Tuesday, October 15, 2013

Safe Boat Operations – Waves and Surf

The ability to recognize wave patterns and characteristics is essential to safe operation in surf and heavy weather.  A person operating in these conditions must be able to determine the timing of lulls and series, and estimate wave heights accurately. There are several forces which create waves at sea; the most significant of which is wind.
The factors that determine the characteristics of wind waves are:
  • Wind speed
  • Wind duration
  • Fetch (the distance over open water which the wind has blown)
As the wind begins to blow, it creates seas, which are typically steep, choppy, and have little pattern. As the wind continues, the seas begin to become more defined.  In heavy weather, observing and measuring waves is important.  If the boat operator can get a general sense of the waves in which they are operating, it will allow the crew to operate accordingly. Heavy weather waves and seas are generated by weather systems, either local or distant.  There are many factors that determine what conditions will be generated by a weather system or series of weather systems.
Factors that will affect wave height include:
  • State of the tide:  Ebb currents often cause wave speed to decrease and wave height to increase.  Conversely, flood currents often cause waves to gain speed and loose height.
  • Rainfall:  Heavy rainfall can reduce the size of waves, but large runoffs from rivers may stop the flood current or drastically change the conditions at inlets or bars.
  • The width of the body of water:  The greater distance the body of water is allows for larger waves to be generated.
  • Depth of water:  Deeper water allows for larger swells to be generated.  As these swells approach shallow water on the coast, they will loose speed and gain height.
  • Air temperature:  Cold air is denser, causing greater impact on the water and building larger swells than warm air.
After waves are generated far out at sea, they move outward, away from their wind source, in ever-increasing curves, and become what are called swells.  The farther the swell moves from its source, the more uniform its characteristics become, as it travels in a series of waves, relatively equidistant, and moving at a more or less constant speed.  Because of this, swells generated from storms far out at sea can be distinguished by their smoothness and uniformity from those that are coarser (peaked and irregular) which have recently originated nearby.  The usual period of these swells is from 6 to 10 seconds.
Interference between different swell systems, which are travelling in nearly the same direction, causes groups of waves to travel outward in patches.  As these groups of several waves (normally 7 to 12) progress outward, those waves in the forefront disappear and new waves, of the same characteristics, appear at the rear of the patch.  This process continues until the waves dissipate their energy at sea, or transfer it to the shore as surf.

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