08 - WAVES Why do they form? What controls their size and...

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Unformatted text preview: WAVES Why do they form? What controls their size and strength? Why do they "break"? What is a wave? disturbance caused by the movement of energy from a source through some medium (solid, liquid, or gas) Wave moving through medium... medium... ex: sound waves, light waves, seismic waves, water waves... waves... Ocean Waves Move Energy across the Sea Surface Ocean waves are visual proof of the transmission of energy across the surface of the ocean. A ribbon of energy moves through the water, but the water itself DOES NOT MOVE! A wave moves from left to right as the gull (and the water in which it is resting) revolves in a circle, moving slightly to the left up the front of an approaching wave, then to the crest, then sliding to the right down the back of the wave. Waves transmit energy, not water mass imagine bird floating on sea surface as wave travels by... it by... moves in circular pattern, as do water molecules that wave energy is passing through 1 Progressive waves waves of moving energy in which the wave form moves in one direction along the surface (or junction) of the transmission medium. Most waves affect only the ocean's surface layer. Movement ocean' ceases at a depth equal to about half the wave's wavelength. wave' Orbital waves are waves in which the particles of water move in closed circles as the wave passes (a type of progressive wave). Note that the water molecules in the crest of the wave move in the same direction as the wave, but molecules in the trough move in the opposite direction. Anatomy of a wave... crest trough height wavelength period frequency Period = Crests passing per time (seconds) Velocity/Wavelength Wavelength= crest-to-crest (meters) crest- to- Observed: 1.5 * Period * Period (or, in mph, = 3.5 * Period) Velocity (km/hr). observed = 5 * Period LONGER WAVES TRAVEL FASTER !! Disturbing & Restoring Forces Free wave- Disturbing force that forms wavewave from a single point which then propagates (ex: tsunami) Forced wave- maintained as it travels by wavecontinuing force (ex: tides) Wave energy is weakened by a restoring force (ex: cohesion (for very small waves), gravity) Table 10-1, p. 268 2 Wave energy... Arranged from short to long wavelengths ocean waves are generated by... by... Wave energy in the ocean as a function of the wave period (and therefore from slowest to fastest), very small disturbances (capillary waves), wind (wind waves), rocking of water in enclosed spaces (seiches), (seiches), seismic and volcanic activity or other sudden displacements (tsunami), and gravitational attraction (tides). The speed of ocean waves usually depends on their wavelength, with long waves moving fastest. Importance of Depth 1. Deep water waves (d > wavelength) 2. Transitional waves (in between) 3. Shallow water waves (d < 1/20 wavelength) Progressive waves The behavior of a wave depends largely on the relation between the wave's size and the depth of water through which it is moving. Classification of progressive waves depends on their wavelength relative to the depth of water through which they are passing. wavelength and depth in determining wave type. Note the importance of the relationship between not to scale... Wavelength vs wave speed Speed is equal to wavelength divided by period. If one characteristic of a wave can be measured, the other two can be calculated. Theoretical relationship among speed, wavelength, & period in deep-water waves deep- The easiest to measure exactly is period... period... in the example shown in red, the speed of a wave with a wavelength of 233 meters and a period of 12 seconds is 19.4 meters per second. 3 Wind waves most common type in ocean gravity waves formed by wind energy usually < 3 m (~10 ft) high grow from capillary waves Average wave velocity = 80% wind Max. Height = 1/7 of wavelength waves mature, organize to form "swell" swell" Wind waves Swell--mature, regular wind waves sorted by dispersion--off the Oregon coast. Small waves superimposed on the large swell are the result of local wind conditions. Wind over the ocean generates waves Wind waves = gravity waves formed by the transfer of wind energy into water. Wind forces convert capillary waves to wind waves. A capillary wave interrupts the smooth sea surface, deflecting surface wind upward, slowing it, and causing some of the wind's wind' energy to be transferred into the water to drive the capillary wave crest forward (point a). The wind may eddy briefly downwind of the tiny crest, creating a slight partial vacuum there ( - ). Atmospheric pressure ( + ) pushes the trailing crest forward (downwind) toward the trough (point b), adding still more energy to the water surface. Wind forces acting on a capillary wave wind speed needs to remain higher than speed of wave crests for energy transfer to continue from air => sea 3 main factors influence wind wave development What factors affect wind wave development? Wind strength - wind must be moving faster than the wave crests for energy transfer to continue Wind duration - winds that blow for a short time will not generate large waves Fetch - the uninterrupted distance over which the wind blows without changing direction Wave size increases with increased wind speed, duration, and fetch. A fetch. strong wind must blow continuously in one direction for nearly three days three for the largest waves to develop fully. the uninterrupted distance over which the wind blows without significant change in direction. Fetch 4 Where can wind waves get biggest? Maximum "theoretical" wind wave height... Global wave height acquired by a radar altimeter aboard the TOPEX/Poseidon TOPEX/Poseidon satellite in October 1992. In this image, the highest waves occur in the occur southern ocean, where waves were more than 6 meters (19.8 ft) high high (represented in white). The lowest waves (indicated by dark blue) are found in blue) the tropical and subtropical ocean, where wind speed is lowest. A wind wave of moderate size shown during the time of its formation. The ratio of height to wavelength, called wave steepness, is 1:7; the crest angle does not exceed 120. Wave trains Waves travel in groups called wave trains. As the leading wave of the group travels forward, it transfers half of its energy forward to initiate motion in the undisturbed surface ahead. The other half is transferred to the wave behind to maintain wave motion. The leading wave in the wave train continuously disappears, while a new wave is continuously formed at the back of the train (Follow wave number 5 in the diagram on R). The wave train travels at half the speed of any individual wave, a speed known as group velocity (v). Interference produces irregular wave motions What happens when waves from different storm systems exist simultaneously? When waves meet up, they interfere with one another. Wave interference can be: Destructive interference two waves that cancel each other out, resulting in reduced or no wave Constructive interference additive interference that results in waves larger than the original waves Rogue waves - these freak waves occur due to interference and result in a wave crest higher than the theoretical maximum Constructive & destructive interference (a) Two overlapping waves of different wavelength are shown, one in blue and one in green. Note that the wave show in blue has a slightly slightly longer wavelength. (b) If both are present in the ocean at the same time, same they will interfere with each other to form a composite wave. At the position of line 1, the two waves in (a) will constructively interfere to form interfere very large crests and troughs, as shown in (b). At the position of line 2, the two waves will destructively interfere, and the crests and troughs will troughs be very small (again shown in b). 5 Deep-Water Waves Change to ShallowDeepShallowWater Waves As They Approach Shore What happens when a wave breaks against the shore? When a wave meets the shore... (1) The swell "feels" bottom when the water is shallower than feels" (2) The wave crests become peaked because the wave's wave' (3) Constraint of circular wave motion by interaction with the (4) (5) ocean floor slows the wave, while waves behind it maintain their original rate. The wave approaches the critical 1:7 ratio of a wave height to wavelength. The wave breaks when the ratio of wave height to water depth is about 3:4 (base slows down when it "feels" feels" seafloor, but top keeps moving at same faster speed). energy is packed into less water depth. half the wavelength. The movement of water particles is shown in red. What different ways can waves break against the shore? "plunger" ex: Plunging waves break violently against the shore, leaving an air-filled tube, or channel, airbetween the crest and foot of the wave. Faster. Plunging waves are formed when waves approach a shore over a steeply sloped bottom. Spilling waves occur on gradually sloping ocean bottoms. The crest of a spilling wave slides down the face of the wave as it breaks on shore. Makua Rothman drops down the face of a 4-meter (13foot) wave at Sunset Beach on the North Shore of the Hawai'ian Island of O'ahu steep sides of underwater volcano and immense amounts of energy in the mid-Pacific = BIG WAVES. "spiller" ex: form when bottom slopes gradually (nice gradual sandy beach) Waves Refract When They Approach a Shore at an Angle What can affect the way that waves travel? Wave refraction - the slowing and bending of waves in shallow water. Wave diffraction - propagation of a wave around an obstacle Wave reflection - occurs when waves "bounce back" from an obstacle they back" encounter. Reflected waves can cause interference with oncoming waves, creating standing waves. waves. 6 Wave refraction Wave refraction around Maili Point, O'ahu, Hawai'i. Note how the wave crests bend almost 90 as they move around the point checkerboard interference Interference from obstacles Diffraction of waves by a breakwater gap at Morro Bay, California. 7 Dec 2007; some of the largest waves on record in N CA (80 ft) storm surge, landslides, flooding, >100mph winds "The perfect storm" storm" "The Why here?? Mavericks: Why BIG waves?? http://www.kqed.org/quest/television/view/249 8 ...
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This note was uploaded on 04/29/2008 for the course EAS 104 taught by Professor Brown during the Spring '08 term at Purdue University-West Lafayette.

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