ONE TWO THREE FOUR FIVE SIX SEVEN EIGHT NINE TEN A Lc c c L 1 s h l J Ii 0 c eo f r t cc inshore ELEVEN TWELVE THIRTEEN FOURTEEN A large ocean a complex CO courtesy of FIFTEEN At the coastal zone of the world ocean air water and solid earth meet and there is endlessinteraction among the geological biological meteorological and oceanicprocesses Each of these environmental processes affects the nature of a coastal region to some degree Consequently a study of the processes at work in a coastalsectorcan help us understand the origin relative age and history of that coast 10 THE OPEN COAST OCEANIC INFLUENCE The diverse in8uencesof ocean waters upon coastal features are related to both the physical attributes of seawater i e waves currents and turbuJence and the chemical properties i e solubility and ooncentration 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 NEARSHORE CmCULATION As wavescarryenergytowardshore th encounter shallow water and their speedof propagation decreases If the 160 Photo I zone The d direction of wardpart of occurs Fig 2 uses wa ve this alignmel pletely para offshore profi 161 FICUU 10 1 A large ocean swell refracts against a complex coastline Photograph courtesy of Barbee Scheibner A B FIGURE 10 2 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 shoreward part of a waveslowsmore than the seawardportion and refraction occurs Fig 10 1 Figure 10 2 showshow the processof refraction causes wavecreststo align themselves parallelto the shoreline However this alignment is only a tendency Wave crestsseldom becomecompletely parallel to the shore exceptwherethe beachhas a long gentle offshoreprofile 161 THE OF f I I j t FIGURE 10 3 Schematicof refractionon an irregularcoast Wavesareapproachingparallel to the coastaltrend Solid lines representwavecrests Dashedlines are waveorthogonals After A N Strahler 1963 The behavior of wavesin expending their energy on a beach can be illustrated by the use of orthogonals These are imaginary lines drawn in such a way that they divide the crest of an unrefracted wave into equal segmentsof length and energy Orthogonals are always perpendicular to wave crestsand thus show the direction of wave propagation Fig 10 3 At the sametime orthogonals indicate the distn bution of energy in a wave train the amount of energy contained between any pair of orthogonals is assumedto be constant regardlessof refraction Where waves attack an FIGURE 10 4 Direction of littoral currentsresultingfrom wavesbreakingat an angleto the shore K irregular coastl focused on it amount of ene However this shorter stretch coastline is inc effect is that time it tends t of wave attack AsaM continually in ally the crest considerations when either of than 1 28 tim passes1 7 A tensely turbulc rather than os the beach is under the inBl water from thc doing the ono The tn the waveswerc is resolved to along the bea backwash wb shore or litt01 current is rela of approach the stronger t On an lands toward c of longshore This accumul which moves This e nate Actual mulates in thc Several factor Dependingon wave are of tell times as it a previous one 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 Eventually 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 surpasses1 7 Mer a wave breaks the water particle motion becomes intensely 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 longshore 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 originate Actually they are associatedwith any situation where water accumulates 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 breakingoa 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 7 V 1 w cresIS FIGtJU 10 5 Net movement of water in an embayment cause rip currenu to foOD r ts a physical obstruction to longshore cunents such as
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