ONETWOTHREEFOURFIVESIXSEVENEIGHTNINETENELEVENTWELVE.. THIRTEENFOURTEENFIFTEEN'~, A.,,:Lc.c#,c.L.~ ~1s.h,.-~ ~; - '.,{... l..J., Ii0 c. eo " f(> r ~ -t . cc.. - ~ ...{ ,inshoreA large ocean :a complex CO&:courtesy ofAt the coastal zone of the world ocean, air, water, and solid earth meet;and there is endless interaction among the geological, biological, meteoro-logical, and oceanic processes. Each of these environmental processesaffects the nature of a coastal region to some degree. Consequently, astudy of the processes at work in a coastal sector can help us understandthe origin, relative age, and history of that coast.10,THE OPEN COAST~Photo (/Izone. The d:OCEANIC INFLUENCEThe diverse in8uences of ocean waters upon coastal features are re-lated to both the physical attributes of seawater (i.e., waves, currents, andturbuJence) and the chemical properties (i.e., solubility and ooncentra-tion). The effect of ocean waves, how~r, is the most important. Theoonfiguration of a coastal area and the offshore floor of the sea is l.argelythe resuJt of wave action. For this reason, the properties of shoaling wavesare emphasized in this section.direction of ,ward part ofoccurs ( Fig.(2 uses wa ve (this alignmelpletely para])offshore profiNEARSHORE CmCULATION. As waves carry energy toward shore, th~encounter shallow water and their speed of propagation decreases. If the160161FICUU 10-1A large ocean swell refracts againsta complex coastline. (Photographcourtesy of Barbee Scheibner).ABFIGURE 10-2Photo (A) and line drawing (B) of wave refraction just seaward of the surfzone. The direction of longshore currents is from right to left along the beach.direction of wave attack is not perpendicular to t~ shoreline, the shore-ward part of a wave slows more than the seaward portion and refractionoccurs (Fig. 10-1). Figure 10-2 shows how the process of refractioncauses wave crests to align themselves parallel to the shoreline. However,this alignment is only a tendency. Wave crests seldom become com-pletely parallel to the shore except where the beach has a long, gentleoffshore profile.161THE OF]irregular coastlfocused on it;amount of eneHowever, thisshorter stretchcoastline is inceffect is that,time, it tends tof wave attack.AsaMcontinually in(ally, the crestconsiderationswhen either ofthan 1.28 tim(passes 1/7. Atensely turbulcrather than osthe beach is (under the inBlwater from thcdoing, the onoThe tnthe waveswercis resolved toalong the beabackwash, wb.shore, or litt01current is rela!of approach.the stronger t}On anlands toward cof longshore,This accumulwhich moves !This enate. Actual]mulates in thcSeveral factor~~~f=::::=:~\ ,., - II/~/~~-~'j~~::~~~+ t-!FIGURE 10-3Schematic of refraction on an irregular coast. Waves are approaching parallelto the coastal trend. Solid lines represent wave crests. Dashed linesare wave orthogonals. (After A. N. Strahler, 1963).The behavior of waves in expending their energy on a beach can beillustrated by the use of orthogonals. These are imaginary lines drawn insuch a way that they divide the crest of an unrefracted wave into equalsegments of length and energy. Orthogonals are always perpendicular towave crests and thus show the direction of wave propagation (Fig. 10-3).At the same time. orthogonals indicate the distn'bution of energy in a wavetrain; the amount of energy contained between any pair of orthogonals isassumed to be constant regardless of refraction. Where waves attack anFIGURE 10-4Direction of littoral currents resulting from waves breaking at anangle to the shore.")K. Depending onwave are of telltimes as it a]previous one.163THE OPEN COASTirregular coastline, a point of land refracts the waves so that energy isfocused on it; an embayment has the opposite effect. In Fig. 10-3, theamount 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 ashorter stretch of beach at C; consequently, the energy per unit length ofcoastline is increased in section C and decreased in section B. The neteffect is that wave erosion is greater on the headlands. Given enoughtime, it tends to straighten an irregular coastline, regardless of the directionof wave attack.As a wave approaches shore, the orbital velocity at the wave crestcontinually increases, whereas the propagation speed decreases. Eventu-ally, the crest "overruns" the trough, and the wave breaks. Theoreticaloonsiderations as well as field observations indicate that breaking occunwhen either of two oonditions are met: (1) the water depth becomes lessthan 1.28 times the wave height, or (2) the wave steepness (H/L) sur-passes 1/7. Mer a wave breaks, the water-particle motion becomes in-tensely turbulent, and energy approaches the beach in a wave of translationrather than oscillation.. The mass of turbulent water that moves uponthe beach is called swczsh. The water that runs back down the beachunder the influence of gravity is termed b«kwash. Backwash removes thewater from the beach and causes it to oppose the onooming waves. In sodoing, the oncoming waves are steepened and break a bit sooner.The transport of water inside the breaker zone is in the directionthe waves were traveling just prior to breaking. In Fig. 10-4, the transportis resolved to show a oomponent onto the beach (B) and a oomponentalong the beach (A). The upbeach transport (swash) is balanced bybackwash, whereas the transport of water along the beach forms a long-shore, or littoral, cu"ent. The quantity of water transported by a littoralcurrent is related to the character of the approaching waves and the angleof 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 convergenceof longshore currents at the mouth of a bay where water accumulates.This accumulation causes a narrow, swift current, called a rip cu"ent,which moves seaward hom the convergence zone (Fig. 10-6).This example shows only one way in which rip currents may origi-nate. Actually, they are associated with any situation where water accu-mulates in the surf zone .to a point where the excess w~ter flows seaward.Several factors can cause' such accumulation: converging longshore
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