vvEPA
United States
Environmental Protection
Agency
Office of Policy
Planning and Evaluation
Washington, DC 20460
October 1985
EPA 230-10-85-013
Potential Impacts of
Sea Level Rise on the
Beach at Ocean City,
Maryland
•...,, >? -jjfa, .
*«r
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Library of Congress Cataloging-in-Publication Data
Main entry under title:
Potential impacts of sea level rise on the beach
at Ocean City, Maryland.
Includes bibliographies.
1. Beaches—Maryland—Ocean City. 2. Sea level—
Maryland—Ocean City. 3. Beach erosion—Maryland—
Ocean City. 4. Greenhouse effect, Atmospheric, I. Titus, James Q.
II. U. S. Environmental Protection Agency.
GB459.4.P67 1985 333.91'7'0975221 85-27563
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POTENTIAL IMPACTS OF SEA LEVEL RISE
ON THE BEACH AT OCEAN CITY, MARYLAND
by
James G. Titus
U.S. Environmental Protection Agency
Stephen P. Leatherman
University of Maryland
Craig H. Everts
Moffatt and Nichol, Engineers
David L. Kriebel
Robert G. Dean
University of Florida
This document has been reviewed in accordance with the U.S.
Environmental Protection Agency's peer and administrative
review policies and approved for publication. Mention of
trade names or commercial products does not constitute
endorsement or recommendation for use. Please send comments
to James G. Titus (PM-220), U.S. Environmental Protection
Agency, Washington, B.C. 20460.
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SUMMARY
Recent reports by the National Academy of Sciences and others have
concluded that increasing atmospheric concentrations of carbon dioxide and
other gases can be expected to cause a global warming that could raise sea
level a few feet in the next century. Unfortunately, it is not yet possible
to accurately predict future sea level. Estimates for the year 2025 range
from five to twenty-one inches above current sea level, while estimates of the
rise by 2100 range from two to eleven feet.
Several issues must be resolved for society to rationally address the
possibility of a significant rise in sea level. Officials in coastal areas
making decisions about near-term projects with long lifetimes must determine
whether the risk of sea level rise justifies a shift to strategies that can
more successfully accommodate a rise in sea level. The research community
needs to decide whether to accelerate studies to more accurately project
future sea level. These decisions require assessments of the adequacy of
existing forecasts, prospects for improving the estimates, and the level of
resources that can be saved if more definitive estimates become available.
These decisions also require an understanding of the consequences of sea
level rise. To futher this understanding, EPA has initiated studies of the
impacts of sea level rise on Charleston, South Carolina; Galveston, Texas;
coastal wetlands; municipal drainage facilities; and salinity of surface and
ground water.
This study examines the potential implications of sea level rise for
efforts to control erosion of the beach at Ocean City, Maryland, a typical
Atlantic Coast resort. Because current trends in sea level and other factors
ire already causing significant erosion at Ocean City and other ocean beach
resorts, strategies for addressing coastal erosion constitute a class of
near-term decisions that may depend on sea level rise. Because land and
improvements are often worth well over one million dollars per acre in these
areas, and erosion increases the likelihood of storm damage and federal
disaster payments, the success of erosion control measures has great economic
importance to the nation. We hope that this report will promote a reasoned
consideration of the long-term consequences of sea level rise, and thereby
enhance the eventual success of erosion control strategies at Ocean City and
other coastal communities.
In this report, three independent teams of coastal process scientists
estimate the erosion that will take place at Ocean City for three scenarios of
future sea level rise: (1) current trends of 1 foot per century along the
Atlantic coast; (2) the National Academy of Sciences estimate of a 2-1/3 foot
global rise in the next century with an 11 inch rise by 2025; and (3) the EPA
mid-high scenario of a global rise of 4-1/2 feet in the next century and 15
inches by 2025. The quantity of sand necessary to maintain the current
shoreline is also estimated for each of the scenarios. Using these estimates
and previous studies by the Corps of Engineers and others, the potential costs
of erosion control are also examined.
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iii
CONCLUSIONS
1. Sea level rise could double the rate of erosion at Ocean City in the
next forty years. If no additional erosion control measures are taken, the
shore will erode 85-153 feet by 2025 assuming current sea level trends. An
11-inch global rise in sea level would increase expected erosion to between
180 and 238 feet, if no additional measures are taken; a 15-inch rise would
increase expected erosion to between 216 and 273 feet.
2. The projected rise in sea level would increase the quantity of sand
necessary to maintain the current shoreline for the next forty years from
5-10 million cubic yards if current trends continue, to 11-15 million cubic
yards for the two scenarios of accelerated sea level rise.
3. Projected sea level rise would increase the priority of erosion
control measures under current policies of the Corps of Engineers. Current
policies place a greater emphasis on flood protection than recreational
benefits provided by proposed projects. Because of the substantial erosion
that could occur from a rise in sea level, the need for flood protection will
be greater if sea level rises.
4. A significant rise in sea level would require a change in the
technology used to control erosion at Ocean City. The current plan to
construct groins was designed to curtail erosion caused by sand moving along
the shore. However, groins do not prevent erosion caused by sea level rise.
Placement of additional sand onto the beach would offset erosion caused by
both sea level rise and alongshore transport.
5. The cost of controlling erosion caused by sea level rise does not
threaten the econcnic viability of Ocean City in the next forty years. Even
the most pessimistic estimate of future erosion control implies a cost of less
than fifty cents for every visitor that comes to Ocean City each year.
Protecting the shore at Ocean City will continue to be economically justified.
6. Understanding the likely impact of sea level rise on Ocean City in
the next century will require identification of the most cost-effective and
environmentally acceptable sources for up to fifty million cubic yards of
sand to be placed on the beach.
7. Better estimates of future sea level rise would enable decision makers
to more adequately determine the most prudent strategy for controlling erosion
at Ocean City.
8. Although improved procedures for estimating erosion are desirable,
current methods are sufficient to yield first-order estimates for use in
long-term planning.
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IV
ACKNOWLEDGMENTS
We wish to express our appreciation for the numerous people who provided
helpful contributions and encouraged us to publish this report. Torrey Brown
and Sarah Taylor of the Maryland Department of Natural Resources initially
requested that EPA use the Ocean City situation as a case study of sea level
rise impacts. Sandy Coyman of Ocean City's Department of Planning and
Community Development provided numerous editorial contributions to make the
report more readable. Ed Fulford and Suzette May of the Army Corps of
Engineers provided several important insights concerning erosion processses
along the Maryland coast.
Chris Zabawa and Earl Bradley of the Maryland Tidewater Administration's
Coastal Resources Division provided general advice throughout the project.
Randall T. Kerhin of the Maryland Geological Survey and Rebecca Hughes of the
Maryland Flood Management Division also commented on the report. Judith
Johnson of the Committee to Preserve Assateague provided a particularly
detailed review of the four chapters in this report. Moffatt and Nichol,
Engineers, provided consulting services in addition to those included in its
contract with EPA.
Dennis Tirpak and John S. Hoffman of EPA's Office of Policy Analysis made
important organizational contributions to Chapter 1. William Hoffman and
Marria 0'Mailey of EPA's Region III reviewed the report from the perspective
of wetland and marine policy. Susan MacMillan of ICF Incorporated and Joan
O'Callaghan of EPA provided detailed editorial contributions. Finally, Margo
Brown prepared the manuscript.
Without the contributions of these and other people who encouraged us
along the way, this report would not have been possible.
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TABLE OF CONTENTS
CHAPTER 1: SEA LEVEL RISE AND THE MARYLAND COAST
James 6. Titus 1
Introduction 2
The Basis for Expecting a Rise in Sea Level 4
Impacts of Sea Level Rise 14
Ocean City Case Study 16
Next Steps 26
Notes 27
References 29
CHAPTER 2: GEOMORPHIC EFFECTS OF ACCELERATED SEA-LEVEL RISE ON
OCEAN CITY, MARYLAND
Stephen P. Leatherman 33
Introduction 34
Site Description 37
Analysis of Shoreline Response 41
Methods 46
Results 48
Summary 57
Appendix I. Nomenclature for Shoreline Interactions with
Sea Level Rise 58
Appendix II. Profile Changes at Ocean City, Maryland:
1929-1978 59
References 64
CHAPTER 3: EFFECT OF SEA LEVEL RISE AND NET SAND VOLUME CHANGE ON
SHO^LINE POSITION AT OCEAN CITY, MARYLAND
Craig H. Everts 67
Introduction 69
Methodologies 69
Temporal and Spatial Averages 74
Data Requirements 76
Calculations 88
Summary 91
References 96
CHAPTER 4: ESTIMATES OF EROSION AND MITIGATION REQUIREMENTS UNDER
VARIOUS SCENARIOS OF SEA LEVEL RISE AND STORM FREQUENCY
FOR OCEAN CITY, MARYLAND
David L. Kriebel and Robert G. Dean 99
Introduction 102
Description of Beach-Dune Erosion Model 103
Calibration -- Seville's Laboratory Experiment .....' 113
Calibration -- Hurricane Eloise Field Data 119
Application to Ocean City, Maryland -- Storm Erosion 132
Application to Ocean City, Maryland -- Erosion Due
to Sea Level Rise 155
Summary and Conclusions 172
References 175
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vi
LIST OF FIGURES
Page
CHAPTER 1
Figure 1. Global Temperatures and Sea Level 6
Figure 2. Measurements of Atmospheric Carbon-Dioxide Abundance
Over Time: 1958-1981 8
Figure 3. Estimated Global Warming Due to a Doubling of
Greenhouse Gases 9
Figure 4. Global Sea Level Rise Scenarios 11
Figure 5. The Bruun Rule 15
Figure 6. Current Shoreline and Projected Erosion at Assateague
Island 18
CHAPTER 2
Figure 1. Recent Sea-Level Changes Along the U.S. Coast 35
Figure 2. Location of Study Area Along the Delmarva Peninsula .... 38
Figure 3. High-Rise Condominimums and Hotels in Ocean City 39
Figure 4. Landward Barrier Migration 40
Figure 5. Shore Adjustment with Sea-Level Rise 42
Figure 6. Shore Adjustment to Change in Water Level 42
Figure 7. Open-Coast Storm Surge Frequency for Ocean City,
Maryland 45
Figure 8. Metric Mapping Technique 47
Figure 9. Comparison of Historical Shoreline Changes Along
Ocean City, Maryland (1850-1980) 49
Figure 10. Index Map of Ocean City Showing Transects Used by
Program that Measures Shoreline Changes 50
Figure 11. Histogram of Historical Shoreline Changes (1929-1942) .. 51
Figure 12. Histogram of Historical Shoreline Changes (1942-1962) .. 52
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vii
LIST OF FIGURES (continued)
Page
CHAPTER 2 (continued)
Figure 13. Histogram of Historical Shoreline Changes (1962-1980) .. 53
Figure 14. Histogram of Historical Shoreline Changes (1850-1980) .. 54
CHAPTER 3
Figure 1. Definition Sketch, Bruun's Method 70
Figure 2. Definition Sketch, Everts' Method 72
Figure 3. Mean Yearly Sea-Level Elevation for Five Tidal Gauges .. 75
Figure 4. Average Shoreface Profiles for the Survey Years
1929, 1965, 1978, and 1979 78
Figure 5. Sediment Size Beneath the Barrier Island and Landward
of the Shoreface 81
Figure 6. Portion of Sand Behind the Shoreface and Above -8.5m
(-28 ft) Mean Sea Level 82
Figure 7. Shore-Parallel Profiles of Shore-Connected Ridges 85
Figure 8. Shoreline Change Rates fox the Period 1929-1980 87
Figure 9. Variations in Shoreline Change Rate 90
Figure 10. Estimated Future Shoreline Retreat and Beach
Nourishment Requirement at Ocean City, Maryland 94
CHAPTER 4
Figure 1. Equilibrium Beach Profile Concepts for Numerical
Erosion Model 104
Figure 2. Equilibrium A Parameter as a Function of Grain Size
and Fall Velocity 105
Figure 3. Previous Method of Estimating Distribution of
Sediment Transport on Beach Face 109
Figure 4. Definition Sketch of Schematic Beach Profile 109
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viii
LIST OF FIGURES (continued)
CHAPTER 4 (continued)
Figure 5. New Method of Estimating Distribution of Sediment
Transport on Beach Face 110
Figure 6. Examples of Estimated Sediment Transport Distributions
on Beach Face 112
Figure 7. Mean Square Error of Volume Eroded Versus Sediment
Transport Coefficient K 115
Figure 8. Comparison of Cumulative Erosion: Calibrated Model
Versus Saville's (1957) Laboratory Experiments 116
Figure 9. Comparison of Profile Forms: Calibrated Model
Versus Saville's (1957) Laboratory Experiments 117
Figure 10. Time-Dependent Evolution of Predicted Profile 118
Figure 11. Hurricane Eloise Storm Surge Hydrograph 120
Figure 12. Pre- and Post-Storm Beach Profile 121
Figure 13. Predicted Volume Eroded for Profile R-41 Versus
Sediment Transport Coefficient K 123
Figure 14. Sensitivity of Predicted Volume Eroded to Wave
Height Description 125
Figure 15. Comparison of Predicted to Observed Post-Storm
Profile Forms 126
Figure 16. Time-Dependent Evolution of Predicted Profile 127
Figure 17. Example of Offshore and Nearshore Predicted Profile
Forms 128
Figure 18. Comparison of Predicted to Observed Erosion for 20
Beach Profiles from Walton County, Florida 130
Figure 19. Nearshore Beach Profile, Ocean City, Maryland 133
Figure 20. Approximate Equilibrium Offshore Profile Forms, Ocean
City, Maryland 135
Figure 21. Storm Erosion Estimates, 14-Foot Dune Height 138
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IX
LIST OF FIGURES (continued)
Page
CHAPTER 4 (continued)
Figure 22. Storm Erosion Estimates, 12-Foot Dune Height 139
Figure 23. Storm Erosion Estimates, 10-Foot Dune Height 140
Figure 24. Estimated Post-Storm Erosion Profiles, 6.3-Foot Peak
Storm Surge 142
Figure 25. Estimated Post-Storm Erosion Profiles, 7.5-Foot Peak
Storm Surge 143
Figure 26. Estimated Post-Storm Erosion Profiles, 8.7-Foot Peak
Storm Surge 144
Figure 27. Estimated Post-Storm Erosion Profiles, 10.3-Foot Peak
Storm Surge 145
Figure 28. Adopted Storm Erorion Estimates for Reference Profile,
Ocean City, Maryland 147
Figure 29. Net Beach Fill Requirements to Prevent Dune
Recession for Reference Profile, Ocean City, Maryland .. 150
Figure 30. Initial and Equilibrium Configurations for Beach Fill
Plan Recommended by U.S. Army Corps of Engineers 153
Figure 31. Estimated Post-Storm Erosion Profiles for Recommended
Beach Fill Plan 154
Figure 32. Response Characteristics of Reference Profile to
Relative Water Level Rise 157
Figure 33. Future Erosion Estimates Due to Sea Level Rise and
Net Sand Volume Losses 162
Figure 34. Future Shoreline Retreat Estimates Due to Sea Level
Rise and Net Sand Volume Losses 163
Figure 35. Future Mitigation Requirements to Prevent Shoreline
Retreat 166
Figure 36. Future Erosion Limits Due to Storms and Long-Term
Erosion for Extension of Current Sea Level Rise Trend .. 168
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LIST OF FIGURES (continued)
Page
CHAPTER 4 (continued)
Figure 37. Future Erosion Limits Due to Storms and Long-Term
Erosion for Mid-Low Sea Level Rise Scenario 169
Figure 38. Future Erosion Limits Due to Storms and Long-Term
Erosion for Mid-High Sea Level Rise Scenario 170
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LIST OF TABLES
Page
CHAPTER 1
Table 1. Scenarios of Worldwide Sea Level Rise 12
Table 2. Relative Sea Level Rise Scenarios for Ocean City,
Maryland 13
Table 3. Retreat of the Beach at Ocean City, Maryland:
1962-1978 20
Table 4. Projected Erosion at Ocean City 21
Table 5. Sand Required to Maintain Current Shoreline 23
CHAPTER 2
Table 1. Relative Sea-Level Rise Scenarios 36
Table 2. Major Storms of Record for Ocean City, Maryland 44
Table 3. Projected Shoreline Recession Along Ocean City,
Maryland 55
Table 4. Contour Shifts (1929-1965) 55
Table II-l. Contour Data from 3rd Street to 145th Street 60
Table II-2. Change in the Position of the Shoreline and -10,
-20, and -30 foot Contours from 1962 to 1978 63
CHAPTER 3
Table 1. Values Used in Calculations 77
Table 2. Calculated Shore Retreat for Ocean City, Maryland,
1930-1980 89
Table 3. Relative Sea Level Rise Scenarios 92
Table 4. Shoreline Retreat Scenarios for Ocean City, Maryland 93
Table 5. Calculated Beachfill Requirements for Ocean City,
Maryland 95
Table 6. Percent of Beachfill Requirement Attributed to
Sea-Level Rise at Ocean City 95
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CHAPTER 4
Table 1.
Table 2.
Table 3.
LIST OF TABLES (continued)
Page
Estimates of Dune Erosion Potential for Recommended
Beach Fill Design of Corps of Engineers 152
Summary of Historical Shoreline Retreat Estimates 160
Relative Sea Level Rise Scenarios 161
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CHAPTER I
SEA LEVEL RISE AND THE MARYLAND COAST
by
James G. Titus
Office of Policy Analysis, U.S. Environmental
Protection Agency, Washington, D.C. 20460
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INTRODUCTION
In the last few decades, Americans have increasingly used the resources
offered by our coastal areas. The popularity of beaches now accounts for a
multibillion dollar industry. Recreational hunting and fishing, while less
significant nationwide, are major attractions in coastal wetlands and
estuaries, such as Louisiana's marshes and swamps, Chesapeake Bay, and
Narragansett Bay.1 Recreational boating has also become more widespread in
coastal areas.2
To accommodate increasing numbers of visitors, modern high-rise hotels and
condominiums, houses, and marinas have replaced the small cottages and vacant
land that once characterized ocean beach resorts and barrier islands. High
land values have sometimes encouraged people to create land by filling marshes
and shallow bays. Many mainland areas within a short commute to the beach are
also being developed extensively.
Increasing development has entailed certain economic and environmental
risks. Buildings in many coastal areas are vulnerable to severe storms which
generally occur every thirty to fifty years (Kunreuther 1978). In many areas,
the beaches are eroding, which gradually removes an important recreational
asset and increases the vulnerability of shorefront property to storms. The
filling of coastal marshes has sometimes destroyed fish and wildlife habitats
and impaired water quality in coastal areas (Office of Technology Assessment
1984). Bulkheads that eliminate natural bay beaches can threaten the food
supply of shore birds.
Congress has enacted several policies to address these risks. In 1968 it
found that "many factors have made it uneconomic for the private insurance
industry alone to make flood insurance available."3 As a result, it enacted
the National Flood Insurance Act which requires property owners with federally
insured mortgages in coastal hazard areas to obtain flood insurance, and
requires participating communities to take measures to ensure that newly
constructed buildings will not be destroyed by a major storm. In 1972
Congress declared that it is national policy to "preserve, protect, develop,
and where possible to restore or enhance, the resources of the nation's
coastal zones for this and succeeding generations"* and passed the Coastal
Zone Management Act, which encourages states to develop coastal policies to
ensure that new development is safe and provides for the conservation of
wetlands and other natural environments. The Coastal Barrier Resources Act
forbids federal subsidies to designated undeveloped barrier islands. Section
404 of the Clean Water Act requires anyone wishing to build on a coastal marsh
to obtain a permit from the Army Corps of Engineers with approval by the
Environmental Protection Agency. Finally, the National Environmental Policy
Act requires an environmental impact statement informing the public of
potential environmental risks for any major federal action, including a permit
under Section 404.
These programs are generally administered by state and local governments.
Over seventeen thousand communities participate in the National Flood
Insurance Program, which requires them to enact zoning and building codes to
prevent excessively hazardous construction. States develop coastal zone
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management plans subject to approval by the federal government. Provided that
the necessary assessments and permits are filed, the decision whether to fill
a marsh is primarily a local land use decision. Many states and localities
have gone beyond federal requirements and effectively prohibited the
construction of bulkheads or filling of coastal marshes.5 These and other
federal, state, and local policies have reduced the economic and environmental
risks of developing coastal areas.
Recent scientific findings, however, suggest that current policies may be
overlooking an environmental impact that could exacerbate the other risks: a
rise in the level of the oceans. Increasing atmospheric concentrations of
carbon dioxide and other gases are expected to warm our planet a few degrees
centigrade in the next century by a mechanism known as the "greenhouse
effect." Such a global warming would probably cause sea level to rise more
rapidly than it is currently. Although estimates of the rise expected in the
next one hundred years range from 38 to 211 centimeters (15 inches to 8 feet),
a precise forecast will not be possible in the foreseeable future.
Even a thirty-centimeter (one-foot) rise in sea level would have important
environmental impacts and would change the consequences of decisions made
today. Along the open coast, beaches could erode 20 to 80 meters (60 to 250
feet), and buildings would be more vulnerable to storms (Bruun 1962). Along
the shores of coastal estuaries, existing marshes would drown and homeowners
in some areas would have to build levees and bulkheads to prevent new marshes
from taking over their properties (Kana, Baca, and Williams 1985).
With a rise of one meter, most coastal communities would have to choose
between several undesirable alternatives: investing substantial resources to
maintain beaches and wetlands in their current locations; building seawalls
and bulkheads to protect property while allowing beaches and marshes to erode
away; or allowing beaches and marshes to encroach inland onto previously
developed land. Fortunately, many of the potential costs can be avoided or
reduced if timely measures are taken in anticipation of sea level rise (Barth
and Titus 1984).
This report examines the erosion that sea level rise could cause the
resort community of Ocean City, Maryland, over the next ninety years. Like
many resorts, Ocean City has an erosion problem. Although city and state
agencies are undertaking measures to reduce erosion, their strategies do not
yet consider the impacts of rising sea level. We hope that this report will
help promote a reasoned consideration of the long-term consequences of sea
level rise, and thereby enhance the eventual success of erosion control
strategies at Ocean City.* We also encourage other coastal communities with
erosion problems to consider the implications of a rising sea.
In the following chapters, three coastal research teams describe their
independent assessments of beach erosion from sea level rise and other
* This report does not consider options for reducing the rise in sea level
due to the greenhouse effect. See Lovins et al. (1981) and Seidel and Keyes
(1983) for discussions of this issue.
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factors. In Chapter 2, Leatherman presents "Geomorphic Effects of Accelerated
Sea Level Rise on Ocean City, Maryland," with an appendix by Bresee. In
Chapter 3, Everts presents "Effect of Sea Level Rise and Net Sand Volume
Changes on Shoreline Position at Ocean City, Maryland." Finally, in Chapter
4, Kriebel and Dean present "Estimates of Erosion and Mitigation Requirements
under Various Scenarios of Sea Level Rise and Storm Frequency for Ocean City,
Maryland."
In this introductory chapter, written for the general reader, we summarize
the results of those studies and other relevant information. We describe the
basis for expecting a significant rise in sea level in the future; provide an
overview of the possible impacts on Maryland and other coastal areas;
summarize the three studies presented in Chapters 2 through 4; and briefly
discuss the implications of these studies and additional steps that could help
Ocean City and similar communities prepare for the consequences of future sea
level rise. Because this study focuses primarily on erosion and beach
nourishment, a more thorough assessment of the long-term economic and policy
implications should be undertaken using the technical data this report
provides.
THE BASIS FOR EXPECTING A RISE IN SEA LEVEL
Past Trends in Sea Level
Throughout geologic history, sea level has risen and fallen by over three
hundred meters (one thousand feet) due to changes in (1) the shape and size of
ocean basins, (2) the amount of water in the oceans, and (3) the average
density of seawater. The emergence and submergence of land has also changed
sea level relative to particular land masses. The first three factors
influence "global sea level"; the latter affects "relative sea level."
In the last 100 million years, changes in the size and shape of ocean
basins have caused the greatest changes in global sea level (Hays and Pitman
1973). However, in the last several thousand years, these processes have
usually been relatively slow and are not likely to accelerate in the near
future.*
Sea level has risen and fallen with past changes in world climate. During
the ice ages, the average global temperature has been 5°C colder than today
(Hansen et al. 1984). With glaciers covering much of the northern hemisphere,
there has been less water in the oceans and the sea level has been one hundred
to one hundred fifty meters (three hundred to five hundred feet) lower than
today (Donn, Farrand, and Ewing 1962). During previous interglacial (warm)
periods, on the other hand, global temperatures have been 1-2°C warmer than
today and sea level has been about six meters (twenty feet) higher (Hollin
1972).
Although the glaciers that covered much of the northern hemisphere during
the last ice age have melted, polar glaciers in Greenland and Antarctica
contain enough water to raise sea level more than seventy meters (over two
hundred feet) (Untersteiner 1975). A complete melting of these glaciers has
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not occurred in the last two million years, and would take tens of thousands
of years even if the earth warmed substantially. However, unlike the other
glaciers which rest on land, the west antarctic ice sheet is marine-based and
more vulnerable to temperature increases. Warmer ocean water would be more
effective than warmer air at melting glaciers, causing West Antarctica to
melt. Mercer (1970) suggests that the west antarctic ice sheet completely
disappeared during the last interglacial period, raising sea level five to
seven meters (about twenty feet) above its present level.
Over relatively short periods of time, climate can influence sea level by
heating and thereby expanding (or cooling and contracting) sea water. In the
last century, tidal gauges have been available to measure relative sea level
in particular locations. Along the Atlantic Coast, sea level has risen about
30 centimeters (one foot) in the last century (Hicks, Debaugh, and Hickman
1983). Studies combining all the measurements have concluded that average
worldwide sea level has risen ten to fifteen centimeters (four to six inches)
in the last one hundred years (Barnett 1983; Gornitz, Lebedeff, and Hansen
1982). At least part of this rise can be explained by the thermal expansion
of the upper layers of the oceans resulting from the observed warming of 0.4°C
in the last century (Gornitz, Lebedeff, and Hansen 1982). Meltwater from
mountain glaciers has also contributed to sea level rise (Meier 1984). Figure
1 shows that global temperature and sea level have been rising in the last
century. Nevertheless, questions remain over the magnitude and causes of sea
level rise in the last century.
The Greenhouse Effect
Concern about a possible acceleration in the rate of sea level rise arises
from measurements that concentrations of carbon dioxide (C02), methane,
chlorofluorocarbons, and other gases released by human activities are
increasing. Because these gases absorb infrared radiation (heat), scientists
generally expect the earth to warm substantially. Although some people have
suggested that unknown or unpredictable factors could offset this warming, the
National Academy of Sciences (NAS) has twice reviewed all the evidence and
concluded that the warming will take place. In 1979, the Academy concluded:
"We have tried but have been unable to find any overlooked physical effect
that could reduce the currently estimated global warming to negligible
proportions" (Charney 1979). In 1982, NAS confirmed the 1979 assessment
(Smagorinsky 1982).
A planet's temperature is determined primarily by the amount of sunlight
it receives, the amount of sunlight it reflects, and the extent to which its
atmosphere retains heat. When sunlight strikes the earth, it warms the
surface, which then reradiates the heat as infrared radiation. However, water
vapor, C02, and other gases in the atmosphere absorb some of the energy rather
than allowing it to pass undeterred through the atmosphere to space. Because
the atmosphere traps heat and warms the earth in a manner somewhat analogous
to the glass panels of a greenhouse, this phenomenon is generally known as the
"greenhouse effect." Without the greenhouse effect of the gases that occur in
the atmosphere naturally, the earth would be approximately 33°C (60°F) colder
than it is currently (Hansen et al. 1984). Thus, the greenhouse effect per
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FIGURE 1
GLOBAL TEMPERATURES AND SEA LEVEL
HAVE RISEN IN THE LAST CENTURY
0.2
Temperature o
-0.2
10
Sea Level
(cm)
-5
1880
I
1920
1960
Year
Sources: Temperature curve from: J.E. Hansen et al., "Climate Impact of
Increasing Atmospheric Carbon Dioxide," Science, 1981, p. 957-966.
Sea level curve adapted from: V. Gornitz, S. Lebedeff, and J.
Hansen, "Global Sea Level Trend in the Past Century," Science,
1982, p. 1611-1614.
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se is not something that will happen; it is a natural characteristic of the
atmosphere.
In recent decades, the concentrations of these "greenhouse gases" have
been increasing. Since the industrial revolution, the combustion of fossil
fuels, deforestation, and cement manufacture have released enough CO2 into the
atmosphere to raise the atmospheric concentration of carbon dioxide by 20
percent (Keeling, Bacastbw, and Whorf 1982). As Figure 2 shows, the
concentration has increased 8 percent since 1958. Recently, the
concentrations of methane, nitrous oxide chlorofluorocarbons and some other
trace gases that also absorb infrared radiation have also been increasing
(Lacis et al. 1981; Ramanathan et al. 1985).
Although there is no doubt that the concentration of greenhouse gases is
increasing, the future rate of that increase is uncertain. A recent report by
the National Academy of Sciences (NAS) examined numerous uncertainties
regarding future energy use patterns, economic growth, and the extent to which
CO2 emissions remain in the atmosphere (Nordhaus and Yohe 1983). The Academy
estimated a 98 percent probability that C02 concentrations will be at least
450 parts per million (1.5 times the preindustrial level) by 2050 and a 55
percent chance that the concentration will be 550 parts per million. The
Academy estimated that the probability of a doubling of COa concentrations by
2100 is 75 percent.
If the impact of the trace gases continues to be equal to the impact of
C02, NAS analysis implies that the effective doubling of all greenhouse gases
has a 98 percent chance of occurring by 2050. 7 However, uncertainties
regarding the emissions of trace gases are greater than those for C02-
Although the sources of chlorofluorocarbon emissions are well documented,
regulatory uncertainties related to their possible impact on stratospheric
ozone depletion make their growth rate -- currently about 5 percent --
impossible to forecast. The current sources of methane, nitrous oxide, and
other trace gases have not yet been fully catalogued.
Considerable uncertainty also exists regarding the impact of a doubling of
greenhouse gases. Physicists and climatologists generally accept the estimate
by Hansen et al. (1984) that a doubling would directly raise the earth's
average temperature 1.2°C if nothing else changed. However, if the earth
warmed 1.2°C, many other aspects of climate would be likely to change,
probably amplifying the direct effect of the greenhouse gases. These indirect
impacts are known as "climatic feedbacks."
Figure 3 shows estimates by Hansen et al. (1984) of the most important
known feedbacks. A warmer atmosphere would retain more water vapor, which is
also a greenhouse gas, warming the earth more. Snow and floating ice would
melt, decreasing the amount of sunlight reflected to space, causing additional
warming. Although the estimates of other researchers differ slightly from
those of Hansen et al., climatologists agree that these two feedbacks would
amplify the global warming from the greenhouse effect. However, the impact of
clouds is far less certain. Although recent investigations have estimated
that changes in cloud height and cloud cover would add to the warming, the
possibility that changes in cloud cover would offset part of the warming
-------
-8-
FIGURE 2
MEASUREMENTS OF ATMOSPHERIC CARBON-DIOXIDE
ABUNDANCE OVER TIME: 1958 to 1981
310
1958
I960
1982
1964
1988
1968
1970
1972
1974
1978
1976
1980 1991
Year
Sources: Mauna Loa Observatory, Hawaii, NOAA, U.S. Department of Commerce.
-------
-9-
FIGURE 3
ESTIMATED GLOBAL WARMING DUE TO A DOUBLING
OF GREENHOUSE GASES: DIRECT EFFECTS AND
CLIMATIC FEEDBACKS
O
o
%_^
-------
-10-
cannot be ruled out. After evaluating the evidence, two panels of the
National Academy of Sciences concluded that the eventual warming from a
doubling of greenhouse gases would be between 1.5° and 4.5°C (3°-8°F).
A global warming of a few degrees could be expected to raise sea level in
the future, as it has in the past. The best understood mechanism is the
warming and resulting expansion of sea water, which could raise sea level
one-half meter in the next century (Hoffman, Keyes, and Titus 1983). Mountain
glaciers could melt and release enough water to raise sea level twelve
centimeters (five inches) (Revelle 1983). Revelle estimates that a 3°C
warming could cause Greenland's glaciers to melt enough water to raise the sea
another twelve centimeters in the next century. Antarctica could contribute
to sea level rise either by meltwater running off or by glaciers sliding into
the oceans.
Recent analysis by the Polar Research Board of the National Academy of
Sciences indicates that glaciers in Greenland and East Antarctica, as well as
those in West Antarctica, could eventually release enough ice into the oceans
to raise sea level two or three centimeters (about one inch) per year.*
However, current thinking holds that such a rapid rise is at least one hundred
years away. Moreover, a complete disintegration of the West Antarctic Ice
Sheet would take several centuries (Bentley 1983; Hughes 1983). It is
possible that snowfall accumulation could partially offset the rise in sea
level.9
In 1983, two independent reports estimated future sea level rise. In the
National Academy of Sciences report Changing Climate, Revelle estimated that
the combined impacts of thermal expansion, Greenland and mountain glaciers
could raise sea level seventy centimeters (two and one-third feet) in the next
century (Revelle 1983). Although he also stated that Antarctica could
contribute two meters per century to sea level starting around 2050, Revelle
did not add this contribution to his estimate.
In a report by the Environmental Protection Agency entitled Projecting
Future Sea Level Rise. Hoffman, Keyes, and Titus (1983) stated that the
uncertainties regarding the factors that could influence sea level are so
numerous that a single estimate of future sea level rise is not practical.
Instead, they consulted the literature to specify high, medium, and low
estimates for all the major uncertainties, including fossil fuel use; the
absorption of carbon dioxide through natural processes; future emissions of
trace gases; the global warming that would result from a doubling of
greenhouse gases (the NAS estimate of 1.5°-4.5°C); the diffusion of heat into
the oceans; and the impact of ice and snow. They estimated that if all of the
low assumptions prove to be correct, the sea will rise 13 cm (5 in) by 2025
and 38 cm (15 in) by 2075 over the 1980 level. If all of the high assumptions
are correct, the sea will rise 55 cm (2 ft) by 2025 and 211 cm (7 ft) by
2075. However, because it is very unlikely that either all the high or all
the low assumptions will prove to be correct, the authors concluded that the
rise in sea level is likely to be between two mid-range scenarios of 26 to 39
cm (11 to 15 in) by 2025 and 91 to 136 cm (3 to 4-1/2 ft) by 2075. Figure 4
and Table 1 illustrate the EPA and NAS estimates. Although neither of these
studies examined options to limit the rise in sea level by curtailing
-------
-11-
FIGURE 4
GLOBAL SEA LEVEL RISE SCENARIOS:
LOW, MID-RANGE LOW, MID-RANGE HIGH, AND HIGH
lnch«»
100
LEVEL
RISE
75
60-
26
CMS
2SO_
200_
1 50
100
50-
HIQM
MID-BANQE
HIGH
MID-RANGE
LOW
• NAS
LOW
2075
Sources: J. Hoffman, D. Keyes, and J. Titus, Projecting Future Sea Level
Rise, Washington, D.C.: Government Printing Office, 1983; Changing
Climate, Washington, B.C.: NAS Press, 1983 (does not include
Antarctica).
-------
-12-
TABLE 1
SCENARIOS OF WORLDWIDE SEA LEVEL RISE
(centimeters)
Current Trends
EPA Scenarios
High
Mid- range
Mid-range
Low
NAS Estimate
2000
2.0-3.0
17.1
high 13.2
low 8 . 8
4.8
-
2025
4.5-6.8
54.9
39.3
26.2
13.0
-
2050
7.0-10.5
116.7
78.9
52.6
23.0
_
2075
9.5-14.3
211.5
136.8
91.2
38.0
_
2080 2100
10-15 12.0-18
345 . 0
216.6
144.4
56.2
70.0
(excluding Antarctic
Contribution)
-------
-13-
etnissions, Seidel and Keyes (1983) estimated that even a ban on coal, shale
oil, and synfuels would only delay the rise in sea level expected through 2050
by twelve years.1°
The East Coast of the United States is slowly sinking (Hoffman, Keyes, and
Titus 1983). Thus relative sea level rise at Ocean City, Maryland, will be
fifteen to twenty centimeters (six to eight inches) greater than global sea
level rise per century. Table 2 displays the projected rise at Ocean City for
the EPA mid-range scenarios and current trends.
TABLE 2
RELATIVE SEA LEVEL RISE SCENARIOS
FOR OCEAN CITY, MARYLAND
(absolute rise over 1980 level in centimeters (feet))
Year
2000
2025
2050
2075
Current Trend
7 (0.24)
16 (0.53)
25 (0.83)
34 (1.13)
Mid-Range Low Rise
12.4 (0.40)
34.3 (1.13)
65.2 (2.14)
108.3 (3.55)
Mid-Range High Rise
16.8 (0.55)
47.4 (1.55)
91.5 (3.00)
153.9 (5.05)
Sources: J. Hoffman, D. Keyes, and J. Titus, Projecting Future Sea Level
Rise, Washington, D.C.: Government Printing Office, 1983.
R. Revelle, "Probable Future Changes in Sea Level Resulting From
Increased Atmospheric Carbon Dioxide," Changing Climate, 1983.
S. Hicks, H. Debaugh, and L. Hickman, Sea Level Variations for the
United States 1855-1980, Rockville, MD: U.S. Department of
Commerce, NOAA-NOS, January 1983.
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-14-
1MPACTS OF SEA LEVEL RISE
The physical impacts of sea level rise can be divided into five
categories: (1) inundation of low-lying area; (2) erosion of beaches,
particularly along the open coast; (3) increased flooding and storm damage;
(4) increased salinity of surface and ground water; and (5) higher water
tables. Most of the land low enough to be inundated in the next century
consists of wetlands, such as the salt marshes along the Chesapeake Bay, and
various coastal estuaries, such as Sinepuxent and Chincoteague Bays near Ocean
City. At the rate of sea level rise of thirty centimeters (one foot) per
century as has occurred in the last century, most salt marshes can keep pace
with the rising sea through sedimentation and growth of vegetation (Orson,
Panageotou, and Leatherman 1985). However, they probably could not keep pace
if the sea rose much more rapidly. In fact, a report by the U.S. Fish and
Wildlife Service cites sea level rise as a cause of marsh loss at Blackwater
Refuge on the Eastern Shore (Pendleton and Stevenson 1983).
Although existing marsh would drown, new marsh could form inland. For
example, Kana, Baca, and Williams (1985) estimate that Charleston, South
Carolina would only lose 50 percent of its marshes with a one-meter rise, as
long as people did not prevent new marsh from forming. However, development
may prevent a landward migration of marshes and force these ecosystems to be
lost. Decision makers might prefer to delay consideration of this issue until
there is more certainty about future sea level rise. However, this strategy
could make it impossible to avoid a future large-scale loss of coastal
wetlands and property. Decisions being made today largely determine whether
or not development will prevent marshes from forming inland. Most building
codes, master plans, and zoning codes assume that once an area just inland of
the marsh is developed, it will remain that way forever; but for wetland
ecosystems to survive, these areas would have to become undeveloped once
again.T1
Sea level rise could also cause land that is above sea level to erode.
Along the coast of Maryland, winter storms and occasional hurricanes erode the
beach and deposit the sand off shore. Waves during calm periods "dredge" the
sand off the nearshore bottom and redeposit it on the beach. Sea level rise
results in a net erosion of the beach by allowing storm waves to strike
further inland and by decreasing the ability of calm waves to rebuild the
beach.12 Figure 5 illustrates the upward and landward shift of the beach
profile that accompanies sea level rise, commonly known as the Bruun Rule
(Bruun 1962). Along most U.S. beaches, a thirty-centimeter (one-foot) rise in
sea level would cause approximately thirty meters (one hundred feet) of
erosion, although the actual amount depends on the wave climate and beach
profile. Rather than erode in place, coastal barrier islands would migrate
landward, as storms push from the ocean side to the bay side.
Perhaps the most economically important consequence of sea level rise
would be increased flooding and storm damage. The direct impact of a
one-meter rise in sea level would be to raise storm flood levels by one
meter. However, several other indirect effects could further increase
damages. Erosion from sea level rise would leave some coastal property more
vulnerable to storm waves. Coastal stormwater drainage systems would operate
-------
•15-
FIGURE 5
THE BRUUN RULE: A RISE IN SEA LEVEL
CAUSES BEACH EROSION
BEACM
SEA LEVEL
\b\ X AFTER RISE /
SEA LEVEL
BOTTOM PROFILE/
AFTER SEA
LEVEL RISE
a « a'
b—b'
INITIAL BOTTOM
PROFILE
LIMITING DEPTH
BETWEEN PREDOMINANT
NEARSHORE AND
OFFSHORE MATERIAL
If the sea rises one foot, so will the offshore bottom. The sand
necessary to raise the bottom (area b1) can be supplied by artificial beach
nourishment or by waves eroding the upper part of the beach (area b).
Source: Adapted from Schwartz, 1967. "The Bruun Theory of Sea Level Rise as
a Cause of Shore Erosion," Journal of Geology, 75:76-92.
-------
-16-
less effectively. Finally, higher water tables and surface water levels would
decrease natural drainage.
Other consequences of a greenhouse warming could also have impacts on
flooding. Warmer temperatures would intensify the hydrologic cycle and
increase worldwide rainfall by 10 percent or more (Rind and Lebedeff 1984).
Although predictions for particular areas are not possible, rainfall would
presumably increase in some coastal areas. Furthermore, because hurricanes
require an ocean temperature of 27°C (79°F) to form (Wendland 1977), a global
warming may extend the hurricane season or result in hurricanes forming at
higher latitudes. However, hurricanes depend upon many other factors, all of
which must be assessed before meaningful statements about future hurricane
frequency will be possible.
EPA has investigated several possible responses to erosion and flooding
caused by sea level rise. Gibbs (1984) estimates that the economic impact on
Charleston, South Carolina, could be one to two billion dollars over the next
century, but that anticipatory zoning and engineering measures could cut the
potential losses in half. Webb and LaRoche examined the drainage systems of a
watershed in Charleston. They concluded that a thirty centimeter (one foot)
rise by 2025 would necessitate modifications (mainly additional pipes) to the
drainage system that would cost $3 million to implement (Webb, LaRoche n.d.).
However, if these modifications are incorporated into the planned overhaul of
the system, the additional cost would only be $300,000.
The possible importance of salinity increases caused by sea level rise is
poorly understood. The Delaware River Basin Commission has estimated that a
thirteen-centimeter (five-inch) rise in sea level would cause the salt front
in the Delaware River to migrate two to four kilometers (one to two miles)
upstream. A rise of one meter could cause salt to move over twenty kilometers
upstream, possibly threatening parts of Philadelphia's water supply, as well
as aquifers in New Jersey recharged by the river (Hull, Titus, and Lennon
n.d.). However, possible responses to such salinity increases have not been
assessed, nor have the impacts on other estuaries.
Finally, a rising sea level would raise water tables. Flooding of
basements and subway systems may be more frequent, necessitating additional
pumps in some areas. No one has investigated the possible impacts on public
sewer systems in coastal areas.13
OCEAN CITY CASE STUDY
Available research indicates that the impacts of even a one-foot rise in
sea level would be important, but that the most adverse consequences could be
avoided if communities take timely actions in anticipation of sea level rise.
Unfortunately, most local governments do not have the resources to undertake
sophisticated assessments of the potential implications. Regardless of the
potential savings, the cost of undertaking a study is a hurdle that can
prevent decision makers from considering the issue.
-------
-17-
Development of low-cost erosion forecasting methods could substantially
reduce the cost of assessing the impact of sea level rise. Although these
methods lack the precision of more sophisticated analyses, their accuracy may
be sufficient for long-range planning, where other variables such as economic
growth and population are also uncertain.
To assess the potential for inexpensive assessments of sea level rise
impacts, EPA contracted with three experts at low-cost erosion forecasts.
This section describes the results of the three studies, each of which could
be applied to other beach communities at a cost of $5,000-$10,000. Chapters 2
through 4 provide additional detail.
Present Trends
Like all ocean beaches, the beach at Ocean City exhibits a seasonal
pattern. Winter storms erode the beach, while the calm waves of spring and
summer rebuild it. In the long run, however, the shoreline has shown a slow
but steady erosion trend. In the last fifty years, the beach has eroded over
thirty meters (one hundred feet).
Leatherman (Chapter 2) and Everts (Chapter 3) offer very different
explanations for the causes of this erosion. Leatherman argues that the
erosion is caused by the long-term sea level rise of thirty-six centimeters
(over one foot) in the last century. Everts estimates that substantial
quantities of sand are being transported along the shore and off Fenwick
Island, and that sea level rise is only causing 20 to 25 percent of the
erosion. Leatherman acknowledges that alongshore losses are taking place, but
suggests that the Delaware portion of the island, not Ocean City, is losing
sand for this reason. Everts1 perspective represents the general viewpoint of
officials in Ocean City and the State of Maryland; however, Leatherman could
also be correct if long-term sea level rise caused the alongshore transport of
sand now observed. 1
-------
-IB-
FIGURE 6
CURRENT SHORELINE AND PROJECTED EROSION
AT ASSATEAGUE ISLAND, ASSUMING CURRENT
TRENDS CONTINUE
1980 Shoreline
Projected Year 2000 Shoreline
Ocean City
'.'•';. •..''.•''•."•' Airport •
;. • .'•,' •'"' Assateague Island.': .v£j
Source: Revised from: Stephen P. Leatherman, "Shoreline Evolution of North
Assateague Island, Maryland," Shore and Beach, (July) 1984, pp. 3-10.
-------
-19-
In an appendix to Chapter 2, Bresee presents data showing the position of
the shoreline and contours where the water is 10, 20, and 30 feet deep, for
the years 1929, 1962, 1965, 1978, and 1979 at seventeen locations along the
beach at Ocean City. Although coverage and season differed from year to year,
it is possible to compare the data for 1962 and 1978 for the area south of
86th Street. Table 3 presents summary statistics of the erosion that has
occurred during that time. Although the shoreline only retreated 9 meters
(35 feet), the underwater portion of the beach eroded 35-45 meters (110-150
feet). In spite of the substantial variation of erosion along the shore,
these results are statistically significant.
Leatherman points out that a continuous erosion rate would not be
expected. Substantial erosion generally occurs during a major storm, with the
calm waves gradually rebuilding (most of) the beach in subsequent years.
Because there has been no major storm since the March 1962 northeaster (the
worst storm on record), one would expect the shoreline to advance (or retreat
more slowly). The slower rate of shoreline retreat does not necessarily imply
that the entire beach system is eroding more slowly. The sand washing from
off shore back onto the shore would generally imply that the offshore part of
the beach system should be eroding more rapidly than the shore itself. For
this reason, Leatherman uses the long-term rate of historical shoreline
retreat in projecting future erosion.
Everts identifies human activities that may also be causing the visible
portion of the beach to erode more slowly than the underwater portion. After
the 1962 storm, the Corps of Engineers placed about one million cubic meters
of sand on the upper part of the beach system. Furthermore, in the last
several years, Ocean City has used bulldozers to push sand landward from the
shore, expanding the visible portion of the beach at the expense of the
underwater portion. Finally, groins may also tend to steepen the profile. If
groins have their intended effect, they slow erosion of the upper part of the
beach; however because they extend at most to the -10 foot contour, they do
nothing to slow erosion of the rest of the profile.
The analyses by Leatherman and Everts imply that current observations of
shoreline retreat may be causing people to underestimate the severity of
current long-term erosion trends. If they are correct in concluding that the
-20 and -30 foot contours have retreated substantially, a severe storm could
restore the profile and cause severe erosion. In Chapter 4, Kriebel & Dean
estimate the erosion that would result from a severe storm, using their storm
climatology model, which accurately predicted the erosion that Hurricane
Eloise caused along the coast of Florida. Kriebel & Dean project that a
recurrence of the March 1962 northeaster (a 50-year storm) would cause the
dune line to erode 20-35 meters (70-120 feet) for dunes with heights of
3.0-4.5 meters (10-14 feet). Even the presumably more imminent 10-year storm
would cause 15 meters (50 feet) of erosion.
Future Projections
Table 4 summarizes the estimates of future erosion presented in Chapters
2, 3, and 4. For current trends, Leatherman's projections are more
conservative than Everts1 or Kriebel & Dean's. Leatherman estimates that the
-------
-20-
TABLE 3
RETREAT OF THE BEACH AT OCEAN CITY, MARYLAND
BETWEEN 21st AND 86th STREETS: 1962 to 19781
Meters (feet)
Shoreline
-10ft
Contours
-20ft
-30ft2
Mean Retreat
Standard Deviation
of Observations
Standard error
of the Estimate
of the Mean
Retreat3
Statistical Confi-
dence Level (CL)
for The Mean
Retreat Exceeding
Zero (%)*
Statistical Confidence
Level (CL) for The
Mean Contour
Retreat Exceeding
the Mean Shoreline
Retreat5
9.1 (30.0)
17.0 (55.9)
5.7 (18.6)
90
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-21-
TABLE 4
PROJECTED EROSION AT OCEAN CITY
Meters (Feet) of Shoreline Retreat Relative to its Current Position
Current Trends
Bruun1
Everts
Leatherman2
Kriebel & Dean
Bruun1
Bruun Adjusted3
Everts
Leatherman
Kriebel & Dean
Bruun1
Bruun Adjusted3
Everts
Leatherman
Kriebel & Dean
2000
4.9 (16)
21.0 (68)
12.0 (39)
20.0 (66)
2025
11.0 ( 36)
46.6 (153)
26.0 ( 85)
46.6 (153)
2050
17.0 ( 57)
72.5 (238)
40.8 (134)
70.4 (231)
Mid-Range Low
2075
23.0 ( 77)
98.5 (323)
55.5 (182)
95.4 (313)
6.7 (22)
23.0 (74)
26.0 (84)
20.0 (64)
22.3 (73)
22.0 ( 72)
57.6 (189)
72.5 (238)
55.5 (182)
54.9 (180)
42.7 (140)
98.1 (322)
132.0 (434)
105.0 (345)
92.7 (304)
70.4 (231)
147.0 (483)
215.0 (707)
174.0 (572)
(460)
Mid-Range High
12
27
29
27
26
.0
.0
.0
.0
.2
(22)
(90)
(95)
(89)
(86)
32.
68.
83.
76.
65.
3
0
2
2
8
(106)
(223)
(273)
(250)
(216)
62
118
156
147
107
.8
.0
.0
.0
.0
(206)
(388)
(511)
(483)
(353)
105.
181.
268.
249.
168.
0
0
0
0
0
(346)
(593)
(878)
(813)
(550)
Bruun Rule is included for completeness. Because it includes only the
impacts of sea level rise, it needs to be adjusted for alongshore and
other losses in areas like Ocean City.
Leatherman 's estimates are based on shoreline maps dating back to 1850.
If he had used only the period since 1962, his estimates would be much
lower. He deemed the longer series more appropriate because the -10, -20,
and -30 foot contours have continued to erode at the long-term rate of
shoreline retreat.
Bruun Rule Adjusted includes 2.6 feet per year due to factors other than
sea level rise. Because 2.6 is derived from Everts, Bruun Adjusted is
equal to Everts for current trends .
-------
-22-
shore would erode 25 meters (85 feet) by 2025, whereas the other researchers
estimate a retreat of about 45 meters (150 feet). However, he projects a
greater increase in erosion due to sea level rise. Using EPA's mid-range low
scenario (which is close to the National Academy of Sciences estimate),
Leatherman, Everts, Kriebel & Dean, and our adjustment of the Bruun Rule
project erosion in the 55 to 72-meter (180 to 238-foot) range for the
30-centimeter (1-foot) rise in sea level that would occur by 2025. For the
mid-range high estimate, the four estimates range from 66 to 83 meters
(216-273 feet). By 2075, the erosion estimates range from 140 to 215 meters
for the mid-range scenario, and from 170 to 250 meters for the mid-range high
scenario.
Because Ocean City's policy is to maintain its current shoreline, Everts
and Kriebel & Dean also estimated the quantity of sand necessary to maintain
the shore at Ocean City in its current location. Although Leatherman did not
estimate sand requirements, we have calculated sand quantities implied by his
estimates of shore retreat. As with the erosion projections, we have also
adjusted Everts1 application of the Bruun Rule to include alongshore losses of
sand.
Table 5 displays the estimates of sand necessary to maintain Ocean City's
shoreline through 2075, assuming that the beach profile remains the same on
average. All of the estimates for the mid-range low scenario are in the range
of 3-4 million cubic meters (4-5 million cubic yards) by 2000 and 8.4-10.0
million cubic meters (11-13 million cubic yards) by 2025. For the mid-range
scenario, the estimates are 4.0-4.6 million cubic meters (5-6 million cubic
yards) by 2025 and 10.0-12.2 million cubic meters (13-16 million cubic yards)
by 2025. However, there is less agreement concerning what sand will be
necessary if current trends continue. Kriebel & Dean's estimates are
approximately twice that implied by the Leatherman analysis. This discrepancy
is probably due to the fact that Kriebel & Dean assume that substantial sand
will continue to be transported out of the area, whereas Leatherman assumes
that on average, only sea level rise will cause a significant loss of sand.
The Corps of Engineers Baltimore District notes that 2-3 million cubic yards
of sand would be necessary to counter losses of sand without sea level
rise.17 To put these quantities into perspective, Kriebel & Dean estimate
that about one million cubic meters would be necessary to protect against a
100-year storm that remained for 24 hours.
All of the methods yield estimates within a factor of two, except for the
unadjusted Bruun rule, which is not designed for communities with significant
alongshore losses of sediment. Although more sophisticated methods may yield
more precise estimates, the estimates provided by the Leatherman, Everts, and
Kriebel & Dean approaches may be adequate for first-order consideration of
seal level rise impacts.
Because the focus of this study is beach erosion, not flooding, the
researchers did not examine other impacts that may also be important to Ocean
City or other coastal communities. These impacts might include bay-side
flooding, wave damage, and the risk of inlet breach.
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-23-
TABLE 5
SAND REQUIRED TO MAINTAIN CURRENT SHORELINE
(millions of cubic yards)
Current Trends
Bruun1
Everts
Kriebel & Dean
Leatherman Adjusted2
Bruun Adjusted3
Everts
Kriebel & Dean
Leatherman Adjusted
Bruun Adjusted
Everts
Kriebel & Dean
Leatherman Adjusted
2000
1.0
4.0
4.8
2.4
2025
2.2
9.3
10.5
5.2
2050
3.3
14.0
11.4
7.8
Mid-Range Low
Mid-Range High
2075
4.6
19.0
22.5
11.0
4.6
4.6
5.5
4.3
12
11
13.3
11
20
19
22.1
21
29
28
33.2
35
5.5
5.2
6.3
5.6
13
13
15
15
23
22
25.9
29
35
34
40.2
48
Bruun Rule is included only for completeness. It is not intended to
estimate erosion in areas with significant alongshore losses.
Leatherman Adjusted is calculated by multiplying the ratio of
Leatherman/Bruun estimates of erosion by the Bruun estimate of beachfill
requirements.
Bruun Adjusted is equal to Everts for current trends.
-------
-24-
Implications
Ocean City's most important asset is probably its beach. Every weekend in
the summer, approximately 250,000 visitors flock to this coastal town to swim
and sunbathe. For this reason, state and local governments have recognized
the beach as a resource that must be maintained. Because moving buildings
back as the shore erodes is economically infeasible, the governments have
opted for erosion control measures.
The expected rise in sea level will substantially increase the costs of
these measures and change the relative merits of various shore protection
strategies. But unlike many less densely developed coastal barriers, Ocean
City's structures (and its stated policy of protecting its shoreline) need not
be threatened by sea level rise. The high recreational and property values
would economically justify shore protection for the foreseeable future.
The Corps of Engineers estimates that the first 4 million cubic meters of
sand would cost approximately $26 million ($6.5 per cubic meter), that the
next 5 million cubic meters would cost about $35 million ($7 per cubic meter),
and that another 2.2 million cubic meters could be obtained for about $25
million ($11.2 per cubic meter) (U.S. Army Corps of Engineers 1980). Thus,
the cost of maintaining the beach at Ocean City would be about $20 million
through 2000 and $60 million through 2025 if the EPA mid-range low scenario
(similar to the National Academy of Sciences estimate) are correct. Even if
the mid-range high scenario occurs, the beach could be protected through 2025
for about $85 million.
Although these cost estimates are not negligible, the implied cost of $1-2
million per year is small when compared with the economic activity that takes
place at Ocean City. At a rate of seven million visitors per year,18 the
cost of protecting Ocean City's shore would appear to be less than 30C per
visitor. If sea level rises as projected, a beach protection plan would thus
almost certainly be cost-beneficial. The Corps of Engineers estimated that
the benefits from their proposed beach restoration would be $8 million per
year, even though they did not consider accelerated sea level rise. The
benefits from addressing the greater erosion that could occur with sea level
rise would be much ,greater.
Ocean City and the State of Maryland have tentatively decided to build
groins at a cost of $400,000 each, as an interim measure until the Corps
beachfill plan is implemented. To the extent that current erosion is caused
by sand moving along the shore and out of Ocean City, these groins might
enable the city to "keep its own sand" and curtail erosion. However, groins
do not prevent erosion caused by sea level rise (Sorensen, Weisman, and Lennon
1984). Although most of the researchers in Chapters 2, 3, and 4 believe that
sea level rise is only causing one quarter of the erosion today, they all
agree that if sea level rises as projected, it will gradually become the
overriding factor. Thus, if sea level rises, pumping sand onto the beach will
eventually be necessary. This sand, however, would bury the groins and
shorten useful lifetimes compared to what previous analyses have indicated.19
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Future sea level rise would also change the types of benefits gained by
undertaking shore protection measures. For example, the Corps of Engineers
determined that the benefits of their recommended beachfill plan would far
exceed the costs; but because most of these benefits would be from increased
recreational use of the beach, not flood protection, they did not consider the
plan to have high priority. The prospect of sea level rise implies that
without additional protection, much of Ocean City will become much more
vulnerable to storm damage. Thus, the flood protection benefits of beach
restoration may be much greater than previously estimated.
In the long run, sea level rise may imply that it will be wise to
construct new buildings somewhat inland of what would otherwise be the
preferred location. For example, it may be advisable to build parking lots on
the seaward side of new high-rises, which would allow a builder to use the
entire lot but leave the building less vulnerable to erosion and flooding (and
the building would cast its afternoon shadow onto the parking lot, not the
beach). The fact that Ocean City officials will probably always be able to
justify expenditures for the protection of Ocean City's many large buildings
does not mean that they should not look for ways of reducing the eventual
costs. After the cheapest twelve million cubic meters of sand are exhausted,
the costs may start to climb. Furthermore, if communities in Delaware follow
Ocean City's example and attempt to keep their own sand, the amount of
Delaware sand washing into Maryland would decrease.
The steepening beach profiles may increase the difficulty of forming a
public consensus to address erosion and sea level rise. Ocean City may become
increasingly vulnerable to storms as the greater part of the beach erodes; yet
as long as the visible part remains stable, few property owners will feel
threatened, even if tidal gauges and scientific reports show a rise in sea
level. A major storm could disrupt this complacency, especially if, as
Leatherman projects, substantial permanent erosion occurs. If major property
damage also occurred, there would be many opportunities to adjust to sea level
rise in the rebuilding phase.
The fundamental difficulty of planning for sea level rise is that the
probability and magnitude of the phenomenon are uncertain. Nevertheless, it
is a risk that should be taken seriously when people make decisions. Although
we have less experience with sea level rise than with other factors such as
storms, our understanding of the causes and our ability to predict the likely
range are already greater for sea level rise than for many factors that are
routinely considered in major decisions, including the severity of the next
major storm.
Sea level rise is a risk against which some policies may provide more
effective insurance than others. Although groins were determined to be more
cost effective than was beach nourishment at controlling Ocean City's
alongshore erosion, the latter would also control erosion caused by sea leve]
rise, whereas groins would not. As with all insurance policies, coastal
decision makers must weigh the costs and risks of various alternatives and
decide on a case-by-case basis whether it is prudent to insure against the
risks of sea level rise.
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NEXT STEPS
A rising sea level could cause the beach at Ocean City to erode hundreds
of feet in the next few decades if control measures are not taken. The cost
of controlling erosion is likely to be tens of millions of dollars through the
year 2000 and perhaps as much as sixty million dollars through 2025. Although
the commercial and recreational resources of Ocean City could easily justify
such expenditures, opportunities to reduce these costs should be
investigated. Erosion control strategies, post-disaster policies, and
long-term planning are all areas where ongoing efforts should consider the
risk of future sea level rise.
Erosion control measures should probably have the highest priority.
Standard analytic procedures can be employed to examine whether the risk of
sea level rise warrants a reconsideration of current strategies. Delaying
such an analysis could have substantial costs: every year the city and state
spend hundreds of thousands of dollars on groins that may be subsequently
buried if sea level rises.
Incorporating sea level rise into post-disaster policies could be very
helpful. In the aftermath of a major storm, people will be much better
educated about the risks of erosion and sea level rise; and an educated public
is much more likely to support efforts that properly address these long-term
risks. However, the need to act quickly may preclude the careful consideration
necessary to adequately adjust to rising sea level. These policies must be
formulated before the storm.
Finally, Ocean City's long-term planning should consider sea level rise.
Over the next 50-100 years, rising sea level could have an impact on coastal
areas as important as the sudden popularity of beaches that took place
starting in the 1950s. Although sufficient sand has been identified to
address erosion expected in the next forty years, the financial health of
Ocean City in the longer run will require identification of additional
low-cost supplies. The ultimate question for coastal barrier communities like
Ocean City will be whether to raise the entire island in place as the sea
rises, or to plan around a retreating shore. But sea level rise also has
important implications for decisions involving building location and design,
future population, roads, canals, and wetland protection.
Adjustments to sea level rise may not always be easy. But they are more
likely to be successful if people start to plan while the phenomenon is still
a future risk, rather than wait until it is a current reality.
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NOTES
1. Expenditures of sport fishermen have increased from $3 billion in 1960-
to $18 billion in 1980. Expenditures of hunters have increased from $1
billion to $9 billion over the same period.
2. The number of recreational boats in U.S. waters has increased from 8.8
million in 1970 to 13.2 million in 1983. Expenditures in 1983 were $9.4
billion.
3. PL 90-448, Section 1302
4. PL 92-53, 16 USC 1451, Section 303.
5. For Massachussets, see M.G.L. Ch. 131, S 40 Reg 310 C.M.R. 9.10(2) or
Mass General Laws.
6. See: Clark, J.A., W.E. Farrell, and W.R. Peltier, 1978. "Global Changes
in Post Glacial Sea Level: A Numerical Calculation." Quarternary
Research 9:265-287. Note, however, that William Tanner of Florida State
University suggests that there is a 3 percent chance that these factors
could cause a rise or fall of one meter in a century. Personal
Communication, William Tanner. Geology Department, Florida State
University.
7. Studies on the greenhouse effect generally discuss the impacts of a C02
doubling. By "effective doubling of all greenhouse gases" we refer to
any combination of increases in the concentrations of the various gases
that causes a warming equal to the warming of a doubling of C02 alone.
If the other gases contribute as much warming as COa, the effective
doubling would occur when CC>2 concentrations have reached 450 ppm, 1.5
times the preindustrial level.
8. Robert Thomas, Jet Propulsion Laboratory, personal communication with
John S. Hoffman, EPA.
9. Robert Thomas, Jet Propulsion Laboratory, personal communication with
John S. Hoffman, EPA.
10. Computer printout underlying calculations from Seidel and Keyes, op.
cit.
11. See: Titus, J.G., 1984. "Planning for Sea Level Rise Before and After a
Coastal Disaster." In Earth, M.C. and J.G. Titus, oj>. cit.
12. The ability of waves to rebuild the beach is reduced in that a complete
restoration of the original profile location would require the nearshore
'water depths to be greater than they had been before the sea rose. As
sea level rises, so must the nearshore bottom.
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NOTES (continued)
13. However, a town planner in Westerley, Rhode Island, estimates that a
thirty-centimeter rise could contaminate over one hundred septic tanks
along the town's shoreline. Griscom, Clement. Presentation to Rhode
Island Sea Grant Conference on Sea Level Rise, November 29, 1984.
14. Sea level rise can contribute to alongshore transport if deeper water
levels create sinks for sand in inlets and tidal shoals. Furthermore,
unless slopes are uniform everywhere, sea level rise will tend to erode
some areas more than others. The areas that erode the least will tend to
later experience alongshore losses to areas that have eroded the most.
15. Conversations with local, state, national park, and Corps of Engineers
officials, as well as citizen groups, indicate that most people believe
that the jetty at the south end of Ocean City has filled with sand that
would have otherwise washed onto Assateague. Robert Whalin, Director of
the Coastal Engineering Research Center, however, states that recent
research by his office shows that the jetties are not the only cause of
erosion. Letter from Robert Whalin, Director of CERC, to James G. Titus,
EPA, May 1985.
16. Although the predominant alongshore drift is to the south, the flow is
occasionally to the north. During these periods, the inlet carries sand
that would otherwise flow to Ocean City to shoals off shore.
17. Ed Fulford, Baltimore District, Corps of Engineers, letter to James G.
Titus, EPA, May 1985.
18. Sandy Coyman, Town of Ocean City, Personal Communication.
19. Ed Fulford, Baltimore District, Corps of Engineers, letter to James G.
Titus, EPA, May 1985.
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REFERENCES
Barnett, T.P., 1983. "Global Sea Level: Estimating and Explaining Apparent
Changes." In Orville T. Magoon, Ed., Coastal Zone 83, New York: American
Society of Civil Engineers, pp. 2777-2795.
Earth, M.C. and J.G. Titus (Eds.), 1984. Greenhouse Effect and Sea Level
Rise: A Challenge for This Generation. New York: Van Nostrand Reinhold.
Bentley, L., 1983. "The West Antarctic Ice Sheet: Diagnosis and Prognosis,"
In Proceedings: Carbon Dioxide Research Conference: Carbon Dioxide,
Science, and Consensus. DOE Conference 820970, Washington, D.C.,
Department of Energy.
Bruun, P., 1962. "Sea Level Rise as a Cause of Shore Erosion." Journal
of Waterways and Harbors Division (ASCE) 1:116-130.
Charney, J., 1979. Carbon Dioxide and Climate: A Scientific Assessment.
Washington, D.C.: National Academy Press.
Donn, W.L., W.R. Farrand, and M. Ewing, 1962. "Pleistocene Ice Volumes and
Sea Level Lowering." Journal of Ecology 70:206-214.
Gibbs, M., 1984. "Economic Analysis of Sea Level Rise: Methods and
Results." In Barth, M.C. and J.G. Titus, op. cit.
Gornitz, V., S. Lebedeff, and J. Hansen, 1982. "Global Sea Level Trend in the
Past Century." Science 255:1611-1614.
Hansen, J.E., A. Lacis, D. Rind, and G. Russell, 1984. "Climate Sensitivity
to Increasing Greenhouse Gases." In Barth, M.C. and J.G. Titus, op. cit.
Hays, J.P. and W.C. Pitman III, 1973. "Lithospheric Plate Motion, Sea Level
Changes, and Climatic and Ecological Consequences." Nature 246:18-22.
Hicks, S.D., H.A. Debaugh, and L.H. Hickman, 1983. Sea Level Variations for
the United States 1855-1980. Rockville, Maryland: U.S. Department of
Commerce, NOAA-NOS.
Hoffman, J.S., D. Keyes, and J.G. Titus, 1983. Projecting Future Sea Level
Rise: Methodology, Estimates to the Year 2100 and Research Needs.
Washington, D.C.: Government Printing Office.
Hollin, J.T., 1972. "interglacial Climates and Antarctic Ice Surges."
Quarternary Research 2:401-408.
Hughes T., 1983. "The Stability of the West Antarctic Ice Sheet: What Has
Happened and What Will Happen." In Proceedings: Carbon Dioxide Research
Conference: Carbon Dioxide, Science, and Consensus. DOE Conference
820970, Washington, D.C., Department of Energy.
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REFERENCES (continued)
Hull, C., and J. Titus, Eds. (n.d.). Greenhouse Effect, Sea Level
Rise, and Salinity in the Delaware Estuary. Environmental Protection
Agency and Delaware River Basin Commission. In press.
Kana, T., B.J. Baca and M.L. Williams, 1985. Potential Impacts of Sea Level
Rise on Wetlands Around Charleston, South Carolina. Washington, D.C.:
U.S. Environmental Protection Agency.
Keeling, C.D., R. Bacastow, and T. Whorf, 1982. "Measurements of the
Concentrations of Carbon Dioxide at Mauna Loa Observatory, Hawaii." In
Clark (Ed.) Carbon Dioxide Review: 1982. New York: Clarendon Press.
Kunreuther, H., 1978. Disaster Insurance Protection. New York: John Wiley
and Sons.
Lacis, A., J.E. Hansen, P. Lee, T. Mitchell, and S. Lebedeff, 1981.
"Greenhouse Effect of Trace Gases, 1970-80." Geophysical Research
Letters 8(10):1035-1038.
Leatherman, S.P., 1984. "Shoreline Evolution of North Assateague Island,
Maryland," Shore and Beach 52:3-10.
Lovins, A., L. Lovins, F. Krause, and W. Bach, 1981. Least-Cost Energy.
Andover, Mass.: Brick House Publishing Co.
Meier, M.F., 1984. "Contribution of Small Glaciers to Global Sea Level."
Science 226(4681):1418-1421.
Mercer, J.F., 1970. "Antarctic Ice and Interglacial High Sea Levels."
Science 168:1605-1606.
Nordhaus, W.D. and G.W. Yohe, 1983. "Future Carbon Dioxide Emissions from
Fossil Fuels." In Changing Climate. Washington, D.C.: National Academy
Press.
Office of Technology Assessment, 1984. Wetlands: Their Use and
Regulation. Washington, D.C.: Government Printing Office.
Orson, R., W. Panageotou, S.P. Leatherman, 1985. "Response of Tidal Salt
Marshes to Rising Sea Levels Along the U.S. Atlantic and Gulf Coasts,"
Journal of Coastal Research 1:29-37.
Pendleton, C. and J.C. Stevenson, 1983. Investigation of Marsh Loss at
Blackwater Refuge. Cambridge, Maryland: Horn Point Environmental
Laboratories of the University of Maryland. (Prepared for Region 5 of
U.S. Fish and Wildlife Service.)
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REFERENCES (continued)
Ramanathan, V., H.B. Singh, R.J. Cicerone, and J.T. Kiehl, 1985. "Trace Gas
Trends and Their Potential Role in Climate Change." Journal of
Geophysical Research (August).
Revelle, R., 1983. "Probable Future Changes in Sea Level Resulting From
Increased Atmospheric Carbon Dioxide." In Changing Climate. Washington,
D.C.: National Academy Press.
Rind, D. and S. Lebedeff, 1984. Potential Climate Impacts of Increasing
Atmospheric CC-2 with Emphasis on Water Availabilityjmd Hydrology in the
United States. Washington, D.C.: Government Printing Office.
Seidel, S. and D. Keyes, 1983. Can We Delay a Greenhouse Warming?
Washington, D.C.: Government Printing Office.
Smagorinsky, J., 1982. Carbon Dioxide and Climate: A Second Assessment.
Washington, D.C.: National Academy Press.
Sorensen, R.M., R.N. Weisman, and G.P. Lennon, 1984. "Control of Erosion,
Inundation, and Salinity Intrusion Caused By Sea Level Rise." In Earth,
M.C. and J.G. Titus (Eds.) O£. cit., pp. 179-214.
Untersteiner, N., 1975. "Sea Ice and Ice Sheets," role in climatic
variations, Appendix 7 of GARP Pub. Series 16: Physical Basis of Climate
Modeling. World Meterological Council of Scientific Unions (April 1975)
206-224.
U.S. Army Corps of Engineers, 1980. Atlantic Coast of Maryland and
Assateague Island, Virginia: Feasibility Report and Final Environmental
Impact Statement. Baltimore: Corps of Engineers, Baltimore District.
Webb, K. and T. LaRoche (n.d.). "impact of Sea Level Rise on Coastal Drainage
Systems." Washington, D.C.: U.S. Environmental Protection Agency. In
press.
Wendland, W.M., 1977. "Tropical Storm Frequencies Related to Sea Surface
Temperatures." Journal of Applied Meteorology 16:480.
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CHAPTER 2
GEOMORPHIC EFFECTS OF ACCELERATED
SEA LEVEL RISE ON OCEAN CITY, MARYLAND
by
Stephen P. Leatherman
Department of Geography
University of Maryland
College Park, Maryland 20742
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INTRODUCTION
Coastal zones are inherently dynamic environments, being characterized by
differing geomorphic processes and coastline configurations. To account for
this wide variability in site and process, this study has combined analyses of
historical trends and empirical approaches to model projected changes along
Ocean City, Maryland. It evaluates the shoreline changes for a range of
projected rates of sea level rise (baseline, mid-low and mid-high) at
particular time periods (2025, 2050, and 2075).
Once digitized and transformed by a sophisticated shoreline mapping
program, Metric Mapping (Leatherman 1983a), former shoreline positions
portrayed on historical maps form the basis for projecting potential shoreline
excursion rates as a result of sea level rise. These extrapolated rates can
then be assessed in light of the possible impact that recent human
modification may have on future trends.
This chapter first describes briefly the physical characteristics of the
study area and then discusses projected shoreline responses to various
EPA-derived sea level scenarios. It also contains an appendix describing the
offshore changes associated with long-term sea level rise.
Sea level has always been rising or falling throughout geologic time
relative to the land surface. The last major change in sea level occurred
during the most recent Ice Age, when sea level was approximately 100 meters
(three hundred feet) lower than at present. Although the rate of rise during
the last several thousand years has apparently slowed, recent sea level
changes based on tidal gauge data show a definite upward trend during this
century (Fig. 1). Sea level may now be rising as fast as at any time during
the last several thousand years (Gornitz, Lebedeff, and Hansen 1982).
An additional reason for concern over the recent rate of sea level rise is
the increasing level of carbon dioxide in the atmosphere. If recent trends
(largely resulting from the burning of fossil fuels) continue, some scientists
believe that the atmospheric COz could double in the next century. The
National Academy of Sciences has estimated that this doubling will raise the
earth's average surface temperature by 1.5°-4.5°C (Charney 1979). Other gases
could double the warming from COz alone.
The sea level rise scenarios were taken from Hoffman et al. (1983); nine
rise/year combinations were selected from the projected sea level rise
curves. Table 1 presents the algebraic sum of the projected sea level rise
and subsidence to yield the relative sea level rise for Ocean City. The table
indicates, for example, that absent any accelerated sea level rise (i.e., the
baseline scenario), by 2025 sea level will have risen by 0.53 feet. In the
mid-range low scenario, sea level will have risen by 1.13 feet by 2025. This
amount of rise would inundate or otherwise.dramatically alter low-lying
coastal regions. Appendix I contains the nomenclature for shoreline
interactions with sea level rise.
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FIGURE 1
RECENT SEA LEVEL CHANGES ALONG THE U.S. COAST,
BASED ON TIDAL GAUGE DATA (from Hicks 1978)
TIME
TIME (r«'»>
1910 1920 '930 1940 1950 I960 1970
Portland. Me
1890 1900 1910 1920 1930 1940 I9SO I960 1970
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TABLE 1
RELATIVE SEA LEVEL RISE SCENARIOS
CUMULATIVE RISE OVER 1980 LEVEL1
Time
20002
20252
20502
20752
Current
Trend
0.24 ft
0.53 ft
0.83 ft
1.13 ft
Mid-Range
Low Estimate
0.40 ft
1.13 ft
2.14 ft
3.55 ft
Mid-Range
High Estimate
0.55 ft
1.55 ft
3.00 ft
5.05 ft
1 Sea level rose 0.59 feet from 1930 to 1980, according to
data from nearby tidal gauges (Hicks, Debaugh, and Hickman
1983) and interpolated using regional crustal deformation data
(Holdahl and Morrison 1974).
2 These estimates, from the Environmental Protection Agency
(Hoffman, Keyes, and Titus 1983), illustrate cumulative rise
and include a 1.8 mm/yr local subsidence rate (1980 is the
base year).
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SITE DESCRIPTION
Ocean City, Maryland, is located on an Atlantic coastal barrier called
Fenwick Island. It extends from the Delaware line to Ocean City Inlet (Fig.
2). Although Ocean City has been a resort community since the 1800s, it has
experienced explosive growth during the last 15 years with the construction of
high-rise condominiums (Fig. 3). The extensively developed barrier
accommodates summer populations that often exceed 250,000 on peak weekends,
although the permanent population is less than 6,000.
Although Ocean City has a tremendous economic investment in new real
estate, there are only limited opportunities for reducing the potential of
losing this existing development to flooding. Strong pressure will continue
to be exerted for the continued development and redevelopment of Ocean City
because of its established position as a major East Coast resort, its
proximity to the major metropolitan areas of Washington, B.C., and Baltimore,
Maryland (Humphries and Johnson, 1984), and because the National Parks Service
owns the rest of Maryland's Atlantic Coast.
Barrier islands are dynamic landforms, subject to storm-surge flooding and
sand transport processes. These coastal features are particularly vulnerable
areas for human habitation, since they extend seaward of the mainland and are
composed entirely of loose sediment (Leatherman, 1982). Coastal hazard
planning on barrier island resorts, such as Ocean City, Maryland, often fails
to recognize natural geological and geomorphic processes and their
consequences on the built environment and related habitation. In defense of
planning methods, coastal hazard analysis often suffers from lack of easily
accessible and comprehensible data.
Physical Processes
Fenwick Island is characterized by low-lying topography fronting a
shallow, microtidal embayment (Isle of Wight Bay). It is subject to flooding
with even small rises in sea level. A slight vertical rise in sea level would
result in significant horizontal displacement of the shoreline (Fig. 4).
Also, storm surges superimposed on higher mean sea levels will tend to
increase shoreline erosion, resulting in major economic losses.
The net transport of sand along the Atlantic Beach of Ocean City is to the
south, although there are several reversals in this trend. The average annual
net longshore transport is estimated to be 150,000 yd3 (U.S. Army Corps of
Engineers 1980). Since the stabilization of Ocean City Inlet with jetties in
1934-35, there has been a pronounced alteration of the adjacent shorelines for
several miles in each direction. Updrift of the jetties at south Ocean City,
a large amount of sedimentation has occurred. This shoreline progradation has
necessitated the lengthening of the Ocean City fishing pier, and the north
jetty is now impounded to capacity. A large portion of the sand moving
southward in the littoral drift system is being swept seaward by the ebb tidal
jet to form an enormous shoal (estimated volume is 8,000,000 cubic yards [Dean
Perlin, and Dally 1978]). Since little of this sand is bypassing Ocean City
Inlet, the northern portion of Assateague Island is being starved of sediment
and pushed landward (Leatherman 1979).
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FIGURE 2
LOCATION OF STUDY AREA ALONG
THE DELMARVA PENINSULA
OCEAN
CITY
CHESAPEAKE
BAY
Cape Charles
Miles
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FIGURE 3
HIGH-RISE CONDOMINIUMS AND HOTELS HAVE BEEN
BUILT ONLY A FEW HUNDRED FEET FROM THE WATER'S EDGE
(1974 photograph near 100 St., Ocean City)
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FICURE 4
LANDWARD BARRIER MIGRATION UP THE
GRADUALLY SLOPING COASTAL PLAIN OVER GEOLOGIC
TIME WITH SEA LEVEL RISE
Past
Shoreline
of Mainland •
Post Location
of Barrier
-Island
OCEAN
f
Present
Shoreline
of Mainland
BAY
Present Location
of Migrating
Barrier Island
Past Location of
Barrier Island at
Past Seo Level
OCEAN
(0 = 100 to 1000 timti H )
H = rise in sea level
0 = horizontal migration of
barrier island
T( : past sea level
T= present sea level
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ANALYSIS OF SHORELINE RESPONSE
Barrier islands, such as Fenwick Island upon which Ocean City has been
constructed, change position and shape, depending upon the relationship
between sand supply, wave energy, and sea level. Since there are essentially
no new sources of sediment for the barrier beyond that already in the
sand-sharing system or in transit through the coastal sector (littoral drift),
shoreline position responds to storms, coupled with long-term changes in water
level.
Although storms are responsible for major coastal alterations, it is not
certain that storms in the absence of water-level changes could continue to
alter the shoreline in an onshore/offshore direction. Wave-driven longshore
transport, which would erode headlands and build spits or fill concavities,
would continue to operate in any case, so that static shoreline conditions
would never be achieved. However, beach stability in a two-dimensional sense
(Bruun Rule; see Chapter 1, Figure 5) should theoretically be reached; Seelig
(1982) has shown that beach equilibrium can be achieved under wave-tank
conditions.
Perhaps a constructive way of viewing the allied roles of sea level sets
the stage for profile adjustments by coastal storms. Long-term sea level rise
places the beach/nearshore profile out of equilibrium, and sporadic storms
accomplish the geologic work in a quantum fashion. Certainly major storms are
required to stir the bottom sands at great depths off shore and hence fully
adjust the profile to the existing water level. Therefore, our underlying
assumption is that beach equilibrium will be the result of water-level
position in a particular wave-climate setting.
Figure 5 illustrates the combined effects of erosion and submergence due
to sea level rise. The term DI represents the landward translation of the
shoreline due to a simple inundation of the land; the response time is
instantaneous. Hence, direct submergence of the land occurs continuously
through time and is particularly evident in coastal bays where freshwater
upland is slowly converted to coastal marshlands. This change is termed
"upland conversion."
The second displacement term, D2, refers to a change in the profile
configuration according to Bruun (1962). The Bruun Rule provides for a
profile of equilibrium in that the volume of material removed during shoreline
retreat is transferred onto the adjacent shoreface/inner shelf, thus
maintaining the original bottom profile and nearshore shallow water
conditions. Figure 6 is a more accurate depiction of this two-dimensional
approach of sediment balancing between eroded and deposited quantities in an
onshore/offshore direction without consideration of longshore transport.
There can be an appreciable lag time in the shoreline's response to
disequilibrium conditions.
Research along the Great Lakes may prove instructive in estimating
response rates of shorelines to water-level changes. Due to climatic periods
of dry and wet conditions, lake levels have fluctuated by as much as six feet
in little over a decade. During 1969 lake levels again were high, resulting
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FIGURE 5
SHORE ADJUSTMENT WITH SEA LEVEL RISE
New Topography Following Sea Level Rise
Original Topography
Sea Level Rise Induced Erosion
New MSL
Original MSL
FIGURE 6
SHORE ADJUSTMENT TO CHANGE IN WATER LEVEL
(after Hands 1976)
. Upper Point of Profile Adjustment
\ Initial Profile
\ Profile Adjusted to Submergence, z
^
\ _ Elevated water »urlace
1 ]_ Initial water surface
Point of profile
closure
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-43-
in significant erosion of sandy beaches and cliffs along many lake shores.
The Great Lakes are not subject to astronomical tides to any degree, so that
this complicating variable was eliminated. Hands (1976) found that the Bruun
Rule is confirmed by field surveys of beach profiles during rising lake
levels. The volume of sand eroded from the beach nearly matched off shore
deposition. Hands (1976) also found that deposition extended off shore to a
distance roughly equal to twice the height of a five-year storm wave. The lag
time in shoreline response to lake level was rather rapid (approximately three
years) because the lakes are subject to frequent storm activity in the fall
and winter before surface icing.
The Great Lakes research may prove to be a useful analog in considering
the response of open ocean shorelines to long-term sea level rise with
qualifications. The Ocean City beaches are characterized by unconsolidated
sandy sediments, which are easily mobilized during major storms. The extent
of beach response depends only on the ability of waves to supply sufficient
energy to the system to accomplish the required work (to obtain profile
equilibrium in accordance with water-level position). Therefore, shore-
response lag times are tied to storm intensity and frequency.
Along the mid-Atlantic Coast, both extratropical (northeasters) and
tropical (hurricanes) storms are responsible for generating large waves
capable of significant beach erosion. Ocean City is subject to several
northeasters each winter, many of which cause moderately high tides and
flooding. The March 1962 northeaster was more severe and damaging than any
previously known storm to have affected the area. This winter storm was
complex in structure and unusual in behavior (Bretschneider 1964). It
produced a storm tide of 7.8 feet NGVD (National Geodetic Vertical Datum),
since the wind-driven tides were superimposed on a high spring tide.
Hurricanes generally produce higher tides than northeasters but are much
less frequent. The last hurricane of significance to affect Ocean City was
Hurricane Donna, which occurred on September 12, 1960 (Table 2).
Figure 7 shows the tidal frequency curve for Ocean City, Maryland. Tidal
elevations for storms with return intervals of between 5 and 500 years are
shown. The annual frequencies of hurricanes and northeasters were determined
separately and then summed to obtain the overall annual frequency at that
level, as depicted on this graph (U.S. Army Corps of Engineers 1980). The
lull in storm occurrence along the mid-Atlantic Coast during the past two and
a half decades has corresponded with the period of major coastal
construction. Ocean City expanded greatly in the early 1970s with the
construction of high-rise condominiums and hotels. Therefore, Ocean City's
beach profile is out of adjustment with sea level changes (by more than 25
years), and this trend will continue until the area is again directly affected
by a major hurricane. Therefore, there is an appreciable time lag in
shoreline response, depending upon the local storm frequency, which can only
be dealt with statistically (at recurring intervals—a frequency/magnitude
approach).
-------
-44-
TABLE 2
MAJOR STORMS OF RECORD
FOR OCEAN CITY, MARYLAND1
Storm
23 Aug. 1933
21 Sept. 1938
14 Sept. 1944
12 Sept. 1960
6-8 March 1962
Type2 Storm Surge3 (ft)
H 6.3
H ?
H ?
H ?
N 7.8
Damage
Estimate
$ 500,000
minor
$ 250,000
$ 350,000
$11,290,000
U.S. Army Corps of Engineers 1980
2Type: H = hurricane; N = northeaster
3Water level above NGVD.
-------
-45-
FIGURE 7
OPEN-COAST STORM SURGE FREQUENCY
FOR OCEAN CITY, MARYLAND
(U.S. Army Corps of Engineers 1980)
1000
10,000 500
12
10
if
*
8
6
100 50 20 10
-------
-46-
METHODS
A shoreline mapping procedure, termed Metric Mapping, has been recently
developed to quantify historical shoreline changes with a high degree of
accuracy (meets or exceeds National Map Accuracy Standards) and relatively low
cost (Leatherman 1983a). This automated technique has been designed to use
the high-speed capabilities of a computer to simulate the best photogrammetric
techniques. A flow chart depicting the steps involved in producing the
computer-plotted maps is shown in Figure 8, and complete discussion of the
procedure may be found in Leatherman (1984).
A large data set on historical shoreline positions (mean-high-water level)
is available from the National Ocean Service. This information included U.S.
Coast & Geodetic Survey charts (now called NOS "T" sheets) for the years
1849/50, 1908, and 1929/33, as well as vertical aerial photographs (1942,
1962/63, and 1977/80). Therefore, six sets of historical shorelines were
available for the study area, spanning approximately the last 130 years
(1850-80).
The Computer Mapping Laboratory of the University of Maryland's Department
of Geography was used for shoreline data manipulation and plotting. The six
shorelines were overlaid and plotted to scale on the computer-generated maps.
Shorelines were differentiated by various dot-dash patterns. As a result of
this research, the mapping program was further refined to provide rates of
shoreline change. This refinement is not trivial, since shorelines are rarely
straight; the base line for measurement must be at all places perpendicular to
the shoreline to provide accurate information. Measurements are taken
orthogonal to the measurement base line (or spine) at a preselected distance,
where the spine is parallel to the shoreline. For each transect, a table of
statistics on shoreline change is generated, and a summary histogram for each
time period is prepared. From these data sets and summary statistics of the
historic trend, a projection of future shoreline changes can be made.*
While this approach is less quantitative for modeling purposes than the
Bruun method, it is more realistic in a geomorphic sense. The Bruun (1962)
concept is essentially a two-dimensional approach, representing the sediment
balance between eroded and deposited quantities in an onshore/offshore
direction, without considering longshore transport. The technique used for
this study involves the empirical determination of projecting new shorelines
using trend lines. In this case, the shoreline response is based on the
historical trend with respect to the local sea level changes during that time
period. This procedure accounts for the inherent variability in shoreline
response based on differing coastal processes, sedimentary environments, and
coastline exposures (Leatherman 1983b).
* This task was accomplished manually for this project, but we plan to
write a computer program to simulate spatial changes in a temporal sense,
using historical shoreline movements and physical relationships as the
required inputs (Leatherman and Clow 1983).
-------
-47-
FIGURE 8
METRIC MAPPING TECHNIQUE
NOS CHARTS
' select control points on
NOS charts
1
AIR PHOTOS
annotate aerial photographs
2
COMMON POINTS
select secondary control points
common on NOS charts
and aerial photos
3
DIGITIZE AIR PHOTOS
digitize air photos, including
secondary control points
and fiducial marks
6
CONVERT PROGRAM
obtain state plane coordinates of
secondary controls from primary
control points with
5 convert program
DIGITIZE CHARTS
digitize primary and secondary
control points on NOS
*T sheets
4
SPACE RESECTION
PROGRAM
use space resection program to
transform air photos to remove
radial and tilt distortion and
scale differences in
7 each photo
MESH PROGRAM
adjust junctions between
adjacent photos with
MESH program
8
PROGRAM PLOT
plot maps with computer
9
-------
-48-
The relationship between sea level rise and shoreline movement is
formulated by assuming that the amount of retreat from the historical record
is directly correlated with the rise rate of sea level. Therefore a 3X rise
in sea level will result in a 3X increase in the retreat rate, assuming lag
effects in shoreline responses are small compared to overall extrapolation
accuracy.
Tidal gauge records document the local (eustatic effects plus isostatic
effects, such as subsidence) rate of sea level change over the period of
record. Records from nearby tidal gauges indicate that sea level rose about
0.59 feet between 1930 and 1980 (Hicks, Debaugh, and Hickman 1983). A portion
of this apparent rise was probably due to subsidence. The relative sea level
rise scenarios for baseline (current trend), mid-range low, and mid-range high
include a 1.8 mm/yr local subsidence rate (Hoffman, Keyes, and Titus 1983).
RESULTS
Historical shoreline changes along Ocean City are shown in Figure 9. The
average rate of oceanside erosion over the 130 years of record has been 1.9
feet per year, but there has been much variation along this shoreline.
Histograms of shoreline change indicate some reversals of this trend,
particularly at stations 1 through 13 (Figs. 10-13). This phenomenon could be
due to large-scale, low-amplitude sand waves migrating downdrift along the
shoreline. However, for most of the Ocean City shoreline, the overall trend
has been long-term erosion (Fig. 14).
There are clearly gradients in the longshore transport of sand due to
differential wave refraction and other effects that give rise to alongshore
variations in shoreline trend (Goldsmith et al. 1974). Since the littoral
nodal point for the Delmarva coastal compartment is believed to be located
near Bethany Beach, Delaware (U.S. Army Corps of Engineers 1980), it can be
assumed that over hundreds of years the littoral influx and outflux of sand at
Ocean City should be approximately equal, except near the jetty. If this is
correct, then the long-term losses of sand to the off shore, evident along the
Ocean City shoreline, are due to historical sea level rise, which has averaged
approximately 1.2 feet per century (Hicks 1978). Therefore, future shoreline
location and erosion rates can be predicted on the basis of anticipated sea
level rise (Leatherman 1983b).
From 1930 to 1980, the relative sea level rise was 0.59 feet (Hicks,
Debaugh and Hickman 1983). This equates to 190 feet of erosion during the
last 100 years with 1.18 feet of rise; thus, a 1-foot rise would correspond to
161 feet of erosion. Using the straight-line method of extrapolation as
previously explained, then shoreline change can be projected for the nine
rise/rate combinations (Table 3). The amount of shoreline recession varies
from 39 feet (baseline) to 89 feet (mid-range high) for the year 2000 and from
182 feet (baseline) to 813 feet (mid-range high) by 2075. At present, the
beaches along Ocean City are critically narrow, particularly during the
high-energy winter months. Therefore, the current trend of recession
exacerbates the problem and increases the vulnerability. Accelerated sea
level rise increases the rate of retreat by two to five times, thereby
-------
-49-
FIGURE 9
COMPARISON OF HISTORICAL SHORELINE CHANGES
ALONG OCEAN CITY, MARYLAND (1850-1980)
-------
FIGURE 10. INDEX MAP OF OCEAN CITY SHOWING TRANSECTS USED BY PROGRAM THAT MEASURES SHORELINE CHANGES
Ln
O
-------
-51-
FIGURE 11
HISTOGRAM OF HISTORICAL SHORELINE CHANGES (1929-1942)
Transects 1 to 45 are Along Ocean City, Maryland.
SHORELINE CHANGE MEASUREMENTS AT OCEAN CITY, MARYLAND
1929-1943
N
E
T see .
s
H
o e .
R
E
L
i -see .
N
E
£ -ieee .
H
A
N
G
E -isee _
I
N
-seee .
F
E
E
T -2500 .
onnffflnrinn
Oil II II IIBII 11111 III IiytPtDQ^'00'"1""
^^MJuUm 1 Uj^^^llr^
^*Til
nrfl
18 y ii B 1 1 B 1 1 n M-^
iflSlilllJU'-P"^^
j||ill-Mj"^^
L^jj-ir^-1
1 1 1 1 1 1 1 1
i i I I I
5 10 15 20 as 30 35 40 45 50 55 60 65 70
STATION NUMBER
-------
-52-
FIGURE 12
HISTOGRAM OF HISTORICAL SHORELINE CHANGES (1942-1962)
Transects 1 to 45 are Along Ocean City, Maryland.
N
E
T
S
H
0
R
E
L
I
N
E
C
H
A
N
G
E
I
N
F
E
E
T
SHORELINE CHANGE MEASUREMENTS AT OCEAN CITV, MARYLAND
194S-196S
see
e .
-see .
-teee _
-isee .
-aeee
-asee .
T
5
T"
15
—r
se
—r
36
~r
35
25 36 35 40
STATION NUMBER
T
45
T
se
55
T
ee
—r
65
~T
?e
-------
-53-
FIGURE 13
HISTOGRAM OF HISTORICAL SHORELINE CHANGES (1962-1980)
Transects 1 to 45 Are Along Ocean City, Maryland.
SHORELINE CHANGE MEASUREMENTS AT OCEAN CITY, MARYLAND
1962-1980
N
E
T
S
H
0
R
E
L
I
N
E
H
A
N
G
E
I
N
see .
e _
-see _
-ieee .
-isee .
-2000 .
-asee .
DffJOP
sog-DQ*
—i 1 1 1 1 1 1 1 1 1 1 1 r
19 15 80 as 30 35 40 45 50 55 60 65 70
STATION NUMBER
-------
-54-
FIGURE 14
HISTOGRAM OF HISTORICAL SHORELINE CHANGES (1850-1980)
Transects 1 to 45 are Along Ocean City, Maryland.
SHORELINE: CHANGES IN OCEAN CITY, MARYLAND
1854-1980
c
H
A
N
c
E
P
E
R
V
E
A
R
I
N
f
E
E
T
e
-s .
-10 .
-is .
-ae ,
JJHP"
r
N
! 4.
u
6
126 112 89 74 57 43 27 14 | I ASSATEAQUE ISLAND
APPROXIMATE STREET NUMBERS
-------
-55-
TABLE 3
PROJECTED SHORELINE RECESSION
ALONG OCEAN CITY, MARYLAND1
Year
2000
2025
2050
2075
Current
Trend
39 ft.
85 ft.
134 ft.
182 ft.
xSee Table 1 for rates
Mid-Range
Low Estimate
64 ft.
182 ft.
345 ft.
572 ft.
of sea level rise.
TABLE 4
Mid-Range
High Estimate
89 ft.
250 ft.
483 ft.
813 ft.
CONTOUR SHIFTS (1929-1965)
From Trident Engineering (1979)
Near
-10
-20
Over 36-
Contour Year Period
High Water line 86 feet
foot contour
foot contour
252 feet
350 feet
Average Shift
per Year
2.4 feet
7.0 feet
9.7 feet
-------
-56-
significantly reducing the planning time for hazard mitigation and
significantly increasing the vulnerability of the urbanized area through time.
While the historical trend of recession has be.en set at 1.9 feet per year,
there has not been an appreciable change in shoreline position since 1961/62
(Fig. 14). In other words, the historical rate of erosion has not been
realized in the last few decades. This marked departure from the trend may be
due to human modifications of the shore, notably groins, sand scraping, and
some beach fill. However, it is more likely that the noted lull in hurricane
activity since 1960 is the key factor.
This proposition is supported by an analysis of historical bathymetric
changes. While these data are not as readily available as shoreline movement
information, and their accuracy is more in question, significant trends emerge
from a historical bathymetric comparison of the area off shore of Ocean City
(Table 4). It is clear that the shoreface is steepening through time. The
landward movement of the 20-foot-deep contour is greater than that of the
10-foot-deep contour, which in turn has migrated farther than the
mean-high-water line.
We have conducted some checks of the Corps of Engineers' profiles, used by
Trident Engineering (1979), as compared to the original Coast and Geodetic
Survey boat sheets and have obtained similar measurements (Appendix II). It
appears that the shoreline remains in approximately the same location for a
period of time, while acting as a hinge as the adjacent shoreface steepens.
It is not known at present what angle of shoreface inclination is the natural
equilibrium orientation. Clearly, the current steepened condition cannot be
considered at equilibrium, since recent bathymetric data have shown that the
steepening trend has continued. Assuming that the equilibrium angle of
inclination for the shoreface was reached at some point during the survey
period (1850-1965), a future major coastal storm should cause the angle to
decrease toward the idealized equilibrium position (Moody 1964).
It is a well established geologic principle that much geomorphic work is
accomplished in quantum steps (Hayes 1967; Leatherman 1981 1982). Therefore,
a major coastal storm would provide the impetus by shifting and redistributing
nearshore sands to reverse the steepening trend of the shoreface. At this
point, the shoreface returns to its minimum angle and then continues to slowly
steepen again through time until the next major storm.
In summary, the shoreface appears to undergo bicyclic adjustment through
time. A long, quiescent steepening phase, during which shoreline position is
relatively stable or slowly retreating, is followed by a brief stormy period
of shoreface flattening and rapid landward migration of the shoreline.
Ongoing research should provide the type of data necessary to quantify this
process and formulate a predictive model.
-------
-57-
SUMMARY
The Atlantic Coast of Ocean City, Maryland, is undergoing long-term
shoreline retreat as a result of sea level rise. During the past 130 years
(1950-1980), the beach has eroded an average of 1.9 feet per year. Inspection
of shoreline movement over this period shows that the recession is not
constant through time or space. Indeed, there were periods of very rapid
shoreline retreat, which probably corresponded to the major storms of record
-- 1902, 1933, and 1962. In addition, the erosional trend at any one point
along the shore has tended to fluctuate through time.
Many areas show reversals in trend, where an area that is characterized by
high recessional rates for a period of time is later retreating more slowly,
as compared to the overall trend, or accreting. These dramatic short-period
(perhaps 20- to 30-year) trends may result from the alongshore migration of
low-amplitude, very long wave length, sand waves. When the trough of the
shoreline meander passes a certain locality, then it is characterized by
erosion in excess of the trend. As the crest of the seaward-projecting horn
of this crescentic feature passes the same point some time later, then the
trend is reversed. Depending upon the amplitude of the sand wave and overall
erosion rate, the area may be so affected as to actually exhibit pronounced
accretion for a period of time. This appears as a flip-flop in the historical
shoreline migrational record.
Analysis of these long-period sand waves can result in much confusion when
we try to interpret short-term information, such as beach profiles. This
analysis indicates that the longest accurate record available should always be
used for determinating shoreline trend. Short-term data are useful in
documenting site-specific and temporal changes, but such data are not the best
indicators of net shoreline response over the long term.
This type of analysis could be undertaken for any sandy shoreline. The
easily eroded unconsolidated sediments of barrier islands make the projections
straightforward, except where modified by coastal engineering structures. The
underlying assumption of this analysis is that shorelines will respond in
similar ways in the future, as was the case in the past, since sea level rise
is the driving function, and all other parameters remain essentially constant.
This analysis has assumed that total shoreline adjustments to sea level
rise would be accomplished at the particular scenario year. Clearly, there
will be some lag in shoreline response to higher water levels. This time
period may be on the order of 25 to 50 years, corresponding to the frequency
of major hurricanes. Better information on storm frequency and magnitude
would improve this analysis. Without an in-depth analysis of site-specific
data on many principal variables, such as offshore profile changes, the simple
extrapolation of historical trends is a reliable technique for forecasting
shoreline changes.
-------
-58-
APPENDIX I
NOMENCLATURE FOR SHORELINE
INTERACTIONS WITH SEA LEVEL RISE
As sea level rises, a number of complex and related phenomena come into
play. In the following enumeration, we present general, intuitive definitions
of the major phenomena and indicate the technical terms which most closely
define each. A variety of shoreline interactions result from the rising
(transgression) and falling (regression) of sea level. Most of these
changes probably act in concert, but individually can be seen to result in
several distinct responses. Rising sea levels are accompanied by general
retreat of the shoreline. This is produced by erosion and/or inundation.
Classically, erosion describes the physical removal of beach and cliff
material, while inundation is the submergence of the otherwise unaltered
shoreline.
During periods of falling (regression) or stable sea level, shorelines may
advance seaward, or prograde, as material is deposited and accrete.
Shoreline propagation generally occurs along river deltas, where sediment
influx is high, unless the rate of sea level rise more than offsets sediment
deposition. The recent dramatic erosion of part of the Nile Delta, resulting
from the loss of sediment trapped behind the Aswan High Dam, reinforces the
importance of sediment supply in maintaining shoreline equilibrium in deltaic
environments. During at least the last century, there has been a significant
rise in sea level which has resulted in pronounced shoreline recession along
most Atlantic Coast beaches (e.g., Leatherman 1979, 1983b) and indeed along
the large majority of sandy beaches worldwide (Bird 1976).
-------
-59-
APPENDIX II
PROFILE CHANGES AT OCEAN CITY, MARYLAND: 1929-1978
by
Susan Bresee
Stephen P. Leather man, Principal Investigator
Graphed profiles were available from the U.S. Army Corps of Engineers for
the years 1965 and 1979. The profiles were drawn from seventeen transects,
measured perpendicularly to the Ocean City coast. The origin and endpoint of
each transect were digitized, along with four latitude and longitude values on
each street map. From these given latitudes and longitudes, the coordinates
for each transect were determined by computer. Thus, the four digitized
rectilinear coordinates defined where the map was in space, and then the
computer let it be known where the transects were, in terms of latitude and
longitude, within that two-dimensional framework.
Map Bathymetry
After transects were determined from the 1965 and 1979 Ocean City street
maps, the seventeen transects were hand plotted on each Ocean City map judged
useful to the project. The other maps chosen were National Ocean Survey maps
for 1929, 1962, and 1978. The 1848 and the 1849 maps were rejected because
depth values did not reach the shoreline, original latitude and longitude
markings were inaccurate, and values were measured sparsely parallel to the
shoreline. Transect numbers are Ocean City street numbers.
Every value on the graphed 1965 and 1979 profiles was digitized. For the
other maps, all values within rectangular envelopes 0.3 miles wide and 0.7
miles long centered along the sketched transects were individually digitized.
each map was oriented in space by digitizing four map coordinates before
transect values were digitized. A modified Surface II program retrieved each
transect within its envelope of stored values. It extrapolated transect
values from observed values and graphed each profile.
The inaccuracies of adjusting map scales and directionally stretching
transposed maps were avoided (Sallenger et al. 1975). Since the transects and
transect values were accurately determined and profiles were accurately
graphed, many errors were eliminated. The largest errors remaining are
mapping errors. For the purpose of slope measurement, extrapolation errors
are not significant. Small irregular depressions or rises would not change
profile slope calculations.
Table II-l shows the position in feet of the shoreline and -10ft., -20ft.,
and -30ft. contours, with respect to an arbitrary origin. Table II-2 shows
the changes between 1962 and 1978, the most recent interval for which the data
permit a meaningful comparison.
-------
-60-
TABLE 11-1
CONTOUR DATA FROM 3RD STREET TO 145TH STREET
(in feet)
S3
Sll
S21
S26
S33
S41
S48
1929
1962
1965
1978
1979
1929
1962
1965
1978
1979
1929
1962
1965
1978
1979
1929
1962
1965
1978
1979
1929
1962
1965
1978
1979
1929
1962
1965
1978
1979
1929
1962
1965
1978
079
Shoreline
0
400
340
400
390
60
200
230
90
120
140
170
140
170
140
60
180
140
90
110
200
200
200
200
220
200
200
250
200
230
200
290
180
310
200
- 10ft.
700
920
790
900
740
880
690
750
620
520
920
810
670
720
450
790
720
660
580
520
950
880
750
630
480
990
710
710
710
810
950
750
650
650
680
- 20ft.
1370
1490
1350
1540
1170
1340
1210
1320
970
800
1300
1030
940
890
690
1320
1070
1050
890
810
1460
1140
1070
910
780
1550
1420
1180
1180
1210
1400
1260
1120
1080
930
- 30ft.
1940
2940
3250
1960
2120
2060
2330
1920
2300
1910
2000
1400
2060
2190
1980
2190
1750
2160
2580
2030
2600
1540
2850
3110
2950
2900
2770
3120
3360
3100
2890
2040
-------
-61-
TABLE 11-1 (Continued)
S55
S65
S76
S86
S94
1929
1962
1965
1978
1979
1929
1962
1965
1978
1979
1929
1962
1965
1978
1979
1929
1962
1965
1978
1979
1929
1965
1978
1979
Shoreline
220
220
180
220
220
280
200
220
220
220
310
120
150
10
150
170
110
140
0
140
80
150
120
- 10ft.
980
780
720
740
530
950
810
720
640
670
1080
720
590
470
470
900
640
670
380
480
830
620
460
- 20ft.
1680
1120
1070
1130
920
1520
1540
1320
1340
1420
1850
1170
1060
900
780
1740
970
1000
1040
890
1260
1010
800
- 30ft.
3320
2510
2410
2630
2540
2700
2920
2730
2630
2770
2260
2950
2930
2690
2650
3330
2100
1780
S100
S119
S129
1929
1965
1978
1979
1929
1965
1978
1979
1929
1965
1978
1979
250
250
10
150
230
280
180
180
250
150
180
150
940
610
460
480
1030
730
720
590
960
570
460
400
1390
1070
1030
860
1430
1150
1030
880
1440
1120
880
670
2400
1870
1780
2390
2330
2330
1550
2420
2550
1190
-------
-62-
TABLE 11-1 (Continued)
Shoreline - 10ft. - 20ft. - 30ft.
S137 1929 150 830 1190 2820
1965 180 63* 850 1770
1978 110 430 730 1970
1979 140 490 720 1320
S145 1929 180 800 1150 1610
1965 120 480 740 1320
1978 0 470 800 1510
1979 140 450 740 1230
-------
-63-
TABLE 11-2
CHANGE IN THE POSITION OF THE SHORELINE AND -10, -20, AND
-30 FOOT CONTOURS FROM 1962 TO 19781
(3rd Street to 86th Street)
Transect
S3
Sll
S21
826
S33
841
848
S55
865
876
886
mean
mean - ad j us t ed 3
Shoreline
0
-110
0
- 90
0
0
+ 20
0
+ 20
-110
-110
- 34.6
- 30.0
-10ft.
- 20
- 70
- 90
-140
-250
0
-100
- 40
-170
-130
-260
-115.5
-131.1
Contours
-20ft.
+ 50
-240
-140
-180
-230
-240
-180
+ 10
-200
-270
+ 70
-140.9
-151.1
-30ft.
+310
+210
+ 90
- 0
+ 20
-210
-470
NA2
NA
+ 40
-260
- 30.0
-112.9
Negative numbers indicate retreat toward the land.
2NA = not available.
3Exludes transects S3 and Sll which are influenced by the jetty at Ocean
City Islet.
-------
-64-
REFERENCES
Bretschneider, C.L., 1964. The Ash Wednesday East Coast Storm, March 5-8,
1962: A Hindcast of Events, Causes, and Effects. In Proc. of 9th Conf.
on Coastal Engineering. New York: Amer. Soc. Civil Engieners, pp.
617-659.
Bruun, P., 1962. Sea level rise as a cause of shore erosion. Journal of
Waterways and Harbors Division (Amer. Soc. Civil Engineers), 1:116-130.
Bird, E.C.F., 1976. Shoreline Changes During the Past Century. In Proc. 23rd
International Geographical Congress, Moscow, 54 pp.
Charney, J., 1979. Carbon Dioxide and Climate: A Scientific Assessment.
Washington, D.C.: Climate Research Board, NAS.
Dean, R.G., M. Perlin, and B. Dally, 1978. A Coastal Engineering Study of
Shoaling in Ocean City Inlet. Dept. of Civil Engineering, Univ. of
Delaware, 135 pp.
Goldsmith, V., W. Morris, R. Byrne, and C. Whitlock, 1974. Wave climate model
of the mid-Atlantic continental shelf and shoreline: model development,
shelf geomorphology and preliminary results. VIMS SRAMSOE No. 38,
Gloucester Point, Va., 146 pp.
Gornitz, V., S. Lebedeff, and J. Hansen, 1982. Global sea level trends in the
past century. Science 215:1611-14.
Hands, E.B., 1976. Predicting adjustments in shore and offshore sand profiles
on the Great Lakes. CERC Tech. Aid 81-4, 25 pp.
/
Hayes, M.O., 1967. Hurricanes as geological agents: Case studies of
hurricanes Carla, 1961, and Cindy, 1963, Report on Investigations No.
61. Austin, Texas: Bureau of Economic Geology, University of Texas.
Hicks, S.D., 1978. An average geopotential sea level series for the United
States. Journal of Geophysical Research 83:1377-1379.
Hicks, S.D., H.A. Debaugh, and L.H. Hickman, 1983. Sea Level Variations for
the United States 1855-1980. Rockville, Maryland: U.S. Department of
Commerce, NOAA-NOS.
Hoffman, J., D. Keyes, and J. Titus, 1983. Projecting Future Sea Level Rise: .
Methodology, Estimates to the Year 2100 and Research Needs. Washington,
D.C.: Government Printing Office.
Holdahl, S.R., and N.L. Morrison, 1974. Regional Investigations of Vertical
Crustal Movements in the U.S. Using Precise Relevelings and Mareograph
Data. Tectonophysics 23:373-390.
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REFERENCES (continued)
Humphries, S.M., and L.R. Johnson, 1984. Reducing the Flood Damage Potential
in Ocean City, Maryland. Annapolis, Md.: Md. Dept. of Natural
Resources, 151 pp.
Leatherman, S.P., 1979. Migration of Assateague Island, Maryland, by inlet and
overwash processes. Geology 7:104-107.
Leatherman, S.P. (Ed.), 1981. Overwash Processes: Benchmark Papers in
Geology, Vol. 58. Stroudsburg, PA: Hutchinson and Ross, 376 pp.
Leatherman, S.P., 1982. Barrier Island Handbook. College Park, Md.:
University of Maryland, 109 pp.
Leatherman, S.P., 1983a. Shoreline mapping: A comparison of techniques.
Shore and Beach 51:28-33.
Leatherman, S.P., 1983b. Historical and Projected Shoreline Mapping. Proc.
of Coastal Zone 83, American Society of Civil Engineers, pp. 2902-2909.
Leatherman, S.P., 1984. Shoreline evolution of North Assateague Island.
Maryland. Shore and Beach 52:3-10.
Leatherman, S.P., and B. Clow, 1983. UMD Shoreline Mapping Project, IEEE
Newsletter 22:5-8.
Moody, D., 1964. Coastal morphology and processes in relation to the
development of submarine sand ridges off Bethany Beach, Delaware. Ph.D.
dissertation, Johns Hopkins University, 167 pp.
Sallenger, A.H., Goldsmith, and C.H. Sutton, 1975. Bathymetric Comparison:
A Manual of Methodology, Error, Criteria, and Techniques. Gloucester
Point, VA: Virginia Institute of Marine Science.
Seelig, Wm., Waterways Experimental Station, Vicksburg, Mississippi; personal
communictaion with author, 1982.
Trident Engineering, 1979. Interim Beach Maintenance at Ocean City, Maryland.
Annapolis, Md.: Maryland Department of Natural Resources (several volumes).
U.S. Army Corps of Engineers, 1980. Beach Erosion Control and Storm
Protection, Atlantic Coast of Maryland and Assateague Island, Virginia.
39 pp. plus appendices.
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CHAPTER 3
EFFECT OF SEA LEVEL RISE AND NET SAND VOLUME CHANGE
ON SHORELINE POSITION AT OCEAN CITY, MARYLAND
by
Craig H. Everts
Moffatt & Nichol, Engineers
250 W. Wardlow Road
Long Beach, CA 90807
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ABSTRACT
An estimate of the impact of past and future sea level rise on the
shoreline of Ocean City, Maryland, is developed in this report. Two
predictive methods are tested on a historical data set for 1930-80 and used to
project future erosion through 2075. These methods are also used to project
future requirements for beach nourishment.
Data consisted of analyzed title records, EPA-supplied global sea level
rise projections, the local land subsidence rate at Ocean City, the net rate
of sand loss by alongshore transport, the net rate of sand loss at the base of
the shoreface and landward to the barrier island by overwash, wind, and
ephemeral inlet processes, volumes added by beach replenishment, shoreface and
barrier island profiles, and the size distribution of sediment landward of the
shoreface.
The average measured shore retreat for the period 1930-80 was 176 ft,
during which sea level rose 0.6 ft along the coast of Ocean City. Everts1
(1984) method predicted a retreat of 184 ft with 19 percent due to sea level
rise and 81 percent resulting from sand losses from the shoreface. Alongshore
losses accounted for 88 percent of the net sand loss; losses to linear,
shoreface-connected ridges at the base of the shoreface accounted for 9
percent. Bruun's (1983) method predicted a shoreline retreat of 43 ft with 91
percent of that value attributed to sea level rise. The Bruun method is not
designed to consider shore retreat resulting from alongshore sediment losses.
The methods were also used to project future shoreline retreat, using sea
level rise projections from Hoffman et al. (1983). These projections imply
that a rise of 1.1-1.6 feet by 2025 and 3.6-5.0 feet by 2075 along the coast
of Ocean City is most likely. The Bruun method projects that the shore will
retreat 72-106 ft by 2025 and 236-346 ft by 2075, if no additional measures
are taken to control erosion. The Everts method projects a retreat of 238-273
ft by 2025 and 707-878 ft by 2075. Shoreline retreat resulting from sea level
rise, with one realistic exception, can only be reduced or stopped by beach
nourishment, i.e., the periodic addition of sand from outside the shoreface
system. The exception, which would reduce, but not eliminate, the retreat, is
a shore-parallel dike that perches the shoreface and reduces its length. That
portion of shoreline retreat caused by sand losses from the shoreface may, in
addition to beach nourishment, be reduced by structural means. For example,
seaward-directed sand losses may be reduced using offshore breakwaters,
especially near the linear, shoreface-connected ridges. Landward-directed
sand losses by overwash, aeolian transport and island breaching during extreme
storms may be reduced with artificial dunes. Sand losses caused by longshore
transport may be substantially reduced by a system employing a sand trap near
Ocean City Inlet and a backpassing procedure to return the sand to the
divergence nodal reach near the north end of Ocean City. The Bruun method
implies that a beach nourishment solution would require 1.5-2.4 million cubic
yards of sand through 2000 and 4.5-6.5 million cubic yards through 2025. The
Everts method projects that 4.6-5.2 million cubic yards of sand will be
necessary through 2000 and 11.3-12.9 million cubic yards through 2025.
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INTRODUCTION
Moffatt & Nichol, Engineers, was requested to: (1) calculate shoreline
retreat to year 2075 at Ocean City, Maryland, and (2) estimate the quantity of
sand that will be necessary to maintain the current shoreline to year 2075.
Two methods were used: (1) Bruun's (1962, 1983) method, and (2) Everts1
(1984) method.
METHODOLOGIES
Bruun's (1962, 1983) Method
In 1962 Bruun proposed a method to predict the effect of a rising relative
sea level (RSL) on a sandy shoreline. He assumes the Inner-Continental Shelf
profile will maintain a constant shape and position relative to the sea
surface by translating landward and up as RSL rises (Fig. 1). To accomplish
this, Bruun maintains the beach and upper shoreface profile will erode and the
lower part of the shoreface profile will acquire an equal volume of sediment.
Furthermore, he assumes a point of intersection of the initial profile and
subsequent profile (Fig. 1) will always exist.
Using the assumptions that (1) beach and offshore profile equilibrium
exists and (2) the shore in question is in a state of quantitative materials
balance, Bruun (1962) determined the practical approximation of shoreline
movement, s (Fig. 1), to be
ad
s = — (1)
h
in which a = RSL rise, h = maximum depth of exchange of material between the
nearshore and the offshore, and £ = length of the profile of exchange. The
cross-sectional area produced by the upward movement of the profile, a, equals
the area swept by the landward movement of the profile, sh. In 1962, Bruun
did not consider balanced sediment transport into and out of the system a
necessity for his method to work, and emphasized that the relationships of
Equation 1 must be considered long term and regional in scope.
In 1983, Bruun discussed the effect of sediment composition on shoreline
change. Where r = percent of material smaller than 0.06 mm (i.e., silt and
mud-sized material), which is eroded from the nearshore area (Fig. 1), Bruun
adjusted Equation 1 such that
an r
s = — (1 + ) (2)
h 100
The requirement for including this materials restriction is based on Bruun's
assumption that very fine material produced in the eroded nearshore zone will
not remain on the equilibrium profile in the depositional offshore zone.
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FIGURE 1
DEFINITION SKETCH, BRUUN'S (1962, 1983) METHOD
OFFSHORE
EARSHQRE
SEA LEVEL RISE
EROSION
INITIAL PROFILE
SUBSEQUENT PROFILE
DEPOSITION
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For a narrow continental shelf, Bruun (1983) also introduced a loss
function R to account for sediment transport beyond the outer edge of the
offshore region. He cited submarine canyon losses as an example. To account
for this (percent) loss seaward of the offshore boundary, Equation 2 was
modified as follows:
at r R
S = — (1 + )(1 + ) (3)
h 100 100
Everts' (1984) Method
This is a sediment budget analysis which considers absolute losses and
gains of sand to a bounded coastal reach. It is coupled with a procedure that
accounts for the apparent loss of sand that occurs as sea level rises relative
to the beach and shoreface. The dependent variable is shoreline position;
independent variables are the RSL rise rate, shoreface and backbeach profile,
percent of sand in the sediment deposit landward of the shoreface, and net
losses and gains of sand from the shoreface and backbeach area (control
volume).
Everts (1984) assumes, as Bruun (1962, 1983) did, that the shoreface will
remain in equilibrium with the sea surface and move vertically upward as sea
level rises. He also assumes no change in profile shape will occur during the
rise. An accretional volume of sand-sized sediment, Vg and/or V'g, as shown
in Figure 2, is required by continuity to move the profile in space. The most
likely source of that sediment is the volume, V-| , eroded as the shoreface
profile translates landward and/or upward. An absolute addition or
subtraction of sand, VQ, may also move to or away from the shoreface by
transport across its boundaries, herein called a control volume. The sediment
balance required to maintain an equilibrium shoreface profile becomes
kV! + Vo - (Vg + V'g ) = 0 (4)
in which k equals that portion of liberated sediment in Vi that is of sand
size or larger.
Sediment volume changes in Equation 4 not caused by a redistribution of
sand on the shoreface as the shoreface retreats are considered in the Vo
term. This term accounts for spatial gradients in various components of
sediment transport through the control volume and includes beach replenishment
and sand mining. The bounds of sediment movement are xm (the landward limit
of sediment transport affecting the shoreline), and xo (the seaward limit of
sediment movement that affects the position of the shoreline, taken at the
base of the shoreface).
If we assume k £ 1, i.e., that sediment eroded as the shoreface retreats
is not all sand-sized or larger, the solution of Equation 4 is
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FIGURE 2
DEFINITION SKETCH, EVERTS1 (1984) METHOD
CONTROL VOLUME
BACKBENCH | SHORE FA
Vs *. s
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x'c
f(x)dx - ( [f(x-s) + a] dx^ + Vo -
xc
'( [f(x-s) + a] dx +y h(x)dx -
[ Xe+S Xm+S
X0+S X0 )
j [f(x-s)+a] dx - j f(x)dx> = 0
^v v I
xc x c ;
C
f(x)dx
j(x)dx
x0+s
(5)
When solved for s, shoreline change is the average for a coastal reach of some
specified length. The three unknowns in Equation 5 are s, xc and x'c •
Because of the site-specific nature of the shape of the initial and later
profiles, the variable nature of the value of k, and other site-specific
considerations including Vo» and because high-speed computers allow a
numerical integration and solution of Eq. 5 by trial and error, it will rarely
be desirable to attempt a simplified solution.
The shape of the equilibrium shoreface profile must be considered when the
value of k is less than 1; that is, the region of fill must be integrated
separately because k is a coefficient applied only to the V-| region. Sediment
deposited in Vg and V'g is assumed to be of sand size or greater because the
depositional surface is on the shoreface or landward of it. Landward
transport to the backbeach area by overwash or wind processes typically
involves only sand.
Boundaries of integration include the seaward limit of the equilibrium
shoreface profile at xo, the landward limit of the shoreface at xe, and the
landward limit of sediment transport on the backbeach, xm, all of which are
known or can be estimated, and two unknown boundaries, xc and x'c ,
respectively, at the landward and seaward limits of the erosional region, V-j
(Fig. 2). The subsequent (after time At) shoreface profile is f(x-s)+a,
which is the initial shoreface profile, f(x), translated upward a distance, a,
and landward a distance, s; the initial and subsequent backbeach profiles,
are, respectively, f(x) and h(x). The subsequent backbeach profile, h(x), is
specified either as an engineered profile in a developed area, as illustrated
in Figure 2, or as a hypothesized natural profile which retains its shape as
it moves landward. Integrations of Equation 5 are made by approximation using
the trapezoidal rule
A = x[l/2 (zz + zn)
(6)
in which z-j, 2-2 ••• zn are surveyed elevations above an arbitrarily-selected
zero datum for a series of equally-spaced parallel chords a distance x apart.
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TEMPORAL AND SPATIAL AVERAGES
Changes in shoreline position obtained using the Bruun or Everts methods,
because of the fluctuating nature of sea level changes and difficulties
inherent in establishing net longshore and cross-shore sediment transport
rates, must be the average obtained over time and space. The time average (in
At) must be such that effects of the net sea level rise are considered.
The spatial average is that obtained for a designated alongshore reach in
which net sediment transport rates can be estimated over the time period, At.
Minimum Time Required
Sea level rise must be considered, and averaged, over a relatively long
time period. The magnitude of changes in water surface elevation decreases as
the frequency of change decreases (Fig. 3). The entire shoreface profile does
not respond by moving vertically upward (and possibly landward) from its base
to the shoreline, as the water surface fluctuates at the frequency of the
tides, a lunar month, or even a few years. Dynamic shoreface equilibrium is
maintained as the shoreface is "swept" to its base and in an alongshore
direction with sand over a long time period. For example, when rates of sea
level rise at Atlantic City, New Jersey (Fig. 3), are compared to rates of
shoreline change at three New Jersey beaches over the 1963-1972 period (Everts
and Czerniak, 1977) no relationship is found between the average sea level
change from one year to the next and the average shoreline change between
years. The variation in yearly sea surface elevation between years (Fig. 3)
is about equal to the net change that occurs over a period of 30 years. In
addition, because waves, winds, and currents vary on scales that certainly
exceed years, averaged values of alongshore sediment transport and cross-shore
sediment transport require averaging at periods in excess of a few years. The
location of nodal "points" in alongshore transport also varies from year to
year depending on wave climate.
Everts (1984) showed that the shoreface at Smith Island, Virginia, about
90 km south of Ocean City, probably maintained a dynamic equilibrium shape as
sea level rose 0.0022 m/yr (0.007 ft/yr) over time intervals of about 70 years
(survey frequency) during a 130-year period when the shoreline retreated 700 m
(2350 ft). Therefore, at a location where the shoreline retreat rate is at
least five times as great as that at Ocean City, the dynamic equilibrium
assumption appears applicable when shoreline retreat and sea level rise was
averaged for 70 years. At an interval of 30 years, it appeared the shoreface
shape varied from its longer-term average at Smith Island.
Because relatively good sea level data are available near Ocean City
starting about 1930 and extending to the present, and because shoreline change
data and surveyed profiles cover the same period, it seems reasonable to
select a shoreline average for 50 years (1930-1980) at Ocean City for this
investigation.
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FIGURE 3
RECENT SEA LEVEL CHANGES ALONG THE U.S. COAST,
BASED ON TIDAL GAUGE DATA
(from Hicks 1978)
TIME
TIME (y«o.«)
TIME (»«'«>
1910 1920 '930 1940 1950 960 I97O 1890 1900 1910 "920 '930 I9«0 I9M> I960 1970 I9ZO 1930 I94O 1950 I960 I»TO
Portland. M«.
Jf
Newport. R.I.
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Alongshore Bounds
To obtain as accurate a long-term average of the net gain or loss of sand
to the system as possible, it is necessary to select alongshore bounds on the
control volume where longshore sediment transport rates are known or can be
estimated over the period selected (50 years). In the study area these
alongshore boundaries are best located at Indian River Inlet (north boundary)
and Ocean City Inlet (south boundary). Sediment transport is out of the study
reach at both locations. The location of the divergent nodal reach is thought
to be between the villages of Fenwick Island and Bethany Beach.
DATA REQUIREMENTS
Table 1 contains the data required to solve Equations 1, 2, 3, and 5.
These data are an average for the 31,190-m long (19.4-mi long) shoreline reach
between Indian river Inlet and Ocean City Inlet for the 50-year period between
1930 and 1980. Most of the data elements are presented separately in the
following sections. All data were obtained from published and unpublished
sources; fieldwork was not a part of this investigation.
Profile Shape
Shoreface profile shape is a variable in Equation 5 when k £ 1.0, which
it is along the study reach. The average shoreface profile was obtained by
averaging, with equal weight and by eye, all the Ocean City profiles for 1979
(Fig. 4). These profiles were obtained by Corps of Engineers surveyors from
the Baltimore District (U.S. Army Corps of Engineers, 1980). A nearly-the-same
average profile was obtained independently by another person (Coyne Foster,
Technical Aide, Moffatt & Nichol, Engineers). His profile for 1979 was within
+l-ft vertical of that given in Figure 4. A similar weighted, average
profile was obtained from 1964 Corps of Engineers-Philadelphia District
profiles and also independently duplicated for the coastal reach of Delaware
from the Maryland border to Indian River Inlet. Depth-distance pairs obtained
from that profile were similar to those of the average 1979 Ocean City
profile. The 1979 average Ocean City profile is used in the Bruun and Everts
analyses.
Both Leatherman (Chapter 2, this report), who supplied the Ocean City
Profiles, and J. Gebert of the Philadelphia District, Corps of Engineers, who
supplied the Delaware profiles, noted that shoreface appeared to be steepening
when profiles of different years were compared. Because of the concern that
the shoreface was steepening, which would negate the dynamic equilibrium
shoreface assumption inherent in using Equation 5, a comparison of profiles
was made to determine whether that assumption was valid for this coastal
reach. All Ocean City profiles for the years 1929, 1965, 1978 and 1979, and
Delaware profiles for 1954 and 1964 were averaged as previously discussed.
The Ocean City profiles are shown in Figure 4. A later set of profiles is
available for Delaware, but was not included in the analysis because it was
provided at a different scale, and time/cost considerations prohibited its
use. The following conclusions were drawn from comparisons of the average
profiles from different survey years:
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TABLE 1. VALUES USED IN CALCULATIONS
1. Time Interval: 50 years (1930-1980)
2. Alongshore Boundaries: south jetty, Indian River Inlet, to north jetty,
Ocean City Inlet (about 31,190 m or 102,300 ft)
3. Base of Shoreface: a) depth below MSL*: z = -8.5 m (-28 ft)
b) distance from MSL shoreline: t, (Fig. 1) =
XQ (Fig. 2) = 700 m (2200 ft)
4. Crest of Foredune: z = z = +4 m (13 ft) above MSL
em '
5. Vertical Profile Dimension: h (Fig. 1) = 12.5 m (41 ft)
6. Relative Sea Level Rise: a = 0.0036 m/yr (0.0118 ft/yr)
7. Portion of Sand Landward of Shoreface: k = 0.75
8. Net loss in Sand Volume in Control Volume: V = 10.6 x 10s m3 (14.0
° x 10s yd3)
a. Longshore Losses: V = 10.2 x 10s m3 (13.4 x 10s yd3)
b. Losses at Base of Shoreface: V = 1.05 x 10s m3 (1.4 x 10s m3)
fl.
c. Losses Resulting From Overwash Transport:
Vw = 0.37 x 10s m3 (0.48 x 10s yd3)
d. Losses Resulting from Aeolian Transport: V =0
3.
e. Losses Through Non-Bounding Inlets: V. = 0
f. Losses from Sand Mining: V =0
m
g. Gains from Beach Replenishment: Vf 10s m3 (1.4 x 10s yd3)
9. Average Shoreline Change: s = -53.5 m (-176 ft)
* MSL = mean sea level.
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FIGURE 4
AVERAGE SHOREFACE PROFILES FOR THE SURVEY
YEARS 1929, 1965, 1978 and 1979
Obtained by Averaging All Profiles Available for a
Given Year Along the Ocean City, Maryland, Coast
(Ocean City inlet to Maryland/Delaware state line)
UI
O ui
> ui
UI CO
uJO
ui r~
28
ui S-
CO I
i
400
1200 1600 2000 2400 2800
DISTANCE FROM GROUND CONTROL POINT
3200 3600
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(1) Because profiles from both north and south of the states line were
observed to "steepen," and because those profiles were obtained independently,
the apparent steepening (explained below) is considered to have occurred and
to not be an artifact of the survey programs.
(2) All profiles at each reach superimpose very well when they are
shifted in a horizontal direction, except near the MSL shoreline. For
example, the 1929 Ocean City profile, when shifted about 220 ft landward, lies
atop the 1965 and 1978 Ocean City profiles from the base of the shoreface to a
depth of -7 ft (MSL). Above -7 ft, the more recent profiles steepen.
(3) It is difficult to explain the large horizontal distance between the
1978 and 1979 profiles. While the 1979 profile is up to several feet lower
(vertical distance) and landward (horizontal distance) than the 1978 profile,
when shape is compared by moving the 1978 profile landward, the profiles are
found to be similar. An apparent survey problem may exist here.
(4) The lack of landward retreat of the upper part of the profile while
the rest of the profile retreated is not explained using the survey data and
is not accounted for volumetrically in shoreline change analyses. A possible
explanation for the apparent shoreline stability in recent years is: (a)
groins constructed at Ocean City have tended to hold the shoreline and
uppermost shoreface in a relatively fixed position; (b) sand scraping, in
which sand is moved landward from near MSL to create a dune to protect
near-beach structures, may also have the short-term effect of keeping sand on
the foreshore and behind it, while wave activity and sea level rise continue
to move sand and maintain an equilibrium shoreface shape in the nearshore and
offshore zone, and (c) beach nourishment in 1961 and 1962 may also have had
the effect of maintaining the upper beach. All three situations result in
human manipulation of the upper beach while the major portion of the shoreface
continues to retreat. The profile shapes shown in Figure 4, using this
not-too-conclusive evidence, are considered to remain dynamically constant as
the profile retreats.
When the results of this analysis are compared to shoreline retreat rates
after 1961, the volume change is accounted for, but the shoreline retreat is
not evident because the shoreline was (apparently) artificially held. When
long-term changes are considered the methodology should correctly predict sand
volume changes. Artificially holding the shoreline without the addition of
beachfill from outside the system cannot continue indefinitely. The shoreface
cannot continue to retreat (and, in essence, steepen) much longer at Ocean
City without shoreline retreat.
Sea Level Rise Rate 1930-1980
The relative sea level rise rate is herein defined as the rate of change
(slope) in the yearly mean sea surface elevation for a period of 50 years
(1930-1980). Sea level changes relative to land at any location are a
function of global changes in the volume (sea surface contraction and
expansion and changes in the geometry of the ocean basins) and mass (polar ice
contributions and withdrawals) of the oceans. On a regional scale the sea
surface may also rise or fall over a relatively long period as the local
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freshwater contribution varies and regional climatic conditions vary, i.e.,
atmospheric pressure, wind-shear stress on the sea surface, and temperature.
In addition, the land may rise or fall relative to the sea surface by tectonic
deformations, compaction, and other factors. Holdahl and Morrison (1974) show
the rate of elevation change (land subsidence) at Lewes, Delaware, to be about
3 mm/yr and 2.4 mm/yr at Hampton Roads, Virginia, between 1920 and 1942 and
1970 and 1971. An analysis of tidal gauge records indicates that the total
relative rate of sea level rise, a, was 0.012 ft/yr (0.0036 m/yr) when
differential subsidence is considered near Ocean City.
Sediment Size Distribution Landward of Shoreface
The Maryland Geological Survey and Professor J. Kraft of the University of
Delaware have obtained borings of the region behind the shoreface.
The sand and the silt/clay regions of Figure 5 were integrated above the
base of the shoreface (-8.5 m, or -28 ft, MSL) to determine the portion of
sand behind the shoreface. This information was supplemented by boring data
collected and described by others. Based on an examination of well borings,
Rasmussen and Slaughter (1955) identified the Pleistocene deposits landward of
the Ocean City shoreface as mud, sand, and "marsh" material, with mud usually
the dominant constituent. Near the Maryland-Delaware line, Weigle and Achmad
(1982) showed the shoreface was backed by sand with minor clay amounts. The
portion of sand in the sand zone of Figure 5 was assumed to be 1.0 of the
total sediment (100%). The sand portion of the silt-clay zone of Figure 5 was
assumed to be 0.1. Figure 6 shows the estimate of the total portion of sand,
by depth, behind the Ocean City shoreface. The total portion of sand behind
this shoreface (above -8.5 m, MSL) is k = 0.75. This value is an
approximation and could be a significant source of error in the shoreline
change test (but will not be a problem in calculating beach nourishment
requirements because k = 1.0).
Perlin et al. (1983) reference Kraft's work which shows that the portion
of sand behind the Delaware shoreface may be slightly lower than that behind
the Ocean City shoreface. Kraft describes the Pleistocene Headlands, which
tend in a northeasterly direction and crop out on the shoreface at Bethany
Beach and North Bethany Beach, as composed of mud, and mud and sand. He also
notes there is a maximum 20 cm (0.7 ft) of sand on the lower and middle
shoreface in this region. This is the "active" surface on the retreating
shoreface. The "active" zone is indicative of the retreat of the shoreface.
Because more substantive information on sand composition is unavailable, the
portion of sand behind the Delaware shoreface is taken to be the same as that
farther south, i.e., k = 0.75.
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FIGURE 5
SEDIMENT SIZE BENEATH THE BARRIER ISLAND AND
LANDWARD OF THE SHOREFACE--AT OCEAN CITY, MARYLAND
STRATIGRAPHIC CROSS-SECTION
ALONG OCEAN CITY, MARYLAND
KEY: . m
............ Surface
CORES:
Mean Sea Level
Sand/Clay
Interface
Sand
(. 1 silt/Clay
I Mixture or
Blank
Unknown
100
Source: Maryland Geological Survey
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FIGURE 6
PORTION OF SAND BEHIND THE SHOREFACE AND ABOVE
-8.5 m (-28 ft) MEAN SEA LEVEL AT OCEAN CITY, MARYLAND
*
.75-
0 10 20 BO
DEPTH BELOW MEAN MA LEVEL (HIT)
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Net Sand Transport Into and Out of Control Volume
Sand transport rates into and out of the control volume are addressed in
this section. The control volume is bounded by the base of the shoreface
(about -8.5 m, or -28 ft, MSL) and the crest of the foredune, and in an
alongshore direction by Ocean City Inlet and Indian River Inlet. Vo in the
time At (1930-1980) is
V0 = Vn + Vc + Vf + Vm + Vw + Va + Vj (7)
in which Vn = net sand volume contributed to or lost from the control volume
by alongshore sand transport, Vc = net sand volume change as the result of
shore-normal transport at the base of the shoreface, Vw= net volume change
behind the foredune as a result of overwash transport, Va = net volume change
behind the foredune as a result of aeolean transport, Vj = net volume change
landward of the foredune as a result of inlet(s) that opened in the control
volume between 1930-1980, Vf = gain as a result of the addition of fill and
from outside the control volume, and Vm = loss as a result of sand mining.
All values used are given in Table 1.
1. Net Loss By Longshore Transport (Vn). Sand transport in an
alongshore direction accounts for the main absolute loss of sand from the
control volume. The net longshore sediment transport rates at Ocean City
Inlet and Indian River Inlet are available from a number of sources using
various lines of evidence.
At Ocean City Inlet, Fulford (personal communication, 1984) said the Corps
of Engineers (Baltimore District) assumes a net longshore transport rate since
1933 (inlet opening) of 1.2 x 10s m3/yr, south (1.6 x 10s yd3/yr,
south). It is reasonable to believe it was also that value prior to inlet
opening in 1933. Dean et al. (1978) using impoundment volumes from north of
the inlet, a sediment budget for the inlet area, and the growth rate of the
ebb-tidal shoal, concluded the net longshore transport rate is probably 0.38 x
105 to 1.14 x 105 m3/yr, south (0.5 to 1.5 x 10s yd3/yr, south).
Everts (1983) in a discussion of shoreline changes south of the inlet
concluded that the rate 14.8 km south of an assumed nodal location at South
Bethany Beach was 1.2 x 105 m3/yr (1.6 x 10s yd3/yr), but that volume
probably includes some material that moved north into the inlet. Based on no
more than an arbitrary selection--near the average of the above estimates--
the average net longshore transport rate at Ocean City Inlet is taken as 1.2 x
10s m3/yr, south (1.6 x 10s yd3/yr, south) or 6 x 10s m3 (7.9 x
10s yd3) between 1930 and 1980.
At Indian River Inlet, Perlin et al. (1983) estimated that the ebb-tidal
shoal is accumulating sand at 0.66 x 10s m3/yr (0.87 x 105 yd3/yr);
the flood-tidal shoal is in equilibrium with the 18-yr average dredging of
0.57 x 10s m3/yr (0.75 x 105 yd3/yr); and the net longshore transport
at the inlet is 1.21 x 10s m3/yr, north (1.6 x 105 yd3/yr, north).
This volume is higher than the 0.84 x 105 m3/yr, north (1.1 x 10s
yd3/yr, north) that the Philadelphia District (Gebert, personal
communication, 1984) estimates is required to stabilize the beaches north of
the inlet. The dynamic nodal position is probably near South Bethany Beach,
-------
-84-
which is closer to Indian River Inlet than to Ocean City Inlet, and if an
equal increase both north and south of the node is assumed, the net longshore
sediment transport rate at Indian River Inlet would be less than that at Ocean
City Inlet. Accordingly, a net longshore transport rate of 0.84 x 10s
m3/yr, north (1.1 x 105 yd3/yr, north) or 4.2 x 105 m3 (5.5 x 10s
yd3) between 1930 and 1980 is used in this investigation.
2. Net Loss at Base of Shoreface. No information exists on net sand
transport across the base of the shoreface. Perlin et al. (1983) show the
Continental Shelf seaward of the shoreface is surfaced partially by
Pleistocene deposits that could not have come from the shoreface, suggesting
negligible transport or possibly onshore transport in those areas (patches).
Bathymetry of other regions on the shoreface and inner-Continental Shelf,
however, is dominated by shoals which trend north-northeast. Some of these
shoals are connected to the shoreface and others, still in line with those at
the shoreface, lie isolated on the Shelf surface. The shoals are composed of
sand which may have come from the shoreface. Along the ocean coast of North
Carolina and Virginia, Everts et al. (1983) found that similar shore-connected
ridges significantly influenced the shoreline. The shoreline north of the
intersection of predominant ridges with the shoreface retreated at an
accelerated rate compared to the average for the entire shore reach;
shorelines south of the ridge intersections prograded or retreated at a lesser
rate than the average for the reach. Generally, the major ridge/shoreface
intersections (ridges which extended far out onto the inner-Continental Shelf)
were located about 5 km south of gentle, convex-outward shorelines, such as
that at Ocean City Inlet.
The only sand that moves across the base of the shoreface in the study
area is assumed to be sand lost to the shoreface-connected ridges. To obtain
the volume lost to those features, five shore-parallel profiles were drawn
between the inlets at 700 m (2250 ft), 1050 m (3450 ft), 1550 m (5100 ft),
2500 m (8200 ft), and 3475 m (11,400 ft) from the shoreline. The extension of
the shore-connected ridges above the -8.5 m (-28 ft) basal depth of the
shoreface is shown in Figure 7. Note the trend of the ridges to the northeast
as the distance from shore increases. The shaded area of the ridges at the
base of the shoreface (Profile 1, Fig. 7) was found to be 7.9 percent of the
total vertical shoreface area. When the volume of sand assumed naturally lost
from the shoreface to nourish the ridges is calculated as this percentage of
the total shoreface depth (-8.5 m; -28 ft) times length (31,190 mi; 102,300
ft) times retreat rate (about 1 m/yr, or -3.4 ft/yr as developed later), the
net yearly loss is 21,000 m3/yr (27,500 yd3/yr) or 10s m3 (1.4 x 106
yd3) between 1930 and 1980. This is about one-tenth the volume lost by
longshore transport out of the system.
3. Net Loss by Overwash Transport. The only storm in which large
quantities of sand were moved landward of the foredune occurred on 7 March
1962. The volume moved is unknown, and most of the sand was subsequently
returned to the beach in developed areas. In undeveloped areas, much of the
overwash deposit was also probably naturally returned by aeolian processes as
Leatherman found occurred on Assateague Island. Assuming that a washover
deposit 300-m deep (1000 ft) and 0.3-m thick (1 ft) formed along 40 percent of
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-85-
FIGURE 7
SHORE-PARALLEL PROFILES OF SHORE-CONNECTED RIDGES
ABOVE THE ELEVATION OF THE BASE OF THE SHOREFACE
(-8.5 m, -28 ft, MSL) BETWEEN OCEAN CITY INLET AND
INDIAN RIVER INLET
PROFILE
PROFILE
PROFILE
PROFILE
PROFILE
0
-10
1 -20
-30
2 -20
-30
3 -20
-30
4 -20
_<»o
5 -20
-30
-40
-50
OCEAN
^ " Jill f\
IV^ ~ /J>^/A * jfiftAM Ji\
el A *y , LA. p\
\- \ \
^\A /\\ fli, A d/\*i
x \ \ \
VA
Al IA \ x
\ A \
II . 1
CITY ikiniAM D
-------
-86-
the shoreline during the 1962 storm, about 1.1 x 10s m3 of sand would have
moved away from the beach. Next, assuming two-thirds of that sand was
naturally or artificially returned to the beach, 3.7 x 10s m3 (4.9 x 105
yd3) would have been "permanently" lost from the control volume.
4. Net Changes Due to Aeolian Transport. Wind-transported sand losses
or gains are assumed to be negligible.
5. Losses Through Non-Bounding Inlets. Between 1930 and 1980 no new
inlets were opened in the control volume so no losses occurred by this
mechanism.
6. Losses by Sand Mining. Apparently the only artificial transport of
sand on the beaches was "sand scraping" that occurred in the 1970Ts and
1980's. This moved sand from the lower foreshore to create or improve the
foredune. There was no net loss to the control volume.
7. Gains from Beach Replenishment. Gains from this source were 0.87 x
10s m3 (1.15 x 10s yd3) from the sound to Ocean City beaches in 1962 and 7.6
x 10* m3 (105 yd3) to Bethany Beach in 1961 and 5 x 104 m3 (7 x 10*
yd3) to Bethany Beach in 1962 and 5 x 10* m3 (7 x 10* yd3) to South
Bethany Beach in 1962. The total quantity of sand from outside sources that
entered the control volume between 1930 and 1980 appears to be 10s m3 (1.4
x 10s yd3), all of which was placed after 1960.
Shoreline Changes (1930-1980)
Shoreline change maps constructed by the National Ocean Services (NOAA)
were used to obtain the average shoreline change rate for the reach between
the inlets and over the 51-yr time period 1929 to 1980 (Fig. 8). This rate
was calculated using 101 equally spaced measurement transects (6 per minute of
latitude on the NOAA maps). The average rate was -0.644 m/yr (-2.1 ft/yr).
The rate north and south of the Maryland-Delaware state's line was similar
(-0.60 m/yr, south reach; -0.68 m/yr, north reach). Leatherman (Chapter 2)
calculated an average rate of -0.58 m/yr (-1.9 ft/yr) for the 1850-1980 period
for the Ocean City reach. In an Environmental Impact Statement, the Corps of
Engineers (1980) stated the historic retreat rate was -0.8 m/yr (-2.3 ft/yr).
Strikingly, the shoreline change rate using the NOAA maps for the period
1961/62 to 1980 was -0.19 m/yr (-0.6 ft/yr) south of the states line and +0.28
m/yr (+0.9 ft/yr) north of the states line. The average for the entire reach
in the 1961/62 to 1980 period was a progradation of +0.08 m/yr (+0.25 ft/yr).
This occurred during a period when sea level rose relative to land at all
nearby sites and when the shore of Assateague Island south of Ocean City Inlet
retreated at a rate equal to the rate it had retreated since the inlet jetties
were constructed in 1933. The 1961/62-1980 period is a time when the
shoreline, as observed by comparing profiles (Fig. 4), was held in place, but
the rest of the shoreface retreated.
Shoreline changes after 1961 do not reflect the general retreat of the
shoreface (Fig. 4) but are controlled by: (1) groin construction at Ocean
City and Bethany Beach, (2) beach replenishment in the early 1960'F "Mch
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-87-
FIGURE 8
SHORELINE CHANGE RATES FOR THE PERIOD 1929-1980
(from NOAA Shoreline change maps, used with permission)
The 1961-1980 average rate is +0.08 m/yr while
the shown 1929-1980 rate is -1.05 m/yr.
HJDlAN
•^•tFl^i^
*IVIR
BAt
BAY
WtftMT
BAY
OMAN
-------
-88-
built up the beach and very shallow portions of the shoreface and was
subsequently held by the groins, and (3) sand scraping (movement of sand from
the foreshore to create an artificial foredune farther landward). All
contributed to holding the uppermost part of the shoreface while the lower
shoreface retreated. The shoreline retreat rate, if the shore was not
modified in these ways, would be greater than -0.64 m/yr (-2.1 ft/yr). The
retreat between 1929 and 1980 (average = -32.8 m) shown on the NOAA maps
occurred between 1929 and 1961/62 (which was measured on the maps), and if we
assume the pre-1961/62 retreat rate would have continued to the present if the
upper shoreface had not been "stabilized," the actual retreat rate would be
-1.05 m/yr (-3.4 ft/yr).
CALCULATIONS
Past Shoreline Changes
Calculations using Bruun's (1962, 1983) method and Everts' (1984) method
with data from Table 1, are given in Table 2. The effect of sea level rise
produced 19 percent of the shoreline retreat (Everts* method) that would have
occurred without human intervention at Ocean City between 1930 and 1980. Sand
volume losses from the system accounted for 81 percent of the calculated
retreat. If the shoreface had been backed with sediment composed entirely of
sand (k = 1.0), the calculated shore retreat would have been 19 percent less.
Bruun's (1962, 1983) methods result in a substantial underprediction of the
shore-retreat rate. Everts' (1984) method predicts a rate that is slightly
higher.
Future Shoreline Changes
A two-step approach was used to provide the requested future shoreline
change rates and beachfill requirements. Using Everts' (1984) method, the
average calculated shoreline change rate (-1.12 m/yr or -3.7 ft/yr) was
checked against the measured rate (-1.03 m/yr or -3.4 ft/yr). The input data
were then adjusted so the past calculated rate was equal to the measured
rate. This adjustment was then made in the independent variable most likely
to have been incorrectly determined. Figure 9 shows the variation in
shoreline change rates as the rate of sea level rise (a) the rate at which net
sand volume increases or decreases (Vo), and the portion of land behind
shoreface (k) are varied individually as others are kept constant at estimated
1930-1980 values (open circles). The shoreline change value varies greatly
with small changes in k, suggesting k may be in error. A k adjustment to k =
0.82 yields a calculated value equal to the "measured" value (-1.03 m/yr).
The value of "a" used in the calculation (Table 1) is probably quite good
(Fig. 3). Likewise, values of the components of Vo (Table 1) are probably
less questionable than the estimated value of k originally used.
Assuming no beachfill (Vf in Table 1=0) and no overwash transport losses
because of anticipated continuing artificial foredune creation and future
development (Vwin Table 1=0), and assuming all other components of Vo will
remain the same as given in Table 1, the net future loss rate of sand along
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TABLE 2. CALCULATED SHORE RETREAT FOR
OCEAN CITY, MARYLAND, 1930-1980
Method
Equat ion
Predicted
Shoreline
Change, Meters
(Feet)
Erosion Caused
By Sea Level
Rise
Percent1
Percent of
Measured Change2
Bruun (1962)
Bruun (1983)
Everts (1984)3
Measured*
1
2
3
5
10 (33)
12 (39)
13 (43)
56 (184)
53.5 (175)
100
100
91
19
18
23
24
105
100
Volume change caused by sand transport into and out of control volume,
equal to R in Equation 3 and Vo in Equations 4 and 5.
The percent given is the calculated change divided by the measured
change times 100.
Shore retreat would have been 19 percent less if eroded shoreface
sediment had been all sand.
Shoreline changes between 1961 and 1980 were greatly affected by groin
construction, beach nourishment, and possibly sand scraping. While the
shoreline was artifically "stabilized," sand volume losses and the effect
of sea level probably continued at the 1930-1961 rate during this period.
This is an important consideration because a shoreline change rate based
on sand volume changes is the essence of the Bruun (1983) and Everts
(1984) methods. The -32.7 m (-107 ft) obtained from shoreline change maps
for 1930-1961 is assumed, for comparison purposes, to have continued in
1980. Ocean City is not an ideal location to test and evaluate predictive
methods because of the large influence of man on the shoreline since about
1961.
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FIGURE 9
VARIATIONS IN SHORELINE CHANGE RATE
(Calculated Using Eq. 5)
WITH SEA LEVEL RISE, NET VOLUME CHANGE IN
CONTROL VOLUME, AND PERCENT SAND IN
SEDIMENTS BEHIND SHOREFACE
Open Circles Reference Data Used in 1930-1980 Analyses
VOLUME CHAHOE
CO 830 0 .1
"
k
4 .6
,5 1.0
0-
-10-
-10-
•90-
-------
-91-
the Ocean City reach, Vo, will be taken as -263,000 m3/yr (-347,000
yd3/yr) or -2.6 m3/m-yr (-3.4 yd3/ft-yr).
Table 3 is a list of two sea level-rise scenarios provided by the
Environmental Protection Agency (EPA). Table 4 shows the calculated (Eq. 5)
cumulative shoreline retreat that is projected for each scenario. These
projected shoreline retreats are averages for the entire shoreline reach and
do not reflect alongshore variations that would surely occur (and have
occurred in the past as shown in Figure 8). Quite likely, the shoreline
retreat would be less than that shown in Table 4 because the jetties would
continue to (and increasingly) act as headlands. The result would be that a
shallow embayment would form in which net alongshore sediment transport losses
(Table 1) would decline at the boundary inlets.
Future Beachfill Requirements
If the shoreline is to be held at its 1984 position, beachfill must be
added to counter the -8.4 m3/m-yr or 18,000 yd3/mi-yr) lost from the Ocean
City beach, plus the increasing effective loss produced by a shift in the
shoreface as the sea level rise accelerates. The k-value of the fill material
is 1.0, so using the VQ at the zero shoreline change rate shown in Figure 9 is
too large (by 19 percent in 1980). Using Equation 5, a continuing net loss by
longshore and cross-shore transport of -2.6 m3/m-yr, (-3.4 yd3/ft-yr), k =
1.0, and the sea level rise scenarios provided by the EPA (Table 3) for Ocean
City from the north jetty of Ocean City Inlet north to the Maryland state
line (14,220 m, 46,650 ft), the calculated future fill requirements are as
given in Table 5. Bruun's (1983) Equation 3 was used with offshore losses
equal R = 12,800 yd3/yr and r = 1.0. Table 6 shows the percent of the
required beachfill material that must be added because of the rise in sea
level and that which must be added because of longshore and cross-shore sand
transport losses. Sand volume losses resulting from sea level rise will
usually require that beachfill be added from outside the control volume.
Losses by longshore and cross-shore (at base of shoreface) transport may
possibly be mitigated using proper structures that slow or halt the transport
of sand, thereby reducing the volume of required beachfill.
SUMMARY
The effect on shoreline position of sea level rise and changes in sand
volume on the shoreface and barrier island of Ocean City, Maryland, are
addressed in this paper. Results are given in a series of Tables and in
Figure 10. Table 1 shows the data used in calculating shoreline change for
the period 1930-1980. Table 2 provides a comparison of the Bruun (1962, 1983)
and Everts prediction of shoreline retreat for that period to the measured
retreat. EPA-provided sea level rise scenarios given in Table 3, are used to
forecast the shoreline changes to 2075 illustrated in Table 4. Table 5 shows
projected beachfill requirements to 2075 using the Table 5 sea level rise
scenarios, and Table 6 shows the percent of the total beachfill requirement
that can be attributed to sea level rise.
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TABLE 3. RELATIVE SEA LEVEL RISE SCENARIOS1
1. ABSOLUTE RISE OVER 1980 LEVEL (Centimeters [feet])
Year Current Trend Mid-Range Low Rise Mid-Range High Rise
20002 7 (0.24) 12.4 (0.41) 16.8 (0.55)
20252 16 (0.53) 34.3 (1.12) 47.4 (1.55)
20502 25 (0.83) 65.2 (2.14) 91.5 (3.00)
20752 34 (1.13) 108.3 (3.55) 153.9 (5.05)
2. RATE OF SEA-LEVEL RISE (Centimeters/year [feet/year])
Mid-Range Low Mid-Range High
1980-20002 0.62 (0.020) 0.84 (0.028)
2000-20252 0.88 (0.029) 1.22 (0.040)
2025-20502 1.24 (0.041) 1.76 (0.058)
2050-20752 1.72 (0.056) 2.50 (0.082)
1 Sea level rose 18 cm between 1930 and 1980 (0.36 cra/yr). Rate obtained
using data from nearby tidal gauge records (Hicks, Debaugh, and Hickman
1983) and interpolated using regional crystal deformation data (Holdahl
and Morrison, 1974).
2 EPA estimates (Hoffman, Keyes, and Titus 1983) which illustrate
cumulative rise and which include a 1.8 mm/yr local subsidence rate as per
Holdahl and Morrison (1974), with 1980 as base year.
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TABLE 4. SHORELINE RETREAT SCENARIOS FOR
OCEAN CITY, MARYLAND1
YEAR
SHORELINE RETREAT RATE (feet/year)
CURRENT TREND
MID-RANGE LOW RISE
MID-RANGE HIGH RISE
1980-2000
2000-2025
2025-2050
2050-2075
Everts
Eq. 5
3.4
3.4
3.4
3.4
Bruun
Eq. 3
0.81
0.81
0.81
0.81
Everts
Eq. 5
4.2
6.2
7.8
10.9
Bruun
Eq. 3
1.10
1.98
2.74
3.83
Everts
Eq. 5
4.8
7.1
9.5
14.7
Bruun
Eq. 3
1.9
2.7
4.0
5.6
YEAR
1980
2000
2025
2050
2075
CUMULATIVE SHORELINE RETREAT (feet)
CURRENT TREND
MID-RANGE LOW RISE
MID-RANGE HIGH RISE
Everts
Eq. 5
0
68
153
238
323
Bruun
Eq. 3
0
16
36
57
77
Everts
Eq. 5
0
84
238
434
707
Bruun
Eq. 3
0
22
72
140
236
Everts
Eq. 5
0
95
273
511
878
Bruun
Eq. 3
0
38
106
206
346
Using EPA sea level rise scenarios (Table 3); assumes no shoreline
manipulation or "hardening" such as has occurred since about 1960, no
beach nourishment, and no overwash losses.
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FIGURE 10
ESTIMATED FUTURE SHORELINE RETREAT AND BEACH
NOURISHMENT REQUIREMENTS AT OCEAN CITY, MARYLAND,
FOR TWO EPA SEA LEVEL RISE SCENARIOS
500
BEACH NOURISHMENT 40° '
REQUIREMENT
(1000s OF CUBIC
YARDS/YEAR)
300-
1980 2000 2020 2040 2060 2080
YEAR
1000
CUMULATIVE 800
SHORELINE RETREAT
(FEET) 600
400
200
1980 2000 2020 2040
YEAR
EVERTS
EVERTS
BRUUN
r CURRENT
-—TREND
I 1
2060 2080
0 08 -
RATE OF SEA 0.06 -
LEVEL RISE
(FEET/YEAR) 0.04 -|
0.02 -
0
1980
EPA MID-RANGE
HIGH ESTIMATE
EPA MID-RANGE
^ ,<•* LOW ESTIMATE
If.''. .QURRENT.IBENfi
I
2000
2020 2040
YEAR
2060
I
2080
Line representations for Everts1 (1984) and Bruun's (1983) models are given
for shoreline retreat and beachfill requirements. The curves are keyed to sea
level scenarios at the bottom of the figure.
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TABLE 5. CALCULATED BEACHFILL REQUIREMENTS
FOR OCEAN CITY, MARYLAND1
YEAR FILL REQUIREMENT (yd*/yr)
CURRENT TREND MID-RANGE LOW RISE MID-RANGE HIGH RISE
Everts Bruun Everts Bruun Everts
Eg. 5 Eg. 3 Eg. 5 Eg. 3 Eg. 5
1980-2000 203,000 48,000 230,000 73,600 260,000 119,000
2000-2025 203,000 48,000 270,000 123,000 310,000 165,000
2025-2050 203,000 48,000 310,000 165,000 380,000 233,000
2050-2075 203,000 48,000 370,000 226,000 470,000 324,000
Using EPA sea level rise scenarios (Table 3) and Equation 5; assumes
size distribution of fill material is equal to size distribution of native
beach sand.
TABLE 6. PERCENT OF BEACHFILL REQUIREMENT ATTRIBUTED
TO SEA LEVEL RISE AT OCEAN CITY1
YEAR CURRENT TREND MID-RANGE LOW RISE MID-RANGE HIGH RISE
Everts Bruun Everts Bruun Everts
Eq. 5 Eq. 3 Eq. 5 Eq. 3 Eq. 5
1980-2000 22 74 31 83 39 89
2000-2025 22 74 40 90 49 92
2025-2050 22 74 49 92 58 95
2050-2075 22 74 57 94 66 96
Remainder is produced by sand losses from the control volume.
-------
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REFERENCES
Bruun, Per, 1962. "Sea Level Rise as a Cause of Erosion," Journal of tht
Waterways and Harbors Division (February), American Society of Civil
Engineers (ASCE) 1:116-130.
Bruun, Per, 1983. "Review of Conditions for Uses of the Bruun Rules of
Erosion," Coastal Engineering, 7:77-89.
Dean, Robert G., Perlin, Marc, and Daly, Bill, 1978. A Coastal Engineering
Study of Shoaling in Ocean City Inlet, Report by Dept. of Civil
Engineering, Univ. of Delaware for Baltimore District, Corps of Engineers,
March 1978, 135 pp.
Everts, Craig H., 1983. "Shoreline changes Downdrift of a Littoral Barrier."
In Proc. of Coastal Structures *83. New York: Amer. Soc. Civil
Engineers, pp. 673-689.
Everts, Craig H., 1985. "Sea Level Rise Effects on Shoreline Position,"
Journal of Waterways, Port, Coastal and Ocean Engineering, ASCE.
Everts, Craig H., and Czerniak, Martin C., 1977. "Spatial and Temporal
Changes in New Jersey Beaches." In Proc. of Coastal Sediments '77. New
York: Amer. Soc. Civil Engineers, pp. 444-459.
Everts, Craig H., Battley, Jeter P., and Gibson, Peter N., 1983. "Shoreline
Movements Cape Harry, Virginia, B. Cape Hatteras, North Carolina, 1949-
1980 Waterways Experiment Station Technical Report CERC -83-1, 111 pp.
Hicks, Steacy D., Debaugh, Henry A. Jr., and Hickman, Leonard H., Jr., 1983.
Sea Level Variations for United States 1855-1980. Rockville, Md.: U.S.
Dept. of Commerce, NOAA-NOS, 170 pp.
Hoffman, John S., Keyes, Dale, and Titus, James G., 1983. Projecting Future
Sea Level Rise: Methodology, Estimates to the Year 2100 and Research
Needs. Washington, D.C.: Government Printing Office, 121 pp.
Holdahl, Sandford R., and Morrison, Nancy L., 1974. "Regional Investigations
of Vertical Crustal Movements in the U.S. Using Precise Relevelings and
Mareograph Data," Tectonophysics 23:373-390.
Perlin, Marc, Chen, Benjamin Y.H., Dalrymple, Robert A., Dean, Robert G., and
Kraft, John C., 1983. Sediment Budget and Sand Bypassing System
Parameters for Delaware's Atlantic Coast, Report prepared for Delaware
Department of Natural Resources, November 1983.
Rasmussen, W.C., and T.H. Slaughter, 1955. The Water Resources of Somerset,
Wicomico, and Worcester Counties, Bulletin 16. Dept. of Geology, Mines,
and Water Resources, 522 pp.
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-97-
REFERENCES (continued)
U.S. Army Corps of Engineers, Baltimore District, 1980. Atlantic Coast of
Maryland and Assateague Island, Virginia: Feasibility Report and Final
Environmental Impact Statement. Baltimore: Corps of Engineers.
Weigle, James M., and Achmad, Grufron, 1982. Geohydrology of the Fresh-Water
Aquifer System in the Vicinity of Ocean City, Maryland, with a Section on
Simulated Water Level Changes, Report of Investigations No. 37, Maryland
Department of Natural Resources, 55 pp.
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CHAPTER 4
ESTIMATES OF EROSION AND MITIGATION
REQUIREMENTS UNDER VARIOUS SCENARIOS OF SEA
LEVEL RISE AND STORM FREQUENCY
FOR OCEAN CITY, MARYLAND
by
David L. Kriebel
Robert G. Dean
Coastal and Oceanographic Engineering Department
336 Weil Hall
University of Florida
Gainesville, Florida 32611
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-100-
ABSTRACT
A numerical erosion model is applied to representative beach profiles for
Ocean City, Maryland, to estimate: 1) the expected erosion impact of severe
storms with 10- to 500-year return periods, 2) the potential erosion impact of
sea level rise to the year 2075, and 3) the mitigation requirements, that is,
volume of beach fill, needed to maintain the current shoreline position based
on predicted erosion due to storms and/or sea level rise.
Storm erosion effects are estimated for the range of possible severe
storms with peak storm surge levels associated with 10- to 500-year storms and
storm durations associated with a typical hurricane, a typical winter storm,
and the longest storm on record--the March 1962 storm. Based on a
representative beach profile for Ocean City, erosion estimates range from 500
to 1400 ft3/ft with probable dune recession of 50 to 100 feet and possibly
to 140 feet. Estimated erosion values seem reasonable when compared to
qualitative descriptions of the March 1962 storm. According to model
verification studies using Hurricane Eloise erosion field data, predictions
are considered average estimates with probable errors of +25 percent to
account for natural variations found under field conditions.
Interpretation of storm erosion estimates indicates a significant
probability for Ocean City dunes to erode quickly and to permit subsequent
overwash, storm flooding, and direct wave propagation into developed areas.
Mitigation requirements are developed to prevent dune breaching based on the
full range of storm conditions of interest. While mitigation requirements
vary according to the desired level of storm protection, required beachfill
volumes over the 8-mile Ocean City shore front range from 3,500,000 to
5,000,000 yd3 to provide protection for the 100-year peak storm surge of all
possible durations. In this respect the 3,500,000 yd3 of beachfill proposed
by the Corps of Engineers is found to be adequate for typical hurricanes but
may not be adequate for longer winter storm durations.
The average long-term erosion trends are estimated for three sea level
rise scenarios corresponding to a continuation of the existing trend and two
accelerated scenarios as suggested by the Environmental Protection Agency.
Hindcasts of erosion due to conditions between 1929 and 1961 provide a
confirmation that methods used to estimate erosion due to sea level rise are
valid. However, since 1961 the shoreline position has been stabilized and has
not eroded as historical trends or predictions would suggest. At the same
time, offshore regions have eroded at what appears to be the historical rate.
The effect of this is a steepening of the profile as the existing shoreline
position is maintained; this is a potentially unstable situation that could
lead to accelerated shoreline erosion to regain a more stable, that is, more
mildly sloping, profile form.
Under the three future sea level rise scenarios, average erosion rates of
3.3, 4.8, and 5.8 ft/yr are predicted under the assumption that the shoreline
will freely respond to a stable position. For comparison, the historical
shoreline erosion rate between 1929 and 1961 was 3.4 ft/yr. By the year 2075,
shoreline recessions of 315, 460, and 550 feet are predicted for the three sea
level rise scenarios. Total mitigation requirements to maintain the existing
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shoreline range from 19,000,000 yd3 to 40,000,000 yd* over the 8-mile
Ocean City shoreline for the 95-year period. Mitigation requirements to
maintain the existing level of storm protection are approximately the same;
these quantities should be increased by 3,500,000 to 5,000,000 yd3 to
provide the additional storm protection for the 100-year design storm.
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INTRODUCTION
Shoreline retreat and dune erosion magnitudes and probabilities are
estimated for Ocean City, Maryland, based on existing and estimated future
conditions regarding sea level rise and storm frequencies. These estimates
are intended to complement similar efforts, by other authors, to estimate the
erosion impacts of projected sea level rise scenarios to the year 2075. The
primary goal of this study, however, is to apply the Kriebel and Dean (1984,
1985) numerical storm erosion model to estimate the short-term erosion
potential of severe storms, and to estimate whether storm effects may
necessitate more direct and immediate erosion mitigation than the sea level
rise induced erosion. The numerical erosion model is then applied to estimate
the erosion potential of sea level rise scenarios. The final results indicate
the relative severity of the two erosion forcing scenarios, that is, sea level
rise versus severe storms, and indicate appropriate mitigation requirements to
maintain existing shoreline positions and current levels of storm protection.
Since the major goal of this study is to estimate the erosion potential of
severe storms, considerable effort has been made to thoroughly develop,
calibrate, and verify the numerical erosion model. Much of this report
provides documentation of this verification procedure, which is considered
essential if the most realistic and reliable estimates of the storm erosion
potential are to be obtained. In Section I, modifications of the numerical
erosion model are described; specifically, methods are included for estimating
beach slope changes and dune steepening during erosion. In Section II, the
modified erosion model is calibrated using Saville's (1957) prototype-scale
wave flume experiments. In Section III, a separate calibration is carried out
using a reference profile from the Hurricane Eloise data set of pre- and
post-storm beach profiles. This section also includes the independent
simulation of dune erosion on an additional 20 profiles from the Hurricane
Eloise data set, which provides a verification of the model and an estimate of
the confidence obtained from model predictions.
In Section IV, the erosion model is applied to estimate the effects of
severe storms on the existing Ocean City beach profile forms. The 10-, 40-,
100-, and 500-year return period storms are investigated explicitly from which
the effects of other storms may be interpolated. Since storm duration is an
important component in determining the storm erosion potential, results are
given for three storm durations, corresponding to a typical hurricane, a
typical northeast storm, and the longest storm duration expected, that is,
that associated with the March 1962 storm. Mitigation requirements for storm
protection are also developed in Section IV. In Section V, the erosion model
is applied to estimate the erosion potential of three sea level rise
scenarios, including net sand volume losses due to longshore sediment
transport. Finally, results are summarized to identify mitigation
requirements for long-term profile adjustment to sea level rise and net sand
volume losses outside of the active profile. Section VI presents a study
summary with final conclusions.
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SECTION I
DESCRIPTION OF BEACH-DUNE EROSION MODEL
Background
The numerical model for predicting beach and dune profile response to
severe storms is based on a theory for equilibrium beach profile development
presented by Dean (1977, 1984). Dean analyzed several mechanisms for the
formation of dynamic equilibrium beach profile forms and concluded that
observed equilibrium profile forms are consistent with the uniform dissipation
of wave energy per unit volume in the surf zone. Based on this argument,
theoretical profile forms may be found to be described by a monotonic curve as:
h = Ax2/3 (1)
where h is the water depth at some distance x seaward of the shoreline (x=0)
and A is a scaling parameter governing the steepness of the profile as in
Figure 1. The A parameter is also related theoretically to a unique value of
the wave energy dissipation per unit volume, D*, which exists at all points in
the profile when the system is in equilibrium. Based on a least-squares
analysis, Dean (1977) determined best-fit A (or D#) values for 502 beach
profiles from the U.S. Atlantic and Gulf coasts. Hughes (1978) and Moore
(1982) have analyzed several hundred additional beach profiles; from these
sources equilibrium A and D* values may be empirically related to mean
sediment size and sediment fall velocity as shown in Figure 2.
For numerical simulation of beach profile response, it is assumed that a
given beach profile will always respond toward its most stable or equilibrium
form relative to given water level or wave height conditions. During a severe
storm, the increased water level permits storm waves to break closer to shore
at first, therefore reducing the width of the surf zone and increasing the
energy dissipation per unit volume at all points in the surf zone as depicted
in Figure 1. The profile is then "out of equilibrium" since energy
dissipation per unit volume is now greater than the equilibrium value D#
throughout the surf zone. Based on the assumption that the profile will
evolve toward an equilibrium, i.e., h = Ax2'3, shape relative to the new
water level and wave height, the net result must be a widening of the surf
zone until the actual energy dissipation per unit volume is reduced to D*.
This can only be achieved by a net redistribution of sand over time, with
erosion of the beach/dune face and deposition pushing the breakpoint farther
off shore.
Based on these concepts, the net offshore sediment flux, Q, is
approximated according to the excess energy dissipation per unit volume at
each point in the surf zone as:
Q = K(D - D#) (2)
This form is similar to the sediment transport equation adopted by Swart
(1974) which approximated Q based on the difference between actual and
equilibrium geometric profile dimensions. In Equation (2), K is a
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-104-
FIGURE 1
EQUILIBRIUM BEACH PROFILE CONCEPTS FOR
NUMERICAL EROSION MODEL
I. EQUILIBRIUM BEACH PROFILE FORM
based on uniform energy dissipation per unit volume where A is related
empirically to sediment size and can be related to D^ by:
A = 24
2/3
5 -21/2
*K g
1 3F
II. ENERGY DISSIPATION PER UNIT VOLUME D = where F is
h 3x
Energy Flux per unit crest length
Storm Surge
D>D.
Erosion
D=D,
•--
;itinn -I
Deposition
III. SEDIMENT TRANSPORT EQUATION Q = K(D -
-------
D P p O
S g g 5
p
r\>
P
A(m^)
p
b
CJl
D
O
cn
P P
P ^
oo P
P
8
A(ft
1 I
m
o
O
2 P
00
m
Q.
cn
O
? £
3 •*
P
'
p
8
po
b
O) p p
P P P
m
O
c
N>
m
•o
Z30
0>
m
O
C
73
m
N>
m
w
§>
O
H
O
O
-n
o
Oi
i
FALL VELOCITY, w (cm/sec)
-------
-106-
proportionality factor that must be determined empirically based on a
comparison of numerical results to observed profile response.
As a final general requirement for modeling profile response, the total
sand volume in a beach profile must be conserved if it is assumed that there
are no longshore transport gradients. For numerical simulation, the equation
for continuity of sand in the onshore-offshore direction is
3x 3Q
— = - — (3)
at ah
which is cast into an implicit, space-centered finite difference form. By
numerically integrating this equation, together with the sediment transport
equation, the change in position, x, of discrete depth contours is determined
over each time step. In general since the actual energy dissipation D and
sediment flux Q vary with water depth and bottom slope, the rate of change of
each discrete contour differs from adjacent contours depending on water level,
wave height, and profile form at the beginning of the time step. By inputting
new water level and wave height conditions at each time step for the duration
of a storm, the time-dependent beach and dune response during the storm may be
estimated.
A description and verification of a numerical beach/dune erosion model
based on the energy dissipation per unit volume and equilibrium beach profile
concepts is presented by Kriebel (1982) and Kriebel and Dean (1984, 1985).
Numerical results are shown to agree qualitatively with observed response
characteristics of natural and laboratory beach profiles for a variety of
water level, wave height, beach slope, and sediment characteristics. A
preliminary quantitative verification of the model was carried out in a
simulation of the time-dependent beach and dune erosion associated with
Hurricane Eloise in Bay and Walton Counties in Florida. It was found that the
model reasonably predicted the magnitude of average storm-induced erosion as
the predicted volumetric erosion compared favorably with observed average
eroded volumes as given by Chiu (1977). Dune steepening during erosion was
not simulated, however, therefore predicted recession of specific evaluation
contours did not agree as closely with observed values.
This verification was considered preliminary since no effort was made to
simulate detailed response of individual pre-storm profiles or to compare
predicted erosion to actual post-storm profiles. Instead, erosion was
simulated using a single average pre-storm profile from the area of interest
and results were then compared to the average profile response characteristics
for the two-county area as given by Chiu (1977). After viewing 110 measured
pre- and post-storm profiles for Walton County, it is evident that there is
great variability in the response of individual profiles. In fact, over 20
profiles showed a net accretion between the two surveys in October 1973 and
October 1975 (post-storm profiles were measured 2-4 weeks after Eloise). The
possible inclusion of these profiles, and others which showed little erosion,
in the average erosion statistics may skew these figures such that published
average erosion characteristics are lower than the actual erosion experienced
by many of the profiles, in many cases by more than a factor of two. These
-------
-107-
differences may be attributed in part to three effects: 1) pre-storm profile
modification between October 1973 and September 1975, 2) localized longshore
transport gradients during the storm, perhaps causing local accumulation of
sand, and 3) beach recovery between the storm and the post-storm surveys.
Chiu (1977) suggests that an average of about 50 ft3/ft may have been
returned above the mean sea level (MSL) contour following the storm but prior
to the post-storm survey.
In order to obtain more realistic estimates of actual storm-induced
erosion, the original erosion model described by Kriebel (1982) and Kriebel
and Dean (1984, 1985) has been modified to include many effects not previously
represented. Specifically, provisions are made in the updated model to
include changing beach slopes and dune steepening to a near vertical slope.
These changes permit a more realistic post-storm profile such that recession
of individual elevation contours more closely agrees with nature. Along with
these changes, a more complete verification of the model is made with actual
pre- and post-storm beach profiles from the Hurricane Eloise data set. The
revised model is first calibrated based on a large-scale laboratory experiment
of Saville (1957) and is then recalibrated using a reference profile, line
R-41, from the Walton County data set. Profile R-41 is used as a calibration
standard since it was also used in two other dune erosion verification studies
by Hughes and Chiu (1981) and Vellinga (1983a). Finally, the calibrated model
is used to hindcast erosion for an additional 20 severely eroded profiles from
the Walton County data set in an effort to test model sensitivity and bias
introduced in the calibration process.
Modified Beach/Dune Erosion Model
Beach slope changes and dune steepening are introduced into the erosion
model based on geometrical arguments similar to those used by Bakker (1968),
Swart (1974), and Perlin and Dean (1983) in other simple numerical schemes for
simulating offshore sediment transport. In general, it is assumed that the
distribution of sediment transport on the active beach face is proportional to
the volumetric difference between the existing profile and an assumed maximum
potential erosion profile for the same region above the still water level.
This maximum potential erosion profile is based on known equilibrium slopes
and the vertical extent of the active profile. The method therefore requires
input of the equilibrium slopes of the active beach and dune face and
approximate runup elevations.
In the original numerical model by Kriebel and Dean (1984, 1985), the
sediment transport distribution on the active beach or dune face was
approximated by a straight-line extension of the Q curve from its calculated
value, Q#, just below the still water level, to zero at the upper limit of the
active profile which was taken as either the berm or the dune crest as in
Figure 3. This linear extension resulted in constant spatial gradients in the
Q curve, 3Q/3h, which then gave uniform retreat of the beach or dune face
based on the continuity equation:
3x 3Q
— = - — = constant
3t 3h
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-108-
Because this uniform retreat, initial beach and dune slopes were maintained
above the depth h#, therefore precluding simulation of slope steepening and
dune scarps.
In the modified erosion model used in this study, the transport curve is
not extended linearly and, instead, an estimate of the sediment transport
distribution is obtained in the following manner:
1) The water level is established and three points in the
profile are located as shown in Figure 4. The depth h^
is established as the breaking depth of incident waves
based on the spilling breaker assumption, i.e., h^ =
1.28 HI,. The depth, h* is established at the depth in
which the equilibrium beach face slope is tangent to the
equilibrium Ax2'3 profile form such that there is a
monotonic decrease in bottom slope from the beach face
to the breakpoint. The elevation hu is then established
at the upper limit of the active profile. This upper
limit is assumed to be the berm crest, the runup limit
on the dune, or the top of the dune scarp after a scarp
is formed .
2) Based on the governing equations, energy dissipation per
unit volume and the sediment transport rate Q are
calculated in the submerged portion of the profile from
hb to h*. The calculated value of h#, denoted Q», then
represents the volume of sand per unit time that must be
eroded from the "dry" beach face, between h^ and hu> and
which must flow past depth h* over the time step. The
estimated volume of sand eroded from the "dry" beach
face over the time step is then:
3) Next, the maximum potential eroded volume, Vp, or the
potential erosion prism of the "dry" beach is estimated
between hu and h#. This is accomplished simply by
establishing the known equilibrium beach or dune slopes
relative to the known active limits. In Figure 5, Case
I indicates that for an equilibrium slope m* steeper
than the actual slope m, the potential erosion prism is
established by taking m* relative to the upper limit,
hu, such that volume Vp is the potential eroded volume.
In Case II, if ra# is less than m, then upper portions of
the profile have a greater erosion potential than the
contours near h*, therefore, the equilibrium slope is
taken relative to h . In Case III, if the actual slope
is already in equilibrium, then it is assumed that all
contours have an equal erosion potential. Since only a
finite volume of material from the potential erosion
prism may pass depth h», it is clear that the entire
potential eroded volume Vp may not be eroded in a
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-109-
FIGURE 3
PREVIOUS METHOD OF ESTIMATING DISTRIBUTION
OF SEDIMENT TRANSPORT ON BEACH FACE
Sediment Flux.Q
.c
•»
i
LU
Q
Linear
Extension
Storm Surge Level
Mean Sea I evel " >_
Calculated
Based on Q=K(D-D^
FIGURE 4
DEFINITION SKETCH OF SCHEMATIC BEACH PROFILE
-------
-110-
FIGURE 5
NEW METHOD OF ESTIMATING DISTRIBUTION OF
SEDIMENT TRANSPORT ON BEACH FACE
Case I
Potential
Erosion Prism V,
Equilibrium Slope m*
Case E
X.nu \/
TJT?:X7:.'r7.A?w »nn
^^fY*/ Y VVY/Ta r
Potential
Erosion Prism Vp
Equilibrium Slope m.
Case IIC
Potential
Erosion Prism Vp
Equilibrium Slope
-------
-Ill-
single time step. In effect only the fraction V*/Vp may
be eroded over the time step.
In some cases, the potential erosion prism for Case
I or Case II may not be large enough to provide the
required volume V*. In these situations, the additional
required volume is obtained by translating the
equilibrium beach slope landward. In this way
combinations of Cases I and III as well as Cases II and
III are used to obtain the final potential volume Vp
which is identically equal to the required volume V#.
4) The estimated distribution of sediment transport on the
beach face is then estimated according to the fraction
V#/Vp of the potential volume that may be eroded above
contour n. Denoting the potential volume above a point,
n, as Vnp the estimate of the eroded volume is
Vn = Vnp —
Vp
and the estimate of the transport rate at n is
Vn
Qn = —
At
For Cases I and II, this method gives the estimated
transport distributions shown in Figure 6. For Case I,
since the beach is assumed to steepen, transport
gradients are greatest near the still waterline and
decrease to near zero at hu. For Case II, since the
beach is assumed to become less steep, the largest
transport gradients occur near the depth hu and decrease
to h*. For Case III, since uniform erosion is expected,
the distribution of Q is a straight line as in the
original model.
5) During the simulation the "dry" beach face evolves
toward the specified equilibrium slopes while the
submerged portion of the profile, between h* and hj,,
evolves toward the equilibrium Ax2'1 configuration.
On the "dry" beach face, estimated Q values are also
converted to equivalent D values based on the transport
equation. The numerical double-sweep procedure,
described by Kriebel (1982) is then applied over the
entire active profile from hu to h|j therefore ensuring
continuity both in terms of the sand volumes
eroded/deposited but also in terms of linking the,
solutions for the geometric (dry) and dynamic
(submerged) regions.
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-112-
FIGURE 6
EXAMPLES OF ESTIMATED SEDIMENT TRANSPORT
DISTRIBUTIONS ON BEACH FACE
CASE I
x
I-
Z
UJ
LLJ On
CASE H
'n
UJ
2
Q
LJ
CO
CASE IE
-------
-113-
SECTION II
CALIBRATION - SAVILLE'S LABORATORY EXPERIMENT
Background
The modified erosion model ir> tested and calibrated based on numerical
simulations of Saville's (1957) large-scale laboratory experiments. The model
calibration is performed by varying the free coefficient, K, in the sediment
transport equation until best-fit, in the least squares sense, is obtained
between predicted and observed beach profile response. Saville's experiments
provide a useful data set for testing the numerical erosion model because they
were conducted at approximately full-scale and because detailed measurements
were made throughout the profile development from which time-dependent erosion
characteristics, including beach slope steepening, can be tested.
Based on copies of Saville's laboratory notes, initial conditions for test
#3 at prototype scale are:
Beach Slope 1:15
Water Depth at toe of slope 14 feet
Berm Crest above Still Water Level 6 feet
Median sand diameter 0.22 mm
Wave height 5.5 feet
Wave period 11.33 sec
Breaker height 6-7 feet
Breaking depth 6 feet
Test duration 50 hours
Approximate equilibrium occurs after about 40 hours with only a minor shifting
of offshore bars occurring between 40 and 50 hours. Waves in the experiment
quickly shoaled on the steep seaward toe of the developing profile such that
plunging breakers with heights of 6 to 7 feet are reported by Saville.
However, the breaking depth and location relative to the outer bar crest also
remained fairly constant. In the numerical model the apparent breaking depth
of 6 feet is used to establish the seaward limit of the concave Ax2'3
profile at equilibrium.
Calibration Procedure
For calibration of the numerical erosion model, all physical parameters
must be specified so that the transport coefficient, K, remains as the only
parameter to be determined. Required physical parameters include the initial
profile form (1:15 slope), the characteristic A parameter, the equilibrium
slopes, runup distance, and breaking depth. Based on Saville's final
equilibrium profile, the equilibrium beach face slope is approximately 1:5 and
the vertical runup distance above the still water level is 4 feet. Seaward of
the breakpoint, the offshore slope is approximated by a uniform slope of 1:5.
The scaling parameter, A, is then determined by a least-squares fit of the
Ax2'3 profile form to the observed profile at 40 hours and the best-fit A
value is found to be 0.160 ft1'3. Based on results from over 700 open-coast
beach profiles analyzed by Dean (1977), Hughes (1978), and Moore (1982), the
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0.22 mm sand used by Saville should correspond to A = 0.155 ft1'3. Given
the close agreement between these two independent estimates, the value used
for calibration is A = 0.160 ft1'3 as obtained from a direct Ax2'3
curve-fit to Saville's equilibrium barred profile.
Calibration of the erosion model is accomplished by a series of
simulations in which separate values of K are used to simulate Saville's
profile development while all other parameters are held constant. The eroded
volume at any time is determined as the cumulative volume of material
displaced between the initial profile and the profile at the current time.
The mean squared error between the predicted and observed eroded volumes is
then obtained after 5, 10, 15, 20, 25, 30, 35, 40, and 50 hours as
1 N
N n=l
Because all other parameters are held constant and are best-fit values
obtained from Saville's equilibrium profile, it is expected that a distinct
best-fit K may be obtained. In Figure 7, results from the calibration test
series are summarized with the mean squared error plotted for eight K values
tested. The five curves shown correspond to the error curves obtained after
10, 20, 30, 40, and 50 hours of simulation since it is desired to determine an
overall K which provides best agreement over the duration of the experiment,
not just after equilibrium is attained at 40 to 50 hours. After 10 hours the
best-fit K is 0.004 to 0.0045 ft*/lb while from 20 to 50 hours, minima of
the error curves occur between 0.0045 and 0.005 ft"/lb. In general, the
broad troughs of the mean squared error curves indicate that varying K by +10
percent is not critical and will give similar results for erosion estimates.
Based on these results, an overall value of K = 0.0045 ft"/lb is adopted
which seems to give near minimum error over all time scales of interest. The
observed and predicted cumulative erosion curves are shown in Figure 8 based
on the selected best-fit value of K.
In Figure 9, the predicted profile is compared to Saville's measured
profile after 40 hours . In general , the monotonic curve approximates the
development of the barred profile quite well in a volumetric sense and in
terms of the width of the surf zone. In the numerical simulation, the
shoreline does not steepen as quickly as the laboratory profiles of early
times; however, at equilibrium the simulated beach face actually recedes 4 to
5 feet farther than the laboratory profiles. In Figure 10, the numerical
evolution of the profile, including the steepening beach face, seems realistic
and provides a confirmation that the sediment transport distributions used in
the model are reasonable approximations.
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-115-
FIGURE 7
MEAN SQUARE ERROR OF VOLUME ERODED VERSUS
SEDIMENT TRANSPORT COEFFICIENT K
2000
1500
1000
500
Hours
10
20
30
4O
50
0
0
0.002
0.004
K(ftl'lb)
0.006
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-116-
FIGURE 8
COMPARISON OF CUMULATIVE EROSION: CALIBRATED
MODEL VERSUS SAVILLE'S (1957) LABORATORY EXPERIMENTS
400
= 300
10
I 1 1
Erosion Model Calibration
vs
Saville(l957) Large-scale
Laboratory Test
a
LJ
a
LJ
UJ
5
ID
200
100
Seville
H=5.5 ft
T=ll.3sec
Calibrated Model
K = 0£>O45 ft4/lb
A=0.160 ft^S
10
15
20 25
TIME (hrs)
30
35
40
45
50
-------
-117-
FIGURE 9
COMPARISON OF PROFILE FORMS: CALIBRATED
MODEL VERSUS SAVILLE'S (1957) LABORATORY EXPERIMENTS
o
Ci
a
in"
H-
ll IO
HO
- I
a
i
o
o
Erosion Model Calibration
vs
Seville's (1957) Large-scale
Laboratory Test
0.00 40.00 80.00 120.00 160.00 200.00
DISTRNCE (FEET)
240.00
280.00
320.00
-------
-118-
FIGURE 10
TIME-DEPENDENT EVOLUTION OF PREDICTED PROFILE
o
o
Erosion Model Calibration
Time Dependent Profile Development
o
o
in"
P
ii ID
U-o"
LU
Jg
o
o
t=IOhrs
20hrs
30hrs
40hrs
50hrs
0.00
40.00
sb.oo
120.00 160.00 200.00 2UO.OO 280.00 320.00
DISTflNCE (FEET)
-------
-119-
SECTION III
CALIBRATION - HURRICANE ELOISE FIELD DATA
Background
Hurricane Eloise made landfall just east of Walton County on September 23,
1975, and was a rapidly moving storm which, while lasting less than 20 hours,
produced estimated peak water levels of between 8 and 10 feet over Walton
County. Although no open-coast storm surge measurements are available,
numerical estimates by Dean and Chiu (1984) range from 8.35 feet at the
western end of Walton County to 9.60 feet at the eastern end of the county.
The predicted storm surge hydrograph near the western end of Walton County is
shown in Figure 11. Significant wave heights recorded during the peak of the
storm are 10 to 14 feet with a dominant period of 11 seconds. For this study,
a significant wave height of 12 feet is used to obtain an estimate of the
offshore limit of sediment deposition.
Pre-storm profiles for the Walton County area are taken from the October
1973 survey of the area by the Florida Department of Natural Resources.
Post-storm surveys were conducted within 3-4 weeks after the storm in October
1975. Due to the timing of these surveys, the "observed" erosion associated
with Hurricane Eloise may be contaminated by two effects. First, it is
probable that some modification of the pre-storm profile occurred in the two
years between October 1973 and September 1975. These effects may include
natural erosion or accretion of the shoreface and possible modification of the
dunes by wind-blown sand or construction activities. Second, in the 3-4 weeks
after the storm, some recovery of the shoreface certainly occurred. Chiu
(1977) estimates that 240,000 yd1 had returned to the beach face over Walton
County, and profiles show a distinct berm of about 30 to 60 ft3/ft between
0- and 4-foot elevations. When discussing agreement between the observed
erosion characteristics and any erosion prediction model, these two effects
must be considered.
Two previous studies of beach/dune erosion have used profile R-41 from
Walton County, Florida, for model verification in reproducing prototype storm
related erosion. Hughes and Chiu (1981) selected R-41 as a representative
profile to be used in verification of small-scale laboratory simulations of
dune erosion. Vellinga (1983a) also used R-41 as a part of a continuing
verification of a computational method for predicted dune erosion due to
severe storms. Since profile R-41 has now become in some respects the
standard reference profile from the Hurricane Eloise data set, it seems
reasonable to test and calibrate the numerical erosion model based on this
profile as well. In this study, however, the erosion model is also tested
against 20 additional profiles from the Walton County data set to determine
possible bias associated with using R-41 as a benchmark for calibration/
verification.
The pre- and post-storm profiles for range R-41 are shown in Figure 12.
This profile is fairly representative of the Walton County area in that it has
broad well-developed dunes, arid a narrow berm with a berm crest elevation of 5
to 6 feet. The post-storm profile is typical in that a distinct break in
-------
-120-
FIGURE 11
HURRICANE ELOISE STORM SURGE hYDROGRAPH
From Dean and Chiu (1984)
10
8
uj 6
o
cc
ID
CO
Hurricane Eloise
Storm Surge Hydrograph
Profile R-2
• i i r i i I
6 8
TIME (hrs)
10
12
14
-------
-121-
FIGURE 12
PRE- AND POST-STORM BEACH PROFILE, PROFILE R-41,
WALTON COUNTY, FLORIDA
From Florida Department of Natural Resources
30
E
E
1
20
10
^U.
^
rt-.
-10
-250
-200
-150
-100
-50
IMOMUHCNl)
50
100 150 200 250
FEET
BEflCH PROFILE
-• •- 01 OCI 7J
— •- 01 OCI 7S
""«"• WflLTON
DIVISION OF BEflri'l -> ( SHORES
nn.oiri.or tiniin .» nisouiicES
BONCEi R ~U 1 l/z
MONUMENT ESTABLISHED: JUN 1973
UEHIIINr.: S 15*00' M IHIIU.)
-------
-122-
slope occurs between the dune scarp and the flattened beach at about 10 feet.
The rebuilt post-storm berm is also clearly evident. The computed eroded
volume of about 400 ft1/ft above the 0-foot contour is perhaps above average
for the entire data set. As noted earlier 20 profiles showed a net accretion
between October 1973 and October 1975. However, R-41 is not the most
severely eroded profile in Walton County.
In this study, water level and wave conditions are assumed to be
reasonably represented by the storm surge hydrograph in Figure 11 and by a
significant breaking wave height of 12 feet. It should be noted that the
storm surge elevations have been interpolated by Dean and Chiu for all 125
profiles in Walton County and the surge hydrograph in Figure 11 is multiplied
by the appropriate factor to give the hydrograph for each site. For profile
R-41, the surge hydrograph is multiplied by 1.083 at all times.
Offshore profile forms for pre-storm conditions are assumed to be in
equilibrium and characterized by an Ax2'3 profile. The scaling parameter A
may be determined from Figure 2 based on the effective grain size of 0.262 mm
given by Hughes and Chiu (1981). In this case the appropriate A value is
0.184 ft1". Hughes and Chiu model the profile form based on a fall
velocity of 4.0 cm/sec for 25°C water while Vellinga (1983a) models the same
profile based on a fall velocity of 3.6 cm/sec. In Figure 2, these fall
velocities correspond to A values of 0.170 ft1'1 to 0.195 ft1" for 20°C
to 30°C water temperatures. Since A = 0.184 ftl/J lies within this range
and is especially good for 25°C temperatures, this value is adopted in this
study.
Calibration/verification of the numerical erosion model is first carried
out on profile R-41. In this case, input to the numerical model consists of
the actual pre-storm profile between the dune crest at 26 feet and mean sea
level, taken to be at 0 feet (National Geodetic Vertical Datum) NGVD. The
offshore profile is established according to h = 0.184 x 2" out to depths
of 20 feet. Additional required input consists of the "equilibrium" slopes of
the dune scarp and beach face as well as an effective runup height. Based on
Figure 12, the equilibrium dune slope is taken as 1.75:1, a 1-foot runup is
assumed (based on peak surge elevation of 9.03 feet and a break in slope at 10
feet), and the post-storm beach face slope is taken to be 1:15. Slopes
seaward of the breaking depth are set at 1:15.
Calibration Procedure Using Profile R-41
Due to the uncertainties involved in the post-storm eroded volumes,
calibration of the erosion model is obtained in a more subjective manner than
in the previous calibration against Seville's data. In this case, since the
estimate of total eroded volume (400 ft'/ft) may not exactly represent the
actual eroded volume, and since the time-history of erosion is not available,
the mean squared error curves as a function of K cannot be developed.
Instead, in Figure 13, predicted maximum volumetric erosion is plotted against
the value of K used in each test, and a smooth curve it drawn through the data
points.
-------
-123-
FIGURE 13
PREDICTED VOLUME ERODED FOR PROFILE R-41
VERSUS SEDIMENT TRANSPORT COEFFICIENT K
600
Hurricane Eloise
Profile R-41
Calibration
Total Eroded
Volume Predicted
Q
UJ
g 400
cc
UJ
UJ
2
ID
o
>
200
x
<
0
0
0.002
OO04
K(ft4/lb)
0.006
-------
-124-
Based on the 400 ft3/ft estimate of total observed eroded volume, the
best prediction is obtained using a value of K = 0.0044 ft*/lb; within
arbitrary 10 percent error bands, the best K values range between 0.0038 and
0.0052 ft*/lb. From this analysis, a value of K between 0.004 and
0.005 ftVlb seems most appropriate. This range is identical to that
suggested by the calibration based on Saville's profile and the previous best
estimate of K = 0.0045 ft*/lb seems equally appropriate for profile R-41.
This surprising agreement between the two calibration runs is fortuitous and
should not be taken as an absolute indication that K = 0.0045 ft"/lb is the
"correct" or universally valid constant for the proposed erosion model. Most
likely, the agreement is the result of assumptions made in each simulation
concerning input parameters.
As a sensitivity test, erosion estimates for R-41 are obtained using
various wave height scenarios. In Figure 14, the eroded volumes obtained by
using a constant wave height over the duration of the storm surge are compared
to estimates obtained by applying a variable wave height. Variable wave
heights are scaled from 3 feet to the maximum height shown according to the
ratio of the storm surge level at each time step to the peak surge level.
Results of this test indicate that a variation in the constant wave height of
+20 percent produces less than a 5 percent change in eroded volume.
Likewise, use of a variable wave height tends to decrease the erosion estimate
by only 5 percent for the range of wave heights of interest. If calibration
had been carried out using a variable wave height, a slightly larger volume of
K would have been required. Due to the small differences between predictions,
all model calibration, verification, and application is performed with a
constant wave height.
In Figure 15, the predicted post-storm profile is compared to the observed
post-storm profile form for K = 0.0045 ft*/lb. There is good agreement
between predicted and observed profiles from the base of the dune scarp across
the shoreface to the point where the berm built by post-storm recovery
occurs. The major difference between the predicted and observed profiles is
the predicted position of the dune scarp, which is about 3 to 4 feet seaward
of its actual position. In this case, a slightly larger value of K would
provide the best agreement with the observed dune scarp.
The time variation of the numerical simulation is depicted in Figure 16.
The predicted profile position is shown after 8, 10, 11 and 12.5 hours of
simulation corresponding to water levels of 4.06,8.99, 6.47, and 3.28 feet
respectively. While the maximum dune recession occurs at 11 hours or 1.5
hours after the peak surge, the maximum eroded volume occurs at 12.5 hours as
a small amount of sand is eroded from lower portions of the beach face as the
water level recedes. In Figure 17, the entire active profile is shown.
Because of the changing water levels and, therefore, the changing position of
the breaking depth, the offshore portion of the profile is smooth with no
discontinuities as would be obtained from a steady-water level estimate as in
the simulation of Saville's profile development.
-------
-125-
FIGURE 14
SENSITIVITY OF PREDICTED VOLUME ERODED
TO WAVE HEIGHT DESCRIPTION
500
I I I I I I I I
I I
400
Q
U
Q
O
DC
LJ
LJ
O
300
200
100
Constant Wave Height
Variable Wave Height
0
Profile R-41
WQlt0n C°-FL K=0.0045ftVlb
i i i i i i i i i i i i it
0
5 10
BREAKING WAVE HEIGHT, Hb (ft)
15
-------
FIGURE 15. COMPARISON OF PREDICTED TO OBSERVED POST-STORM PROFILE FORMS
tprra:|::PrafiJe!::::!::::!::::
-10
-250
-200
-150
-100
(MONUMENT)
200
FEET
250
NJ
O\
I
BEflCH PROFILE
01 OCT 73
01 OCT 75
WflLTON
DIVISION OF BEfiCHES « SHORES
FLfl.OEPT.OF NRTURfU. BESOUHCES
RflNCEi
R-41
1/2
MONUMENT ESTABLISHED) JUN 1973
BERRINC: S 15*00' H (HRG.)
-------
FIGURE 16. TIME-DEPENDENT EVOLUTION OF PREDICTED PROFILE
Ul
8
8
Initial Profile
-Profile After 10 hrs.
Vofile After 8hrs.
Profile After II hrs.-
Hurricane Eloise
Profile R-4I Walton Co.
Initial Profile
Profile After 12.5 hrs.
I
l-»
N>
°100.00 110.00 KO.OO 130.00 IW.OO 150.00 ICO-00 I7O.OO I8O.90 190.00 200.00 210.00 Z2D.OB E30.00 2VO.OD ZSO.M Z80.00 270.00 280.00
DISTANCE (FEET)
-------
FIGURE 17. EXAMPLE OF OFFSHORE AND NEARSHORE PREDICTED PROFILE FORMS
8
o
Hurricane Eloise
Profile R-41 Walton County
Initial Profile
Profile After
12.5 Hrs
00
I
.00
123.72 157.1(3 191.15 22V.87 258.58 292.30 326.02
359.73 393.45 H27.17 460.
OlSTflNCE (FEET)
494.60 528.32 562.03 595.7
-------
-129-
Verification Using Additional Field Profiles
As noted, numerical simulations are also carried out on an additional 20
profiles from the Hurricane Eloise data set. These profiles are selected to
be representative of those profiles showing the maximum erosion either in
terms of volume eroded or, in the case of low dunes, in terms of contour
recession. The selected profiles cover the entire Walton County shoreline and
several groups of closely spaced profiles are chosen to indicate the natural
variability that may exist between adjacent profiles. All tests are made with
K = 0.0045 ftVlb as established by calibration tests. Input in each case
consists of idealized pre-storm profiles, that is, described by a dune crest
position and dune face slope and a berm crest position and beach face slope.
Offshore profiles are all simulated initially by A = 0.184 ft1'3. Breaking
depths and offshore slopes are established at 12 feet and 1:15 respectively,
as in the case of profile R-41. Storm surge hydrographs are identical in form
to Figure 11 but increased by a multiplicative constant suggested by Dean and
Chiu for each profile. Other input variables include the observed post-storm
dune slope, beach slope, and runup distance as determined for each profile.
Estimates of total eroded volumes are obtained from the pre- and post-storm
profiles and are subject to uncertainties as discussed previously.
In Figure 18, predicted eroded volumes are compared to observed values for
the 20 profiles and for the calibration profile R-41. The diagonal line
represents perfect agreement between predicted and observed values and falls
through profile R-41. There is considerable scatter in the data points about
the line of complete agreement; however, there seems to be little bias as the
numerical model overpredicts erosion in 11 cases and underpredicts in 9
cases. In 5 cases, predictions are within 10 percent of the assumed observed
values; in 16 cases predictions are within 25 percent of observed values; and
all 20 cases are within a 40 percent margin of error.
Errors outside the 25 percent range may be attributed to several factors.
For profile R-8, the model underpredicts erosion substantially. However, the
large eroded volume at R-8 appears to be a local anomaly; adjacent profile R-9
shows about 300 ft3/ft of erosion while adjacent profile R-7 shows net
accretion. For profiles R-114, R-15, and R-123 the model overpredicts erosion
by 30 to 40 percent. The explanation for this seems to be that these are
among the steepest profiles simulated and, as discussed by Kriebel (1982), the
numerical scheme does tend to predict greater erosion in areas of steep berm
or dune slopes. Again, however, local longshore effects may play a
considerable role; profile R-122 just 1,000 feet west of R-123, shows the
greatest observed erosion of the 21 profiles and is underpredicted by the
numerical model by about 25 percent.
Results are somewhat biased in the prediction of dune recession. Of the
20 tests, the location of the dune scarp is correctly predicted on 4 profiles;
is slightly overpredicted on 5 profiles and is underpredicted on 11 profiles.
On average, the position of the dune scarp was underpredicted by 5.4 feet.
Extreme estimates range from an underprediction of 18 feet (out of a total
observed recession of about 55 feet for a 33 percent error) to an overestimate
of 5 feet (out of a total observed recession of 36 feet for a 14 percent
-------
-130-
FIGURE 18
COMPARISON OF PREDICTED TO OBSERVED EROSION
FOR 20 BEACH PROFILES FROM WALTON COUNTY, FLORIDA
600
O
LJ
Q
O
o:
LU
LJ
400
Q
LU
LJ
CO
m
o
200
Hurricane Eloise
Walton Co., FL.
K =0.0045 fr/lb
i i i
200
400
600
PREDICTED VOLUME ERODED (ft /ft)
c R-5
e R-8
e R-14
§B:S
V R-36
T7 R-38
O R-41
V R-44
^ R-47
I gill
0 R-74
Q R-78
S R-80
D R-85
A R-87
A R-M6
A R-122
A R-123
-------
-131-
error). Based on these results, dune scarp location might be better predicted
with a slightly larger K value.
Summary of Calibration-Verification Phase
The modified numerical erosion model seems to be well-calibrated and
verified for application to other areas. The first calibration, against
Seville's large-scale laboratory profile, provides some indication of the
validity of the model for predicting time-dependent profile development,
including beach face slope steepening. Saville's tests are quite controlled
relative to prototype conditions, yet the best-fit K value, 0.0045 ft*/lb,
should be representative of full-scale (prototype) events.
The second calibration, against profile R-41 from the Hurricane Eloise
data set, is less conclusive in terms of precise numerical calibration.
However, it also indicates a best-fit K value of approximately 0.0045 ft*/lb
for prototype erosion under severe storm conditions. Certainly, the agreement
between this calibration test and the Saville calibration should be viewed, in
part, as a fortuitous correlation. This conclusion is supported by the
comparison of numerical predictions to observed erosion for an additional 20
profiles from the Hurricane Eloise data set where the best-fit K = 0.0045
ftu/lb gives agreement to within a 25 percent error on 16 profiles. Larger
errors on the other 4 profiles are mainly attributed to localized longshore
erosion effects that cannot be accounted for in the model. A further
conclusion reached is that beach slope changes and dune scarps are reasonably
approximated by the model.
For application to other dune erosion predictions, the following
guidelines are recommended:
1) For the modified erosion model, K = 0.0045 ft*/lb
should be used to obtain average dune erosion
characteristics.
2) Actual dune erosion is highly variable due to a number
of natural factors. Numerical predictions are also
sensitive to some parameters such as very steep slope.
Therefore, all erosion estimates should be considered
average estimates with probable errors of around 25
percent and possible errors 40 percent or more.
3) While individual contour recession predictions are much
better than those previously obtained (Kriebel 1982),
estimates of dune recession should also be considered
average within probable error limits of 5 to 20 feet or
25 percent and perhaps more. The model also is somewhat
biased, however, and tends to underpredict dune
recession based on the profiles tested.
-------
-132-
SECTION IV
APPLICATION TO OCEAN CITY, MARYLAND --
STORM EROSION
Background
The numerical erosion model is applied to Ocean City, Maryland, to obtain
estimates of the storm erosion potential of existing beach profile forms.
With the calibration and verification of the model, it is expected that the
time-dependent erosion due to severe storms at Ocean City may be estimated to
within the same level of accuracy obtained in the hindcast of erosion
associated with Hurricane Eloise.
In this study, a typical or representative profile form, similar to that
suggested by Everts (1984), is adopted. The shoreface and backshore are taken
from the 55th Street profile which is described as "typical" by the U.S. Army
Corps of Engineers (1980). This profile is characterized by a low, broad dune
with a crest elevation of +14 feet NGVD (National Geodetic Vertical Datum), a
linear dune face with a 1:10 slope, and a 40-foot wide berm at +5 to +6 feet
NGVD. The beach slope is taken to be 1:10 as given by the Corps of Engineers
and Trident Engineering (1979). The set of Ocean City beach profiles provided
by Leatherman (1984b) indicate that average dune heights vary between about 8
and 15 feet NGVD. Therefore, numerical simulations are also performed on
beach profiles with crest elevations of 10 and 12 feet in addition to the
reference profile, with a 14-foot crest elevation. The range of dune
configurations tested is depicted in Figure 19.
The offshore regions of Ocean City beach profiles are highly variable as
suggested by the overlays of the 1929, 1965, 1978, and 1979 profiles provided
by Leatherman (1984b). General profile characteristics include a fairly mild
slope to depths of -7 feet followed by steeper slopes to -15 to -20 feet
contours, after which the profiles flatten considerably. The apparent
steepening below -7 foot depths is a subject of some interest. It appears
that over the past several decades this portion of the profile has steepened
considerably while the nearshore zone, i.e., above -7 foot contour, has
remained fairly stable.
In this study, two scenarios for the observed profile forms are
investigated in which: 1) the existing profile is considered to be in
approximate equilibrium and 2) the existing profile is considered to be
artificially steepening away from a more gently sloping equilibrium profiles
form. For each scenario, the equilibrium form of the profile is approximated
by a monotonic profile of the Ax2'3 form.
For the equilibrium scenario, the average profile of Everts and the
profiles provided by Leatherman have been analyzed, and approximate best-fit A
values are obtained between the shoreline and an assumed closure depth of 28
ft. For the 1979 profile, an A value of about 0.250 ft1'3 seems to provide
the best fit; however, 1965 and 1978 profiles suggest an A value of about
0.200 ft1'3. Due to the significant variation between the 1978 and 1979
profiles and the close agreement between 1965 and 1978 profiles, the 1979
-------
ELEVATION (Ft, NGVD)
O O 5
ro
O
Z
m
X
O
73
m
B>
m
>
O
O 73
m
O)
m
O
O
m
O
H
N
2
73
|
O
-------
-134-
profiles have been disregarded and the representative A value of 0.200 ft1/3
is adopted. The monotonic Ax2/3 curve obtained using A = 0.200 ft1/3
typically overpredicts depths nearshore and slightly underpredicts depths
offshore, but represents approximately the total volume of sand in the profile.
For the second scenario, in which the existing profile is assumed to be
steeper than a representative Ax2'3 equilibrium form, separate analyses
indicate that A = 0.175 ft1'3 provides a reasonable equilibrium
approximation. Based on the median sand grain diameter, given by the U.S.
Army Corps of Engineers (1980) as 2.020, or 0.24 mm, the corresponding A
value from Figure 2 is about 0.180 ft1/3. Likewise when an Ax1/3 form is
fitted through the nearshore portion of the profiles provided by Leatherman,
best-fit A values seem to range from about 0.150 to 0.200 ft1'3 with most
common values of about 0.175 ft1'3. Based on these two inconclusive, but
supporting sources, an A value of 0.175 ft1'3 is adopted as being
representative of an equilibrium profile form that is milder than most
observed profiles.
It should be noted that A = 0.175 ft1'3 does provide a better overall
fit to profiles S-65, S-48, and S-3 than A = 0.200 ft173. On the remainder
of the profiles, A = 0.175 ft1'3 provides a good fit nearshore to depths of
-7 to -10 feet, then predicts milder slopes than are observed. In effect, if
A = 0.175 ft1'3 is a reasonable approximation of the equilibrium form, a
large sand deficit exists offshore which must be filled by shifting sand from
the beach face. It appears that this readjustment is being prevented or
delayed in the active surf zone, i.e. out to depths of about -7 feet, perhaps
by the presence of shoreline stabilization structures.
In Figure 20, the two proposed equilibrium profile forms are shown along
with the average profile used by Everts. For subsequent analysis, the storm
erosion potential is determined for each profile form but it is expected that,
since storms affect the upper portion of the profile, the results for the
milder profile, A = 0.175 ft1'3, will be most realistic. The erosion
potential for profile adjustment to equilibrium and sea level rise is also
considered for either scenario and results may be interpreted as providing a
range of estimates only, as the appropriate equilibrium form for long-term
profile development is unknown.
In order to develop storm erosion probabilities, storm surge elevations
corresponding to 10-, 40-, 100-, and 500-year return periods are used. These
values provide four data points with approximately equal spacing on a
log-normal plot of storm erosion magnitudes so that erosion associated with
other return periods may be easily interpolated. Based on the National
Weather Service (Ho et al. 1976) joint storm tide analysis, the combined
probabilities of hurricanes and winter storms are represented by a single
frequency curve. In general, hurricanes may be considered the most severe
storms in terms of peak storm surge elevations. Therefore, hurricanes
dominate the higher, 100- to 500-year, return periods, while winter storms are
predominant at lower, 10- to 50-year, return periods. The National Weather
Service predictions, adopted by the U.S. Army Corps of Engineers (1980), are
used in this study, and appropriate peak storm tide elevations for the four
return periods tested are given as:
-------
-135-
FIGURE 20
APPROXIMATE EQUILIBRIUM OFFSHORE PROFILE FORMS
Ocean City, Maryland
20
10
o -10
UJ
_l
UJ
-20
-30
-40
Ocean City.MD.
i
^
0.175 ft 3
A=0.200ft'/3
Average Profile
from Everts
_L
_L
_L
_L
_L
_L
'0 400 800 1200 1600 2000 2400 2800 3200 3600
DISTANCE SEAWARD OF CONTROL POINT (ft)
-------
-136-
Return Periods Frequency Per Year Peak Surge (ft)
10 0.1 6.3
40 0.025 7.5
100 0.01 8.7
500 0.002 10.3
Since the time-characteristics of the storm surge have a major influence
on the extent of erosion during a severe storm, it is also necessary to
evaluate the effects of various surge durations. For this purpose, the storm
surge hydrograph is approximated by a sine-squared distribution as
S(t) = Sp sin2(irt/T)
where S(t) is the surge level at time t, Sp is the peak surge level, and T is
the total storm surge duration defined as the time between departure from a
no-surge condition and return to the same condition.
In this study, three surge durations, T, are evaluated. For typical
hurricanes, a 24-hour total duration is assumed due to the general fast motion
of hurricanes in an alongshore direction at Ocean City. For typical winter
storms, a 48-hour total duration is assumed, based on the surge hydrograph for
the 1956 storm presented by the National Weather Service (Ho et al. 1976).
Finally, since at least two winter storms have had total durations of at least
4 days, a duration of 96 hours is also evaluated. This surge duration
corresponds to the March 1962 storm, generally the most destructive storm
experienced along the U.S. East Coast in recent times. Based on an estimated
peak surge elevation of 7.8 feet NGVD at Ocean City, the March 1962 storm
surge elevations are expected to be equalled or exceeded about once in 50
years. Due to the unusually long duration of this storm, the actual
probability of reoccurrence is probably much less than 0.02; however, without
statistical description of joint storm surge/duration probabilities, it is
difficult to estimate the probability more precisely. In any event, the fact
that such a severe storm did occur in 1962 is sufficient reason for many
planners and engineers to consider its reoccurrence.
Other required input data include an estimate of the runup limit and an
estimate of storm breaking wave heights that effectively limit offshore
sediment transport. Based on the results of Hurricane Eloise simulations, an
approximate location of the break between the dune scarp and the beach face is
about 2-3 feet above the peak surge level for cases with tall dunes and
slightly higher for low dunes when swash may overtop the dune. For Ocean
City, where dune crests are low, an effective runup of 4 feet is assumed. For
an estimate of the breaking wave heights, wave data given by Bretschneider
(1964) for the March 1962 storm at Bethany Beach, Delaware, are used. In that
storm, wave heights in 17 feet of water increased rapidly to 10 to 12 feet and
were maintained at that level throughout most of the storm. As noted,
estimates of erosion during Hurricane Eloise based on variable wave heights
were about 5 percent lower than estimates based on a constant wave height.
Since model calibration is based on a constant wave height, however, a
constant breaking wave height of 10 feet is used to estimate storm erosion for
Ocean City.
-------
-137-
Erosion Estimates
Estimates of storm-induced erosion are obtained for 72 cases covering the
range of profile forms, storm surge elevations, and storm durations of
interest. As a brief summary, the simulated conditions correspond to all
possible permutations associated with the following input variables:
Variable Units Values Tested
A parameter ft1'3 0.175, 0.200
Dune Elevations ft 10, 12, 14
Peak Storm Surge Elevations ft 6.3, 7.5, 8.7, 10.3
Storm Durations hours 24, 48, 96
Output from the erosion model in each of the 72 cases includes the post-storm
profile at the time of maximum erosion, the total volume of sand eroded above
the 0-ft contour, and the recession of the dune crest. Numerical results for
the volume eroded and dune recession are presented in Figures 21 through 23
where each set of figures corresponds to a given dune configuration. Primary
discussion in this study is directed toward results obtained for the reference
profile, in Figure 21, with a dune height of 14 feet. Other results are
discussed briefly to indicate the range of erosion predictions that might be
expected over the Ocean City coastline where dune heights differ from the
reference profile.
As a guide to interpretation of Figures 21 through 23, the six curves in
each figure represent a given set of storm duration and profile conditions for
the range of peak surge levels tested. Smooth curves have been drawn through
the four data points obtained from the four peak surge levels to aid
interpolation of erosion estimates for other surge levels. Of the six curves
in each figure, the three solid curves correspond to erosion predictions for
A = 0.175 ft1'3 and the three storm durations of 96, 48, and 24 hours; the
three dashed curves represent erosion estimates for A = 0.200 ft1'3 for the
three storm durations.
The curves are plotted with respect to the return periods for the four
peak surge elevations but do not represent the probability of erosion
associated with given return periods. Instead, for a given peak surge
elevation and return period, there is a distribution of expected erosion
events corresponding to the distribution of expected storm durations. If the
distribution of storm durations were known, such that the probabilities of
24-, 48-, or 96-hour storm durations could be determined, then the total
erosion frequency distribution could be obtained by considering the joint
probabilities of storm surge and storm duration. Due to the limited scope of
this study, the three erosion estimates are given for each storm surge return
period to indicate the range of erosion that may be expected for storms with a
given magnitude, or peak surge level.
Erosion estimates, in Figures 21 through 23, vary in a predictable fashion
according to storm surge characteristics and assumed dune crest heights.
Based on the reference profile, erosion estimates for the shortest storm
duration range from 469 to 741 cubic feet of sand per linear foot of shoreline
-------
-138-
FIGURE 21
STORM EROSION ESTIMATES, 14-FOOT DUNE HEIGHT
rt
Q
Ul
UJ
1600
1200
800
400
10 20 50 100 200 500
RETURN PERIOD (Yrs)
160
120
80
UJ
40
•96Hr
48Hr
}24Hr
10 20
50 100 200 500
RETURN PERIOO(Yrs)
-------
-139-
FIGURE 22
STORM EROSION ESTIMATES, 12 FOOT DUNE HEIGHT
O
UJ
a
o
cr
LJ
UJ
2
5
1600
1200
800'
400
10 20 50 100 200 SCO
RETURN PERIOD (Yrs)
20
50 100 200 500
RETURN PERIOD(Yrs)
-------
DUNE CREST RECESSION (ft)
VOLUME ERODEDtft /ft)
CO
m
73
O
co
O
z
m
CO
H
m
CO
O
o
H
D
C
2
m
I
m
o
Tl
O
C
73
m
o
I
-------
-141-
(ft3/ft) with corresponding dune recession of 0 to 53 feet. Erosion
estimates for the longest storm duration, comparable to the March 1962 storm,
range from 770 to 1361 ft3/ft with dune recession of 32 to 189 feet. Best
estimates for erosion values that might be expected for a repeat of the March
1962 storm, that is, a 7.5-ft peak surge for a 96-hour storm duration, are
volumetric erosion of 974 to 1129 ft3/ft and a recession of 58 to 71 feet
for the 14-foot dune crest.
In Figures 24 through 27, estimated post-storm profiles are shown for the
four peak storm surge levels and the three durations tested. Results are for
an A value of 0.175 ft1'3 and for the reference profile with dune height of
14 feet. Erosion estimates for the reference profile vary from 516 to 1361
ft3/ft of erosion with dune recession of 2 to 136 feet.
Although field data are not available to conclusively verify these erosion
estimates, predicted values appear reasonable when compared to observations
from storms of record. Bretschneider (1964) presents estimated beach profiles
for the Delaware coast before and after the March 1962 storm where dune
recession of 50 to 100 feet occurred. Hayes (1967) has also documented
average dune recession of 100 feet on central Padre Island, Texas, after
Hurricane Carla, which had a duration of more than 80 hours. Vellinga (1983b)
presents field measurements of erosion in the Netherlands in which volumes of
up to 1600 ft3/ft (up to 150 m3/m) were eroded along with 30 to 70 feet of
dune recession. Finally, the Shore Protection Manual (U.S. Army Corps of
Engineers 1977) suggests the following guidelines for storm related erosion:
Storm Class Volume Eroded (ft3/ft)
Moderate 108 - 270
Extreme (or moderate that
persists for long duration) 270 - 540
Rare 540 - 1350
While quantification of the above storm classes is not available, and
would vary with location, all storms tested in this study have long durations
typical of the most severe storms of record and fall easily within the Extreme
to Rare classifications. Based on this brief and inconclusive comparison, it
does appear that numerical estimates are of the correct order-of-magnitude;
and, based on descriptions of historical storms, estimates seem as likely to
underestimate as to overestimate erosion. Since this finding is also
substantiated by application of the numerical model to the Hurricane Eloise
field data set, the numerical estimates for storm erosion at Ocean City are
considered to be subject to the same +25 percent probable errors found during
the verification phase of this study.
In Figures 21 through 23 the erosion estimates for A = 0.175 ft1'3 and
A = 0.200 ft1'3 are offset almost linearly for each storm duration, with
A = 175 ft1'3 always giving the larger erosion magnitudes. Results for the
two A values agree very closely for the 24-hour storm duration estimates and
eroded volume differs by less than 20 percent for 96-hour storm durations.
While these two A values may provide high- and low-range erosion estimates,
-------
-142-
FIGURE 24
ESTIMATED POST-STORM EROSION PROFILES,
6.3-FOOT PEAK STORM SURGE
20
T
6.3 Ft Peak Storm Surge
UJ
10
9 0
I
Storm Duration
24hrs
48hrs
96hrs
-10
-20
Reference Profile
Ocean City, MO.
1
_L
_L
200
400
DISTANCE (Ft)
600
800
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-143-
FIGURE 25
ESTIMATED POST-STORM EROSION PROFILES,
7.5-FOOT PEAK STORM SURGE
20
f 10
o
z
I
fef
UJ
-10
Storm Duration
24 hrs
48hrs
96 hrs
Reference Profile
Ocean City, MO.
200
75 Ft. Peak Storm Surge
400
DISTANCE (Ft)
600
800
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-144-
FIGURE 26
ESTIMATED POST-STORM EROSION PROFILES,
8.7-FOOT PEAK STORM SURGE
20
Q
O
8.7 Ft. Peak Storm Surge
10
2
g 0
1
ui
-10
Storm Duration
24hrs
48hrs
96hrs
Reference Profile
Ocean City,MO.
-20
200
400
DISTANCE (Ft)
600
800
-------
-145-
FIGURE 27
ESTIMATED POST-STORM EROSION PROFILES,
10.3-FOOT PEAK STORM SURGE
Q
20
10
9 0
-10
-20
10.3 Ft. Peak Storm Surge
Storm Duration
24hrs
" 48 hrs
96hrs
Reference Profile
Ocean City, MO.
200
400
DISTANCE (Ft)
600
800
-------
-146-
the results for A * 0.175 ft1'1 are considered the most realistic for
estimating storm erosion magnitudes. This conclusion is based on the
comparisons of the two monotonic Ax2'3 profile forms to the Ocean City beach
profiles, where A = 0.175 ft1" provides the best fit for nearshore portions
of the profile that are dominated by normal wave activity.
Storm surge elevation and duration are both found to be important
parameters in determining the storm erosion potential. When considering the
volume of sand eroded, variations in storm duration are of greater importance
than variations in storm surge elevation over the range of values tested.
However, dune recession seems to be almost equally influenced by storm
duration and magnitude. These conclusions emphasize the need to consider the
joint probability of occurrence of storm duration for a given storm surge
magnitude when estimating storm erosion potential.
Numerical results for different dune height scenarios, in Figures 21
through 23, exhibit expected results where smaller dunes erode farther but
with less total volume eroded. Reductions in dune height from 14 to 10 feet
cause a decrease in eroded volume of about 10 to 15 percent while leading to
increases in dune recession of over 50 percent for the most extreme storm
durations tested, with absolute increases in dune recession of 45 to 50 feet
for all scenarios considered. These results have rather important
implications for erosion mitigation. Protective dunes that are high and
narrow must store a greater sand volume than low, broad dunes. However, the
benefit in reduction of sand volume achieved by construction of wide berms or
dunes must be weighed against the potential for overwash and flooding that may
be more effectively controlled by high dunes.
It is emphasized that effects of dune breaching and overwash are not
simulated in the numerical model. The 10-foot dune will certainly be
overtopped by the 100- to 500-year storms and overtopping may occur with other
dune heights as well. Likewise, since dunes at Ocean City are narrow,
numerical estimates indicate that complete erosion of existing dunes may occur
under several storm scenarios. Numerical results assume a uniform profile
landward of the dune crest and do not consider structures that may be
present. For cases where these assumptions are not valid, numerical results
indicate the erosion potential that exists and may be used to identify areas
in which dune breaching, overwash, or structural undermining is expected. For
example, a typical cross-section of the Ocean City area shows that elevations
decrease to nearly +8 feet about 200 feet landward of the dune crest. From
Figures 24 through 27, severe overwash and flooding can be expected for the
10.5-foot storm surge of all durations while the 6.5-foot storm surge seems
likely to result in significant overwash for the 96-hour duration storm only.
Summary of Storm Erosion Calculations
Erosion estimates adopted for the reference profile are summarized in
Figure 28 for the range of storm conditions tested. Guidelines for use of
these estimates are summarized as follows:
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-147-
FIGURE 28
ADOPTED STORM EROSION ESTIMATES FOR
REFERENCE PROFILE, OCEAN CITY, MARYLAND
o
o
-------
-148-
(1) Storm erosion estimates are based on the modified
Kriebel and Dean numerical erosion model. The model
has been calibrated and verified, and is considered to
be accurate to within probable errors of +25 percent
and possible errors of +50 percent.
2) The erosion estimates do not account for longshore
sediment transport gradients during a severe storm;
however, based on estimates of the effect of oblique
waves by Swart (1974) and Vellinga (1983b) it is
believed that most longshore effects may be accounted
for by applying the error margins listed above.
3) The erosion estimates do not account for the presence
of structures and assume that the eroding dune is able
to supply sand freely to the active profile. The
influence of shore stabilization structures (i.e.,
groins, seawalls, or revetments) cannot be tested;
likewise, the effect of residential or commercial
structures or paved roads cannot be determined.
4) The erosion estimates do not consider overwash
processes that may move a considerable sand volume over
the dune crest or through breaches in the dune line.
For high dune scenarios, this may not be a critical
omission; however, for low dunes, overwash is known to
be an important process. In numerical calculations, if
the storm surge elevation exceeds the dune crest
elevation, as it may for the 10-ft dune scenario, the
limit of active sediment transport is established at
the dune crest. In these cases, large offshore
sediment transport rates occur seaward of the dune
crest while no sediment transport is assumed landward
of the dune crest. This creates a large sand deficit
at the dune crest which erodes the dune crest and
adjacent contours rapidly, thus planing off the profile
and giving large dune recession predictions. However,
no sand losses in a landward direction are included;
therefore, erosion estimates for low dune scenarios may
substantially underpredict the horizontal extent of the
active profile.
Mitigation Requirements - Storm Erosion
Mitigation requirements are estimated for the reference profile based on
the storm erosion estimates presented in Figure 28 and a simple sensitivity
analysis. In this analysis, beach fill requirements are estimated as the net
volume of sand that must be added above mean sea level in order to prevent
recession of the dune crest. Other mitigation requirements may exist, such as
the elimination of flooding or overwash; however, it is assumed here that the
interest is to prevent erosion of the dune crest.
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-149-
Beach fill designs typically specify placement of sand in the form of an
elevated berm with a linear beach face and a slope near the existing natural
slope. It is well known that waves will tend to reshape the fill slope, such
that some material is deposited offshore and an apparent erosion of the berm
crest occurs. Based on energy dissipation arguments, the fill slope should
achieve a general concave form which will then be in dynamic equilibrium with
respect to the normal wave climate. During this reshaping process some sand
is "lost" from the exposed portions of the beach face; thus, the net volume of
sand added to the berm or dune is less than that which is actually placed.
Other losses may also occur due to the incompatibility of fill material or due
to longshore transport out of the fill area. These effects are not considered
in this analysis.
For storm erosion protection, it is the net sand volume remaining above
mean sea level after adjusting to equilibrium, that provides additional storm
erosion protection. The shape of the fill also may have some impact on the
effectiveness of the fill in preventing recession of the dune crest. In this
study, the general fill requirements are developed independent of initial fill
configuration; specific fill designs should be tested individually to estimate
their effectiveness.
A total of eight beach fill schemes are analyzed to estimate basic fill
requirements. These schemes include the design recommended by the U.S. Army
Corps of Engineers (1980), where a 100-foot wide berm is constructed at
+9 feet NGVD to establish a 200-foot wide beach between the dune slope and the
mean water line. The other seven schemes are selected somewhat arbitrarily
and include designs for a wide natural berm at +5 feet, a wide storm berm at
+9 feet, as well as an addition of sand to the dune face. In each case, the
volume of sand added to the reference profile is allowed to remold to
equilibrium out to a 10-foot depth, after which storm conditions are applied.
A combination of the 24-, 48-, and 96-hour storm surges are applied, with peak
surge elevations of 6.3, 8.7 and 10.3 feet. Results for each run consist of
the erosion characteristics of the 14-foot dune crest.
Guidelines for mitigation are developed by correlating the net sand volume
added above mean sea level after equilibrium is attained to the erosion
potential of the storm on the reference profile without mitigation. In
general, of relatively small water level variations, the net volume of sand
that should be added to the beach face (after remolding to equilibrium) should
be equal to or greater than the volume of sand that is expected to be eroded
from the profile without mitigation. For the storm and profile conditions
tested, this seems to apply for all storm durations for peak water levels of
up to 6.5 feet. For higher peak surge levels, wave uprush reaches higher on
the profile and recession of the upper elevations is more pronounced. For the
8.7-foot peak surge, the net volume of sand added to the beach face should be
at least 30 percent greater than the volume that is expected to be eroded
without mitigation. For the largest peak surge levels, the net volume added
should be about 50 to 60 percent greater than the expected eroded volume
without mitigation. Suggested net fill volumes required to provide protection
for the reference profile for the range of storm conditions considered are
determined as in Figure 29.
-------
-ISO-
FIGURE 29
NET BEACH FILL REQUIREMENTS TO PREVENT DUNE
RECESSION FOR REFERENCE PROFILE, OCEAN CITY, MARYLAND
2400
2000
10
Q
UJ
cr
ID
o
LU
DC
_J
-I
u.
o
UJ
2
§
H
UJ
1600
1200
800
400
20 50 100 200
RETURN PERIOD (Yrs)
500
-------
-151-
According to numerical estimates, the beach nourishment design recommended
by the U.S. Army Corps of Engineers (1980) seems to provide adequate storm
erosion protection for the 10-year storm surge with all possible durations and
the 100-year storm surge for 24-hour durations. For more intense or longer
duration storms, some erosion of the original dune crest, at +14 feet, is
expected. The recommended beach fill initially adds about 1450 ft3/ft to
the profile below the +9-ft NGVD elevation. After remolding to equilibrium
about 930 ft3/ft remains above the still water level to provide protection
from storm erosion. For the 6.3-ft peak surge, the dune crest does not recede
for 24- to 48-hour storm surge levels and recedes 4 feet for the 96-hour storm
as shown in Table 1. For the design storm, with 8.7-ft peak surge, the
24-hour storm causes minor dune recession of 5 feet, while the 48-hour and
96-hour storms cause erosion of 27 and 57 feet, respectively. Dune erosion
for the 10.3-ft peak surge ranges varies from 35 to 52 to 93 feet for the 24-,
48-, and 96-hour storms, respectively.
Based on guidelines for beach fill volumes in Figure 29, complete
protection is provided by the net addition above the still water level of
1000 ft3/ft for the 10-year storm, 1600 ft3/ft for the 100-year storm, and
2050 ft3/ft for the 500-year storm. However, for practical application,
where the probability of a 96-hour duration storm may be small, the Corps'
design does seem to provide reasonable protection from typical duration storms
for the 100-year surge level. The initial fill design and equilibrium fill
configuration are shown in Figure 30. Estimated eroded profiles for the
8.7-foot peak storm surge are shown in Figure 31.
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-152-
TABLE 1
ESTIMATES OF DUNE EROSION POTENTIAL FOR U.S. ARMY
CORPS OF ENGINEERS' RECOMMENDED BEACH FILL DESIGN1
Peak Surge Level Storm Duration
(ft) (hrs)
6.3 24
48
96
8.7 24
48
96
10.3 24
48
96
Erosion of +14-Foot
NGVD
Dune Crest
(ft)
0
0
4
5
27
57
35
52
93
Numerical estimates do not include the storm dune proposed by
the U.S. Army Corps of Engineers at +16 feet NGVD with a 25-foot
crest width.
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-153-
FIGURE 30
INITIAL AND EQUILIBRIUM CONFIGURATIONS FOR
BEACH FILL PLAN RECOMMENDED BY U.S. ARMY
CORPS OF ENGINEERS (I960)
20
O
z
UJ
_)
UJ
10
9 0
-10
200' Beach
Initial Reference
Profile
-20-
Recommended Plan
Corps of Engineers
Ocean City, MO.
I
•Initial Beach Fill Configuration
-Beach Fill at Equilibrium
J_
200
400
DISTANCE (Ft)
600
800
-------
-154-
FIGURE 31
ESTIMATED POST-STORM EROSION PROFILES
FOR RECOMMENDED BEACH FILL PLAN
20
o
z
Ul
10
0
I
Storm Duration
24hrs
48hrs
96hrs
-10
Recommended Plan
Corps of Engineers
Ocean City, MO.
-20
200
Design 8.7 Ft. Peak Storm Surge
Beach Fill at Equilibrium
400
DISTANCE (Ft)
600
800
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-155-
SECTION V
APPLICATION TO OCEAN CITY, MARYLAND --
EROSION DUE TO SEA LEVEL RISE
Background
Estimates of the erosion potential due to long-term change in mean sea
level are obtained by adjusting the numerical model to determine beach profile
response to a new equilibrium given a steady water level that is raised
relative to the original still water level. The basic computational procedure
determines profile changes in a manner comparable to that suggested by Bruun
(1962). The initial profile is characterized by a monotonic Ax2^3 form out
to an effective depth of closure. In this study, two initial profile forms
are established; one with A = 0.175 ft1'3 representing the more mildly
sloping Ocean City beach profiles and the other with A = 0.200 ft1'3
representing the majority of the 1965 and 1978 profiles. Depth of closure is
assumed to be 28 feet. The berm and dune are established according to the
reference profile with an initial dune height of 14 feet.
Numerical computations proceed by raising the water level 1-foot and
allowing the profile to achieve a new equilibrium relative to the increased
water level; this procedure is then repeated for successive 1-foot water level
changes up to a 6-foot increase over the initial level. Final results include
the net volume of sand displaced between the initial and final equilibrium
profile forms, as well as the estimated dune crest recession and the change in
location of the shoreline, defined as the elevation contour which intersects
the still water line at any time. In order to maximize the usefulness of
erosion calculations for comparison to similar work performed by Leatherman
and Everts, results are first presented for basic cases of offshore sediment
transport only. These estimates are then modified by accounting for other
transport processes to obtain more realistic estimates of potential shoreline
adjustment.
Based on Bruun1s method, long-term, spatially averaged profile adjustment
due to sea level rise occurs in two spatial dimensions as profile adjustment
occurs horizontally and vertically. As the nearshore portion of the profile
erodes and shifts landward, deposition is assumed to occur both off shore and
landward such that the original profile form is always maintained relative to
the rising water level. A major assumption is that landward transport of
sediment by overwash and/or aeolian processes occurs in sufficient quantities
to cause the dune and berm to grow vertically as the profile retreats. For
barrier island systems, this implies that the barrier island width and form
are also maintained as the entire island cross-section slowly shifts both
landward and vertically in response to the slowly rising sea level.
While the theory of barrier island migration may be applicable for many
natural areas, there is also evidence of barrier islands "drowning" in place
rather than migrating in response to sea level rise. For highly urbanized
barrier islands, it is likely that the island will not grow vertically, as it
is doubtful that the elevation of the barrier island will be permitted to
increase substantially given the existence of roads and residential or
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-156-
commercial structures. In effect, any sand deposited inland of the dune lines
by overwash or aeolian transport will probably be returned to the beach face
or dune to help prevent further erosion as occurred in Ocean City after the
March 1962 storm. Given this scenario, erosion due to sea level rise will
occur at a slower pace; however, long-term net erosion would cause the barrier
island to narrow and, in effect, drown in place as the sea level continued to
rise.
For Ocean City, the drown-in-place scenario seems likely for increases in
sea level rise that are expected over the next several decades. In this
study, numerical estimates are obtained for a "drowned" profile scenario where
the stepwise numerical procedure is applied to the reference profile with an
initial dune crest elevation of 14 feet. It is assumed that this elevation
will not change appreciably as sea level rise occurs, therefore, the relative
dune height continually decreases. For example, after a 3-foot water level
rise, the effective dune height is 11 feet. The berm elevation and width are
adjusted and maintained at 5 feet above the still water level and 40 feet from
the dune toe. Based on these assumptions, the profile form after a 2-foot
water level rise corresponds to an existing profile with a 12-foot dune crest;
the profile form after a 4-foot water level is identical in form to an
existing profile with a 10-foot dune crest, and so on. Therefore, the storm
erosion potential of eroded profiles are identical to those already developed
for various dune height scenarios. As expected, the magnitudes of dune
erosion increase for future conditions.
Sea Level Rise Erosion Estimates
Based on the "drowned" profile scenario, erosion estimates due to sea
level rises of 1 to 6 feet are presented in Figure 32. Results are presented
for three offshore profile scenarios, based on the two equilibrium Ax2'3
profile forms, as described below.
Case I - The existing profile is in equilibrium, characterized by the
steeper slope with A = 0.200 ft1'3. The profile after sea
level rise also assumes an equilibrium form with A = 0.300
ft1'3.
Case II - The existing profile is in equilibrium, characterized by the
milder slope with A = 0.175 ft1'3. The profile after sea
level rise also assumes an equilibrium form with A = 0.175
ft113.
Case III - The existing profile is not in equilibrium but is
characterized by a steeper slope represented by A = 0.200
ft1'3. The profile form readjusts to its equilibrium form
characterized by A = 0.175 ft1/3 which is then maintained
during sea level rise. This scenario is included to
demonstrate the potential impact of a profile shift from an
assumed artificially steepened state to a more natural mild
equilibrium slope.
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-157-
FIGURE 32
RESPONSE CHARACTERISTICS OF REFERENCE PROFILE
TO RELATIVE WATER LEVEL RISE
UI
10
UI
>
UJ
UI
co
VOLUME ERODED (fr/ft)
~ 6
ui
£
cr
£ 4
ui
>
ui
UI
CO
200 400
SHORELINE CHANGE (ft)
ui
CO
UI
>
UI
UI
CO
200 400
DUNE RECESSION (ft)
-------
-158-
Other assumptions used to generate Figure 32 include: 1) profile
adjustment is due to offshore sediment transport only, 2) eroded volumes are
assumed to be composed of all sand-sized material, 3) erosion is assumed to
respond to the mean rate of sea level rise averaged over a long time period,
and 4) the effect of shore stabilization structures is neglected.
The results in Figure 32 also indicate that, as might be expected, the
cases with A = 0.175 ft1/3 have a greater erosion potential than the case
with A = 0.200 ft1/3; erosion estimates for A = 0.200 ft1/3 are about 83
percent of the erosion estimates for A = 0.175 ft1'3. The differences
between the two estimates is probably less than the errors involved in the
calculations and the absolute differences are so small, in terms of
implications to Ocean City, that either estimate may be considered equally
valid. Erosion curves for Case III are determined by first allowing the
initial steepened profile, with A = 0.200 ft1'3, to evolve into an
equilibrium form, with A = 0.175 ft1/3 at the initial time. This produces a
linear offset between curves for Case II and Case III corresponding to the
sand deficit that may exist off shore due to recent profile steepening. If
this scenario is valid, an erosion potential of 2300 ft3/ft, corresponding
to a potential shoreline recession of 99 feet exists over much of the Ocean
City coastline. The mechanisms for this quantum change in profile form and
position may occur over short time periods during severe storms and/or may
occur over longer periods in response to the steep slope reaching instability
limits. Again, this case is included as a hypothetical scenario; the
equilibrium characteristics of Ocean City beaches are not known with
certainty. In the rest of this report, only Case II is considered in detail.
Corrections to Account for Other Sand Volume Losses
As noted, erosion estimates in Figure 32 are subject to a number of
simplifying assumptions. In order to obtain more realistic estimates of
erosion magnitudes, several corrections should be applied to the estimates
presented. First, since it is known that mud, silt, and clay deposits exist
underneath the active sand layers at Ocean City, results should be modified to
account for the percentage, p, of material eroded which is sand sized and the
percentage, 1-p, which is expected to be removed from the active region after
erosion. During storm erosion, these ancient marsh deposits are sometimes
exposed on the shoreface; but no corrections are used in storm erosion
estimates. During large-scale profile changes to sea level rise, the large
volumes of material eroded from the shoreface will normally include greater
percentages of non-sand-sized sediment.
Given the percentage of material that is sand of a size compatible with
natural sand, the erosion estimates, Ve, in Figure 32 may be modified such
that the total volume of material eroded from the beach face is
Ve
Vt = —
P
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-159-
and the total recession of the shoreline or dune may be found to be that
originally predicted, Re, plus an additional amount due to the loss of fine
sediment:
Rt = R€
where D is the dune crest height and ho is the depth of closure, in this case
assumed to be 28 feet. Note that as the sea level rise increases, D decreases
such that the additional shoreline recession will increase more quickly as sea
level rise continues.
Perhaps the most critical corrections that may be made to the erosion
estimates in Figure 32 are the inclusion of net sand volume gains or losses
over time. Since this analysis is performed along a beach profile of unit
width, corrections to the erosion estimates are easily made if net sand volume
losses are expressed on a volume per linear foot basis. If the net gains or
losses of sediment per linear foot are known to be V^ over some time period,
then the total volume of sand eroded from the beach profile due to sea level
rise and the net gains or losses is found to be
Ve V^
Vt = — + -i-
P P2
where the net gains or losses may be adjusted for the percentage of sand sized
material, p_. If net sand gains are experienced, then p2 = 1.0; if net sand
losses are experienced then p2 may equal p, which is a weighted percentage
over the profile. Net gains or losses to the profile are reflected as a
linear accretion or erosion of the profile over total depth, D+ho, such that
the total expected shoreline recession is:
1 1-p 11
Rt = Re + ( ) Ve + Vp
D+h0
In this discussion, net gains or losses may be attributed to natural and
man-induced causes, including longshore transport gradients, beach
nourishment, overwash, aeolian transport, or offshore losses in deeper water.
Estimate of Historical Shoreline Retreat
Based on data presented by Everts (1984), historical shoreline retreat for
the period 1930-1980 may be estimated. In order that the methods may be
compared directly, input data obtained by Everts (1984) are used to adjust the
predicted erosion estimates in Figure 32. According to Everts, the relative
sea level rise between 1930 and 1980 was 0.59 feet. From Figure 32, using
Case II, this corresponds to a predicted erosion of 370 ft3/ft and an
estimated shoreline change of 31 feet, based only on the profile adjustment to
sea level rise. Based on Everts assumed percentage of sand in the profile,
p = 0.75, an additional volume of 125 ft3/ft is expected, corresponding to
-------
-160-
an additional shoreline retreat of 3 feet. Finally, Everts estimates net
losses to the 102,300-ft region between Ocean City Inlet and Indian River
Inlet, Delaware, to be 1.388 x 107 yd3 over 50 years for an average loss
per linear foot of 3660 ft3/ft. Adjusting this value for the percentage of
sand in the profile, the average eroded volume due to all other losses is 4880
ft3/ft corresponding to an average shoreline retreat of 116 feet based on
the 42-foot total active profile (14-foot dune crest to 28-foot closure
depth). Finally, the total estimated eroded volume is 370 + 125 + 4880 = 5375
ft3/ft with an average shoreline retreat of 31 + 3 + 116 = 150 feet.
The observed average shoreline retreat ranges from about 98 to 115 feet
based on low- and high-range average retreat rates 1.9 ft/yr and 2.3 ft/yr
from Leatherman (1984a) and the U.S. Army Corps of Engineers (1980). However,
since 1961/1962 the shoreline retreat has slowed, seemingly due to groin
construction. By extrapolating the 1930-1961 erosion trends of 3.4 ft/yr to
1980 as suggested by Everts, the shoreline would have retreated 170 feet over
the 50-year period if the shoreline had responded freely. Based on Everts1
method of predicting shoreline retreat due to sea level rise and other volume
losses, a calculated shoreline retreat of 184 ft is obtained. A summary of
shoreline retreat rates, both observed and predicted is given in Table 2.
Table 2. Summary of Historical Shoreline Retreat Estimates
Leatherman (1984a)
Corps of Engineers (1980)
NOS maps, Everts (1984)
Everts estimated (1984)
Kriebel/Dean estimated
Avg. Retreat
Rate
1929 - 1980
1.9 ft/yr
2.3 ft/yr
Avg. Retreat
Rate
1929 - 1961
3.4 ft/yr
Total Retreat
Over
50 Years
98
115
170
184
150
The differences between Everts1 estimated 184-ft retreat and the 150-ft
retreat found in this study appear to be due to calculation procedures and
assumed profile forms. The most noticeable difference between the two methods
is that the Kriebel/Dean method does not consider sand deposition landward of
the dune crest while Everts apparently considers deposition of sand in this
region, thus requiring a larger volume of sand to be eroded from the shoreface.
The predicted values of shoreline recession also account for an estimated
1.4 x 106 yd3 of sand added to the profile in various beach nourishment
projects. Since the 3.4 ft/yr observed erosion rate does not include the
effects of this net addition of sand, numerical estimates should also be made
without this sand volume. After accounting for this sand volume added by
beach nourishment, the total net loss of sand lost from the profile is 5210
ft3/ft over a 50-year period. This corresponds to a 124-ft shoreline
recession which, when added to the 34-ft recession due to sea level rise,
gives a total estimated shoreline recession of 158 feet, somewhat closer to
the 170-ft extrapolated value.
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Calculation of Future Erosion Rates
Estimates of future erosion trends are obtained by combining the result^
of profile adjustment to increased water levels for Case II with the projected
net sand volume losses to the profile. This analysis is carried out for three
sea level rise scenarios, as indicated in Table 3, corresponding to a
continuation of the existing trend and two more severe scenarios developed by
the EPA and modified by Everts to account for subsidence. In order to compare
the results of this study with those of Everts, the net sand volume losses
identified by Everts are also adopted. These include a net loss of 270,000
yd3 per year due to longshore transport out of Fenwick Island and losses of
about 30,000 yd3 to offshore areas. After adjusting for the loss of
non-sand-sized material, net sand volume losses from the profile are taken to
be 106 ft3/ft per year. This loss rate is applied uniformly over the
95-year period from 1980 to 2075. As Everts discusses, however, there are
several estimates of net losses available, and future net losses may vary
considerably from present estimates.
Table 3. Relative Sea Level Rise Scenarios
Cumulative Change in Mean Sea Level Elevation
Current Mid-Low Mid-High
Period Trend Estimate Estimate
1980 - 2000 0.24 ft 0.40 ft 0.55 ft
2000 - 2025 0.53 ft 1.13 ft 1.55 ft
2025 - 2050 0.83 ft 2.14 ft 3.00 ft
2050 - 2075 1.13 ft 3.55 ft 5.05 ft
Figures 33 and 34 indicate the projected future erosion trends based on
the three sea level rise scenarios and the estimated net sand volume losses.
The most striking result of the analysis is the large erosion potential that
exists due to the projected net sand volume losses. Over the 95-year period,
a total loss of 10,070 ft3/ft is estimated for a total loss of 17,600,000
yd3 over the Ocean City shoreline (based on a 46,650-ft total length from
Ocean City Inlet to the Maryland-Delaware state line). In Figure 34, these
net volume losses are assumed to be applied uniformly over the profile
cross-section from the dune crest out to the depth of closure and, while
causing a net retreat of the profile, do not change the shape or vertical
position. A profile retreat of 2.6 ft/yr or 246 feet over the 95-year period
is projected due to net sand losses from the profile.
In contrast to the net sand volume losses, the volume of material eroded
due to sea level rise is not lost from the profile but is simply redistributed
from the beach face to off shore. Numerical estimates indicate that erosion
will occur between the dune crest and a nodal depth of about -7 feet, with
deposition occurring from -7 feet to the depth of closure. From Figures 33
and 34, it is evident that this redistribution of sand erodes a volume of sand
from the beach face that is small compared to the net sand volume lost due to
longshore or offshore transport. However, because this volume is removed from
the beach face and not over the full depth of profile, the shoreline retreat
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FIGURE 33
FUTURE EROSION ESTIMATES DUE TO SEA
LEVEL RISE AND NET SAND VOLUME LOSSES
20,000| 1 r
Reference Profile
Ocean City.MD.
Mid-High Sea Level Rise.
Plus Net Loss
a
LU
a
ui
UJ
5
i
Mid-Low Sea Level Rise,
Plus Net Loss
10,000 -
Current Sea Level Trend,
Plus Net Loss
0
Projected Net Sand
Volume Loss
0
20
J_
40
YEARS FROM
60
PRESENT(1980 Base Year)
80
100
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FIGURE 34
FUTURE SHORELINE RETREAT ESTIMATES
DUE TO SEA LEVEL RISE AND NET SAND VOLUME LOSSES
1000
800
600
a:
UJ
UJ
a:
o
400
200
1 r
Reference Profile
Ocean City, MD.
Mid-High Sea Level Rise^
Mid-Low Sea Level Rise.
Current Sea Level Trend-
Projected Retreat Due to Net
Sand Volume Loss
40 60 80
YEARS FROM PRESENT (1980 Base Year)
100
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due to sea level rise may be comparable to that caused by net sand volume
losses.
A continuation of the existing sea level rise trend is expected to produce
an average erosion of about 10 ft3/ft per yr and a shoreline recession of
about 0.7 ft/yr. The combined effects of this sea level rise trend with net
sand volume losses are an average shoreline recession of about 3.3 ft/yr,
comparable to the 3.4 ft/yr observed from 1929 to 1961. The existing rate of
sea level rise therefore accounts for about 20 percent of the historical
shoreline retreat rate and about 10 percent of the volume of material eroded.
Under the scenarios of accelerated sea level rise, erosion response, and
particularly shoreline response, is increased substantially. Based on the
mid-low range scenario an eroded volume of 2940 ft3/ft and a shoreline
recession of 210 feet are projected in addition to erosion expected due to net
sand volume losses over 95 years. When combined, the net sand volume losses
and the erosion due to the mid-low sea level rise produce a total potential
shoreline recession of about 460 ft, corresponding to a 4.8 ft/yr average.
The mid-high range sea level rise scenario yields a sizeable 4075 ft3/ft
additional eroded volume nearshore with a 305-ft potential shoreline
recession. When combined with net sand volume losses, a total shoreline
recession of 550 ft is expected after 95 years for an average of nearly 5.8
ft/yr. Due to the non-linear erosion response to the accelerated sea level
rise scenarios, projected erosion rates also accelerate over the 95-year
period. Over the first 20 to 50 years, erosion rates are closer to the
existing trend. For example, over the first 50 years erosion rates of 4.0
ft/yr and 4.8 ft/yr are projected for the mid-low and mid-high scenarios
respectively. In later years, erosion rates increase rapidly as sea level
rise accelerates and as the assumed active profile depth decreases due to the
"drowned" profile assumption.
As noted, numerical model does not account for the actual barrier island
cross-section landward of the dune crest and, instead, this area is
approximated by a uniform elevation. It is recognized that this is not a
realistic condition since average island elevations decrease to +8 feet and +4
feet at distances of 200 and 400 feet landward of the dune crest, respectively.
Because of this natural decrease in elevations toward the bayside of the
barrier island, actual shoreline retreat should be increased over values shown
in Figure 34 since a progressively smaller sand reservoir is available in
reality. However, the errors involved are on the order of only 5 to 10
percent for a shoreline retreat of up to 200 feet. Predictions in Figure 34
are therefore assumed to be reasonable for periods of up to 40 years for the
mid-high case, 50 years for the mid-low case, and over 60 years for the
extension of the current trend. Beyond these time periods, conditions are so
uncertain in terms of human response, such as possible beach nourishment, dike
construction, or seawall construction, that errors in predictions of shoreline
retreat are of little concern.
In summary, the impacts of projected sea level rise scenarios are
significant, with the mid-high scenario resulting in a near doubling of the
existing (i.e., pre-1961) erosion rate. Sea level rise erosion is most easily
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interpreted in terms of the erosion potential due to net sand volume losses.
As noted, the existing sea level rise trend has contributed to only 20 percent
of the observed shoreline retreat between 1929 and 1961. For the mid-low and
mid-high sea level rise scenarios, sea level rise accounts for 44 to 55
percent of the total projected shoreline response to the year 2075.
Mitigation Requirements - Long-term Erosion Trends
Mitigation requirements for sea level rise and net sand volume losses are
estimated based on the premise that the existing shoreline position is to be
maintained. Net sand volumes that must be added to the profile to restore the
shoreline position are estimated according to the predicted shoreline retreat
times the active profile depth of about 42 feet. This total volume is larger
than the estimated eroded volume since the equilibrium profile form must be
maintained at all depths, and this cannot be accomplished by simply returning
the eroded volume back to the profile. The shoreline position could
temporarily be re-established by adding the eroded volume back to the
nearshore portion of the profile, but since this would result in a steepened
profile, it would tend to be unstable in the long-term.
In Figure 35, estimated beach nourishment requirements are given for the
range of scenarios considered to the year 2075. The estimated volumes are
quite large and range from a minimum addition of 106 ft3/ft per yr to
replace sand lost due to longshore or offshore transport, to a maximum
addition of 245 ft3/ft per yr for the mid-high range sea level rise
scenario. Total sand volumes that must be added over the 46,650-ft Ocean City
shoreline range from an average of 185,000 yd3/yr to 425,000 yd3/yr for
total volumes of 17,600,000 yd3 to about 40,000,000 yd3 by the year 2075.
These volumes are for fill material that has a grain size distribution
compatible to the existing natural grain size distribution; larger volumes are
required if a fraction of the fill material is too fine to remain stable under
the existing wave climate.
Based on the estimated mitigation requirements, significant quantities of
sand must be added to the area from outside of the active region considered,
bounded by Ocean City Inlet, the Maryland-Delaware state line, and the closure
depth of 28 feet. While a portion of the 17,600,000 yd3 lost from the
system may be recycled by dredging from bounding inlets and returning to
source areas or retained through additional holding structures such as groin
fields, the impact of this remedial action on adjacent beaches outside the
system should be considered. Similarly, the dredging of material from
offshore sand sources should take place outside of the active profile so that
it does not contribute to a sand deficit in offshore regions. In this case,
dredging should not take place landward of the depth of closure.
Finally, it should be emphasized that natural processes may aid in
returning sand back into the system. As the profile retreats landward,
offshore slopes become milder and a broad shelf is created near the closure
depth. As this occurs, a net onshore transport of sand may occur in this
region. This would tend to offset the net sand volume now lost to offshore
areas. This response mechanism is not included in simulations but may slow
profile retreat.
-------
BEACH FILL REQUIREMENTS TO MAINTAIN
POSITION (ft3/ft)
SHORELINE
H"n
°5
m
OH
m T
O
C
73
m
CO
Ol
ON
m
C
CO
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Relative Effects of Long-term and Short-term Erosion Scenarios
The combined effects of long-term profile adjustment, which is
attributable to sea level rise and net sand volume losses, and short-term,
storm-induced erosion are summarized in Figures 36 through 38. In these
examples, the range of erosion characteristics for all storm surge elevation
and duration scenarios is superimposed on the estimated long-term eroded
volume and dune recession for each sea level rise scenario. For this
comparison, dune recession is used instead of shoreline recession since,
during storms, the shoreline position may advance seaward as the beach face is
flattened and as the dune recedes. As noted earlier, storm erosion estimates
range from 500 to 1400 ft3/ft with dune recession of 0 to 136 ft for the
existing reference profile with a 14-ft dune crest. As sea level rise
advances on the dune crest, the relative dune height decreases under the
drowned profile scenario, therefore the potential for dune retreat increases
over time.
For interpretation of erosion effects, Figures 36 through 38 indicate that
the erosion potential of even the worst storm scenario is small relative to
long-term erosion expected over the 95-year period. In terms of eroded
volume, the storm erosion potential that exists today is roughly equivalent to
about 10 years of the more progressive erosion expected due to sea level rise
and net sand volume losses. In terms of dune recession, a severe storm may
cause the equivalent of 30 to 50 years of long-term erosion. It should be
emphasized, however, that while a portion (perhaps all) of the material eroded
during a storm may be returned to the beach face by natural recovery
processes, most sand lost due to longshore transport gradients or sea level
rise is permanently removed from the beach face.
For erosion mitigation, these results suggest that appropriate planning
must address two erosion scenarios on two far different time scales.
Short-term erosion mitigation requirements should concentrate on the potential
impact of severe storms where sudden, dramatic erosion may occur at any time.
By today's standards, mitigation requirements to provide protection against
severe storms represent substantial but not prohibitively large sand volumes.
As an example, mitigation requirements for the storm scenarios tested range
from a net addition of about 1000 ft3/ft to about 2100 ft3/ft for the
reference profile based on Figure 29. Since this is the net addition above
the still water level after the fill has achieved equilibrium, an initial
overfill of about 700 to 900 ft3/ft is required to provide sand necessary to
achieve equilibrium over the normal active profile. As a conservative
estimate, a total addition of 2000 ft3/ft to 3000 ft3/ft is required to
provide storm protection. Over the 46,650-ft Ocean City shoreline, an
addition of 3,500,000 to 5,000,000 yd1 is then required.
When these short-term requirements are compared to long-term mitigation
requirements of 17,600,000 to 40,000,000 yd3, however, the importance of
long-term planning is evident. It has been found that mitigation requirements
estimated on the premise that the current level of storm protection is to be
maintained, are nearly identical to those required to maintain the existing
shoreline position. Although maintenance of the existing dune crest position
does not require as large a volume as maintenance of the existing shoreline
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FIGURE 36
FUTURE EROSION LIMITS DUE TO STORMS AND
LONG-TERM EROSION FOR EXTENSION OF
CURRENT SEA LEVEL RISE TREND
15000
8 10000
cr
ui
i
5000
-Mid-Low Range Sea Level Rise
Ocean City, MO.
Range of Values
for Severe Storms
Eroded Volume Due
to Sea Level Rise
and Net Losses
20 40 60 80
YEARS FROM PRESENT(I980 Base Year)
OO
c 600
(75
CO
UJ
UJ
=>
a
400
200
Range of Values
for Severe Storms
Dune Retreat Due to
Sea Level Rise and
Net Losses ,
20 40 60 80
YEARS FROM PRESENT(I980 BaseYear)
100
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FIGURE 37
FUTURE EROSION LIMITS DUE TO STORMS AND
LONG-TERM EROSION FOR MID-LOW
SEA LEVEL RISE SCENARIO
15000
S 10000
Q
o
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FIGURE 38
FUGURE EROSION LIMITS DUE TO STORMS
AND LONG-TERM EROSION FOR MID-HIGH
SEA LEVEL RISE SCENARIO
15000
8 10000
8
S
UJ
i 5000
§
-Mid-High Range Sea Level Rise
Ocean City, MD.
Ranqe of Values
for Severe Storms
Eroded Volume Due
to Sea Level Rise
and Net Losses
20 40 60 80
YEARS FROM PRESENT( 1980 Base Year)
100
5= 600
UJ
400
200
Range of Values
for Severe Storms
Dune Retreat Due to
Sea Level Rise and
Net Losses,
20 40 60 80
YEARS FROM PRESENT* 1980 Base Year)
100
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position, it has been found that an additional sand volume is required to
raise the "drowned" dune crest back to the 14-ft elevation at the existing
location. These two effects tend to offset each other yielding, for practical
purposes, identical mitigation volume requirements even though fill
configurations are slightly different. Based on the general mitigation
requirements in Figure 35, it is evident that beach fill volumes required to
provide complete short-term storm protection today are small in comparison to
beach fill volumes required to maintain existing levels of protection over 50
or 100 years.
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SECTION VI
SUMMARY AND CONCLUSIONS
Based on numerical analyses of the erosion potential due to severe storms
and various sea level rise scenarios the following general conclusions are
made:
1) The erosion potential of severe storms of 10- to
500-year return periods ranges from 500 to 1400 ft3/ft
eroded above the mean sea level contour with
corresponding probable dune retreat of 50 to 100 feet
and possible dune retreat of 140 feet. These estimates
are for a reference profile with a 14-foot dune crest
height; dune retreat distances increase substantially
for lower dunes.
2) The estimated erosion potential of the March 1962 storm
is 974 to 1129 ft3/ft with dune recession of 58 to 71
feet for the reference profile. These estimates appear
reasonable compared to available qualitative
descriptions of storm damage; however, significant dune
breaching and overwash occurred during the 1962 storm.
These natural occurrences are not simulated in the
numerical model.
3) Based on estimates of the erosion potential of typical
Ocean City profiles, existing dunes will be quickly
eroded or overtopped by most of the severe storms
tested. A useful application of the predictions would
be a mapping of dune erosion, overtopping, overwash, and
storm flooding potential to identify areas in need of
dune restoration.
4) Mitigation requirements for storm erosion are developed
based on the predicted volume eroded during a storm in
the absence of mitigation. In general, safe beach fill
designs require a net addition of 1.0 to 1.6 times the
predicted eroded volume to be placed above the mean sea
level contour. Actual in-place beach fill requirements
are larger due to the natural readjustment of artificial
fill to the incident wave climate. Storm protection for
the 100-year storm surge seems to require initial beach
fill volumes of 3,500,000 to 5,000,000 yd3 of sand
over the Ocean City shoreline.
5) The beach fill plan recommended by the U.S. Army Corps
of Engineers is found to be adequate for design
conditions of the 100-year storm surge with up to
24-hour durations. Although the design is not adequate
for more severe storms, the probability of occurrence of
more severe conditions is small; for practical design
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the Corps' plan seems to be sufficient although periodic
renourishment schemes have not been evaluated.
6) The erosion potential due to three sea level rise
scenarios and net longshore or offshore sand volume
losses result in average shoreline retreat rates of 3.3
to 5.8 ft/yr. These estimates reflect the potential
retreat rates without mitigation measures.
7) Based on extrapolation of existing sea level rise
trends, predicted shoreline erosion rates of 3.3 ft/yr
agree with observed rates of 3.4 ft/yr between 1929 and
1961. Since 1961, groin construction and beach
nourishment seem to have slowed the average shoreline
retreat rate. However, offshore areas have continued to
retreat at historical rates, resulting in what appears
to be an artificially steepened profile.
8) Based on two accelerated sea level rise scenarios, the
potential shoreline retreat rate is slightly greater
than the historical retreat rate over the next 20 to 40
years but accelerates rapidly in later years. While
existing erosion rates are dominated by net longshore or
offshore sand volume losses, erosion rates under
accelerated sea level rise scenarios are almost equally
influenced by net volume losses and profile readjustment
to sea level rise.
9) Mitigation requirements for maintaining the existing
shoreline position are found to be 116 ft3/ft per yr
to 245 ft3/ft per yr on average with total sand
volumes of 19,000,000 yd3 to over 40,000,000 yd3
required over the 46,650-ft Ocean City shoreline by the
year 2075. Since sea level rise scenarios predict
accelerated water level rise over time, mitigation
requirements also increase non-linearly over time.
Non-linear shoreline response over time also occurs due
to the assumed fixed dune crest elevation as sea level
rises.
10) The erosion potential of severe storms is found to be
equal to approximately 10 to 30 years of long-term
erosion due to sea level rise and net volume losses.
However, profiles will recover somewhat after storms,
whereas long-term erosion is permanent. Mitigation
requirements for providing the current level of storm
protection are found to be approximately equal to
mitigation requirements for maintaining the existing
shoreline position. Therefore long-term sand volumes of
19,000,000 to 40,000,000 yd3 are required to maintain
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the current level of storm protection over the 95-year
period. An additional 3,500,000 to 5,000,000 yd3 are
required to provide additional storm protection for the
100-year storm surge.
While storm erosion estimates are believed to be accurate to within
probable errors of +25 percent, erosion estimates due to sea level rise are
considered less accurate. It must be recognized that the ability to forecast
erosion at any location is not precise due to a general lack of long-term data
on beach profile response to sea level rise. For an urbanized area like Ocean
City, apparent steenpening of offshore profiles and attempts to stabilize the
shoreline complicate prediction further. Erosion estimates due to sea level
rise and net sand volume losses confirm that current erosion problems will
accelerate under accelerated sea level rise conditions. However, the absolute
values of the actual shoreline retreat should be considered highly variable;
estimates given in this report represent a best estimate based on current
methodologies and field data.
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REFERENCES
Bakker, W.T., 1968. "The Dynamics of a Coast with a Groyne System." In Proc.
llth Conf. on Coastal Engineering. New York: Amer. Soc. Civil Engineers.
Bretschneider, C.L., 1964. "The Ash Wednesday East Coast Storm, March 5-8,
1962: A Hindcast of Events, Causes, and Effects." In Proc. 9th Conf. on
Coastal Engineering. New York: Amer. Soc. Civil Engineers, pp. 617-659,
Bruun, P., 1962. "Sea-Level Rise as a Cause of Shore Erosion," Journal of
Waterways and Harbors Division, Amer. Soc. Civil Engineers, 1:116-130.
Caldwell, J.M., 1959. "Shore Erosion by Storm Waves," Dept. of the Army,
Beach Erosion Board, MP 1-59.
Chiu, T.Y., 1977. "Beach and Dune Response to Hurricane Eloise of September
1975." Coastal Sediments '77, Amer. Soc. Civil Engineers.
Dean, R.G., 1977. "Equilibrium Beach Profiles: U.S. Atlantic and Gulf
Coasts," Ocean Engineering Report No. 12, Department of Civil Engineering,
University of Delaware.
Dean, R.G. and Chiu, T.Y., Personal Communication, 1984.
Dean, R.G., 1984. "Application of Equilibrium Beach Profile Concepts." In
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Engineers.
Hayes, M.O., 1967. Hurricanes as Geological Agents; Case Studies of
Hurricanes Carla, 1961, and Cindy, 1963, Report on Investigations No.
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Ho, F.P., Tracey, R.J., Myers, V.A., and Foat, N.S., 1976. Storm Tide
Frequency Analysis for the Open Coast of Virginia, Maryland, and
Delaware. NOAA Tech. Memo NWS HYDRO-32. U.S. Dept. of Commerce, National
Weather Service.
Hoffman, J.S., Keyes, D., and Titus, J.G., 1983. Projecting Future Sea Level
Rise: Methodology, Estimates to the Year 2100 and Research Needs.
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Hughes, S.A., 1978. "The Variation of Beach Profiles when Approximated by a
Theoretical Curve," M.S. Thesis, University of Florida.
Hughes, S.A. and Chiu, T.Y., 1981. Beach and Dune Erosion During Severe
Storms, UFL/COEL-TR/043, Dept. of Coastal and Oceanographic Engineering,
University of Florida.
Kriebel, D.L., 1982. "Beach and Dune Response to Hurricanes," M.S. Thesis,
University of Delaware, Newark.
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REFERENCES (continued)
Kriebel, D.L. and Dean, R.G., 1984. "Beach and Dune Response to Severe
Storms." In Proc. 19th Conf. on Coastal Engineering. New York: Amer.
Soc. Civil Engineers.
Kriebel, D.L. and Dean, R.G. , 1985. "Numerical Simulation of Time Dependent
Beach and Dune Erosion," Journal of Coastal Engineering.
Leatherman, S.P., 1984a. "Shoreline Evolution of North Assateague Island,
Maryland," Shore and Beach 52:3-10.
Leatherman, S.P., 1984b. "Historical Bathymetric Comparison, Ocean City,
Maryland." (document provided by Dr. Leatherman, University of Maryland,
College Park).
Moore, B., 1982. "Beach Profile Evolution in Response to Changes in Water
Level and Wave Height," M.S. Thesis, University of Delaware.
Perlin, M. and Dean, R.G, 1983. A Numerical Model to Simulate Sediment
Transport in the Vincinity of Coastal Structures, U.S. Army Corps of
Engineers, Coastal Engineering Research Center, Misc. Rpt. 83-10.
Saville, T., 1957. "Scale Effects in Two-Dimensional Beach Studies." In
Trans. 7th Meeting of Intl. Assoc. of Hydraulic Research, Lisbon.
Swart, D.H., 1974. Offshore Sediment Transport and Equilibrium Beach
Profiles, Publication No. 131, Delft Hydraulics Lab., Delft University of
Technology.
Trident Engineering Associates, Inc., 1979. Interim Beach Maintenance at
Ocean City (prepared for Department of Natural Resources, State of
Maryland).
U.S. Army Corps of Engineers, 1977. Shore Protection Manual, Coastal
Engineering Research Center.
U.S. Army Corps of Engineers, 1980. Atlantic Coast of Maryland and Assateague
Island, Virginia: Feasibility Report and Final Environmental Impact
Statement. Baltimore: Baltimore District, Corps of Engineers.
Vellinga, P., 1983a. Verification of Predictive Computational Model for Beach
and Dune Erosion During Storm Surges, Delft Hydraulics Lab., Delft
University of Technology.
Vellinga, P., 1983b. "Predictive Computational Model for Beach and Dune
Erosion During Storm Surges." In Proc. Coastal Structures '83. New
York: Amer. Soc. Civil Engineers.
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