2B.4A: Waves and Beach Morphology

• Waves in general

  • A wave is created through friction between the wind and water surface, transferring energy from the wind into the water. This generates ripples, which grow into waves when the wind is sustained. 
  • A wave is the transfer of energy from one water particle to its neighbour with individual water particles moving in a circular orbit
  • The size of the wave particle orbit decreases with depth
  • Wave height is the vertical distance from peak to trough
  • it's determined by the energy transferred from the wind, and the water depth
• ​Waves in open sea

  • Waves are simply energy moving through water
  • The water itself only moves up and down, not horizontally
  • There is some orbital water particle motion within the wave, but no net forward water particle motion. 
• Wave size depends upon:

  • the strength of the wind
  • the duration for which the wind blows
  • water depth
  • wave fetch
  • this is the uninterrupted distance across water over which the wind blows, and therefore the distance waves have to grow in size. 
• How a wave breaks:

  When waves approach shore at half their wavelength depth:

  - Internal water motion touches sea bed.

  - Friction distorts wave to elliptical shape.

  - Wave speed decreases; enters offshore zone.

  Further changes as wave depth decreases:

  - Reduced wave speed.

  - Shorter wavelength and increased height.

  - Waves group closely together.

  Wave crest:

  - Moves faster than trough.

  - Leads to wave breaking in nearshore zone.

  • Resulting effects:

  - Swash flows up beach.

  - Backwash pulls water back due to diminished energy and gravity.
• Constructive waves

  • Low energy
  • Cause deposition of material
  • Strong swash, weak backwash
  • Low wave frequency (about 6-9 per minute)
  • A strong swash that pushes sediment up the beach, but a weaker backwash is unable to transport all particles back down, so they are deposited it as a ridge of sediment (berm) at the top of the beach
• Tides

  • Caused by gravitational pull of the moon and sun
  • 2 tides every 12 hours & 25 minutes
  • Spring tides - when the moon and sun are in line - large range
  • Neap tides - when sun & moon are at right angles - small range
  • Tidal range varies spatially. Britain has a large range (macrotidal)
• Tidal currents

  • Rising tide pick up & carry sediment inland
  • Falling tide carries material in opposite direction
  • Velocities low at start & end cycle, max in middle
• Shore-normal currents

  • Waves approach shore with crests parallel to shape of coast
  • Water carried up beach, return flow
  • At fairly evenly spaced locations rip currents flow back through advancing waves
• Longshore currents

  • Where waves approach shore at an angle, predominant movement of water along the shore
• River currents

  • In estuaries, rivers transport fluvial sediment and freshwater into coastal zone
• Summer beaches

  • Constructive waves more common
  • Waves are less frequent (6-9 per min)
  • Berm builds up, water dissipates through shingle berm so weaker backwash
  • Wave energy dissipates and deposits over a wide area
  • Swash deposits larger material at top of beach creates a berm
  • Weaker backwash
  • Material gets sorted as less energy, smaller towards the shoreline
• Winter beaches

  • Steeper beach angle
  • Sometimes backwash drags sediment back as next wave arrives over top
  • Higher frequency waves (11-16 per min)
  • Berms get eroded by plunging waves and high energy swash
  • Sediment dragged offshore
  • Deposits in offshore bars
  • Strong backwash transports sediment offshore
  • Destructive waves more common