Coastal Processes

About 70% of Earth’s surface is covered by water, and the coast, which forms the interface between land and water, is the sight of a particular array of geomorphic processes and a range of landforms. For example, waves and tides involve movement and dissipation of large amounts of energy capable of causing rapid and spectacular changes in landforms along coasts.

 

Waves

Waves are characterized by their length, height (amplitude), velocity (rate of forward motion of the wave peak), and period (the interval of time between successive wave peaks passing the same point). These properties, and the relationships between them, vary greatly depending on the nature of the mechanism generating the wave, the intensity of this generating mechanism, and the environment in which the wave exists.

Wind-generation of waves involves a transfer of energy from moving air to a water surface. Although a very familiar process, the way in which this occurs is still not fully understood (Summerfield 1991). The amount of energy exchanged depends mainly on velocity, duration, and fetch (the distance over which the wind blows, which has an important influence on wave height and period) of the wind. The highest waves are produced by strong winds blowing in the same direction over a long distance; such waves can reach heights of 50 feet (15 m). Waves also are generated by low atmospheric pressure (storm surges) and displacement of the ocean floor, in particular by earthquakes (tsunami).

In the deep ocean, little forward motion of water in waves occurs because the wave form moves rather than the water. As waves move toward shallower water, however, their mode of movement changes dramatically. The seafloor starts to interfere with the oscillatory motion of waves where the water depth decreases to less than half that of the wave length. The orbit of individual water particles becomes less circular and more elliptical. Forward movement of water now becomes important as the oscillatory (deep-ocean) waves are transformed into translatory waves. As the water depth becomes progressively more shallow, wave length and velocity decrease, wave height increases and consequently the wave steepens. Eventually the wave is over-steepened to the stage where it breaks as its crest crashes forward creating surf. The zone of active breaking waves is known as the surf zone. Once the wave form has been destroyed, the remaining water moves up the shore as swash and returns under the force of gravity as undertow.

 

Tides

Tides result from the gravitational attraction exerted on ocean water by the Moon and the Sun. Because the Moon is closer to Earth, it has more than twice the gravitational effect of the distant Sun, despite the immense size and mass of the Sun. The motions of Earth, Moon, and Sun with respect to one another produce semi-diurnal tides along most coasts in which there are two lows and two highs approximately every 24 hours. Tides higher than normal, known as spring tides, occur every 14–17 days when the Sun and Moon are aligned. In between these periods, lower than normal—or neap tides—occur when the Sun and Moon are positioned at an angle of 90° with respect to Earth. Spring and neap tides involve deviations of about 20% above and below normal tidal range.

Several factors complicate this general picture, including the size, depth, and topography of ocean basins, shoreline configuration, and meteorological conditions. Much of the Pacific coast, for instance, experiences a regime of mixed tides in which highs and lows of each 24-hour period are of different magnitudes. Other coastlines, such as much of Antarctica, have diurnal tides with only one high and one low per 24 hours.

Although contrasts between tidal types are important in some coastal processes, of much greater overall geomorphic significance is tidal range. The most extreme ranges occur where coastal configuration and submarine topography induce an oscillation of water in phase with the tidal period. This effect is particularly pronounced in the Bay of Fundy, an inlet of the Atlantic Ocean in southeastern Canada, where the typical tidal range is more than 50 feet (16 m).

Tidal range and type are important for several reasons. Tidal type determines the interval between tides and therefore the time available for the shore to dry after high tide, which is significant for shoreline weathering processes and biological activity. Additionally, tidal type affects the intensity of tidal currents since, for a given tidal range, the velocity of water movement will be greater in semi-diurnal regimes than for mixed or diurnal types because a shorter interval between high and low tides occurs. This effect is particularly important in narrow coastal embayments where tidal flows are concentrated. Tidal range is important because it controls the vertical distance over which waves and currents are effective in shaping shorelines, and in conjunction with the slope of a shoreline, tidal range determines the extent of the intertidal zone, that is, the area between high and low tide (Summerfield 1991).

 

Currents

The horizontal movement of water (or air) is called a current. Reflected, or turned back, by the beach slope, water from waves becomes undertow or cross-shore currents, flowing seaward. As cross-shore currents meet with incoming waves, some water spreads sideward and merges with other sideward-moving water. The combined waters form an elongated cell from which water flows seaward as a rip current, which extends to the so-called rip end, as much as half mile (0.80 km) offshore, where the water disperses in various directions. Rip currents disperse outside of the surf zone.

Meanwhile, some water from undertow and incoming waves flow sideward parallel to the shore as longshore currents. These are created in part by waves meeting the shore obliquely. Longshore currents can be very strong; they can transport sediments and people along the coast. In areas with offshore mounds of sand, known as sandbars, longshore currents are often very strong in the trough that separates the sandbar from the beach. Longshore currents commonly feed into rip currents, mainly those on the downwind side.

Waves are often known as the “shakers” of the beach environment. The motions of waves in shallow water act to suspend sediment, while currents move or transport the sediments. Currents associated with tides can transport and erode sediment where flow velocities are high. This is usually confined to estuaries or other enclosed sections of coast that experience semi-diurnal tides with a high range.

 

Shoreline Weathering

A shore zone may be subject to the same range of physical and chemical weathering processes that occur on land, but the presence of seawater and the cycle of wetting and drying produced by tides introduces additional significant factors. The tidal cycle of wetting and drying is instrumental in a variety of weathering processes. The zone affected extends from low water mark to the furthest limit reached by waves and spray at high tide. Its areal extent is therefore controlled largely by tidal range, but tidal type and meteorological factors are also important because these affect, respectively, the time available for drying between tides and the rate of evaporation. The most aggressive regime for shoreline weathering probably occurs along coasts characterized by high evaporation rates and mixed or diurnal tides (Summerfield 1991).

An important process in shoreline weathering is salt weathering, although the effectiveness depends on the ability of shoreline rocks to absorb seawater and spray. Chemical weathering also plays a role: the nature of the chemical reactions being controlled by rock mineralogy, and their rate being influenced by temperature and variations in micro-environmental factors such as organic activity and pH along the shore. Solution is also a significant process on limestone and calcium carbonate-cemented rocks.

In high latitudes frost weathering is a potentially significant shoreline weathering process because of the frequency of wetting of bare rock surfaces in the intertidal zone. Seawater alone is not very effective because, upon freezing, the salts it contains become segregated and produce a rather soft ice incapable of transmitting high stresses to rocks. Frost weathering is apparently more effective where freshwater is available from melting snow banks or permafrost (Summerfield 1991).

 

Coastal Erosion

Waves are the most important erosive agent along most coasts but their effect varies with wave energy and characteristics, and with the nature of the material exposed to wave attack. Where a coast is formed by steep cliffs that plunge straight into deep water, waves are not forced to break before they impact. As virtually no forward mass displacement of water exists in such waves, they are reflected with little loss of energy and accomplish negligible geomorphic work. Much more commonly, coasts are subject to breaking waves. These involve significant mass displacement and a considerable loss of kinetic energy as they break on a shoreline. This energy is dissipated over a short distance where the shore gradient is steep, but over a greater horizontal distance where it is shallow. Of the main types of breaking waves, plunging breakers produce the highest instantaneous pressure since air can be trapped and compressed between the leading wave front and the shore (Summerfield 1991). A plunging breaker is the sensational, curling type commonly sought after by surfers. Because the energy of the plunging breaker is concentrated in a small or narrow area of the seafloor, it is able to move large amounts of sand.

The combined effect of air compression and impact of a considerable mass of water is capable of dislodging fractured rock and other loose particles, a process known as quarrying. Heavily jointed rocks and unconsolidated or weakly consolidated sediments are particularly susceptible. Breaking waves also may throw particles against the shore, which leads to the abrasion of shoreline materials. The effectiveness of abrasion is highly dependent on wave energy and on the availability of suitable material, such as pebbles, along the shore. Large boulders can be moved only during the most intense storms, whereas small boulders and pebbles can be moved much more frequently by waves of less energy.

 

Sediment Transport and Deposition

The main sources of sediment along coasts are: (1) the coastal landforms themselves, including cliffs and beaches; (2) the nearshore zone; and (3) the offshore zone and beyond. The erosion of coastal landforms, especially cliffs, can locally provide abundant sediment in environments with high wave energies (especially where unconsolidated sediments are being eroded).

Land-derived sediment can be provided by mass movement (e.g., rockfalls, landslides, and debris flows), especially where cliffs composed of material susceptible to such processes are being actively undercut. In periglacial environments, such as the Arctic coast, gelifluction is a particularly important means of transporting sediment into the nearshore zone, while in other high-latitude environments material can be supplied by glaciers. Nevertheless, by far the most significant source of sediment overall is from rivers. Scientists have estimated that on a global basis, rivers contribute about 100 times as much sediment as marine erosion. This ratio varies latitudinally, with fluvial sources being of even greater relative importance throughout most of the tropics where low wave energies make marine erosion less effective. However, even most beaches in the midlatitudes, where wave energies are on the average significantly higher, usually contain less than 5% of material derived from cliff erosion.

Cross-shore currents may return previously eroded beach material or fluvial sediments to the nearshore zone initially deposited offshore. Particularly severe storm waves, storm surges, and tsunami may occasionally bring sediment from beyond the offshore zone.

While sediment is constantly being moved more or less perpendicular to or from shorelines by tidal and wave action, the predominant net movement of sediment along most coasts is parallel with the shore through the effects of longshore currents. The movement is called longshore sediment transport and its rate is dependent on wave energy and the angle at which waves strike the coast (an angle around 30° being the most effective).

The overall significance of longshore sediment transport depends on whether it occurs along coasts of free or impeded transport. Impeded transport is characteristic of coasts with irregular configurations, the amount of longshore transport being limited by the trapping of sediment against headlands or coastal engineering structures. This contrasts with unimpeded longshore sediment transport along straight coasts. Given a particular coastal configuration the rate of longshore transport will be related to the predominant angle of the wave approach and the constancy and velocity of longshore currents.

 

Organic Activity

A variety of seashore organisms, including mollusks, sponges, and sea urchins, can destroy rocks by physically boring into them. Their effectiveness is influenced by rock type: most sedimentary rocks being much more susceptible than igneous rocks, for example. The relative importance of biological erosion is much greater along coasts characterized by low wave energies because here abrasion and quarrying operate at only moderate or low intensities.

Many organisms are either directly or indirectly responsible for the construction of some coastal landforms. The most spectacular example of direct construction is by corals and other carbonate-secreting organisms that form coral reefs. These structures can attain immense sizes, as in the case of the Great Barrier Reef, which extends along much of the northeastern coast of Australia. The national park system also protects coral environments.

A range of plants are adapted to saltwater and these form salt-marsh communities in the intertidal zone along sheltered, muddy coasts. In the tropics mangroves are an important element in coastal vegetation and, together with other halophytic (salt-tolerant) plants, they play a geomorphic role by trapping sediment within their root systems and thereby aid the process of deposition. Their precise role, however, is uncertain (Summerfield 1991). More certain is the role of plants in stabilizing coastal dune systems and contributing to sand deposition.

 

Changes in Sea Level

The position and height of the sea relative to land, that is relative sea level, determines the location of shorelines. Though global fluctuations in sea level may result from the growth and melting of continental glaciers and large-scale changes in the configuration of continental margins and ocean floors, many regional processes result in rise or fall of relative sea level that affect one coastline and not another. These include: thermal expansion of ocean waters, changes in meltwater load, crustal rebound from glaciation, uplift or subsidence in coastal areas related to various tectonic processes (e.g., seismic disturbance and volcanic action), fluid withdrawal, and sediment deposition and compaction. Variations in relative sea level also may result from geodetic changes such as fluctuations in the angular velocity of Earth or polar drift. Tide-gauge records suggest an average global sea-level rise over the past century of 0 to 0.12 inches (3 mm) per year, though these rates provide no firm evidence of acceleration. Indeed, a recent study by the U.S. Environmental Protection Agency predicts that global sea level is likely to rise 5.9 inches (15 cm) by 2050 as a result of human-induced climate warming (Berger and Iams 1996).

Changes in relative sea level may alter the position and morphology of coastlines, causing coastal flooding, waterlogged soils, and a loss or gain of land. They also may create or destroy coastal wetlands and salt marshes, inundate coastal settlements, and induce saltwater intrusion into aquifers, leading to salinization of groundwater. Coastal ecosystems are bound to be affected, for example, by increased salt stress on plants. A changing relative sea level also may have profound effects on coastal structures and communities. Low-lying coastal and island states are particularly susceptible to sea-level rise. Scientists estimate that 70% of the world’s sandy beaches are affected by coastal erosion induced by relative sea-level rise (Berger and Iams 1996).

Much of the present-day onshore movement of sediment arises from post-glacial rise in sea level. Sediments deposited on previously exposed continental shelves are still being moved inland in response to Holocene rise in sea level and the adjustment of the nearshore zone to present-day tidal and wave regimes. In some localities this sediment supply seems to have been exhausted and some depositional landforms constructed during this period of sea-level rise are now being eroded (Summerfield 1991).

For more information, visit “Global Warming and Sea-Level Rise” on the National Park Service Coastal Geology Web site.

 

Coastal Hazards

Natural geomorphic processes are considered hazards when human populations are affected by them. For example, storms, hurricanes, and tsunamis are natural driving forces of coastal processes and landforms, but also cause loss of life and property in coastal communities. Storms provide much of the sediment to shallow-marine and estuarine ecosystems. Many wetland environments and barrier islands depend on storm activities for sediment build-up and survival when faced with rising sea levels. When combined with increasing sea levels, these events may have tremendous impacts on coastal environments and beaches.

 

Rip Currents

Rip currents are strong and swift, moving 3 to 6 feet (1 to 2 m) per second, which is faster than an average person can swim! These currents are created because of “set-up” near the shoreline. Set-up is a slight increase in water levels compared to those found seaward of the surf zone and creates unstable conditions that eventually are relieved through the formation of rip currents. These dangerous currents generally form at a low point or saddle in a sandbar. Because rip currents tend to be narrow, swimmers caught in a rip current should swim parallel to the shore to escape being taken out to sea. Swimmers should be especially cautious during storm events, which may increase the frequency and strength of rip currents.

 

Storm Surges

Storm surges—extraordinarily high water levels—are generated by the combined effects of low atmospheric pressure and very high wind speeds. The strong onshore winds that accompany tropical storms, hurricanes, and frontal storms of the midlatitudes drag and stack water against coasts, creating a storm surge. In the center of tropical-storm systems, atmospheric pressure may drop as much as 100 millibars below normal, and this can “suck up” sea level below the center of the cyclone by as much as 3.3 feet (1 m) (Summerfield 1991). Winds blowing toward coastal embayments produce the largest storm surges, which are accentuated if they coincide with high tides. By the time the waves generated by the storm surge reach the coast, they may have built up to a height of nearly 10 feet (several meters) above normal high tides.

When low-pressure storm systems approach land, strong winds can affect a coast in a variety of ways. High velocity onshore winds, particularly the kind associated with hurricanes, drive water ashore and elevate the water line well above the predicted tidal variation. The effect of storm surges can be catastrophic because the elevated water surface results in widespread coastal flooding and allows waves to break much further inland than they would normally. In addition, torrential rainfall associated with the storm causes widespread river flooding. The combination of these effects can result in extensive property damage and loss of life (Pinet 1992).

Although comparatively uncommon on a global basis, storm surges occur repeatedly along coasts experiencing tropical cyclones and in midlatitude areas subject to intense storms where the coastal configuration is particularly favorable. Storm surges are most destructive along very low-lying coasts where their effects can extend many miles (kilometers) inland, but their geomorphic significance arises in large part from the way in which they lead to wave attack at much higher levels along a shoreline than is reached by normal waves.

For more information about hurricanes, check out these Web sites:

http://science.nasa.gov/headlines/y2003/18sep_isabel.htm: NASA satellite photos

http://www.nasa.gov/vision/earth/environment/HURRICANE_RECIPE.html: NASA “recipe for a hurricane”

http://www.nasa.gov/vision/earth/lookingatearth/Isabels_Engine.html: NASA information on Hurricane Isabel

http://www.hurricanehunters.com/isabel.htm: Story and photos of Hurricane Isabel by Hurricane Hunters

http://www.hurricanehunters.com/wx_links.htm: Listing of more Web sites on hurricanes

http://www.hurricanehunters.com/welcome.htm: Hurricane Hunters home page

 

Tsunamis

Tsunamis are very large seismic ocean waves that are radially generated from volcanic eruptions, earthquakes, or subaqueous slumping. In the open ocean, these waves may travel at speeds in excess of 493 miles per hour (793 kph)! Strangely, sailors on deep ocean vessels may not notice the passing of these waves on account of the waves’ flat, low propagation. In contrast, when tsunamis reach shallow water, they slow down considerably and may reach great heights [up to 33 ft (10 m)]. Tsunamis have caused great destruction and loss of life because of abrupt changes in water levels above the normal high water mark. Numerous areas in the United States have experienced tsunamis including Hawaii, the Pacific Northwest, and Alaska. A new system, the International Tsunami Warning System, is used now to alert the public to impending tsunamis.

Use these Web links for more information about tsunamis: where they occur, why they occur, and what happens in a tsunami.

http://walrus.wr.usgs.gov/tsunami/cascadia.html: Local tsunamis of the Pacific Northwest – USGS

http://walrus.wr.usgs.gov/tsunami/links.html: Tsunami and Earthquake Links – USGS

http://www.fema.gov/hazards/tsunamis/tsunami.shtm: Backgrounder: Tsunami – FEMA

http://www.nws.noaa.gov./om/brochures/tsunami.htm: Tsunami: the Great Waves - NOAA

 

Human Modifications of Coastal Processes

People love living near the beach. More than 50% of the U.S. population lives within 50 miles (80 km) of a shoreline. Once developed, communities make an effort to protect their beach homes and coastal businesses. Throughout history, humans have attempted to slow or alter the dynamic coastal zone. The anthropogenic (human-influenced) changes to coastal environments may take many forms: creation or stabilization of inlets, beach nourishment and sediment bypassing, creation of dunes for property protection, dredging of waterways for shipping and commerce, and introduction of hard structures such as jetties, groins, and seawalls. These modifications change coastal features and have far-reaching effects on coastal processes and ecosystems. An understanding of how human changes alter shoreline environments and park resources is vital for the protection and preservation of coastal areas.

Variations in sea levels are natural responses to climate change, geodetic variations, movements of the sea floor, and other Earth processes. Human actions, including drainage of wetlands, withdrawal of groundwater (which eventually flows to the sea), and deforestation (which reduces terrestrial water-storage capacity), may also contribute to global rise in sea level. Additionally, human-induced climate change, primarily through the burning of fossil fuels, is also of importance. Local changes may be caused by large engineering works nearby, such as river channeling or dam construction that influence sediment delivery and deposition in deltaic areas.

The National Park Service allows natural coastal processes to continue without interference. However, when natural processes, including coastal erosion and storm events, interfere with the preservation of cultural resources and park infrastructure, modifications to coastal dynamics may be necessary. How coastal modifications will affect natural park resources (biological and physical) must be investigated thoroughly in order that wise decisions are made. Park managers in coastal parks strive to achieve a balance between preservation of historic landmarks (e.g., forts and lighthouses) and the protection of natural ecosystems. In addition, a history of long-term human alteration, combined with a lack of historical documentation, makes defining a natural coastal system difficult. An understanding of how anthropogenic modifications will alter shoreline environments and park resources is vital for effective coastal management.

 

Soft Structures


Dredging

Dredging, the removal of sediment, including sand, silt, rock, and other subaqueous materials from our coastal waterways is a hotly debated topic in coastal management. The effects of dredging waterways and ports to benefit shipping, transport, and recreation are not fully understood. Opponents claim that coastal dredging may have detrimental environmental impacts and may interfere with sediment transport and flow dynamics in coastal and marine systems.

Dredged sediments may include harmful contaminants and pollutants. After dredging, these sediments are often redeposited offshore or used for the creation of dredge spoil islands adjacent to the scoured waterways. In addition, dredged sediment may be incorporated into beach nourishment projects; however, the sediment grain size of dredged materials may not be compatible with native beach sediment. Grain-size alterations and contamination may exceed flora and fauna tolerances, negatively impacting native ecosystem functions.

Proponents of dredging cite that this method of removal is necessary for commerce, recreation, and national defense. Interagency partnerships, such as the Nature Conservancy and the U.S. Army Corps of Engineers http://www.sandandgravel.com/news, have been established to promote a better understanding of how dredging could impact coastal environments.

For more information about dredging, check out the Web sites:

http://www.wes.army.mil/el/dots/: information on dredging research, benefits, and resources; Dredging Operations Technical Support (DOTS) provides direct environmental and engineering support to the U.S. Army Corps of Engineers’ Operations and Maintenance (O&M) dredging missions

http://bonita.mbnms.nos.noaa.gov/resourcepro/resmanissues/dredge.html: overview of dredging by the Monterey Bay National Marine Sanctuary (NOAA)

 

Beach Nourishment

Beach nourishment is the process of placing additional sediment on a beach. This material is obtained from another source that either lies inland or is dredged offshore. Nourishment entails the removal of sediment from “borrow sites,” and the subsequent transport of the sediment to beach areas. Borrow sites may alter sediment transport, hydrodynamic patterns, marine ecosystems, and sediment transport, such as creating erosional “hot spots” on adjacent shorelines.

Subaqueous nourishment is an alternative form of replenishment. The creation of offshore berms (mounds) may be utilized for the subsequent landward migration of sediments, often leading to sediment accretion on adjacent beaches. Although still a fairly new replenishment method, and not documented as fully effective, subaqueous nourishment may be substituted on account of cost limitations or biotic complications (e.g., migration and preserving endangered species) resulting from direct beach nourishment.

Often, beach nourishment is needed to counteract the effects of the hard-structure stabilization of coastlines. These structures (e.g., jetties, groins, and seawalls) typically increase downdrift erosion rates, promoting a need for continued coastal modifications through nourishment. The need for beach nourishment after human alteration is quite evident at Assateague Island National Seashore (Maryland). In the mid-1930s, the U.S. Army Corps of Engineers established a jetty system to stabilize the Ocean City inlet. While sediment transported by north-to-south longshore currents increased sediment accretion updrift of the jetties at Ocean City, excessive erosion and barrier island migration have occurred on the island to the south of the jetties. That is, Assateague Island has migrated westward more than 1,148 feet (350 m) since 1933! The deterrence of longshore sediment transport to Assateague Island National Park has had numerous detrimental impacts on biological, geologic, and cultural resources. Both short-term and long-term beach nourishment plans are in place to mitigate the destructive effects of jetty placement.

For more information about Assateague Island National Seashore, check out these Web sites:

http://bigfoot.wes.army.mil/6718.html: Ocean City–Assateague Island, Maryland, studies by Gregory Bass, U.S. Army Corps of Engineers

http://pubs.usgs.gov/circular/c1075/conflicts.html: report by the U.S. Geological Survey on coastal conflicts that highlights Ocean City jetty construction and Assateague Island migration

http://www.newsline.umd.edu/etcetera/specialreports/reachbeach/assateague041301.htm: article in Maryland Newsline that describes nourishment efforts by the U.S. Army Corps of Engineers on Assateague Island

For more information about methods, environmental impacts, and costs of beach nourishment, check out these Web sites:

http://www.csc.noaa.gov/opis/html/descrip.htm: overview of NOAA dredging activities, including legal and political constraints, sand resources for beach nourishment, and potential impacts of nourishment projects

http://www.ocrm.nos.noaa.gov/pdf/finalbeach.pdf: NOAA Web site containing a national overview of state, territory, and commonwealth beach nourishment programs

http://www.usace.army.mil/inet/functions/cw/hot_topics/beachnourishment.htm: environmental review of beach nourishing projects by the U.S. Army Corps of Engineers

 

Beach Scraping

Beach scraping (i.e., grading and bulldozing) is the process of reshaping beach and dune landforms with heavy machinery. Usually a layer of sand from the lower beach is moved to the upper beach. Beach scraping creates dunes, which are used to give property owners some security from beach erosion, severe storms, and winter washover events. During the summers, the created sandbanks may be bulldozed flat, providing water views to property owners. However, the effects of beach scraping on coastal environments are little known, and this procedure may be harmful to coastal biota and habitats. Proponents claim that beach scraping is a time and cost-effective method to ensure shoreline protection, while opponents state that this method may be the most ecologically destructive form of coastal manipulation to date.

Please see this Web site for more information on beach scraping:

http://www.ncsu.edu/seagrant/FRG/98ep-05.html: brief summary of how beach scraping affects biology and turbidity in North Carolina

 

Hard Structures

Hard structures are often placed in marine environments to counteract erosion in sediment-deficient areas, or to deter accretion in sediment-rich areas such as inlets. Unfortunately, these anthropogenic modifications may accelerate erosion in adjacent downdrift areas, increasing the need for additional hard structures. The creation of new hard structures is currently banned in many states, or strongly discouraged as coastal management practice.

Groins are shore-perpendicular structures, used to maintain updrift beaches or to restrict longshore sediment transport. Permeable groins are becoming popular, and may negate some of the negative effects of impermeable groins. Another type of shore-perpendicular hard structure are jetties, which are normally placed adjacent to tidal inlets to control inlet migration and to minimize sediment deposition within the inlet.

Shore parallel structures include seawalls, bulkheads, and revetments. These structures are designed to protect coastal property. Development permits are relatively easy to obtain in many states because seawalls may be built above the high-water mark on private property, and they are relatively inexpensive, compared to beach nourishment. Ironically, seawalls usually accelerate erosion on beaches they are intended to protect. Wave energy is reflected off seawalls, increasing erosion in front of them. The placement of a seawall will decrease the sediment supply near seawalls, increasing erosion on adjacent beaches. In many areas, beaches have completely eroded and disappeared on account of seawalls.

Other anthropogenic structures that are used to stop or alter natural coastal changes include breakwaters, headlands, sills, and reefs. These structures are composed of either natural or artificial materials, and are designed to alter the effects of waves and slow coastline erosion and change. Submerged reefs and sills dampen wave energy and may create new habitat, which is significant for local fisheries. However, the long-term effects of these structures, on both physical and biological processes, are not understood and require thorough examination.

For more information concerning these and other anthropogenic modifications, check out these Web sites:

http://www.beachbrowser.com/Archives/Environment/August-99/BEACHES-OR-BEDROOMS.htm: Wilmington Morning Star article concerning the construction of hard structures on North Carolina shores; contains links to other articles discussing coastal management issues, Dr. Orrin Pilkey, and barrier-island geomorphology

http://www.crcwater.org/issues4/19980412coastalerosion.html: Associated Press article “Development-protecting seawalls debated”

 

Links

Introduction to Coastal Geology

Coastal National Parks

Coastal Processes

Coastal Materials

Coastal Environments

Challenge Your Understanding

 

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