The Violent Ocean – Waves and Tsunami
Surface currents: the wind blows over ocean, drags on surface and transfers energy from atmosphere
to water- permanent winds prod surface currents
- Waves defined as mechanical expressions of energy
Wave anatomy:
Wave characteristics:
Wave energy:
- H : energy transported by waves related to wave height (H)
- Waves w/ great heights transport more energy, also have long wavelengths. -
Classifying waves
Generating/ Disturbing force
- Waves classified accord to force that gen waves, force that disturbs water cause waves to form
- Winds blow over ocean constantly- range in str, amnt of time blowing over ocean, and area it
blows over.
- Wind –gen waves can range from tiny surface wrinkles, gentle rolling waves to waves that can
break ships in two. Restoring Force
- Force that causes water to return to undisturbed state
Capillary waves: tiny waves w/ wavelength size of fingernail that are restored by surface tension.
- Imp in generation of wind-driven waves- when wind blows over calm sea, cap waves generated
first.
- As more energy transferred only gravity can restore the waves that grow.
Sea State Development
- When wind speed incr. Or when wind blows over area for long period, more energy transferred
to surface ocean & waves grow larger.
- Assessment of ocean surface = “Sea State”
- Amount of energy transferred to ocean estimated by wave height
o V. Large waves form when wind speeds v.high and blow over long period of time over
uninterrupted direction
Sea State depends on:
- Wind str./speed
- Fetch
- Wind duration, length of time wind been blowing over fetch.
When wind blows over fetch- transformed from capillary waves to ripples to chop (short waves): once
excess energy input by wind equals the energy dissipated from waves breaking-= sea is fully developed
Fully Developed waves = chaotic
- Develop swells and propagate away from fetch area in groups called wave trains. - Constrained by 3 factors: windspeed, fetch and duration
- Wind blowing at 32 knots must blow for about 28 hours over a fetch of about 600km to
generate fully developed sea state.
-
Significant Wave Height
- Average height of the highest 1/3 of waves (from trough to crest)
- b/c SWH is avg #, expect to encounter group of waves whose maximum wave height can be
twice the significant wave height.
NASA and CNES produces satellite altimetry information which are maps of significant wave height
- Altimeter measures height of sea surface from which SWH calculated.
- Monthly analyses for globe show avg wave height in tropical Pacific typically < 3m whereas wave
height in high latitudes in winter hemisphere reaches 3-6m due to large storms.
- Ex.
o Signif wave height off coast of BC is 2-3 m, means max wave height we can expect is 6m.
Beaufort Scale
- Qualitative measure of sea state -
Wave Motion
Water particles move forward then downward in direction of wave as the wave crest passes
Backward and upward as trough passes
Net movement =0 because wave ends up in the same place.- Water particles move in orbital(circular
motion)
Orbits Attenuate (Decrease) : and is at minimum at a depth called Wave Base- below WB you cannot
feel wave Deep water waves:
- Waves travel in depths of D = or deeper than L/2
- Waves do not feel the bottom because wave bases shallower than ocean bottom
Shallow water wave:
- D equal or shallower than L/20
- Shallow waves feel bottom as particles are dragging on the bottom as they move. Ex.
Transitional/Intermediate Waves:
- Transition between a deep-water wave and a shallow-water wave
- D shallower than L/2 but greater than L/20
Wave speed
- As wave enters into shallow water enviro, speed dependent on depth of water it is travelling
thru
- Deep water waves also called disperse waves because speed of travel related to
wavelength(distance from crest to crest)
o Speed of deep water wave:
o
L = wavelength G= gravitational constant = 9.8 meters/ sec squared
o S= 1.56 x T, T is in units of metres/seconds
- Shallow water waves: non disperse wave b/c speed controlled only by depth of water.
o All shallow water waves regardless of length travel SAME speed when they are at same
depth.
D = depth of water calc in unites of meters/seconds—D must covert to meters
Shoaling Waves
- When waves move from deep ocean to coast line, char change called SHOALING
o Waves slow down o Wavelengths shorten
o Orbitals flatten
Water particles only move forward and back
o Energy conserved
Virtually no change in energy transported by wave
o Wave period remains same
Slows down at same time
o Wave height incr.
To compensate for water depth
o Waves steepen
When waves become too high for own wavelength it becomes unstable and
collapses.
H/L >= 1/7
Point where steepness equals or exceeds 1/7 = breaks and wave ceases
to exist and turns into surf
o Point described as : H/d= ¾
Surf : turbulent mass of agitated water rushing onshore during and
after wave break
- After break, energy transferred to water, w/ production of surf wave energy converted to
kinetic energy- also transformed to potential energy contained in shore, ocean bottom, and surf.
Storm Surge: Characteristics
Storm Surge: abrupt bulge of water driven ashore by hurricane, tropical cyclone or typhoon
- Gen by 2 processess occurring @ same time :
o a) hurricane wind pushing water into large tall mound called wind-driven surge
o b) raising of ocean water underneath eye of storm b/c of low air pressure at ocean
surface- produces small effect of air pressure called pressure surge. - Surge technically not wave, only has crest but behaves like shallow-water wave as bulge
becomes bigger as depth gets shallower
o Does not break into surf
o Most disastrous during high tide b/c height(and mass of water) of tide adds on to that
of surge
o Hurricanes storm surge last
More
Less
