UC San DiegoCoastal Morphology Group

TitleNearshore Processes

Permalinkhttps://escholarship.org/uc/item/204201x5

AuthorInman, Douglas L.

Publication Date2002

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Nearshore processes 1

Nearshore processesProcesses that shape the shore features of coastlinesand begin the mixing, sorting, and transportation ofsediments and runoff from land. In particular, theprocesses include those interactions among waves,winds, tides, currents, and land that relate to thewaters, sediments, and organisms of the nearshoreportions of the continental shelf. The nearshore ex-tends from the landward limit of storm-wave influ-ence, seaward to depths where wave shoaling be-gins. See COASTAL LANDFORMS.

The energy for nearshore processes comes fromthe sea and is produced by the force of winds blow-ing over the ocean, by the gravitational attraction ofMoon and Sun acting on the mass of the ocean, andby various impulsive disturbances at the atmosphericand terrestrial boundaries of the ocean. These forcesproduce waves and currents that transport energytoward the coast. The configuration of the landmassand adjacent shelves modifies and focuses the flow ofenergy and determines the intensity of wave and cur-rent action in coastal waters. Rivers and winds trans-port erosion products from the land to the coast,where they are sorted and dispersed by waves andcurrents.

In temperate latitudes, the dispersive mechanismsoperative in the nearshore waters of oceans, bays,and lakes are all quite similar, differing only in inten-sity and scale, and are determined primarily by thenature of the wave action and the dimensions ofthe surfzone. The most important mechanisms arethe orbital motion of the waves, the basic mecha-nism by which wave energy is expended on the shal-low sea bottom, and the currents of the nearshorecirculation system that produce a continuous inter-change of water between the surfzone and offshoreareas. The dispersion of water and sediments nearthe coast and the formation and erosion of sandybeaches are some of the common manifestations ofnearshore processes.

Erosional and depositional nearshore processesplay an important role in determining the configu-ration of coastlines. Whether deposition or erosionwill be predominant in any particular place dependsupon a number of interrelated factors: the amount ofavailable beach sand and the location of its source;the configuration of the coastline and of the adjoin-ing ocean floor; and the effects of wave, current,wind, and tidal action. The establishment and persis-tence of natural sand beaches are often the result ofa delicate balance among a number of these factors,and any changes, natural or anthropogenic, tend toupset this equilibrium. See DEPOSITIONAL SYSTEMS

AND ENVIRONMENTS; EROSION.Waves. Waves and the currents that they generate

are the most important factors in the transportationand deposition of nearshore sediments. Waves areeffective in moving material along the bottom andin placing it in suspension for weaker currents totransport. In the absence of beaches, the direct forceof the breaking waves erodes cliffs and sea walls.

Wave action along most coasts is seasonal, re-sponding to changing wind systems over the waterswhere the waves are generated. The height and pe-riod of the waves depend on the speed and durationof the winds generating them, and the fetch, or dis-tance, over which the wind blows. Consequently,the nature and intensity of wave attack against coast-lines vary with the size of the water body, as wellas with latitude and exposure. Waves generated bywinter storms in the Southern Hemisphere of thePacific Ocean may travel 10,000 km (6000 mi) be-fore breaking on the shores of California, where theyare common summer waves for the Northern Hemi-sphere.

The profiles of ocean waves in deep water arelong and low, approaching a sinusoidal form. As thewaves enter shallow water, the propagation speedand wavelength decrease, the wave steepens, andthe wave height increases until the wave train con-sists of peaked crests separated by flat troughs. Nearthe breaker zone, the process of steepening is ac-celerated so that the breaking wave usually attains aheight greater than the deep-water wave. This trans-formation is particularly pronounced for long-periodwaves from a distant storm. However, the profilesof local storm waves and the waves generated oversmall water bodies, such as lakes, show considerablesteepness even in deep water.

Wave shoaling, that is, the shallow-water transfor-mation of waves, commences at the depth wherethe waves “feel bottom.” This depth is about one-halfthe deep-water wavelength, where the wavelengthis the horizontal distance from wave crest to crest.Upon entering shallow water, waves are also sub-jected to refraction, a process in which the wavecrests tend to parallel the depth contours, and towave diffraction, which causes a flow of energy alongthe wave crest. For straight coasts with parallel con-tours, refraction decreases the angle between the ap-proaching wave and the coast, and causes a spread-ing of the energy along the crests. The wave heightis decreased by this process, but the effect is uni-form along the coast (Fig. 1). The amount of waverefraction and diffraction and the consequent changein wave height and direction at any point along thecoast is a function of wave period, direction of ap-proach, and the configuration of the bottom topog-raphy. See OCEAN WAVES; WAVE MOTION IN LIQUIDS.

Nearshore circulation. When waves break so thatthere is an angle between the crest of the breakingwave and the beach, the momentum of the breakingwave has a component along the beach in the direc-tion of wave propagation. This results in the gener-ation of longshore currents that flow parallel to thebeach inside the breaker zone (Fig. 2a). After flow-ing parallel to the beach as longshore currents, thewater returns seaward along relatively narrow zonesas rip currents. The net onshore transport of waterby wave action in the breaker zone, the lateral trans-port inside the breaker zone by longshore currents,the seaward return of the flow through the surfzoneby rip currents, and the longshore movement in the

2 Nearshore processes

Fig. 1. Longshore currents, generated when wavesapproach the beach at an angle. At Oceanside, California,the longshore current is flowing toward the observer.(Department of Engineering, University of California,Berkeley)

expanding head of the rip current together consti-tute the nearshore circulation system. The patternthat results from this circulation commonly takes theform of an eddy or cell with a vertical axis. The di-mensions of the cell are related to the width of thesurfzone and the spacing between rip currents. Thespacing between rip currents is usually two to eighttimes the width of the surfzone.

When waves break with their crests parallel toa straight beach, the flow pattern of the nearshorecirculation cell becomes symmetrical (Fig. 2b).

longshore current

beach

onshore transport by waves

break

ing

wave

riphead

surf

-zo

nesu

rf-

zone

c u rren

t

(b)

(a)

rip

Fig. 2. Definition sketches for nearshore circulation cells.(a) Asymmetrical cell for breakers oblique to the shore.(b) Symmetrical cell for breakers parallel to the shore.

Longshore currents occur within each cell, but thereis little longshore exchange of water or sedimentfrom cell to cell.

The nearshore cirulation system produces a con-tinuous interchange between the waters of the surfand offshore zones, acting as a distributing mecha-nism for nutrients and as a dispersing mechanism forland runoff. Offshore water is transported into thesurfzone by breaking waves, and particulate matteris filtered out on the sands of the beach face. Runofffrom land and pollutants introduced into the surf-zone are carried along the shore and mixed withthe offshore waters by the seaward-flowing rip cur-rents. These currents are a danger to swimmers whomay be unexpectedly carried seaward. Longshorecurrents may attain velocities in excess of 2.5 m/s(8 ft/s), while rip current velocities in excess of1.5 m/s (5ft/s) have been measured. Periodicity orfluctuation of current velocity and direction is a char-acteristic of flow in the nearshore system. This vari-ability is primarily due to the grouping of high wavesfollowed by low waves, a phenomenon called surfbeat that gives rise to a pulsation of water level inthe surfzone.

Formation of circulation cells. Nearshore circulationcells result from differences in mean water level inthe surfzone associated with changes in breakerheight along the beach. Waves transmit momentumin the direction of their travel, and their passagethrough water produces second-order pressure fieldsthat change the mean water level near the shore.Near the breakpoint, the presence of the pressurefield produces a decrease in mean water level (wavesetdown) that is proportional to the square of thewave height and, for waves near the surfzone, hasa maximum value that is about one-sixteenth that ofthe breaker height. Shoreward of the breakpoint, theonshore flow of momentum against the beach pro-duces a rise in mean water level over the beach face,called setup.

If the wave height varies along a beach, the setupwill also vary, causing a longshore gradient in meanwater level within the surfzone. Longshore currentsflow from regions of high water to regions of lowwater, and thus flow away from zones of high waves.The longshore currents flow seaward as rip currentswhere the breakers are lower. Pronounced changesin breaker height along beaches usually result fromwave refraction over irregular offshore topography.However, on a smaller scale, uniformly spaced zonesof high and low breakers occur along straightbeaches with parallel offshore contours. It has beenshown that these alternate zones of high and lowwaves are due to the interaction of the incident wavestraveling toward the beach from deep water withone of the many possible modes of oscillation of thenearshore zone known as edge waves. Edge wavesare trapped modes of oscillation that travel along theshore. Circulation in the nearshore cell is enhancedby edge waves having the period of the incidentwaves, or that of their surf beat, because these in-teractions produce alternate zones of high and low

Nearshore processes 3

rockyheadland

coastal mountains

cliffs

tidalmarsh

(a) (b)

submarinebasin

longshore transport

littoral cell

submarine

canyon

bay

offshoreshoal

longshore transport

sink

littoral cell

Fig. 3. Typical (a) collision and (b) trailing-edge coasts and their littoral cells. Solid arrows show sediment transport paths;dotted arrows indicate occasional onshore and offshore transport modes. (After D. L. Inman, Types of coastal zones:Similarities and differences, in National Research Council, Environmental Science in the Coastal Zone, National AcademyPress, 1994)

breakers whose positions are stationary along thebeach. It appears that the edge waves can be stand-ing or progressive. In either case, the spacing be-tween zones of high waves (and hence between ripcurrents) is related to the wavelength of the edgewave.

Headlands, breakwaters, and piers influence thecirculation pattern and alter the direction of the cur-rents flowing along the shore. In general, these ob-structions determine the position of one side of thecirculation cell. In places where a relatively straightbeach is terminated on the down-current side bypoints or other obstructions, a pronounced rip cur-rent extends seaward. During periods of large waveshaving a diagonal approach, these rip currents canbe traced seaward for distances of 1.6 km (1 mi) ormore.

Types of coasts. The geologic setting and theexposure to waves are the two most significantfactors in determining nearshore processes. Thelarge-scale features of a coast are associated with itsposition relative to the edges of the Earth’s movingtectonic plates. Accordingly, plate tectonics providesa convenient basis for the first-order classification ofcoasts, with longshore dimensions of about 1000 km(600 mi). Tectonic classification leads to the defini-tion of three general types: collision coasts, trailing-edge coasts, and marginal seacoasts. See PLATE TEC-

TONICS.Collision coasts. These coasts occur along active plate

margins, where the two plates are in collision or im-pinging upon each other (Fig. 3a). This area is one ofcrustal compression and consumption. These coastsare characterized by narrow continental shelves bor-dered by deep basins and ocean trenches. Subma-rine canyons cut across the narrow shelves and enterdeep water. The shore is often rugged and backed byseacliffs and coastal mountain ranges; earthquakesand volcanism are common. The coastal mountainsoften contain elevated wave-cut platforms and seaterraces representing former relations between thelevel of the sea and the land. The west coasts of theAmericas are typical examples of collision coasts. SeeSUBMARINE CANYON.

Trailing-edge coasts. These occur on the trailing edgeof a landmass that moves with the plate (Fig. 3b).They are thus situated upon passive continental mar-gins that form the stable portion of the plate, wellaway from the plate boundaries. The east coasts ofNorth and South America are examples of mature,trailing-edge coasts. These coasts typically havebroad continental shelves that slope into deeperwater without a bordering trench. The coastal plainis also typically wide and low-lying and usuallycontains lagoons and barrier islands, as on the east-ern coasts of the Americas. See BARRIER ISLANDS;COASTAL PLAIN; CONTINENTAL MARGIN.

Marginal seacoasts. These coasts develop along theshores of seas enclosed by continents and island arcs.These coasts are typically bordered by wide shelvesand shallow seas. The coastal plains of marginal sea-coasts vary in width and may be bordered by hillsand low mountains. Rivers entering the sea alongmarginal seacoasts often develop extensive deltas be-cause of the reduced intensity of wave action asso-ciated with small bodies of water. Typical marginalseacoasts border the South China and East Chinaseas, the Sea of Okhotsk, the Mediterranean Sea, andthe Gulf of Mexico. See BASIN; DELTA.

Other types. A complete classification would also in-clude coasts formed by other agents, such as glacialscour, ice-push, and reef-building organisms, addingtwo other types of coast: cryogenic and biogenic.Common examples of these two coastal types arearctic coasts and coral reef coasts.

Beaches. Beaches consist of transient clastic mate-rial (unconsolidated fragments) that reposes near theinterface between the land and the sea and is subjectto wave action. The material is in dynamic reposerather than in a stable deposit, and thus the width andthickness of beaches is subject to rapid fluctuations,depending upon the amount and rigor of erosion andtransportation of beach material. Beaches along colli-sion coasts are essentially long rivers of sand that aremoved by waves and currents and are derived fromthe material eroded from the coast and brought tothe sea by streams. The coast may be cliffed (Fig. 3a),or it may contain a ridge of windblown sand dunes

4 Nearshore processes

storm bar longshore bar

longshore trough

terraceface

wave-cutplatform

stormscarp

seacliff

closure depth

shorerise

breaker

shorezone

beach or shore

foreshore

surfzone

backshore

berms

nearshore

shoreline

Fig. 4. Beach profile, showing characteristic features.

and be backed by marshes and water (Fig. 3b). Alongmany low sandy coasts, such as the east and Gulfcoasts of the United States, the beach is separatedfrom the mainland by water or by a natural coastalcanal. Such beaches are called barrier islands. Barrierbeaches are “braided” forms of the “river of sand”with transgressive rollover caused by sea-level riseand washover processes. A beach that extends fromland and terminates in open water is referred to asa spit, while a beach that connects an island or rockto the mainland or another island is a tombolo.

Beach nomenclature. While differing in detail,beaches worldwide have certain characteristic fea-tures which allow application of a general terminol-ogy to their profile (Fig. 4). The beach or shoreextends landward from the breakpoint bar to theeffective limit of attack by storm waves. The regionseaward is termed the shorerise; that landward is thecoast. The coast is part of the coastal zone, whichincludes the continental shelf and the coast. Thebeach includes a backshore and foreshore. The back-shore is the highest portion and is acted upon bywaves only during storms. The foreshore extendsfrom the crest of the berm to the breakpoint barand is the active portion of the beach traversed bythe broken waves and the uprush and backwash ofthe swash on the beach face. The foreshore con-sists of a steep seaward-dipping face, related to thesize of the beach material and the rigor of theuprush, and of a more gentle seaward terrace, some-times referred to as the low-tide terrace, over whichthe waves break and surge. In most localitiesthe foreshore face and terrace merge into one con-tinuous curve; in others there is a discontinuity atthe toe of the beach face. The former condition ischaracteristic of fine sand beaches and of coastswhere the wave height is equal to or greater thanthe tidal range. The latter is typical of coastlineswhere the tidal range is large compared with the

wave height, as along the Patagonian coast of SouthAmerica and portions of the Gulf of California. Theforeshore frequently contains one or more bars andtroughs that parallel the beach; these are referredto as longshore bars and longshore troughs. Long-shore bars commonly form at the plunge point ofthe wave, and their position is thus influenced by thebreaker height and the nature of the tidal fluctuation.See TIDE.

The shorerise is the transition between the conti-nental shelf and the beach, and is marked by the in-crease in slope leading from the gently sloping shelfup to the beach proper. The shorerise extends sea-ward from the breakpoint bar to the closure depththat marks the seaward extent of depth changes be-tween winter (storm) and summer beach profiles(Fig. 4). Together, the shorerise, foreshore, and back-shore comprise the shorezone, which is the zone ofactive transport of beach material and the resultingareas of beach accretion and erosion.

Beach cycles. Waves are effective in causing sandto be transported laterally along the beach by long-shore currents and in causing movements of sandfrom the beach to the shorerise and back again to thebeach. Although these two types of transport are in-terrelated, for convenience longshore movement ofsand is discussed separately.

Along most coasts, there is a seasonal migrationof sand between the beaches and the shorerise inresponse to the changes in the character and direc-tion of approach of the waves. In general, the beachface builds seaward during the small waves of sum-mer and is cut back by high storm waves in winter.There are also shorter cycles of cut-and-fill associ-ated with spring and neap tides and with nonsea-sonal waves and storms. Bottom surveys indicate thatmost offshore-onshore interchange of sand occurs indepths of 10–15 m (33-45 ft) but that some effectsmay extend to depths of 30 m (100 ft) or more.

Nearshore processes 5

A typical summer beach is built seaward by lowwaves (Fig. 4). During stormy seasons, the beachface is eroded, sometimes forming a beach scarp.Subsequent low waves build the beach face seawardagain. The beach face is a depositional feature, and itshighest point, the berm crest, represents the maxi-mum height of the runup of water on the beach.The height of wave runup above still-water level isabout equal to the height of the breaking wave. Sincethe height of the berm depends on wave height, thehigher berm, if it is present, is sometimes referred toas the winter or storm berm, and the lower berm asthe summer berm. The entire beach may be cut backto the underlying country rock during severe storms.Under such conditions, the waves erode the coastand form seacliffs and wave-cut platforms. These fea-tures and their terrace deposits are frequently pre-served in the geologic record and serve as markersfor the past relations between the levels of the seaand the land.

Mechanics of beach formation. Beaches form wher-ever there are waves and an adequate supply of sandor coarser material. Even anthropogenic fills andstructures are effectively eroded and reformed bythe waves. The initial event in the formation of a newbeach from a heterogeneous sediment is the sortingof the material, with coarse material remaining onthe beach and fine material being carried away. Con-current with the sorting action, the material is rear-ranged, some being piled high above the water levelby the runup of the waves to form the beach berm,some carried back down the face to form the fore-shore terrace. In a relatively short time, the beach as-sumes a profile that is in equilibrium with the forcesgenerating it.

The beach face is frequently characterized by lam-inations (closely spaced layers) that show slight dif-ferences in the size, shape, or density of the sandgrains. The laminations parallel the beach face andrepresent shear sorting within the granular load as itis transported by the swash and backwash of thewaves over the beach face. Detailed examinationshows that each lamina of fine-grained minerals delin-eates the plane of shear between moving and residualsand. The mechanism concentrating the heavy finegrains at the shear plane is partly the effect of grav-ity acting during shearing, causing the small grainsto work their way down through the interstices be-tween larger grains, and partly the dependence of thenormal dispersive pressure upon grain size. Whengrains are sheared, the normal dispersive pressurebetween grains, which varies as the square of thegrain diameter, causes large grains to drift toward thezone of least shear strain, that is, the free surface, andthe smaller grains toward the zone of greatest shearstrain, that is, the shear plane.

Equilibrium profile. The action of waves on a slop-ing beach eventually produces a profile that achievesequilibrium with the energy dissipation associatedwith the oscillatory motion of the waves over thesand bottom. The slope of the beach face is related to

the dissipation of energy by the swash andbackwash over the beach face. Percolation of theswash into a permeable beach reduces the amountof flow in the backwash and is thus conducive todeposition of the sand transported by the swash. Ifin addition the beach is dry, this action is accentu-ated. Coarse sands are more permeable and conse-quently more conducive to deposition and the for-mation of steep beach faces. Large waves elevate thewater table in the beach. When the beach is satu-rated, the backwash has a higher velocity, a con-dition conducive to erosion. From the foregoing itfollows that the slope of the beach foreshore in-creases with increasing sediment size and with de-creasing wave height. If an artificial slope exceedsthe natural equilibrium slope, an offshore transportof sand will result from the gravity component ofthe sand load until the slope reaches equilibrium.Conversely, if an artificial slope is less than the natu-ral equilibrium slope, a shoreward transport of sandby waves and currents will result, and the beachslope will steepen. An equilibrium slope is attainedwhen the up-slope and down-slope transports areequal.

Beaches respond to wave-forcing by adjustingtheir form to an equilibrium or constant shape at-tributable to a given type of incident wave. The sea-sonal changes in beach profile in response to thehigh waves of winter and the lower waves of sum-mer are expressions of the beach form tending to-ward a seasonal equilibrium with the changing char-acter of the prevailing waves. Field studies showthat the equilibrium beach consists of two conjoinedparabolic curves that intersect at the breakpoint bar,one curve for the shorerise that extends from theclosure depth to the breakpoint bar, and anotherfor the foreshore that extends from the bar to theberm (Fig. 4). The principal differences betweenseasonal profiles are that in winter (higher waves)the breakpoint bar is deeper and farther offshore,while the berm crest is displaced landward. Thus, thechanges in seasonal equilibria are manifest by sim-ple, self-similar displacements of the bar-berm andshorerise curves. Also, since the equilibrium profileis parabolic, for any given sand size, the slope in-creases from the bar to the berm.

Longshore movement of sand. The movement ofsand along the shore occurs in the form of bed load(material rolled and dragged along the bottom) andsuspended load (material stirred up and carried withthe current). Suspended-load transportation occursprimarily in the surfzone, where the turbulence andvertical mixing of water are most effective in plac-ing sand in suspension and where the longshore cur-rents that transport the sediment-laden waters havethe highest velocity. The longshore transport rate ofsand is directly proportional to the longshore compo-nent of wave power. Thus, the longshore transportrate of sand can be estimated from a knowledge ofthe wave climate, that is, the budget of wave energyincident upon the beach.

6 Nearshore processes

Fig. 5. Effect of headlands on the accretion of beach sandat Point Mugu, California. The point forms a naturalobstruction that interrupts the longshore transport of sand,causing accretion and a wide beach to form (foreground).The regularly spaced scallops are swash cusps on thebeach face. (Department of Engineering, University ofCalifornia, Berkeley)

The volume of littoral transport along oceaniccoasts is usually estimated from the observed ratesof erosion or accretion, most often in the vicinity ofnatural obstructions, such as headlands (Fig. 5), orof coastal engineering structures, such as groins orjetties. In general, beaches build seaward up-currentfrom obstructions and are eroded on the currentlee where the supply of sand is diminished. Suchobservations indicate that the transport rate variesfrom almost nothing to several million cubic metersper year, with average values of 150,000–600,000 m3

(200,000–800,000 yd3) per year. Along the shores ofsmaller bodies of water, such as the Great Lakes ofthe United States, the littoral transport rate can beexpected to range about 7000–150,000 m3 (9000–200,000 yd3) per year.

The large quantity of sand moved along the shoreand the pattern of accretion and erosion thatoccurs when the flow is interrupted pose seriousproblems for coastal engineers. The problem is par-ticularly acute when jetties are constructed to stabi-lize and maintain deep navigation channels throughsandy beaches. A common remedial procedure isto dredge sand periodically from the accretion onthe up-current side of the obstruction and deposit iton the eroding beaches in the current lee. Anothermethod is the installation of sand bypassing systems,which continually remove the accreting sand and

transport it by hydraulic pipeline to the beaches inthe lee of the obstruction. See COASTAL ENGINEER-

ING; SEDIMENTOLOGY.Sources and sinks of beach sediment. The princi-

pal sources of beach and nearshore sediments arethe rivers that bring quantities of sand directly tothe ocean; the seacliffs and blufflands of unconsoli-dated material that are eroded by waves; and mate-rial of biogenous origin (shell and coral fragmentsand skeletons of small marine animals). In places,sediment may be supplied by the erosion of uncon-solidated deposits in shallow water (Fig. 3b). Beachsediments on the coasts of the Netherlands are de-rived in part from the shallow waters of the NorthSea. Windblown sand may be a source of beach sed-iment, although winds are usually more effective inremoving sand from beaches than in supplying it. Intropical latitudes, many beaches are composed en-tirely of grains of calcium carbonate of biogenousorigin. Generally the material consists of fragmentsof shells, corals, and calcareous algae growing onor near fringing reefs. The material is carried to thebeach by wave action over the reef. Some beaches arecomposed mainly of the tests (shells) of foraminiferathat live on sandy bottom offshore from the reefs.See NORTH SEA; REEF.

Streams and rivers may be important sources ofsand for beaches in temperate latitudes (Fig. 3a).Surprisingly, the contribution of sand by streams inarid climates is quite high (Fig. 6). Arid weatheringproduces sand-size material from areas with a min-imum cover of vegetation, so that occasional flashfloods may transport large volumes of sand. The max-imum sediment yield occurs from drainage basinswhere the mean annual precipitation is about 30 cm(12 in.) per year.

There is a pronounced multidecadal variability inthe amount of river-borne sediment transported tothe beach. The variability is associated with global cli-mate changes related to the El Nino/Southern Oscil-lation (ENSO) phenomena. ENSO drives large-scaleevents such as the Pacific/North American (PNA) pat-terns of atmospheric pressure that lead to wet and

Fig. 6. Sand delta at Rio de la Concepcion on the arid coastof the Gulf of California. Such deltas are important sourcesof sand for beaches. (Courtesy of D. L. Inman)

Nearshore processes 7

dry climate along the Pacific coast of NorthAmerica. The 20 coastal rivers of central and south-ern California had streamflow and sediment fluxesduring the wet phase of PNA (1969–1998) that ex-ceed those during the preceding dry phase (1944–1968) by factors of 3 and 5, respectively. Thesediment flux during the three major El Ninoevents of the wet phase were on average 27 timesgreater than the annual sediment flux during the dryphase. Also, the wave climate in southern Califor-nia changed with the shift from dry to wet phase ofPNA. The prevailing northwesterly winter waves ofthe dry phase were replaced by high-energy wavesapproaching from the west or southwest during thewet phase. Wave climate along the east coast of NorthAmerica responds to shifts in the atmospheric pat-terns of the North Atlantic Oscillation (NAO), whichis generally out of phase with the west coast climate.See CLIMATIC PREDICTION; EL NINO; TROPICAL METE-

OROLOGY; WEATHER FORECASTING AND PREDICTION.Wave erosion of rocky coasts is usually slow, even

where the rocks are relatively soft shales. Therefore,cliff erosion usually does not account for more thanabout 10% of the material on most beaches. How-ever, retreats greater than 1 m (3.3 ft) per year arenot uncommon in unconsolidated seacliffs. The mostdramatic modern example of coastal erosion is foundalong the delta of the Nile River in Egypt. The HighAswan Dam, constructed in 1964, has interceptedthe sediment that was previously brought down theNile to the coast. Lacking source material, the deltacoast is eroding, and waves and currents may causean entire city block of Ras El Bar to be lost to the seain one year.

The sand carried along the coast by waves andlongshore currents may be deposited in continentalembayments, or it may be diverted to deeper waterby submarine canyons which traverse the continen-tal shelf and effectively tap the supply of sand(Fig. 3a). Most of the deep sediments on the abyssalplains along a 400-km (250-mi) section of the Califor-nia coastline are probably derived from two subma-rine canyons, Delgada Submarine Canyon in north-ern California and Monterey Submarine Canyon incentral California. See MARINE SEDIMENTS.

Littoral cells and the budget of sediment. The bud-get of sediment for a region is obtained by assess-ing the sedimentary contributions and losses to theregion and their relation to the various sedimentsources and transport mechanisms. Determinationof the budget of sediment is not a simple matter,since it requires knowledge of the rates of erosionand deposition as well as understanding of the capac-ity of various transport agents. Studies of the budgetof sediment show that coastal areas can be dividedinto a series of discrete sedimentation compartmentscalled littoral cells. Each cell contains a completecycle of littoral transportation and sedimentation, in-cluding transport paths and sources and sinks of sed-iment. Littoral cells take a variety of forms, but thereare two basic types. One is characteristic of collisioncoasts with submarine canyons (Fig. 3a), while the

other is more typical of trailing-edge coasts whererivers empty into large estuaries (Fig. 3b). The con-cept of a littoral cell (or a subcell) with its budget ofsediment and transport paths provides objective cri-teria for making choices among various coastal con-servation methods. See ESTUARINE OCEANOGRAPHY.

Biological effects. The rigor of wave action and thecontinually shifting substrate make the sand beach aunique biological environment. Because few largeplants can survive, the beach is occupied mostly byanimals and microscopic plants. Much of the foodsupply for the animals consists of particulate matterthat is brought to the beach by the nearshore circu-lation system and is trapped in the sand. The beachacts as a giant sand filter straining out particulate mat-ter from the water that percolates through the beachface.

Since the beach-forming processes and the trap-ping of material by currents and sand are much thesame everywhere, the animals found on sandbeaches throughout the world are similar in aspectsand habits, although different species are presentin different localities. In addition, since the slopesand other physical properties of beaches are closelyrelated to elevation, the sea animals also exhibit amarked horizontal zonation. Organisms on the ac-tive portion of the beach face tend to be of twogeneral types insofar as the procurement of foodis concerned: those that burrow into the sand, us-ing it for refuge while they filter particulate matterfrom the water through siphons or other appendagesthat protrude above the sandy bottom, and thosethat remove organic material from the surfaceof the sand grains by ingesting them or by “licking.”There are usually few species, which may be veryabundant.

In tropical seas, the entire shore may be composedof the cemented and interlocking skeletons of reef-building corals and calcareous algae. When this oc-curs, the nearshore current system is controlled bythe configuration of the reefs. Where there are fring-ing reefs, breaking waves carry water over the edgeof the reef, generating currents that flow along theshore inside of the reef and then flow back to seathrough deep channels between reefs. Under suchconditions, beaches are usually restricted to a bermand foreshore face bordering the shoreward edge ofthe reef. Douglas L. Inman

Bibliography. W. Bascom, Waves and Beaches: TheDynamics of the Ocean Surface, rev. ed., 1980; D. C.Conley and D. L. Inman, Field observations of thefluid-granular boundary layer under nearbreakingwaves, J. Geophys. Res., 97(C6):9631–43, 1992; R. A.Davis, Jr., The Evolving Coast, 1994; K. Horikawa(ed.), Nearshore Dynamics and Coastal Processes,1988; D. L. Inman and B. M. Brush, The coastal chal-lenge, Science, 181:20–32, 1973; D. L. Inman and R.Dolan, The Outer Banks of North Carolina: Budget ofsediment and inlet dynamics along a migrating bar-rier system, J. Coastal Res., 5(2):193–237, 1989; D. L.Inman, M. H. S. Elwany, and S. A. Jenkins, Shoreriseand bar-berm profiles on ocean beaches, J. Geophys.

8 Nearshore processes

Res., 9(C10):18,181–18,199, 1993; D. L. Inman andS. A. Jenkins, Climate change and the episodicityof sediment flux of small California rivers, J. Geol.,107(3):251–70, 1999; J. P. Kennett, Marine Geology,1982; P. D. Komar, Beach Processes and Sedimen-tation, rev. ed., 1998; National Research Council,

Environmental Science in the Coastal Zone: Issuesfor Further Research, 1994; R. J. Seymour (ed.),Nearshore Sediment Transport, 1989; U.S. ArmyCorps of Engineers, Engineering and Design—Coastal Littoral Transport; L. D. Wright, Morpho-dynamics of Inner Continental Shelves, 1995.

Reprinted from the McGraw-Hill Encyclopedia of

Science & Technology, 9th Edition. Copyright c© 2002 by

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