Erosion Of Barriers
The barrier beaches, which line the south
shore of Long Island are in a constant
state of change due to factor's such
as wind, tides and wave action. "The
term 'barrier' identifies one that
protects other features, such as lagoons,
salt marshes, and bays from direct
attack from the open ocean" (Leatherman
1). The pebbles and sand of which
these beaches are made constantly lifted and
deposited in other areas.
Currents created by tides and waves carry sediment and
deposit it on beaches
and in shallow water areas along the shoreline. In this
paper I will discuss
wave activity, the most prevalent factor of beach erosion.
In addition, I
will examine the roll of sand dunes and other ways of preventing
erosion. The
problem which many Long Island beaches face today is the element of
erosion.
More sediment is carried away from the shoreline than is deposited.
These
elements of nature make the barrier beaches very unstable features of
Long
Island's south shore. Wave action, tides, and winds constantly
change beaches
and shorelines, and are the cause of the many devastating
effects of erosion.
The barrier beach which I have chosen to focus my
research on is TOBAY beach, a
beach located just east of Jones Beach. Like
TOBAY, barrier beaches are usually
the result of a sandbar which is built up
and develops as an islands. They are
continuously gaining and losing sand,
and slowly move landward. This is known as
barrier island migration. "Sand
from the ocean side of the barrier is
transported by water and wind toward
the backside of the island so that the
whole landform gradually changes its
location" (Leatherman 47). The
constant movement of sediment from the front
of the beach to the back is known
as rollover. IF the barrier beaches did not
move, they would eventually be
covered by water. In a way, barrier island
migration acts against the effects of
erosion. (McCormick 23) Wave action is
a major cause of beach erosion. The
strength of these waves is determined by
the weather. "Waves are created by
the wind blowing over the surface of the
water, transferring the energy of
motion from air to water" (Press 421). Calm
weather creates smaller waves
which approach the shoreline at regular
intervals. Stormy weather creates large
waves which move at high speeds. The
power if wind creates the power the waves.
If week winds prevail, small
ripples form. However, if heavy winds persist,
large, destructive waves are
created. "The height of waves increase as the
wind speed increases, the wind
blows for longer times, and the distance over
which the wind blows is
increased" (Press 421). The breaking of waves is an
important aspect of wave
action. Since waves greatly effect erosion and the
condition of our beaches,
it is important to understand how certain waves
"break". As a wave moves
closer to a shoreline they break. The wave
breaking action occurs when the
wave becomes so steep it can no longer support
its weight. "Gently sloping
bottoms cause waves to break further away from
the shore, and steeply sloping
bottoms make waves break closer to the
shore" (Press 423). Breaking waves fit
the coastline carrying sand away and
breaking up solid rocks. After the wave
breaks, its height is reduced. This
spreads more water into direct contact
with the sand. This is when erosion takes
place. When the water spreads
across the sloping front of the shoreline, it is
called swash. Swash creates
a backwash of water flowing back into the ocean.
This backwash carries
sand along with it. "Fine sand can be moved by wave
action in water up to
about twenty inches deep" (Press 423). This process
is not only capable of
carrying sand grains, but small rocks as well. At the
beach, I built a mound
of sand up near the water's edge. As waves came and
smothered my pile, the
mound began to get smaller and smaller. Within 5-6 waves
there was no trace
of any mound that I had built. This experiment portrays
"swash" and how sand
is carried away. Another important aspect of wave
action is refraction. Wave
refraction is the bending of waves as they approach
the shoreline from an
angle. The shallow bottom, the part of the wave closest to
the shore, hits
first causing the wave particles to become more elliptical.
Thus, the
wave and its particles are slowed down. This process continues as the
wave
approaches the shoreline, and cause the wave to bend its face landward.
The
swash then runs up the beach at an angle, and runs down in a similar
manner. The
repetition of this process results in the displacement of sand
particles. Long
shore drift is the cause of this displacement. Long shore
drift and long shore
currents working together are potent processes in the
transport of large amounts
of sand on beaches and in very shallow waters"
(Press 425). Long shore
current is a shallow watered current which runs
parallel to the shoreline. It
results from the angled nature of the waves'
approach. The problem which the
beaches of Long Island face is the movement
of sand westward from Montauk Point.
The currents which flow parallel to
the shoreline run westward and carry
sediment to beaches farther down the
coast. The sediment will eventually build
itself up again, but their size,
position and location will not be the same. The
landward motion of rollover
will eventually cause part of the beach to be lost
to erosion (Leatherman
47). In reaction to the drastic amounts of erosion which
have taken place,
many efforts have been made to prevent this from continuing.
Natural
resistance to the effects of the tides and waves comes from sand
dunes.
These large piles of sand line the back of the beach. They provide
protection
from eroding winds and provide a supply of sand to the blown down
and replace
the sand lost to wave activity on the shoreline. Dunes are often
held in place
by the vegetation which grows on them. This vegetation, known
as American
Beachgrass, stabilizes sediment which moves along the coast.
Its strength lies
in its roots, which spread wide beneath the sand and help
give rise to new
plants. The vigorous growth of American Beahgrass makes it
able to grow up
through heavy deposits of sand. Another form of dune
stabilization lies in the
deposition of discarded Christmas trees in the
dunes. This is a fairly new
practice which I have witnesses and taken part in
past years. The trees are
placed on top of the sand dunes, and over time
their branches become covered
with sand deposited by wind and water. In
actuality, he tree's branches have an
effect similar to that of American
Beachgrass. In addition to the protection
provided by the dunes, many
man-made structures provide alternatives. The
practice of creating these
structures is known as shoreline stabilization. The
most common examples of
these structures are jetties and groins. Standing on the
beach, I recognized
that Jetties seem to prevent the occurrence of Long Shore
Drifts. A wave
it only as wide as the Jetties and Groins allow them to be. A
groin is a
shore protection structure built perpendicular to the shore to trap
sediment
and retard shore erosion" (Leatherman 51). Time has proven,
however, that
groins and jetties have negative aspects. An article in The New
York
Times states: Critics say the groins and their longer sister
structure,
Jetties, tend to trap sand moving in currents running parallel
to the shore and
dam it on one side of the structure, creating a fat beach
there and a skinny,
sand starved beach on the other side (New York Times).
Another form of
prevention of erosion is the shore strengthening structures.
Examples include
bulkhead and seawalls. These structures are similar in that
they shield land
from erosion. However, the problem is that they are believed
to deflect wave
energy rather than absorb it. Wave energy is deflected, not
absorb, when it
strikes a hard surface so the force is simply
redirected-energy defelcted
downward, this can scour away sand under a wall.
And storm waves that wash
around sea walls erode property at either end (New
York Times). In effect sea
walls cause erosion to occur on the beach adjacent
to the sea wall. The final
and most recent development in shoreline
stabilization is known as GEOTUBE. The
GEOTUBE is a "huge fabric tube
pumped full of wet sand, creating a
temporary barrier" (New York Times). It
is placed underneath the existing
sand dunes. It has primarily been used to
save highways which run along many
coastlines near beaches. Growing up on
Long Island, I have seen the
deterioration of the beaches over many years. I
never thought twice about what
causes this erosion, until I went and did my
field study. Looking at the ebaches
from a scientific point of view made me
realize that beach erosion is a constant
factor in Long Island's beaches.
Every wave has an effect the condition of
our
beaches.
Bibliography
1. Hanley, Roberts. As beaches Erode,
a Debate on Who'll Oay for
Repairs." The NEW YORK TIMES. 20 Apr. 1998 2.
Press, Frank. Siever,
Raymond. Understanding Earth. New York. W.H.
Freeman and Company. 1998 3.
Leatherman, Stephen P. Barrier Island
Handbook. Maryland. University of
Maryland, 1982.