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inshore
A large ocean : a complex CO&: courtesy of
At the coastal zone of the world ocean, air, water, and solid earth meet;
and there is endlessinteraction among the geological, biological, meteoro-
logical, and oceanicprocesses.Each of these environmental processes
affects the nature of a coastal region to some degree. Consequently, a
study of the processesat work in a coastalsectorcan help us understand
the origin, relative age, and history of that coast.
10, THE OPEN COAST
Photo (/I zone. The d:
OCEANIC INFLUENCE
The diverse in8uencesof ocean waters upon coastal features are re- lated to both the physical attributes of seawater (i.e., waves,currents, and turbuJence) and the chemical properties (i.e., solubility and ooncentra- tion). The effect of ocean waves,how~r, is the most important. The oonfiguration of a coastal area and the offshore floor of the sea is l.argely the resuJtof wave action. For this reason,the properties of shoaling waves are emphasizedin this section.
direction of , wardpart of occurs ( Fig. (2 uses wa ve ( this alignmel pletely para]) offshore profi
NEARSHORE CmCULATION. As wavescarryenergytowardshore,th~ encounter shallow water and their speedof propagation decreases. If the
(^160 )
FICUU 10- A large ocean swell refracts against a complex coastline. (Photograph courtesy of Barbee Scheibner).
A (^) B
FIGURE 10- Photo (A) and line drawing (B) of waverefractionjust seawardof the surf zone. The direction of longshorecurrentsis from right to left alongthe beach.
directionof waveattack is not perpendicularto t~ shoreline,the shore-
ward part of a waveslowsmore than the seawardportion and refraction
occurs (Fig. 10-1). Figure 10-2 showshow the processof refraction
causeswavecreststo align themselvesparallelto the shoreline. However,
this alignment is only a tendency. Wave crestsseldom becomecom-
pletely parallel to the shore exceptwherethe beachhas a long, gentle
offshoreprofile.
161
THE OPEN COAST 163
irregular coastline, a point of land refracts the waves so that energy is focused on it; an embayment has the opposite effect. In Fig. 10-3, the
amount of energy moving onshore is the same for sections A. B, and C.
However, this energy is spread over a longer stretch of beach at B and a shorter stretch of beach at C; consequently, the energy per unit length of coastline is increasedin section C and decreasedin section B. The net effect is that wave erosion is greater on the headlands. Given enough time, it tends to straighten an irregular coastline, regardlessof the direction of wave attack. As a wave approachesshore, the orbital velocity at the wave crest continually increases,whereas the propagation speed decreases. Eventu- ally, the crest "overruns" the trough, and the wave breaks. Theoretical oonsiderationsas well as field observations indicate that breaking occun when either of two oonditions are met: (1) the water depth becomesless than 1.28 times the wave height, or (2) the wave steepness(H/L) sur- passes1/7. Mer a wave breaks, the water-particle motion becomes in- tensely turbulent, and energyapproachesthe beach in a wave of translation rather than oscillation.. The mass of turbulent water that moves upon the beach is called swczsh. The water that runs back down the beach under the influence of gravity is termed b«kwash. Backwashremovesthe water from the beach and causesit to oppose the onooming waves. In so doing, the oncoming wavesare steepenedand break a bit sooner. The transport of water inside the breaker zone is in the direction the waveswere traveling just prior to breaking. In Fig. 10-4,the transport is resolved to show a oomponent onto the beach (B) and a oomponent along the beach (A). The upbeach transport (swash) is balanced by backwash,whereasthe transport of water along the beach forms a long- shore, or littoral, cu"ent. The quantity of water transported by a littoral current is related to the character of the approaching wavesand the angle of approach. The larger the waves or the greater the angle of approach, the stronger the longshore current. On an irregular shoreline, the net movement of water is from head. lands toward embayments (Fig. 10-5). As a result, there is a convergence of longshore currents at the mouth of a bay where water accumulates. This accumulation causesa narrow, swift current, called a rip cu"ent, which movesseawardhom the convergencezone (Fig. 10-6). This example shows only one way in which rip currents may origi- nate. Actually, they are associatedwith any situation where water accu- mulates in the surf zone .to a point where the excessw~ter flows seaward. Several factors can cause'such accumulation: converging longshore cur-
. Dependineon the way in which breaking oa:urs, the ~tory characteristicsof a waveare often maintainedto the extent that the wavemay reform and break several times as it approachesthe beach. Each succeedingbreaker is smaJJerthan the previousone.
THE on
Some riJ width where th These currents of their velocit) The we)" as shown in Fi may last only a sing1ecauseof (a1socalled an parallel to the s: current; the 1at To sum: 1ine is the fon wave approach by the waves c within the surf 1ands) to low c
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FIGtJU 10- Net movement of water in an embayment cause rip currenu to foOD.
r~ts; a physical obstruction to longshore cunents, such as a rock groin (a barrier, illustrated in Fig. 10-18), breakwater, or headland; or just the continuous addition of water from breakers approaching parallel to the beach. Rip currents can be recognized in severalways: discoloration due to suspendedsand, premature steepeningof the approachingwavesand the seawarddisplacement of the breaker line, or the accumulation of foam at the head of the rip. A small cusp of sand may also occur on the beach where the convergenceor obstruction occurs.
NEARSH
tern is veryeft
the ooastline:
Jongshore curr-
of suspension;
shore,or litto
piricalJythat ,
waveenergy 0
Moving
FIGURE 10- Photographof rip currentsin an embayment.
Schematic(
(From:
164
(^166) INSHORE OCEANOGRAPHY
A
'FIGURE 10- A steeprockybeach(A) and Bat sandybeach (B) illustrate the association of beachslopewith sedimentsize. (B courtesyof Clifford E..Moon).
B
FIGURE 10- The sediment size in an embayment decreasesfrom rocks on the exposed headland where wave energy is high to sand at the back of the bay where energy is low.
size of material that can be moved by a given current, and according to its capacity, which is the quantity of material that the Bow is capable of moving. With increasing current speed, both competency and capacity increase. Hence, large waves are usually associatedwith higher rates of littoral drift and move coarsersediment than small wavesdo. In general, beacheson stormy coasts (e.g., in high temperate lati- tudes or polar regions) are steep, narrow, and consist of coarsesand, peb- bles, or cobbles (Fig. 10-8A). Protected beaches and those located in
many tropical areas,where winds and waveshave low energy, frequently are broad, have gentle slopes,and are composedof medium to fine sand
(Fig. l0-8B). Similarly, around headlands, wave energy is high; in em- bayments,wavesare smaller. Bays, therefore, have sandy beaches,whereas the coast along headlands is rocky or consists of quite coarse material (Fig. 10-9). ' , In the swash-backwashzone, sand is transported up the beach face with the swash and moves directly downslope during backwash. Figure 10-10 shows how this motion results in a net driftin~f sand along the beach. This beach drift is generally paraJIelto the direction of longshore drift but is highly irregular. Note that the direction of these two modes of sedimentmovement is dependent upon the direction of wave attack. If the direction of wave attack is resolved into components parallel and per- pendicular to the direction of the beach, the direction of the parallel com-
167
THE OPEN OOAST.
in winter to being progradedduring the summer, becausethe nature of the waves changes seasonally. Fig\Jre 1001lA shows a beach profile as it changesthroughout the year. From April to September,the beach is pro- grading, whereasit retreatsduring the winter season(Septemberto March). Laboratory studies and measurementson beachesshow that short- period, steepwavesfrom local winter storms occurring close to shore have an increasedcapacity and tend to keep sand suspended. Under these ron- ditions, sand is removed from the beach by rip currents and deposited in the seawardpart of the littoral zone. During the summer, the beach is attacked by swells with long wavelengths and low steepnessthat arrive from distant storms. These waves inBuence the Boor of the sea at great distancesoffshoreand gradually return sand shoreward to repair the beach erosion of the previous winter. A winter profile is recognizableby a steep beach face, relatively roarse sediment, and a sea cliff or winter berm (a narrow shelf). A summer beach profile is characterizedby finer sediments, less steep beach face, and smaller berm. Sometimes certain portions of
FIC1JU10-
The beachconfigurationchangesthroughout the yearasa function of wave
conditions (A). The characteristicelementsof a beachprofile ~B).
(A after Office of Naval Research,1957;B after A. N. Strahler, 1963). e20r-- Carmel. calif. IUtion 1-
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170 INSHORE OCEANOGRAPHY
the winter profile (i.e., the winter benD or some offshore bars) remain as remnant features becauseof the inability of the summer waves to affect them (Fig. 1001IB). We can often determine the state of the sediment and the short. term processesacting on a beach sector by examining certain features of a beach. Its configuration in profile and plan, distnoution by grain size of beach sediments,and types of vegetation at the shore indicate whether a beach is being eroded or accreted. In addition, a beach'sfeatures reveal the types of wavesacting on it. The slope of the beach and the height of the berm are determined by the energy in the swashof the waves. An energetic swash will carry sand to the top of the beach berm and deposit it there, thus steepeningthe face of the beach. If the energy of the waves increasessuddenly, a sea cliff quickly forms becauseof the increasedrate of erosion. Under prolonged attack, however, a smooth slope eventually results. If waves are so ener. getic that the swash overtops the benD, the beach will build upward in response. If this action occurs infrequently, coastal winds may erode the berm and build a dune field behind the beach. When conditions remain relatively constant for several years, certain types of vegetation, crawling plants, and beach grasses(Fig. 10-12) will become established and sta- bilize the beach in that configuration. The distribution of various types of sand on a beach reflect the pre- vailing regimen of waves. Extremely coarseand heavy materials tend to
Sedimental by waveact and density, bands p; Examples bands of shell layering minel21s(B). (B) are aPl
FIGURE10. Grassesgrowing on the beach face help to stabilize the sand and promote further deposition.
remain when berm. Extre and so are rei normal wave present in m remain after and stringers zones of con
171
;\I"uUI,;I.~U. ~tnvUS" IJal.UlaJ Dl;:aI,;IIQ UU not OiJways conooor4l~ oDseIVa. tions of beach behavior in models,at least some broad generalizationscan
be drawn. Empirical measurementson natural, beachesare generallynot
satisfactory, becausethe generalizations derived usually apply only to a single location. The movement of sand along the coast is the result of many in- dividual littoral drifts acting on discrete coastal segments. Locally, the cunents move sand from promontories toward embayments. Regionally, the drift tendency is in the direction of the longshore component due to the prevailing wind waves. Most littoral sand is introduced at the mouths of streams. Sand is lost as it is carried by steep, high waves into deep water or submarine canyons,or as it is removed to dunes by onshorewinds (if rainfall is not excessive)as seenin FiSt 10-14. The long-term effect of all these factors is a straightening of an inegular coastline. Whenever either a longshore cunent is interrupted or the waves that provide the energy to maintain the longshore cunent are intenupted, the capacity of that Bow to transport sediment is decreasedand deposition occurs. Figure 10-15 shows'how an embayment interrupting a straight
172
mE OPEN COAST 173
coastaltersthe waverefraction in such a way that depositionof littoral
drift occurs,causingthe formation of a spit or small point of sandacross the mouth of the bay (Fig. 10-16). In caseswherethe embaymenthasa vigorouslyBowingriver at its head, the estuarywill neverbecomecom. pletely sealed. However,the river may be forced to migrate becauseof fluctuationin the dynamicequilibriumbetweensedimentdepositionfrom littoral drift and sedimenterosionby outflowingriver water (Fig. 10-17). Often the way in which a spit has been built indicatesthe direction of sedimentdrift that haspersistedin that areafor a considerablenumberof years. If we extendthis reasoning,we can sometimesinfer the direction of the prevailingwindsor waves. Frequently,the natural tendencyof littoral processesis not de.
FICtJaE 10.. Fonnation of a spit acrossan estuary. The spit builds into the estuarybecauseof waverefractioninto the mouth of the bay. (A) initial situation; (B) wel1-developedspit.
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B FJCVU 10- Photographand line drawingof (A) nearshorerefractionand sedimentation arounda singlegroin; (B) sedimentationassociatedwith a serits of groins.
sirable. Coastal installations are jeopardized if excessiveerosion occurs where they are built or if they are built on an unstable beach, in which wide variations in accretion and erosion take place yearly or over a few years. Sometimes a man-made interruption of littoral drift precipitates sudden changesin the configuration of the coastline. In all these cases,it is desirable to stabilize the beach and prevent excessiveerosion or accre- tion. In other words, littoral drift must be stabilized. In practice, it is erosion of beachesthat must be controlled, for lossesof sand during a prolonged period of erosion tend to be permanent losses. Techniques for controlling beach erosion involve such proceduresas retarding the littoral drift rate, trapping sand or otherwise diverting sand from the heads of submarine canyons,building jetties and breakwaters to protect segmentsof beach exposedto concentrated wave attack, or bring- ing in sand to nourish eroding beaches. Figure 10-18A illustrates the effects of a deh'berateinterruption of sediment drift. This figure showsa groin, or barrier, constructed to extend from the beach out into the breaker zone. To be effective, this groin must extend from the bottom of the surf zone to high-tide level. When a groin is installed, it dams the littoral drift. Thus, sand accumulatesas sedi-
175
.76 (^) INSHORE OCEANOGRAPHY THE OPE
mentation on its updrift side; subsequently,sand is removed (erosion) on its downdrift side. Figure lo.18B shows the effect of a series of groins constructed to increasethe area of a sand beach. Deposition of littoral drift also occurs when the wavesthat provide the energy to move sand are interrupted. A simple breakwater parallel to shore may decreasewave surge in a harbor entrance,but it also causessand deposition as shown in Fig. 10.19. Some other examples of common breakwater configurations are also shown. Note that both a zone of accre- tion and a zone of erosion are associatedwith tbe breakwater. Generally, sand is dredged or pumped from the site of deposition to the zone of ero- sion. Although the cost is considerable,this procedure maintains the bar- bor entrance and minimizes damageby erosion.
OrIgIn
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FLUCTUATIONSIN SEA LEVEL
Sea-levelchangeshave a considerableinftuenceon coastal processes and on beaches. A large tidal range is often associatedwith broad beaches and extensivesalt marshes. Wave energy is greatly diminished by friction over the broad intertidal platfonn, and coastal erosion is intennittent. The strength of tidal currents is associatedwith tidal range. In coastal areasexposedto both large wavesand a large tidal range, erosiveprocesses can be significant in the nearshorezone. Changesin sealevel have occurred in past geologic time as a result of vertical movements of the land and changesin the volume of water in the ocean basins. The land can move either up or down and submergeor rajse coastal areas. Such movements can be related to faulting, folding, and tilting of the earth or to isostatic adjustmen.ts of the crust when heavy loads of ice, sediment, or lava are added or removed. For example. crustal subsidencegenerally occurs in the vicinity of large deltas; crustal rebound follows a period of glaciation. Coasts that have been influenced by recent earth movements are recognized in New Zealand, New Guinea, Japan, California, and around the Mediterranean Sea. Isostatic movements from the melting of conti- nental glaciers are recognizedin Scandinaviaand Canada. The continental glaciers of the Pleistoceneepoch removed signifi- cant volumes of water from the ocean basins. Geological evidence from the land has led scientiststo conclude that the Pleistoceneepoch was char- acterized by alternate advances and retreats of the ice of continental glaciers. Sea-levelfluctuations accompanied these oscillations in ice for-. ~ macon.
Calculationsof the volumeof water frozen in glaciersduring the
last glacial stageindicate that the sealevel must have been loweredby
110to 130m. Thesecalculationsare conoboratedby fathogramsof sub-
mergedshorelinesand by beach sandsdredgedfrom depths to 130 m
throughoutthe world.
The ice of the last glacial stagebeganto melt about 20,000years
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. Tex.. . HoIl~ . E8tem Argentine (peat) . S W LouisilM Cheniers .Austrllia OFIoridI 6 Ceylon D W. Mexico. off Broed CO8StIIPllin 1~L I I I I 1 1 1 1 1 1 1 1 I I 1 I 1 I 1 °1- s.CMXt 1o,CMXt 15.CMXt m.CMXt ~ of v-s befOle ttIe pr-.nt Flcuu 10- Estimatesof the Holocenemarinetransgressionduring the past 20,000years. (Permissionof F. P. Shepard).
Numerous
evidenceof coastdu
ago. Since then, the sea level has risen approximately 120 meters (Fig. 10-20). All of the estuaries,embayments,and ooastalregions of the world have also been inundated by the Holocene marine transgressionof the past 20,000years. Most coastal features representeffects of erosion, depo- sition, and biological growth that have occurred in the past few thousand years.
Wh en rock I durable, cliff ell slowly. Here planes of the ]a orientationof
GEOLOGIC INFLUENCE The degree of erosion in a coastal zone depends on the differences in hardness and durability of rocks. Variations in the character or structure of coastal rocks exposed to waves and seawater thus are accentuated by differential physical or chemical erosion.. The resistance of a rock can be~measured by (1) its hardness or ability to withstand consbnt wave attack, (2) its chemical resistance to the solvent properties of seawater, and (3) its ability to undergo processes of continuous wetting and drying. On exposed coasts where wave forces are great, the harder rocks stand as rocky promontories after the softer ones have been removed (Fig. 10-21), or they form bold cliffs surrounded by shore platfonns. The durability of rocks exposed to identical oceanic con- ditions can be compared in Figs. 100n and 10-23.
178
