Waves, Beaches Waves, Beaches & & Coastal ErosionCoastal Erosion

Would You Live Here?Would You Live Here?

Florida Keys

WavesWaves

• Transport energy over a body of waterTransport energy over a body of water• Can cause coastal erosionCan cause coastal erosion

WavesWavesGenerationGeneration

•Generated by Force Generated by Force

– Sea Wrinkles Sea Wrinkles ripples ripples wind waves wind waves swells swells

• Generated at storm centersGenerated at storm centers

• Separation of waves by Separation of waves by differing rates of travel - differing rates of travel - dispersiondispersion

– wind or airwind or air

WavesWavesSizeSize

•Factors determining size:Factors determining size:º FetchFetch

º Wind velocityWind velocityº DurationDuration

– Distance wind blows over open oceanDistance wind blows over open ocean– Speed of windSpeed of wind

– Length of time wind blowsLength of time wind blows

Wave BehaviorWave BehaviorRefractionRefraction

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

• Therefore, Wave bendsTherefore, Wave bends

• Crest over shallow water slows; crest over deeper water Crest over shallow water slows; crest over deeper water does notdoes not

Wave BehaviorWave BehaviorBeach ErosionBeach Erosion

• Wave rays travel perpendicular to wave crestWave rays travel perpendicular to wave crest• Energy focused on headlands Energy focused on headlands erosion erosion

Wave BehaviorWave BehaviorLongshore Current and TransportLongshore Current and Transport

•Current that moves sediment parallel to shoreCurrent that moves sediment 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• Zig-zag motion has an overall net transport; longshore transportZig-zag motion has an overall net transport; longshore transport

Beach compartments in Beach compartments in southern Californiasouthern California

• Beach Beach compartmecompartments include:nts include:– RiversRivers– BeachesBeaches– Submarine Submarine

canyonscanyons

Figure 10-12

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

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 beachNarrow beach

– Carry sand to shore but Carry sand to shore but too weak to carry back too weak to carry back to seato sea– Wider beach resultsWider beach results

winterwintersummersummer

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

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 downcurrent of a downcurrent of a headlandheadland

Barrier IslandsBarrier Islands

• 15,000 years ago sea level was 15,000 years ago sea level was ~~85 m below present85 m below present

• As sea level rose, a breach in ridge resulted in floodingAs sea level rose, a breach in ridge resulted in flooding

• As sea level continues to rise, ridge migrates landwardAs sea level continues to rise, ridge migrates landward

North CarolinaGalveston, Texas

Baymouth BarsBaymouth Bars

• Bars are temporary structuresBars are temporary structures• Beach through barBeach through bar

Alabama

SandspitSandspit

• Depositional features are constantly migratingDepositional features are constantly migrating

New Zealand

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

Wave Cut TerraceWave Cut Terrace

Santa Barbara, California

Beach ErosionCaliforniaBeach ErosionCalifornia

– 86% of California coasts are 86% of California coasts are classified as actively erodingclassified as actively eroding

– 1 in 4 homes within 500 1 in 4 homes within 500 feet of coast will be lost to feet of coast will be lost to erosion in 60 yearserosion in 60 years

• Beach erosion is a serious Beach erosion is a serious problem for coastal problem for coastal communitiescommunities

• Some causes:Some causes:– Sea level riseSea level rise

– Building near coastBuilding near coast

– Efforts to reduce erosionEfforts to reduce erosion

Palos Verdes PeninsulaPalos Verdes PeninsulaMalibu, California

Beach ErosionBeach ErosionCaliforniaCalifornia

Santa Barbara, CaliforniaSanta Barbara, California

200220022004200420052005

Sand Sources and Beach Sand Sources and Beach SupplySupply

• Rivers and StreamsRivers and Streams– In Southern California beaches are wider near river mouthsIn Southern California beaches are wider near river mouths

• Eroding beach cliffs – 5 – 10% of beach sandEroding beach cliffs – 5 – 10% of beach sand– Generally sandstone and granitic cliffs are good sand Generally sandstone and granitic cliffs are good sand

sourcessources– Off-shore reefs may supply sandOff-shore reefs may supply sand

• Sand from the seafloor immediately offshoreSand from the seafloor immediately offshore– Barrier islands of the Atlantic coast are examplesBarrier islands of the Atlantic coast are examples

Threats to the Sand SupplyThreats to the Sand Supply • Flood control damsFlood control dams

– They have virtually eliminated the sand supply to S. They have virtually eliminated the sand supply to S. California beachesCalifornia beaches

– Ventura River sand supply is reduced to 66% of Ventura River sand supply is reduced to 66% of normalnormal

• Southern California has 77 sand and gravel quarries in Southern California has 77 sand and gravel quarries in stream channelsstream channels– An annual sand and gravel production of 20 million An annual sand and gravel production of 20 million

tonstons• Paved river channels – reduces channel wideningPaved river channels – reduces channel widening• Seawalls and riprap reduce cliff erosion and focus wave Seawalls and riprap reduce cliff erosion and focus wave

energy onto the beachenergy onto the beach

Coastal ErosionCoastal Erosion

• Erosion may be SeasonalErosion may be Seasonal– Worst erosion may occur during severe stormWorst erosion may occur during severe storm– Low pressure raises water levelLow pressure raises water level– Waves stronger during stormWaves stronger during storm

• A wave 10 feet high and 90 feet in wave length can A wave 10 feet high and 90 feet in wave length can exert 10 tons of pressure/mexert 10 tons of pressure/m22 on a seacliff on a seacliff

Mechanism of ErosionMechanism of Erosion

• Hydraulic impact – water driven into Hydraulic impact – water driven into cracks and rock breakscracks and rock breaks

• Abrasion – sand and boulders Abrasion – sand and boulders abrade cliffabrade cliff

• Solution-- chemically dissolving Solution-- chemically dissolving rockrock

• Biological activity – animal borings, Biological activity – animal borings, peoplepeople  

Factors Affecting How Much Factors Affecting How Much Erosion OccursErosion Occurs

• Natural factors Natural factors – Width of the beachWidth of the beach– Amount of wave energyAmount of wave energy– Nature of the rock presentNature of the rock present

Factors Affecting How Much Factors Affecting How Much Erosion OccursErosion Occurs

• Humans ImpactsHumans Impacts– Cliff tops are loaded with buildingsCliff tops are loaded with buildings– Surface drainage is alteredSurface drainage is altered– Roads, seawalls, etc. are constructedRoads, seawalls, etc. are constructed

Pacific Coast ErosionPacific Coast Erosion

• Sea cliff retreat rates vary from nearly Sea cliff retreat rates vary from nearly zero to 9 feet per yearzero to 9 feet per year– Marine terraces in many places are Marine terraces in many places are

eroding 1 foot/yreroding 1 foot/yr

• Erosion has caused some Erosion has caused some developments to establish a setback developments to establish a setback calculated to last 50 years.calculated to last 50 years.– The setback serves as a common park The setback serves as a common park

areaarea

Planning for ErosionPlanning for Erosion

• Erosion must be incorporated into Erosion must be incorporated into coastal land use planscoastal land use plans– Setbacks must be establishedSetbacks must be established

• Real estate agents must by law reveal Real estate agents must by law reveal problems to coastal buyersproblems to coastal buyers

Control of Coastal ErosionControl of Coastal Erosion

• Riprap – large boulders at base of sea cliff Riprap – large boulders at base of sea cliff – Riprap should not be discontinuousRiprap should not be discontinuous

• Seawalls Seawalls – Accelerate erosion of adjacent propertyAccelerate erosion of adjacent property

• Groins – are built to block the longshore driftGroins – are built to block the longshore drift• BreakwatersBreakwaters

– Reduce wave action on the coastReduce wave action on the coast– Santa Monica breakwater requires continuous Santa Monica breakwater requires continuous

dredgingdredging– Santa Barbara breakwater is attached to the coast Santa Barbara breakwater is attached to the coast

and requires 350,000 Mand requires 350,000 M33/yr of dredging/yr of dredging  

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– Restore erosionRestore erosion

– Erodes downcurrent sideErodes downcurrent side

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

– Erosion and accretionErosion and accretion

TsunamisTsunamis

• Cause erosion, destruction of property, loss of Cause erosion, destruction of property, loss of liveslives

Coastal Zone ManagementCoastal Zone Management

• Federal EffortsFederal Efforts• Coastal Zone management Act of 1973 passed Coastal Zone management Act of 1973 passed

to support state efforts to manage coastal to support state efforts to manage coastal developmentdevelopment

• Flood Insurance Act of 1973Flood Insurance Act of 1973– Requires mapping and avoidance of coastal hazardsRequires mapping and avoidance of coastal hazards– Program has not worked well, owners rebuilding in sites of Program has not worked well, owners rebuilding in sites of

homes destroyed by stormshomes destroyed by storms

• State EffortsState Efforts– The Coastal Initiative passed in 1972The Coastal Initiative passed in 1972– California Coastal Act established the Coastal Commission in California Coastal Act established the Coastal Commission in

19761976

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

Let Nature Takes Its CourseLet Nature Takes Its Course

• Remove Artificial BarriersRemove Artificial Barriers

• Move houses away or backMove houses away or back

• Learn to live with nature not change itLearn to live with nature not change it