Motion in the Sea -- Waves

I. Characteristics of Wave Motion

  1. a wave transfers a disturbance from one part of a material to another (i.e., the disturbance caused by a stone dropping into a pond is transmitted across the pond by ripples)
  2. the disturbance is propagated through the material without any substantial overall motion of the material itself
    • when under a wave crest, the water moves up and forward; under the troughs, it moves down and back; thus, on the whole, water particles don't go anywhere at all as the wave passes, but move in circles -- however, the energy of this movement is transmitted to succeeding water particles
      • it is this orbital motion of the water particles that causes an object to bob up and down, forward and backward as waves pass under it; however, this motion does not extend far downward (the particles experience a displacing force and a restoring force)
  3. the disturbance is propagated without any significant distortion of the wave form
  4. the disturbance appears to be propagated with constant speed



II. Wave Forms -- Idealized

III. Wave Speed

L = (g/2?) *T2
where T=period and g=9.8 m/sec2 (or 32 ft/sec2) -- the equation can be simplified to L = 1.56 T2 (for meters) or 5.12 T2 (for feet)
  • deep water waves that have the greatest wavelengths and longest periods, travel the fastest and are the first to arrive in regions distant from the storm which generated them
  • if you toss a stone into a pond, a band of ripples is produced -- this band gets wider with increasing distance from the original disturbance and the ripples of greater wavelength progressively outdistance those with shorter wavelengths (they outrun the shorter ones)
    • this is dispersion (separation of waves by their differing rates of travel) and it produces swells
  • progressive groups of swell with the same origin and wavelength are called wave trains
    • the leading wave in a train is drained of energy (because it must set up the circular motion), but after the wave train passes, it leaves behind enough energy to generate a new wave
    • as a wave advances through the group, each wave moves through the group to die out at the front -- the distance traveled by each individual wave as it travels from rear to front of the group is twice that traveled by the group as a whole
      • hence the wave speed is 2 x the group speed or the group speed is 1/2 the wave speed
  • it is inevitable that swells from different disturbances will run together -- when this happens, they will interfere with each other
  • when crests of two wave trains coincide, the amplitudes are added (constructive interference)
  • when the wave trains are out of phase--the crests coincide with the troughs -- the amplitudes cancel out (destructive interference); get mixed interference when crests and troughs do not coincide
  • thus, the wave trains interact, lose their individual identities and form a new group of waves separated by regions free from waves

IV. Wave Types

  • interaction with the bottom slows the wave, but waves behind this first wave continue at their original speed -- thus period remains unchanged while wavelength decreases; the wave then becomes too high for its decreasing wavelength
  • depth continues to decrease and the crest of the wave breaks -- the turbulent mass of water rushing shoreward after the break is known as surf and the surf zone is that region between the shore and the breaking waves
  • there are 4 types of breaking waves:
    1. spilling breakers -- the crest moves forward faster than the wave as a whole; foam eventually covers the leading face of the wave; characteristically found on a gently sloping shoreline
    2. plunging breakers -- the crest curls over and plunges downward with considerable force
    3. collapsing breakers -- similar to plunging breakers, but instead of the crest curling over, it collapses; occur on beaches with moderately steep slopes
    4. surging breakers -- crests remain relatively unbroken as the waves slide up the steep beach

V. Wave Refraction

VI. Seiches (sayshes)

  • but if a basin is open at one end, it is possible for a node to occur at the entrance of the basin and an antinode at the landward end; water levels will then rhythmically oscillate between node and antinode and will result in damage on the landward side

VII. Tsunami