Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Sea-Level changePast 40,000 ySea-level rise and migration of barriersShorelines migrate in response to:sea-level risesediment supply (usually small for most barriers, no coastal mountains on trailing-edge margin)shoreline erosion (waves, tidal currents, storms)tectonic motions (not important for most barriers, on trailing-edge margins tectonics are weak)consolidation (not important for most barriers, dominated by sand)Landward migration found for most barriersSeaward migration occurs where much sediment supplied (e.g., near rivers)Mechanisms for landward migrationMust move beach sediment landward:Wind transport to and through dunesWashover during large stormsTidal transport through inletsFlood-tide deltaBeach sand carried by longshore transport reaches inletIf tide is flooding (rising), sand carried into lagoon – where waves are weakSand stops moving and forms flood-tide delta, with distributary channelsSome sand reaches inlet during ebb (falling) tide and some sand is transported out of lagoon by ebbing currents. This sediment forms an ebb-tide deltaEbb-tide deltas are small, due to continued reworking by ocean wavesInlet MigrationSand is removed from longshore transport by: accumulation on upstream side entrapment in the flood-tide deltaRemoval of sand starves the longshore transport system, and causes erosion of the downstream side…causing the inlet to migrate in direction of longshore transportBarrier cross sectionNatural dunes are not continuous, they have breaks that allow washover sediment to nourish some parts of the barrier flatOther parts of the flats are protected and develop maritime forestsTidal flats, salt marshes (temperate) and mangrove forests (tropical) are found near sea level on the lagoon side – contain mud and peatAs barrier migrates landward, mud and peat are buried, and then exhumed on beachDaily tidal fluctuations(actually a little more than 24 hours)Most areas have semidiurnal fluctuations, with two nearly equal high and low tides each daySome areas have diurnal fluctuations, with one high and one low each dayOther areas have mixed semidiurnal fluctuations, with two highs and two lows of unequal elevationCause of TidesGravitational attraction of moon/sun creates bulge of ocean waterCentrifugal force creates second bulgeEarth rotates through both bulges in ~24 hours, causing two high and two low tides each dayMonthly fluctuations in tidesOver ~28 days, orientation of moon and sun changes with respect to EarthThis causes two periods of large tidal range (spring tides) and two periods of small tidal range (neap tides) each monthWhy we have monthly changes in tidesGravitational attraction from moon and sun pull water toward themThis creates two bulgesAs the Earth rotates through these bulges each day, locations experience changing sea levelOver a ~28-day period, the orientation of the moon and sun change, creating different tidal ranges through monthSpring tide = large differencesNeap tide = small differencesTidal range(vertical difference between high and low tide)Macro > 4 m Meso = 2-4 m Micro <2 mLocal differences in geometry of seabed can increase or decrease tidal rangeLagoonal environmentsIntertidal environments (between high and low tide) surround lagoonThey trap and accumulate sediment, filling lagoonTidal-flat sedimentationMud transported as suspended load accumulates on high flatSand transported as bedload accumulates on low flatUpward growth ultimately controlled by rate of sea-level riseAsymmetry between flood and ebb currentsFrictional interaction with seabed commonly causes flood current to be strongerThis causes more sediment to be transported into the lagoon and onto the tidal flats, enhancing accumulationMarsh vegetationMany niches develop, depending on many variables, e.g.,: salt and soaking tolerance, and current velocityVegetation helps to baffle flow, reduce tidal current velocity, and enhance sediment accumulationMangrove vegetationMangrove distributionFound in warm, tropical settingsOcean circulation extends latitudinal distribution on west sides of ocean basins, and reduces distribution on east sidesSediment Budget for Beaches and CoastsSediment Sources:Longshore drift (local source)Cliff erosionRiversBiogenic shellsContinental shelfSediment Sinks:DunesLagoon (washover, tidal inlets)Submarine canyons (unusual)Longshore drift (local sink)Prograding shoreline, building seawardRequires sediment supply to exceed processes leading toward landward movement of shoreline.Rivers are most common supply mechanism(example from east Texas coastline, downstream of Mississippi supply)Prograding beach ridgesImpact of Groins to ShorelineDeposit sediment on upcurrent side, erode beach on downcurrent sideDeflects longshore transport farther offshoreJetty entrapment of sedimentSediment trapped on upstream side, due to longshore transportLoss of sediment causes erosion on downstream side, to resupply longshore transport systemSimilar to entrapment associated with groins, but on larger scalegroinsHuman Beach
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