Coast and Beach

Coast and Beach

Observing erosion and deposition

Emergent coasts

Submergent coasts

The sand budget

Barrier islands

Society and the beach environment

Resources

The coast and beach, where the continents meet the sea, are dynamic environments where agents of erosion vie with processes of deposition to produce a set of features reflecting their complex interplay and the influences of changes in sea level, climate and sediment supply. “Coast” usually refers to the entire region of a continent or island that is significantly

affected by its proximity to the sea, whereas “beach” refers to a much smaller region, usually just the areas directly affected by wave action.

Observing erosion and deposition

Earth is constantly changing. Mountains are built up by tectonic forces, weathered, and eroded away. The erosional debris is deposited in the sea. In most places these changes occur so slowly they are barely noticeable, but at the beach, they are often observable.

Most features of the beach environment are temporary, steady-state features. This means that although features on beaches may appear to be permanent structures, they are constantly in a state of flux. For example, the size and shape of a spit, which is a body of sand stretching out from a point perpendicular to the shore, is similar to the level of the water in this example. To stay the same size, the rate at which sand is being added to the spit must be exactly balanced by the rate at which it is being removed. Failure to recognize this has often led to serious degradation of the coastal environment.

Sea level is the point from which elevation is measured. A minor change in elevation high on a mountain is undetectable without sophisticated surveying equipment. The environment at 4, 320 ft (1, 316.7 m) above sea level is not much different from that at 4, 310 ft (1, 313.6 m). The same 10-ft (3-m) change in the elevation of a beach would expose formerly submerged land, or inundate formerly exposed land, making it easy to notice. Not only is the environment different, but also the dominant geologic processes are different. Erosion occurs above sea level, deposition occurs below sea level. As a result, coasts where the land is rising relative to sea level (emergent

coasts) are usually very different from those where the land is sinking relative to sea level (submergent coasts).

Emergent coasts

If the coast rises, or sea level goes down, areas that were once covered by the sea will emerge and form part of the landscape. The erosive action of the waves will attack surfaces that previously lay safely below them. This wave attack occurs at sea level, but its effects extend beyond sea level. Waves may undercut a cliff, and eventually the cliff will fail and fall into the sea, removing material from higher elevations. In this manner, the cliff retreats, while the beach profile is extended at its base. The rate at which this process continues depends on the material of the cliff and the profile of the beach. As the process continues, the gradual slope of the bottom extends farther and farther until most waves break far from shore and the rate of cliff retreat slows, resulting in a stable profile that may persist for long periods of time. Eventually another episode of uplift is likely to occur, and the process repeats.

Emergent coasts, such as the coast along much of California, often exhibit a series of terraces, each consisting of a former beach and wave cut cliff. This provides evidence of both the total uplift of the coast, and its incremental nature.

Softer rocks erode more easily, leaving resistant rock that forms points of land called headlands jutting out into the sea. Subsurface depth contours mimic that of the shoreline, resulting in wave refraction when the change in depth causes the waves to change the direction of their approach. This refraction concentrates wave energy on the headlands, and spreads it out across the areas in between. The “pocket beaches” separated by jagged headlands, which characterize much of the scenic coastline of Oregon and northern California were formed in this way. Wave refraction explains the fact that waves on both sides of a headland may approach it from nearly opposite directions, producing some spectacular displays when they break.

Submergent coasts

If sea level rises, or the elevation of the coast falls, formerly exposed topography will be inundated. Valleys carved out by rivers will become estuaries like Chesapeake Bay. Hilly terrains will become collections of islands, such as those off the coast of Maine.

The ability of rivers to transport sediment depends on their velocities. When rivers flow into a deep body of water, they slow down and deposit their sediment in what will eventually become a delta. Thus, the flooding of estuaries causes deposition further inland. As the estuary fills in with sediment, the depth of the water will decrease, and the velocity of the water flowing across the top of the delta will increase. This permits further sediment transport. The delta builds out toward, and eventually into, the sea. The additional load of all the sediment may cause the crust of Earth to deform, submerging the coast further.

The sand budget

Wave action moves incredible amounts of sand. As waves approach shallow water, they slow down because of friction with the bottom, then get steeper, and finally break. It is during this slowing and breaking that sand is transported.

When a breaking wave washes up onto the beach at a slight angle it moves sand on the beach with it. This movement is mostly towards shore, but also slightly down the beach. When the water sloshes back, it goes directly down the slope, without any oblique component. As a result, sand moves in a zigzag path with a net motion parallel to the beach. This is called “longshore drift.” Although most easily observed and understood in the swash zone, the area of the beach that gets alternately wet and dry with each passing wave, longshore drift is active in any water shallow enough to slow waves down.

Many features of sandy coasts are the result of longshore drift. Spits build out from projecting land-masses, sometimes becoming hooked at their end, as sand moves parallel to the shore. At Cape Cod, Massachusetts, glacial debris deposited thousands of years ago is still being eroded and redistributed by wave action.

An artificial jetty or “groin” can trap sand on one side of it, broadening the beach there. On the other side, however, wave action will transport sand away. Because of the jetty it will not be replenished, and erosion of the beach will result.

The magnitude and direction of transport of long-shore drift depends on the strength and direction of approach of waves, and these may vary with the season. A beach with a very gentle slope, covered with fine sand every July may be a steep pebble beach in February.

Barrier islands

Long, linear islands parallel to the shore are common along the Atlantic coast. Attractive sites for resorts and real estate developments, these barrier islands are in constant flux. A hurricane can drive storm waves over low spots, cutting islands in two. Conversely, migration of sand can extend a spit across the channel between two islands, merging them into one.

Interruptions in sand supply can result in erosion. This has happened off the coast of Maryland, where Assateague Island has become thinner and moved shoreward since jetties were installed at Ocean City, just to the north.

Society and the beach environment

Coastal areas of the continental United States comprise only 17% of the land area of the country, but house over one-half of the population. In 2003, the population of this zone was 153 million, an increase of 33 million since 1980. The coast is attractive for a wide variety of reasons, and economic growth of the zone typically follows the growth of the population. The impacts on the coastal environment can result in environmental degradation, particularly within environmentally sensitive areas.

Often, the dynamic nature of the beach environment is not properly assessed when beach areas are developed. In places away from the coasts, where rates of erosion and deposition are much slower, development projects can change the topology of the land and the results will persist for centuries. In a beach environment, however, modifications are ephemeral. Maintaining a parking lot where winds would produce a dune requires removal of tons of sand every year. Even more significantly, because the flow of sediment is so great, modifications intended to have only a local, beneficial effect may influence erosion and deposition far down the beach.

Coastal retreat is a significant issue along many areas of coastline. Efforts are ongoing to quantify the rate of retreat along the coast, to designate areas of

KEY TERMS

Emergent coast— A coast rising relative to sea level, characterized by exposed terraces consisting of older wave cut cliffs and formerly submerged beaches.

Longshore drift— Movement of sand parallel to the shore, caused by waves approaching the shore obliquely, slowing and breaking.

Refraction— The bending of light that occurs when traveling from one medium to another, such as air to glass or air to water.

Submergent coast— A coast sinking relative to sea level, characterized by drowned river valleys.

particular risk, and to match appropriate uses with the location. Even in areas with minimal retreat, the movement of sediment along the shore can impact the property owner significantly. Utilization of engineered shoreline protection can affect nearby properties. Sediment budgets for a shoreline can be impacted by the damming of rivers upstream. Even artificial means of beach nourishment can have unintended environmental impacts. One might be able to protect the beach in front of a beach house by installing a concrete barrier, but this might result in eroding the supports to the highway giving access to the beach house.

The costs of shoreline protection are high and may not be factored in during development. Furthermore, these costs may be borne by the taxpayer rather than the property owner. The long-range outlook for all such costs is that they may in some cases exceed the value of the property and may be exacerbated by rising sea levels associated with global climate alteration. Many scientists and engineers are concerned with the migratory nature of the coast, and caution that structures built upon a moving coastline are subject to the same fluctuating forces that control the beaches themselves.

See also Beach nourishment; Ocean; Shoreline protection;Tides.

Resources

BOOKS

Bird, Eric. Coastal Geomorphology: An Introduction. West Sussex, England: John Wiley & Sons Ltd. 2000.

U.S. Army Corps of Engineers. Coastal Geology. Honolulu, Hawaii: University Press of the Pacific, 2004.

OTHER

Geological Society of America (GSA). Beach Nourishment: The Wave of the Future for Erosion Control (Part A). Southeast Section Annual Meeting Abstracts. 2001 (accessed October 19, 2002). <gsa.confex.com/gsa/2001SE/finalprogram/session_77.htm>. National Oceanic and Atmospheric Administration

(NOAA). Managing Coastal Resources. NOAA’s State of the Coast Report, 1998. <http://state-of-coast.-noaa.gov/bulletins/html/crm_13/crm.html> (accessed October 19, 2002).

National Oceanic and Atmospheric Administration (NOAA). Monitoring the Coastal Environment. NOAA’s State of the Coast Report. 1998. <http://state-of-coast.noaa.gov/bulletins/html/mcwq_12/mcwq.html> (accessed October 18, 2002).

National Oceanic and Atmospheric Administration (NOAA). Population at Risk from Natural Hazards. NOAA’s State of the Coast Report. 1998. <http://state-of-coast.noaa.gov/bulletins/html/par_02/par.html> (accessed October 19, 2002).

National Oceanic and Atmospheric Administration (NOAA). “Population Trends Along the Coastal United States: 1980-2008.” September 2004. <http://marineeconomics.noaa.gov/socioeconomics/assessment/population.html> (accessed October 11, 2006).

Otto H. Muller