Seawalls and Beach Erosion
Seawalls and Beach Erosion
Beaches are the slightly sloping portions of land that border oceans and other bodies of water. Seawalls are protective walls or embankments specifically constructed by humans to reduce the destructive force of strong waves, especially during large storms, and to prevent erosion of shorelines, primarily beaches. They may take several structural forms, but generally the ocean-facing side is either curved, straight sloping, or vertical.
Seawalls are constructed from a variety of materials, including reinforced concrete, boulders or stones, steel, rock-filled wire cages, or a combination of these materials. Some seawalls are built in the water, while others are built on the land just off the shoreline. They sometimes extend out from the shore and into the water so that
beach erosion in minimized or prevented. Most seawalls are built to a height that is equal to or greater than the height difference between the beach and the mainland.
Historical Background and Scientific Foundations
Beaches are increased in size by the sand deposited by tides, waves, and winds. However, sometimes, they are reduced in size by these same natural forces. Seawalls have been constructed by humans in the past primarily as a way to protect human occupation in coastal areas, but also to make a clear dividing line between the low-lying beach, left in its natural state, and the higher mainland, which has been developed. Generally, beaches in front of a seawall do not erode, and buildings, roadways, and people behind the seawall are protected from waves crashing onto the shore.
A now famous seawall was constructed in response to the September 8, 1900 hurricane that brought 120 mph (190 km/h) winds to the city of Galveston, on Galveston Island, in Texas. During the storm, tidal waves destroyed most of the city's structures, producing millions of dollars of damage and killing at least 6,000 citizens of the city. A reinforced concrete seawall was built, and the entire city was raised above sea level using sand taken from the Gulf of Mexico. Eight miles (13 km) in length and 17 ft (5 m) in height, the Galveston Seawall resists the strong hurricane winds and enveloping waters of the Gulf of Mexico.
Thus, seawalls are important to the preservation of beaches, which are highly valued economic and environmental areas along coastlines. The United States, alone, contains tens of thousands of miles of beaches, and the majority of its citizens live and work in close proximity to these coastlines. Although seawalls are designed to protect humans and property and prevent erosion, they are not always successful in meeting these goals.
Impacts and Issues
Using computer programs, U.S. scientists have discovered that most of the country's coasts have been eroding for at least 100 years. With the possibility of continued global warming, this erosion is likely to increase as sea levels rise. Seawalls can help to reduce this erosion if properly designed and constructed.
However, even though seawalls are designed to prevent beach erosion, they can actually be the cause of such erosion. If improperly built, the energy of waves hitting a seawall can be reflected onto the beach material underneath and in front of the seawall. These materials soon are eroded unless they are frequently replenishedwithotherbeachmaterials,suchassand. Building seawalls so that the energy of the waves is directed back into the sea helps to minimize such unintentional erosion.
In other instances, beach erosion can be prevented by using rock aprons, which reduce the energy of the waves by forcing water to flow slowly through gaps in the structure. Such actions stop the deluge of water that can flow over long periods of time or during briefer episodes of severe storms. Scientific studies with mathematical modeling of seawalls have found that curved structures are best at eliminating or minimizing beach erosion because wave energy is dissipated back out to sea. Geologists, environmental scientists and engineers, and other professionals continue to develop better ways to construct seawalls and to prevent beach erosion.
WORDS TO KNOW
BEACH: A gently sloping band of rock or shell particles that is bounded by land on one side and a river, lake, or ocean on the other.
EROSION: Processes (mechanical and chemical) responsible for the wearing away, loosening, and dissolving of materials of Earth's crust.
HURRICANE: Large, rotating system of thunderstorms whose highest windspeed exceeds 74 mph (119 km/h). Globally, such storms are termed tropical cyclones: the word “hurricane” is often reserved for tropical cyclones in the Atlantic.
ROCK APRON: Area of crushed rock laid on the ground surface in order to prevent erosion by moving surface water or waves.
TIDES: Daily or twice-daily rise and fall of local sea level. Tides are caused by gravitational and centrifugal forces as the moon and Earth swing around their common center of gravity like a pair of dancers holding hands. The moon pulls water towards itself, creating a moon-facing tide, while centrifugal force dominates on the far side of Earth, causing a second tide directed away from the moon. As Earth rotates on its axis, each point on its surface passes through each of these two tidal bulges once a day, creating two tides at each local point. Because of local variations in shorelines and sea-floor topography, however, not all regions experience the same tidal effects.
Conversely, some environmentalists argue that sea-walls can actually promote beach erosion and that some beaches with seawalls may be eroded faster than ones without seawalls. Although the science behind the proper design and construction of seawalls has not been perfected, geologists contend that if erosion is out of control, seawalls, along with the continued use of replenishment measures (such as the addition of sand), can dramatically help to save property and reduce erosion. Currently, neither side in this debate has sufficient facts to make a definite decision concerning the usefulness of seawalls in preventing beach erosion. However, in most cases, a properly built seawall will help to stop beach erosion, whileanimproperlybuilt onemaycausemore problems than leaving the beach in its natural state without a seawall.
Davis, Richard A. Beaches and Coasts. Malden, MA: Blackwell Publishing, 2004.
Dronkers, Job J. Dynamics of Coastal Systems. Hackensack, NJ: World Scientific, 2005.
Valiela, Ivan. Global Coastal Change. Malden, MA: Blackwell Publishing, 2006.
William Arthur Atkins