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Progressive waves move across the sea surface.
Submarine disturbance
Gravitational attraction of sun and moon
Simply put, wave size increases as the strength and duration of the wind, and distance over which it blows increases.
Swell - more regular waves beyond area of generation
Surf - waves that have reached the coast, grow in height, and break
When wave passes, no net displacement of water.
It is the energy of the wave, not its mass that is in motion.
- Constructive
- Destructive
- Wavelength shortens
Formula on pg. 239
Bending of the wave crest as waves enter shallow water. It is due to:
- Drag along the bottom.
Wave energy is focused where the lines converge.
Wave energy is defocused where the lines diverge.
Wave height/wave length.
When this value is about 1/7, waves begin to break.
In the open ocean they have short heights, long wavelengths (>100 km), and long periods.
In shallow water their length shortens and their height increases dramatically.
- Persistent onshore winds.
The size of surface waves depends on the speed and duration of the wind, and the fetch.
Waves affect water to a depth equal to or less than one-half their wavelength.
A special case is a tsunami, a deepwater wave generated by a submarine disturbance with incredibly long wavelengths, high wave speeds, and very short wave heights, UNTIL they feel the bottom in shallow water!
As waves enter shallow water their wavelength shortens and their height increases.
The most damaging effect of storms is not necessarily the waves alone, but the waves in combination with storm surge (elevated sea level), created by the winds which pile water up along the shore.
Standing waves (seiches) do not move horizontally. They are stationary and
Oscillate back and forth about a node i.e., a fixed point.
Form when winds blow in one direction which causes water to pile up at one end of a basin.
Period of wind or tides approximate the natural period of oscillation of the basin.
The result is amplification of the wave size which can lead to flooding.
Think of sloshing in bathtubs and or swinging in swing...
Tsunamis which are seismic sea waves.
They occur when water masses slip over one another.
They have very long periods and very large heights.
http://dusk.geo.orst.edu/oceans/waves.html
Standing waves oscillate about a fixed point.
Wave Parameters (Figure 7-1a)
What Causes Waves?
Wind
Wind Generation of Waves
The type of wave generated by wind is determined by:
- Wind velocity
- Wind duration
- Fetch (distance over which wind blows)
Progressive Waves
Wind-generated waves are progressive waves because they travel across the sea surface.
Progressive Wave Types
Sea - irregular waves in the area of generation
Wave Motions
Two basic motions associated with an ocean wave:
The forward movement of the wave form.
The orbital motion of water particles beneath the wave.
It is wave energy not water molecules that moves across the sea surface.
Wave Motion with Depth (Figure 7-3a)
Click here for ANIMATION
Motion of Water Particles Beneath Waves (Figure 7-3b)
Deep Water Waves (Figure 7-4a)
In deep water most waves do not interact with the sea bottom and are called deep-water waves. The orbits of the water molecules are circular.
Shallow Water Waves (Figure 7-4b)
Waves which interact with the sea floor are known as shallow-water waves. The orbits of the water molecules become elliptical.
Wave Speed
The speed (celerity) of the wave form can be calculated by
Dividing the wavelength by the wave period.
Life History of Ocean Waves
Waves originate in the fetch area. This area is characterized by a "confused" sea state with extensive wave interference. This interference may be:
Rogue Waves? (Figure 7-6b)
Constructive wave interference occurs when several wave crests or troughs coincide. The crests build up and the troughs build down.
Destructive Waves (Figure 7-6c)
Destructive wave interference occurs when the crest of one wave coincides with the trough of another wave resulting in a cancellation.
Transformation of Deep-water Waves: Dispersion (Figure 7-7a)
Shallow Waves Affected by Bottom
In shallow water, the sea bottom transforms the wave's properties. This leads to wave refraction and waves collapse forming surf (breakers).
Shallow-water Wave Transformations
Interaction with the sea bottom.
Water depths are less than 1/20 the wave length.
Bottom friction alters both the
- Wave form
- Celerity
Waves Entering Shallow Water
As waves enter shallow water:
- Height increases
- Speed decreases
Transformation of Shallow-water Waves (Figure 7-7b)
Wave Refraction (Figure 7-8a)
Click here for ANIMATION
- Differential speed along the crest.
Wave Orthogonals (Figure 7-8)
Refraction can be assessed by drawing wave orthogonals, imaginary lines perpendicular to the wave crest.
Life History of Ocean Waves (Cont.)
Shore breakers (surf) are collapsing waves. Breaking is determined by wave steepness
Waves Break by Spilling Plunging, or Surging (Figure 7-9)
depending on the slope of the bottom
Surfing Video: Condition Black
Tsunamis
Sudden shifting of the ocean floor due to earthquakes, volcanic eruptions, and submarine slumping.
Storm Surges
Storms affect shorelines in a variety of ways. Some of the greatest destruction comes from the storm surge or extreme high water levels due to the:
- Low pressure which leads to a rise in sea level.
Chapter 7 Summary
Waves are disturbances that are generated on or beneath the sea surface. Progressive waves move along the sea surface.
Standing Waves
Seas, swell, and surf are progressive waves.
Node = Fixed Point, Antinode = maximum displacement (Figure 7-11)
Resonance
Standing waves can be dangerous due to the phenomenon of resonance.
Other Types of Progressive Waves
There are two other notable types of progressive waves.
Internal waves which occur underwater and move along pycnoclines.
Internal Waves
Slower speeds than surface waves because the density difference between water masses is less than between air and water.