|
Items in cart [0] |
Questions? Call 888-624-8373 |
| ||||||||||||
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 20
2
Coastal Erosion:
Its Causes, Effects, and Distribution
INTRODUCTION
This chapter discusses how beaches are formed and factors that
determine coastal erosion, stability, or accretion. It also contains a
summary of U.S. coastline characteristics, which serves to empha-
size the diversity of shore types that must be considered in erosion
management policies.
Historical shoreline changes along the coasts of the United States
range from highly erosional to accretional. Superimposed on these
Tong-term trends, however, can be rapid, extreme erosion caused
by coastal storms from which the shore may or may not recover.
In addition, the high likelihood of significant increases in sea level
also has the potential to affect future shore erosion trends (National
Research Council, 1987a).
A quantitative understanding of these short- and long-term
shoreline changes is essential for the establishment of rational poli-
cies to regulate development in the coastal zone. Shoreline changes
can be due to natural causes or they can be human-induced. Several
common causes of human-induced shoreline change are
poses,
regions,
construction or modification of inlets for navigational pur-
construction of harbors with breakwaters built in nearshore
20
OCR for page 21
CAUSES,EFFECTS,AND DISTRIBUTION
construction of dams on rivers with steep gradients,
sand mining from riverbeds in the near coastal area, and
extraction of ground fluids resulting in coastal subsidence.
21
The human-induced causes are particularly relevant for policy makers
to consider.
Beaches can change on various time scales from short-duration,
dramatic changes to slow, almost imperceptible evolution that over
time yields significant displacements. An important part of the
FEMA program implementation is determination of the long-term
trend of shoreline change. Unfortunately, storm-induced short-term
beach variations can be so large that they may mask Tong-term
trends. Another complicating factor is that at some locations the
shoreline change trend rate itself has changed during the past several
decades; quite often these changes are human-induced. Table 2-1
summarizes the possible natural contributions to shoreline change.
REGIONAL VARIATION
Types of Beaches
The United States has three general types of beaches: pocket,
mainland, and barrier beaches. Beaches are composed of loose sed-
iment particles, ranging in grain size from fine sand to large cow
blest Pocket beaches form between erosion-resistant headlands and
are usually quite small. Pocket beaches are common along the rocky
coast of New England and the cliffed coasts of California and Oregon.
Because the sediment that constitutes pocket beaches is trapped by
adjacent headlands, these beaches respond to prevailing waves; there
is little movement of littoral sediment to or from adjacent beaches.
Mainland (also called strandplain) beaches are the most common
type along the Pacific coast and on the Great Lakes, where the
adjacent bluffs often are over 100 feet high. These beaches develop
anywhere that ample sediment supply allows for accumulation along
the shoreline. The beach usually is derived from the adjacent erodible
chit material. Mainland beaches backed by high eroding bluffs are
well displayed along outer Cape Cod, Massachusetts. Elsewhere,
mainland beaches can be quite low, such as those found in northern
New Jersey and Delaware and along parts of the Gulf coastal plain.
The mainland beaches of Holly Beach, Louisiana, are particularly
Tow lying and susceptible to storm flooding.
OCR for page 22
22
a
V
A_
o ~
~ E
o
Ct
Z
o
U)
Cal
to
C)
Cal
an:
C)
._
—~ C) U) ~ Cal
o ~ Ct ~ o ~
~ U ~ ~ C ~r~ ~ ~ ~ ~ ~ ~ ~ i ~ U
7 ~ ' ~ ; '; ' , ' ~ ~ — , o ~ ~
·~ C) ,c~ ~
o.= ° =~ ~ ~ E E E E E ~ ~ ~ ~ E c ~ ~
U. ~ ou ~ ~ _ ~o _ _ _ _ o o o _ o
~ ~ ~ u~ u~ u~ ~q u: c~ ~ ~ .~
~ £ -= ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
~ C,) ° o o o o o o o C) o o ~ ~ ~
~ V ~ =~= ~ ~ ~ ~ ~ ~ ~ ~V
U'
U' e~
~.o e ~ _ ~ c C ~ c c ° °
C) O O O ~ O O _ G) C~ —O 0— C; ~ O O O O O
-= 0 0 0 () C) 0 0 0 _ ~ O 0 0 0 0
`:Q ~ <: ~ ~ Z
= =
~ ~ ~ :; r~ ~ 3 ~ o c ~ C ' 3 o ~ _ ~ c
U) V) V) U) ~ U) ~ ~ ~ ~ ~ O ~ ~
OCR for page 23
CAUSES, EFFECTS, AND DISTRIBUTION
23
Slope instability is a major concern along erodible mainland
coasts. Slope instability is largely controlled by the local geology,
water level, wave action, and ground water movement. Bluff failure,
concomitant loss of land, and sometimes houses are a continual prob-
lem along outer Cape Cod (Leatherman, 1987), the western shore of
the Chesapeake Bay (Leatherman, 1986), and parts of the California
coast (e.g., San Diego and Los Angeles county beaches). Extreme
instability problems occur along the Great Lakes, where nearly 65
percent of the 16,047-kilometer-Iong shoreline is designated as having
significant erosion (Edil, 1982~.
Barrier beaches are perhaps the most dynamic coastal land
masses along the open-ocean coast. These land forms predominate
the U.S. East and Gulf coasts from Long Island, New York, to Texas.
Barrier beaches can extend continuously for 10 to 100 miles, inter-
rupted only by tide] inlets. Physically separated barrier islands often
are linked by the longshore sediment transport system, so that an
engineering action taken in any one beach area can have major im-
pacts on adjacent downdrift beaches. For example, the south shore
of Long Island, New York, is considered a single littoral cell. The
eroding headlands and mainland beaches at Montauk Point to the
east supply a portion of the sand that moves westward along the bar-
rier beach chain (Southampton/Westhampton beaches, Fire Island,
Jones Beach, the Rockaways) and is eventually deposited in New
York harbor (Leatherman, 1985~. Barrier islands are typically Tow
lying, flood prone, and underlain by easily erodible, unconsolidated
sediments. Thus, these land forms are especially difficult to develop
because they are so dynamic.
BEACH PROCESSES—THE NATURAL SYSTEM
Natural beaches are formed by the accumulation of loose sed-
iment, primarily sand, along the U.S. coasts. Their morphology is
the result of antecedent conditions and sediment supply as well as
the forces of waves, tides, currents, and winds. Beaches respond to
changes in these forces and conditions on time scales ranging from
hours to rn~lennia. A discussion of the formation and processes of
beach change follows.
Beach Sands: Sources and Sinks
Beaches are formed by an accumulation of sediment at the shore-
line. The factors that determine coastal change are the rate of rise
OCR for page 24
24
MANAGING COASTAL EROSION
or fall in sea level relative to the land, the frequency and severity
of storms, and the total volume of sand size and coarser sediments
available in the sand-sharing system. Many coastal regions can be
segmented into compartments; the boundaries are defined by the
geologic features and processes that isolate the transport of littoral
sediments from adjacent coastal compartments. Each compartment
normally is composed of one or more sand sources and sand sinks,
and the beach and nearshore serve as a conduit for the flow of sand
between the sources and sinks.
Many factors are involved In the natured processes that provide
sandy sediment to the coast. Often, the sand sources are local
and transport distances are short; however, sometunes sedunents
are carried great distances before deposition occurs. There are five
general sources of beach sediment: (1) terrestrial, (2) headlands, (3)
shoreface, (4) biogenic production, and (5) the inner shelf. Their
contributions vary with geographic location.
Terrestrial erosion and runoff provide rivers with large quanti-
ties of sediment of widely varying grain size and composition. These
coarse-grained sediments then are carried toward the coast and may
eventually reach the shore and be dispersed to adjacent beaches by
littoral transport processes. However, to be significant sources of
sand, rivers must have fairly high gradients. Many rivers along the
U.S. Pacific coast were major contributors of sand, but dam bui23-
ing has greatly reduced the sediment that reaches the beaches. In
contrast, most rivers along the Atlantic and Gulf coastal plains have
Tow gradients and limited capacities to transport coarse sediment.
Nearly all of the sand entering these rivers and transported seaward
is deposited In their flood plains or estuaries and never reaches the
open-coast beaches; the Mississippi River in Louisiana is an obvious
exception.
Headland and linear bluff areas along coasts offer another major
source of beach sand; wave undercutting and slumping make available
large volumes of sediment for redistribution by wave action. Sand-
size and coarser materials are carried by longshore currents along
the beach, while the finer sifts and clays are transported seaward
into deep water. These finer materials also may be deposited in
backbarrier lagoons if mIets are present.
The shoreface (i.e., the subaqueous portion of a beach) is another
source of coastal sediment. Wave action erodes sand from a beach
and shoreface, and longshore currents transport it downdrift. In this
manner sand is recycled continually as the shore retreats.
OCR for page 25
CAUSES, EFFECTS, AND DISTRIBUTION
25
Calcium carbonate (CaCO3, such as found in shelIs) accounts
for much of the beach sand in Tower latitude tropical and subtropical
regions and can contribute to beaches in middle- and even high-
latitude areas as well. In most cases the shell debris is provided by
organisms living in shallow areas close to the beach. The majority
of carbonate particles are derived from disintegration of calcareous
hard parts of invertebrate fauna such as molluscs, brachipods, corals,
echinoderms, and foraminifera. Some beaches, however, also contain
significant contributions of shell derived from older estuarine or la-
goonal deposits that crop out and are eroded along the shoreface.
The inner shelf offshore of wide and gently sloping coastal plains
also can be an important source of beach materials where there is an
abundance of relict sand on the sea bottom. During the gradual rise
in sea level over the past 15,000 years since glacial times, marine sand
bodies have been eroded and the sediment redistributed by coastal
currents. Over the long term, sand may be moved landward across
the shelf where it can be incorporated eventually into the littoral
system.
In contrast to sediment sources, littoral sinks function to re-
duce the volume of sand along the coast. The most common sinks
to beaches are landward transport of sand through tidal inlets to
form flood-tide shoals, storm-generated overwash deposits, landward-
migrating sand dunes, losses down submarine canyons that extend
close to shore (Pacific coast only), Tosses from sediment abrasion
(largely the CaCO3 fraction), and losses from human-induced causes
such as mining, dredging, and breakwaters and jetties (Dolan et al.,
1987~.
Seasonal Fluctuations
Beaches respond quickly to changing wave conditions. In par-
ticular, steep (i.e., storm) waves formed by a combination of large
wave heights and short wave periods tend to result in seaward sed-
iment transport and shoreline recession. Thus, stormy winter (and
hurricane) waves generally cause erosion, whereas milder and longer
period summer waves promote beach recovery. Thus, beach width
fluctuates on a seasonal basis for many U.S. beaches. These natu-
ral, interannual changes in shoreline position should not be confused
with net long-term changes. Relatively poor documentation exists
to quantify seasonal beach changes around the U.S. coastline; beach
width may fluctuate by 100 feet or more (Johnson, 1971), but the
national average is probably about half of this amount.
OCR for page 26
26
A~4NAGING COASTAL EROSION
Storm-Related Beach Changes
Storm surges also contribute substantially to the beach erosion
process. These above-normal tides are caused primarily by the high
winds (i.e., shoreward-directed wind stress) and the reduced baro-
metric pressures associated with major tropical or extratropical (i.e.,
low pressure) storms. Along the Atlantic and Gulf coast shorelines,
the Goodyear return period storm surges are approximately 12 to 15
feet above mean sea ferret. The Goodyear storm surges along the Pa-
cific shore are much smaller because of the narrow continental shelf.
The largest documented storm surge along the U.S. coast was caused
by Hurricane Camille in 1969, when the water was elevated 22.4 feet
above normal at Pass Christian, Mississippi.
The three most important factors contributing to beach and dune
erosion during storms are (1) storm surge heights, (2) storm surge
duration, and (3) wave steepness (ratio of wave height to length).
Almost all hurricane-induced erosion is limited because the time
scale of the erosion process is shorter than the duration of the near-
peak storm tides. Therefore, only a percent of the potential erosion
actually may be realized. "Northeasters (i.e., severe storms coming
from the northeast along the AtIantic Coast) can last for days and
therefore can achieve their full erosional potential.
Trends of Shoreline Change
The long-term trends of shoreline change depend on a number
of factors, and all the causative processes cannot be quantified at
present. Relevant factors include the antecedent topography (geo-
morphology) and the geologic rise of sea level, which has caused the
shoreline to shift landward across the present-day continental shelf
during the last 15,000 years. In some areas submerged sand on the
inner shelf still is being transported shoreward and thus contributes
to overall shoreline stability or accretion (Williams and Meisburger,
1987~. In other areas there are no offshore sources of sand, and
the slowly rising sea level induces beach erosion. Local land subsi-
dence caused by natural or human-induced processes also can cause
shoreline recession. Finally, the equilibrium beach profile is not well
established along some (particularly glaciated) coasts, and sand is
transported seaward for this reason alone even if there are no other
causes.
Beach stability also must be considered in terms of alongshore
discontinuities, which can cause areas of long-term erosion (e.g.,
OCR for page 27
CAUSES, EFFECTS, AND DISTRIBUTION
27
headlands) to be in close proximity to areas of long-term accretion
(e.g., sand spits). For example, headland erosion along the outer
Cape Cod shoreline supplies the sand necessary for the continued
accretion of Provincetown spit (Leatherman, 1987~. Elsewhere, ero-
sion may be pervasive on one flank of a coastal land form, such as
the severe erosion on the northern section at Cape Hatteras, North
Carolina, while the adjacent southern flank experiences long-term
accretion.
Because of these complexities, the only reliable basis at present
for determining long-term shoreline changes or stability is through
analysis of site-specific data. The methods of obtaining such data
are described in Chapter 6.
Natural Subsidence
Numerous examples of naturally occurring subsidence can be
seen around the nation. For example, in the Mississippi River delta
the weight of the accumulating sediment causes continued com-
paction and sinking. Earthquakes can result in rapid downward
displacement of the land surface. An example of this type of tec-
tonic subsidence occurred during the 1964 earthquake at Homer
Spit, Alaska. This severe earthquake caused differential subsidence
amounting to about 3 feet near the headland attachment and nearly
7 feet at the tip of the spit (Smith et al., 1985~.
Subsidence usually results in a similar impact on the shore: beach
erosion. When nearshore elevations drop, it is equivalent to a sea
level rise of the same magnitude; the beach profile is thrown out of
equilibrium by the creation of a sand sink offshore, and this induces
offshore sediment transport and shore recession.
Stinson Beach, north of San Francisco, California, demonstrates
that beach erosion does not always follow land subsidence if am-
ple sand supplies are available from other sources. During the 1906
earthquake, the land dropped as much as 1 foot and moved 13.5
feet horizontally (Ritter, 1969~. Residents reported that waves over-
topped the spit far more frequently after the earthquake than before,
but because adequate quantities of sand are supplied from contiguous
regions, the spit is relatively stable on average.
Beach erosion at Stinson Beach was severe during the winter
of 1982/1983 because of a series of large storms, several of which
occurred when tides were abnormally high (i.e., spring tides). E!
Nino also contributed to high water conditions. Near the end of
OCR for page 28
28
MANAGING COASTAL EROSION
FIGURE 2-1 Several homes in jeopardy and vertical scarp, western end of
Seadrift, Stinson Beach, California, January 27, 1983. SOURCE: Photo pro-
vided by Robert E. Wiegel.
~ _
,. ~
FIGURE 2-2 Riprap largely covered by accretion of sand, Seadrift, Stinson
Beach, California, November 15, 1987. SOURCE: Photo provided by Robert E.
Wiegel.
OCR for page 29
OA USES, EFFECTS, AND DISTRIBUTION
29
January 1983, several homes were nearly lost (Figure 2-1), and an
emergency-engineered seawall was built. The beach has gradually
recovered, and much of the quarry-rock seawall has been covered
naturally by sand (Figure 2-2~. This example further illustrates the
dynamic nature of the beach, which varies on a daily, monthly, and
yearly basis.
HUMAN-INDUC1DD CHANGES
~lets, Jetties, and Dredged Entrances
Natural channel entrances have a substantial capacity to modify
sediment transport in their vicinity. However, artificially dredged
channel entrances, structurally modified for navigational purposes,
have a much greater potential for affecting the adjacent shores. These
impacts can have a different magnitude depending on the character-
istics of the particular entrance (Table 2-2~. Effects can extend miles
from the entrance and are greatest where there is substantial net
longshore sediment transport. The following three examples illus-
trate how human-induced changes affect beaches.
O C EAN C ITY INLET, MARYLAND
The barrier island breach that later became Ocean City Inlet
was caused by a major hurricane in September 1933. The inlet was
stabilized by jetties soon after the breach occurred. Net longshore
sediment transport is toward the south and estimated at 140,000
m3/year. No sustained effort has been made to carry out a sand-
bypassing program. The impacts on the adjacent shoreline have
been substantial: the immediate downdrift (i.e., south) shoreline has
migrated landward a distance equal to the complete width of the
barrier island in the last 50 years (Leatherman, 1984~. Figure 2-3
shows changes in the Ocean City shoreline from 1931 to 1972 and
presents a sediment budget for the area. All of the factors listed in
Table 2-2 have contributed to these changes, with the exception of
dredge disposal In deep water.
ST. MARY'S ENTRANCE, FLORIDA
The natural river entrance at St. Mary's was stabilized by jetty
construction between 1881 and 1902. The jetties, which are low
and permeable, allow flood waters to flow over them into the inlet.
OCR for page 30
30
MANAGING COASTAL EROSION
TABLE 2-2 Mechanisms by Which Modified Inlets Can Affect the Sediment
Budget of Adjacent Shorelines
Mechanism
Net Deficit to Adjacent Shorelines?
1. Storage against updrift jetties
2. Ebb tidal shoal growth
3. Flood tidal shoal growth
4. Dredge disposal in deep water
5. Leaky jetties
No, balanced by downdrift erosion
Generally
Yes
Yes (very large sand quantities
have already been permanently
lost)
Yes
SOURCE: Modified from Dean, 1989.
During ebb flow the ocean tide is low, and the seaward-directed
flow is confined between the jetties. As a result, there has been a
major alteration of the ebb tidal shoals, which are large depositional
features formed seaward of the inlet by sand transport of the ebb tidal
currents. A total of 90 million cubic meters of sand was removed from
the nearshore and displaced farther offshore in the ebb tidal shoals
(Olsen, 1977~.
P ORT CANAVERAL, F LORIDA
This inlet was cut in 1951, the jetties were constructed in 1953
and 1954, and a beach nourishment project was carried out in 1974.
The net longshore sediment transport has been estimated by the U.S.
Army Corps of Engineers (1967) to be 270,000 m3/year toward the
south. Results of shoreline change rates over the periods 1877-1951
(Ion" term before entrance), 1955-1987 (postentrance establishment
to prenourishment), and 1974-1986 (postnourishment) are presented
in Figure 2-4. Primary impacts include interruption of the longshore
sediment transport, impoundment of the north jetty to capacity, and
offshore disposal of maintenance dredging material.
Sand Disposal Onshore
Disposal of beach-quality dredged sand offshore often results in
a significant adverse impact on adjacent shorelines. The primary
erosional effects occur on the downdrift shoreline, but erosion also
OCR for page 33
CA USES, EFFECTS, AND DISTRIBUTION
33
the beach to attempt to reestablish equilibrium. If sand removed
from the channel is deposited offshore rather than back into the
sand-sharing system, erosion can be the only result.
Wholesale losses of beach sand have been caused by the indis-
criminate dumping of inIet-dredged material onshore. Figure 2-5
estimates the amount of beach-quality sand dredged and deposited
offshore from Florida's east coast inlets. This 56 million cubic yards
would be sufficient to advance the sandy beach about 25 feet seaward
along the entire 375-m~le east coast of Florida.
Sand Mining
The loss of sand from beaches because of mining for construction
and other purposes can be considerable. The effects of sand Coning
are similar to the effects caused when dredge material is deposited
offshore.
California illustrates the scale and impacts of sand and gravel
mining. For example, about 145 million tons of sand was mined in
California in 1972; about 2 percent of this material was derived from
beaches and dunes (Magoon et al., 1972~. It is further estimated that
about 9 million tons has been taken from the south end of Monterey
Bay alone since the inception of the industry; about 500,000 cubic
yards per year is taken from this area (Oradiwe, 1986~. In many
other regions sand is mined from riverbeds, and this material might
otherwise be transported to beaches during periods of flooding. This
mining also can produce a sand deficit, accelerating beach erosion
along the coast.
HI Induced Subsidence
Human-induced subsidence can be caused by extraction of hy-
drocarbons; water extraction for industrial, agricultural, and ~nari-
cultural use; and loading by earth overburden. An example of ex-
treme subsidence caused by hydrocarbon extraction is found in the
Terminal Island-Long Beach region in California (National Research
Council, 1987a) where the ground surface above the center of of!
production subsided about 9 meters (29.5 feet) over 27 years. The
subsidence was finally arrested through the use of water injection,
and some rebound occurred, which in conjunction with pump pres-
sures lifted the surface as much as 33.5 cm (1.1 feet) over an 8-year
interval.
OCR for page 34
34
MANAGING COASTAL EROSION
. _
Berm
( 1 ~=
/
a
St. Mary's Entrance 12.5
St. John's Entrance 20.8
. ~
\
at-- ~ ~ ''.
~r-: ~.~:
~ 1
on ~
) .1:
v' ,: ~
-;
-
0 Boom
0 00o
~ Port Canaveral Entrance 10+
Am_ Sebastian Inlet 1.8
i_ Fort Pierce 2.7
,]
o
Ohio
-_ I sky Worth Entrance R~
) _
I — Hillsboro Inlet 0.5
Port Everglades Entrance 2.8
~ Bakers Haulover 0.2
- - O (1) Quantities in Millions of Cubic Yards
(2) Total in Excess of 56 Million Cubic Yards
FIGURE 2-5 Estimated quantities of maintenance dredging disposed of in the
deep water off the east coast of Florida. SOURCE: Dean, 1989.
Coastal land subsidence is a worldwide problem. Combined wa-
ter and hydrocarbon extraction has been responsible for subsidence
in the Po River Delta, Italy (Carbognin et al., 1984), and in the
Galveston Bay-Houston, Texas, area. As much as 5 feet of subsidence
occurred in the Galveston region between 1943 and 1964 (Gabrysch,
1969~. The south end of San Francisco Bay subsided about 3 feet
OCR for page 35
CAUSES, EFFECTS, AND DISTRIBUTION
35
between 1934 and 1967; subsidence a few miles south of the bay was
as much as 8 feet during the same interval, which was caused by
ground water withdrawal for agricultural use (Seltz-Petrash, 1980~.
Dams
Rivers are a major source of sand for U.S. Pacific coast beaches.
The rate at which sand is transported to the coast increases rapidly
with increased river flows; therefore, infrequent large floods are re-
sponsible for supplying most of the sand to the coast. During floods,
rivers deposit sand in a river mouth delta, and waves and currents
act on this episodic source, gradually transporting the sand along
the coast.
The building of dams for flood and debris control, water supply,
and hydroelectric power reduces the supply of sand available to the
coast. By preventing floods, dam operations substantially decrease
the supply of sand to beaches (National Research Council, 1987b).
The Santa Clara River in southern California illustrates the effects of
dams on sediment transport downstream. The average annual trans-
port of sand and grave] from 1928 to 1975 was estimated to be 0.96
million tons (about 600,000 cubic meters). During the years when
dams operated (1956 to 1975), the estimated average annual deficit of
beach material was 270,000 tons (170,000 cubic meters), about a 28
percent decrease (BrownTie and Taylor, 1981~. Thus, water resource
planners and national decision makers should consider carefully the
effects of dams on the supply of sand to beaches. Increased research
and development of economical means to transport sand to beaches
and alternative operations of dams would provide the information
needed to make future decisions wisely.
Groins, SeawalIs, and Breakwaters
An array of coastal engineering structures have been used with
varying degrees of success to stabilize beaches and control erosion
around U.S. coasts. Their design and utility are discussed in Chap-
ter 3; the problem, however, is that many of these rigid structures
may induce downdrift beach erosion. Because no new sand is cre-
ated, their purpose is to redistribute sand along and across the beach
profile or to prevent further erosion of the coast. The use of prop-
erly engineered structures has proven to be useful where properly
designed, constructed, and maintained; however, their effects on ad-
jacent shores must be carefully evaluated.
OCR for page 36
36
MANAGING COASTAL EROSION
U.S. COASTLINE CHARACTERISTICS
The U.S. continental coast is highly variable in character because
of differences in the geology and coastal processes. The Pacific coast
(including Hawaii and Alaska) is tectonically active and is subject
to earthquakes, volcanic eruptions, and tsunamis in contrast to the
Atlantic and Gulf of Mexico coastal plains and the Great Lakes.
The continental shelf is narrow and in places essentially nonexistent,
which results in minimal attenuation of deep-water wave energy.
Also, the continual passage of low-pressure cells, centered along the
northern Pacific Ocean, generates large oceanic swells. The resulting
wave energy is significantly higher on average than for the Atlantic
and Gulf of Mexico coastlines.
Atlantic Coast
The Atlantic coast ~ composed of two parts: the glacial north-
east coast and the southern coastal plain extending from New Jersey
to Florida. Barrier islands are the dominant coastal land forms in
the southern portion.
The glaciated coast extends from Maine to northern New Jersey.
Scattered small pocket beaches can be found at shoreline reentrants
along the erosion-resistant crystalline rock of northern New England.
In contrast, the cliffs along southern New England and New York
are erodible glacial deposits, with some notable exceptions such as
the rocky headlands at Point Judith, Rhode Island, and Cape Anne,
Massachusetts. These glacial sediments have been shaped by waves
and currents into sandy barriers across embayments. Therefore, the
New England coast is highly irregular, the outer shoreline becoming
more smoothed by headland erosion and barrier beach accretion. For
instance, the relatively small state of Massachusetts has over 1500
miles of open-coast shoreline due to its complex coastal configuration.
This results in a wide range of beach types and susceptibility to storm
waves and surges. In fact, Massachusetts has the largest number of
recreational beaches in New England, but those along the Rhode
Island coast are more urbanized and have suffered severe darnage
during hurricanes.
The mid-Atlantic coast, which extends from New York to Vir-
ginia, is the most urbanized shore in the country except for parts of
Florida and southern California. The recreational beaches in New
OCR for page 37
CAUSES, EFFECTS, AND DISTRIBUTION
37
York and northern New Jersey serve as playgrounds for the 15 mil-
lion people in the greater New York metropolitan area. The New
York to Washington metropolitan corridor exerts heavy demands
for coastal recreational opportunities. As a result, land prices have
soared, and there has been a coastal building boom for the last three
decades. Beach erosion is a chronic problem, perhaps averaging 2
to 3 feet of recession per year along these sandy beaches. Numerous
shoreline engineering projects have been attempted, particularly in
northern New Jersey, to stop the shore recession. Now, however,
planners are shifting emphasis away from rigid structures (e.g., sea-
walis, groins, etc.), and relying more on "soft" techniques such as
beach nourishment.
The U.S. southeastern coast (i.e., from North Carolina to Florida)
is the least urbanized along the Atlantic Coast, but this area has sig-
nificant growth potential because of the availability of beachfront
property. The Outer Banks of North Carolina are a long chain of
barrier islands with development spread out along the shoreline. Al-
though an increasing number of multistory condominiums are being
built, the traditional building is a wooden single-family house that
can be readily moved. Therefore, in this area retreating from the
shore often is more attractive than beach stabilization. This alter-
native is plausible to a lesser extent in South Carolina and Georgia,
but many islands already are too urbanized for this approach (e.g.,
Hilton Head, South Carolina). Also, the barrier islands in the Geor-
gia bight (southern South Carolina to northern Florida) are generally
higher in elevation, much wider, and more stable than the microtidal
barriers found elsewhere along the U.S. Atlantic coast (Leatherman,
1989~.
Florida should be considered separately from the other south-
eastern states. Its long coastline is perhaps the most important in
the United States as it serves as a national and even international
resort area. Recreational beaches are a major source of revenue for
Florida, and state officials are considering spending tens of millions of
dollars each year for beach nourishment. The Miami Beach project,
completed in 1980 at a cost of $65 million for 10 miles of beach,
represents the scale and magnitude of potential future projects along
this rapidly urbanizing coast, which is becoming dominated by high-
density, high-rise-type developments. The southern two counties,
Dade and Broward, have nourished over 68 percent of their total (45
mile) shoreline.
OCR for page 38
38
MANAGING COASTAL EROSION
Gulf Coast
The Gulf Coast is the lowest lying area in the United States and
consequently is the most susceptible to flooding. One of the earli-
est extensive beach nourishment projects undertaken in the United
States was in Harrison County, Mississippi, in the 1950s. The beaches
have narrowed substantially since this time, and renourishment was
required after Hurricane Camille in 1969 and again after Hurricanes
Elena and Kate in 1985.
Louisiana has the most complex coastline in the region and also
holds the distinction of having the most rapid rate of coastal erosion
in the nation. This is largely a result of regional subsidence. The
state of Louisiana has only two recreational beaches: Grand Isle
and Holly Beach. Although Grand Isle recently was nourished, it is
unlikely that the economics (i.e., the relative high cost of sand fill
versus the value of property to be protected) will make such future
projects feasible.
Texas has the most extensive sandy coastline in the Gulf, but
much of the area is not inhabited nor easily accessible. Clearly, the
city of Galveston will be maintained; the nearly century-old seawall
and landfi~! generally have been effective in protecting this urbanized
area. Elsewhere, retreat from the eroding beaches probably is the
most viable alternative because land on the barrier islands is usually
available for relocation.
Pacific Coast
The Pacific coast can be divided into two sections: southern Cali-
fornia and the rest. Southern California, which extends roughly from
Santa Barbara to San Diego, may be the most modified coastline in
the country (although some could argue the same is true of northern
New Jersey). This semiarid area has been transformed into one of
the largest population centers in the United States, and explosive
growth still is occurring. Because of extensive and widespread nour-
ishment projects, many beaches reportedly are wider today than
they were a century ago. The long-term trend of shore recession
has been reversed successfully through coastal engineering projects
(largely beach nourishment), primarily as a by-product of harbor
construction. Considering the value of this real estate, the potential
for continued growth, and the history of coastal projects, these public
recreational beaches undoubtedly will be maintained in the future
(Herron, 1980~.
OCR for page 39
CAUSES, EFFECTS, AND DISTRIBUTION
39
The northern California, Oregon, and Washington coasts can be
divided into regions exposed directly to the waves and currents of the
Pacific Ocean and those within tidal estuaries (e.g., San Francisco
Bay arid Grays Harbor), tidal lagoons, and the Puget Sound-Strait of
Georgia area. The Pacific coast is mountainous and the continental
shelf is narrow (Inman and Nordstrom, 1971~. The major sand
beaches are associated with large rivers. There are also hundreds
of miles of rocky headlands and rugged mountainous regions with
small steep rivers and small narrow beaches, but barrier beaches are
very limited. The beaches may consist of fine sand, coarse sand, or
cobbles, and some are composed of sand with cobbles underneath.
The coast of Oregon is mostly mountainous rugged shoreline, but
there are some sand beaches in the south. The central coastline is
characterized by narrow sand beaches, low cliffs, and marine terraces.
Alternating regions of both rock coast and beaches continue to the
north, where wide, flat-sIoped, fine sandy beaches are associated with
the Columbia River.
The coast of Washington can be divided into three geomorphic
regions: southern, central, and northern. The southern region is
composed of wide beaches; the sand is transported as littoral drift
from the Columbia River mouth. The central region consists of Tong
beaches backed by steep seacliffs. Some sand is derived from the
Columbia River mouth, but most is supplied by five other rivers that
discharge into this region. The northern region is rugged, with high
seacTiffs and small pocket beaches of pebbles and cobbles.
Earthquake faults play an important role in the coastal geology
of the Pacific coast. For example, the San Andreas Fault crosses the
coastline just south of San Francisco and then crosses the coastline
again and forms Bolinas Lagoon. The fault continues northwest,
forms Tomales Bay, and then moves out to sea and back across land,
forrn~ng Bodega Bay. The region is tectonically active, which affects
the relative change in mean sea level. Also, earthquakes can be a
major factor in cliff erosion as is evident from the reconnaissance
survey by members of the U.S. Geological Survey, made a few days
after the October 17, 1989, earthquake (Richter Scale 7.1) centered
in the Santa Cruz mountains (Flinn, 1989~.
The U.S. Pacific coast can be divided into a series of littoral
cells, such as the Santa Monica and San Pedro cells. Sources of
sand within littoral cells can be quite complex. Local rivers and
coastal bluff erosion are obvious sources, together with some biogenic
material. There are two major types of sinks of beach sand along the
OCR for page 40
40
MANAGING COASTAL EROSION
Pacific coast: submarine canyons, such as the Monterey Canyon and
the Scripps Canyon (Shepard and WanIess, 1971), and sand dunes
(Cooper, 1967~.
Natural events sometimes occur that have a major anp act on
the sources of sand in a littoral cell; for example, the Los Angeles
River flowed through Ballona Gap until 1825, when it was diverted
to the south during a severe flood, joining the San Gabriel River
discharging into San Pedro Bay. During severe floods in 1862 and
1884, some of the Los Angeles River waters again flowed to the sea
via Ballona Creek, but since then it has discharged only into San
Pedro Bay (Kenyon, 1951~. Historical changes in this river course
also changed the location of sand discharge from the Santa Monica
Cell to the San Pedro Cell.
Great [ekes
The Great Lakes coasts are composed of a wide variety of shore
types, ranging from high rock bluffs to low plains and wetlands. The
general character of the coasts is related directly to erosional and de-
positional influences of the last period of glaciation. Stream mouths
and shore lakes are a distinctive feature of the coastal corridor bor-
dering the Great Lakes. Stream mouths generally are associated with
low gradient streams and in many cases form freshwater estuaries.
Some shore lakes are drowned river mouths formed by the great melt
of the last Ice Age; others are erosional and/or depositional features
from the same period. River mouths and shore lakes contribute little
sediment to the Great Lakes littoral system. Most sediment com-
prising Great Lakes beaches and transported in the littoral system
comes directly from erosion of coastal bluffs and dunes.
A detailed inventory of shore types that compose the U.S. Great
Lakes has been prepared (Great Lakes Basin Comrn~ssion Framework
Study, 1975~. Classification of shore types was based on shore height,
slope, composition, and erodibility. Shores were simply divided into
nonerodible (e.g., rock bluffs) and erodible (e.g., glacial deposits and
sand dunes) categories. Rates of bluff and dune erosion along Great
Lakes shores vary from near zero to tens of feet per year because of
annual changes in wave climate and lake level.
The U.S. coastline of Lake Superior has approximately 400 miles
of nonerodible shore with areas of steep rock cliffs such as Pictured
Rocks National Lakeshore. The remaining 487 miles of Lake Superior
OCR for page 41
CAUSES' EFFECTS, AND DISTRIBUTION
41
shore is erodible and varies from low-lying clay and grave! bluffs to
sandy bluff-backed beaches.
Lake Michigan's 1,362 miles of shorelands has every shore type
characteristic of the Great Lakes. Most impressive is the expanse of
sand dunes that extend almost continuously from the Indiana Dunes
National Lakeshore on southern Lake Michigan northward along
Michigan's western shore to the Leelanau Peninsula. Large areas of
high erodible bluffs exist along both the Michigan and Wisconsin
shores, which all too often are used as prime building sites because
of their exceptional natural view. By contrast, Lake Huron's 565
miles of coastline is characterized by rocky and boulder areas with
some high cliff-backed beaches; elsewhere, the shore is sandy with
low dunes and bluffs.
Lake Erie with 342 miles of coastline is predominately high and
low erodible bluff. The southwestern area contains wetlands and a
Tow erodible plain. This shore type changes to a low bluff and sparse
dune area in western Ohio, before becoming a high erodible bluff in
central and eastern Ohio. Approximately 12 percent of Lake Erie's
shore is artificial fin.
Lake Ontario's 290 miles of coastline consists of bluffs of glacial
material and rock outcrops at the shore. A bluff shore type fronted
with narrow grave} beaches predominates along the southern shore
of Lake Ontario. Bluff heights range from 20 to 60 feet and are
occasionally broken by low marshes. A short reach of low dunes
and barrier beaches separates this erodible bluff type shore from
the erosion-resistant rock outcrops extending northward to the St.
Lawrence River.
SUMMARY
The U.S. coastline exhibits a great diversity of shore types, and
these variations must be considered when establishing an erosion
management program. Differences in the level of development, use,
and engineering structures at the shore complicate this natural di-
versity. Sediment sources and sinks, which are highly susceptible to
human activities at the shore and in adjoining rivers and waterways,
are also a major concern for erosion zone management. As a result
of these multiple factors, it is necessary to consider both local con-
ditions and broad regional issues when establishing a coastal erosion
zone management program.
OCR for page 42
42
MANAGING COASTAL EROSION
REFERENCES
Brownlie, W. R., and B. D. Taylor. 1981. Coastal Sediment Delivery by Major
Rivers in Southern California. Sediment Management for Southern Califor-
nia Mountains, Coastal Plains and Shoreline, Part C, California Institute
of Technology, Environmental Quality Laboratory, EQL Report No. 17-C.
Carbognin, L., P. Gatto, and F. Marabini. 1984. Guidebook of the Eastern
Po Plain (Italy): A Shore Illustration About Environmental and Land
Subsidence. Published for use at the Third International Symposium on
Land Subsidence, Venice, Italy, March 19-25, 1984. Printed by Officio
Stampa, Comunicazione e Informazione, Comune di Madera.
Cooper, W. S. 1967. Coastal Dunes of California. Geological Society of America,
Memoir No. 104.
Dean, R. G. 1989. Sediment interaction at modified coastal inlets: Processes
and policies. Pp. 412-439 in Hydrodynamics and Sediment Dynamics of
Tidal Inlets, Lecture Notes on Coastal Estuarine Studies, 29, D. Aubrey
and L. Weishar, eds. Berlin: Springer-Verlag.
Dean, R. G., and M. Perlin. 1977. Coastal Engineering Study of Ocean City,
Maryland. Pp. 520-542 in Proceedings of ASCE Specialty Conference on
Coastal Sediments '77.
Dolan, T. J., P. G. Castens, C. J. Sonu, and A. K. Egense. 1987. Review of
sediment budget methodology: Oceanside littoral cell, California. Coastal
Sediments '87, ASCE, Vol. II, pp. 1289-1304.
Edil, T. B. 1982. Causes and Mechanics of Coastal Bluff Recession in the Great
Lakes. Proceedings of Workshop on Bluff Slumping, Michigan Sea Grant
Report 901, pp. 1-48.
Flinn, J. October 26, 1989. Beaches Imperiled by Weak Cliffs. San Francisco
Examiner, pp. A1 and A9.
Gabrysch, R. K. 1969. Land-Surface Subsidence in the Houston-Galveston
Region, Texas. Land Subsidence: Proceedings of the Tokyo Symposium,
September 1969, Paris: UNESCO, Vol. 1, pp. 43-54.
Great Lakes Basin Commission Framework Study. 1975. Shore Use and Erosion,
Appendix 12.
Herron, W. J. 1980. Artificial beaches in Southern California. Shore Beach
48:3-12.
Inman, D. L., and C. E. Nordstrom. 1971. On the tectonic and morphologic
classification of coasts. J. Geol. 79:1-21.
Johnson, J. W. 1971. The significance of seasonal beach changes in tidal
boundaries. Shore Beach 39:26-31.
Kenyon, E. C., Jr. 1951. History of Ocean Outlets, Los Angeles Flood Control
District. Proceedings of First Conference on Coastal Engineering, Long
Beach, California, October 1950. Edited by J. W. Johnson, Council on
Wave Research, The Engineering Foundation, pp. 277-282.
Leatherman, S. P. 1984. Shoreline evolution of North Assateague Island, Mary-
land. Shore Beach 52:3-10.
Leatherman, S. P. 1985. Geomorphic and stratigraphic analysis of fire island,
New York. Marine Geol. 63:173-195.
Leatherman, S. P. 1986. Cliff stability along western Chesapeake Bay, Maryland.
Marine Tech. Soc. J. 20:28-36.
OCR for page 43
CAUSES,EFFECTS,AND DISTRIBUTION
43
Leatherman, S. P. 1987. Reworking of glacial sediments along outer Cape Code:
Development of Provincetown spit. Pp. 307-325 in Treatise of Glaciated
Coasts, D. M. Fitzgerald and P. S. Rosen, eds. New York: Academic Press.
Leatherman, S. P. 1989 (in press). Coasts and beaches. In Heritage of Engineer-
ing Geology: The First Hundred Years. Geological Society of America.
Magoon, O. T., J. C. Haugen, and R. L. Sloan. 1972. Coastal Sand Mining
in Northern California, U.S.A. Proceedings of the Thirteenth Coastal
Engineering Conference, July 10-14, 1972, Vancouver, Canada, ASCE,
Chapter 87, pp. 1571-1597.
National Research Council. 1987a. Responding to Changes in Sea Level: Engi-
neering Implications. Washington, D.C.: National Academy Press.
National Research Council. 1987b. River and Dam Management: A Review of
the Bureau of Reclamation's Glen Canyon Environmental Studies. Wash-
ington, D.C.: National Academy Press.
Olsen, E. J. 1977. A Study of the Effects of Inlet Stabilization at St. Mary's En-
trance, Florida. Pp. 311-329 in Proceedings of ASCE Specialty Conference
on Coastal Sediments '77.
Oradiwe, E. N. 1986. Sediment Budget for Monterey Bay. M.S. thesis. U.S.
Naval Postgraduate School, Monterey, California.
Ritter, J. R. 1969. Preliminary Studies of Sedimentation and Hydrology in
Bolinas Lagoon, Marin County, California, May 1967-June 1968. U.S.
Geological Survey Open-File Report.
Seltz-Petrash, A. 1980. Subsidence a geological problem with a political solu-
tion. Civil Eng. 52:60-63.
Shepard, F. P., and H. R. Wanless. 1971. Our Changing Coastlines. New York:
McGraw-Hill.
Smith, O. P., J. M. Smith, M. A. Cialone, J. Pope, and T. L. Walton. 1985.
Engineering Analysis of Beach Erosion at Homer Spit, Alaska. U.S. Army
Corps of Engineers, Coastal Engineering Research Center, Misc. Paper
CERC-85-13.
U.S. Army Corps of Engineers. 1967. Beach Erosion Control Study on Brevard
County, Florida. Jacksonville, Fla.
Williams, S. J., and E. P. Meisburger. 1987. Sand Sources for the ~ansgressive
Barrier Coast of Long Island, N.Y.: Evidence for Landward Transport
of Shelf Sediments. Proceedings, ASCE Specialty Conference on Coastal
Sediments '87, New Orleans, Louisiana.
Representative terms from entire chapter:
pacific coast
