Waves & Tsunamis Flashcards
All waves
T = period (S) one wave crest to the next
steepness = H / L
speed = L / T
Deep water wave speed
speed S = 1.25 * swrt(L)
S = 1.56 * T
Longer wavelengths travel faster
Shallow water wave speed
speed S = sqrt(gd)
S = 3.1 * swrt(d)
g = gravitational constant, d = water depth
waves travel faster in deeper water (no dependence on wave length)
examples: tides & tsunamis
Generation of wind-driven waves
Wave energy imparted to ocean depends:
- wind speed
- duration (longer the wind blows)
- fetch (area over which the wind blows)
Wave height plot
Beaufort scale
Relates wind energy and wind speed to appearance of sea surface
How do waves go from chop to swell?
- storm puts more energy into ocean, waves move away from storm
Wave dispersion: - waves with longer wavelengths travel faster
- waves are sorted by wavelength/speed
- longer wavelengths reach the shore first
Deep water wave train speed
Wave energy travels more slowly than each wave crest (for deep water waves)
- leading wave dies out, new wave forms behind
- wave train (and wave energy) travels at 1/2 speed of individual wave
Interference - what happens when two wave systems (swells) collide?
Destructive interference
Constructive interference
Mixed wave inteference
Patterns of small and tall eaves along coast
Non-linear affects
- Linear: The combined height equals the sum of the individual waves
- Non-linear: create rogue waves that are tller than the sum of the individual waves
Rogue waves
- an anomalously large wave that is over twice as big as the significant wave height of the surrounding sea state
- caused by a combination of multiple waves and non-linear effects
- occur in open ocean with multiple swells
Onshore progression
Deep waves become shallow
- wave speed decreases as become shallow-water waves
- wavelength decreases
- wave height increases
- waves break when height: wavelength ratios exceeds ~1.7
- speed goes doen bc dependent on water depth
Types of breakers
- spilling breakers: foam, not a dramatic break
- plunging breakers: steeper beach, drmatic steepening of waves, depth changes fast = speed changes fast
- surging breakers: too steep, lots of foam, not a clean break
What makes the world’s biggest waves surfable?
The underwater canyon focuses wave energy like a magnifying glass focuses light energy
Wave refraction
- waves usually appraoch the coast at an angle
- one end of the wave shoals before the other = refraction
Tsunamis
- long wavelength ocean waves generated by submarine earthquakes, landslides, volcanic eruptions, and meteor impacts
- shallow water waves, wave orbitals smushes at the bottom (long wavelength and low wave period)
- behaves as a shallow water wave (depth is <1/20 wavelength)
- they can pass unoticed at sea (not very tall compared to its long wavelength)
Which waves are the most deadly?
- rogue waves: abnormally large waves in the open ocean observed by mariners
- tidal waves: the wave that creates the twice-daily up and down motion of the tides (often incorrectly used to describe a rogue wave)
- seiches: large-scale sloshing of water in lakes
- tsunamis: large waves creates by earthquakes or landslides
- sneaker waves: multiple waves crashing together on a beach
- wind-driven waves: the most common type of wave ___
Why are tsunamis so destructive?
- energy compresses into smaller area, creating larger wave with steeper crest
- small amplitudes in the deep ocean, but as they approach land they slow down and create waves up to 40 m
Tsunami detection
Bottom pressure recorder: detects tsunamis with amplitudes as small as 30 mm in 6,000 m of water
-DART: record pressure
How fast do tsunamis travel?
example:
propogation speed: 196
Pacific ring of fire
80% of tsunamis occure in Pacififc ocean
