Oceanography-Chapter 6 (Waves).pptx

Waves Ocean surface waves are surface waves that occur on the free surface of the ocean. They usually result from wind, and are also referred to as wind waves. Some waves can travel thousands of miles before reaching land. They range in size from small ripples to huge rogue waves. The winds cause wa...

Waves Ocean surface waves are surface waves that occur on the free surface of the ocean. They usually result from wind, and are also referred to as wind waves. Some waves can travel thousands of miles before reaching land. They range in size from small ripples to huge rogue waves. The winds cause waves on the surface of the ocean (and on lakes). The wind transfers some of its energy to the water, through friction between the air molecules and the water molecules. Stronger winds (like storm surges) cause larger waves. There is little actual forward motion of individual water particles in a wave, despite the large amount of energy it may carry forward. Waves of water do not move horizontally, they only move up and down (a wave does not represent a flow of water). You can see a demonstration of this by watching a floating buoy bob up and down with a wave; it does not, however, move horizontally with the wave. Wave height of a surface wave denotes the difference between the elevations of a crest and a neighbouring trough Wavelength is the distance between repeating units of a propagating wave of a given frequency. A crest is the point on a wave with the greatest positive value or upward displacement in a cycle. A trough is the opposite of a crest. The velocity of idealized traveling waves on the ocean is wavelength dependent and for shallow enough depths, it also depends upon the depth of the water. The wave speed relationship is: Tsunamis are a specific type of wave not caused by wind but by geological effects. In deep water, tsunamis are not visible because they are small in height and very long in wavelength. They may grow to devastating proportions at the coast due to reduced water depth. Wave formation The great majority of large breakers one observes on an ocean beach result from distant winds. Four factors influence the formation of wind waves: wind speed distance of open water that the wind has blown over — called fetch time duration the wind has blown over a given area water depth. All of these factors work together to determine the size of ocean waves. The greater each of the variables, the larger the waves. Waves are characterized by: Wave height (from trough to crest), Wavelength (from crest to crest), Period (time interval between arrival of consecutive crests at a stationary point), The direction of wave propagation. Types of wind waves Three different types of wind waves develop over time: Capillary waves, or ripples Seas Swells Ripples appear on smooth water when the wind blows, but will die quickly if the wind stops. The restoring force that allows them to propagate is surface tension. Seas are the larger-scale, often irregular motions that form under sustained winds. They tend to last much longer, even after the wind has died, and the restoring force that allows them to persist is gravity. As seas propagate away from their area of origin, they naturally separate according to their direction and wavelength. The regular wave motions formed in this way are known as swells. Individual "rogue waves" (also called "freak waves", "monster waves", "killer waves", and "king waves") sometimes occur in the ocean, up to heights near 30 meters, and being much higher than the other waves in the sea state. Such waves are distinct from tides, caused by the Moon and Sun's gravitational pull, tsunamis that are caused by underwater earthquakes or landslides, and waves generated by underwater explosions or the fall of meteorites — all having far longer wavelengths than ocean surface waves. Wave breaking (surfing) Some waves undergo a phenomenon called "breaking". A breaking wave is one whose base can no longer support its top, causing it to collapse. A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces. When the slope, or steepness ratio, of a wave is too great, breaking is inevitable. As waves move toward a beach, the shallower water decreases the wavespeed, so the wavelength becomes shorter and the peak heights increase. The wavepeaks become unstable and, moving faster than the water below, they break forward. DEEP WATER SWELL ZONE OF TRANSITION BREAKING Refraction Refraction is the bending of waves because of varying water depths underneath. The part of a wave in shallow water moves slower than the part of a wave in deeper water. So when the depth under a wave crest varies along the crest, the wave bends. Wave refraction also occurs around a circular island. The wave approaching from one direction will wrap around the island so the wave crest will approach the beach close to parallel on all sides of the island. In Figure, the wave crests are shown (the first crest is a horizontal line at the top of the figure), and the vertical lines are wave orthogonals (lines which would be traced out by following the wave direction). Note the criss-crossed wave pattern behind the island. Diffraction Diffraction usually happens when waves encounter surface-piercing obstacle, such as a breakwater or an island. It would seem that on the lee side of the island, the water would be perfectly calm; however it is not. The waves, after passing the island, turn into the region behind the island and carry wave energy and the wave crest into this so-called 'shadow zone.' The turning of the waves into the sheltered region is due to the changes in wave height (say along the crest) in the same wave. If the sides of the island are sloping under the water, then refraction would also be present. Diffraction Wave reflection When a wave encounters a boundary in its medium, it reflects off that boundary. That is, the wave is sent back in the direction it came from. If the ends of the wave machine are free, then a wave crest striking the end will be reflected as a crest, and a trough will be reflected as a trough. The wave will retain its shape as it returns in the other direction. Waves are a means by which energy travels. Many different particles move in waves. The waves on an ocean are physical waves caused mainly by wind. Physical waves reflect, refract and diffract energy. These properties of wave motion are important because they explain how waves move. Diffraction is when a wave goes through a small hole and has a flared out geometric shadow of the slit. Diffraction is a characteristic of waves of all types. We can hear around a corner because of the diffraction of sound waves. For instance, if a wall is next to you when you yell, the sound will parallel the wall. The wall may stop, but the voice doesn't; sound will almost turn the corner of the wall. This is diffraction. Reflection is when waves, whether physical or electromagnetic, bounce from a surface back toward the source. A mirror reflects the image of the observer. Refraction is when waves, whether physical or electromagnetic, are deflected when the waves go through a substance. The wave generally changes the angle of its general direction.