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Chapt10_Waves

# Chapt10_Waves - Waves Waves Chap 10 s s s s s The energy of...

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Unformatted text preview: Waves Waves Chap 10 s s s s s The energy of the wind is transferred to the water. The wind blows over the water and piles it up into waves. The stronger (faster) the wind, the bigger the waves. The longer distance over which the wind can blow ('fetch'), the bigger the waves. The more days a given wind blows (time), the bigger the waves. Waves have identifiable parts Waves have identifiable parts s s s s s s crest (p.222, f.10.2­3) trough wavelength (L) wave height (also called amplitude) (H) period celerity In deep water, longer wavelengths and longer periods make for faster waves (greater celerity). s Waves generally move in sets, rather than as single waves. s These sets of waves are called wave trains. s How water moves in a wave? s s s s Waves don't actually move water from one place to another. Water in a wave moves up and down. The water moves up as a crest passes, and moves down as a trough passes. Each molecule of water draws a circle as one complete wave (crest to crest) passes. s s s s s waves are moving from left to right (p.233, f.10­1,3) As the wave crest approaches the seagull, the bird moves UP. As the trough approaches, the bird moves DOWN. There is a small amount of side­to­side motion (more with longer wavelengths), but mainly the water motion is up and down. This is called orbital motion. s s s The circle (=orbit) of water motion at the surface sets in motion another orbit of water beneath it. However, some of the energy from the first orbit is lost, so the next orbit is somewhat smaller Eventually the orbit of motion gets so small, it disappears altogether, and there is no motion. s s s This water depth is called wave base and is usually ~1/2 the wavelength. This means that waves in deep water do not touch bottom!!! They do not touch bottom unless the water depth is less than 1/2 the wave's wavelength. Waves start out in the open ocean, and eventually move toward land, across the continental shelf and up onto the beach. s s s s As waves approach the beach, water depth decreases (it gets shallow!), and waves start to touch bottom where water depth is around 1/2 the wavelength. When waves begin to touch the seafloor, friction starts to distort the perfect circles of the orbits. The orbits change from circles to ellipses (oval­shaped) s s s Only where water is more shallow than the wave base can waves move sediment around. In shallow water, waves can erode sediment or deposit it. It depends on the size of the wave and the size of the sediment (sediment texture). Waves tend to be bigger in the winter because big winter storms are more frequent than hurricanes. s s Beaches tend to be eroded during the winter, but during the summer smaller, more gentle waves will bring the sand back to be deposited. This is a natural cycle, but try telling that to someone who just paid \$100,000 for a little lot on the beach, and sees what happens after the first winter storm!!! Beach Dynamics Beach Dynamics s s Eroded in winter because waves are bigger during the winter, because there are more big storms Deposited in the summer because big storms are less frequent, and the waves are usually smaller Life History of Ocean Waves Life History of Ocean Waves s s s s Out in the deep ocean, winds are blowing. We've seen how consistent wind patterns form the Trade Winds and the Westerlies, but low pressure systems can develop just about anywhere in the ocean, and this means a storm. This also means strong winds, which whip waves up. When the wind first blows over the water, it produces small ripples on the water surface: s s s s s s As the wind continues to blow, waves are piled up and are short and choppy. This is called a sea (as in, 'the seas are 3­4 feet today'): Over hours to days, the waves get bigger. They become so big, waves break and collapse on themselves. The wind may whip the tops over into a foam. These waves are called whitecaps and the sea is called a fully developed sea. s s Eventually, after the waves have traveled some distance, they become smoother, wavelength increases, the period increases, and waves appear more organized. These waves are called swell, and they can travel great distances. (p.238, f.10.8) Waves Interfere With Each Waves Interfere With Each Other s s s Waves generated by a storm march out away from the storm in all directions. A storm off the coast of Africa might generate a swell, and a storm off the coast of Spain the same day or a few days later will generate another swell. These two sets of waves will very likely have different wavelengths, different amplitudes, and different periods. s s s s Longer wavelength waves travel faster than waves with shorter wavelengths. When two wave trains meet, crests and troughs can add together to make a bigger wave Crests meeting troughs will cancel each other out These phenomena together are called interference. s s s When waves interfere and a bigger wave results, we call it constructive interference. When waves interfere and cancel each other out, we call it destructive interference. When you see waves at the beach (or at sea when you are in a vessel), you are observing not a single wave train but perhaps a dozen or more wave trains that are interfering and producing an irregular array of waves. Waves Approaching the Shore s s s Waves moving out away from a storm eventually organize themselves into a swell, and eventually, if they are not destroyed by interference, they reach the shore. The seafloor shallows as the waves approach shore, and eventually the waves touch bottom (they reach wave base). At this point we shift from "deep water" to "shallow water" (from the wave's perspective). s s s s Once at wave base, the orbital motion of the water particles changes to ellipses The waves begin to slow down (celerity decreases) due to friction and wave celerity now depends on water depth (remember, in deep water wave celerity depends on wavelength and period: C= L/T). The shallower the water, the more the waves slow down. Wavelengths shorten, but period remains the same. The wave height increases and the trough flattens out. s The wave gets so tall it can't support itself, and the water crashes over the top. s This is called a breaker, and breakers form in an area called the surf zone. s The wave loses most of its energy by breaking (it actually gives off some light and heat), and the remaining energy causes the water to rush up the shore. s It loses the rest of its energy to friction in this manner, then gravity pulls the water back out to sea. s s The surge onshore is called swash; the slump back to sea is called backwash. Swash and backwash occur in the swash zone. s So as the the waves come to shore from the sea, • they change from deep­water waves to shallow water waves at wave base, where water depth = 1/2 wavelength, • they slow down (celerity decreases) • wavelength decreases • period stays the same • height increases • wave breaks and becomes • swash, then backwash s s The appearance of a wave breaking at the shore depends on the size of the wave and how steep the shore and continental shelf are. Flat slopes for shore and shelf encourage spilling breakers: s s Somewhat steep bottoms encourage plunging breakers. Very steep bottoms encourage surging breakers ­ these don't actually break, since the bottom slopes too abruptly s s s s s Note surfers prefer spilling and plunging waves­but they have to be sizeable. The east coast has spilling waves, but they are only 1­2 feet high most of the time. There are a number of reasons for this: the continental shelf there is broad and shallow; the fetch of the Atlantic Ocean is less than that for the Pacific Waves Refraction Waves Refraction Wave base is not a fixed point in the ocean! s Wave base moves onshore during high tide, and moves offshore during low tide. s It is closer to shore for waves with short wavelengths, and farther offshore for waves with long wavelengths. s s s s Imagine that a single wave train is coming towards shore. If the seafloor were a perfect incline, the wave would reach wave base at the same distance offshore, no matter how far left or right you looked. The wave would steeped at the same place offshore, and break at the same place at the shore. It might look like the photo below: s s s s s In many places, the seafloor is NOT a perfect incline. Even though it is under water, it has hills and valleys as you walk along, parallel to the shore. That means that a given wave train will reach wave base at different distances from shore. In the photo below, some waves are breaking a few feet farther offshore than others. Waves break where water depth is about 1/5 wave height. If you drew a line under all the breaking waves, you would draw a line where water depth = 1/5 wave height. s When the area just offshore of the beach has topography, waves reach wave base farther offshore. s At this point, orbits change to ellipses, and the wave slows down s s s The part of the wave that hasn't touched bottom yet continues to move toward the beach, now moving faster than the part of the wave that has slowed down. This is similar to playing 'crack the whip' while ice skating: the faster waves bend (=refract) toward the slower­moving waves s s s In this photo, the land just out to sea, making wave base farther offshore than it is for the waves entering this small bay. Waves in the forefront of the photo are bending toward you; on the opposite side, they bend away. In either case, the waves are bending TOWARD the shallowest part of the bay (nearest to them). This bending of waves around objects is called wave refraction. s Wave refraction causes waves to strike as near to parallel to shore as they can get. s s s s s In the photo above, waves bend around a peninsula of sand that juts out to sea. Wave refraction helps erode land that juts out to sea, and deposit sediment in bays. Waves bending toward shore converge on headlands (which are bits of rocky land jutting out to sea). Converging waves concentrate energy, so the waves pound the headlands, weathering them to sedimentary particles. Waves bending toward shore diverge in the bays. s This dilutes the energy of the waves, and the bays are quiet places where sediment accumulates as a beach. s ...
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