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OCR for page 40
s
Erects of Sea Level Rise
in the Coastal Zone
Sea level rise wiD have different effects along various portions
of the U.S. coastline depending on conditions such as sediment
type and coastal planform. It Is possible to divide the coasts
into physiographic regions for consideration of their response to
relative sea level rise. For instance, the conditions in Louisiana do
not apply to the coast of Mane because the Mississippi delta region
is very flat, undergoing pronounced compaction and subsidence,
while northern New England is characterized by nonerodible cliffs
and portions are experiencing neotectonic uplift.
The present rise in water level is a complex phenomenon,
including local, regional, ~d global components, as detailed pre-
viously. Shoreline position will respond to the cumulative effect of
vertical motions, termed the relative mean sea level rise, regard-
less of their cause. However, it is instructive to divide the coasts
into regions that will behave In a similar manner due to particular
processes and materials.
The U.S. continental coastline is highly variable in character
but certain regional trends are apparent. Tectonic mapping clearly
indicates the reasons for the prominent differences between the
Pacific coastal range as compared to the Atlantic and Gulf coastal
prams. The tectonically active Pacific run ~ a coast where one
plate is being subducted below another (Inman and Nordstrom,
40
OCR for page 41
EFFECTS O1? SEA LEVEL RISE IN THE COASTAL ZONE
41
1971), resulting in a narrow continental shelf and an essentially
nonexistent coastal plain. This coast is characterized by headlands
and intervening pocket beaches. By comparison, the Atiantic
and Gulf coasts have long been tectonically stable and constitute
trailing edge and marginal sea coasts, respectively.
The AtIantic coastal plain, which extends over 100 miles in-
land, is characterized by a gently sloping surface with gradients of
only several feet per mile near the shore. The prominent landforms
from Long Island, New York to Miami Beach, Florida along the
Atlantic coastal plain are barrier islands. The Gulf coastal plain
exhibits the lowest average relief and gentlest gradients. Barrier
islands are again the dominant coastal landforms, but the chain
is less continuous than on the Atlantic Coast, as broken by the
Mississippi River deltaic sediments of Louisiana, the marshy out-
crops along the northeast Gulf coast of Florida (Tanner, 1960),
and the broad outflowing of the Everglades along the limestone
rocky coast of the southern peninsula of Florida.
These three coasts can be further subdivided into physio-
graphic regions on the basis of geologic history and coastal mor-
phodynam~cs. Basically 11 types of coasts can be defined for the
U.S. contment (Figure 5-1) using a modification of the ciassifica-
tion by Shepard and WanIess (1971~. The glaciated coast extends
from northern New Jersey to Mane. This physiographic region can
be further subdivided into the erosion-res~stant crystalline (gran-
ite) rock of northern New England and the mostly unconsolidated
glacial tiD of southern New England. While coastal barriers have
developed by spit growth across many embayments, the main-
land is often fairly high near the shore, forming cliffs. The low
areas, which are subject to storm surges, are clearly marked on
topographic maps.
The coastal compartment barrier chains (Swift, 1968) of New
Jersey and the Delmarva Peninsula are characterized by four sec-
tors: (1) terminal north spit, (2) low, eroding headland, (3) long
barrier islands backed by open lagoons, and finally to the south (4)
short, stubby barriers with marsh-fired embayments. Within these
compartments, there is variable shoreline vulnerability in response
to cliffering rates and patterns of shoreline erosion (Leatherman
et al., 1982), storm surge flooding, and inlet breaching potential.
The south shore of Long Island, New York can also be included
in this sector since the glacial outwash plain there has a similar
physiography to that of the Atiantic coastal plain.
OCR for page 42
42
RESPONDING TO CHANGES IN SEA LEVEL
/1 ~ ~' \ 1
W~
If.
~ -at/
; ~ _ /% Cuspate Coast
~ 1
)0f \~)
w~~W
'A W./
WL ~ \\ __~__~
/ ~ 4 MesoSIdal Coast \
-
-
'` ~ ~ ~ \~\ 5 Stralght Barrler
~ ~~ ~~ Florlda 8 \ L ~\ Islands 0 2~ 4
>\ SO 9 DelSalc ~ Panhandle ~ ~ I ~ ~ l
N~ Hi / Stralght
~ CoasS
Panhandle ~~ ~ .
Barrlers At\
=lA-~_ x~ V
miles
Guii 7~ti j 6 Florlda Keys \
| Peninsula \ \
.~
FIGURE 5-1 The Atlantic and Gulf coastlines classified according to geo-
logical and morphological criteria. Source: Adapted from Shepard and
Wanless (1971~.
The cuspate coast, which is best exemplified by Cape Hatteras,
North Carolina, extends from Norfolk, Virginia to Cape Romain,
South Carolina (Figure 5-1~. The Outer Banks" type barrier
islands form a string of sand that protrudes far from the mainland
coast along the northern part until merging with the mainland at
Myrtle Beach, South Carolina. The historical record shows that
the Outer Banks of North Carolina have been breached by many
inlets (Swift, 1968~.
The mesotidal coast of the Georgia bight extends from south-
ern South Carolina to northern Florida. This physiographic region
has the only tide-dominated barrier islands along the U.S. Atlantic
and Gulf coastal plains. These islands tend to be relatively short
and stubby with marsh-fi~led lagoons; they often display a "drum-
stick shaper (Hayes, 1979~. The famed Sea Islands (Hoyt and
Henry, 1967) are also present in this region. These coastal land-
forms are distinctive by being composed of a Pleistocene core
(often with land surface above the Midyear flood level). Holocene
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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE
43
(modern) sediments are plastered onto the seaward face of the old
Pleistocene barrier or separated from it by small salt marshes and
tidal creeks.
The Sea Islands and mesotidal barriers are more stable than
their m~crotidal counterparts due to the fact that the tidal inlets
are located in pre-Holocene drainage channel, and their locations
shift within narrowly defined limits (Oertel, 19793. That these in-
lets are tidally adjusted to better accommodate storm surge flood-
ing, in combination with the antecedent topography and possibly
some present day river~ne sources of coarse sediment, makes these
islands more stable than barriers along the rest of the Atlantic and
Gulf coasts.
Straight, narrow barrier islands persist from northern Florida
to the Florida Keys, with Cape Canaveral being the only anomaly.
The lagoons are narrow and shallow, and the presence of existing
inlets, maIly of which are stabilized, largely precludes future in-
let breaching. The coast is low and sandy, with only occasional
outcrops of erosion-res~st ant limestone (Anastasia coquina rock
formation) in the beach face.
The Florida Keys are founded on coral reefs, often capped
with coral rubble and, in lirn~ted areas, faced with sandy beaches
produced by the hurricane destruction and wave-abrasion of living
or dead corals in the surf zone. Coral reef platforms are more
resistant to erosion than barrier islands, but their low elevations
make them especially vulnerable to increased flooding and over-
wash with relative sea level rme. However, coral reefs can continue
to grow vertically, which is nature's response to sea level rise, as
long as anthropogenic pollution does not kill these ecosystems.
The Florida Gulf peninsula contains a diverse coast of sandy
barrier beaches and swamps. The Everglades empties directly into
the Florida Bay along a wide front. This physiographic section
is largely controlled by Pleistocene limestone rock, which in some
places outcrops near sea ferret to shelter and allow formation of
coastal swamps and In others lies tens of feet below the present
water surface (Evans et al., 1985~. The swampy coast is little
developed, but the intermittent, low elevation, m}crotidal barrier
islands have been highly urbanized near such population centers
as Tampa.
The Florida panhandle barrier-island system actually extends
to the Mississippi delta off Gulfport, Mississippi. The barriers are
largely Holocene in origin, except for a Pleistocene core, which
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44
RESPONDING TO CHANGES IN SEA LEVEL
constitutes the eastern, bulbous end of Dauphin Island, Alabama.
These beach-ridge dorn~nated barrier islands indicate periods of
past geological accretion and are presently experiencing strong
westward rn~gration due to littoral drift.
The deltaic coast of Louisiana is the most vulnerable to rela-
tive sea level rise of the entire U.S. continental area. The sed~rnents
are largely fine grapnel (silts and clays), very organically rich, and
subject to compaction (and hence suffer subsidence) and erosion
along the shoreline. Much of the Louisiana coast, except for the
small area of active delta building, is retreating landward on the
order of many meters per year (May et al., 1983~. The natu-
ral problem of compaction, predominantly a result of loading by
sediments deposited in the Mississippi delta region, is greatly com-
pounded by the artificial withdrawal of subsurface fluids. Hence,
some areas are sinking at rates of 1 cm/yr or more, drowning salt
marshes and pushing the small sandy barriers as thin sand wedges
over the adjacent back-barrier serpents. The Chancleleur Islands
and Isles Dernieres barrier chains are being fragmented by hur-
ricanes, and it appears that these islands will be lost during the
next 100 years even under the present conditions of relative sea
level rise (PenIand et al., 1985~.
The straight barrier coast of Texas has been wed studied
(McGowen et al., 1977~. The small delta of the Brazos River is the
only major interruption of these very long, but fairly wide barrier
islands. The existing inlets front the mouths of large bays or
rivers emptying into the sea (e.g., Brazos and Rio Grande rivers).
Generally, the great widths (several miles across) of the Texas
barriers, mucrotidal conditions, and shallow lagoons preclude most
inlet activity. Unlike the condition on most of the Atiantic and
Gulf coasts, rivers are still providing some coarse-grained material
(sand) to the open-ocean coast, so that the barrier islands are not
nearly as sand-starved as those found elsewhere. Locally, erosion is
a problem, as at Sargents Beach (Herbich, 1975) and as evidenced
by the lack of beach in front of portions of the Galveston sea wall.
The Pacific Coast of California, Oregon, and Washington is
quite irregular and diverse. There is little to no coastal plain, and
cliffs of resistant hard rock or unconsolidated river-fi~} sediments
predominate along this tectonically active Pacific rim. Within
small embayments, sandy to gravelly spits can grow, but these
landforms are ephemeral, geologically speaking, and are highly
OCR for page 45
EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE
45
unstable features over the short term (Komar, 1976~. The geo-
morphic diversity over short distances largely precludes the typing
of this coast into natural units as physiographic regions. Generally,
there are three sections.
From the Mexican border to Point Conception, California,
the coast is a nearly continuous, albeit very narrow, beach
interrupted by a few headlands.
From central California to the Columbia River, Oregon,
headlands predorn~nate.
From the Columbia River to the Olympic Peninsula, Wash-
ington, the fine-grained, flat-sIoped, sandy beaches are
composed of river-derived material.
The Alaskan Coast can be roughly divided into four physio-
graphic regions (Shepard and WanIess, 1971~:
fjords of the south coast,
Aleutian hard rock islands,
perma£rost-dominated lowlands of the west coast, and
low barriers, spits, and permafrost mainland of the north
coast.
~ assessing the vulnerability of a coastal area to sea level rise,
the best guide is to consider the nature of the sedunents (erosion-
resistant bedrock or unconsolidated sands, gravels, and clays) and
the topography (high to low clips versus low sandy barrier spits).
Additionally, the degree of existing erosion may serve as an index
if future problems result from an accelerated sea level rise.
SANDY COASTLINE:S
Mean sea level is one of the principal determinants of shoreline
position. Swift et al. (1972) suggested that a relationship exists
among several factors: sediment supply, wave energy, sea level,
and shoreline position. Rising relative sea level tends to cause
shoreline recession, except where this trend is offset by an influx
of sediment.
The primary reason that a sea level rise would induce beach
erosion ~ that natural beach profiles are concave upward; this
geometry results in the wave energy being dissipated in a smaller
water volume than without sea level rise, and thus the turbulence
generated within the surf zone is greater. The profile responds
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46
RESPONDING TO CHANGES IN SEA LEVEL
by conforming to a more gentle nearshore slope, which requires
additional sedunent to be eroded from the beach.
Most sandy shorelines worIdw~de have retreated during the
past century (Bird, 1976~. Progradation has been restricted to
coastal areas where excess sediment is supplied by river sources
or where the land is being elevated due to tectonic uplift or iso-
static glacial rebound. Human interference cannot be considered
a primary cause of erosion worldwide since retreat also occurs on
sparsely populated and little-developed sandy coasts (Bird, 1976~.
Such recession could result from an increase in storminess, but this
trend would have to be almost worldwide to account for erosion
on geographically dispersed sandy shorelines. Therefore, in view
of the demonstrated general relative rise of sea level along the U.S.
shoreline, the link between shore retreat and sea level rise is based
on more than circumstantial evidence; it can be stated that the
relationship is causal in nature.
III some areas, it is clear that human actions have caused sum
spatial erosional pressures. Undoubtedly the principal contribu-
tar has been the construction of jettied inlets and the deepening of
channel entrances for navigation. Along shorelines with high rates
of longshore sediment transport, these constructed features trap
sediment at the uplift jetty and, if material dredged from the
navigation channel is not placed on the downdrift beaches, cause
an amount of downdrift erosion equal to the reduction In trans-
port. At some Florida entrances, tens of millions of cubic yards of
dredged material have been placed in water depths outside the lit-
toral system. This has resulted In very high erosion concentrated
downdrift of the entrances. Some of these shorelines were stable
or accret~ng in their natural condition, prior to mIet modification.
Geologic Indicators
The geologic record of the Atlantic and Gulf coastal plains over
the last 10 million years indicates that sea levels have fluctuated
by 200 m or so during this time period. Five distinct transgressive
coastal systems have been identified on the DeLnarva peninsula
from geomorphic and subsurface data. Each was produced during
interglacial high sea levels and range in age from more than 1
minion years to 60,000 years (Demarest and Leatherman, 1985~.
Sedimentologica1 and historical evidence for four minor transgres-
sive phases or pulses, with sea level fluctuations of less than 1 m,
OCR for page 47
EFFECTS OF SEA L,EYEL RISE IN THE COASTAL ZONE
47
during the last 2,000 years have been found along the Eriesland
barrier Glands in the Netherlands (Bakker, 19813. The modern
transgressive pulse during the overall Holocene transgression pre-
sumably began in the eighteenth century, when history indicates
an increase In storm surge damage and coastal flooding.
Long periods of sedimentary accretion resulting in beach
ridges have been arrested or the trend reversed during the past cen-
tury. Teichert (1947) reported that beach ridge formation ceased
slightly more than 100 years ago, and the western Australian
corall~ne shore is now subject to erosion by the sea. In Nigeria,
Pugh (1954) noted that earlier progradation had similarly given
way to retrogradation on sandy shorelines. Bogue Banks, along
the Outer Banks of North Carolina, Is a barrier island composed
of parallel sets of beach ridges, which have prograded seaward
during the past 3,000 4,000 years, but now the beaches are nar-
row and dunes are actively wave cut during annual winter storms
(Steele, 1980~. Similar reversals in trend, from long-term accretion
to recession, have been noted by many investigators working along
sedimentary coots in the United States (e.g., Tanner and Stapor,
1971) and worse (Davies, 1957~.
Onset of the present transgressive pulse, attested to by marked
beach and dune erosion, has varied geographically depending upon
local differences In sand supply ~d wave energy. Information
from 73 correspondents in 39 coastal countries showed that less
than 10 percent of the length of the worId's sandy shorelines
have prograded, more than 60 percent have retrograded, and the
balance have been relatively stable or have shown no consistent
trend during the past century (Bird, 1976~.
Other geologic indicators of shore retreat are woodcut cliffs,
which occur worldwide (Sunamura, 1983~. Exhumation of salt
marsh peat on beach faces indicates upward and landward bar-
rier migration. Most barrier island coasts have been retreating
for at least the past few hundred years, as clearly indicated by
these back-barrier peat outcrops, exposed on the lower beach fore-
shore after severe storms. Peat outcrops have been reported in
widely dispersed areas Tong the U.S. AtIantic and Gulf coasts, in-
cluding Nauset Spit, Massachusetts (Leatherman, 1979b); coastal
Delaware (Kraft, 1971~; Assateague Island, Maryland (Leather-
man, 1979a); Cape Hatteras, North Carolina (Swift, 19683; and
Sargent Beach, Texas (Herbich, 1975~.
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48
RESPONDING TO CHANGES IN SEA LEVEL
TABLE 5-1 National Assessment of Shore Erosion (Jules)
Total
Location Shoreline Erosional Nonerosional
North Atlantic 8,620 7,460 1,160
South Atlantic Gulf 14,620 2,820 11,800
Lower Mississippi 1,940 1,580 360
Texas Gulf 2,500 360 2,140
Great Lakes 3,680 1,260 2,420
California 1,810 1,550 260
North Pacific 2,840 260 2,580
Alaska 47,300 5,100 42,200
Hawaii 930 110 820
Total 84,240 20,500 63,740
SOURCE: U.S. Army Corps of Engineers (1971~.
Historical Records
Historical records also indicate the prevalence of shore reces-
sion during at least the past century. The National Shoreline
Study by the U.S. Army Corps of Engineers (1971) was the first
overall national appraisal of shore erosion problems. Of the over
84,000 miles of United States ocean and Great Lakes shorelines,
significant erosion occurs along 20,500 miles or 25 percent of the
total (Table ~1~. Excluding Alaska, it shows that 43 percent of the
shoreline is undergoing significant erosion. It should also be noted
that a significant portion of the shoreline Is categorized by the
U.S. Army Corps of Engineers (1971) as noncritical, which does
not connote nonserious. ~ these cases the problems appear to be
amenable to land use controls and other management techniques
rather than relying upon engineering measures to halt erosion.
More recently, May et al. (1983) have assembled data derived
from aerial photography of the U.S. Geological Survey dating
back to the late 1930s, providing a maximum record of 40-50
years. The National Ocean Service of the National Oceanic and
Atmospheric Administration (NOAA) also has made efforts to
determine the historical rate of shoreline change along portions of
the U.S. continental coast using historical maps and charts (NOS
"T~ sheets). The data base includes most of the rriid-Atiantic
Coast as well as South Carolina and parts of California. These
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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE
49
data aDow for the quantification of historical shoreline changes over
100 150 years of record. The NOS maps show a general pattern
of pervasive shore recession except for local anomalies (Everts et
al., 1983~.
The existing data sets (Table 5-2) have been grouped by state
for comparative purposes (May et al., 1983~. The national average
(unweighted) shoreline erosion rate is 0.4 m/yr. Along the Atlantic
Coast, the average erosion rate is about 0.8 m/yr with the Virginia
barrier islands exhibiting the highest rates of erosion (Leatherman
et al., 1982~. The Gulf Coast states are distinguished by the high-
est average erosion rate in the nation (1.8 m/yr). The delta~c coast
of Louisiana is by far the most dynamic (4.2 m/yr erosion; May
et al., 1983~. The Pacific coastline is essentially stable, although
more than half of the shore is hard rock. The erosion rate can be
tabulates} by landform type for comparative purposes (Table 5-3~.
Table ~3 may be useful In ascertaining the shoreline erosion rate
for a site-specific area, such as along the geomorphically diverse
Pacific Coast.
Ter~n;ques of Projectmg Shoreline Retreat
Due to Sea [eve! Rise
Rising sea level ~ accompanied by a general recession of the
shoreline due to inundation or erosion. Inundation is the sum
mergence of the otherwise unaltered shore, while erosion is the
physical remove of beach material. Direct submergence of the
land occurs continuously through time and is particularly evident
in coastal bays where upland is slowly converted to coastal marsh-
lands. Submergence, however, accounts for only a small portion of
the net shore recession Tong exposed, sed~rnentary coasts (Hands,
1976~.
Several different approaches can be used to mode! the result-
ing shoreline configuration as a function of sea level rise. The
simplest method uses the drowned-vaBey concept (Figure 5-2), in
which preexisting topography along shorelines is considered fixed
and combined with increased sea level to project new shorelines
(Kane et al., 1984~. Slope is the controlling variable: steep-sloped
areas wig experience little horizontal shoreline displacement with
each increment of water level rise, while gently sloping shores will
undergo a much broader area of dooding for a given sea level rise.
This Is the preferred methodology for immobile substrates, such as
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so
RESPONDING TO CHANGES IN SEA LEVEL
TABLE 5-2 Shoreline Erosion Rate Based on Historical Aerial Photographs by State
and Region
Standard
Average Deviation Extreme Shoreline Change Number of
Shoreline of Shoreline Rates (m/yr)a Sample
Change Rate Change Rate Maximum Maximum Data b
Region (m/yr) (m/yr) Accretion Erosion Points
Atlantic Coast -0.8 3.2 25.5 -24.6 510
Maine -0.4 0.6 1.9 -0.S 16
New Hampshire -0.5 -- -0.5 -0.S 4
Massachusetts -0.9 1.9 4.5 -4.5 48
Rhode Island -0.5 0.1 -0.3 -0.7 17
New Yorlc 0.1 3.2 163.8 -2.2 42
New Jersey -1.0 S.4 25.5 -15.0 39
Delaware 0.1 2.4 5.0 -2.3 7
Maryland -1.5 3.0 1.3 -8.8 9
Virginia -4.2 5.5 0.9 -24.6 34
North Carolina -0.6 2.1 9.4 -6.0 101
South Carolina -2.0 3.8 5.9 -17.7 s?
Georgia 0.7 2.8 5.0 -4.0 31
Florida -0.1 1.2 5.O -2.9 105
Gulf of Mexico -1.8 2.7 8.8 -15.3 358
Florida -0.4 1.6 8.8 -4.5 118
Alabama -1.1 0.6 0.S -3.1 16
Mississippi -0.6 2.0 0.6 -6.4 12
Louisiana -4.2 3.3 3.4 -15.3 106
Texas -1.2 1.4 0.8 -5.0 106
Pacific Coast -0.0 1.5 10.0 -5.0 305
California -0.1 1.3 10.0 -4.2 164
Oregon 0.1 1.1 5.0 -5.0 86
Washington 0.5 2.2 5.0 -3.9 46
Alaska -2.4 2.0 2.9 -6.0 69
baNegative values indicate erosion and positive values indicate accretion.
Total number of 3-min grid cells over which statistics are calculated.
SOURCE: May et al. ( 1983).
rocky or armored shorelines, or where the wave climate is subdued,
as on the sheltered coasts of embayments.
Several approaches to shoreline recession that have been em-
ployed to date are largely based on the erosional potential of
sea level rise: (1) extrapolation of historical trends (Leatherman,
1984b), (2) the Bruun rule (Hands, 1981, Weggel, 1979; Bruun,
1962), (3) the sediment budget method (Everts, 1985), and (4) the
dynamic equilibrium model (Dean, 1983~. These methodologies,
including applications and limitations, will be discussed in the
order outlined.
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EFFECTS OF SEA LEVEL RISE IN TWIT COASTAL ZONE
TABLE 5-4 Projected Erosion at Ocean City, Maryland in Meters
(ft) of Shoreline Retreat Relative to Its Current Position
Current Trends
2000
2025
2050
2075
Bruun~ 5 (16) 11 (36) 17 (57) 23 (75)
Everts 21 (68) 47 (153) 73 (238) 99 (323)
Leatherman 12 (39) 26 (85) 41 (134) 56 (182)
Kriebel/Dean 20 (66) 47 (153) 70 (231) 95 (102)
Mid-ran~e Low
Bruuna b 7 (22) 22 (72) 43 (140) 70 (231)
Bruun adjusted 23 (74) 58 (189) 98 (322) 147 (482)
Exerts 26 (84) 73 (238) 132 (434) 215 (205)
Leatherman 20 (64) 56 (182) 105 (345) 174 (571)
Kriebel/Dean NC 55 (180) NC 140 (460)
Mid-ran~e High
Bound b 12 (38) 32 (106) 63 (206) 105 (346)
Bruun adjusted 27 (90) 68 (223) 118 (388) 181 (592)
Everts 29 (95) 83 (273) 156 (511) 268 (878)
Leatherman 27 (89) 76 (250) 147 (483) 249 (812)
Kriebel/Dean NC 66 (216) NC 168 (550)
_
NOTE: NC = not calculated.
’3ruun rule includes only the impacts of sea level rise.
Brulm rule adjusted includes 2.6 ft/yr due to factors
other than sea level rise. Because 2.6 ft/yr is derived from
Exerts, Bruun adjusted is equal to Everts for current trends.
SOURCE: Titus (1985).
61
rates near the end of the time span considered and substantial
potential shoreline changes within the entire time span. The long-
shore losses along this portion of the coastline me believed to be
due to the presence of a nodal point located at South Bethany
Beach (U.S. Army Corps of Engineers, 19803.
BLUFF AND CLIFF RETREAT
While most of the attention by coastal geomorphologists and
engineers has been directed at studying sandy beaches, cliff retreat
is a significant problem along large portions of the nation's coast
(i.e., the Pacific Coast, the Great Lakes, and parts of the New
England and New York coasts). Creases In water level will only
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62
RESPONDING TO CHANGES IN SEA LEVEL
accelerate the erosion rate as has been clearly shown by Hands
(1981) along the Lake Michigan shore.
Elsewhere, the high cliffs of unconsolidated sands and grave]
along outer Cape Cod, Massachusetts are eroding at an average
rate of 2.2 ft/yr based on more than 100 years of field survey data.
Dalrymple et al. (1986) indicate that bluff recession in Chesapeake
Bay is related to the heights of the bluffs and their compositions,
as well as the available wave energy.
Kuhn and Shepard (1981) showed that the unconsolidated
sedimentary cliffs of southern California recede in an episodic
manner, corresponding to rainfall and storm wave attack during
unusually severe winter storms. Thornton et al. (198S) derived an
empirical relation between surge level and wave runup and cliff
retreat based on studies of Monterey Bay, California.
As previously mentioned, cliffs of crystalline rock are essen-
tially stable with response times to sea level rise much longer than
those of sandy shorelines. Thus, for parts of the Pacific Coast and
ahnost Al of the rocky Maine coast, cliff position is unchanged over
historical periods of record. Sunamura (1983) provides a review of
cliff erosion processes.
TIDAL INLETS
Along the barrier island coastlines of the United States, inlets
provide hydraulic connections between the back-barrier environ-
ments and the ocean. In their natural conditions, inlets can mi-
grate along the shoreline, whereas when stabilized by jetties, they
are fixed in position to provide reliable navigation channels.
An inlet can be characterized by its tidal prom, the toted flow
Of water through the inlet from low to high tide, and the amount
of sand moving locally in the littoral transport system. Inlets
with small tidal prisms have little ability to scour and erode sand
transported into the inlet Tom the adjacent shores. Often these
inlets have very pronounced ebb tidal deltas, shallow enough to
permit waves to move sand past the inlet. Inlets with reduced
sediment transport environments or large tidal prisms have ebb
tidal deltas located in deeper water or farther out to sea. In either
case the amount of sand capable of bypassing ~ inlet modified
for unproved navigation is very much less, and severe downdrift
erosion can result (Bruun and Gerritsen, 1960~.
The ebb tidal delta and the flood tidal delta in the backbay
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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE
63
consist of platforms (~wash-platforms) on the ebb tidal delta or
ramps (flood ramps), which are separated by channels kept clear
by the tidal currents.
The size of the ebb tidal delta is roughly proportional to
the tidal prism (Walton and Adams, 1976), and often represents
the trapping of millions of cubic meters of sand unavailable to
the neighboring beaches. The development of new inlets or the
stabilization of existing inlets generally results In the development
of large or larger ebb tidal deltas, impounding greater amounts of
sand, thereby reducing the sand available to the beaches. Artificial
sand bypassing, consisting of a Boating or land-based dredge pump
which Recharges sand onto a downdrift beach, is used at several
inlets (e.g., Lake Worth Entrance, South Lake Worth Inlet, and
Hilisboro ~let, Florida) to augment any natural bypassing of sand;
however, as a general rule a tidal inlet represents a sink of beach
sand. A recent study by the state of Florida (1986) showed that
most of the state's eroding areas were next to tidal miets, implying
that effective bypassing of sand at the inlets would reduce many
of the erosion problems.
The basic effects of sea level change on tidal prism and inlet
cross-sectional area were discussed in general terms earlier In this
report. The magnitude of change in tidal prism in response to sea
level rise is highly dependent on conditions along the bay shoreline.
Bays surrounded by Pleistocene uplands generally have relatively
steep shorelines, so that rising sea level will have only a minor
impact on changes in tidal prism. For example, an estimated 1 m
rise in local sea level at Indian River Inlet, Delaware may cause
only a 2 percent change in prism of Indian River Inlet.
Shallow bays surrounded by extensive wetlands will expand
rapidly in response to a rise both because of the gentle slope
and the deterioration of the marshes in response to water level
increases. Barataria Bay, Louisiana has increased its surface area
about 1~15 percent over the last century in response to about 1
m of local relative sea level rise in that area.
Of perhaps greater importance is the change in sand storage
volume of the ebb and flood deltas. If the prism increases, there
is likely to be a corresponding increase in the volume of these
shoab. Furthermore, as the sea level rises the deltas must grow
in elevation to keep up with the rise, implying that any natural
bypassing of sand will be reduced and that downdrift erosion will
increase.
OCR for page 64
64
RESPONDING TO CHANGES IN SEA LEVEL
Stabilized inlets will be afl3ected strongly by a large sea level
rise. The protective jetties, which retard the ability of the lit-
toral drift to enter the navigational channel and reduce the wave
climate in the channel, will become less effective as they are sum
merged. Also, the stability of the jetties is reduced due to the
aforementioned greater wave heights as a result of sea level rise.
WETLANDS
Wetlands account for most of the land less than 1 m above
sea level. These extensive marshes, swamps, and mangrove forests
fringe most of the U.S. coastline, particularly along the Atlantic
and Gulf coasts. Coastal wetlands serve as nurseries for fish and
shrimp, many birds, and fur-bear~ng animals. They are vital to
coastal recreation, to the maintenance of water quality, and as a
buffer against shore erosion.
Their estimated original extent in the United States was 5
million acres or about 20,200 km2 (7,800 rni2) (Hoese, 1967~. This
acreage has been significantly reduced through a variety of actions
including an early widespread practice of filling marshlands in
urban areas. Wetlands loss has also been caused by other human
actions, such as the construction of canals and waterways and the
diversion of fluvial sediment to the offshore.
III response to this loss, several federal and state programs
have been designed to prevent wetness destruction. Specifically,
Section 10 of the federal Rivers and Harbors Act, Section 404 of
the federal Clean Water Act, and Executive Order 11988 on flood-
plain management all establish permit requirements for actions
affecting waterways and wetlands. In general, the wetlands policy
of both the U.S. Army Corps of Engineers and the Environmental
Protection Agency (EPA) ~ to discourage issuance of a permit for
an activity that would involve alteration of wetlands. However,
the effectiveness of this permitting process has been questioned.
The congressional Office of Technology Assessment (OTA) con-
cluded that permit applications for wetlands alteration are still
rarely denied (OTA, 1984~. The continuing human destruction of
wetlands should be kept in mind for the proper perspective when
considering sea level rise and its potential effects on wetlands de-
terioration.
Ecological conditions in coastal marshes range from marine to
nearly terrestrial. A change in controlling factors, such as water
OCR for page 65
EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE
65
salinity or tidal and wave energy, will cause a displacement in
marsh zonation. Generally, coastal marshes are divided into low
and high marsh based on their elevation relative to sea level (Red-
field, 1972~. Since marsh plants are attuned to particular mean
water levels (e.g., spatting patents, salt meadow grass, grows at
mean high tide), a rise in sea level will shift the distribution of
plant species proportionally landward. Beyond this fundamental
response to variation in relative sea level, however, a more com-
plex set of attendant responses may occur, tied to the type of
marsh considered. Thus, anticipated changes in coastal marshes
must be assessed within the context of the basic marsh types that
characterize U.S. coasts.
Marshes have been classified on the basis of the flora present
(Redfield, 1972) and salinity and floristic relations (Chabreck,
1972), and functionally on the basis of geologic/geomorphic pro-
cesses (Stevenson et al., 1986~. Nevertheless, with respect to the
future effects of a rise in sea level, coastal marshes may be broadly
divided into back-barrier marshes, estuarine (brackish) marshes,
and tidal freshwater marshes.
Back-Barrier Marshes
Back-barrier marshes occur along the bay sides of barrier
systems of the Atlantic and Gulf coasts. Studies (e.g., Zaremba
and Leatherman, 1986) show that these marshes are formed and
destroyed rapidly in such dynamic environments. Maintenance of
these marshes is therefore more a function of barrier stability than
the pace of upward growth of the marsh surface, since sediment
supplies are ample (Letzsch and Frey, 1980~. For barriers rapidly
migrating landward, there may be a net decline in back-barrier
marshes. This has been found to be the case at north Assateague
Island, Maryland, where sediment blockage by jetties has greatly
increased the rate of landward barrier migration (Leatherman,
1984a), and the same qualitative result would be anticipated as a
result of accelerated sea level rise.
Estuarine (Brackish) Marshes
Estuarine marshes embrace a wide variety of floristic species
in diverse geologic settings where salinities are less than 30 ppt.
These marshes, comprising integral components of major estuarine
OCR for page 66
66
RESPONDING TO CHANGES IN SEA LEVEL
systems such as the Chesapeake Bay, occur in areas of quiescent
waters and ample sediment supply. Accretionary budgets differ
widely (Table 5-5), but in a dynamic equilibrium condition onsite
production of organic materials and influx of mineral sediments
cause vertical accretion, balancing the loch rate of sea level rise.
In view of the geographic range of the measurement sites and the
local variability within coastal marshes, it is rather remarkable
that the measured sedimentation rates all fall within the same
order of magnitude. Accretion rates are generally found to vary
from 1 mm/yr in high marsh at Duplin River, Georgia to 11 mm/yr
in the Savannah River estuary, Georgia.
The data In Table 5-5 demonstrate that, in general, the mea-
sured accretion rates do exceed the locally determined relative
rates of sea level rise. Consequently, most marshes do receive ad-
equate sediment supply to compensate for current sea level rise.
This must have been the case over the last few hundred years since
"naturals marsh loss has not historically been reported to be a
problem. Three notable exceptions occur at Barn Island, Con-
necticut; Blackwater Marsh, Maryland; and in I,ouisiana, where
the present short-term rates of marsh accretion are lower than the
local rates of sea level rise. ~ Louisiana there is widespread loss of
coastal wetlands, in part attributable to a sediment deficit. Marsh
deterioration is also known In the Blackwater Wildlife Refuge,
Maryland, but no such problems are yet reported at Barn Island.
Exceptionally low local rates of sediment accretion appear to be
the cause in both Connecticut and Maryland.
Tidal Freshwater Marshes
Tidal freshwater marshes are located in the upper reaches of
estuaries and other areas where ambient salinities are less than
5 ppt. The flora of these marshes is varied and lacks the typical
vegetation zonation of open-coast marshes. The effects of rising
sea levels will be saltwater intrusion and the eventual dominance of
higher salt-tolerant plants. However, the effects of canalization on
tidal freshwater marshes in the Mississippi delta demonstrate that
dramatic increases in salinity over a comparatively short period
exceed the capability of these marshes to adjust so that rapid
losses ensue.
OCR for page 67
67
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level rise
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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE
Processes of Marsh Loss with Sea [eve] Rise
69
Land losses in most marshes result from a combination of
mechanisms. Shoreline erosion at the seaward edge of the marsh,
being the most obvious process, could be expected to accelerate
with increased water levels. Nationally, however, shoreline erosion
probably accounts for about 1 percent of all marsh losses annually.
The comparative resistance of marshy shorelines to wave attack
suggests that with rapidly rising sea levels, most marshes will be
Tong since submerged before extensive shoreline erosion occurs.
A more probable catastrophic mechanism of marsh loss with
a large increase In sea levels will be the formation of extensive
interior ponds allied with general tidal creek bank erosion and
headward growth as tidal prisms increase. The rapid enTarge-
ment and coalescence of interior ponds in marshes subject to rapid
coastal submergence has been amply documented in the Missis-
sippi delta (DeLaune et al., 1983) and at the Blackwater Wildlife
Refuge (Stevenson et al., 1986~. The magnitude of marsh losses
from interior poking is instructive. At the Blackwater Wildlife
Refuge in Maryland, over one-third of the total marsh area (about
5,000 acres) was lost between 1938 and 1979 by the growth of in-
terior ponds, largely occurring during a midyear period. The phys-
iolog~cal mechanism behind the development of interior ponds is
believed to be anoxia, and ult~rnate root death of marsh plants, as
sea levels outpace the ability of the marsh to maintain elevation.
Hn~n-~duced Changes
The most dramatic changes ~ wetlands have historically re-
sulted from human alterations. Over half the salt marshes in
New England have been lost because of dredge and fill activities.
Elsewhere, the expanse of marshes has actually been increased by
poor land practices. Early settlers felled large tracts of forest for
agricultural fields, resulting ~ massive siltation of some bays and
estuaries. This pattern Is especially true of the Chesapeake Bay,
where the colonial port at Gunpowder River is now separated from
navigable waters by several miles of intertidal flats, colonized by
marsh grasses.
Present human activities are mainly preventing sediments
from reaching wetlands areas. Moreover, soil conservation prac-
tices through contour plowing, buffer strips, and no-till agriculture
have substantially reduced the influx of soil into adjacent water
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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE
~1
bodies and wetlands. Darns and levees on major rivers trap mate-
ri~ upstream and prevent over-bank flow of muds and fine sand
during flood conditions. This Is a particularly acute problem in
coastal Louisiana, where the marshes have been established on
deltaic sediments and have continued to accrete upward by sedi-
ments delivered during flood stage of the Mississippi River. With-
out these levees and other engineered structures, some cities built
on floodplains, such as New OrIeans, would be subject to massive
and frequent flooding.
Finally, wetlands are being lost ~ coastal Louisiana because
of pipeline and navigation canals that now lace much of the area,
allowing saltwater intrusion. The resulting interactions are com-
plicated, but there is no doubt that this practice has significantly
contributed to the dramatic loss of wetlands presently being expe-
· · T e
rlencec . In Louisiana.
Although salt marshes are protected by federal legislation,
major losses of estuarme marshes can be anticipated in the fu-
ture because of bulkheading along bay shores (Figure 5-4~. With
Holocene sea level rise, these salt marshes have been naturally
translated landward through tune. With the construction of
landward-flank~g bulkheads, which are prevalent along the main-
land bay shores of many coastal states, these marshes will literally
be squeezed out of existence with a sea level rise (Figure 5-4~.
Prospects for Wetlands
The prospect for wetlands is bleak in light of existing con-
ditions and projected changes. The present situation in coastal
I.ouisiana can be used to forecast qualitatively the expected wet-
lands changes elsewhere. Due predominantly to subsidence from
loading by the Mississippi delta and elirrunation of sediment sum
ply by levee construction, the land surface has been subsiding
about 1 cm/yr. Without the influx of massive quantities of in-
organic riverborne sediments, the marsh surface can only accrete
vertically by biogenic production, which is limited. Therefore,
marshes are not able to keep pace with relative sea ieve! rise (over
90 percent due to subsidence at present) and are being drowned in
place. A rapidly subsiding substrate or accelerating sea level rise
can yield similar results. Marsh grasses cannot accrete vertically
fast enough to keep pace with sea level rise. This will likely be
the fate for extensive estuarine marshes elsewhere in the United
States if substantially higher rates of sea level rise are realized in
the future.
