Chapter 7 Waves in the Ocean

Chapter 7
Waves in the
©2003 Jones and Bartlett Publishers
7-1 Properties of Ocean Waves
Definition: Waves are the undulatory
motion of a water surface.
• Two general wave categories:
– Progressive waves
• Surface waves
• Internal waves
• Tsunamis
– Standing waves
• Seiches
7-1 Properties of Ocean Waves
Characteristics of progressive waves:
• Parts of a wave are:
– wave crest
– wave trough
• Wave parameters:
– wave
– wave
– wave
– wave
height (H)
amplitude (1/2H)
length (L)
period (T).
• Wave period provides a basis for
classifying waves as capillary waves, chop,
swell, seiches, tsunamis, and.
Idealized Wave Spectrum
Idealized Wave Spectrum
7-1 Properties of Ocean Waves
Most of the waves present on the
ocean’s surface are wind-generated
• Size and type of wind-generated waves are
controlled by:
– wind velocity
– wind duration
– Fetch
– original state of the sea surface.
• As wind velocity increases wavelength,
period and height increase, but only if
wind duration and fetch are sufficient.
7-1 Properties of Ocean Waves
• A fully developed sea is a sea state where
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 of
the highest 1/3 of the waves present.
– Good indicator of potential for wave damage to
ships and for erosion of shorelines.
7-2 Wave Motions
Progressive waves are waves that
“move” forward across a surface.
• As waves pass, wave form and wave
energy move forward, but 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 depth below
the water surface.
7-2 Wave Motions
Orbit Diameter and Stokes Drift
7-2 Wave Motions
• Wave base is the depth to which a surface
wave can move water.
• If the water is deeper than wave base:
– orbits are circular
– no interaction between the bottom and the wave.
• If the water is shallower than wave base
– orbits are elliptical
– orbits become increasingly flattened towards the
Deep- and Shallow-Water Motion
7-2 Wave Motions
• There are three types of waves defined by
water depth
– Deep-water wave (d>or=1/2 of L)
– Intermediate-water wave (d>1/20 and <1/2 of L)
– Shallow-water wave (d<or= 1/20 of L)
• Celerity is the velocity of the wave form and
not of the water.
– Celerity equations.
7-3 Life History of Ocean Waves
Fetch is the area of contact between the
wind and the water and is where windgenerated 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.
• Wave interference is the momentary interaction
between waves as they pass through each other.
Wave interference can be constructive or
• Chaotic Sea exhibiting complex surface wave forms.
7-3 Life History of Ocean Waves
•Dispersion: Gradual separation of wave
types based on their relative wavelengths
and speeds
•Because celerity increases as
wavelength increases:
-long waves travel faster than short waves.
-This causes dispersion outside of the fetch
and regular ocean swell.
7-3 Life History of Ocean Waves
7-3 Life History of Ocean Waves
Chaotic seas
inside fetch area.
Swells: wave
type found
outside the fetch.
7-3 Life History of Ocean Waves
A Rogue wave occurs when there is a momentary
appearance of an unusually large wave formed
by constructive interference of many smaller
7-3 Life History of Ocean Waves
7-3 Life History of Ocean Waves
7-3 Life History of Ocean Waves
The shallower the water, the greater the
interaction between the wave and the
bottom alters the wave properties,
eventually causing the wave to collapse.
• Wave speed decreases as depth decreases.
• Wavelength decreases as depth decreases.
• Wave height increases as depth decreases.
• Troughs become flattened and the wave profile
becomes extremely asymmetrical.
• Period remains unchanged. Period is a fundamental
property of a wave.
• Celerity equation of shallow water wave.
•Wave refraction is the bending of a wave crest into
an area where it travels more slowly.
7-3 Life History of Ocean Waves
Wave steepness is a ratio of wave height
divided by wavelength (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
and breaks.
• There are three types of breakers: spilling
breakers, plunging breakers, and surging
Spilling, Plunging and Surging Breakers
•Spilling breaker: Top of wave
crest ‘spills over’ wave. Energy
released gradually across entire
surf zone.
•Plunging breaker: Crest ‘curls
over’ front of wave. Energy
dissipates quickly. Common at
shorelines with steep slopes
•Surging breaker: Never breaks
as it never attains critical wave
steepness. Common along
upwardly sloping beach faces or
seawalls. Energy released
7-3 Life History of Ocean Waves
Storm surge is the rise in sea level
resulting from low atmospheric pressure
and the accumulation of water driven
shoreward by storm winds.
• Water is deeper at the shore area, allowing
waves to progress farther inland.
• Storm surge is especially severe when
superimposed upon a spring high tide.
Storm surge damage
Storm surge damage
7-4 Standing Waves
Standing waves or seiches consist of a
water surface “seesawing” back and
• Node : The line about which the surface
– Located in centers of enclosed basins and toward
the seaward side of open basins.
• Antinodes: Points where there are the
maximum displacement of the surface as it
– Antinodes usually located at the edge of the
Natural Period of Standing Waves
7-4 Standing Waves
• 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
• 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.
Other Types of
7-5 Progressive Waves
Internal waves form within the water
column along the pycnocline.
• Because of the small density difference between
the water masses above and below the pycnocline,
wave properties are different compared to surface
• 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.
Other Types of
7-5 Progressive Waves
Internal waves form within the water
column along the pycnocline.
Other Types of
7-5 Progressive 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 wavelength (up to 100
km) and high speed (up to 760 km/hr) in the deep
• Because of their large wavelength, 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.
Generation of a Tsunami
Generation of a Tsunami
Tsunami damage

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