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Waves-Lecture Notes
Ocean waves are on one hand infinitely complex in their behavior and form, and on the other hand, follow simple rules of physics. For this lecture, we will first summarize the physical attributes of ocean waves and then focus in on the various types. General wave terminology The height of a wave is the vertical measurement from its crest (highest peak) to its trough (lowest valley). The wavelength is the horizontal measurement between successive crests or troughs. The frequency is a measure of the number of waves that pass a stationary point in a given amount of time, usually one second. The period is the amount of time it takes successive waves to pass the same point, such as a stationary pier piling. In other words, if a wave passes a particular pier piling and you start your stopwatch, you would stop it when the next wave passes the same spot. Motion of surface ocean waves Common ocean waves form
from wind blowing across the surface, disturbing the atmosphere-ocean
interface. For wind waves to form, the wind has to be strong, and
blow form a long length of time, known as the "duration."
The distance that the wind steadily blows is called the "fetch."
As the wave is being propelled, it is referred to as a "forced
wave." Once the wave gets momentum and moves on its own, it is
called a "free wave." It is important to realize that although
it looks like waves are "moving" across the surface, the
individual water molecules actually only move in circles, or orbits.
Out on the ocean, the wind pushes the water up and then either the
surface tension (for small waves) or gravity, pulls the water back
down. Because the molecules slide into the trough and are pushed a
bit by the crest, the actual motion is an enclosed circle. As you
look down in the water column below the surface, the circular path
that a water molecule moves becomes more squashed in the vertical
direction. Eventually, the water molecules only move in a horizontal,
back and forth motion. The lowest point in which the water molecules
moves is called the "wave base." Interestingly, the wave
base is determined by the wavelength, not the height, and is ½
of the wavelength. If the depth to the seafloor is less than 1/20
of the wavelength, all of the water motion is horizontal. Types of waves Capillary waves-the
smallest of wind-driven waves with wavelengths less than 1.73 centimeters. Additionally, you should note that the various waves have distinctly different wavelengths. Tides One of the most peculiar
types of waves is the common tide. Unfortunately, like most subjects
in this class, we can only skim the surface of tides. Tides are ultimately
driven by the gravitational pull of the moon, and to a lesser degree,
the pull of the sun. If the Earth was a simple ball of water, we would
see two bulges, or wave crests, aligned with the position of the moon.
As the Earth rotates, the bulges do not move with respect to the moon,
but pass by stationary points on the Earth. In this ideal model, each
spot on the Earth (except the poles) would receive two high tides
(crests) and two low tides (troughs) each day. But, we are not on
an ideal Earth…Many complications such as the Coriolis Effect,
shape of the ocean basins and shoreline, differences in the distance
between the Earth and moon, etc., affect the timing and heights of
the tides. Key Points: 1. The height of a wave
is the vertical measurement from its crest (highest peak) to its trough
(lowest valley). The wavelength is the horizontal measurement between
successive crests or troughs. The frequency is a measure of the number
of waves that pass a stationary point in a given amount of time and
the period is the amount of time it takes successive waves to pass
the same point 3. Water molecules within a passing wave move in enclosed orbits. 4. The orbits become more squashed vertically with increasing depth. Wave base is the depth in the ocean equal to ½ of the wavelength. If the water depth is less than 1/20th of the wavelength, all the motion is entirely horizontal. 5. Waves refract or bend because of friction due to drag. In addition to bending, the drag causes the wavelength to decrease. When the height to length ration reaches 1/7, the wave is too steep, and breaks, forming "surf." 6. In addition to refraction, waves can also be "reflected." 7. Waves originating from different sources can cross each other. The energy reflected in the crests and troughs is added (crest and crest or trough and trough) or subtracted (crest and trough). 8. Capillary waves are the smallest of wind-driven waves with wavelengths less than 1.73 centimeters. 9. Wind waves are driven by winds. If they pass over water greater than ½ of the wavelength they are called "deep-water waves", otherwise they are "shallow-water waves." 10. Seismic sea waves or Tsunamis are driven by large displacements of water (e.g., underwater avalanches, breaking off of icebergs, or vertical seafloor movements along faults). 11. Seiches are waves that sloshes back and forth across an enclosed basin. 12. Rogue waves are anomalously large waves made by adding together the heights of several waves which meet simultaneously (constructive interference). 13. Internal waves form at the contact between watermasses of different densities. 14. The term sea refers to the area of the ocean riddled with forming wind waves. 15. TRUE tidal waves form when the incoming or outgoing tide is forced into a narrow area forming a wave front. True tidal waves are also called tidal bores. 16. Tides are driven by the gravitational pull of the moon, and to a lesser degree, the pull of the sun. 17. Many complications such as the Coriolis Effect, shape of the ocean basins and shoreline, differences in the distance between the Earth and moon, etc., affect the timing and heights of the tides. 18. Diurnal tides are one high tide and one low tide in a day. 19. Semidiurnal tides are two high tides and two low tides in a day, with the highs being roughly similar and the lows also being roughly similar. 20. Mixed tides refer to two high tides and two low tides in a day, but there are big differences between them. 21. Spring tides occur when the moon is closest to the Earth and the tides are high HIGH and low LOW. Spring tides occur year round (twice a month), not just in the spring 22. Neap tides are when the difference between the high and low tides is not so great. WWW Sites: http://www.humguide.com/weather.shtml Site to get lots of links to Humboldt County marine (and weather) conditions. http://www.nws.fsu.edu/B/buoy?station=46022 Bouy off of the Eel River http://www.opsd.nos.noaa.gov/tides/westHB.html Tides for Humbodt Bay, in month by month listings. http://tbone.biol.sc.edu/tide/tideshow.cgi?site=Humboldt+Bay%2C+California Another good site for tides in Humboldt Bay. http://www.geophys.washington.edu/tsunami/intro.html Site for lots of Tsunami information. http://cwis.usc.edu/dept/tsunamis/ More tsunamis, including MOVIES!! http://www.jodc.jhd.go.jp/online.html Site for data from the Japanese Oceanographic folks. http://www.amsci.org/amsci/articles/95articles/bjkorgen.html Good site for an informative article on seiches.
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