Waves and Tides

Waves

(Waves are created by the transfer of energy from wind to the surface of the sea. As the strength of the wind increases wave size increases, and frictional drag increases.)

• Swell- waves which result from distant storms, and strong winds, these waves can travel long distances. Characterised by waves with low wave height, gentle steepness, long wave length, and long period.
• Sea- waves which result from local wind, and therefore only travel short distances. Opposite characteristics to swell, occurring in higher energy waves.
• Fetch- maximum distance of open water over which wind can blow. (higher fetch leads to more wind and waves with higher energy)
• Wave height (H)- the vertical distance between the wave crest, and the trough
• Wave period (T)- time taken for a wave to travel one wavelength.
• Wave length (L)- the distance between 2 successive crests.
• Wave velocity (C)- the speed of movement of a wave.
• Wave steepness (H÷L)- ratio of wave height to wave length, cannot exceed 1:7, or wave will break.
• Wave energy (E)- the energy possessed by a wave in deep water expressed as EµLH².

Wave behaviour

• Deep water.Water is considered deep if D>½L

  The drag of wind causes floating objects to move in an orbital pattern. Waves are surface features and therefore the size of the orbits decreases rapidly with depth.

• Shallow water.

  Water is considered shallow if D<½L

  In shallow water friction with the sea bed increases and as the base of the wave slows down the ellipse becomes more acute, as the water depth decreases, so does the wavelength. The steepness of the wave increases until the the upper part spills over, or plunges over. At the plunge line, the depth of water and the height of the wave are almost equal. The water which rushes up the beach is called the swash, and the water retuning on the surface is called the backwash.

• Wave refraction

  As waves approach an irregular coastline they become increasingly parallel to the coastline. As each wave crest gets close to the coast the part of the wave in the shallower moves more slowly due to increased frictional drag, so the wave crests bend to become parallel to the coastline. In the diagram below the convergence of wave fronts on the headland leads to a concentration of energy on the headland, leading to accentuated erosion. The diagram also shows the formation of littoral currents, which carry sediment away from the headland.

  

{Main Page|Coastal Erosion|Coastal Deposition|Case Study}