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.