A. BACKGROUND
Erosion along marine coasts is a worldwide
problem
for two
main reasons: (1) sea
level is gradually rising and (2) many
people live along sea coasts. Lake
shores also experience
erosion, but because lake levels rise
and fall in
response to
rainfall, the severity of lake shore erosion varies over
time.
U.S. Coastal Erosion
B. WAVES
1. WAVE FORMS
a. Wavelength:
b. Wave height:
2. ORIGIN OF WAVES
a. High winds (especially storms)
b. Earthquakes near the coastline
c. Volcanic eruptions in the ocean
3. WAVE MOTION
Waves cause an oscillatory motion as they pass through
deep water.
Wave motion becomes less noticeable with
increasing depth below the water surface. The
depth at
which no
motion can be detected (called the wave base)
is equal to
roughly 1/2 of the
wavelength.
4. WHY WAVES BREAK
As a wave approaches the shoreline, its
wave base will
encounter
the bottom and begin to slow down before the
wave
crest. Eventually the wave crest overrides its base
and
"breaks" within the surf zone.
5. TSUNAMIS
Surprisingly, tsunamis are not very high in the open ocean
(wave heights
are usually between 3 and 6 feet); however,
they have
very long wavelengths (up to 100 miles!) and can
travel very
fast (up to 500 mph). When tsunamis reach the
shoreline,
they can reach heights of 100 feet. Before they
arrive,
tsunamis often cause the ocean to recede, leaving a
deceptively
dry beach! (Historic
tsunamis)
C. SHORELINE PROCESSES
Wave refraction occurs when waves approach an irregular
shoreline. Waves touch bottom and begin to slow down off
the headlands
while they are continue to move ahead in the
bay
areas. This causes the crest line to be
refracted or bent
parallel to
the irregular shoreline. The result is an unequal
distribution
of wave energy that leads to focused erosion
of
the
headlands and deposition in bays. (Eroded
headlands)
Longshore currents are the result of waves hitting a beach
at some angle
to the shoreline. The waves push sand up the
beach face at
an angle, but the backwash is directly down
the beach
face. The resulting net motion of sand is parallel
to the
shoreline.
D. FACTORS AFFECTING EROSION
Although sea level is rising, land elevations at the coastline
can also
rise
or
fall. Reasons for changing land elevations
include:
plate tectonics,
isostatic
adjustments (e.g., Maine ,
Louisiana), and natural or
human-induced land
subsidence.
2. WAVE ENERGY
Wave energy is directly related to wave height, and erosive
power is directly related to wave height
times velocity. The
heights and
velocities of waves often reflects storm energy,
which changes
seasonally (i.e.,
summer vs. winter)
For longshore
currents, the ability to transport sediment is
also directly
related to the angle of approach.
The shoreline's resistance to erosion varies with the type of
material
present at the coast. For example, soft sediments
are more easily eroded
than hard rock. California's
coast.
A sediment budget includes
all sources of
sediment input to
and
sediment loss from any given section of
shoreline. If a
sediment
budget is negative (meaning more sediment is lost
than added),
shoreline erosion is
likely to be a problem.
A recent study of Florida's beaches illustrates this point.
1. DESCRIPTION
Barrier islands (also called barrier
beaches) are long and
narrow
islands that occur parallel to the shoreline. For the
most part
these islands are close to sea
level, perhaps only
10 to 15 feet
above sea level along their dune
lines.
2. ORIGIN
Barrier islands are believed to have formed as a result of
sea level
rise since the time of continental glaciation. The
rising sea
level has pushed sediment toward the coast, and
the sediment
eventually an offshore island.
3. NATURAL PROCESSES
a. Landward Migration: a result of sea level rise
b. Inlet Migration: a result of longshore currents
c.
Washovers: the result of
large storms (Hurricane
Andrew)
The Cape Hatteras Lighthouse saga: map of Outer Banks
Because so many people live along shorelines, much effort
has gone into protecting them from erosion.
These efforts
include engineering structures (which have
proven largely
ineffective) and non-structural approaches
(such as beach
nourishment).
1. SEAWALLS / REVETMENTS
The purpose of
seawalls or
revetments is to protect coasts
from
erosion caused by
direct wave attack. However, the
benefit is
usually temporary, because waves will eventually
undermine the structure (picture of
undermined seawall).
2. BREAKWATERS
Breakwaters protect harbors from wave action, creating
a
quiet water area where
longshore drift can accumulate.
In cases
where longshore drift does accumulate, increased
erosion is
likely in the downcurrent direction.
Groins and jetties are used to
trap longshore drift and thus
widen or
maintain a beach. However, trapping sediments
on one side
of these structures leads to
increased erosion
on the
downcurrent side. (See
Ocean City, MD)
Beach nourishment is an attempt to balance the sediment
budget by
adding sand to a beach.
Sources of sand include
inland
quarries and the
dredging of areas where sand has
accumulated
(e.g., offshore or on the
upcurrent side of an
engineering
structure).
Miami Beach has managed to survive
rising sea level only
by
periodically adding sand to its beach. Other East Coast
communities
have tried the same approach, with varying
degrees of
success (e.g., Ocean City, NJ).
