Wind-Caused Waves Misleads Us Energy Is Transferred to the Wave
Total Page：16
File Type：pdf, Size：1020Kb
WAVES IN WATER is quite similar. Most waves are created by the frictional drag of wind blowing across the water surface. A wave begins as Tsunami can be the most overwhelming of all waves, but a tiny ripple. Once formed, the side of a ripple increases the their origins and behaviors differ from those of the every day waves we see at the seashore or lakeshore. The familiar surface area of water, allowing the wind to push the ripple into waves are caused by wind blowing over the water surface. a higher and higher wave. As a wave gets bigger, more wind Our experience with these wind-caused waves misleads us energy is transferred to the wave. How tall a wave becomes in understanding tsunami. Let us first understand everyday, depends on (1) the velocity of the wind, (2) the duration of wind-caused waves and then contrast them with tsunami. time the wind blows, (3) the length of water surface (fetch) the wind blows across, and (4) the consistency of wind direction. Once waves are formed, their energy pulses can travel thou Wind-Caused Waves sands of kilometers away from the winds that created them. Waves transfer energy away from some disturbance. Waves moving through a water mass cause water particles to rotate in WHY A WIND-BLOWN WAVE BREAKS place, similar to the passage of seismic waves (figure 8.5; see Waves undergo changes when they move into shallow water figure 3.18). You can feel the orbital motion within waves by water with depths less than one-half their wavelength. Wave standing chest-deep in the ocean. An incoming wave will pick friction on the floor of the shallow ocean interferes with the you up and carry you shoreward and then drop you downward orbital motions of water particles, so waves begin slowing and back as it passes. At the water surface, the diameter of the (figure 8.6). Friction with the bottom flattens the circular water-particle orbit is the same as the wave height. The diameters motions of the water into elliptical and horizontal movements. of water orbits decrease rapidly as water deepens; wave orbital As waves slow down, their wavelengths decrease, thus motion ceases at a depth of about one-half of the wavelength. concentrating water and energy into shorter lengths and Waves vary from tiny, wind-blown ripples to monster causing the waves to grow higher. When the wave height rogue waves, but the rotational motion of water within a wave to-wavelength ratio (H:L) reaches about 1:7, the wave front has grown too steep, and it topples forward as a breaker 1...--- Wavelength (L) ----. 1 (figures 8.7 and 8.8). Note that the 1:7 ratio is reached by Wave direction Figure 8.5 Waves are energy fronts passing through water, causing water particles to rotate in place. Rotational Figure 8.6 As a wave moves into shallower water, it rises movement becomes insignificant at depth about one-half of the higher. Circular rotating water touches bottom, flattening into wavelength. a back-and-forth motion. Deep water __..,..r-- Shallow water •._ __ T_r_a_ns_i_tio_n __ ~•~r••B_re_a_k_e~~~ii~•t----s_u_rt __...... ;:n~sh~ r zone zone zone 1 I - Figure 8.7 Schematic cross-section of deep-water waves entering shallow water. Wavelengths decrease and wave heights increase, causing the water to pitch forward as breakers. 204 Chapter 8 Tsunami Versus Wind-Caused Waves produce a sea swell that is the result of the constructive and destructive interference of multiple sets of ocean waves (figure 8.9a). However, every once in a while, the various waves become briefly synchronized, with their energies united to form a spectacular tall wave, the so-called rogue wave (figure 8.9b). The moving waves quickly disunite, and the short-lived rogue wave is but a memory. But if a ship is present at the wrong time, a disaster may occur. During World War II, the Queen Elizabeth was operating as a troop transport passing Greenland when a rogue wave hit, causing numerous deaths and injuries. On 3 June 1984, the three-masted Marques was sailing 120 km (75 mi) north of Bermuda when two rogue waves quickly sent the ship under, drowning 19 of the 28 people on board. In 1987, the recre Figure 8.8 A breaking wave. Circular rotating water is ational fishing boat Fish-n-Fool sank beneath a sudden "wall slowed at the base but rolls forward on top. of water" in the Pacific Ocean near a Baja California island. © Royalty-Free/Corbis. On 10 April 2005 in New York, 2,300 eager passengers boarded the 295 m (965 ft) long Norwegian Dawn for a one week vacation cruise to the Bahamas. On the return trip, the changes in both wave height and wavelength; wave height seas became rough. Then a thunderous disruption shocked is increasing at the same time that wavelength is decreasing. people as a freak 22 m (70 ft) high wave slammed into the The depth of water beneath a breaker is roughly 1.3 times ship, breaking windows, sending furniture flying, flood the wave height as measured from the still-water level. At this ing more than 60 cabins, and injuring four passengers. The depth, the velocity of water-particle motion in the wave crest wave even ripped out whirlpools on deck 10. Damage to the is greater than the wave velocity, thus the faster-moving wave hull forced an emergency stop for inspection and repairs in crest outraces its bottom and falls forward as a turbulent mass. Charleston, South Carolina. Spring vacation was interrupted by a rogue wave. ROGUE WAVES On occasion, rogue waves strike the shoreline and carry An ocean is such an extensive body of water that different people away from the beach. On 4 July 1992, a rogue wave storms are likely to be operating in different areas. Each 5.5 m (18 ft) high rose out of a calm sea at Daytona Beach, storm creates its own wave sets. As waves from different Florida, crashed ashore, and smashed hundreds of cars parked storms collide, they interfere with each other and usually on the beach, causing injuries to 75 of the fleeing people. Figure 8.9 Waves on the sea surface. (a) At ~\\\\\\1111111111////////1, #'''''1111111111111111//1111,,,,!. ~,,,,~ ~ ... -.... # ~--.... ~/ any time, there usually are several different -~~·.--... ' · ··· ~~ ··· L_; · ··· ' .•··. ~~ - · ~ ... ' ~~ storms, each producing its own waves of e-- .. .. .. \. # .. , .. .. · . ' ·. characteristic wavelength. Different wave sets ·...... ' ·.. ~ ,, .... ... \ ~~... ,, .... •... , ' ~ , # , \ usually int erfere with each other. (b) On rare .... _... ~ ~~ --; ' occasions, the various wave sets combine to ~ -#' ~4't. ~,,,,~ produce an unexpected giant- a rogue wave. W/1111111111111111\\~ (a) and (b) are separate drawings; they do not (a) Multiple wave sets directly correlate. (b) Rogue wave Wind-Caused Waves 205 In Greater Depth Deep-Water Wave Velocity, Length, Higher-velocity waves carry more energy, but how much more? Wave energies per unit length can be computed wit h the Period, and Energy following relationship: Waves moving through water deeper than one-half their wave Ew = 0.125p gH2L length are essentially unaffected by friction with the bottom. The waves move as low, broad, evenly spaced, rounded swells with where Ew equals wave energy, p (rho) equals density of water, g velocities related to wavelength by: equals gravitational acceleration, H equals wave height in meters, and L equals wavelength. Some representative values computed Vw = 1.25VL from this equation are listed in table 8.1. Notice that doubling the wavelength doubles the wave energy, but that doubling the wave where Vw equals wave velocity and L equals wavelength. A swell height quadruples the wave energy. w ith a wavelength of 64 m would have a velocity of the square root of 64 (i.e., 8) times 1.25, or 10 m/sec (22.4 mph). The equa tion is telling us that wave ve locity in deep water depends on the TABLE 8.1 wave's length- as wavelength increases, so does velocity. The period (T) is the amount of time it takes for two successive Some Ocean-Wave Energies wave crests to pass a given point Since the distance between suc Wave Wave- Wave Energy cessive wave crests is the w avelength, there must be a relationship Period (T) length (L) Height (H) in Joules between period (T) in seconds and wavelength (L) in meters. This relationship may be defined by: 10 seconds 156m lm 2.39 x 105 V = distance traveled/time = UT 2m 9.57 X II which may be simplified to: 3m 21.54 X II 4.79 x 105 L = 1.56T2 14.l seconds 312m Im 2m 19.15 x II As a rule of thumb, the velocity of waves in miles per hour may be estimated as 3.5 times the wave period in seconds. For exam 3m 43.08 X II ple, waves with a period of 10 seconds are moving about 35 mph. Rogue waves have been measured at 34 m (112 ft) in A deadly example hit Japan on 15 June 1896, a sum height. The problems they present also include the steepness mer day when fishermen were out to sea and beaches were of the wave front descending into the wave trough. A small, crowded with vacationers. An offshore earthquake swayed short boat is maneuverable and in good position to ride over the seafloor; then, about 20 minutes later, the sea withdrew, the rogue wave, as long as it does not get hit sideways and only to return in 45 minutes with a sound like a powerful rolled, or tossed from the front of one wave onto the back of rainstorm.