BeachesBeaches

What is a beach?What is a beach?

• Area along coast where sediment accumulatesArea along coast where sediment accumulates• Coastal zones begin when waves interact with seafloorCoastal zones begin when waves interact with seafloor

BeachesBeachesShoreline DivisionsShoreline Divisions

•Littoral zoneLittoral zone

o ShorefaceShoreface

o ForeshoreForeshore

o Backshore Backshore

– Part of coast that stretches from land plants to where Part of coast that stretches from land plants to where waves start to breakwaves start to break

– Permanently covered by waterPermanently covered by water

– Exposed at low tide and covered at high tideExposed at low tide and covered at high tide

– above mean high tide – only affected by stormsabove mean high tide – only affected by storms

BeachesBeachesShoreline DivisionsShoreline Divisions

Divided by wave action:Divided by wave action:• Breaker zoneBreaker zone– Where waves become unstable and breakWhere waves become unstable and break• Surf zoneSurf zone– Shallower waves move up beach faceShallower waves move up beach face• Swash zoneSwash zone– alternately covered by “swash” and uncovered by “backwash”alternately covered by “swash” and uncovered by “backwash”

BeachesBeachesSediment ProfileSediment Profile

• Longshore barsLongshore bars

•BermBerm– Flat-top ridge of backshoreFlat-top ridge of backshore

• Berm crestBerm crest– High point on a beachHigh point on a beach

• Beach faceBeach face– Sloping portion of the beachSloping portion of the beach

– In winter, sediment from berm is In winter, sediment from berm is transported below breaker zonetransported below breaker zone

BeachesBeachesTypesTypes

•Shape and structureShape and structure

•CompositionComposition

•SizeSize

•ColorColor

widwidee

narrownarrow

QuartzQuartzCobbleCobblePink sandPink sand

ShellShellsandsandBlack sandBlack sand

BeachesBeachesSummer and WinterSummer and Winter

• Summer Summer •Winter Winter – Erosive storm wavesErosive storm waves– Gentle wavesGentle waves

– Sand carried seaward Sand carried seaward to offshore sandbarsto offshore sandbars

– Narrow, rocky beachNarrow, rocky beach

– Carry sand to shore but Carry sand to shore but too weak to carry back too weak to carry back to seato sea– Wide, sandy beachWide, sandy beach

summersummer winterwinter

Beach ProcessesBeach ProcessesRefractionRefraction

•Wave crests approach shore at an angleWave crests approach shore at an angle

• Energy focused on headlands Energy focused on headlands erosion erosion

• Wave rays travel perpendicular to wave crestWave rays travel perpendicular to wave crest• Wave bendsWave bends• Slows over shallow waterSlows over shallow water

Beach ProcessesBeach ProcessesRip CurrentsRip Currents

• Incoming waves create underwater Incoming waves create underwater sandbar close to shoresandbar close to shore

• Waves cause sandbar to collapseWaves cause sandbar to collapse

• Excess water is pushed through gapExcess water is pushed through gap

• Creates a strong, narrow Creates a strong, narrow current away from shorecurrent away from shore

Beach ProcessesBeach ProcessesLongshore Current and TransportLongshore Current and Transport

•Longshore current moves parallel to shoreLongshore current moves parallel to shore

– SwashSwash moves sediment onto beach at an angle moves sediment onto beach at an angle– BackwashBackwash moves sediment down perpendicular to shore moves sediment down perpendicular to shore

• Longshore transport moves sediment along shore by LS current Longshore transport moves sediment along shore by LS current • Zig-zag motion has an overall net transportZig-zag motion has an overall net transport

Beach DynamicsBeach DynamicsDeposition and ErosionDeposition and Erosion

•DepositionDeposition

•ErosionErosion

– Reshaping of Reshaping of coastline by wave coastline by wave actionaction

– Coasts build out by Coasts build out by waves and currentswaves and currents

Sandy shoreSandy shore

Rocky shoreRocky shore

– Little incoming sand, Little incoming sand, usually no major usually no major river river

– Accumulation of sedimentAccumulation of sediment

– Sediment is removedSediment is removed

BeachesBeachesDepositional FeaturesDepositional Features

•Sand spitsSand spits

•Barrier islandBarrier island– Long sand island Long sand island

enclosing a bay or enclosing a bay or lagoonlagoon

•Bay mouth barsBay mouth bars– When a sand spit When a sand spit

closes off a baycloses off a bay

– Longshore current Longshore current deposits sand in a line deposits sand in a line downcurrentdowncurrent

BeachesBeachesErosional FeaturesErosional Features

•ArchesArches

•TomboloTombolo– Sand bar connecting to Sand bar connecting to

a rock or offshore a rock or offshore outcropoutcrop

•StacksStacks– Offshore column Offshore column

formed from collapsed formed from collapsed archarch

– Weaker rocks erode Weaker rocks erode

ArchArchStackStack

TomboloTombolo

BeachesBeachesErosional FeaturesErosional Features

•Wave cut benchWave cut bench

•CliffCliff– Cliff bottom attacked by wavesCliff bottom attacked by waves– Waves undercut cliff to form Waves undercut cliff to form wave cut notchwave cut notch– Overhanging rock collapseOverhanging rock collapse

– As cliff retreats, area of flat rock exposedAs cliff retreats, area of flat rock exposed

CliffCliff Wave cut benchWave cut bench

BeachesBeachesErosional FeaturesErosional Features

•Marine terraceMarine terrace

– If sea level lowers If sea level lowers or land uplifted, or land uplifted, wave cut bench wave cut bench becomes marine becomes marine terraceterrace

BeachesBeachesHuman ImpactsHuman Impacts•Sea wallsSea walls

• BreakwatersBreakwaters– Wall parallel to shoreWall parallel to shore– Reduce wave energyReduce wave energy– Interrupts beach driftInterrupts beach drift

• JettiesJetties– Designed to Designed to

maintain navigation maintain navigation routesroutes

• GroinsGroins– Wall perpendicular to shoreWall perpendicular to shore– Stops erosionStops erosion– Erodes downcurrent sideErodes downcurrent side

– Parallel concrete structuresParallel concrete structures– Absorbs wave energyAbsorbs wave energy– Actually promotes erosionActually promotes erosion

– Erosion and depositionErosion and deposition

BeachesHuman intervention

•Beach nourishmentBeach nourishment

•DredgingDredging– Import sand into coastal areasImport sand into coastal areas

– Underwater excavation to clear Underwater excavation to clear waterwayswaterways

beforebefore afterafter

Miami BeachMiami Beach