2 The Shoreline: A Dynamic Boundary The shoreline is a dynamic boundary between air, land, and the ocean.The shoreline is constantly changing.The coastal zonesees intense humanactivity.
3 Ocean Water Composition – Marine salinity is 3.5 % (35 ppt, 35,000 ppm)Mostly Cl and NaGreater at tropics; less at polesTimes Atlas of the WorldW. W. Norton
4 Ocean Water Temperature Average 17 °C (63 °F), colder at poles, warmer at tropicsWater moderates temperature fluctuationsBottom waters are < 4 °C (39 °F)Times Atlas of the WorldW. W. Norton
5 Ocean water movements Surface circulation Ocean currents are masses of water that flow from one place to anotherSurface currents develop from friction between the ocean and the wind that blows across the surfaceHuge, slowly moving gyres
6 Ocean water movements Surface circulation Five main gyres North Pacific GyreSouth Pacific GyreNorth Atlantic GyreSouth Atlantic GyreIndian Ocean GyreRelated to atmospheric circulation
7 Average ocean surface currents in February–March
8 Ocean water movements Surface circulation Deflected by the Coriolis effectTo the right in the Northern HemisphereTo the left in the Southern HemisphereFour main currents generally exist within each gyreImportance of surface currentsClimateCurrents from low latitudes into higher latitudes (warm currents) transfer heat from warmer to cooler areas
9 Ocean water movements Surface circulation Importance of surface currentsClimateInfluence of cold currents is most pronounced in the tropics or during the summer months in the middle latitudesUpwellingThe rising of cold water from deeper layersMost characteristic along west coasts of continentsBrings greater concentrations of dissolved nutrients to the ocean surface
10 Ocean water movements Deep-ocean circulation A response to density differencesFactors creating a dense mass of waterTemperature – cold water is denseSalinity – density increases with increasing salinityCalled thermohaline circulation
11 Ocean water movements Deep-ocean circulation Most water involved in deep-ocean currents begins in high latitudes at the surfaceA simplified model of ocean circulation is similar to a conveyor belt that travels from the Atlantic Ocean, through the Indian and Pacific Oceans, and back again
12 Idealized “conveyor belt” model of ocean circulation
13 The coastal zone The land–sea boundary Shoreline – contact between land and seaShore – area between lowest tidal level and highest areas affected by storm wavesCoastline – the seaward edge of the coastBeach – accumulation of sediment along the landward margin of the ocean
15 Ocean water movements Waves Energy traveling along the interface between ocean and atmosphereDerive their energy and motion from windPartsCrestTrough
16 Ocean water movements Waves Measurements of a wave Wave height – the distance between a trough and a crestWavelength – the horizontal distance between successive crests (or troughs)Wave period – the time interval for one full wave to pass a fixed position
18 Ocean water movements Waves Wave height, length, and period depend on Wind speedLength of time the wind blowsFetch – the distance that the wind travelsAs the wave travels, the water passes energy along by moving in a circleWaveform moves forwardAt a depth of about one-half the wavelength, the movement of water particles becomes negligible (the wave base)
19 Changes that occur when a wave moves onto shore
20 Wave erosion Wave erosion Caused by Wave impact and pressure Breaks down rock material and supplies sand to beachesAbrasion – sawing and grinding action of water armed with rock fragments
21 Sand movement on the beach Beaches are composed of whatever material is availableSome beaches have a significant biological componentMaterial does not stay in one placeWave energy moves large quantities of sand parallel and perpendicular to the shoreline
22 Beaches and shoreline processes Wave refractionBending of a waveWave arrives parallel to shoreResultsWave energy is concentrated against the sides and ends of headlandWave erosion straightens an irregular shoreline
24 Shoreline EvolutionShorelines are continually modified by erosional and depositional processes.Over time, a geologically stable coastline will become less jagged.Headlands erode…Creating sediments…Deposited in embayments…
25 Beaches and shoreline processes Longshore transportBeach drift – sediment moves in a zigzag pattern along the beach faceLongshore currentCurrent in surf zoneFlows parallel to the shoreMoves substantially more sediment than beach drift
31 Sea Stacks Isolated remnants of headland rock formed when sea arches collapse
32 Shoreline features Depositional features Spit – a ridge of sand extending from the land into the mouth of an adjacent bay with an end that often hooks landwardBaymouth bar – a sand bar that completely crosses a bayTombolo – a ridge of sand that connects an island to the mainland
33 Spit Elongated ridges of sand extending from the land into the mouth of an adjacent bay.
34 Aerial view of a spit and baymouth bar along the Massachusetts coastline
36 Shoreline features Depositional features Barrier islands Mainly along the Atlantic and Gulf Coastal PlainsParallel the coastOriginate in several ways
37 Barrier Islands Coast parallel sand ridges 3 to 30 km offshore Barrier Islands Coast parallel sand ridges 3 to 30 km offshore. Protect a quiet lagoon. Heavily utilized.
38 Shoreline EvolutionBecause shorelines are continually being modified, they are notoriously unstable in human terms.This creates problems because of intense development in the coastal zone.These problems result from erosion during…HurricanesNormal wave attack
39 Shoreline EvolutionBecause shorelines are continually being modified, they are notoriously unstable in human terms.This creates problems because of intense development in the coastal zone.These problems result from erosion during…HurricanesNormal wave attack
40 Shoreline Erosion Shoreline erosion is influenced by the local factors Proximity to sediment-laden riversDegree of tectonic activityTopography and composition of the landPrevailing wind and weather patternsConfiguration of the coastline
41 Responses to Shoreline Erosion 3 Fundamental Responses to Erosion ProblemsBuild control structures.Beach nourishment.Abandonment and relocation.
42 Stabilizing the shoreHard stabilization –Responses to erosion problemsTypes of structuresGroins – barriers built at a right angle to the beach that are designed to trap sandBreakwaters – barriers built offshore and parallel to the coast to protect boats from breaking wavesSeawalls – Armors the coast against the force of breaking wavesOften these structures are not effective
44 Responses to Shoreline Erosion GroinsBuilt to maintain or widen beaches.Constructed at a right angle to the beach to trap sand.Create erosion problems “downdrift.”
45 Responses to Shoreline Erosion SeawallBarrier parallel to shore and close to the beach to protect property.Stops waves from reaching the beach behind the wall.Results in…Destruction of beachesEnhanced damage when seawall is undermined and removed.Accelerating expenditures to rebuild bigger seawalls.
46 Stabilizing the shore Responses to erosion problems Alternatives to hard stabilizationBeach nourishment by adding sand to the beach systemRelocating buildings away from beachErosion problems along U.S. coastsShoreline erosion problems are different along the opposite coasts
49 Stabilizing the shore Erosion problems along U.S. coasts Atlantic and Gulf CoastsDevelopment occurs mainly on barrier islandsFace open oceanReceive full force of stormsDevelopment has taken place more rapidly than our understanding of barrier island dynamics
50 Stabilizing the shore Erosion problems along U.S. coasts Pacific Coast Characterized by relatively narrow beaches backed by steep cliffs and mountain rangesMajor problem is the narrowing of the beachesSediment for beaches is interrupted by dams and reservoirsRapid erosion occurs along the beaches
51 Coastal classification Shoreline classification is difficultClassification based on changes with respect to sea levelEmergent coastCaused byUplift of the land, orA drop in sea level
52 Coastal classification Classification based on changes with respect to sea levelEmergent coastFeatures of an emergent coastWave-cut cliffsMarine terraces
53 Coastal classification Classification based on changes with respect to sea levelSubmergent coastCaused byLand adjacent to sea subsides, orSea level risesFeatures of a submergent coastHighly irregular shorelineEstuaries – drowned river mouths
54 Major estuaries along the East Coast of the United States
55 Emergent Coasts Develop due to land uplift or sea level fall Emergent coast featuresWave-cut cliffsWave-cut platforms
56 Tides Changes in elevation of the ocean surface Caused by the gravitational forces exerted upon the Earth by theMoon, and to a lesser extent by theSun
61 Tides Monthly tidal cycle Tidal patterns Neap tide First and third quarters of the MoonGravitational forces are offsetDaily tidal range is leastTidal patternsMany factors influence the tidesShape of the coastlineConfiguration of the ocean basinWater depth
62 High tide in the Bay of Fundy along the Nova Scotia coast
63 Low tide in the Bay of Fundy along the Nova Scotia coast
64 Tides Tidal patterns Main tidal patterns Diurnal tidal pattern A single high and low tide each tidal dayOccurs along the northern shore of the Gulf of MexicoSemidiurnal tidal patternTwo high and two low tides each tidal dayLittle difference in the high and low water heightsCommon along the Atlantic Coast of the U.S.
65 Tides Tidal patterns Main tidal patterns Mixed tidal pattern Two high and two low waters each dayLarge inequality in high-water heights, low-water heights, or bothPrevalent along the Pacific Coast of the U.S.
66 Tides Tidal patterns Tidal currents Horizontal flow accompanying the rise and fall of tidesTypes of tidal currentsFlood current – advances into the coastal zoneEbb current – seaward-moving waterSometimes tidal deltas are created by tidal currents