Chapter 19 Shorelines and Coastal Processes Coastal Processes ...

Chapter 19Shorelines and CoastalProcessesCoastal Processesand TerrainCoastal ProcessesWaves, tides, longshore drift, seasonal anddecadal-scale changes, dunes, mass wastingCoastal LandformsDepositional: spits, baymouth bars, barrierbars, tombolosErosional: headlands, sea stacks, arches,marine terraces, wave-cut benchesShore-Shaping Forces• Waves• Changes in Water Level• Tides• Stream Outflow• Ice Push• Organic Secretions•ErosionSea Waves• Due to wind blowing across water• Wave motion– wave length– wave height– circular motion in deep water– wave base- no motion below this depth– motion in shallow water- elliptical• causes surfFigure 20-4Waves

Development of WavesIdealized Beach ProfileFigure 20-2Figure 20-14TidesNormal High TidesFigure 20-8Figure 20-A(1)

Spring and Neap TidesHighest Tidal RangesFigure 20-BWave RefractionFigure 20-5StreamOutflowtomboloFigure 20-9

Coastal Landforms• Depositional Landforms–Beaches– Barrier Islands/Lagoons–Spits• Shorelines of Submergence– Fjorded Coasts– Wave-Cut Cliffs and Platforms– Marine Terraces• Coral CoastsSand Drift• Surf zone• Longshore currents– due to waves striking coast at an angle•Spit• Baymouth bar• Effects of blocking transportWave refractionThe effects of groins and littoral (longshore) drift

…jetties can have the same effect!Barrier IslandsRequaPointView to southPhoto by John WhitmerView to northPhoto by John Whitmer

Photo by John WhitmerPhoto by John WhitmerPhoto by John WhitmerPhoto by John WhitmerBarrier Islands and LagoonsPhoto by John Whitmer

SpitsErosionFigure 20-18Formationof aWave-CutNotchWave-Cut NotchFormation of ArchesFigure 20-6

Marine Terrace DevelopmentFigure 20-24Coral CoastsCoral Reefs of the WorldFigure 20-27Figure 20-25Fringing ReefsBarrier Reefs and Atolls

Photo by John WhitmerPhoto by John WhitmerPhoto by John Whitmer19.2 How do waves form and move in water?Tsunami coming to shore in Thailand, Dec. 2004. These curiousunfortunates went to explore the expanded beach as the troughexposed it. The crest is seen in the background.Photo by John WhitmerFig 11.12

• Earthquakes and Tsunami– Motion along sub-sea faultdisplaces crust.– Rapidly displaced crust moveswater.– The Dec. 2004 event picturedpreviously displaced ocean floor,and the sea above, by 5 metersover an area approximately 400km long by 50 km wide.– Displaced water mass was ~100billion metric tons.– Such high energy input creates awave of very long wavelengthand low wave height. Thus, theyare difficult to “see” in the openocean, yet the wave energy isvery fast.• Tsunami can also be caused byundersea mass movements.19.2 How do waves form and move inwater?Tsunami have a very deepwave base, and begin tobreak well offshore. Theinshore deceleration andwave crest buildup areboth very large as a result.Fig 19.11Fig 19.1219.5 Why does shoreline location changethrough time?Uplift and subsidence along the Oregon coast. Two surveys, 57 yearsapart, show marked vertical change along the entire coast of the state.19.6 How do we know ... that global sealevel is rising?100 years of tide gage data for NYC indicate a definite trend in sea level.This could, of course, indicate the land is subsiding (as it measuresrelative sea-level change), but generally geologists try to avoid using datafrom known, tectonically active or otherwise vertically active locations.Data that doesinclude uplift ordownwardmovement can beadjusted to astable point,however.Fig 19.41Fig 19.4419.6 How do we know ... that global sealevel is rising?Geologists use coral as “dipsticks” of former sea levels, as theyare often very limited in the depth they can survive. Acroporapalmata is one such species and lives up to 5 meters depth. Oldcoral reefs found at 120 meters down indicates a change in sealevel.19.6 How do we know ... that global sealevel is rising?Using carbon-14 dating techniques, scientists are able toaccurately date the corals. They show a clear relation of depth totime. Other similar data sets have been collected and supportthese results.Fig 19.45Fig 19.45

19.7 What are the consequences ofrising sea level?Low-lying islands such as theFlorida Keys would all butdisappear with a two-meterincrease of sea level.19.7 What are the consequences ofrising sea level?Sea-level rise would cause a net loss of beach area due to erosion.On average, 1 cm of rise equals 1.5 meters of landward incursion.Fig 19.47Fig 19.46Fig 11.2919.7 What are the consequences of risingsea level?• Barrier island effects– Higher waves from risingseas wash sand acrossisland.– Island migrates shoreward.– Islands merge with shoreafter filling lagoon.– Full wave effects arethereafter felt on shore.– Geologists find lagoondeposits indicating this hasoccurred in the past.19.7 What are the consequences ofrising sea level?Fig 19.49• Estuary effects– For an estuary to persist,sea-level rise must exceedsedimentation from river.– Submergence of wetlands(pictured 50 years apart)– In a natural system, tidalflats and salt marsheswould move inland. Withhumans in the environmentthis does not occur.Fig 11.29 Fig 19.48Fig 19.52• What do we do? Thereare three generalresponses.– Armor the shoreline(seawalls, rip-rap, boulders,various deflectors)– Add sediment to erodingbeaches to maintain them– Abandon the coastNote that armoring, suchas seawalls, protectsspecific property but doesnot save the beach or stoperosion.