Wave Development
Leif Lie
WSFO, Juneau
When a wind begins to blow across a flat body of water riplets form almost immediately, then wavelets, and finally nearly fully developed waves, having almost definite wave lengths and shapes. Surface waves will also form, during calm wind, where opposing currents meet. This is known as current rips and can be dangerous for small or overloaded boats.
A good estimate of the wind speed can be obtained by observing its effect on the water. For example, at 5 knots only small wavelets can be observed. A 10 knot wind speed will produce a few whitecaps. At 15 knots whitecaps can be seen up to 30% of the area. At 20 knots whitecaps can be seen up
to 60% of the area. At 30 knots whitecaps and spray can be seen all over and foam from the breakers begins to form. At 40 knots streaks of foam can easily be seen. At 50 knots visibility is reduced and the sea begins to take on a white appearance from all the foam. Current directions and speed will greatly affect the areas covered by spray and breakers.
Factors Affecting Wave Heights:
1. Wind.
2. Wind duration.
3. Length and width of wind fetch. (This is the distance over the water in which the wind blows in nearly the same direction.)
4. Depth of the water.
5. Direction and speed of the current.
6. Rate of precipitation.
7. Air temperature.
8. Amounts of flotsam, ice, or seaweed in the fetch area.
1. How high will the waves get?
Maximum wave heights (in feet) will not get any higher than * the wind
speed in knots. This is correct over 90% of the time. Note: Wave height
begins to increase when the depth of the water is less than or equal to
1/4 of the wavelength because the wave starts to touch the sea floor.
2. What is the difference between waves and swells?
Waves occur where the wind is blowing. Swells are waves that have moved
away from the wind generating area and frequently give warnings of an approaching
storm.
3. What is the significant wave height? Significant
wave height (SWH) is the average height of 1/3 of the highest waves, or
the height of the waves an observer is most likely to report. The significant
wave height is used in the marine forecasts.
Table 1.Wave heights in relationship to the significant
wave height (SWH)
Most frequent wave height .5 x SWH
Average wave height .6 x SWH
Significant wave height 1.0 x SWH
Height of the highest 10% 1.3 x SWH
Heights of highest 1% of waves 1.7 x SWH
One wave in about 1200 1.9 x SWH
4. How fast do the waves move?
This can easily be answered when we know the wave length (ft) and the period
in seconds (sec).
Wave length in feet = 3 x P2 (sec) for wind waves
= 5 x P (sec) for swell
Wave speed in mph = roughly 3.5 x P (in sec)
Example: The speed of a surface wave with a period
of
5 sec = 3.5 x 5 = 17.5 mph
Wave length is the distance between two adjacent
crests. Period is the time it takes for two adjacent crests to pass a particular
point.
Waves can be classified according to the period.
The period for wind waves generated over the ocean generally ranges from
1 second to 30 seconds. The longest period of swell reported is 22 seconds.
This will correspond to a wavelength of 2,478 feet and a speed of 77 mph.
Average period of swell along the coast of Southeast Alaska is about 10
seconds which corresponds to a wavelength of 512 feet. Along the coast
of Europe the period is a little less than in the Pacific.
5. What are the most dangerous waves?
The steepest waves that normally occur at the beginning of a storm will
cause more damage to ships than waves of the same height that occur at
a later cycle during the storm.
6. What is the combined sea height (CSH)?
CSH =* (wind
wave height)2 + (swell height)2
Example: When Sea = 3 ft. and Swell = 4 ft.
CSH = * (3)2 + (4)2
= * 25
= 5 ft. (not 3 + 4)
