Waves

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Waves
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Wave characteristics and terminology

• Wavelength (L)
• Still water level
• Orbital motion

• Crest
• Trough
• Wave height (H)

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Wave characteristics and terminology (continued)

• If wave steepness exceeds 1/7, the wave breaks
• Period (T) the time it takes one full waveone
  wavelengthto pass a fixed position

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• Orbital size decreases with depth to zero at wave
  base
• Depth of wave base ½ wavelength, measured from
  still water level

Calm water
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Deep- and shallow-water waves

• Deep-water waves
• Water depth gt wave base
• Shallow-water waves
• Water depth lt 1/20 of wavelength
• Transitional waves
• Water depth lt wave base but also gt 1/20 of
  wavelength

Figure 8-6a b
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Wave speed (S)

• General formula
• Deep-water waves
• Wave speed (S) in meters per second 1.56 T in
  seconds
• Wave speed (S) in feet per second 5.12 T in
  seconds
• Shallow-water waves (d water depth)

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Wave Classification

• Ocean waves can be classified in various ways
• Disturbing Force- the forces which generate the
  waves.
• Meteorological forcing (wind, air pressure) sea
  and swell belong to this category.
• Earthquakes they generate tsunamis, which are
  shallow water or long waves.
• Tides (astronomical forcing) they are always
  shallow water or long waves.

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The sea and swell

• Waves originate in a sea area
• Swell describes waves that
• Have traveled out of their area of origination
• Exhibit a uniform and symmetrical shape

Figure 8-9
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Hurricane Katrina
Hurricane Iniki
September 1992
August 29, 2005
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"Tsunami" - a Japanese word meaning "great wave
in harbor". It is a series of ocean waves
commonly caused by violent movement of the sea
floor by submarine faulting, landslides, or
volcanic activity. A tsunami travels at the speed
of nearly 500 miles per hour outward from the
site of the violent movement.
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Tsunami

• Tsunami terminology
• Often called tidal waves but have nothing to do
  with the tides
• Japanese term meaning harbor wave
• Also called seismic sea waves
• Created by movement of the ocean floor by
• Underwater fault movement
• Underwater avalanches
• Underwater volcanic eruptions

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Most tsunami originate from underwater fault
movement
Figure 8-21a
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Tsunami

Fault displacement under water displaces water,
water moves to fill vacuum, generating large
waves.
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Tsunami characteristics

• Affect entire water column, so carry more energy
  than surface waves
• Can travel at speeds over 700 kilometers (435
  miles) per hour
• Small wave height in the open ocean, so pass
  beneath ships unnoticed
• Build up to extreme heights in shallow coastal
  areas

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Coastal effects of tsunami

• If trough arrives first, appear as a strong
  withdrawal of water (similar to an extreme and
  suddenly-occurring low tide)
• If crest arrives first, appear as a strong surge
  of water that can raise sea level many meters and
  flood inland areas
• Tsunami often occur as a series of surges and
  withdrawals

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Tsunami since 1900

• Most tsunami are created near the margins of the
  Pacific Ocean along the Pacific Ring of Fire

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Maui Puunene AvenueAftermath of a Tsunami in
Kahului, 1960
Tsunamis struck Kahului in 1946, 1957, 1960, and
1964. The earliest historically recorded tsunami
in Kahului occurred on November 7, 1837, when a
large tsunami traveled 800 yards inland and
destroyed a Hawaiian village.The 1960 tsunami
was caused by a violent earthquake in Chile on
May 22, 1960. It took approximately 15 hours for
the tsunami to travel from Chile to the Hawaiian
Islands. The tsunami killed 61 people in Hilo on
the Big Island, but there were no other human
casualties on any of the other islands. The
tsunami caused moderate damage in Kahului.
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A giant wave engulfs the Hilo pier during the
1946 tsunami. The red arrow points to a man who
was swept away seconds later.
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Tsunami warning system

• Seismic listening stations track underwater
  earthquakes that could produce tsunami
• Once a large earthquake occurs, the tsunami must
  be verified at a nearby station
• If verified, a tsunami warning is issued
• Successful in preventing loss of life (if people
  heed warnings)
• Damage to property has been increasing

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tides w/out tsunami
Earthquake originated in Anchorage, AK
Passage of a tsunami as seen in a sea level
record from Hilo, Hawaii. The observed sea level
shows high frequency variations with a period of
approximately 20 minutes and an initial amplitude
of nearly two meters (total tsunami wave height
3.7 m)
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This mathematical simulation (above) shows the
tsunami created by the Cascadia Subduction Zone
earthquake on January 26, 1700, as it reaches
Hawaii on its way across the Pacific Ocean (5
hrs).
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Tsunami India Ocean Dec. 26, 2004
No tsunami ocean buoy in Indian Ocean

• The most powerful earthquake in 40 years
• Measured 9.0 on the Richter scale
• Occurred approximately four miles below the
  Indian Ocean near the Indonesian Island of
  Sumatra.

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Tsunami Thailand
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• Free Waves, Forced Waves
• Free waves- a wave that is formed by a disturbing
  force such as a storm. Waves continue to move
  without additional wind energy
• Forced wave- a wave that is maintained by its
  disturbing force, e.g., tides

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• Restoring Force
• Force necessary to restore the water surface to
  flatness after a wave has formed in it
• Capillary waves- wavelength lt 1.73 cm
• Gravity waves- wavelength gt 1.73 cm

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Deep-water, Transitional, Shallow-water
waves Wavelength- determines the size of the
orbits of water molecules within a wave Water
depth- determines the shape of the orbits

• Deep-water waves
• Water depth gt wave base
• More circular orbits
• Shallow-water waves
• Water depth lt 1/20 of wavelength
• Orbits are more flattened
• Transitional waves
• Water depth lt wave base but also gt 1/20 of
  wavelength
• Intermediate-shaped orbits

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• Wind Waves- gravity waves formed by the transfer
  of wind energy into water
• Wave ht- usually lt3m
• Wave length- 60-150m
• Factors that affect wind wave development
• Wind strength
• Wind duration
• Fetch- the uninterrupted distance the wind blows

http//www.newportsurf.com/tides.html
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Fetch
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Interference and Rogue Waves
Interference waves when waves from different
storm systems overtake one another. They add
(constructive interference) or subtract
(destructive interference) from the other.
Constructive
Destructive
Mixed
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• Rogue waves
• freak waves that come out of nowhere
• created by constructive interference
• formed by the interaction of a wind wave and a
  swift surface current
• common in southeastern tip of Africa

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Waves approaching shore

• Types of Breaking Waves
• Plunging breaker
• Spilling breaker
• Surging breaker
• Factors that determine the position and nature of
  the breaking wave
• Slope
• Contour
• Composition

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a gradual sloping bottom generates a milder wave
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Surging Breaker

• doesn't break, because it never reaches critical
  wave steepness
• breaker diminishes in size and looses momentum
• Found on beach with a very steep or near vertical
  slope

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Sunset Beach
Waikiki
What type wave are these?
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Wave refraction

• As waves approach shore, the part of the wave in
  shallow water slows down
• The part of the wave in deep water continues at
  its original speed
• Causes wave crests to refract (bend)
• Results in waves lining up nearly parallel to
  shore
• Creates odd surf patterns

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Wave Refraction
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Wave Diffraction- Propagation of a wave around an
obstacle


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Wave Reflection

• Wave energy is reflected (bounced back) when it
  hits a solid object

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Rip Current
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Internal Waves- at thermocline/pycnocline layer
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Internal Waves- surface view
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Wave exposed environment

• constant coral species turnover associated with
  mortality
• and recruitment
• rarely thicker than a single coral colony

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Mortality on wave exposed environment due to

• Breakage
• Scour
• Abrasion

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Depth- lack of coral accretion in shallow open
ocean coastline due to wave energy Absence of
mature barrier reef in Hawaiian Islands
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Wave climate in Hawaii

• 5 types of open ocean swells that cause
  disturbance to coral
• Destructive waves-causes high mortality on reef
  building corals
• North Pacific winter waves on north and western
  coastline
• hurricane generated swells on south or southwest
  coastline (40 yr cycle)

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Low moderate nondestructive waves- optimizes
mixing and nutrient uptake or exchange, usually
beneficial due to increased circulation and
nutrients between water and organisms 3.
Tradewinds generated from northeast or east ht.
of 1-3 m, occurs 90 of summertime and 55-65 of
wintertime 4. Long period southerly swell from
southern ocean during the Austral winter common
between April and September (1-2 m in ht) 5.
Kona storm generated waves (4m) occasionally
may be destructive and cause beach and shoreline
erosion 
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Reef Front in a Low Energy Environment
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Reef Front in a High Energy Environment
Algal Ridge
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Upper Reef Slope of a High Energy Environment
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Upper Reef Slope
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Upper Reef Slope of a Lower Energy Environment
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Table 1. Community structure and growth of coral
reef at sites selected for study. Attributes of
community structure are based on one 50 m
transect at each station. Annual coral growth
rates are averages of 10 colonies.
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Inquiry

• Tsunamis are caused by ______.
• A fetch is _______.
• Waves that approach shore and bend so that they
  are parallel with the shore line are called
  _____.
• Waves with a wavelength greater than 1.73 cm are
  ______ waves.
• Internal waves occur at the __________.
• Waves from different storms systems that dont
  have coinciding troughs or crests form
  _____waves.
• Waves that approach a steep shore and never
  really break are _______ waves.