Coastal processes: waves and tides

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Lecture focuses on shallow-water area

Coastal margins- shallow areas are affected by winds and tidal currents

  • Typically covered by sand and mud.
  • If there is little terrigenous (land-derived) sediment input- carbonates may form. 
  • Continental shelf slopes off into the abyss

Landscapes are highly variable, depending upon:

  • stability of coastal region (uplifting, subsiding, stable)
  • Nature of rocks or sediments at the shoreline
  • Long-term changes in sea level
  • Wave energy 
  • Tidal energy

Long time scales- Ice ages can shape the coastline e.g. isostatic subsidence and rebound. - Scotland = rebounding + South England is subsiding. T affects relative sea level.

Waves and Tides in Shallow waters

Energy:

Waves- energy is controlled by wind velocity+ duration and the fetch (distance wave can propogate)

e.g. Over the N. Atlantic blowing west to east- long fetch for waves to build

wind pushes water up and along, gravity is pulling water down- maximum wave height to be recorded- 34m in the pacific by a US Navy tanker.

enormous waves may be responsible for the loss of ships that just go missing- rogue waves- frak event

Motion

Waves have wave lengths- L (distance between two crests) and wave height- H (crest to trough). The particle motion is in a circle. Down through the wave, these rotations get smaller as the wave's influence dwindles. Wave period-(T) how long for 2 crests to pass a point.

Wave base= L/2. Water depth=D

Shallow water waves will influence the sea bed D<L/2                                                         Deep water waves have no influence on the sea bottom D>L/2

When deep water waves become shallow water waves- water slows down on contact with the sea bed- friction- and wave builds. - Heightened and sharpened crests. Length decreases. 

In the surf zone- gravty acts on wave crests- water topples over

When various wave frequencies coalesce and disperse- swells can build into waves with…

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