Coastal Migration Sea Level change Past 40 000 y Sea level rise and migration of barriers Shorelines migrate in response to sea level rise sediment supply usually small for 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 barriers Seaward migration occurs where much sediment supplied e g near rivers Mechanisms for landward migration Must move beach sediment landward Wind transport to and through dunes Washover during large storms Tidal transport through inlets Flood tide delta Beach sand carried by longshore transport reaches inlet If tide is flooding rising sand carried into lagoon where waves are weak Sand stops moving and forms flood tide delta with distributary channels Some sand reaches inlet during ebb falling tide and some sand is transported out of lagoon by ebbing currents This sediment forms an ebbtide delta Ebb tide deltas are small due to continued reworking by ocean waves Inlet Migration Sand is removed from longshore transport by accumulation on upstream side entrapment in the flood tide delta Removal of sand starves the longshore transport system and causes erosion of the downstream side causing the inlet to migrate in direction of longshore transport Barrier cross section Natural dunes are not continuous they have breaks that allow washover sediment to nourish some parts of the barrier flat Other parts of the flats are protected and develop maritime forests Tidal flats salt marshes temperate and mangrove forests tropical are found near sea level on the lagoon side contain mud and peat As barrier migrates landward mud and peat are buried and then exhumed on beach Tides Daily tidal fluctuations actually a little more than 24 hours Most areas have semidiurnal fluctuations with two nearly equal high and low tides each day Some areas have diurnal fluctuations with one high and one low each day Other areas have mixed semidiurnal fluctuations with two highs and two lows of unequal elevation Cause of Tides Gravitational attraction of moon sun creates bulge of ocean water Centrifugal force creates second bulge Earth rotates through both bulges in 24 hours causing two high and two low tides each day Monthly fluctuations in tides Over 28 days orientation of moon and sun changes with respect to Earth This causes two periods of large tidal range spring tides and two periods of small tidal range neap tides each month Why we have monthly changes in tides Gravitational attraction from moon and sun pull water toward them This creates two bulges As the Earth rotates through these bulges each day locations experience changing sea level Over a 28 day period the orientation of the moon and sun change creating different tidal ranges through month Spring tide large differences Neap tide small differences Tidal range vertical difference between high and low tide Macro 4 m Meso 2 4 m Micro 2 m Local differences in geometry of seabed can increase or decrease tidal range Tidal Flats and Marshes Lagoonal environments Intertidal environments between high and low tide surround lagoon They trap and accumulate sediment filling lagoon Tidal flat sedimentation Mud transported as suspended load accumulates on high flat Sand transported as bedload accumulates on low flat Upward growth ultimately controlled by rate of sea level rise Asymmetry between flood and ebb currents Frictional interaction with seabed commonly causes flood current to be stronger This results in more sediment being transported into the lagoon and onto the tidal flats enhancing accumulation Marsh vegetation Many niches develop depending on many variables e g salt and soaking tolerance and current velocity Vegetation helps to baffle flow reduce tidal current velocity and enhance sediment accumulation Mangrove vegetation Mangrove distribution Found in warm tropical settings Ocean circulation extends latitudinal distribution on west sides of ocean basins and reduces distribution on east sides Sediment Budgets for Beaches and Coasts Sediment Sources Longshore drift local source Cliff erosion Rivers Biogenic shells Continental shelf Sediment Losses Dunes Lagoon washover tidal inlets Submarine canyons unusual Longshore drift local sink Prograding shoreline building seaward Requires 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 ridges Human Structures Impact of Groins to Shoreline Deposit sediment on upcurrent side erode beach on downcurrent side Deflects longshore transport farther offshore Jetty entrapment of sediment Sediment trapped on upstream side due to longshore transport Loss of sediment causes erosion on downstream side to resupply longshore transport system Similar to entrapment associated with groins but on larger scale Human Beach Structures groins
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