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Planet earth waves, beaches and coasts notes Planet earth waves, beaches and coasts notes Document Transcript

  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 1The OceansImportance of the Ocean.1. Plays a significant role in the hydrologic cycle (more water is evaporated off the ocean than on land2. Plays a major role in collecting, storing and distributing heat3. It is a major driver of climate, the maritime influence4. Significant producer of oxygen to our atmosphere (phytoplankton and primary production) and a significant absorber of carbon dioxide from the atmosphere (by plants, algae and in carbonate rock formation)5. Source of food, minerals and resources for the world.71% of the earth is covered by oceans. The land comprising the remaining 29% isunevenly distributed. The uneven distribution of land and water plays an importantrole in determining the paths along which water circulates.Ocean Geography (See world map)The oceans contain 97% of the earth’s water supply. Most of the water iscontained in three huge interconnected basins – the pacific, Atlantic, and Indianoceans. All three are connected with the southern ocean that completely encirclesAntarctica.• The smaller water bodies connected with the Atlantic Ocean include the Mediterranean, Black, North, Baltic, Norwegian, and Caribbean Seas, the Gulf of Mexico, and the Baffin and Hudson Bays.• The smaller water bodies connected with the Pacific Ocean include the Gulf of California, Bering Sea, Sea of Okhotsk, Sea of Japan, and the East China, South China, Coral, and Tasman seas.• The smaller water bodies connected with the Indian Ocean include the Persian Gulf, Red Sea, and Arabian Sea. All these seas and gulfs vary considerably in shape and size, some are almost completely surrounded by land, whereas others are only partly enclosed.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 2Depth and Volume of the OceansWhat we thought.It was once thought that the deep ocean was flat in bathymetry (topography ofthe ocean floor). Most knowledge of the earth’s ocean developed as a result ofWorld War II technology - sonar, deep diving vehicles, coring devices andsatellites.In the 1960’s the first physiographic diagram was constructed, utilizing thousandsof echo sounding ships to map out the ocean floor. The bathymetry of the oceanfloor is as irregular as the topography on the continents. The features are largerand more complex than any continental features. But the high relief of the oceanfloor is masked by large, thick layers of sediments and ooze that accumulate, andfill in depressions. There are no denudation processes that break down featuresunder the ocean, they are merely covered up by sediments.The average depth of the sea floor is about 3800m (3.8 km). The deepest oceandepth yet measured at 11000m (11 km) is along the Mariana trench near the islandof Guam in the western Pacific.METHODS OF STUDYING THE SEA FLOORDepth and Bathymetry of the Sea Floor• Examination of the sea floor via submersibles, cameras, and robots. Observe, photograph and move around on the sea floor.• Examining sea floor via echo sounding equipment. Sound waves are sent out, hit the bottom, and are reflected back up to the instrument. This instrument allows you to measure depth and bathymetry. Because the speed of sound traveling through water is known, the water depth can be calculated. By transferring time to depth you can obtain a two dimensional profile of the sea floor along a line producing an image of the bathymetry.• Seismic profiler works in a similar way as a echo sounding machine but uses shorter frequency seismic waves that penetrate the bottom and reflects through the sediments and rocks and gets a view of the rock layers below the sediment surface. It records water depth and reveals the internal structure of the rocks and sediments of the sea floor, including bedding planes, faults etc.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 3The oceans vary considerably in their depth. The deepest part of the ocean iscalled the abyssal plain. As the seafloor starts to rise toward continental marginsit is called the continental rise. The continental slope is the steep slope risingtoward continual margins. The gently sloping area along the margin of a continent iscalled the continental shelf. In addition, deep trenches that occur along zoneswhere oceanic lithosphere descends back into the mantle are called oceanictrenches. And, ridges in the deep oceans that rise above the abyssal plains andwhere new oceanic lithosphere is created are called oceanic ridges.Planet Earth Waves, Beaches and Coasts Notes View slide
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 4Ocean SalinitySalinity is the measure of the ocean’s saltiness expressed in parts per million (ppm)The salinity of seawater normally ranges from 33 to 37 ppm. The principalelements that contribute to the oceans salinity are sodium and chlorine (formingNaCl). Other dissolved solutes include Magnesium, Sulfur, Calcium, Potassium,dissolved oxygen and carbon dioxide.Sources of ions.• Chemical weathering of crustal rocks leaches cations out to become part of the dissolved load of a stream. Each year streams carry 2.5 billion tons of dissolved substances to the oceans.• Volcanic eruptions erupt water vapor, carbon dioxide, and chloride and sulfate (SO4-2) anions into the atmosphere where they dissolve in atmospheric water and return to earth through precipitation.• Volcanic gases release anions directly into the oceans via submarine eruption.• Human activities produce gaseous, liquid, and solid pollutants that are carried by streams, atmospheric circulation and precipitation, or are directly released into the oceans.Removal of ions.• Aquatic plants and animals extract silicon, calcium, and phosphorus from seawater to build their tests, shells, and skeletons.• Charged clay particles and other minerals attract charged ions (such as sodium and potassium) and remove them from solution as they settle to the sea floor.• Ions are directly precipitated from solution to form minerals.Because the processes that contribute to the salinity of seawater areessentially equal to the processes that remove ions, the composition ofseawater remains virtually unchanged.Planet Earth Waves, Beaches and Coasts Notes View slide
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 5Salinity of Surface Waters (*See map)The salinity of surface waters is related to latitude, climate and topography.Factors affecting salinity include1. evaporation (which removes water and leaves the remaining water saltier)2. precipitation (which adds fresh water, diluting seawater making it less salty)3. inflow of fresh water by rivers (diluting seawater and reducing salinity)4. freezing of sea ice (salts are excluded from the ice leaving the unfrozen water saltier)5. melting of sea ice (which adds fresh water, diluting seawater making it less salty)Therefore• Salinity is lower near the equator due to higher precipitation.• Salinity is lower at high latitudes due to low temperatures.• Salinity is low near the mouths of large rivers.• Salinity is high at desert latitudes due to high evaporation.• Salinity is high in restricted bodies where there is little inflow of fresh water.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 6Temperature and Heat Capacity of the Ocean (*See map)Surface TemperaturesIsotherms (lines connecting points of equal temperature) approximately parallelthe equator.• In summer, the warmest waters (~28C or 82F) occur in the region between about 30N and 10S latitude where solar radiation is at a maximum.• In winter, as the belt of maximum incoming solar radiation shift southward, so does the belt of warmest waters.• Waters become progressively cooler north and south of the warm water belt (to less than 10C or 50F).Heat CapacityHeat capacity is the amount of heat required to raise or lower the temperature ofa material. Water has a high heat capacity- it therefore has the ability to absorband release large amounts of heat with very little change in temperature.Therefore both the total range and the seasonal changes in ocean temperaturesfluctuate very little. Ocean temperature range varies only about 38C from the equator to the poles. The temperature of surface seawater varies with latitude, from near 0o C near the poles to 29oC near the equator. But restricted areas can have temperatures up to 37oC Annual change in sea surface temperatures varies only a few degrees – 0-2C in the tropics, 5-8c in the mid-latitudes, and 2-4C at the poles.In contrast, extreme temperatures occur on land. Land has a low heat capacityand heats and cools much faster than water. Latitude temperature differences onland can vary almost 150C, while seasonal temperature ranges on the continents canexceed 50C.Interaction of the hydrosphere, atmosphere, land surface, and biosphere –*See map of global ocean and surface temperatures.Ocean temperatures affect the climate, and climate ultimately is a major factor incontrolling the distribution of plants and animalsPlanet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 7Vertical Stratification of the OceansThe physical properties of seawater vary with depth due to variations in density.There are three major depth zones. A surface zone extending from the surface down to a depth of 100 to 500m where winds, waves, and temperature changes cause extensive mixing. A zone characterize by dramatic changes in temperature, salinity, and density with increasing depth. A name is applied based on the given property. – Thermocline – zone where temperature decreases with depth. Seawater becomes denser as temperature decreases. – Halocline – zone where salinity increases with depth. Seawater becomes denser as salinity increases. – Pycnocline – zone where density increase with depth. Based on a decrease in temperature, an increase in salinity, or both. A Deep zone where waters are dense as a result of their low temperatures and high salinity.Surface Currents of the Open OceanSurface Ocean currents are result of drift of the upper 50 to 100 m of the oceandue to the prevailing surface winds. Thus, surface ocean currents generally followthe same patterns as atmospheric circulation, with the exception that atmosphericcurrents continue over the land surface while ocean currents are deflected by theland.The ultimate source of this motion is the sun, which heats the Earth unequally,thereby setting in motion the planetary wind system. Therefore, ocean circulationresults from the interplay of a number of earth processes:1. Radiation from the sun provides heat energy to the atmosphere.2. Non-uniform heating generates winds.3. The winds, in turn, drive the motion of the ocean’s surface water.The troposphere (the portion of the atmosphere at the earth’s surface to analtitude of 10 to 16km) undergoes circulation because of convection. Convection inthe atmosphere is mainly the result of the fact that more of the Suns heatenergy is received by parts of the Earth near the Equator than at the poles. Thusair at the equator is heated reducing its the density causing it to rise. At the topof the troposphere this air spreads toward the poles.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 8If the Earth were not rotating, this would result in a convection cell, with warmmoist air rising at the equator, spreading toward the poles along the top of thetroposphere, cooling as it moves poleward, then descending at the poles, exceptthat the figure in your text mistakenly shows a rotating Earth). Once back at thesurface of the Earth, the dry cold air would circulate back toward the equator tobecome warmed once again.The Coriolis Effect - Again, the diagram above would only apply to a non-rotatingEarth. Since the Earth is in fact rotating, atmospheric circulation patterns aremuch more complex. The reason for this is the Coriolis Effect. The Coriolis Effectcauses any body that moves on a rotating planet to turn to the right (clockwise) inthe northern hemisphere and to the left (counterclockwise) in the southernhemisphere. The effect is negligible at the equator and increases both north andsouth toward the poles.EG. Globe Demo. A body at the poles will rotatecompletely around as a result of the earth’s rotation.A body on the equator, however, would not rotate atall, but rather would exhibit an end-over-end motion.At any latitude between the equator and the pole,some rotation and some end-over-end motion occursand for this reason the Coriolis effect, which is dueto rotation, is latitude dependent, ranging from zeroat the equator to a maximum at the poles.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 9The surface of the oceans move in response to winds blowing over the surface. Thewinds, in effect, drag the surface of oceans creating a current of water that isusually no more than about 50 meters deep. Thus, surface ocean currents tend toflow in patterns similar to the winds as discussed above, and are reinforced by theCoriolis Effect. But, unlike winds, the ocean currents are diverted when theyencounter a continental land mass.The surface currents have thefollowing properties:• Circulation is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.• In each hemisphere cooler waters from higher latitudes circulate toward the equator where they are warmed and circulate back toward the poles.• Low latitude regions are dominated by the warm, westward flowing North and South Equatorial currents. In each major ocean basin, westward flowing equatorial currents are deflected poleward as they encounter land. Each current thereby is transformed into a current that flows generally poleward.• In the middle latitudes ocean currents run generally eastward, flowing clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. Such easterly flowing currents are deflected by the continents and thus most of the flow of water occurs parallel to the coasts along the margins of continents. Only in the southern oceans, between South America, Africa, Australia, and Antarctica are these surface currents unimpeded by continents, so the flow is generally in an easterly direction around the continent of Antarctica.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 10Ekman TransportWind blowing across the ocean affects the uppermost layers of the water column,producing a net water flow that is at an angle to the wind. The surface water layerdragging on the water immediately beneath sets the lower layer in motion, and theprocess continues downward. Internal friction, however, decreases currentvelocity with increasing depth. The Coriolis effect shifts each successive, slowermoving layer farther to the right (in the northern hemisphere, left in the southernhemisphere) producing a spiraling current pattern (called the Ekman spiral) whenseen from above. The average flow over the full depth of the spiral, called theEkman transport, moves at 90o to the wind direction.Upwelling and DownwellingNear coasts, Ekman transport can lead to vertical movement of ocean water.Upwelling - If the net Ekman transport is away from the land, subsurface watersflow upward and replace the water moving away.Downwelling – If the net Ekman transport is toward the land, the surface watersinks.Ekman Transport in ActionEl Nino –The fishing grounds off the coast of Peru, among the riches in the world, aresustained by upwelling cold waters filled with nutrients. The prevailing currentalong the coast of Peru is northward sustained by the southeast trade winds.Because Peru is in the southern hemisphere, the Ekman transport is 90 degrees tothe left of the prevailing surface current away from south America, causingsubsurface waters to flow upward (upwelling). During El Nino years, thetradewinds slacken, upwelling is reduced, the fish population declines followed by adie-off of the coastal bird population that feed on the fish.• During normal years, the tradewinds blowing across the Pacific pile up a large pool of warm water in the western Pacific while cooler water upwells in the eastern pacific. The warm water promotes a large center of heavy rains around Indonesia.• With El Nino, tradewinds slacken, upwelling is reduced, and the waters of the Eastern and Central pacific warm as the warm water in the western pool sloshes back eastward. The eastern movement of warm water causes the zone of high rainfall to shift to the central Pacific causing draught in Indonesia and Australia, heavy rainfall in arid regions of Peru and Ecuador, and increased rainfall in the middle Pacific including Hawaii among other things.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 11The Global Ocean Conveyor SystemIn addition to surface circulation, seawater also circulates vertically as a result ofchanges in density resulting from changing salinity and temperature. Suchcirculation, because it controlled by both temperature differences and differencesin salinity of the water, is called thermohaline circulation. The thermohalinecirculation is a density-driven, global ocean circulation. Sometimes called theConveyor Belt, the thermohaline circulation moves warmer, less salty surfacewaters poleward and cold, salty bottom waters equator-ward• When we simplify this to the basic flow, we see a model showing a "conveyor belt" of surface water sinking in the North Atlantic, flowing south to the South Atlantic, then east with the Antarctic Circumpolar Current into the South Pacific and then the North Pacific where it comes to the surface and returns through the southwest Pacific and back into the South Atlantic and eventually completing the flow into the North Atlantic.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 12Ocean WavesMajor currents are large-scale geographic features of the ocean surface. Finer-scale ocean waves are also a response to the interaction of the atmosphere andocean surface.• Coasts are the most dynamic environment in the world (continually changing).• Hydrosphere, lithosphere and atmosphere all meet and interact.Beach Profile• A beach is a strip of sediment that extends from the low water line inland to a cliff or dune line of permanent vegetation. The sediment comprising a beach consists of loose material such as sand or gravel that has accumulated by wave action at the shoreline. The variety of loose material depends on the environment.• Beaches derive their sediments from stream systems depositing sediments in the ocean from eroding the continents and from erosion of headlands. • NJ – granite – quartz sand beaches • Hawaii – volcanic rock including basalt, pumice, obsidian – black sand beaches• Active versus Passive Continental Margins. Active margins – Cliffed coasts Passive margins – barrier islandsPlanet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 13The landward extension of the beach terminates at a natural topographic andmorphologic change such as a seacliff or dune line. From that topographictermination seaward the beach profile includes The Berm is the upper part of the beach landward of the high tide line (the areas where people sunbathe). It is a wave deposited platform that is flat or slopes landward. Berms are formed by deposition of sediment as waves rush up and expend the last of their energy. The Beach face is the sloping portion of the beach below the berm. The beach face is the section exposed to wave action, particularly at high tide. Specifically, the part of the beach face that is exposed by the uprush and backwash of waves is called the swash zone. Seaward beyond the swash zone is the surf zone. The surf zone is that portion of the seashore environment where turbulent translational waves move toward the shore after the incoming waves break. The area where incoming waves become unstable, peak and break is the breaker zone. The longshore trough and longshore bar are an elongated depression and adjacent ridge of sand produced by wave action. (feel this when you go out in water which is deep and then shallows far offshore)Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 14WAVE PHYSICSWaves continuously erode, transport and deposit materials. The goal of waves isto straighten and smooth the shoreline, so anything that sticks out such aspeninsulas, headlands, etc are weathered and bays and estuaries are filled in.How waves break. Waves are the result of the transfer of energy from wind towater. As wind blows on top of water, energy is transferred to water waves. Theheight of the waves and their speed are controlled by (1) wind speed. The greater the wind velocity, the larger the waves. (2) wind duration or the time period the wind blows. Storms of longer duration have more time to impart wave energy to water. (3) fetch, the distance that the wind blows across the surface of the water. The longer the fetch, the larger the waves are likely to be.The wave height is a good key in determining wave energy. During storms such ashurricanes or northeasters, waves are generally larger, because there is moreenergy in the wind. On the open ocean, wave heights 1 – 15 feet, during violentstorms, <50 feet.Wave FormWave height – difference in height between wave trough and peak (crest)Wave length – distance between successive peaksWave period – the time in seconds for successive waves to pass a reference point(5 to 10 sec.)Wave base – depth of orbital motion, equal to half the wavelength.Wave front – wave approaching parallel to shorelineWave normal – direction of wave travel. Perpendicular to wave front.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 15• A wave is generated as wind energy is transferred to water. As the wind energy travels through the water, the water is not displaced. Rather, a particle of water moves in an orbit, a nearly circular path, and as the wave passes the particle returns to its original position.• At the surface, the diameter of the orbital path is equal to the height of the wave.• Below the water surface, the orbits decrease in size until motionless where the depth is 1/2L. *Duck under waves.• As a deeper water wave approaches the shoreline, water depth decreases. The wave begins to ‘feel’ the bottom when the depth equals ½ the wavelength. Circular orbits near the base elongate and the wave slows down due to friction at the bottom, causing the length of the wave to decrease. Additional to that, the sloping sea floor wedges the water upward, increasing the wave height. Because the height is increasing while the length is decreasing, the waves become steeper until they break. The water washes up the beach as swash and back flow is backwash.• Energy is then transferred to moving and eroding along the shoreline.Wave feels bottom at the level of lowest orbital motion. Circular orbits flatteninto ovals. Waves slow down and their length decreases. The upper part of thewave continues to move landward, while the lower portion is slowed as the slopingbottom wedges the moving water upwards, increasing the wave height.. The wavessteepen until they break.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 16Coastal Erosion Processes.Coastal erosion is a serious national and worldwide problem. It is a continuous andpredicable process, which is affected by a number of factors. Severe storms Interference with material flow of sand from inland areas due to coastal manipulation. Sea level riseCoastal erosion and Cliffed coastsThe active margins of the Pacific Ocean along the North and South Americancontinents are characteristic coastlines affected by erosional landform processes.Erosional coastlines tend to be rugged , of high relief, and tectonically active, asexpected from their association with the leading edge of drifting lithosphericplates. Cliffed coasts are formed as tectonic uplift raises the shoreline above sealevel.Where a seacliff is present along a coastline it is exposed to both marine and landprocesses which work together to erode the cliff. Along a Cliffed coast in thesummer, the beach is composed of a wide berm of an approximately 1 meter thicklayer of loose material that overlays a wave cut platform of bedrock. The wideberm acts to protect the cliff from wave erosion. In the winter the waves removethe sand exposing the base of the cliff.Sea Cliffs are formed by theundercutting action of the sea. Asindentations are produced at water level,such a cliff becomes notched, leading tosubsequent collapse and retreat of thecliff. Wave action can cut a horizontalbench in the tidal zone, extending from asea cliff out into the sea. Such astructure is called a wave-cut platformor marine terraces. If the relationshipbetween the land and sea level changesover time, multiple platforms or terracesmay rise like stairsteps back from thecoast.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 17A terrace above sea level represents the former abrasion platform that has beenraised above sea level by tectonic uplift (or sea level drop). These form as waveerosion at the cliff base forms wave cut notches, which may lead to the collapseof cliff walls when they are undercut. The erosive action of the waves create anotch and then gradually undercuts the land, creating steep coastal cliffs.Discuss wave refraction.• Bending of waves as they approach a shore (shallow water).• Consequence, energy concentrated along parts of shore and reduced in others.• Part of the wave reaches shallow water first, feels bottom, and slows down before the rest of the wave (which is continuing at the regular velocity).• As more of the wave approaches shore, more of it slows down. This causes the waves to bend or refract.• Headland areas (shallow water) receive more energy (wears them back and increases erosion here) and energy is low in bays (therefore deposition predominates). The concentration of erosion on headlands wears them back, while beach deposition in bays fills them seaward. This evens out the shoreline.Erosional Features along Cliffed Coasts• Erosional forms that evolve along cliff-dominated coastlines include Notches/Caves/Arches/Stacks - Sequential landforms produced by wave erosion of a headlands area.• Wave erosion of headlands at the base will chew out a notch on the sides, as waves swing around from refraction. Notches become deeper and deeper into the headland creating caves (deep notches). Sea arches result where two caves meet from either side of a headland. When the top of an arch collapses, or a sea cliff retreats and a resistant pillar is left standing surrounded by water a sea stack remains.Material eroded from the sea cliff comes to rest in the beaches of the coastalindentations (bays), or is carried out to sea by backwash.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 18Littoral Transport and Barrier Beaches.The sand on beaches is not static. Wave action constantly keeps the sand movingin the surf and swash zones, and when waves strike the coast at an angle, theresult is a longshore drift, which collectively moves beach material along a coast ina process called littoral transport. The movement of sediment by the longshorecurrent is called longshore drift.As waves approach the shore on an angle (of relatively straight coasts), the waterin the wave pushes onshore at an angle carrying sediment with it. When the waveretreats, it retreats parallel to the coastline pulling the sediment perpendicular toshore. Every strike of the wave results in a net movement of sediments in onedirection.• Long shore transport results in tons of sediment movement in a year. 750,000 tons of sediment are moved every year at Sandy Hook.• Longshore transport results in a variety of landforms (spits, bay mouth bar, fills in tombolos).• Areas that do not experience longshore transport (waves are parallel) are known as nodal zones.Barrier Island Beaches.• Barrier island beaches are very common along the eastern seaboard of the United States including New Jersey. They consist of ridges of sand that parallel the shoreline and extend above sea level.• These coasts are primarily formed by sediment deposits derived from river deposits and continental shelf sediments pushed onshore, which are moved by longshore transport.• Long shore transport moves sediments laterally along coast forming barrier chains with many features.Barrier chains are long, narrow, depositional features, generally of sand, that formoffshore roughly parallel to the coast separating a body of water between itselfand the mainland. The name “barrier” is appropriate, for they take the brunt ofstorm energy and actually act as protection for the mainland. They usually havewide beaches and sand dunes backed by grasses, shrubs, and trees, followed by a(tidal) marsh, (tidal) flat, and lagoon or bay.Characteristic wave- and current-deposited landforms includePlanet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 19 A barrier spit consists of material deposited in a long ridge extending out from a coast attached at one end. It partially crosses and blocks the mouth of a bay. A spit becomes a bay barrier, sometimes referred to as a baymouth bar, if it completely cuts off the bay from the ocean and forms an inland lagoon. Breaks in barriers form inlets, connecting a bay with the ocean. Inlets allow water to pass between the bay and ocean during high and low tides. During severe storms, new inlets may form while tidal inlets may be reclosed by littoral drift.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 20.SEA LEVEL CHANGESChanges in sea level. Erosion and deposition along a coast, and many coastalfeatures could be attributed to changes in sea level. Short term (daily) changesare the result of tides, but changing global climates result in eustatic sea levelchanges.Tides. The gravitational systems of the earth, moon, and sun, and theirinteractions generate tides. The sun appears overhead once every 24 hours, whilethe moon takes about 24 hours and 52 minutes to return to an overhead positiondue to earth’s rotation. Centrifugal forces acting on the earth between theearth – moon and sun – earth causes the earth’s fluid to bulge out producingtides. It is the earth’s 24 hour rotation that result in two high tides and two lowtides every 24 hours and 50 minutes - bulge on one side due to the centrifugalforce between earth and moon, and the other bulge due to gravitational pull bymoon.• Two sets of tidal bulges will coincide twice a month, at the new and full moons, when the sun and moon align with the earth.. These are the highest tides of the month called the spring tides. During spring tides, the sun, moon and earth are all lined up, having a maximum gravitational pull by the moon and sun resulting in the highest high tides and lowest low tides, (largest tidal range)• In the first and third quarters of the moon, the sun and moon are at right angle to earth, thus producing the lowest tidal ranges called the neap tides. During neap tides, the moon makes a right angle with the sun and the earth. Some of its gravitational pull is directed in the opposite direction of the sun’s. During these times, the sun and moon’s forces act against each other, resulting in the lowest high tides and highest low tides (smallest tidal range).Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 21Eustatic sea level change. Increasing or decreasing the amount of water in theocean causes variations in the ocean surface elevation called eustatic sea levelchanges.• Changes in sea level can be caused by changes in global climates which melt or freeze glacial ice. When glaciers melt, water is added to the oceans • Over the past 15,000 years, glaciers have been melting, and sea level has already risen 400 feet, sea level is rising today at about 1.5 inches per century). Sea level rise along stable and subsiding coasts is causing shoreline erosion further inland.• Cliff retreat. Rising sea level elevates waves so they can cut into cliffs at a higher level, initiating mass movements in poorly consolidated sediment.• Coastal submergence. As sea level rises, it will inundate present coastal areas. Unprotected low-lying areas of coastal cities- Boston, New York, Charleston, Miami, San Diego, Los Angeles- will suffer serious damage to their waterfront areas.• Barrier Rollover. Sea level rise is a special problem along the low-lying, sandy, barrier islands of much of the Atlantic and Gulf coasts. Barrier islands respond to rising sea level by erosion on their oceanic sides and deposition of the material by overwash on their back sides. Thus with time, the ocean front recedes and the back side of the island grows landward in a process called barrier rollover. The entire island is displaced landward with time. • Barrier rollover applies only to coasts where engineering structures have not been built to control erosion. Engineered structures such as seawalls prevents the overwash that allows the backside of the island to build landward as sea level rises. In short, a barrier island that is protected by engineered structures will only get narrower with time. Ironically the only way to preserve barrier islands as sea level rises is to let their fronts erode and allow the sediment to overwash across the island to build out the bayward side.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 22Pseudo sea level change.Isostasy. Vertical movements of the rigid lithosphere floating on the flexibleasthenosphere are a result of isostacy. Just as an iceberg juts up out of theocean while most of its floating mass is beneath sea level, so does a floatingcontinent jut upward at the same time it has a thick root beneath it. Add a load onthe surface of the earth and measure the downward adjustment, remove a load andmeasure the uplift – isostatic equilibrium.• As erosion occurs and the mass of the lithosphere is reduced, isostatic compensation occurs and the lithosphere rises in response. Additional weight on the lithosphere has the inverse effect. • Where river deltas discharge, we know that large quantities of sediment become heavy, when continuously deposited in one area. Isostasy acts upon these heavy sediments, and they sink, isostatically. New Orleans is built on the Mississippi River delta, which is sinking. New Orleans has problems because it is being isostatically dropped, and sea level ‘rises’.• Tectonic uplift or down drop of lithosphere along active continental margins can have the same pseudo sea level rise/fall effect. In areas where a chunk of lithosphere drops, it may become submerged below sea level. Tectonic movements along converging plate boundaries have uplifted beaches and tropical reefs far above sea level.Planet Earth Waves, Beaches and Coasts Notes
  • Prof. C.ValentiPlanet Earth Waves, Beaches, and Coasts Notes 23Submergent versus emergent coastlines. Changes in sea level, or pseudo sealevel may have great effects on coastline geomorphology. Generally, coastlines canbe classified as submergent (seas transgress onshore) or emergent (seas regressoffshore). • Coastlines of emergence occur where the water level has been lowered OR land along the coastal zone has risen. Land that was once covered by sea water no longer is. Areas such as California, Washington and Oregon are good examples of emergent coasts due to tectonic activity (active continental margin). Coastal features are found above the present sea level. • Coastlines of submergence occurs from sea level rise OR tectonic down drop, where the coastal zone drops below water. Louisiana, San Francisco Bay, Atlantic Gulf Coast.Planet Earth Waves, Beaches and Coasts Notes