Chapter 7 Waves in the Ocean
Eric Gevaert/ShutterStock, Inc. Figure 07.COPCO: Waves in the Ocean
Maverick s in Half Moon Bay off central California is rated as the world s top big wave surf spot.
Waves Are the Undulatory Motion of a Water Surface Parts of a wave are: Wave crest Wave trough Wave height (H) Wave amplitude Wave length (L) Wave period (T) Wave period provides a basis for classifying waves as: Capillary waves Chop Swell Seiches Tsunamis Tides
Waves and Their Properties Figure 07.01a: Regular, symmetrical waves can be described by their height, wavelength, and period (the time one wavelength takes to pass a fixed point).
Waves and Their Properties Figure 07.01b: Waves can be classified according to their wave period.
Table 07.T01: Wave Classification
Most of the Waves Present on the Ocean s Surface Are Wind-generated Waves Size and type of wind-generated waves are controlled by: Wind velocity Wind duration Fetch, or distance over which the wind blows Original state of the sea surface As wind velocity increases: Wavelength Period Height All increase, but only if wind duration and fetch are sufficient.
A fully developed sea means the wind-generated waves are as large as they can be under current wind velocity and fetch conditions. Significant wave height is the average of the highest one-third of the waves present. It is a good indicator of potential for: Wave damage to ships Erosion of shorelines Waves (Continued)
Table 07.T02: Waves in Fully Developed Seas Source: Adapted from Thurman, H. V. Introductory Oceanography (Westerville, OH: Charles E. Merrill, 1988).
Progressive Waves Waves that move across a surface As waves pass across a surface, wave forms and wave energy move rapidly forward, but not the material (water, in this case) through which the wave is moving. The material moves very little or not at all. Figure 07.02: The waves created by flicking a rope represent the flow of energy and not mass.
Progressive Waves Progressive waves oscillate uniformly and progress without breaking Longitudinal Transverse Orbital
Longitudinal Waves Also called push-pull waves Compress and decompress as they travel, like a coiled spring Transverse Waves Also called side-to-side waves Energy travels at right angles to direction of moving particles. Generally only transmit through solids, not liquids Orbital Waves Also called interface waves Waves on ocean surface
Waves and Orbital Motion Water molecules move in an orbital motion as the wave passes. The sum of all motions experienced by the molecule is ~0, so there is no net motion of the water particles. Diameter of orbit: Increases with increasing wave size Decreases with depth below the water surface
Wave Terminology Still water level: The horizontal surface halfway between crest and trough of a wave, also known as the zero energy level. Troughs: The parts of an ocean wave that are displaced below the still water level. Crests: The portion of an ocean wave that is displaced above the still water level. Wave height: The vertical distance between a crest and the adjoining trough. Wave length: The horizontal distance between two neighboring crests.
Orbital Wave Characteristics Wave steepness = H/L If wave steepness > 1 / 7, wave breaks Wave period (T) = time for one wavelength to pass fixed point Wave frequency = inverse of period or 1/T
Orbital Wave Characteristics Diameter of orbital motion decreases with depth of water. Wave base = ½ L Hardly any motion below wave base due to wave activity
Circular Orbital Motion Wave particles move in a circle. Waveform travels forward. Wave energy advances.
The Motion of Water Particles Beneath Waves Figure 07.03a: The arrows at the sea surface denote the motion of water particles beneath waves.
The Motion of Water Particles Beneath Waves Figure 07.03b: The orbital diameters described by water particles beneath waves decrease rapidly with distance below the water surface. Figure 07.03c: The orbits described by water particles beneath waves are not closed, but slightly open.
Wave Base The depth to which energy related to a surface wave can be measured (below this depth, there is no orbital motion of water molecules). If the water is deeper than wave base: Orbits are circular There is no interaction between the seabed and the wave If the water is shallower than wave base: Orbits are elliptical Orbits become increasingly flattened toward the bottom
The Distortion of Water-Particle Orbits in Shallow Water Figure 07.04a: For deep-water waves, the orbits described by water particles beneath waves are circles.
The Distortion of Water-Particle Orbits in Shallow Water Figure 07.04b: For shallow-water waves, the orbits of water particles are greatly compressed vertically into elongated ellipses.
The Distortion of Water-Particle Orbits in Shallow Water Figure 07.04a: For deep-water waves, the orbits described by water particles beneath waves are circles. Figure 07.04b: For shallow-water waves, the orbits of water particles are greatly compressed vertically into elongated ellipses.
There are three types of waves defined by water depth: Deep-water wave Waves and Wave Grouping Intermediate-water wave Shallow-water wave Celerity is the velocity of the wave form, not of the water. The celerity of a group of waves all traveling at the same speed in the same direction is less than the speed of individual waves within the group.
Characteristi cs of deepwater, shallowwater, and intermediate waves *** Deep-water waves Transitional/ Intermediate waves Shallow-water waves
Figure 07.B01_02: Satellite imagery. Waves can now be monitored by radar instruments mounted in satellites. The irregular sea surface created by waves reflects and distorts a pulse of radio energy, which is used as an indirect measure of wave height. Courtesy of Space Science and Engineering Center, University of Wisconsin-Madison
Types of Waves Figure 07.05a: The chaotic patterns of seas are produced by waves of many sizes interfering with one another. Courtesy of NOAA This is in the fetch area and is not a nice place to be! Figure 07.05b: A regular ocean swell produced by dispersion that separates waves on the basis of their celerity. Hemera Technologies/AbleStock.com/Thinkstock This is outside of the fetch area and is more regular and orderly Figure 07.05c: Breakers dissipate their energy at the shoreline by wave collapse. Digital Vision Waves break when they reach shallow water
Fetch is the area of contact between the wind and the water. It is where wind-generated waves begin. Seas is the term applied to the sea state of the fetch when there is a chaotic jumble of new waves. Waves continue to grow until the sea is fully developed or becomes limited by fetch restriction or wind duration. Figure 07.06a: Seas in the fetch appear chaotic, as indicated by the irregular wave profile at the top of the diagram. Adapted from Deacon, G. E. R. and Sutton, G., ed. The Sea and Its Problems in The World Around Us. English University Press, 1960.
Wind-Generated Wave Development Capillary waves Wind generates stress on sea surface, V-shaped troughs, wavelength < 1.74 cm Gravity waves Increasing wave energy,, wavelength >= 1.74 cm Trochoidal waveforms Increased energy, pointed crests & rounded troughs
Sea and Swell Sea or sea area where wind-driven waves are generated; choppy waves moving to different directions with different periods and wavelengths due to wind changes Swell uniform, symmetrical waves originating from sea area Factors affecting wave energy: Wind speed Wind duration Fetch distance over which wind blows
A fully developed sea with large waves on the order of 13 to 14 meters (~43 to 46 feet) high will arise for a wind blowing at 20 meters per second (~45 mph), provided the fetch is equal or greater than 1,500 kilometers (~930 miles) and the wind blows at that high speed for at least 40 hours. A smaller fetch or shorter wind duration will result in waves that are smaller than the theoretical maximum for a fully developed sea. In the case of a wind blowing at 20 meters per second (~45 mph), the largest waves to form, if the fetch is limited to 500 kilometers (~310 miles) or the wind duration to 22 hours, will be no higher than about 7 meters (~21 feet).
Wave Height Directly related to wave energy Wave heights usually less than 2 meters (6.6 feet), sometime up to 10 meters. Breakers called whitecaps form when wave reaches critical steepness If wave steepness H/L > 1 / 7, wave breaks
Beaufort Wind Scale
Maximum Wave Height USS Ramapo (1933): 152-meters (500 feet) long ship caught in Pacific typhoon Waves 34 meters (112 feet) high Previously thought waves could not exceed 60 feet
Wave Damage USS Ramapo undamaged Other craft not as lucky Ships damaged or disappear annually due to high storm waves
Wave Damage USS Ramapo undamaged Other craft not as lucky Ships damaged or disappear annually due to high storm waves
Wave Energy Fully developed sea Equilibrium condition Waves can grow no further Swell Uniform, symmetrical waves that travel outward from storm area Long crests Transport energy long distances
Fully Developed Sea
Wave Speed and Celerity Figure 07.07a: Seas are irregular in the fetch area because of constructive and destructive wave interference.
Swells Longer wavelength (deep water) waves travel faster and outdistance other waves. Wave train a group of waves with similar characteristics Wave dispersion sorting of waves by wavelengths Decay distance distance over which waves change from choppy sea to uniform swell Wave train speed is ~½ speed of individual wave.
Wave Train Movement Front wave loses energy in Setting calm water in motion Among swells, longer wavelength waves travel faster and leave the sea area first. Then they are followed by slower & shorter wave trains. Wave train a group of waves with similar characteristics Wave dispersion sorting of waves by wavelengths Wave train speed is ½ speed of individual wave Rear wave develops using energy left By advancing wave group
Collision of two or more wave systems Constructive interference In-phase wave trains with about the same wavelengths Destructive interference Out-of-phase wave trains with about the same wavelengths Wave Interference Patterns
Wave Interference Patterns Mixed interference Two swells with different wavelengths and different wave heights
Wave Interference The momentary interaction between waves as they pass through each other. Wave interference can be constructive, destructive, or complex. Figure 07.06b: Constructive wave interference deepens the wave troughs and raises the wave crests, producing a larger wave. Figure 07.06c: Destructive wave interference reduces the height of the larger wave. Figure 07.06d: Typically, both construction and destructive wave interference occur simultaneously, resulting in complex, irregular wave form.
Wave Speed and Depth Figure 07.07b: This diagram shows how shallow-water waves are transformed by shoaling of the seafloor.
Waves Approaching Shore
Waves in Surf Zone Surf zone zone of breaking waves near shore Shoaling water water becoming gradually more shallow When deep water waves encounter shoaling water less than ½ their wavelength, they become transitional waves.
Waves Approaching Shore As a deep-water wave becomes a shallowwater wave: Wave speed decreases Wavelength decreases Wave height increases Wave steepness (height/wavelength) increases When steepness > 1 / 7, wave breaks
Wave Steepness Is a Ratio of Wave Height Divided by Wavelength (H/L) In shallow water: Wave height increases Wave length decreases When H/L 1/7, the wave becomes unstable and breaks.
There Are Three Types of Breakers Gentle slope Figure 07.09a: The crest of the spilling breaker becomes oversteepened, breaks, and cascades down the front of the wave as it proceeds through the surf zone. steeper slope Figure 07.09b: The plunging breaker evolves into the classic pipeline shape, as the crest curls over the front of the advancing wave. Figure 07.09c: The surging breaker does not break, because it never attains a critical wave steepness. Very steep slope (art a c) Adapted from Galvin, C.J., J Geophys Res. 73 (1968): 3651 3659. (a and b) Digital Vision; (c) Jason McCartney/Shutterstock, Inc.
Three Types of Breakers Spilling Plunging Surging
Spilling Breakers Gently sloping sea floor Wave energy expended over longer distance Water slides down front slope of wave
Plunging Breakers Moderately steep sea floor Wave energy expended over shorter distance Best for board surfers Curling wave crest
Surging Breakers Steepest sea floor Energy spread over shortest distance Best for body surfing Waves break on the shore Return some energy back to sea
Surfing Like riding a gravity-operated water sled Balance of gravity and buoyancy Skilled surfers position board on wave front Can achieve speeds up to 40 km/hour (25 miles/hour) Big waves Man made
Helmets only work if you wear them Foreign exchange program gone awry
2010 return, older, fatter, way out of shape
In shallow water: Troughs become flattened Crests higher Speed lower Wave becomes extremely asymmetrical Period remains unchanged Period is a fundamental property of a wave. Refraction is the bending of a wave crest into an area where it travels more slowly. Figure 07.08a: This block diagram shows the pattern of wave refraction along an irregular shoreline. Adapted from Earle, M. D. and Madsen, O. S. Georges Bank. MIT Press, 1987.
Wave Refraction Waves rarely approach shore at a perfect 90- degree angle. As waves approach shore, they bend so wave crests are nearly parallel to shore. Wave speed is proportional to the depth of water (shallow-water wave). Different segments of the wave crest travel at different speeds.
Wave Refraction
Wave Refraction Wave energy unevenly distributed on shore Orthogonal lines or wave rays drawn perpendicular to wave crests More energy released on headlands Energy more dissipated in bays
Wave Refraction Gradually erodes headlands Sediment accumulates in bays
Wave Reflection Waves and wave energy bounced back from barrier Reflected wave can interfere with next incoming wave. With constructive interference, can create dangerous plunging breakers
Storm Surge The rise in sea level resulting from: Low atmospheric pressure Accumulation of water driven shoreward by storm winds Water is deeper at the shore, allowing waves to progress farther inland. Storm surge is especially severe when it occurs during a spring high tide.
Hurricane Damage Related to Storm Surge Figure 07.10a: Damage caused by the 1900 Galveston hurricane and storm surge. Figure 07.10b: Boats in Gulfport, Mississippi, were washed up on shore by the force of Hurricane Camille in 1969. (a and b) Courtesy of NOAA
Standing Waves or Seiches Consist of a Water Surface Seesawing Back and Forth A node is an imaginary line across the surface that experiences no change in elevation as the standing wave oscillates. It is the line about which the surface oscillates. Antinodes are where there is maximum displacement of the surface as it oscillates. They are usually located at the edge of the basin.
Natural Period of Standing Waves Figure 07.11: The natural period of standing waves in closed and open basins is a function directly of the basin length and inversely of water depth.
Standing Waves Two waves with same wavelength moving in opposite directions Water particles move vertically and horizontally. Water sloshes back and forth. Nodes have no vertical movement Antinodes are alternating crests and troughs.
A standing wave in a lake, harbor, or estuary is called a seiche. Storm winds blowing persistently in one direction drag and pile up water at the downwind end of a basin, creating a storm surge. When the wind dies after the storm, the water surface in the basin may slosh back and forth.
Lake Superior Seiche https://www.youtube.com/watch?v=byi1zijjr4g Time lapse https://www.youtube.com/watch?v=ujurpczskuq
Waves, Nodes, and Surges Geometry of the basin controls the period of the standing wave. A basin can be closed or open. Standing waves can be generated by storm surges. Resonance amplifies the displacement at the nodes. It occurs when the natural period of the basin is similar to the period of the force producing the standing wave.
Internal Waves Form Within the Water Column Along the Pycnocline There is small density difference between the water masses above and below the pycnocline. Therefore, properties of internal waves are different from surface waves. They travel more slowly. They can be much larger. (USS Thresher) Periods measured in minutes Internal waves display all the properties of surface progressive waves including: Reflection Refraction Interference Breaking
Any disturbance to the pycnocline can generate internal waves, including: Flow of water related to the tides Flow of water masses past each other Storms Submarine landslides Internal Waves Figure 07.12: Echo returns from water layers containing dense assemblages of small organisms (plankton) reveal a wavelike pattern that is attributed to the passage of internal waves. Adapted from Longhurst, A. R., and Pauly, D., Ecology of Tropical Oceans. Academic Press, 1987.
Internal Waves Associated with pycnocline Larger than surface waves Caused by tides, turbidity currents, winds, ships Possible hazard for submarines
Tsunamis Tsunamis were previously called tidal waves, but are unrelated to tides. Tsunamis consist of a series of long-period waves characterized by: Very long wavelength (up to 100 km) High speed (up to 760 km/hr) in the deep ocean Because of their large wavelength, tsunamis are shallow-water waves as they travel across the ocean basin. They only become a danger when reaching coastal areas where wave height can reach 10 m or more.
Tsunami Seismic sea waves Originate from sudden sea floor topography changes Earthquakes most common cause Underwater landslides Underwater volcano collapse Underwater volcanic eruption Meteorite impact splash waves
Tsunami Characteristics Long wavelengths (> 200 km or 125 miles) Behaves as a shallow-water wave Encompasses entire water column, regardless of ocean depth Can pass undetected under boats in open ocean Speed proportional to water depth Very fast in open ocean
Tsunamis Originate from Earthquakes, Volcanic Explosions, or Submarine Landslides On December 26, 2004, an earthquake with a magnitude of over 9.0 on the Richter scale triggered a megatsunami that had an impact on coastlines throughout the Indian Ocean. Figure 07.13a: Slumping of a large mass of sediment disturbs the overlying water surface and produces a series of flat, long-period waves, known as a tsunami. Adapted from Ilda, K. and Iwaski, T., eds. Tsunamis: Their Science and Engineering. Springer, 1983
Tsunami Earthquake at Sundra trench, Subduction plate boundary where India collides with the EuroAsian plate. Persisted for over 10 minutes. Between 10 and 30 meter waves
The great Indian Ocean tsunami of December 2004 began when a rupture along a plate junction lifted the sea surface above. The wave moved outward at a speed of about 212 m/sec (472 miles/hour). At this speed, it took only about 15 minutes to reach the nearest Sumatran coast and 28 minutes to travel to the city of Banda Aceh.
Tsunami Destruction Sea level can rise up to 40 meters (131 feet) when a tsunami reaches shore. Sequence of photos of the 2004 Indian Ocean Tsunami inundating the Chedi Resort in Phuket, Thailand, on December 26, 2004.
Tsunami Most occur in Pacific Ocean More earthquakes and volcanic eruptions Damaging to coastal areas Loss of human lives Tsunami Damage in Hilo, Hawaii. Flattened parking meters in Hilo, Hawaii, caused by the 1945 Tsunami that resulted in more than $25 million in damage and 159 deaths.
Historical Tsunami Krakatau 1883 Indonesian volcanic eruption Scotch Cap, Alaska/Hilo, Hawaii 1946 Magnitude 7.3 earthquake in Aleutian Trench Papua New Guinea 1998 Pacific Ring of Fire magnitude 7.1 earthquake
Historical Large Tsunami
Historical Large Tsunami
Indian Ocean Tsunami December 26, 2004 Magnitude 9.2 earthquake off coast of Sumatra 1200 km seafloor displaced between two tectonic plates Deadliest tsunami in history Coastal villages completely wiped out
Indian Ocean Tsunami Detected by Jason-1 satellite Traveled more than 5000 km (3000 mi) Wavelength about 500 km (300 mi) 230,000 300,000 people in 11 countries killed Lack of warning system in Indian Ocean
Japan Tsunami March 11, 2011 Tohoku Earthquake Magnitude 9.0 earthquake in Japan Trench Felt throughout Pacific basin Most expensive tsunami in history Initial surge 15 meters (49 ft) Topped harbor-protecting tsunami walls Amplified by local topography
Japan Tsunami Killed 19,508 people Disrupted power at Fukushima Daiichi nuclear power plant Reactors exploded Radioactivity problem initiated