As we will see in section Refraction can also explain why waves tend to be larger off of points and headlands, and smaller in bays. A wave front approaching shore will touch the bottom off of the point before it touches bottom in a bay. Once again, the shallower part of the wave front will slow down, and cause the rest of the wave front to refract towards the slower region the point. In the bay, the refraction has caused the wave fronts to refract away from each other, dispersing the wave energy, and leading to calmer water and smaller waves.
The wave height is the overall vertical change in height between the crest and the trough and distance between two successive crests or troughs is the length of the wave or wavelength.
While one normally associates an up and down motion with the passage of each wave. Actually, a circular motion occurs. It is this orbital motion of the water or objects on the surface of the water that causes an object to bob up and down, forward and backward as waves pass under it. But even this motion is not exactly circular but is trochoidal line form traced by a point on a rolling wheel.
Ocean waves offer a very large source of renewable energy. Technologies that efficiently harvest this energy resource are actively being researched and developed by scientists. By watching a buoy anchored in a wave zone one can see how water moves in a series of waves. The passing swells do not move the buoy toward shore; instead, the waves move the buoy in a circular fashion, first up and forward, then down, and finally back to a place near the original position.
Neither the buoy nor the water advances toward shore. As the energy of a wave passes through water, the energy sets water particles into orbital motion as shown in Fig. Notice that water particles near the surface move in circular orbits with diameters approximately equal to the wave height. Notice also that the orbital diameter, and the wave energy, decreases deeper in the water. The energy of a deep-water wave does not touch the bottom in the open water Fig.
When deep-water waves move into shallow water, they change into breaking waves. When the energy of the waves touches the ocean floor, the water particles drag along the bottom and flatten their orbit Fig. At this point the water movement of particles on the surface transitions from swells to steeper waves called peaking waves Fig. Because of the friction of the deeper part of the wave with particles on the bottom, the top of the wave begins to move faster than the deeper parts of the wave.
When this happens, the front surface of the wave gradually becomes steeper than the back surface. At this point, the top of the wave travels so much faster than the bottom of the wave that top of the wave begins to spill over and fall down the front surface.
This is called a breaking wave. A breaking wave occurs when one of three things happen:. In some ways a breaking wave is similar to what happens when a person trips and falls. Thus, a long swell wave may act as a shallow water wave in depths of 10 m, but also a very short wave may act as a deep water wave in depths of 1 m.
The tangent hyperbolic function has some convenient properties for the limiting values of its argument:. The dispersion relationship then reduces to:. The dispersion relationship is now:. Feel the bottom refers to the fact that the wave-induced velocity field extends all the way from the top of the water column to the bottom of the water column. When the wave "feels the bottom" it means that there is some interaction with the bottom boundary.
A very thin boundary layer develops at the bottom where vorticity is generated due to the velocity field interacting with the bed roughness. The vorticity can diffuse up into the interior of the fluid and is responsible for turbulent activity inside the boundary layer.
Due to the persistent nature of waves, their action is thought to play an important role in the redistribution of particulate matter in the nearshore, such as nutrients, larvae, sediment and even contaminants. This picture exemplifies it pretty well. Sign up to join this community.
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