beaches - wordpress.com · 2018-05-20 · beaches: • depositional landform • extends from the...
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Beaches:• Depositionallandform• Extendsfromthehighesthightidetothelowestlowtide.• Veryimportanttemporarystoreincoastalsystem.• Accretion:sedimentreturningtothevisibleportionofabeach• Beachaccretionwilltakeplaceduringaprolongedperiodofconstructivewaves,driven
bystorms100’sofmilesaway.• Destructivewaves,resultinginlocalisedstormsmayexcavate(dig/remove)thebeach
removingalotofsedimentandcouldexposewave-cutplatforms.
ChesilBeach,DorsetUKHighenergy,shinglecoastline
BrightonBeach,SussexUKPebblebeach 1
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Swash-alignedBeaches:• Forminlowenergyenvironmentse.g.
bays• Wavesarriveparalleltotheshore• Couldbesandorshingledependingon
natureofsedimentandpowerofwaves• Highenergywavestransportsandbut
depositcoarsershingle• Lowenergywavesdepositsandor
mud.
Drift-alignedBeaches:• Formwhenwavesapproachthecoast
atanangle.• Longshoredriftmovessedimentalong
thebeach,oftenformingaspitwhichisasedimentstoreorsink.
• Sedimentmaybegradedonthedrift-alignedbeach.Finershingleparticlesarecarriedfurtherbylongshoredriftandbecomeincreasinglyroundedastheymove.
Beachesaredescribedasswash-alignedordrift-alignedwhichrelatestotheirorientationrelativetotheprevailingwindandwavedirection.
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Beachforms:• Beachesarepartofabiggerdepositionareaextendingoffshore.• Characteristicsofbeachesare:berms,cusps,runnels.• Bermsareridgesthatdeveloponthebeachparalleltothewater,showingtheaverage
hightidemark,resultingfromdeposition.• Bermsrepresentdifferenttidallevels.• Stormbermsshowthehighestpointonabeach.• Bermsaremadefromsandorpebbles.
BrightonBeachUK,(abovecentre,andright)hasalotofBerms.Whathappenedinthispicture?
Above=sandyberm
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Cusps=semi-circulardepressionsformedbyacollectionofwavesreachingthesamepoint.Thesideschanneltheincomingswashandproducesastrongerbackwashwhichdragsmaterialdownthebeachfromthecentreofthecusp.
Runnels(ripples)=thespreadingoutofwavesenergyacrossawideareaofbeachcreatingridgesandinterveningdepressions.Theyarecommononshallow,sandybeaches.
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Spits:• Longnarrowfeature• Madeofsandorshingle• Extendsfromlandintothesea,
orpartwayacrossanestuary.• Formondrift-alignedbeaches• Formedbylongshoredrift
continuingwhenthecoastlinechangesdirectione.g.becauseofariverestuary.Sedimentbuildsupacrosstheestuarymouthandaspitwillform.Theflowoftheriverwillstopthespitfromextendingacrosstheentiremouth.
• Arecurvedtipiswhentheendofthespitcurvesaroundbecausewaverefractioncariesmaterialroundintothemoreshelteredwaterbehindthespit. 5
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Tombolo:• Isabeachorridgeofsandandshinglethathasbeenformedbetweenasmall
islandandthemainland.• Depositionoccurswherewavesloseenergyandthetombolobeginstobuildup.• Tombolo’smaybecoveredathightide.E.g.stNinian’sintheShetlandIslands
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Offshorebars:• Alsoknownassandbars• Submergedorpartlyexposedridgesofsandorcoarsesediment• Createdbywavesoffshorefromthecoast.• Destructivewaveserodesandfromthebeachwiththeirstrongbackwashanddepositit
offshore• Offshorebarsactasbothsedimentsinksandpotentiallysedimentinputstores.• Theycanabsorbwaveenergytherebyreducingtheimpactsofwavesonthecoastline.
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BarrierBeaches:• Alsocalledbar/barrierbar.• Whereabeachorspitextendsacrossabaytojointwoheadlands.• E.g.StartBayDevon,whichis9kmlongandisformedfromroundedshingledeposits
(mostlyofflintandquartzgravel)• CantrapwaterbehindandcreatealagoonsuchasSlaptonLey.• BarrierbeachesandbarsonthesouthcoastofEnglandarebelievedtohavebeen
depositedfollowingrisingsealevelsafterthelastglacialperiod.SedimentdepositedbymeltwaterinwhatisnowtheEnglishChannelwasbulldozedonshoretoformthepresent-daybarrierbeachorbar.
• Laterlongshoredrifthasaddedmorematerialandreworkedthesediment.• Thisisagoodillustrationofenergyflowsanditalsodemonstratestheimportanceof
timeintheformationofpresent-daylandforms.• Wecannotalwaysassumethatlandformsaretheresultsolelyofprocessesoperating
atthepresenttime.
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BarrierIslands:• Whereabeachbecomesseparatedfromthemainlanditisreferredtoasabarrier
island.• Barrierislandsvaryinscaleandform.• Theyareusuallysandorshinglefeatures• Commoninareaswithlowtidalrangesandwheretheoffshorecoastlineisgently
sloping.• Large-scalebarrierislandscanbefoundalongthecoastoftheNetherlands,andin
NorthAmericaalongtheSouthTexascoast.
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Sanddunes:• Sandybeachesmaybebackedbysanddunes.E.g.StudlandinDorset.• Sandhasbeenblownoffthebeachbyanonshorewind.• Anoffshoresandbarisanidealsourceofsandforasanddune.• Prerequisitesforsanddunestoform:
q Largequantitiesofavailablesand,washedonshorebyconstructivewaves.q Largetidalrangecreatinglargeexposureofsandthatcandryoutatlow
tideq Dominantonshorewindsthatwillblowdriedsandtothebackofthebeach
• Dunesdevelopwheresandisinitiallytrappedbydebristowardsthebackofthebeach.
• Vegetationhelpstostabilisethesandandgraduallydunesdevelop.
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Bolonia,Andalucia,SouthSpain
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Sanddunevegetationsuccession:• Thefirstcolonisingspeciesarecalledpioneer
species• Pioneerspecieshaveadaptationstohelpthem
survivethehostileconditions.• Searocketandcouchgrassareplantswhichareable
tocopewithverydry,saltyandexposedconditions.• Embryodunesarethefirstdunestodevelop• Foredunesareoncethesanddunebecomesbigger,
andisyellowincolourbutovertimedarkentogreyduetodecayingplansaddinghumus.Marramgrassisfoundinthiszoneandisadaptedwithlongtaprootstoseekwater.Therootshelpbindthesandandaddstabilitytothedune.
• Depressionsbetweendunescandevelopintoduneslackswherethereismoredampconditionsbecausethewatertableisclosetoitoratthesurface.
• Thefinalcommunitywillbeadjustedtotheclimaticconditionsoftheareaandisknownastheclimaticclimaxcommunity.
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Estuarinemudflats:² Riverestuariesareimportantsedimentstores(sinks)wherehugequantitiesofriver
sedimentisdepositedinwaterclosetotheedgesoftheriverawayfromthefastertidalcurrentsthatscourthechannels.
² Risingtidescreateabuffertotheriverflow,slowingvelocityandleadingtoconsiderabledeposition.
² Mostofthesedimentthataccumulateshereismud,duetothelowvelocities,andovertime,expansivemudflatscanformthatthendevelopintosaltmarshes.
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Saltmarshes:q Saltmarshesareareasofflat,siltysedimentsthataccumulatearound
estuariesorlagoons.q Theydevelopinthreetypesofenvironment:
q Shelteredareaswheredepositionoccurs(e.g.leeofaspit)q Wheresaltandfreshwatermeet(e.g.estuaries)q Wheretherearenostrongtidesorcurrentstopreventsediment
depositionandaccumulation.q TheyarecoveredathightideandcommonaroundtheBritishcoast.
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Saltmarshvegetationsuccession:q Theydevelopovertimewithvegetationsuccession.q Tostartmudisdepositedclosetothehightidelinedroppingoutofthewaterby
aprocessknownasflocculation.Flocculationiswhentinyparticlesofclay/mudsticktogethertoeventuallyhavethecombinedmasstosinktotheseabed.
q Eelgrassandcordgrassstarttocolonisethetransitionzonebetweenhightideandlowtide.Theseplantscantolerateinundationbysaltwaterandtheyalsohelptrapinthemuddeposits.
q Oncethemudlevelrisesabovethehightidemarkalowersaltmarshdevelopsawiderrangeofplantsthatnolongerneedtobewelladaptedtosaltyconditions.
q Soilconditionsimproveandvegetationsuccessioncontinuestoformameadow.q Eventuallyshrubsandtreeswillcolonisetheareaasthesuccessionreachesits
climaticclimax.