Oceanography Chp 7

• Wave


  • Requires a flexible medium to move through
  • Transmits energy, not water mass across the ocean's surface

  
• Orbital wave


  Water moves in closed circles

  
• Orbital motion


  • Extends to depth of 1/2 of wavelength
  • Diameter of circles only 1/23 of those at surface
• Factors used to classify ocean waves

  • Disturbing force that creates them
  • Restoring force that flattens them
  • Wavelength

  
• Disturbing force


  The energy that causes ocean waves to form (e.g. wind, change in atmospheric pressure, landslides, volcanic eruptions, faulting of seafloor, changes in gravitational force)

  
• Restoring force


  Dominant force that returns the water surface to flatness after a wave has formed (surface tension for wavelengths < 1.73 cm, gravity for wavelengths > 1.73 cm)
• Types of ocean waves

  • Capillary
  • Wind
  • Seiches
  • Seismic waves
  • Tidal waves

  
• Wave characteristics


  • Depend on depth and wavelength
  • Wavelength determines size of orbits
  • Depth determines the shape of orbits

  
• Wave types based on depth

  • Deep water: depth is greater than L/2 (only capillary and wind waves)
  • Transitional: depth is less than L/2 but greater than L/20
  • Shallow water: depth is less than L/20
• Celerity or Speed (C)


  In deep water waves, the longer the wavelength the faster the wave energy will move through water (C= L/T or C=1.56T or C=5T)
• Speed in shallow water waves


  C=√ g*d or C=3.1√ d
• Wind waves


  Gravity waves formed from transfer of energy from wind to water
• Sea waves


  Irregular peaked waves formed by simultaneous wind waves of many wavelengths, periods and heights

  
• Swells


  Mature wind waves with crests becoming rounded and regular when wind slows down away from storm
• Wind wave development


  • Determined by wind strength, wind duration, and fetch
  • Fully developed sea is maximum wave size theoretically possible for a wind of a certain strength, duration and fetch

  
• Wave steepness


  Maximum ratio of wave height to wavelength is 1:7 for moderately sized wind waves

  
• Waves approaching shore


  1. Wave train moves towards shore and feels bottom at D=L/2

  2. Circular motion becomes elliptical and wave crests become peaked

  3. Interaction with the bottom slows the wave

  4. Wave becomes too high

  5. Break occurs in 3:4 ratio of wave height to water depth

  
• Wave refraction


  Redistributes energy and straightens a coastline
• Wave reflection


  Bounces off obstacle and moves towards area of initial propagation
• Wave diffraction


  When wave meets an edge it will wrap around the object, most pronounced when gap width is similar to wavelength or smaller

  
• Storm surge


  Abrupt bulge of water driven ashore by storm, not a progressive wave as only a crest, can reach up to 7.5m, affected by strength of storm, strong onshore winds, low atmospheric pressure, high tide, and bottom contour

  
• Seiche


  Sloshes from side to side in a confined space, called a standing wave as it oscillates vertically with no forward movement, damage is rare on coastal regions, height in open ocean rarely more than a few inches

  
• Tsunami and seismic sea waves

  Also called "harbor waves", long wavelength shallow water progressive waves caused by rapid displacement of water, speed = √ acceleration due to gravity (G) * depth

  
• Tsunami characteristics


  Extremely low steepness (ratio of height to wavelength), long period (5-20 minutes), when approaching shore period remains constant, wave speed decreases and wave height greatly increases

  
• Other tsunamis in history

  • 1883- Krakatoa, Indonesia
  • 1946 fracture along Aleutian trench
  • 1960 earthquake along Peru-Chile trench
  • 1992- Nicaragua
  • 1993- earthquake in sea of Japan
  • 1998 wave in New Guinea
  • 2010 earthquake in Chile
• DART (Deep Ocean Assessment and Reporting of Tsunamis)


  Warning network
• Tides


  Periodic, short-term changes in the height of the ocean surface due to gravitational force of moon & Sun and rotation of the Earth, wavelength is 1/2 circumference of Earth, also called force waves as never free of forces that form them

  
• Newton's law of gravity

  Pull of gravity between 2 bodies is proportional to the mass of bodies and inversely proportional to the square of the distance between them
• Earth-Moon system


  Revolves around center of mass, revolves once a month around system's mass (27.3 days)
• Lunar tides


  A complete tidal day is 24 hours 50 minutes as moon rises 50 minutes later each day
• Sun's role in tides

  Sun's influence is only 46% of the moon, causes smaller bulges, highest point moves between 23.5o above & below the equator, position changes slowly as Earth only revolves around the Sun once a year
• Spring vs Neap tides

  • Spring tides occur when Sun & moon are in conjunction, greatest high tides and greater tidal ranges, occur at new and full moon
  • Neap tides occur when Sun & Moon not in conjunction or opposite, lesser tidal range, arrive a week after spring tides
• Dynamic theory of tides

  Lunar tidal bulge approx 55 cm, solar tidal bulge approx 24 cm, avg tidal range approx 2 m
• Tidal patterns


  • Tidal waves are shallow waves affected by ocean bottom and continents, shape of basin also contributes
  • Semi-diurnal: 2 high, 2 low of equal heights
  • Diurnal: 1 high, 1 low
  • Mixed: 2 high, 2 low with significantly different heights
• Tidal patterns and Coriolis effect

  Water moving in a tidal wave tends to stay to the right of an ocean basin, as it moves north in a Northern Hemisphere ocean it moves toward the eastern boundary, as it moves from north to south it moves toward western boundary

  
• Tidal range


  Varies with basin configuration, small basins have small tidal range, moderate in large confined areas, varies from coast to centers of the ocean, largest tidal range occurs on the edges of the largest ocean basin especially in narrow bays or inlets
• Tides in confined narrow basins

  Bay of Fundy: Tidal Range 15 meters (50 ft)
• Tides and marine organisms

  • Dealing with changing salinity, water availability, wave shock, temperature changes, light
  • Adaptations include having shells, moving to avoid predation, being sessile and attaching, being flexible, having protective mucous coatings, clumping together, and zonations according to conditions

  
• Grunion runs


  Reproduction timed with spring tides
• Tidal power


  As water flows from higher level to lower level, it can be used to spin an electricity-generating turbine (minimum 5 m/16ft range), suitable sites are limited as tidal changes are greatest near the poles and accentuated in narrow bays and estuaries