seated 

movement can trigger additional, shallow slides. Reactivation 

is related to regional ground-water levels and appears to require extended periods of wet weather, possibly extending over 

years (Shipman, 2001).

Factors Affecting Slides

The occurrence of landslides is governed by numerous 

factors, though geology, hydrology, and slope steepness are 

the most signifi cant. Most landslides on Puget Sound occur in 

response to either heavy precipitation or elevated groundwater 

conditions (Thorsen, 1987). Different rainfall regimes may 

lead to different kinds of slides, refl ecting the ability of heavy 

precipitation to saturate shallow soils or of extended wet periods to lead to a rise in regional groundwater levels. During 

the winter of 1996-1997, two major episodes of landsliding 

followed heavy rainfalls, a majority of which were relatively 

shallow failures. In contrast, during the winter of 1998-1999, 

shallow landslides were infrequent, but prolonged wet conditions led to the reactivation of numerous large, deep-seated 

landslides (Shipman, 2001).

The geology of the bluffs affects the geotechnical properties of the bluff soils, but its most signifi cant impact on stability appears to be stratigraphic and hydrologic. Most landslides 

in the region occur where permeable sand and gravel units 

lie directly on top of less permeable silts and clays, allowing 

a perched water table to develop and soils to become locally 

saturated (Tubbs, 1974). The most common scenario is where 

advance outwash overlies proglacial lakebed clay. Groundwater percolates downward in the porous outwash and laterally 

toward the bluff face along the contact with the fi ner grained 

underlying material. When water levels rise, increased pore 

pressures lead to weakness and failure. Similar geologic 

conditions exist where glacial sediments overlie bedrock and 

where recessional outwash is found above impermeable glacial 

till.

Steeper slopes are generally more prone to failure as gravitational stresses are greater, but variations in rock strength 

and differences in hydrologic conditions make it diffi cult to 

predict landslides based on slope alone. On coastal bluffs, 

erosion of the toe by wave action ultimately leads to steepening of the slope and the increasing likelihood of failure, but 

whereas toe erosion is a relatively slow process on most Puget 

Sound bluffs, landslides typically occur in response to transient increases in groundwater or soil saturation. As a result, 

wave action and undercutting may set the stage for future 

slope failures but rarely precipitate landslides. The common 

practice of constructing shoreline bulkheads to prevent coastal 

bluff erosion often overemphasizes the role of waves in determining slope stability.

Earthquakes

The Puget Sound region is a seismically active region, 

but the role of earthquakes on the bluffs is poorly understood. 

Chleborad (1994) describes a large landslide on the Tacoma 

Narrows that is believed to be associated with the 1949 Olympia earthquake (magnitude 7.1). This slide involved as much 

as 100,000 m3 of material and narrowly missed a residential 

community built along the shore. Relatively few landslides 

occurred following the Nisqually Earthquake of February 2000 

(magnitude 6.8). This has been attributed to a dry winter and 

less observed groundshaking than expected.

Karlin and Abella (1992) dated large landslides in Lake 

Washington (east of Seattle) to the last major earthquake on 

the Seattle Fault, about A.D. 1,000, and it is reasonable to 

expect that similar landslides may have also occurred along 

the marine shoreline in the vicinity of the fault (which runs 

east and west across Puget Sound from Seattle to the Bremerton area). Such features may not be as well preserved in the 

more active marine environment, where tidal currents modify 

the submarine deposits of slides and wave action gradually 

removes subaerial exposures. Recent laser-based (LIDAR) 

topographic mapping has identifi ed or confi rmed the presence 

of several large landslide features along the shoreline in close 

proximity to mapped faults.

Rates of Bluff Recession

Long-term bluff recession rates on Puget Sound refl ect 

three primary factors, including wave action, bluff geology, 

and beach characteristics (Shipman, 1995; Washington Department of Ecology, 1978-80; Keuler, 1988). Waves provide the 

energy necessary to erode the toe of the bluff and to remove 

eroded sediment from the site. Geology determines the resistance of the bluff to erosion and its susceptibility to masswasting processes that deliver easily erodible material to the 

base of the slope. The width of the beach and the height of 

the beach berm control the frequency and intensity with which 

waves can reach the bluff toe.

Wave Exposure

Wave action within Puget Sound is generated almost 

exclusively by local storms, as the infl uence of ocean swell 

diminishes rapidly eastward within the Strait of Juan de Fuca. 

Wave energy during storms is related to wind speed and duration and the length of open water across which the wind blows. 

As a result, the exposure of particular sites along Puget Sound 

is a function of their orientation to dominant winds and the local fetch. The relatively small size of waves (compared to open 

ocean waves), combined with the presence of deep water close 

to shore, means that most wave energy reaches the beaches 

and is not dissipated offshore. At extreme high tides, storm 

waves can overtop the beach berm and directly erode the toe of 

Coastal Bluffs and Sea Cliffs on Puget Sound, Washington
88 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

the bluff or colluvial debris deposited at the slope toe by mass 

wasting. 

Geology

Some geologic materials resist the erosive action of waves 

better than others. Erosion rates on rocky shorelines are at 

least an order of magnitude less (Keuler, 1979; Shipman, 

1995) than on shorelines consisting of poorly consolidated 

Pleistocene sediments. More resistant lithologies, such as 

crystalline rocks, well-cemented gravels, and highly indurated 

diamicton (pre-Vashon tills, in particular), often form distinct 

protuberances along the shoreline. Lateral changes in the 

lithology exposed at the toe of the bluff can result in irregularities in shoreline shape (fi g. 10).

Beach Conditions

A wide beach can protect the bluff from wave action. Energy is dissipated over a larger area and in the movement of 

beach materials. Similarly, a high gravel berm can isolate the 

bluff toe from all but the most severe wave events. On Puget 

Sound, drift logs that commonly accumulate on the berm can 

redirect or absorb wave action prior to its reaching the bluff 

face. Beaches vary greatly along the Puget Sound shoreline, 

both in morphology and in sediment type, leading to lateral 

changes in beach height and berm width. This in turn affects 

how waves interact with the bluff toe.

Where beaches are broad, due to a recent infl ux of sediment or proximity to a groin or other drift obstruction, bluff 

erosion may be locally reduced (fi g. 11). Conversely, where 

beaches are starved of sediment due to either natural or artifi cial circumstances, the erosion rate of associated bluffs may 

accelerate. Jacobsen and Schwartz (1981) noted that bluff 

morphology systematically changes through individual littoral 

cells — that beaches generally widen and bluff erosion rates 

decrease in a downdrift direction. In general, on the sound, 

rapid erosion rates are most common on bluffs at the origin of 

littoral cells where beaches are minimal and eroded sediment 

is readily carried away by longshore transport.

Long-term bluff retreat depends on continued downcutting of the nearshore platform (Davidson-Arnott and Ollerhead, 1995; Trenhaile, 1997; Kamphius, 1987). In some locations on Puget Sound, beach sediments are suffi ciently thick 

and continuous that they appear to protect the platform from 

abrasion and scouring, whereas in others, the platform is exposed or only intermittently covered with sediment. Similarly, 

on some shorelines a coarse cobble and boulder lag deposit 

armors the lower intertidal platform, limiting platform erosion 

and therefore bluff retreat rates.

Figure 10. Irregular shoreline along western shore of Whidbey 

Island. Wave exposure is relatively uniform along this reach, and 

shape of shoreline is related to lithology and longshore redistribution 

of sediment by littoral processes. Beach in foreground is a barrier. 

Sharp point in mid-distance occurs where a resistant glacial till unit 

emerges at the toe of the bluff. (Photo by Gerald Thorsen).

Figure 11. Offset in bluffs resulting from the presence of large glacial 

erratic in the nearshore. Wave action and longshore drift are from 

left to right. The boulder acts as a groin—on the left side the beach is 

relative stable, but erosion has been exacerbated on its downdrift side.
89

Rates

Little systematic study of bluff recession rates has been 

carried out within the Puget Sound region, limiting knowledge 

of actual rates and understanding of the relative importance of 

different factors in determining rates. No regular monitoring 

of bluff erosion occurs, although interest has been expressed 

in doing this by local volunteer groups (Thorsen and Shipman, 

1998). Relative erosion rates have been assessed qualitatively, 

typically using bluff steepness or vegetation (Keuler, 1988; 

Washington Department of Ecology, 1978-1980; Terich, 1987), 

but quantitative erosion rates are limited to just a few studies 

(summarized in Shipman, 1995).

Historical aerial photographs have been of limited use for 

evaluating bluff recession rates on Puget Sound. Few good 

photos are available prior to the 1950s. Heavy vegetation 

often obscures both the bluff toe and the top edge of the bluff 

and the highly irregular orientation of the shoreline makes 

consistently good lighting conditions unlikely. As with other 

methods, the slow, but highly episodic, character of bluff recession requires long-term records in order to get suffi cient 

recession distances to document reliably (Keuler, 1988). 

Keuler’s study (1988) of erosion rates in the Port 

Townsend and Whidbey Island areas involved revisiting survey 

monuments for which original descriptions and location information provided clues as to the position of the shoreline, typically the toe or top edge of the bluff. Monuments had often 

been lost or could not be relocated, but where they could be 

found, an erosion rate could be established for time frames that 

in some cases spanned many decades. 

The total amount of late-Holocene bluff recession along 

some shorelines can be estimated from the width of the 

nearshore platform, at least in places where a distinct erosional 

edge to the platform can be reliably identifi ed (Keuler, 1979). 

These platforms range from a tens of meters to hundreds of 

meters in width. Inferring modern rates from platform width 

is problematic, as rates may have changed over time due to the 

widening platform, variation in geology and topography, and 

possible changes in rates of sea level change.

The highest erosion rates measured on Puget Sound and in 

the Georgia Strait occur in poorly consolidated late Pleistocene 

sediments where wave exposure is high. Van Osch (1990) noted bluff recession rates of 60 cm/yr at Cowichan Head north of 

Victoria and 30-50 cm/yr at Point Grey near Vancouver, B.C. 

Galster and Schwartz (1990) found that erosion rates of bluffs 

west of Port Angeles were as much as one meter per year before the shoreline was armored. Keuler (1988) determined 

rates of over 30 cm/yr on Smith Island, the western shore of 

Whidbey Island, and the northern side of Protection Island, all 

with substantial exposures along the Strait of Juan de Fuca.

These rates are not typical, however, and recession rates 

appear more commonly to be on the order of a few centimeters 

a year, or less, in most areas. Rates vary temporally and at 

any given site, retreat is likely to occur as a single mass-wasting event every few decades. Wave erosion is highly episodic, 

driven by combinations of unusually severe storms and high 

tides (or even temporarily elevated sea levels as was observed 

during the 1998 El Niño). Slope failures are also episodic 

and tied to heavy precipitation. Beach fl uctuations that might 

effect bluff erosion can also vary over periods of years due to 

storm conditions or sediment supply variations.

Spatial variability in erosion rates appears remarkably 

high. Even along shorelines with generally similar exposure 

and lithology, rates can vary signifi cantly (Keuler, 1988). We 

believe this refl ects small variations in shoreline orientation 

and beach characteristics, combined with lateral variability in 

the geology exposed on the platform and at the bluff toe (fi g. 

10).

Development on Coastal Bluffs

Pressure to build along coastal bluffs is rising rapidly 

with the increasing population growth and urbanization of the 

Puget Sound region. Much of the shoreline lies within a short 

distance of the major metropolitan centers of Seattle, Tacoma, 

and Everett. The area has seen a signifi cant shift in the character of shoreline development from small seasonal retreats and 

retirement cabins to large primary residences. The style and 

size of new waterfront homes and the extent of landscaping 

is typical of that seen in affl uent suburban developments in 

nonshoreline areas. The demand for waterfront and bluff property is driven primarily by access to the water and unimpeded 

views of the Sound and nearby mountains.

Development along bluffs most commonly occurs at the 

top of the bluff (fi g. 12). The distance a building is set back 

from the bluff edge depends on local regulations, the history 

and age of the structure, the topography of the site, lot lines, 

and the original property owner’s concept of risk and their 

desire for views. Property owners often build as close to the 

Figure 12. Homes built along the top edge of a bluff in southern 

Puget Sound. Setback requirements vary among jurisdictions. The 

desire for views typically leads property owners to build as close to 

the bluff as regulations allow.

Coastal Bluffs and Sea Cliffs on Puget Sound, Washington
90 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

edge as allowed, in large part to maximize views in an otherwise forested area. The risk to bluff top homes is relatively 

low as a consequence of slow erosion rates, although a property owner’s perception of danger may be greatly enhanced by 

periodic landslides or related bluff failures.

In several locations around the sound, development has 

occurred at the base of steep coastal bluffs. In some cases, 

homes are built on spits, stream deltas, or related depositional 

landforms that have accreted waterward of the bluff toe. In 

other cases, development occurs on artifi cial fi ll placed across 

the backshore or beach. On Whidbey and Camano Islands, in 

central Puget Sound, numerous residential developments were 

created in the 1950s and 1960s by constructing bulkheads on 

the beach below a high bluff and then using hydraulic methods to wash bluff material in as landfi ll. The legacy of such 

development is rows of homes at water level, constructed on 

unengineered hydraulic fi ll, and located at the base of unstable 

bluffs 40-70 m high (fi g. 13).

In some locations, homes (and in the case of Tacoma’s 

Salmon Beach, an entire community) were constructed on 

piles over the beach at the base of high bluffs (fi g. 14). Such 

development would not be allowed today for many safety and 

environmental reasons, but where it already exists, we observe 

regular conversion of small cabins into large homes and periodic slide damage to homes.

Although the steeper coastal bluffs largely preclude development on the slopes themselves, development can and 

does occur in less extreme situations. Building is common 

on more gradually sloping portions of complex coastal slopes 

and, in particular, on the mid-slope benches that characterize 

many bluff shorelines. These areas often appear to offer prime 

building sites in otherwise diffi cult to build areas. Unfortunately, these benched slopes often refl ect unstable geologic 

conditions (fi g. 15). Another circumstance where building 

occurs on the slopes themselves is in areas where property 

lines, old unregulated building practices, or modern heavily 

engineered development have led to homes being constructed 

on piles or multilevel foundations either above or into the face 

of steep slopes.

Human Impacts On Bluffs

Humans are in themselves an agent of bluff erosion, at 

least in their capacity to trigger landslides or increase erosion 

through careless or imprudent development practices. The 

occurrence of landslides and the continued erosion of coastal 

bluffs is a natural process, but humans, primarily through their 

propensity to modify natural hydrology, can easily exacerbate 

unstable conditions or trigger slides.

Puget Lowland is a heavily forested area with high precipitation. Surface runoff and subsurface saturation are highly 

Figure 13. Residential development on artifi cial fi ll below high bluffs 

on eastern Whidbey Island.

Figure 14. Salmon Beach community in Tacoma. Homes built over 

beach on piles at base of high bluff are periodically damaged or 

destroyed by landslides.

Figure 15. Homes destroyed by landsliding along Magnolia Bluff in 

Seattle during the winter of 1996-97. Homes had been built on a midslope bench formed by past erosion and landsliding.
91

sensitive to the abundance and type of vegetation. Land 

development and clearing of vegetation can result in changes 

in subsurface hydrology that increase the likelihood of slope 

failures. Collecting runoff in drain systems can reduce localized saturation of bluff soils and thereby increase stability, 

but conveyance systems concentrate fl ow and are subject to 

failure if not designed, constructed, or maintained properly. 

In Seattle, more than 70 percent of slope failures in the heavy 

rainfall events of early 1997 were at least in part due to human actions (Shannon and Wilson, 2000). Less frequently, 

direct modifi cation of the bluff by placing fi ll on the upper 

slope or by excavating into lower portions of the slope triggers failures.

Bluff Stabilization

A wide variety of techniques are employed to stabilize 

coastal bluffs on Puget Sound. Some of these techniques 

address waves and toe erosion, whereas others deal more 

specifi cally with mass-wasting and slope stability. Most bluff 

stabilization and erosion control on Puget Sound occurs on 

residential property, which generally dictates the scale (in 

size and cost) of particular solutions. Increasing property 

values during the last decade have led to an increase in both 

the quality of site evaluations and the sophistication of technical fi xes.

Drainage

The least expensive and most common measure taken by 

bluff top property owners to reduce slope problems is to collect surface drainage from gutters, drives, and French drains 

and to convey it directly to the beach, reducing opportunities 

for surface erosion or saturation of bluff top sediments. On 

residential sites, this is typically done with fl exible pipe and 

is rarely engineered. Such methods can be effective but often 

create new problems when pipes are inadequately designed or 

are not maintained, because failures result in collected fl ows 

discharging directly onto soils high on the bluff.

Increasingly sophisticated drainage measures have been 

employed in recent years, both on private sites and on public 

projects. Vertical dewatering wells (either gravity drained or 

pumped) are occasionally used and the region is seeing an increase in the use of directional drilling to construct horizontal 

drain systems. Variability in subsurface conditions and fl ow 

makes the success of such systems dependent on accurate 

geologic analyses of water bearing strata. Whereas short 

horizontal wells drilled into the bluff face were traditionally 

diffi cult and expensive to construct due to equipment access, 

newer directional drilling techniques allow wells to be drilled 

from several hundred meters landward of a bluff, under any 

structures, and then out the bluff face. This may also better 

facilitate cleanouts and maintenance, a common problem with 

horizontal drains. 

Bulkheads

Shoreline bulkheads are used extensively on Puget Sound 

to address wave-induced toe erosion. Numerous materials 

are used, including concrete, wood, and rock, and a variety of 

designs are employed, including gravity walls, cantilevered 

structures, riprap revetments, and sheet pile walls. Currently, 

the most commonly built structure is a steep rock bulkhead 

or rockery, usually built from readily available basalt. These 

structures are typically less than 2 m high and are required by 

regulations to be located as close to the bluff toe as possible.

The effectiveness of bulkheads varies considerably. The 

wave environment in most of Puget Sound is suffi ciently 

protected that structures need not be massive to address local 

wave conditions, but bulkheads are often seen as a panacea 

for slope stability problems that are only indirectly associated 

with wave action. On many shoreline bluffs, particularly those 

where recent erosion has been notable and where property 

owners are likely to consider bulkheads, the slope is already 

over-steepened and more likely to fail during a heavy rainfall 

than during high wave conditions. Many of the landslides during the heavy rainstorms of 1996-97 occurred on slopes where 

bulkheads had protected the toe for many decades.

Slope Engineering

Although bulkheads are commonly used to protect the toe 

of slopes from wave action, in some cases (for example, after 

a failure of a coastal bluff already protected by a bulkhead) 

property owners have built multiple-stage retaining walls, 

reinforced soil embankments, or have otherwise modifi ed the 

geometry of the entire bluff. In the case of deeper sliding, toe 

buttresses have been constructed, but regulations preventing 

encroachment across the beach increasingly discourage such 

solutions. Biotechnical stabilization methods have received 

much interest in recent years, in part because of their potential 

for addressing slope stability problems in environmentally sensitive areas, but their actual application has been limited.

Management and Regulation

Development along coastal bluffs presents a variety of 

problems for coastal planners and resource managers. These 

range from protecting people from natural hazards to protecting nearshore ecology from the impacts of human land 

use practices. Several regulations affect development along 

coastal bluffs on Puget Sound. The Shoreline Management 

Act (SMA) and the Growth Management Act (GMA) are 

both State laws that provide guidelines under which local 

regulations are developed and implemented. Because local 

governments differ signifi cantly in their approaches to land 

use planning and in their technical capabilities, there is much 

variability in how individual counties and cities manage their 

coastal bluffs, despite the common basis in state-level regulaCoastal Bluffs and Sea Cliffs on Puget Sound, Washington
92 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

tion. Identifi cation of potentially unstable coastal bluffs is 

often guided by maps developed in the 1970s shortly after passage of the State’s Shoreline Management Act (fi g. 16). This 

mapping still provides the basis for many local planning decisions1

.

Construction setbacks are the standard approach for guiding new development away from bluff hazards, but setbacks 

vary considerably between jurisdictions and property owners 

often seek and obtain variances to build closer to bluffs than 

the code recommends. Setbacks can range from arbitrary 

minimums to distances based on the height of the slope. Recent updates to Critical Areas Ordinances (under Growth Management) and Shoreline Master Programs (under the Shoreline 

Management Act) in some jurisdictions have increased setbacks, driven both by renewed awareness of landslide hazards 

brought by the winter of 1997-98 and by greater emphasis on 

protecting shoreline habitat through avoiding development 

that is likely to require shoreline structures in the foreseeable 

future.

Bulkheading of coastal bluffs has become a signifi cant 

management issue in recent years on the sound (Canning and 

Shipman, 1995). The practice has been a standard tool for 

addressing bluff erosion for decades, but increased awareness 

of environmental problems associated with these structures 

(Macdonald and others, 1994) has led to scrutiny of individual 

projects and review of broader policies. In addition, numerous failures of bluffs above existing bulkheads raises questions 

about the effi cacy and safety of these solutions in certain situations. 

Concerns about the environmental impacts of constructing 

bulkheads on coastal bluffs include possible loss of sediment 

supplies to downdrift shorelines, changes in beach profi les and 

beach substrate, modifi cations to riparian vegetation or beach 

hydrology, and simply the gradual loss of the upper beach as 

shoreline retreat continues in front of fi xed structures. Geologically, sediment starvation is the primary issue as most 

Puget Sound beaches are fed by bluff erosion. At Ediz Hook 

in Port Angeles, armoring of eroding bluffs was the major 

cause of extensive beach erosion and expensive mitigation in 

the form of beach feeding with cobble-size material (Galster 

and Schwartz, 1990).

Summary

Much of the Puget Sound shoreline is characterized by 

coastal bluffs cut into poorly consolidated glacial and interglacial sediments. Bluff recession rates are relatively slow, 

in part due to the protected nature of the sound, and erosion 

is dominated by hillslope processes and landslides. Erosion 

rates are controlled by wave exposure, bluff geology, and 

beach characteristics. Because bluff erosion both supplies 

sediment to the beach and is regulated by the condition of the 

beach, a complex relationship exists between bluff and beach 

processes. 

Bluffs are in high demand for residential development 

due to their proximity to the water and their spectacular views. 

The extensive development of coastal bluffs, however, sets 

the stage for serious long-term management problems. Large 

numbers of homes have been constructed in locations that if 

not hazardous now, will be in several decades. In addition, 

engineering measures intended to address bluff erosion pose 

serious implications for the long-term health of the region’s 

beaches.

Research Needs

Remarkably little systematic research has been carried out 

on Puget Sound bluffs, despite their prevalence, the hazards 

associated with their development, and the growing interest in 

the relationship between bluff erosion and nearshore ecological health. Types of research that would be useful include:

• Existing geologic mapping of the sound is outdated 

and often inaccurate. Traditional mapping that 

emphasizes the spatial distribution of units does not 

 1 These maps are available on the Washington 

 Department of Ecology’s Puget Sound Landslides 

 website (http://www.ecy.wa.gov/programs/sea landslides/).

Figure 16. Map of coastal slope stability for residential portion of 

Seattle, from Coastal Zone Atlas of Washington (Washington Department of Ecology, 1978-1980). U indicates Unstable; Urs indicates 

recent landslides (as of late 1970s). Such maps exist for most of 

Puget Sound. 
93

necessarily present stratigraphic information well. 

New mapping, including more detailed examination 

and portrayal of shoreline stratigraphy, is critical to 

understanding coastal bluff processes.

• Recently, high resolution topographic data have 

been collected for much of the Puget Lowland using 

LIDAR (Light Detection and Ranging) technology. 

These data provide valuable information about bluff 

morphology and slope processes that were not available before. Little detailed analysis of these data has 

been carried out so far.

• Erosion rates have been acquired for only a few locations. A long-term monitoring program, coupled with 

detailed studies of specifi c sites, could provide a basis 

for estimating erosion rates throughout the Puget 

Sound region.

• Little is understood of littoral processes, sediment 

budgets, or of shoreline evolution on the sound. 

What information is available is largely qualitative. 

Quantitative, process-oriented studies will greatly 

improve our understanding of the bluffs and their 

change over time.

References

Arndt, B., 1999, Determination of the conditions necessary 

for slope failure of a deep-seated landslide at Woodway, 

Washington: Golden, Colorado, Colorado School of Mines, 

Masters thesis.

Baum, R.L., Chleborad, A.F., and Schuster, R.L., 1998, Landslides triggered by the winter 1996-97 storms in the Puget 

Lowland, Washington: United States Geological Survey 

Open File Report 98-239, 16 p.

Booth, D.B., 1994, Glaciofl uvial infi lling and scour of the Puget 

Lowland, Washington, during ice-sheet glaciation: Geology, v. 22, p. 695-698.

Burns, R., 1985, The shape and form of Puget Sound: Seattle, 

Washington Sea Grant, University of Washington Press, 

100 p.

Canning, D.J. and Shipman, H., 1995, Coastal erosion management studies in Puget Sound, Washington — executive 

summary: Washington Department of Ecology, Shorelands 

and Water Resources Program.

Chleborad, A.F., 1994, Modeling and analysis of the 1949 Narrows Landslide, Tacoma, Washington: Bulletin of the Association of Engineering Geologists, v. 31, p. 305-327.

Coe, J.A., Michael, J.A., Crovelli, R.A., and Savage, W.Z., 2000, 

Preliminary map showing landslide densities, mean recurrence intervals, and exceedance probabilities as determined 

from historic records, Seattle, Washington: United States 

Geological Survey Open File Report 00-303, 25 p., map 

sheet scale 1:25,000.

Davidson-Arnott, R.G.D., and Ollerhead, J., 1995, Nearshore 

erosion on a cohesive shoreline: Marine Geology, v. 122, 

p. 349-365.

Downing, J., 1983, The Coast of Puget Sound — its processes 

and development: Seattle, Washington, Washington Sea 

Grant, University of Washington Press.

Easterbrook, D.J., 1986, Stratigraphy and chronology of Quaternary deposits of the Puget Lowland and Olympic 

Mountains of Washington and the Cascade Mountains of 

Washington and Oregon: Quaternary Science Reviews, v. 

5, p. 145-159.

Easterbrook, D.J., 1994, Chronology of pre-late Wisconsin 

Pleistocene sediments in the Puget Lowland, Washington, 

in Lasmanis, R. and Cheney, E.S., eds., Regional geology 

of Washington State: Washington Division of Geology and 

Earth Resources, Bulletin 80, p. 191-206.

Galster, R.W., and Schwartz, M.L., 1990, Ediz Hook — a case 

history of coastal erosion and rehabilitation: Journal of 

Coastal Research, Special Issue, no. 6, p. 103-113.

Gerstel, W.J., Brunengo, M.J., Lingley, W.S., Jr., Logan, R.L., 

Shipman, H. and Walsh, T.J., 1997, Puget Sound bluffs 

— the where, why, and when of landslides following the 

holiday 1996/97 storms: Washington Geology, v. 25, no. 

1, p. 17-31.

Jacobsen, E.E. and Schwartz, M.L., 1981, The use of geomorphic 

indicators to determine the direction of net shore-drift: 

Shore and Beach, v. 49, p. 38-43.

Kamphuis, J.W., 1987, Recession rate of glacial till bluffs: Journal of Waterway, Port, Coastal and Ocean Engineering, v. 

113, p. 60-73.

Karlin, R.E. and Abella, S.E.B., 1992, Paleoearthquakes in the 

Puget Sound region recorded in sediments from Lake 

Washington: Science, v. 258, p. 1617-1620.

Keuler, R.F., 1979, Coastal zone processes and geomorphology of Skagit County, Washington: Bellingham, Western 

Washington University, Masters thesis.

Keuler, R.F., 1988, Map showing coastal erosion, sediment supply, and longshore transport in the Port Townsend 30- by 

60-minute quadrangle, Puget Sound Region, Washington: 

United States Geological Survey Miscellaneous Investigation I-1198-E.

Macdonald, K., Simpson, D., Paulsen, B., Cox, J., and Gendron, 

J., 1994, Shoreline armoring effects on physical coastal 

processes in Puget Sound, Washington: Shorelands and 

Water Resources Program, Washington Department of 

Ecology.

Schwartz, M.L., Wallace, R.S., and Jacobsen, E.E., 1989, Net 

Shore-Drift in Puget Sound, in Engineering Geology in 

Washington: Washington Division of Geology and Earth 

Resources Bulletin 78, p. 1137-1146.

Shannon and Wilson, Inc., 2000, Seattle landslide study (prepared 

for Seattle Public Utilities): Seattle, Washington.

Shipman, H., 1990, Vertical land movement in coastal Washington: Washington Geologic Newsletter, v. 18, p. 26-33.

Shipman, H., 1995, The rate and character of shoreline erosion 

on Puget Sound: Olympia, Washington, Puget Sound 

Water Quality Action Team, Puget Sound Research ‘95, 

p. 77-83.

Shipman, H., 2001, Coastal landsliding on Puget Sound — a 

Coastal Bluffs and Sea Cliffs on Puget Sound, Washington
94 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

review of landslides occurring between 1996 and 1999: 

Washington Department of Ecology, Shorelands and 

Environmental Assistance Program, Publication No. 01-

06-019.

Terich, T.A., 1987, Living with the Shore of Puget Sound and 

the Georgia Strait: Durham, N.C., Duke University Press, 

165 p.

Thorsen, G.W., 1987, Soil bluffs + rain = slide hazards: Washington Geologic Newsletter, v. 15, p. 3-11.

Thorsen, G.W., 1989, Landslide provinces in Washington, in 

Engineering Geology in Washington: Washington Division 

of Geology and Earth Resources Bulletin 78, p. 71-89.

Thorsen, G., and Shipman, H., 1998, Bluff erosion monitoring 

on Puget Sound — a guide for volunteers (prepared for 

Island County Beach Watchers): Washington Department 

of Ecology, Shorelands and Environmental Assistance 

Program.

Trenhaile, A.S., 1997, Coastal Dynamics and Landforms: Oxford, Oxford University Press, 366 p.

Tubbs, D., 1974, Landslides in Seattle: Department of Natural 

Resources, Division of Geology and Earth Resources, 

Information Circular 52.

Van Osch, E., 1990, Coastal bluff erosion in the Strait of Georgia 

region — geomorphology and management: Vancouver, 

Simon Fraser University, Masters thesis.

Washington Department of Ecology, 1978-1980, Washington 

State Coastal Zone Atlas (12 volumes): Washington Shorelands and Coastal Zone Management Program.
95

Although its shoreline differs in many ways from that of 

the mid-Atlantic and southeastern states, New England is part 

of the United States East Coast passive continental margin. 

South of New England, the outer coast of the United States 

is dominated by barrier islands, but bedrock frames all of 

the coast north of New York City with the exception of Cape 

Cod and nearby islands. The rocks of this region are all part 

of the Appalachian Mountains, which formed primarily during several plate collisions in the early-middle Paleozoic Era. 

North America grew as a consequence of these collisions, 

and the rocks of the New England coast are a patchwork of 

exotic terranes derived from a variety of sources. Following 

the initial formation of the Appalachians, several basins within 

them fi lled with sedimentary rocks in the late Paleozoic. The 

Atlantic Ocean opened up in the early Mesozoic and failed 

rift basins, fi lled with sandstones and basaltic volcanic rocks, 

remain along the coast and offshore from that time. The last 

igneous rocks formed in the late Mesozoic, and the region has 

generally undergone erosion since then.

The weathered products of the erosion of the Appalachian 

Mountains, the coastal plain sediments of the south, are not exposed on the New England coast, and were presumably eroded 

themselves. It is not a coincidence that the latest agents of 

erosion, continental glaciers, covered all of New England, and 

reached only as far south as Long Island, New York. Although 

the glaciers removed the Coastal Plain material, they left in its 

place a heterogeneous assemblage of deposits partly mantling 

the bedrock. Contemporary reworking of these glacial deposits 

by coastal processes provides materials for the highly variable 

modern environments of the New England coastal zone.

The irregular shape of the New England coast is mostly 

due to the structure and differential erosion of its bedrock 

skeleton. Rocks that have most resisted erosion by glaciers 

(igneous rocks, quartzites) tend to form peninsulas, islands, 

and relatively high headlands. More easily eroded rocks (sedimentary rocks and slates/shists) underlie embayments and 

estuaries. Because these rock types are associated with numerous exotic terranes, and often separated by ancient fault zones, 

there is great variation in topography and shoreline orientation 

throughout New England (fi g. 1). Despite this overall heterogeneity, the coast may simply be described as a series of bedrock compartments that are internally relatively homogeneous, 

but distinct from their neighbors (fi g. 1). 

Extending southwest from the Bay of Fundy, Canada, 

the cliffed coastline (fi g. 1) is framed by fault zones, and is a 

relatively high-relief shoreline of igneous rocks with few embayments (Kelley, 1987, 1993; Kelley and others, 1989, 1995, 

2003). As a result of the high rock cliffs, bluffs of unconsolidated sediment are relatively rare. 

Coastal Bluffs of New England

By Joseph T. Kelley

Introduction

Figure 1. Generalized map of the New England coast, with geomorphic compartments of similar bedrock and glacial materials marked 

(modifi ed from Kelley, 1987; Kelley and others, 1995).

 U.S. Geological Survey Professional Paper 1693
96 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Where large granitic bodies crop out in central Maine, the 

island-bay compartment consists of many broad embayments 

underlain by metamorphic rocks that are protected by granitic 

islands (fi gs. 1, 2). South of Penobscot Bay, Maine, layered 

metamorphic rocks of varying resistance to erosion form a 

closely spaced series of narrow peninsulas and estuaries (fi gs. 

1, 3). Bluffs of glacial materials are extremely common in the 

many sheltered coves of these two compartments. Because of 

the highly irregular nature of this stretch of shoreline, it is approximately as long (4,098 km; Kelley, 1987) as the coastlines 

of all of the other New England states combined (Ringold and 

Clark, 1980).

South of Portland, Maine, these rocks abruptly change. 

With the exception of Cape Cod, the rocks from here to Connecticut consist of headlands of low-relief igneous rocks and 

embayments of more deeply eroded sedimentary and metamorphic rocks. Sand beaches are common in the embayments 

of this stretch of coast, and straighten the bedrock outline. The 

Connecticut coast consists of low-lying metamorphic rocks of 

similar resistance to erosion that, as a consequence, provide a 

relatively straight shoreline with few large embayments. Cape 

Cod, Long Island, Block Island, Nantucket, and Marthas Vineyard (and numerous nearby smaller islands) are entirely composed of glacial deposits with no exposed bedrock.

New England has experienced many glaciations during 

the Pleistocene, but deposits from the last event, the Wisconsinan, dominate the coastline. Long Island (New York), Block 

Island (Rhode Island), and Nantucket, Marthas Vineyard and 

Cape Cod (Mass.), along with many smaller nearby islands, 

are large moraines with outwash plains on their southern 

sides (Stone and Borns, 1986; Uchupi and others, 2001; fi g. 

4). The moraines contain boulder- to clay-size sediment and 

were thrust, or “bulldozed” into place about 21,000 years ago. 

Some older glacial and nonglacial sediment is included in the 

moraines (Uchupi and others, 1996; Oldale, 1992). The associated outwash deposits are of low relief except on the eastern 

and northern shores of Cape Cod. Here, bluffs of fl uvial sand 

and gravel are as great as 50 m high as a result of fl ow from a 

glacier into an ice and moraine-dammed lake in present-day 

Cape Cod Bay (Uchupi and others, 1996, 2001; Oldale, 1992; 

fi g. 5).

Drumlins are common near the coast north of Cape Cod 

to southern Maine. These features are composed of till of 

heterogeneous sediment textures and lithologies. In Boston 

Harbor, a large fi eld of drumlins forms many islands and 

headlands commonly up to 10 m in height. Erosion of these 

drumlins has formed many of the large tombolos and spits in 

this area (fi g. 6).

The coastal lowlands north of Boston experienced a marine inundation during deglaciation between about 14,000 

and 11,000 years ago (Belknap and others, 1987; Dorion and 

others, 2001; Stone and Borns, 1986). This resulted from 

isostatic depression of the land by thick glacial ice. Because 

of the late-glacial fl ooding, moraines in this region are often 

stratifi ed combinations of subaqueous outwash (underwater 

fans of sand and gravel) and till (fi g. 7) (Ashley and others, 

1991). Most of the coastal till deposits are relatively low-relief 

features, less than 5 m in height above sea level (though often 

extending well below the sea surface). In a few places, large 

moraines partly block embayments and have signifi cantly controlled the Holocene evolution of the shoreline (fi g. 8). Bluffs 

of glacial-marine muddy sediment occur in association with 

moraines and are extremely common in the coastal zone north 

of Portland (fi g. 9). These bluffs range up to 15 m high and are 

most abundant in the protected, inner reaches of embayments 

(Kelley and Dickson, 2000).

Northern and southern New England experienced differing sea-level histories as a consequence of the differing thickFigure 2. The island-bay coastline at Mount Desert Island, 

Maine.

Figure 3. The indented-shoreline coast near 

Wiscasset, Maine.
97

ness of ice in the two areas. Because Maine was covered by 

relatively thick ice, it was isostatically depressed and drowned 

in late glacial times (Dorion and others, 2001). Once the load 

of the ice was removed, the land rebounded and sea level fell 

to a lowstand around 60 m below present sea level (Kelley and 

others, 1992, 2003; Barnhardt and others, 1995, 1997). Sea 

Figure 4. Bluff cut into a glacial moraine on Block Island, Rhode 

Island (photograph by Jon Boothroyd, University of Rhode Island).

Figure 5. Highland Light on outer Cape Cod. Eroding bluff is mostly 

composed of outwash sand and gravel. The lighthouse was moved 

back from the bluff shortly after this photo was taken (photograph 

from James Allen, U. S. Geological Survey).

Figure 6. Drumlin islands in Boston harbor. Erosion of these till deposits leads to the formation of the associated spits and tombolos.

Figure 7. Stratifi ed coastal moraine in Kennebunk, Maine.

Figure 8. Sprague Neck moraine has eroded for a long time, but still 

blocks a large part of the entrance of Machias Bay, Maine. Note the 

2-km-long beach (left) derived from erosion of the till.

Figure 9. Eroding bluff of glacial-marine sediment, Brunswick, 

Maine.

Coastal Bluffs of New England
98 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

level has risen to the present day at an uneven rate, possibly 

because of delayed isostatic responses (Barnhardt and others, 

1995; Balco and others, 1998). Present sea-level rise ranges 

from 2 to 3 mm/yr in the Gulf of Maine.

In southern New England, the thin ice load did not depress 

the crust signifi cantly, and the region was isostatically uplifted 

by the peripheral bulge of material squeezed from beneath 

areas to the north. Thus, the late-glacial coastal environments 

were terrestrial, and sea-level rise has occurred more or less 

continuously since glacial times. Sea-level rise today ranges 

from 3.0 to 4.0 mm/yr in southern New England, as peripheral 

bulge collapse augments the worldwide rise of sea level (Emery and Aubrey, 1991; Peltier, 2002).

Bluff Erosion and Failure

Because of its highly irregular outline, varying orientation, differing rates of sea-level rise and heterogeneous collection of glacial materials, New England’s bluffed coast erodes 

at spatially variable rates and through many mechanisms. The 

most rapid and persistent bluff retreat is caused by high wave 

energy on the outer coast in the Cape Cod (and nearby islands) 

region (fi gs. 5, 10). During storms, waves directly strike the 

base of the sandy bluffs, and undermine them. The collapsed 

material forms a beach, but strong longshore currents continuously transport sand away, exposing the bluff to further erosion 

(fi g. 11). 

Wave erosion of till deposits is usually a slower process 

because boulders eroded from the till remain nearby, acting as 

a seawall and inhibiting further wave impact. Finer constituents of the till are winnowed away, however, and a lag deposit 

of gravel often marks the retreat of till bluffs (fi g. 12). Where 

sand and fi ne gravel is abundant within till deposits, large 

beaches may grow and protect the coast. This is the case in 

Boston Harbor, where drumlin till is the source of sediment 

(fi g. 6). During thousands of years of sea-level rise, the coast 

retreats in a stepwise fashion from one glacial sediment source 

to another (Boyd and others, 1987). For a time, beaches may 

protect bluffs from wave attack, but when one source of beach 

material is gone, the next bluff begins to erode.

In sheltered areas bluffs do not experience signifi cant 

wave energy. Gravity acts on all exposed cliff edges, however, 

Figure 10. Marconi Station, outer Cape Cod. A, A representation of 

the original Marconi apparatus and its disappearance is shown in this 

National Park Service diagram. B, Eroding bluff of glacial-fl uvial sand 

and gravel at Marconi Station. The most landward part of Marconi’s 

wireless transmitter’s foundation (arrow) disappeared in 1993. 

Figure 11. Landslide on Block Island, Rhode Island. The large 

volume of eroded material disappeared soon after the bluff collapse 

(photograph from Jon Boothroyd, Univ. Rhode Island).
99

and creep of bluff materials leads to slow bluff erosion (fi g. 

13). Creep is a complex process aided by wetting and drying, 

as well as freezing and thawing of ground water, in coastal 

bluffs. Creep is too slow to be observed directly, but the bending of tree trunks as they slide down a slope is a distinct symptom of creep (fi g. 13).

Erosion of surface materials by rain or snowmelt is another 

mechanism causing bluff retreat. When it is the dominant process, rill marks cover a slope (fi g. 14). Runoff-induced erosion 

is abetted by a lack of vegetation. Plants impede downslope water movement and help to dry soils by removing water from the 

ground. People cut trees and brush to improve views, however, 

and hiking and bike trails on bluff slopes also aid in the erosion 

of bluffs by inhibiting plant growth and loosening soil materials.

Ground water is the most important agent infl uencing 

bluff erosion where wave action is weak. Seepage of ground 

water from bluffs occurs through coarse-grained units and at 

contacts between different materials, especially at the bluffbedrock contact. Seepage may remove sediment and allow 

it to fl ow down the bluff slope. This is especially important 

in northern areas where the frozen ground water thaws in the 

spring and large amounts of water and sediment are released 

in a brief period of time (fi g. 15). Ultimately, ground water 

reduces sediment strength and is always associated with largescale mass movements like landslides. 

Landslides occur in all glacial materials (fi gs. 5, 11), but 

are most common in bluffs of glacial-marine sediment (fi g. 

16; Kelley and Dickson, 2000). Gravity is the force causing 

landslides, and they occur largely in materials with at least 5 m 

of relief (Berry and others, 1996). Gravity is resisted only by 

friction within the sediment of bluffs. Water reduces the shear 

strength of sediment and allows gravity to overcome sediment 

friction, and snow melt during spring or winter thaws has often 

been implicated as a cause of landslides in Maine’s glacial-marine sediment. This material is generally muddy and relatively 

impermeable, but fractures or sandy beds must exist to allow 

water to enter the muddy sediment (Berry and others, 1996).

Figure 12. Aerial photo of eroded moraine (surrounded by arrows) in 

Casco Bay, Maine. 

Figure 13. Large block of glacial sand and gravel creeping down the 

slope of an esker in Prospect, Maine.

Figure 14. Small gullies on bluff of moraine in Cutler, Maine. This 

moraine is protected from direct wave attack by a beach of gravel 

eroded from the till.

Figure 15. Frozen ground water in bluff of glacial-marine mud is 

thawing and fl owing down the face of the bluff in Lubec, Maine. 

Coastal Bluffs of New England
100 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Documenting Coastal Bluff 

Erosion Rates

The rate of erosion of bluffs in New England is controlled 

to a large degree by the rate of removal of eroded materials 

(Sunamura, 1983). These materials may be slump blocks from 

a large mass movement or sand formed by waves onto a beach. 

In sheltered areas, salt marshes colonize intertidal mud deposits and landslide debris above mean tide level and inhibit further erosion (fi g. 17; Kelley and others, 1989). Thus, the longterm rate of bluff retreat is often the average of short bursts of 

erosion and long intervals of stability (Sunamura, 1983). Although there are no published studies that have evaluated bluff 

retreat and storm occurrence, it is reasonable to believe that 

once a bluff has lost the protection of a salt marsh or beach, 

retreat occurs during a large storm event.

The best-documented rates of bluff retreat are in Massachusetts, where the State coastal zone management offi ce 

has measured shoreline positions on historic maps and aerial 

photographs since the nineteenth century (http://www.appgeo.

com/atlas/project_source/czmcc/ccindex.html). Rates vary 

from greater than 1.0 m/yr on the outer bluffs on Cape Cod to 

0.1 m/yr in sheltered locations.

At six locations in Maine, glacial-marine sediment bluffs 

were specifi cally studied by photogrammatic and direct 

surveying methods (Smith, 1990; Kelley and Dickson, 2000). 

Rates of erosion averaged 0.5 m/yr between 1985-1988 by 

direct survey methods and 0.22 m/yr and 0.40 m/yr between 

1940-1972 and 1972-1985, respectively, by photogrammatic 

methods. These are not representative of all Maine bluffs, but 

were selected partly because of easy access across private 

property. Prior to a landslide in 1996, which involved 180 m 

of erosion in one day (fi g. 16), the bluffs at Rockland were 

probably eroding at rates less than 0.5 m/yr (Berry and others, 

1996; Kelley and Dickson, 2000). Landslides comparable in 

size to the Rockland event and involving property are documented in Maine from 1973, 1983, and 1989 (Berry and 

others, 1996); earlier large events are not well documented.

There are no published descriptions of bluff erosion 

in New Hampshire, Rhode Island, or Connecticut. New 

Hampshire’s outer coast is largely composed of bedrock and 

beaches, but small bluffs of glacial sediment similar to those in 

Maine probably exist in the few estuaries of the State. Eroding 

till bluffs were probably common in Rhode Island and Connecticut (fi g. 18), but human development has protected most 

bluffs from erosion with seawalls. 

Human Occupancy of the Coast and 

Erosion Hotspots

The 9,847 km of tidally infl uenced shoreline in New England was the fi rst coastal region in the United States settled by 

Europeans (Ringold and Clark, 1980). In some areas use of 

the coast has grown until the present day, but in many of the 

earliest settlement areas, the intensity of human occupation of 

the coast has declined since colonial times. Land use in contemporary coastal areas ranges from urban in Boston (Mass.), 

Figure 17. Salt marsh deposit protecting a bluff from erosion in 

Brunswick, Maine.

Figure 16. Landslide in glacial-marine sediment, Rockland, Maine. 

Two houses were lost when erosion due to the event reached more 

than 100 m landward from the edge of the bluff in April 1996.

Figure 18. Eroding bluff of till, Pine Island, Conn. (photograph by 

Nate Gardner, University of Maine).
101

Providence (Rhode Island), Bridgeport (Conn.), and Portland 

(Maine) to largely undeveloped in many locations in eastern 

Maine (fi g. 1). Suburban residential development is probably 

most common, and is widespread across Connecticut, Rhode 

Island, and Massachusetts. Even formerly remote regions in 

central Maine are beginning to experience growing numbers of 

vacation homes along the coast.

Early settlers apparently shied away from unstable bluffs, 

although by the 19th century accounts of landslides in glacial-marine sediment were described near Portland, Maine 

(Bouve and Jackson, 1859; Morse, 1869). By the 20th century, 

construction of large-scale protective, engineering structures 

and extensive fi lling of intertidal areas near cities had removed 

any erosion hazard from urban areas. Early suburban residents 

constructed houses near eroding bluffs and began to armor 

bluffs as the threat to residences increased (fi g. 19). For most 

low-relief bluffs of glacial sediment in sheltered locations, 

well maintained seawalls are adequate to stop bluff erosion 

for a hundred years or more. In several locations, however, the 

scale of the bluffs and consequences of seawall construction 

have posed larger problems by cutting off sand supply to adjacent beaches.

The outer coasts of Cape Cod, Martha’s Vineyard, and

Nantucket and Block Islands are especially precarious. Erosion 

rates on the order of a meter per year are common, and measurements of erosion have led to the movement of several lighthouses prior to their collapse. Highland Light, constructed in 1797, 

for example, was recently moved 150 m to lengthen its lifetime 

(fi g. 5). Short-term rates of retreat are even more extreme, and 

greater than 10 m of retreat has been observed during individual 

storms (fi g. 20; Sunamura, 1983).

In many places bluff erosion directly provides sand for 

nearby beaches (Duffy and others, 1989). Humarock Beach, in 

Scituate, Mass., has eroded and lost many buildings since engineering structures were built to stablize nearby drumlins that 

had acted as sediment sources (Woods Hole Oceanographic 

Institute Sea Grant, 2001). Nearby in the Plymouth area, the 

erosion of high bluffs of outwash threatens buildings (fi g. 21), 

but stabilization will eliminate beaches and is generally not allowed under Massachussetts law (J. O’Connell, oral commun., 

2002). On Siaconset Beach, eastern Nantucket Island, a costly 

“dewatering” system was emplaced to induce accumulation of 

beach sand by waves because stable beaches ultimately protect 

the bluffs behind them (Allen, 1996). 

In Maine, Rockland Harbor has been a landslide erosion hotspot for decades (Berry and others, 1996; Kelley and 

Dickson, 2000; Kelley and others, 1989). Here 10 m bluffs of 

glacial-marine mud fail catastrophically from time to time (fi g. 

Figure 19. Typical response of homeowner to bluff erosion in Jonesport, Maine. A, 1983. B, 1985. C, 1989. D, 1993.

Coastal Bluffs of New England
102 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

16). Even without landslides, the bluffs are retreating through 

creep (fi g. 22). Similar bluffs comprise extensive stretches 

of the Maine coast. In undeveloped areas there is little concern about bluff retreat. In the suburban areas near Portland, 

Maine, however, more than 25 km of bluff shoreline is deemed 

“highly unstable” by the State (Kelley and Dickson, 2000), and 

valuable properties are now at risk (fi g. 23).

Human Responses to Bluff Erosion

The initial response to bluff erosion in most places in New 

England was to armor the bluff. In urban areas massive engineering structures and artifi cial fi ll eliminated the problem of 

erosion. In areas where bluffs supplied beaches with sediment, 

there was no early connection made between sediment source 

and sink. Winthrop, Mass., for example, eliminated the supply 

of sand to its beaches by the early 20th century and has used 

seawalls, groins, breakwaters, and replenishment to hold the 

beach shoreline in place (fi g. 24). Because so many beaches 

in New England are associated with eroding bluffs (Duffy 

and others, 1989), bluff stabilization may be a major cause 

of chronic beach erosion and the growing need to replenish 

beaches (Haddad and Pilkey, 1998). In many residential coastal areas, all of the original eroding bluffs of glacial sediment 

are armored. In Maine, 20 percent of the 1,250 km of Casco 

Bay’s shoreline is armored (Kelley and Dickson, 2000); an additional 20 percent is bedrock. 

Massachusetts has mapped the erosion rate of its entire 

coastline and placed the data on a web site (Massachusetts 

Coastal Zone Management, 2002; Theiler and others, 2001). 

The maps on this site depict shoreline positions from 19th 

century maps and 20th century aerial photographs (fi g. 25). 

Connecticut is presently mapping the rate of shoreline change 

along its coast, but no products are yet available from this 

effort (Ralph Lewis, Connecticut State Geologist, oral commun., 2002). New Hampshire and Rhode Island have no mapFigure 20. Bluff erosion on the south shore of Cape Cod, Mass. 

threatened condominiums during the “Halloween Storm” of 1991. The 

bluff retreated at least a meter at the beginning of the storm, and sand 

was dumped onto the beach to protect the buildings.

Figure 21. Bluff of outwash sand and gravel at the White Cliffs area 

of Plymouth, Mass., have historically eroded at rates between 1m/yr 

and 2 m/yr (James O’Connell, Woods Hole Sea Grant, oral commun., 

2002). The gabion wall was built prior to laws prohibiting such structures to slow shoreline retreat.

Figure 22. Erosion of glacial-marine mud bluffs in Rockland, Maine, 

proceeds relentlessly between large landslide events.

Figure 23. Aerial photo of Cumberland Foreside, Maine, with slowly 

retreating bluffs and valuable nearby houses.
103

ping or other programs in existence regarding bluff erosion 

(Jon Boothroyd, Rhode Island State Geologist, oral commun., 

2002).

The Maine Geological Survey and University of Maine 

have mapped bluff stability for several years (Kelley and 

Dickson, 2000) and hope to complete the mapping in 2004. 

They map (1) presence or absence of a bluff, (2) the relatively 

Figure 24. Stabilization of the eroding drumlins in Winthrop, Mass., 

cut off the supply of sediment to adjacent beaches. Seawalls, groins, 

and offshore detached segmented breakwaters are needed, along 

with occasional beach replenishment, to maintain the shoreline position.

Figure 25. An example of the Massachusetts Coastal Zone Management Web site on bluff erosion (Massachusetts Coastal Zone Management, 2002). The lines paralleling the coast represent shoreline 

positions in the past. Erosion rates were calculated at the location of 

lines perpendicular to the coast.

stability of the bluff, (3) the nature of the intertidal zone at 

the base of the bluff, and (4) the possibility of a landslide at 

the location (fi g. 26). In Maine a permit is required to armor 

a bluff, and the Natural Resources Protection Act precludes 

“unreasonably interfering with the natural transfer of soil from 

the land to the sea,” but this has not deterred construction of 

protective structures on eroding bluffs. 

Figure 26. An example of the bluff stability maps produced 

by the Maine Geological Survey (from Kelley and Dickson, 

2000).

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eastern Maine, in Weddle, T.K., and Retelle, M.J., eds., 

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New England and adjacent Canada: Geological Society 

of America Special Paper 351, p. 215-242.

Duffy, W., Belknap, D.F., and Kelley, J.T., 1989, Morphology 

and stratigraphy of small barrier-lagoon systems in Maine: 

Marine Geology, v. 88, p. 243-262.

Emery, K.O., and Aubrey, D.G., 1991, Sea levels, land levels and 

tide gauges: New York, Springer-Verlag, 237 p.

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England beach experience (1935-1996): Journal of Coastal 

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Kelley, J.T., and Dickson, S.M., 2000, Low-cost bluff stability 

mapping in coastal Maine— providing geological hazard 

information without alarming the public: Environmental 

Geosciences, v. 7, p. 46-56.

Kelley, J.T., Kelley, A.R., and Pilkey, O. H., 1989, Living with 

the Maine coast: Duke University Press, 174 p.

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shelf, in Wehmiller, J. and Fletcher, C., eds., Quaternary 

coasts of the United States, Society of Economic Paleontologists and Mineralogists, Special Paper 48, p. 23-34.

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to the Bay of Fundy; an environmental atlas of the Gulf of 

Maine: Cambridge, Mass., The MIT Press, p. 19-39.

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and Barber, D.C., 2003, Distribution and volume of sand 

bodies on the rocky, glaciated inner continental shelf of the 

northwestern Gulf of Maine: Journal of Coastal Research, 

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changes in Massachusetts: http://www.appgeo.com/atlas/

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story: East Orleans, Mass., Parnassus Imprints, 208 p.

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maximum to Holocene: Quaternary Science Reviews v. 

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Francisco, Calif., W.H. Freeman and Company, 172 p.

Smith, R.V., 1990, Geomorphic trends and shoreline dynamics 

in three Maine embayments: University of Maine, Orono, 

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Stone, R.D., and Borns, H.W., Jr., 1986, Pleistocene glacial and 

interglacial stratigraphy of New England, Long Island, and 

adjacent Georges Bank and Gulf of Maine, in Sibrava, 

V., Bowen, D.Q., and Richmond, G.M., eds., Quaternary 

glaciations in the northern hemisphere: Quaternary Science Review, v. 5, p. 39-52.

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in Komar, P.D., ed., CRC handbook of coastal processes 

and erosion: Boca Raton, Fla., CRC Press, p. 233-265.

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Massachusetts coastal zone change project—1800’s to 

1994: Woods Hole Oceanographic Institute Sea Grant, 

WHOI-T-01-001, 39 p.

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The Late Quaternary construction of Cape Cod, Massachusetts— a reconsideration of the W.M. Davis model: 

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Coastal Bluffs of New England
106 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends
107

Introduction

Nearly 65 percent (10,444 km) of the 16,047-km-long 

Great Lakes shoreline is designated as having signifi cant 

erosion; about 5.4 percent (860 km) of it is critical. Adjusted 

to 1990 dollars, estimated total costs of damage along U.S. 

Great Lakes shores due to shore erosion between 1959 and 

1990 range from $286.6 million to $2.9 billion (Angel, 

1995). Nearly 32 percent of the U.S. shoreline of the Great 

Lakes, not including the islands, consists of erodible cliffs or 

bluffs. These range from a few to tens of meters high and are 

typically fronted by a narrow beach. The extent of the shoreline with erodible bluffs and dunes and the often complex 

response of this type of shoreline to wave erosion make slope 

processes an important part of the shore recession problem. 

Shore recession, in turn, affects the planning, design, and 

maintenance of transportation facilities and all types of 

development in coastal areas. Understanding coastal bluff 

processes is fundamental to solving land-use confl icts on the 

shoreline. 

Geological Setting

The Great Lakes lie in the craton, the stable core of the 

North American continent. The impacts of plate tectonic setting on coastal erosion and bluff instability are insignifi cant 

compared to the west coast of the United States. On the other 

hand, unlike much of the east and west coasts, eroding bluff 

shorelines along the Great Lakes are composed of Quaternary glacial till (much of it clayey), silt and clay lake sediment, and outwash sand and gravel that erode readily. Most 

of the bedrock shore is fairly competent, and erosion on 

these shores is slow relative to the bluffs of unconsolidated 

deposits. 

Along the northern shore of Lake Superior and Georgian 

Bay of Lake Huron, Precambrian igneous and metamorphic 

rocks are at or near the surface, and there are relatively few 

areas of eroding bluff. Much of the shore of northern Lakes 

Huron and Michigan is composed of fairly resistant dolomite and limestone and thus also is eroding slowly. Although 

there are Paleozoic sedimentary rocks exposed along a few 

reaches, most of the lakeshore bluffs along southern Lakes 

Michigan and Huron and most of Lakes Erie and Ontario are 

composed of relatively thick unconsolidated sediment, resulting in greater susceptibility to erosion. An overview of the 

distribution of eroding bluff shore is shown in fi gure 1, and 

a map of the Great Lakes showing shore type and recession 

rates is given by Pope and others (2001).

Origin of the Modern Great Lakes

All of the Great Lakes except Lake Superior were river 

valleys about two million years ago when glaciers fi rst entered the region. The Lake Superior basin was formed by 

faulting long before the last glacial period and may have 

been a lake basin when glaciers fi rst advanced. Perhaps 15 

or 20 times, the Laurentide Ice Sheet formed and advanced 

from the north. Each glacial advance carved the lake basins 

deeper until the basins reached their present size beneath the 

last major ice advance during the late Wisconsin glaciation, 

which occurred between 25,000 and 10,000 years ago. 

Glaciers have the ability to erode rock and soil and 

carry it along with the fl owing ice to the glacier edge where 

it is deposited as till, a mixture of sand, silt and clay grains 

released from the melting ice. As the glaciers receded from 

the Great Lakes about 15,000 to 10,000 years ago, there were 

numerous minor readvances of the ice edge. Each readvance 

deposited till with a slightly different composition than previous or later advances, and these till layers are now exposed 

Erosion of Coastal Bluffs in the Great Lakes

By David M. Mickelson, Tuncer B. Edil, and Donald E. Guy

Figure 1. Map of Great Lakes showing the distribution of cohesive 

bluffs and banks (from Pope and others, 2001).

 U.S. Geological Survey Professional Paper 1693
108 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

in eroding coastal bluffs. Between these till layers there are 

typically layers or lenses of sand and gravel. These sandy, 

stratifi ed sediments were deposited in water as beaches and 

deltas in front of the retreating glacier. Ground water preferentially drains laterally through these permeable layers, creating bluff instability. Between glacial advances, laminated 

silt and clay were also deposited in proglacial lakes formed 

along the margin of the ice sheet. These lakes had elevations 

up to 20 m higher than the present level of the Great Lakes. 

Draining of these proglacial lakes set the stage for the modern Great Lakes. Erosion along the shores of these lakes for 

about the last 10,000 years by waves and slope processes has 

produced the coastal bluffs as they are seen around the present Great Lakes.

Fluctuation of Lake Levels and Recession of 

Bluff Shorelines

The present position of the bluff and beach is not the 

shoreline position of the past. Shorelines change position 

for several reasons. Water level can rise or fall, causing the 

position of the water/land interface to migrate landward or 

lakeward. Although the Great Lakes do not have signifi cant 

astronomical tides like ocean shorelines, they experience 

water level changes on several time and spatial scales. When 

glacial ice melted away to the north about 10,000 years ago, 

it left the Great Lakes basins much as they are today. Over 

the last 10,000 years water levels have fl uctuated tens of 

meters because of outlet changes, formation and removal of 

dams produced by glacial deposits (and by the glacier itself), 

climate variations, and tilting of the basins due to glacial 

isostatic rebound. Low, wave-cut terraces were covered by 

sand during ancient higher water levels and lie in front of no 

longer active shoreline bluffs in some places. Former beaches 

and beach ridges are preserved kilometers inland from the 

present shore in some places around the Great Lakes where 

the land is low and gently sloping. Many early footpaths and 

some modern roads follow these old beach ridges. 

Isostatic rebound is the upward movement of the land 

that was depressed by the weight of glacial ice up to 1.5 km 

thick along the northern edge of the Great Lakes. The land is 

still rising millimeters per year in the north, but in the south 

most rebound has been completed because thicker ice in the 

north depressed the land more and because deglaciation occurred later. This differential rebound has caused water level 

change in historic time, mostly affecting Lake Superior. For 

instance, the outlet of Lake Superior at Sault Saint Marie is 

rising faster than the western part of the lake causing a continuous, slow rise of water level with climatically driven lake 

level changes superimposed on them. The Lake Ontario shore 

at Port Colborne, Ontario has been raised by 0.11 m per century (U.S. Army Coastal Engineering Research Center, 1984) 

and the northern part of the basin has risen more than 0.53 m 

per century (U.S. Army Corps of Engineers, 1999).

Shorter-term fl uctuations of water level also have occurred. On a time scale of weeks to years, water level mostly 

varies due to changes in precipitation (fi g. 2). Lakes Huron, 

Michigan, and Ontario have experienced the largest fl uctuations, about 2 m from high to low since the 1860s when 

record keeping began. These fl uctuations are extremely important in infl uencing erosion rates and the nature of bluff 

processes.

A strong unidirectional wind lasting from hours to 

several days can cause water to rise signifi cantly for a few 

tens of hours. Lake Erie, because it is shallow and oriented 

southwest-northeast is especially susceptible to storm surges. 

For example, Pope and others (2001) cite a major storm in 

Lake Erie in 1985 that for a few hours raised water level by 

2 m at Buffalo, at the northeast end of the lake, and dropped 

Figure 2. Fluctuation of Great Lakes levels (from Pope and others, 

2001).

Figure 3. Positive and negative storm surges and seiche after storms 

in Lake Erie (from Pope and others, 2001).
109

water level by about the same amount at Toledo, at the southwest end. Figure 3 shows positive and negative storm surges 

and the 14-hour seiche that occurs after storm surges. These 

short-lived lake level changes can have a dramatic effect on 

already eroding bluffs.

Shoreline position can also change because of erosion or 

deposition. Bluff shorelines are erosional in the long term, 

otherwise there would be no actively eroding bluff. Where 

the bluff contains unconsolidated sediment, the shoreline 

may have retreated several kilometers since deglaciation. 

Even bedrock shorelines have been eroded by waves, though 

to a lesser extent. Along some parts of the coast, where littoral currents have deposited large amounts of sand, the shoreline has migrated lakeward protecting the bluffs from wave 

attack. Because this paper concentrates on eroding bluff 

shorelines, sandy, depositional shorelines are not considered 

further. 

Beach and Nearshore Erosion 

Processes

The most signifi cant geomorphic process along the Great 

Lakes bluff shore is the erosion and transport of shoreline 

sediment by waves. Wave action is important, both in itself 

and in initiating and perpetuating other geomorphic processes in coastal bluffs. The most notable factors that affect wave 

erosion in the Great Lakes are water-level changes and storm 

activity. As described earlier, there are several time scales of 

water level change in the Great Lakes, but the fl uctuations 

that occur in response to climatic variations over intervals of 

10 to 30 years and with magnitudes of up to 2 m present the 

most immediate concern (Brown, 2000).

Effect of Lake Level Change

Change in lake level is commonly considered to be a major factor controlling changes in bluff retreat rate (Bray and 

Hooke, 1997; Kirk and others, 2000; Carter, 1976), although 

there have been reports showing no statistically signifi cant 

causal relationship. As lake level rises, the beach narrows 

and more waves that arrive at the shore impact the bluff toe. 

Davis and others (1973) applied models developed for sand 

beaches and dunes (such as those along the Florida coast) to 

the eastern Lake Michigan shore, where similar conditions 

exist, and showed that the local rates of erosion are related 

more to the presence or absence of nearshore sand bars, manmade coastal structures, and the frequency of intense storms 

than to the relatively long-term fl uctuations of lake level. 

Lake level, they contend, plays only a passive role in coastal 

erosion in such settings, not a causative one. This conclusion 

would probably not hold for most bluff shorelines because 

erosion events occur with suffi cient frequency to prevent 

bluffs from reaching equilibrium as sand shorelines commonly do.

Unlike tidal changes in oceans, Great Lakes water level 

fl uctuations are almost never regular and are diffi cult to 

predict accurately in the long term due to complex climatic 

and weather impacts and the diffi culty in predicting climate. 

However, records of lake level more than 100 years long 

are available for all of the Great Lakes as shown in fi gure 2. 

Lake-level highstands, although they have no regular interval, seem to occur every 10 to 30 years, and the magnitudes 

are 3 to 6 times greater than the average seasonal variation 

— on the order of about 2 m. Brown (2000) has shown a 

correlation between bluff toe recession and lake level fl uctuations for clayey till bluffs of the western Lake Michigan 

shore. Carter and Guy (1982) found that erosion along the 

bluff toe was episodic and that signifi cant erosion events occurred when the storm surge exceeded 0.15 m and lake level 

exceeded 1.5 m above chart datum.

Wave Erosion

The weather directly infl uences bluff erosion rates 

(Dewberry and Davis, 1994; Carter, 1976; Jibson and others, 

1994; Davidson-Arnott and Pollard, 1980; Powers, 1958). 

Wind-generated waves exert powerful erosive forces at the 

base of the bluff, beach, and nearshore. Continual wave action undercuts the toe of the slope and deepens the nearshore 

zone (lake-bed downcutting), both of which ultimately lead 

to slope failure. Extreme storm events are associated with 

increased wave attack due in part to the higher wave energy 

associated with higher winds and associated higher waves.

 Wave erosion affects bluff stability and recession in 

two major ways: erosion at the bluff toe and erosion in the 

nearshore (lake-bed) zone.

Bluff Toe Erosion 

Surface waves generated by wind blowing across the 

lake surface coupled with suffi ciently high water levels are 

considered a principal cause of shore recession (Jibson and 

others, 1994; Dewberry and Davis, 1994; Davidson-Arnott 

and Pollard, 1980). The average wave height and period increase as the wind velocity or fetch increases, and wave energy on the beach is directly related to water depth and wave 

height. Wind waves generated on the Great Lakes are capable 

of eroding bluffs both directly and indirectly (Kamphuis, 

1987; Carter, 1976). Waves deform and break when the water 

depth is 1 to 1.5 times the wave height. Thus, for given size 

waves, the gentler the nearshore slope is, the farther offshore 

the waves will break and the less energy the waves will have 

upon reaching the beach. Kamphuis (1987) derived a theoretical expression that relates recession rate of a till bluff to 

incident wave power. 

The continuous onslaught of waves serves to erode and 

wash away the intact, exposed lower bluff face and to remove 

slumped material at the base of the bluff. Thus, the erosion 

Erosion of Coastal Bluffs in the Great Lakes 
110 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

of the bluffs is a continuing, but not a continuous, process. 

Oversteepening of the wave-cut lower bluff then leads to sliding, which may retrogressively move up the bluff or result in 

a single large slide that reaches the bluff top. Concurrently, 

native vegetation is lost and the bluff face becomes open to 

many erosional processes. 

Lake Bed Downcutting 

For cohesive shorelines, nearshore downcutting impacts bluff stability indirectly. The nearshore zone is 

compositionally and geotechnically closely related to the 

lower layers of the bluff, but can be weathered and covered 

discontinuously with deposits of sand or gravel of varying 

thickness (Kamphuis, 1987). Nearshore downcutting is the 

general planing down of the lake bed due to (1) drag forces 

on the particles caused by foreshore wave motion and (2) 

abrasion by coarse sediments over the cohesive lake bed 

(Davidson-Arnott and Askin, 1980). Kamphuis (1987) and 

Nairn and others (1999) showed that the wave-related processes taking place on the foreshore could actually control 

the long-term rate of bluff erosion. 

Lake-bed and fl ume tests by Bishop and others (1992) 

also clearly document the importance of lake bed downcutting in determining recession rate. Signifi cant downcutting 

produces deeper water closer to the beach. This creates a 

condition where, for a given size wave, more wave energy 

impacts the bluff toe than previously would have taken place. 

For any given lake level, if downcutting of the nearshore did 

not take place, waves would break farther and farther from 

the bluff toe as the bluff receded, and bluff retreat would 

eventually stop as a very wide, gently sloping wave cut 

platform developed. This clearly does not happen, so downcutting of the wave cut platform must keep pace with bluff 

recession. However, the real threat of nearshore downcutting 

exists for all till shorelines where there is an insuffi cient sand 

supply to provide adequate protection of the cohesive-till 

lake bed from wave action. Kamphuis and others (1990) have 

quantifi ed the importance of the abrasion of granular material as it moves in the waves on the erosion rate of a platform 

of cohesive till or lake sediment. The amount of sand on the 

lake bottom has a signifi cant effect on the nearshore erosion 

rate. A limited amount of sand, acting as an abrasive, causes 

a much higher erosion rate than on bare cohesive sediment, 

whereas a 15-to-20-cm thickness of sandy sediment protects 

the cohesive sediment from erosion (Davidson-Arnott and 

Askin, 1980). 

Wind Erosion

Wind erosion along the Great Lakes is most prevalent 

on sandy slopes, such as those along the eastern and southeastern Lake Michigan shore, where sand dunes are impacted and shaped signifi cantly by wind action. Defl ation of 

mechanically weathered material from the face of cohesive 

bluffs on Lake Erie was also reported by Carter and Guy 

(1988). The process occurred during the winter months when 

repeated cycles of freeze/thaw and dehydration created a mechanically weathered surface layer 5 to 10 mm thick on the 

face of the bluff. This weathered surfi cial sediment then was 

dislodged by wind and accumulated at the toe of the bluff in 

a small talus slope. As a process, defl ation of grains from the 

face of cohesive bluffs is likely of minor importance. 

Ice Erosion

A nearshore ice complex consisting of several parts often develops on the Great Lakes in the winter. On beaches 

exposed to waves, an ice foot forms against the beach as 

slush ice is driven to shore by waves. Ice ridges form where 

waves break, such as over nearshore sandbars, and may provide a lakeward boundary to the ice. The outer ice ridges 

at times rise to 5 m by slush ice driven onto them by waves. 

They may disappear abruptly during major storm events and 

can be destroyed and rebuilt several times during the winter. 

During winter months, the nearshore ice complex tends to 

protect the beach and bluff from wave erosion. Because the 

largest wave heights on the Great Lakes generally occur between November and March, freeze-up and break-up dates 

can substantially affect the extent of wave action at the shore. 

This protection of the beach by the nearshore ice complex is 

offset by incorporation of sand and gravel particles by ice. 

Barnes and others (1994) showed that coastal ice plays a 

signifi cant role in removing and transporting sediment from 

the coast, at least along the southern Lake Michigan shore. In 

addition, waves breaking against grounded ice ridges scour 

the lakebed. The lake-bed sediment is often gouged by contact with the keels of ice blocks moved by the wind. Some of 

this sediment is frozen into the ice and drifts into deeper water, transporting signifi cant quantities of sediment out of the 

beach/nearshore system. Ice shove may also occur when lake 

ice, moved by water currents or wind, comes into contact 

with the shore and bluffs (Keillor, 2003). 

Bluff Erosion Processes 

The interaction between driving forces (gravity and climate) and shear resistance of the soils that form the coastal 

bluffs results in a number of processes that produce and 

remove debris from bluffs. This erosion is an important process, not only because it leads to confl ict with human activity 

on the bluff top, but because the sediment produced feeds the 

beach and nearshore with sand and gravel. The commonly 

encountered processes in Great Lakes coastal bluffs can be 

separated into two broad groups — individual particle and 

mass movements. Sediment transport by waves, currents, 

rain, groundwater, wind, and ice is generally as single par-
111

ticles. In mass movement, debris moves as a coherent unit. 

Mass movement on Great Lakes bluffs occurs in the form of 

landslides (a rigid body movement along a failure surface), 

fl ows, and solifl uction (a slow, shallow creep of saturated soil 

often enhanced by the freeze-thaw process). Table 1 summarizes these processes. Offshore, nearshore, bluff toe, bluff 

face, and topland erosional processes are all affected by erosion that occurs continually on the bluff shores of the Great 

Lakes region (Edil, 1982, 1992). 

Rill and Sheet Erosion

Rill and sheetwash are important on sparsely vegetated 

coastal slopes of the Great Lakes. During periods of intense 

precipitation, rainwater running down the slopes as surface 

runoff carries sediments from the bluff face to the lakeshore. 

A description of these processes is given by Sterrett (1980), 

who determined on the basis of fi eld observations that most 

of the material removed from the slopes during summer is 

by sheet-wash and rill erosion. Sterrett (1980) found that the 

universal soil-loss equation, in its modifi ed form as suggested by Foster and Wischmeier (1974), is useful in predicting 

soil loss from steep bluff slopes. On slopes formed in granular material, rill and sheet erosion are dominant. On cohesive 

bluffs, mass wasting processes tend to be more important.

Groundwater Sapping

In addition to its direct effect on the slope face, and on 

lake level, precipitation affects local groundwater conditions. 

Rain percolates into the bluff face and into the ground behind 

the bluff top. Water-table fl uctuation affects slope stability, 

as discussed later, and ground water, exiting the bluff face 

as springs, locally causes substantial erosion by sapping. 

Ground water generally fl ows toward the bluff face. In places 

where sand or gravel lies between more clayey sediment layers, ground-water fl ow is concentrated in the coarse units. 

Sand-size grains in particular can be carried off the bluff and 

onto the beach by this sapping process. If this process takes 

place low in the bluff, it removes support for the overlying 

mass, which breaks off into large blocks and falls from the 

cliff face. This is a signifi cant process along many reaches 

of the Great Lakes bluffs where groundwater seeps all of the 

time, independent of rainfall events. 

Sliding and Slumping

Slides (both rotational and translational) are mass movements commonly encountered on high bluffs in the Great 

Lakes region. Rotational slides involving approximately 

circular rupture surfaces have been observed and analyzed 

in Great Lakes bluffs formed in cohesive soils (Quigley and 

Tutt, 1968; Edil and Vallejo, 1977; Edil and Haas, 1980; 

Chapman and others, 1997). Deep-seated rotational slips 

occur only in clay soils and are not observed in sand. One 

method of analysis of rotational slides that is accurate for 

most purposes is that advanced by Bishop (1955). The failure arc predicted by the Bishop method has been found to 

compare very well with actual failure surfaces in bluffs of 

the Great Lakes and other places. Edil and Vallejo (1977) 

described bluff stability at two sites on the shore of Lake 

Michigan. In general, stability of the slopes was predictable. In places where unexpected stability occurred, lack of 

failure could be explained in a rational manner by a poorly 

understood process of delayed failure. That is, even though a 

bluff appears to be unstable, it can stand for some time. Slow 

strength changes may result from the unloading of clays by 

erosion of sediment above, and perhaps a “trigger” or threshold level of stress is needed before actual failure takes place. 

Using the effective stress approach and the Bishop method, 

Vallejo and Edil (1979) developed stability charts for rapid 

evaluation of the state of stability of actively evolving Great 

Lakes coastal slopes. These charts indicate the stability status 

as well as the type of potential failure (for example, deep or 

shallow) that might occur. 

A translational slide, in which the moving mass consists 

of a single unit or a few closely related units that are not 

greatly deformed, is called a block slide. An example of such 

a failure involving a block of fractured till in the upper part 

of a coastal bluff in Milwaukee County, Wisconsin, was reported by Sterrett and Edil (1982). 

Translational slides, in which shallow mass of surface 

material slides along a failure surface parallel to the slope 

surface occur in many different materials. Granular materials, such as sand and gravel, fail by surface raveling in this 

manner. Similar failures also occur in a mantle of weathered 

or colluvial material on clay slopes and are referred to as slab 

slides. 

An infi nite slope approach is often appropriate to use for 

analysis and prediction of translational slides. This assumes 

a planar slide surface with a predicted thickness, but infi nite 

width and length. Sterrett (1980) reported slab slides with 

a depth of about 0.6 m from Milwaukee County. This depth 

coincides closely with the depth of desiccation cracking and 

soil structure change from prismatic and blocky above to 

Forces:

Gravity

Vibration

Climate

Resistance:

Shear strength

Vegetation

Structural systems

Processes:

Mass Movement

Slide

Rotational Slump

Translation Block

Slide Slab slide

Flow

Solifluction

Debris flow

Particle Movement

Wave erosion

Wind erosion

Ice erosion

Rill erosion and sheetwash

Sapping

Table 1. Slope processes on Great Lakes bluff shorelines.

Erosion of Coastal Bluffs in the Great Lakes 
112 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

massive intact blocks below. This upper sediment is softened 

by freeze-thaw and desiccation, so when it is again saturated 

it tends to fail readily. Sterrett (1980) also observed that frozen slabs of soil measuring about 0.6 m x 10 m x 10 m failed 

in early spring, and he attributed this failure to surface thawing of bluff sediment and accumulation of pore water above 

still frozen ground. Along the north shore of Lake Erie, 

midway between Port Stanley and Port Bruce, Ontario, high 

velocity slides have been observed generating debris fans 

that extend 90 m offshore, more than twice the bluff height 

(Quigley and others, 1977).

Flows and Solifluction

Flows commonly result from heavy precipitation and thaw 

of snow or frozen soil and they can be rapid (mud or 

debris fl ow) or slow (solifl uction or creep). The fl ows observed 

in the Great Lakes bluffs occur mostly in spring and result 

primarily from ground thawing and snow and ice melting. 

Many of them begin as shallow slides high on the bluff and 

become fl ows lower on the slope. The size of the fl ows along 

the western Lake Michigan shoreline varies from less than 1 m 

wide to as much as 15 to 20 m wide and 20 m long. Many large 

ones are mobile enough to cross beaches more than 10 m wide. 

Solifl uction is the slow down-slope movement of watersaturated materials that follows thawing in previously frozen 

slopes. Like fl ows, they take place almost exclusively in the 

Spring. Vegetation that is deep-rooted appears to provide the 

most resistance to failure by this process. Very slow creep probably is most active in spring as well, but it may occur throughout the year on moist, steep slopes. A number of approaches 

for the analysis of solifl uction failures have been suggested. 

Vallejo (1980) introduced a new approach to the analysis of 

solifl uction that refl ects the particulate structure of the fl owing mass. Vallejo and Edil (1981) applied this analysis, with 

successful fi eld verifi cation, to a coastal bluff in Kewaukee, 

Wisconsin. The critical depth of thaw normal to the slope face 

above which failure occurred was found to be about 0.25 m. 

Bluff Types and Recession Rates

Recession Rates 

Erosion of bluffs by waves supplies sediment that nourishes the longshore transport system in all of the Great Lakes 

basins. Although lake bed downcutting certainly takes place, 

its volumetric contribution varies based on the relief and geology. Recession of the Ohio lakeshore introduced 1.19 million 

cubic yards to the lake between 1876/77 and 1973; of this 

total, about 27 percent came from subaqueous erosion (Carter, 

1977). The volume of sediment derived from subaqueous 

erosion constituted a larger percentage, as high as 73 percent, 

of the total sediment load along low-relief reaches. Conversely, subaqueous erosion contributed a smaller percentage, less 

than 21 percent, of the total sediment load along high-relief 

reaches or where the nearshore substrate was shale. In areas of 

low bluff, or where bluffs are protected or for other reasons not 

eroding, lake bed erosion is a more signifi cant source of sediment. Thus, other things being equal, the volume and coarse 

grain-size distribution of sediment produced from eroding 

bluffs determines the grain-size character of the beach and 

therefore the response of the shoreline to big waves. 

In a much more immediate sense, bluff shorelines are a 

focus of attention, because recession of the bluff top often 

threatens houses and other buildings, roads, and other components of our infrastructure. Any prediction of potential risk 

must include recognition of bluff processes and the response 

of the bluff top to wave erosion and other processes on the 

bluff face. Although instability on the Great Lakes bluff shorelines is ultimately perpetuated by the wave erosion that takes 

place at the base of the bluff, the actual delivery of sediment to 

the beach and the longshore drift system and the rate of retreat 

of the bluff top is complex. Factors such as engineering properties of soils, water-table elevation, bluff height, and nature 

of failure dramatically infl uence the rate at which the bluff top 

recedes.

Rates of bluff erosion vary substantially from place to 

place and over different time periods. The cumulative average annual rate of change for all shorelines on the Great Lakes 

is between 0.1 and 0.29 m per year (Pope and others, 2001). 

Lake Erie has the highest rates of erosion, with long stretches 

of shoreline having erosion rates greater than 2 m per year. 

Many eroding bluffs have average recession rates of 0.5 to 

more than 1 m per year. Maps of generalized recession rates 

are shown for all of the Great Lakes shorelines in Pope and 

others (2001).

Determining Recession Rates

Bluff recession has grown in importance as urban and 

suburban development has increased in the coastal zone. For 

example, about 25 percent of the lakefront homes in Ohio are 

within 7.5 m of the bluff edge. There is no way to measure 

rates of bluff recession over periods longer than the last 150 

years (when original surveys were made), because there is no 

geologic record of bluff position before that time; all trace has 

been eroded. Therefore, “long-term” measurements are really 

no longer than 150 years, and those are only at widely spaced 

points where survey lines actually measure the distance from 

section corners or other survey markers. In some cases, old 

maps provide some information as well, but they are generally crude and of limited value (Haras and others, 1976). For 

example, Gelinas and Quigley (1973) compared early survey 

maps of the north shore of Lake Erie with aerial photos to 

determine recession at 3-km intervals for a period of more 

than 150 years. They estimated a maximum probable error 

of about 0.25 m/year. Berg and Collinson (1976) used the 

distance measured from old topographic maps between cultural features such as roads and railroad tracks and the bluff 
113

edge over a 4-km stretch of bluff in northern Illinois. In Ohio, 

surveys conducted in the mid 1870s as part of the Survey of 

the Northwest Territories by the U.S. Topographic Engineers 

contain suffi cient geographic features that the maps can be 

referenced to aerial photographs taken in the 1930s, and thus 

provide a means of measuring recession of the Ohio lakeshore 

between 1876 and the present (Carter, 1976; Benson, 1978; 

Carter and Guy, 1980, 1986).

Much more detailed information is obtained from aerial 

photographs (see, for example, Sunamura and Horikawa, 1969; 

Bird and Armstrong, 1970; Carter, 1976; Berg and Collinson, 

1976; Mackey and Guy, 1994; Guy, 1999; Kruepke, 2000; 

Brown, 2000). Since the late 1930s and early 1940s, vertical 

photos have been taken at irregular intervals. The distance between bluff top or bluff base as identifi ed on two sets of photos 

is used to calculate a recession rate. Early studies used unrectifi ed imagery, whereas more recent studies use orthorectifi ed 

imagery to provide more accurate measurements. This process 

requires that the bluff toe or bluff base be clearly visible on the 

photos used. Technology developed in the last 10 years allows 

orthophotos to be constructed from scanned older imagery, 

improving accuracy and greatly reducing the time necessary 

to make spatial corrections (Burrough and McDonnell, 1998; 

Brown, 2000) and putting the data in digital form, so that 

numerous closely spaced measurements can be made using 

Arcview and Arcinfo or other software packages. Brown (2000) 

calculated recession rates at 10 to 20 m intervals along the bluff 

over a distance of several hundred meters to compare with each 

measured profi le. In the future, this approach will greatly simplify measurements from aerial photos (Hapke, this volume). 

The methodology is, however, still limited by the quality of 

the airphotos, by how well the top and base of the bluff can be 

discerned, and by the resolution of the digital elevation model 

(DEM) used for the corrections. In addition, correlation of recession rates with wave data are limited by the rather large time 

intervals between photos. This is especially problematic in high 

bluff areas where failure is episodic, with a low frequency of 

occurrence, but a high magnitude of failure.

Finally, an analysis of recession rate can also be made by 

frequent measurement from a monument or even pegs on the 

bluff that are surveyed. These can be used to produce highfrequency data over relatively short time periods that air-photo 

studies generally cannot produce (Keillor and DeGroot, 1978; 

Carter and Guy, 1988; Amin, 1991; Highman and Shakoor, 

1998). Although great accuracy can be obtained, these measurements are time consuming and pegs or rods are subject to 

loss due to large wave-erosion events, slope failures, or vandalism.

Interpretation and Prediction of Recession 

Rates for Different Bluff Types

One can argue that over a fairly long time period, bluff top 

recession is equal to recession at the base of the bluff because 

unless materials or ground-water level change, there is no 

reason to think that the bluff angle was substantially different 

at times in the past than it is now. Thus, for periods of hundreds of years, the rate of retreat of the bluff top can be taken 

as equal to the amount of erosion at the base of the bluff. On 

shorter time scales and especially for high bluffs, this is often 

not the case.

Table 2 lists bluff types recognized particularly on Lake 

Michigan, but on the other Great Lakes as well. Figures 4 

through 10 show examples of these bluff types. This list does 

not include some erodible bluff shore types, such as unconsolidated sediment over rock, which occurs along parts of the 

Lake Erie and Lake Ontario shorelines. A critical consideration when estimating bluff recession rates is the frequency at 

which failure takes place and whether bluff face recession is 

parallel or if slope angle changes with time. If slope angle is 

constant, then bluff-top recession rate measured over relatively 

short times (but long enough to include one or two high/low 

water cycles) may be representative of long-term behavior and 

could be used to forecast future bluff recession rate. This is 

because failure mode is dominated by creep, solifl uction, shallow slides, and fl ows that occur frequently. Bluffs in categories 

L2 and H2 (table 2; fi gs. 5, 8) are composed of sand that is 

noncohesive and that fails as shallow slides that occur frequently unless the slope is vegetation covered. Bluffs in categories L1 and H1 (table 2; fi gs. 4, 7) are composed at least 

in part of cohesive sediments but fail by shallow slides and 

fl ows. These are frequent and shallow, producing parallel retreat. Reasonable recession rate measurements can probably 

be made over relatively short periods for these bluffs as well. 

However, if the bluff fails primarily by large, episodic slumps, 

then bluff-top recession rates are probably not meaningful, 

even if measured over 50 years (categories L3 and H3 in table 

2; fi gs. 6, 9).

High bluffs (many over 35 m high) north of Milwaukee 

on Lake Michigan are particularly good examples of bluff 

category H3. They contain clayey till and lake sediment. They 

are subject to failure by large (50-100 m wide), deep-seated 

slumps that occur infrequently. This is the most diffi cult bluff 

type in which to predict future recession rates because of the 

episodic nature of slumping. Bluff top recession measurements from vertical air photos in many places indicate no bluff 

top recession, yet these bluffs are obviously subject to failure. 

Using the same sets of photos, adjacent properties that have 

undergone failure between times the airphotos were taken have 

high recession rates (Brown, 2000). Even for high lake-level 

periods as were experienced in the mid 1970s and late 1980s, 

the main bluff top in many places was unaffected by wave erosion because of the protection offered by a slump block at the 

base. Many bluff profi les have safety factors greater than 1.0 

for large deep-seated slumps, giving a false sense of security 

(Mickelson and others, 1977; Chapman, 1996; Chapman and 

others, 1997).

These high, complex bluffs appear to evolve through 

time and the factors of safety change as the slope evolves as 

described in fi gure 11. The cycle begins with a steep, unstable 

bluff having a low factor of safety. After a large-scale slump 

Erosion of Coastal Bluffs in the Great Lakes 
114 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

Table 2. Simple classifi cation of failing bluffs on Great Lakes shorelines (not including bluffs 

with bedrock).

Category Bluff

material

Profile

morphology

Failure mo de Retreat type

Low bluffs (less than about 20 m in height)

L1 Mostly

cohesive soils

Simple profiles

Generally unstable

Shallow slides

and small

shallow slumps

and associated

flows

Parallel retreat

L2 Mostly sand Simple profiles

Generally unstable

Shallow slides

and associated

sand flows or

simply wind

and water

erosion

More-or-less

parallel retreat

L3 Cohesive soils

or

interstratified

sand and

cohesive

sediments

Complex profiles

Unstable

Shallow slides

and shallow to

deeper-seated

slumps and

associated flows

Slope angles

vary in time and

along a profile

from top to

bottom as retreat

takes place

High bluffs (greater than about 20m in height)

H1 Mostly

cohesive soils

Simple profiles

Unstable

Shallow slides More-or-less

parallel retreat

H2 Sand Simple profiles

Unstable

Shallow slides

and associated

sand flows or

simply wind

and water

erosion

Parallel retreat

H3 Mostly

cohesive soils

Complex profiles

Unstable

Deep-seated

slumps of

various age and

in various

stages of

evolution

Bluff angles are

complex,

varying in time

and space

Figure 4. Low cohesive bluff (10 m) with simple profi le and parallel retreat on the southeastern shore of Lake Michigan in Kenosha 

County, Wisconsin.

Figure 5. Low (8 m) bluff composed mostly of sand on the eastern 

shore of Lake Michigan near Luddington, Michigan.
115

Figure 6. Low (12 m), rapidly receding bluff with complex profi le that 

fails by slump, falls, and fl ows on the south shore of Lake Erie, near 

Cleveland, Ohio.

Figure 7. High (35 m) bluff of cohesive till and lake sediment on the 

western shore of Lake Michigan north of Milwaukee, Wisconsin. 

Figure 8. High (> 40 m) bluff of sand and gravel capped by sand on 

the south shore of Lake Superior near Munising, Michigan.

Figure 9. High (>30 m), complex bluff that failed in the early 1990s 

by a large deep-seated slump north of Milwaukee, Wisconsin, on the 

western shore of Lake Michigan.

Figure 10. High (>30 m), complex bluff protected by large, deepseated slump block that failed more than 50 years ago. Note that top 

of bluff is above white snow patch; slump block is below.

Erosion of Coastal Bluffs in the Great Lakes 
116 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

event, the slump block begins to be affected by wave erosion. 

As waves erode the base of the slump block, the actively eroding face of the slump block increases in height as the lakeward 

part of the slump block is removed. At the same time, the 

actual bluff top, now isolated from wave action, makes minor 

adjustments and becomes stable. Continued wave erosion 

eventually removes the slump block over what can be a long 

period (probably 50 to 100 years near Milwaukee; Chapman 

and others, 1997), eventually producing an unstable slope, 

which fails again.

It seems likely that extent of growth of the actively eroding lower bluff segment (that is, the rate of removal of the 

slump block) is an expression of the extent of bluff evolution 

toward a major failure (Mickelson and Edil, 1998, 1999). As 

far as we know there has been no attempt to quantitatively 

include this model of bluff evolution into recession-rate measurements, although the results of Brown (2000) indicate that 

it should be done for high, cohesive bluffs. A somewhat similar classifi cation of cliffs was proposed by Hutchinson (1973, 

in Trenhaile, 1997). He attributed the cyclic behavior and 

deep-seated slumps of cliffs in the London Clay to extensive 

erosion at the base of the cliff and parallel retreat mostly by 

shallow slides to less intense wave erosion. Trenhaile (1997) 

points out several other models of cliff retreat as well, attributing different styles of bluff behavior to differences in materials 

and groundwater hydrology.

Correlation of Recession Rate to Environmental 

Factors

Measurement of recession rates as described above does 

not allow for prediction of future recession, because recession rate depends on factors such as bluff height and angle, 

frequency of failure, composition, height of ground-water 

table, lake level, wave climate, nature of toe materials, and 

bathymetry of the nearshore (Quigley, 1976a,b; Quigley and 

Zeman, 1980; Edil and Haas, 1980). One approach to solve 

this problem is to correlate the erosion rate with these factors. 

If a strong correlation exists with one or more variables, and 

if that variable can be predicted, then bluff retreat rate can be 

predicted as well. Several studies have attempted to do this, 

with mixed success. It seems clear at this point that there are 

many variables that infl uence the rate of bluff retreat, and that 

it is unlikely that a single factor will explain all variation in 

rate. Nonetheless, it appears that some aspect of wave climate, 

perhaps wave impact height, will predict a substantial amount 

of erosion rate change through time (Brown, 2000).

It appears that higher bluffs have higher recession rates 

(Edil and Vallejo, 1977). Relationships between bluff sediments and rates of bluff recession are outlined by Gelinas and 

Quigley (1973), Montgomery (1998), Edil and Haas (1980) 

and are too complex to discuss here. Ground water clearly 

has an effect, and for a given size bluff, higher water tables 

produce lower factors of safety and increased likelihood of 

failure. This is especially true of bluffs containing fi ne-grained 

till or lake sediment overlying and underlying sand or gravel. 

The presence of fractures in the upper unit increases the rapidity at which water pressure can build (Sterrett, 1980; Montgomery, 1998). Sediment softening during freeze/thaw is also 

an important process that weakens sediment on the bluff face. 

The nature of the toe materials also appears to be important in 

determining rates of recession (Carter and Guy, 1988). 

The above factors infl uence the stability and likelihood of 

failure of the bluff at any given time. Factors infl uencing wave 

erosion at the base are probably better predictors of long-term 

recession rates. Of particular importance is the bathymetry of 

the nearshore area. Most cohesive bluffs are fronted by a waveeroded platform covered by only a thin, often discontinuous, 

granular sediment layer. In addition to the lakebed downcutting described previously, the bathymetry of the nearshore can 

be modifi ed with changing lake levels. When lake level is low, 

sand accumulates as low-lying shore terraces and in nearshore 

bars. As water level rises, a new nearshore profi le develops as 

the sand erodes (Davis, 1976).

In addition to bathymetry, the character of the waves, or 

wave climate, is critical. Wave power, or wave power in combination with bathymetry, show the best relationship with recession rate. For example, Kamphuis (1987) reported that the 

1 2

3 4

5 6

Figure 11. Phases of episodic changes for a high cohesive coastal 

bluff subject to continuing toe erosion by waves. (1) Steep unstable 

bluff; (2) large, deep-seated slump takes place causing up to 50 feet 

of bluff recession; (3) wave erosion of toe begins; (4) wave erosion 

continues, lower bluff steepens; (5) wave erosion continues, lower 

steep segment of bluff grows higher; (6) failure occurs again. Cycle 

may take more than 50 years to be completed.
117

recession rate of cohesive till bluffs was proportional to wave 

power to the exponent 1.4. This must be modifi ed for different sites with different bathymetry. This appears to be a fairly 

good predictor, but as pointed out by Quigley (1976a), it has 

limitations for several reasons, the most important of which 

is that it does not take into account lake level change. A more 

comprehensive predictor, and one that appears to be a somewhat better predictor of erosion rate, is wave impact height. 

This is expressed as: 

Wave impact height = (wave runup+still lake elevation+wind 

setup) – base of bluff elevation

Brown (2000) analyzed recession over 5 to 6 time intervals from about 1940 to 2000 for two stretches of the Lake 

Michigan shoreline in Wisconsin. There is a relatively good 

relationship between bluff recession rate and wave impact 

height (fi g.12) for the low bluff site near Two Rivers, but a 

poor relationship between bluff-top recession and wave impact 

height at the high-bluff (Ozaukee) site for reasons described in 

the previous section. The high bluff site does, however, show 

a good relation between bluff toe recession rate and wave impact height (Brown, 2000). In a similar study, Carter and Guy 

(1988) found that maximum lake level was crucial to erosion 

of the bluff toe.

Mitigating Bluff Failure and Recession 

in the Great Lakes

The most signifi cant characteristic of coastal bluffs on the 

Great Lakes is the fact that they are actively evolving natural 

slopes that continually retreat at varying rates with constant or 

evolving geometry. This characteristic sets these slopes apart 

from other natural slopes in terms of stabilization approaches. 

There are basically two approaches to minimize impact on 

humans of actively retreating coastal slopes. Structural approaches are typically developed on a site-specifi c basis. 

Nonstructural approaches typically involve planning and management decisions on a broader scale. The solution strategies 

for actively eroding coastal slopes are summarized in table 3. 

Advice is available to riparian property owners and interested 

professionals on the coastal environment and how to protect 

coastal investments (Keillor, 1998, 2003).

Structural (Stabilization) Approach

The structural approach, with some additional considerations, is similar to other natural slope stabilization efforts. 

A proper stabilization program should include (1) protection 

against wave action in all cases, (2) slope stabilization against 

deep slips if needed (important in the delayed instability often observed in high bluffs formed in stiff clay soils), and (3) 

stabilization against face degradation and shallow slips (including control of surface water) (table 3 and fi g. 13). Shore 

protection is a major component and may be more costly than 

slope stabilization. Problems associated with the execution of 

these solutions are of two types: (1) many attempts are not engineered and fail to anticipate the problems that will arise, and 

(2) engineered solutions often neglect to consider all aspects 

of the problem, thus have deleterious effects on another part of 

the system. 

Numerous erosion control structures have been built to 

protect cohesive bluffs in the Great Lakes, particularly where 

urban development is greatest. These structures fi t into two 

broad categories — shore-normal structures (for example, 

groins and harbor jetties) built to trap sand and shore-parallel 

structures (for example, seawalls, bulkheads, and revetments) 

built to create a physical barrier between attacking waves 

and cohesive shore deposits. Offshore breakwaters built to 

trap sand and prevent wave attack fi t into both categories. In 

more recent years, awareness of the impact of such structures 

on neighboring coastal reaches and nearshore ecology has 

increased, and typically structures that stop all longshore transport of sand are discouraged. Rock (riprap) revetments and 

offshore breakwaters (including submerged breakwaters) that 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Wave-Impact Height, m

1930 1940 1950 1960 1970 1980 1990 2000

Recession Rate, WTR-1

Recession Rate, WTR-2 

Recession Rate, WTR-3

Recession Rate, WTR-4 

Recession Rate, WTR-5 

WIH, WTR-1, no setup

WIH, WTR-2, no setup

WIH, WTR-3, no setup

WIH, WTR-4, no setup

WIH, WTR-5, no setup

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

Recession Rate [m/yr]

38-52 52-61 61-6767-75 92-99 75-92

Annual Average of Peak Monthly Wave-Impact Height

Crest Recession Rate

Manitowoc Site

Figure 12. Relationship between bluff-recession rate and waveimpact height from a 10-km-long reach along the western shore of 

Lake Michigan, near Manitowoc, Wisconsin (from Brown, 2000). 

Indicated negative rates are an artifact of measurement accuracy 

and are essentially zero.

Erosion of Coastal Bluffs in the Great Lakes 
118 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

allow some longshore transport are common forms currently 

favored. Additionally, recent awareness of the importance 

of lake bed downcutting has suggested armoring or paving 

lakebed by use of densely packed cobble-size (15 to 45 cm in 

diameter) stones. So far, it has been used on an experimental 

basis in the Great Lakes.

 Several variables determine the long term effectiveness of 

shore protection structures:

(1) The structure must have enough mass to withstand 

 the forces exerted on the structure by waves imping- 

 ing on the lakeward side of the structure and by the 

 forces exerted by downslope movement of cohesive 

 bluff material behind the structure,

(2) The structure must have suffi cient height to prevent 

 wave overtopping and consequent erosion of cohesive 

 bluff material behind the structure, and

(3) If the fi rst two conditions are met, then issues such as 

 adequate foundation design to support the structure 

 and installation of weep holes to relieve hydraulic 

 pressures become important.

A variety of approaches are available to stabilize the bluff 

once the bluff toe is protected. Prevention of mass movement 

requires an anticipation of the type of movement, location of 

potential failure surface, size of potential failing block, and 

anticipation of the likely triggering mechanism(s). Bluff stabilization approaches typically include:

(1) Modifi cation of slope by reduction of the slope angle 

 by cutting back the top of the slope or buttressing it 

 against sliding by fi lling at the toe to reduce 

 driving stress,

(2) Controlling surface water running onto the slope, 

(3) Revegetating the slope to protect slope face, and

(4) Lowering the ground-water table, thereby reducing 

 pore pressure and increasing resistance to sliding.

Use of structural means such as retaining walls and drilled 

shafts to increase resistance to sliding has been limited, though 

the use of stabilizing berms or buttresses (sometimes internally reinforced) is on the rise.

An integrated approach, as shown in fi gure 14, assures the 

effectiveness of shore protection over a suffi ciently long period 

of time with proper maintenance. This site-specifi c approach 

to protection, if not undertaken over a reach of shoreline (that 

is, a segment with similar wave climate, geomorphology, and 

geologic setting), will likely result in outfl anking of the protected segment by continued recession of the neighboring unprotected shoreline and result in eventual failure.

Management Approach

The nonstructural planning and management approach is 

particularly suitable for undeveloped land where mitigation 

of hazards to transportation, housing, and commercial facilities can be planned and managed over an extensive part of the 

shoreline (the size of a county or at least several kilometers are 

usually considered). These projects are usually aimed at minimizing future structural damage while allowing erosion to take 

Process Solution/Mitigation

Structural/ Stabilization

(design)

Nonstructural/Management

(prediction)

Toe erosion Shore protection

(revetments, breakwaters

groins, seawalls, beach

nourishment)

Shore recession rate

(long-term and cyclic)

Deep rotational slides Slope stabilization

(regrading, buttressing,

dewatering)

Stable slope angle against

steep slides

Face degradation and

shallow slides and flows

Surface protection

(vegetation, surface water

management, berms)

Ultimate angle of stability

for shallow slides and flows

Table 3. Strategies for mitigating bluff failure and recession.

Figure 13. Three steps to stabilization of coastal bluffs on the Great 

Lakes. (1) Toe protection against waves ensures no further steepening; (2) slope must be stabilized with respect to deep slumps if 

the present slope angle is greater than safe stable slope angle; (3) 

bluff face is protected to reduce or prevent shallow slides and face 

degradation.
119

place, thus avoiding problems with structures described in the 

next section. In this case, the need for understanding bluff processes is critical because predictions of future recession over 

a long period of time with changing water level and climate 

conditions are necessary. This approach necessitates an understanding of bluff processes and development of qualitative 

(and preferably quantitative) models of bluff evolution. The 

main problem of prediction of slope evolution is understanding the response times to environmental changes and the time 

necessary for bluffs to pass through an evolutionary sequence. 

The main tool used in the nonstructural or management 

approach is the establishment of a setback requirement for 

new buildings or infrastructure. This requires knowledge of 

coastal recession over a long time, at least 30 to 50 years, and 

the determination of stable slope angles. Typically, historical 

aerial photographs are used to establish the recession rates and 

geological and geotechnical analyses are used to determine 

the stable slope angles. Research conducted primarily during 

the last few decades has identifi ed the operating processes and 

their possible magnitudes (Edil, 1982), and a nonstructural setback distance can be estimated as shown in fi gure 15 (Southeastern Wisconsin Regional Planning Commission, 1989). In 

this case, the setback distance consists of two components. 

Erosion risk distance is the distance from the existing bluff 

edge that could be affected by recession of the bluff over some 

appropriate time (50 years?) plus the setback necessary to regrade the bluff to a stable slope angle. The minimum facility 

setback distance is an additional safety zone. 

Environmental and Ecological Impacts of Shore 

Protection Structures

Although sparsely developed areas along the Great Lakes 

shorelines remain unprotected by structures, numerous attempts have been made over the past 150 years to stop erosion 

in more developed areas. Along the south shore of Lake Erie, 

where construction of shore protection structures began well 

before the turn of the century, armoring of the lakeshore has 

followed a progression. The earliest shore protection structures 

were seawalls and bulkheads built along harbor water fronts. 

In urban areas away from the harbors, groins were constructed 

to trap sand to create or stabilize the beach. If the groins did 

not maintain a beach of adequate width, or if rising lake levFigure 14. Typical methods of coastal bluff stabilization used along the western Lake Michigan shore (from Southeast Wisconsin Regional 

Planning Commission, 1989).

Erosion of Coastal Bluffs in the Great Lakes 
120 Formation, Evolution, and Stability of Coastal Cliffs—Status and Trends

els inundated part of the beach, seawalls and bulkheads were 

installed to prevent erosion of the bluff toe. As seawalls failed, 

they were typically replaced by small breakwaters or rock revetments. 

All of these structures have had a severe impact on the 

beach/nearshore system. Shore-normal structures, such as 

groins and harbor structures, trap sand to create a beach. This 

commonly creates or aggravates erosion along the downdrift 

shore. Eroding bluffs and erosional embayments are typical 

features downdrift of shore-normal structures in the Great 

Lakes. For groins, this effect may extend hundreds of meters. 

For long harbor jetties, the effect may extend for kilometers.

Most shore-parallel structures do not trap sand (breakwaters are the exception). However, they may adversely affect 

coastal processes. Downward defl ection of wave energy along 

vertically faced structures scours the lake bed unless a scour 

apron is installed along the base of the structure. If the structure is built at the back of a beach too narrow to dissipate wave 

energy, turbulence along the face of the structure may erode 

the beach. Spray generated by waves hitting vertically faced 

structures may saturate the bluff face and erode loose material. 

Vertically faced structures also refl ect wave energy offshore 

and against an adjacent shore. Using armor-stone construction 

reduces problems of wave scour, wave spray, and wave refl ection, but the irregular surface of the structure restricts access 

to the lake. 

A less apparent impact of shore-parallel structures occurs where a structure completely covers the beach, an all too 

typical mode of construction in the Great Lakes. In order for a 

beach to reform in front of the structure, the nearshore profi le 

must build upward to reestablish an equilibrium profi le. For 

many reaches of the Great Lakes, there is insuffi cient sand to 

build up the nearshore profi le, resulting in a permanent beachless shore.

Recreational use of the lake is adversely affected by 

structures. As just noted, the irregular surface of armor-stone 

(or concrete-rubble) structures restricts access to the lake. 

Where concrete rubble used to build a structure is dispersed 

by wave action, exposed reinforcing bars poses a serious threat 

to swimming and boating. Where the shore is armored with 

vertically faced structures, constructive interference of incoming and refl ected waves increases wave height and may locally 

affect recreational boating. With proper design, structures can 

be designed to minimize adverse impacts, limit erosion, and 

provide access to the lake.

Armoring a cohesive-bluff shore cuts off an important 

source of sand for the littoral system. For example, cohesive 

bluffs in Ohio contain about 20 percent sand-size or coarser 

material, and as the bluffs erode, this sand nourishes the littoral system. For the United States shore of Lake Erie, erosion 

of cohesive bluff material between the 1870s and the 1970s annually contributed about 3,350 m3

 of sand and gravel per km 

of shore; erosion of cohesive nearshore deposits contributed an 

additional 725 m3 per km (Carter, 1977). Armoring these bluffs 

cuts off this source of sand and results in a sand-starved littoral 

system. Along some reaches of the Great Lakes, particularly 

along the south shore of Lake Erie, impoundment of sand by 

structures and armoring of the shore has created sand-starved 

reaches that extend kilometers downdrift.

Loss of sand from the beach and nearshore also results in 

greater turbidity, as the sand-starved shore and nearshore are 

exposed to erosion by frequent, small-wave events. This adFigure 15. Procedure used to estimate nonstructural setback distance for management option of mitigating bluff recession impacts 

(from Southeast Wisconsin Regional Planning Commission, 1989).
121

versely affects water quality. Loss of sand from the nearshore 

also alters the nearshore biologic habitat. Many organisms that 

inhabit the nearshore are adapted to a mobile sand substrate 

and the bar and trough system that forms where sand is present. Loss of this sand and replacement by a cobble and boulder 

covered wave-cut platform has a negative effect on these organisms and encourages growth of nuisance species like zebra 

mussels. Armoring the lakeshore to prevent shore erosion has 

had signifi cant impacts on coastal sand resources and coastal 

habitats. The full extent and nature of these impacts are still 

not fully understood.

Future Research Needs

Several areas seem to have great promise as future avenues of research. Prediction of future recession is an overarching concern for coastal planners and managers, and listed 

below are several ways in which our ability to predict recession could be improved:

(1) One approach is to keep better track of what is hap 

 pening now on the shoreline. Advanced digital 

 technologies provide a much greater capability of 

 determining past rates of recession, limited only by 

 the availability of aerial photos or satellite im 

 ages, the frequency at which they have been taken, 

 and geomorphic interpretation of changes. Frequency 

 could be increased by regular, high quality air 

 photography by drones. 

(2) Our ability to connect lake level and more complex 

 variables, such as wave impact height, to beach and 

 bluff-toe erosion can be improved. This would help 

 with prediction of future bluff recession, if lake 

 level changes could be predicted. That presumably 

 would require linking lake level to climate change 

 models and using this approach to forecast potential 

 scenarios.

(3) Although there are models of sediment transport in 

 the nearshore zone, there still is no comprehensive 

 model that integrates bluff process with processes 

 with beach and nearshore processes. 

(4) In order to predict future behavior of high, cohesive 

 bluffs, we must understand the geomorphic changes 

 that take place as the bluff shape evolves after a 

 massive failure. We must also understand the 

 timescale over which these changes take place. 

Another important future research need is quantifying the 

cumulative and secondary impacts of armoring cohesive bluffs 

on coastal sand budgets. Most Great Lakes shorelines are still 

not protected, but as population increases, more and more of 

the shoreline will be protected from erosion, thus reducing 

sand supply. We don’t fully understand the physical and biological impacts of this trend. 

There may be ways of minimizing the impacts of shore 

protection by changing the way we solve problems of slope 

instability. For instance, it may be possible to identify incipient bluff failure, so that remedial measures can be taken to 

locally stabilize the bluff. One example would be to map the 

topography on cohesive bluff materials that underlie sandy 

stratigraphic units, so that one or two strategically placed wells 

might be used to dewater the bluff, thereby increasing bluff 

stability without building structures that interfere with the 

beach/nearshore system.

Finally, geologists and engineers need to research better 

ways to educate the public about hazard and resource issues 

along the Great Lakes shores. Most negative impacts result 

from confl icts between human and natural process, and a better educated public might lead to a reduction of these confl icts.

Acknowledgements

We would like to thank A.S. Trenhaile and Laura Moore 

for very helpful comments during the review process. E.A. 

(Lisa) Brown was helpful in assembling some of the background information that is not specifi cally cited in the text. 

Anoushka Sarrazin and Leslie Pearson helped with an extensive literature search. Lindsay Anderson and Jessica Darter 

provided helpful comments on the manuscript. Mary Diman 

and Adam Kiehn provided excellent graphics and editorial 

support.

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Erosion of Coastal Bluffs in the Great Lakes