Beach and Shoreline
Beach and Shoreline
The beach, also known as the shoreline or strand, is the line of intersection between a standing body of water (an ocean, gulf, lagoon, bay, estuary, or lake) and the surrounding land. The beach may consist of rock fragments (pebbles), sand, or mud (a mixture of sand and clay). The pebbles, sand, and mud may be derived from local bedrock, sediment, or soil, or the beach may be made of local bedrock, which is usually referred to as a rocky shoreline. In some instances, the sand and mud at the shoreline may be organically derived from biologic components in the nearby ocean (for example, broken up shells or coral fragments). There are various parts to a typical beach, including the berm, beach face, and shallow offshore bars. The location of a beach changes with the changing level of the adjacent ocean or lake, or can change with changing land level as well.
Historical Background and Scientific Foundations
Beaches and Their Features
Beaches, especially oceanic beaches and those on large lakes, have a profile that usually consists of a berm (high ridge above the water level), which slopes toward the water. The berm gives way to the beach face, which continues the slope down to the water. Waves break on the beach face and the slope of the beach face changes with the intensity of waves associated with different seasons of the year.
Beyond the beach face is a wide area of sand known as the lower shoreface, which includes one or more parallel sets of offshore bars. The lower shoreface grades imperceptibly into the shallow shelf region of oceans or the more basinal parts of large lakes.
Beaches and Longshore Drift
Waves approaching the beach break upon the beach face at an angle that sends water up the beach face (called
swash) and then runs back down (a process called back-wash). Because waves approach the beach at an angle most of the time on most beaches, the swash is at an angle. However, backwash, which is controlled by gravity, flows directly back to the water. Continual swash and backwash over time moves sediment comprising the beach in a flow direction that is parallel to the beach. This is called longshore drift.
Longshore drift is essential to the continued existence of the beach because sediment is moved from points where it is first placed on the beach (for example at the mouth of a river or stream) to points farther down the beach. Anything that interrupts this flow can deprive the beach of sediment and cause its destruction.
Beaches and Other Processes
In addition to changes in relative water-land level and sediment supply, beaches are affected by other processes including large cyclonic storms (hurricanes and tropical storms), tides, and tsunamis. Cyclonic storms most often affect beaches on the eastern sides of continental land-masses and areas at moderate to low latitudes. The passage of such storms, which is usually accompanied by a surge of high water and strong waves, can redistribute beach sediment in such a way as to reconfigure the beach in a very short time.
This is particularly true with barrier-island beaches, a type of beach that exists on a narrow sand island, which is detached from the mainland by a lagoon or bay. Such low-lying sand islands are particularly susceptible to devastation by storm surges and waves. Tides, the continual ebb and flow of water due to celestial gravity, also affect beaches and their processes. Tsunamis, or seismic sea waves (called seiche in large lakes), can also cause sudden erosion of beaches and their reconfiguration.
Impacts and Issues
Beaches have existed on Earth since the first water accumulated on the surface a few billion years ago. There are many examples of ancient beaches, which are preserved in ancient sedimentary rock. The action of waves washing on the beach face produces distinctive fine layers in the beach sediment, which is in turn preserved in sedimentary rock. Continuous changes in the world's sea level over time have resulted in widespread rock formations that contain ancient beach deposits. Today, sea-level change is causing movement and relocation of modern beaches. To a lesser extent and in smaller areas, land-level changes affect beaches as well.
WORDS TO KNOW
: Portion of a beach that slopes downward to the water.
: A platform of wave-deposited sediment that is flat or slopes slightly landward.
: Solid unconsolidated rock and mineral fragments that come from the weathering of rocks and are transported by water, air, or ice and form layers on Earth's surface. Sediments can also result from chemical precipitation or secretion by organisms.
: A standing wave in a body of water (pronounced SAYSH). In water, a standing wave is a stationary raised mass of water on the surface of an otherwise flat water mass that is sustained by some source of vibration agitating the body of water.
: Ocean wave caused by a large displacement of mass under the surface of the water, such as an earthquake or volcanic eruption.
As Earth's climates change, so do Earth's beaches and shorelines. Beaches and shorelines change location as sea level rises or falls, which may be a response to either climatic warming or cooling. Global warming may intensify tropical storms and hurricanes (typhoons in the Pacific), which impacts beaches and shorelines as well.
Bascom, W. Waves and Beaches. Garden City, NY: Anchor Press/Doubleday, 1980.
Davis, R. A., and D. M. Fitzgerald. Beaches and Coasts. Oxford, UK: Blackwell, 2004.
“Coastal Zones and Sea Level Rise.” U.S. Environmental Protection Agency, October 19, 2006. < http://www.epa.gov/climatechange/effects/coastal/index.html> (accessed November 30, 2007.
David T. King Jr.