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Waves

Waves. Transport energy over a body of water. Height. Still water line. Wave Terminology. Still water line – level of ocean if it were flat w/o waves. Crest – highest part of wave Trough – lowest part of wave. Wave height (H) – vertical distance between crest and trough.

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Waves

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  1. Waves • Transport energy over a body of water

  2. Height Still water line Wave Terminology • Still water line – level of ocean if it were flat w/o waves • Crest – highest part of wave • Trough – lowest part of wave • Wave height (H) – vertical distance between crest and trough • Amplitude – distance between crest and still water line • ½ the wave height • Wavelength (L) – horizontal distance from each crest or each trough • Or any point with the same successive point • Steepness = Height (H)/length (L)

  3. Wave Parameters • Period (T) – the time it takes for two successive waves to pass a particular point • Frequency (f) – the # of waves that pass a particular point in any given time period

  4. Deep Water WaveMotion • Water particles move in orbits • Waves transmit energy, not water mass • Diameter of orbits decrease with depth • Particle motion ceases at ½ wavelength

  5. Shallow Water Waves 3. Orbits progressively flatten at depth 1. Swell feels bottom at depth < ½ wavelength 2. Wave crest peaks and wave slows 4. Wave height (H) increases and wavelength (L) decreases 5. Wave breaks when H/L ratio > 1/7 6. Just above seafloor particles move in back-and-forth motion

  6. Breaking Waves

  7. TsunamiCharacteristics • Energy passes through entire water column • Shallow water wave • Long periods (T) • T = 10-20 min. • Small Height (H) • H = 1-2 m • long wavelengths (L) • L = 100-200 km • Deep wave base • Travel at great speeds • c = 200 m/s

  8. TsunamiCrest and Trough

  9. Beaches

  10. What is a beach? • Area along coast where sediment accumulates • Coastal zones begin when waves interact with seafloor

  11. Quartz wide Cobble Pink sand narrow sand Black sand Shell BeachesTypes • Shape and structure • Composition • Size • Color

  12. summer winter BeachesSummer and Winter • Summer • Winter • Gentle waves • Erosive storm waves • Carry sand to shore but too weak to carry back to sea • Sand carried seaward to offshore sandbars • Wider beach results • Narrow beach

  13. Beach ProcessesRefraction • Wave crests approach shore at an angle • Wave crest over shallow water slows but attached crest over deeper water does not • Wave bends • Wave rays travel perpendicular to wave crest • Energy focused on headlands  erosion

  14. Beach ProcessesLongshore Current and Transport • Current that moves sediment parallel to shore • Swash moves sediment onto beach at an angle • Backwash moves sediment down perpendicular to shore • Zig-zag motion has an overall net transport; longshore transport

  15. Beach DynamicsDeposition and Erosion • Deposition • Coasts build out by waves and currents Sandy shore • Erosion • Reshaping of coastline by wave action • Little incoming sand, usually no major river Rocky shore

  16. BeachesDepositional Features • Longshore current deposits sand in a line downcurrent of a headland • Sand spits • Bay mouth bars • When a sand spit closes off a bay • Barrier island • Long sand island enclosing a bay or lagoon

  17. BeachesErosional Features Arch • Arches • Weaker rocks erode Stack • Stacks • Offshore column formed from collapsed arch

  18. Cliff Wave cut bench BeachesErosional Features • Wave cut bench • Cliff • Cliff bottom attacked by waves • Waves undercut cliff to form wave cut notch • Overhanging rock collapse • As cliff retreats, area of flat rock exposed

  19. BeachesErosional Features • Marine terrace • If sea level lowers or land uplifted, wave cut bench become marine terrace

  20. BeachesHuman Impacts • Parallel concrete structures • Absorbs wave energy • Sea walls • Actually promotes erosion • Groins • Wall perpendicular to shore • Restore erosion • Erodes downcurrent side • Jetties • Designed to maintain navigation routes • Erosion and accretion • Breakwaters • Wall parallel to shore • Reduce wave energy • Interrupts beach drift

  21. Miami Beach after before BeachesHuman intervention • Beach nourishment • Import sand into coastal areas • Dredging • Underwater excavation to clear waterways

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