# Introduction to Oceanography Dynamic Oceanography: Waves.

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Introduction to Oceanography Dynamic Oceanography: Waves

Overview * Waves transmit energy, not water mass, across the ocean’s surface. * Waves are classified by several characteristics. * The behavior of a wave depends on the relation between the wave’s size and the depth of water through which it is moving. * Wind waves form when energy is transferred from wind to water. * Waves can change direction by refraction and diffraction, can interfere with one another, and reflect from solid objects.

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),and Wave period (T). Wave period provides a basis for the wave classifications: Capillary waves, Chop, Swell, Tsunamis, Seiches. 7-1 Properties of Ocean Waves 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, and Original state of sea surface. As wind velocity increases wave length, period and height increase, but only if wind duration and fetch are sufficient. Fully developed sea is when the waves generated by the wind are as large as they can be under current conditions of wind velocity and fetch. Significant wave height is the average wave height of the highest 1/3 of the waves present and is a good indicator of potential for wave damage.

Ocean Waves What are the parts of an ocean wave?

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. Classifying Waves

Ocean Waves Orbital waves are waves in which the particles of water move in closed circles as the wave passes. Progressive waves are waves of moving energy in which the wave form moves in one direction along the surface (or junction) of the transmission medium. Orbital waves are a type of progressive wave because the waveform moves forward

Progressive waves are waves that move forward across the surface. As waves pass, wave form and wave energy move rapidly forward, not the water. Water molecules move in an orbital motion as the wave passes. Diameter of orbit increases with increasing wave size and decreases with decreasing water depth. Wave base is the depth to which a wave can move water. If the water is deeper than wave base, orbits are circular and there is no interaction between the bottom and the wave, but if the water is shallower than wave base, orbits are elliptical and become increasingly flattened towards the bottom. 7-2 Wave Motions

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. Ocean Waves 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.

There are three types of waves defined by water depth: Deep- water wave, Intermediate-water wave, and Shallow-water wave. Celerity is the velocity of the wave form, not the water. C = L/T The celerity of a group of waves all traveling at the same speed in the same direction is less than the speed of the waves within the group. 7-2 Wave Motions

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep Water Waves, Shallow Water Waves Note the importance of the relationship between wavelength and depth in determining wave type.

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. 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 Factors Affecting Wind Wave Development

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. Factors Affecting Wind Wave Development

Fetch is the area of contact between the wind and the water and is where wind-generated waves begin. Seas is the term applied when the fetch has 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. Wave interference is the momentary interaction between waves as they pass through each other. Wave interference can be constructive or destructive. Because celerity increases as wave length increases, longer waves travel faster than short waves. 7-3 Life History of Ocean Waves

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. Wind Waves Wind waves are gravity waves formed by the transfer of wind energy into water. Wind forces convert capillary waves to wind waves.

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. 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 Interference And Rogue Waves

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. Swell Formation and Dispersion Wave separation, or dispersion, is a function of wavelength. Waves with the longest wavelength move the fastest and leave the area of wave formation sooner. The smooth undulation of ocean water caused by wave dispersion is called swell. (left) The process known as a wave train.

The shallower the water, the greater the interaction between the wave and the bottom alters the wave properties, eventually causing the wave to collapse. Celerity decreases as depth decreases. Wave length decreases as depth decreases. Wave height increases as depth decreases. Troughs become flattened and wave profile becomes extremely asymmetrical. Period remains unchanged. Period is a fundamental property of a wave Refraction is the bending of a wave into an area where it travels more slowly. 7-3 Life History of Ocean Waves

Wave steepness (stability) is a ratio of wave height divided by wave length (= H/L). In shallow water, wave height increases and wave length decreases. When H/L is larger than or equals 1/7 (H/L  1/7), the wave becomes unstable. There are three types of breakers:, Spilling breakers, Plunging breakers, and Surging breakers. 7-3 Life History of Ocean Waves

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. What different ways can waves break against the shore? Plunging waves break violently against the shore, leaving an air-filled tube, or channel, between the crest and foot of the wave. 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. Wind Waves Approaching Shore

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. 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 encounter. Reflected waves can cause interference with oncoming waves, creating standing waves. Wave Refraction, Diffraction, and Reflection

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. Wind Waves Approaching Shore What happens when wind waves break against the shore?

Storm surge is the rise in sea level resulting from low atmospheric pressure associated with storms and the accumulation of water driven shoreward by the winds. Water is deeper at the shore area, allowing waves to progress farther inland. Storm surge is especially severe when superimposed upon a high tide. 7-3 Life History of Ocean Waves

Standing waves or seiches consist of a water surface “seesawing” back and forth. A node is an imaginary line across the surface which experiences no change in elevation as the standing wave oscillates. It is the line about which the surface oscillates. Antinodes are where there is the maximum displacement of the surface as it oscillates and are usually located at the edge of the basin. 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. 7-4 Standing Waves

Internal waves form within the water column on the pycnocline. Because of the small density difference between the water masses above and below the pycnocline, wave properties are different compared to surface waves. Internal waves display all the properties of surface progressive waves including reflection, refraction, interference, breaking, etc. 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, or Submarine landslides. Resonance amplifies the displacement at the nodes and occurs when the period of the basin is similar to the period of the force producing the standing wave. 7-5 Other Types of Progressive Waves

© 2002 Brooks/Cole, a division of Thomson Learning, Inc. Internal Waves Waves that occur at the boundaries of water layers with different densities are called internal waves.

Tsunamis were previously called tidal waves, but are unrelated to tides. Tsunamis consist of a series of long-period waves characterized by very long wave length (up to 100 km) and high speed (up to 760 km/hr) in the deep ocean. Because of their large wave length, tsunamis are shallow- water to intermediate-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. Tsunamis originate from earthquakes, volcanic explosions, or submarine landslides. 7-5 Other Types of Progressive Waves