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Wave Characteristics

Wave Characteristics


  Tidal movement, river outflow and ocean currents such as the East Australia current all move large amounts of water. This movement of the water can change the characteristics of surface waves. Waves moving against a current tend to become higher and have steeper faces. 
  This effect is well known through the Whitsunday Island group where strong tidal flows are funnelled between islands. Inexperienced mariners that hire vessels in this area can give themselves a bit of a scare if they experience these conditions, even in moderate seas. 

Shoaling Waves 

  When vessels are operating in the near-shore zone particular care needs to be taken. As waves move into shallower water at a depth of approximately one half of their wavelength they begin to interact with the sea floor. The wave slows down and the face begins to become steeper and wave grows in height. This process is called shoaling. 
Shoaling Waves  As the wave moves further inshore and continues to ‘shoal’ it grows to a size where it can no longer maintain its height and the wave breaks. Waves standing up (shoaling) in relatively open water can be an indicator to the mariner of an undersea obstruction such as a reef or large wreck. 
  Breaking waves take three main forms: surging, plunging and spilling. A surging wave occurs where the sea is relatively deep (compared to the wave length of the wave) until into the near-shore zone. This type of wave will give a fast moving, steep and smooth faced wave as it begins to shoal. It delivers a lot of energy very quickly, it is the type of wave that a surfer would look for. However, for vessels in the near-shore zone this type of wave can lead to broaching or pooping. 
  The surging wave will eventually break and will most likely become a plunging wave as it breaks. These are as dangerous as surging waves as they have the original initial energy of the movement of wave plus the energy from the wave breaking. 
  The spilling type wave breaks and continues to break as the wave moves through the near shore. This continued breaking spreads the energy of the wave over a large area and is the more benign of the three types of wave. 

Wind and Swell WavesWind and Swell Waves 

  In the near shore zone, a breeze travelling in the same direction as the swell can ‘blow the top’ of the wave as it begins to shoal. This will cause the wave to break earlier than it otherwise would have done. As it is not the whole wave breaking just the top this is likely to bring about a spilling type wave that breaks over a large area. 
  Conversely wind against a swell will sharpen the face of the wave and hold up the wave that will tend to break as a plunging wave. 

Long Period Swell 

  Deep water swell waves travel at a speed proportional to their wave period. 
  The speed of the wave in knots is approximately three times the wave period in seconds. A deep water swell with a period of ten seconds travels at approximately thirty knots, one with a period of fifteen seconds travels at approximately forty five knots. 
  In the near-shore zone long period swell will move a greater mass of water and move it faster (per wave) than a shorter period swell of the same height. 
  At many locations around the Australian coastline, mariners cross river barways to get out to sea. The wave conditions mentioned above could all combine to cause the transiting of barways to be extremely hazardous. 
  Firstly the tide can be falling with water exiting the river mouth, the rate of this flow is greatest midway between tides. Heavy rainfall in the catchments may add to the current exiting the river. This outgoing tide will make the waves or swell steepen and get larger. A breeze blowing out to sea will add to this process (swell waves only). 
  The waves will shoal as they move into shallower water further increasing the size of the waves. 
  If the waves are long period swells they can move a lot of water very quickly in the bar where the above processes will increase their size and be a danger to mariners. 
  There are excellent pamphlets available about crossing barways. Contact your local marine safety authority for more information. 

Measuring waves 

  I am often asked why the Bureau of Meteorology wave heights in the marine forecasts and warnings are the significant wave height (average of the highest one third of waves). Why not just use the maximum wave or the average wave? 
  Measuring waves is complex as there are many variables in forming a single wave and of course there are many, many waves. In any area of sea there are large number of possible combinations of currents winds, fetches, swell and the time and distance that all of them have been affecting each other. Also all of these elements are changing in strength all the time.
  I came across this following example of how complex this is, in the book Oceanography and Seamanship by William G. Van Dorn (available in State Reference Libraries):
  If there was a fully developed sea, this means that the wind has been blowing from the same direction over a long distance for a large amount of time and the waves have reached an equilibrium with the winds and will not grow any further.
  In this example a steady wind of 30 knots blowing in the same direction for 24 hours over 340 nautical miles. This wind would produce a significant wave (type the Bureau forecasts) of 5 metres.
  However, the average wave height would be 3 metres.
  The most frequent wave height would be 2.5 metres.
  10 % of all waves would be less than 1 metre.
  10 % of all waves would be higher than 5 metres.
  The average height of the highest ten percent of all waves would be 7 metres.  A 5% probability of encountering a wave greater than ten metres every thirty minutes.
 What this means for interpreting the Bureau forecast is that most waves will be smaller than the forecast height. Some will be greater than 5m and every now and then a very large, 10 metre wave is possible.  This is why forecasts have the preamble “Please be Aware, Wind gusts can be a further 40 percent stronger than the averages given here, and maximum wave heights may be up to twice the height”.
  Why does the Bureau of Meteorology use the significant wave height?
  It is all down to history.
  The original definition of significant wave height resulted from work by the oceanographer Walter Munk during World War II (who also did surf forecasts for the D-Day landings). The significant wave height was intended to mathematically express the height that a “trained observer” would intuitively see.  It is now the most common way of recording wave height worldwide.

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of QLD and is a Master V. He gives education courses on Marine Meteorology.

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