Waves Flashcards
Key properties of waves
Wavelength Height Amplitude Period (one full cycle time) Steepness (H/L) Frequency (cycles/second) Celerity (L/T) wavelength per time period, speed! (m/s)
Forces that generate waves
Generating forces
- wind, seismic action, tides (moon pulling, gravity)
Restoring forces
-surface tension (1.7cm) and gravity
Restoring force overshoots to create a trough, which in turn creates an oscillation
How waves move matter and energy
In a circular motion, backwards, up, forwards and down as the crest passes. There is NO NET movement of water.
In open water, waves move in a circular/orbital manner, at 1/2 L, they stop. this is called wave base.
At the top, orbitals diameter is equal to wave height.
Wave speed and behavior in shallow or deep water
Deep water waves: Depth is > 1/2 of wavelength. When water depth is greater than wavebase. Waves like these don’t feel the bottom. Their speed is dependant only on their wavelength. The longer the wavelength the faster the wave. c = 1.25*sqrt(L)
Shallow water waves
Ocean depth is < 1/20th of L. Waves feel the bottom, oscilations flatten and turn into ellipses and then just go back and forth. Speed depends only on depth of water.
c=3.1*sqrt(depth) - this works for intermediate waves too. Travel faster in deeper water and slower in shallower water.
Changes in wave behavior as they shoal
As waves approach the shore they turn from deep water waves to intermediate to shallow water waves. The height increases, speed decreases, steepness increases. everything changes except the period!
Breakers happen when H/depth = 3/4 or when H/L = 1/7 (whichever happens first)
Waves break because the speed of motion in the wave crest outruns the wave velocity at bottom and falls forwards.
what factors determine roughness of the sea?
- high wind speed
- long wind duration
- long wind fetch (distance traveled)
Use beaufort scale of wind speeds, waves increase with speed increase. however they reach a cap: each wind speed will only generate so big of a wave that duration and fetch won’t make it any bigger. This is a maximum/fully developed sea. i.e. 40knots, 50 hours, and 650 miles create a fully developed sea.
Wave heights are bigger in the east due to longer fetch from antarctia.
BREAKING: in open ocean, if H/L is > 1/7 (0.14) wave will break.
Wind wavelengths have up to 150 m.
Explain how waves interact? (C and D)
Waves come from storms, and can come from all directions, they are sorted by wavelength and sent out. If two waves interact, they will either be constructive or destructively interacting. Constructive (crests match up with crests etc) or Descructive (crests match with troughs) to create bigger/smaller waves.
Can be complex too (lots of interferences producing a mixture of waves)
How are rogue waves generated?
Constructive interferenece that generates a huge random wave. Can come from nowhere, super dangerous. 3/4times the size of surrounding waves. Hazardous for ships at sea.
Cape of Good Hope is bad for this because of winds from antarctic and Angulhas current merging together.
Whats a seiche?
A standing oscillating wave in a ‘closed’ body of water. Observed in the great lakes! The longer the basin, the longer the wavelength.
Depends totally on the basins contour, length/depth
Seiches can be generated by seismic action, meteorologically (wind, pressure changes)
EQUATION: 0.64*(L/sqrt(depth)) is the period of a seiche!!
What are interference and resonance to marine hazards? how are the hazards.
Resonance: every object/system has a natural resonant frequency at which is oscillates at a greater amplitude in a given period. Larger bodies of water have larger resonance. i.e. swingset example.
Seiches can be hazardous because if they blow at the right frequency or multiple of that frequency, they can be really really big. i.e. in lake erie, the period of 18H, so if winds blow 18h or in multiples of that, it will be really really big.
What is a tsunami
A big wave generated by quakes, eruptions, landslides, meteors, icebergs. 2-400,000m wavelengths. i.e verticle changes in sea floor suddenly.
for a quake: flows away radially from source. propegates as a wave train (short burst of localized action) and disturbs entire water column. Very small height (0.5-1m) Very flat. Very fast.
All ocean waves are shallow water waves. Since ocean depth is about 4.5 km,
Speed of a tsunami is dependant on depth of the water. 3.1*sqrt(depth)
How are tusnamis different than wind waves?
They are all shallow water waves. have huge wavelengths, generated by different things.
Long period (1hour) vs. 10seconds
Long wavelength (400km) vs. 150m
At shore, L goes to 20km and speed goes to 80km/hour
Faster (750km/hour) vs 60km/hour
Huge mass of water comes to shore.
Tsunamis don’t actually ever really break, since in open ocean it needs to be greater than 1/7 (h/l) that will never happen, and at the coast they typically just rush up as surges of water that last many minutes.
Why do tsunamis come ashore so violently?
Because the H increases from 0.75 to 30 meters and becase they last for multiple minutes. Advance and retreat of water can be deadly.
Why are tsunamis more devastating in some bays?
Tsunamis can be exaggerated by the natural resonance of a semi enclosed basin, if the wave period if a multiple of that bay’s resonant frequency. I.E. in Hilo bay! the 15m tsunami and bay 30m period led to problems! can cross oceans to get places. cool but deadly.
How are tsunamis generated?
Mostly occur in the ring of fire, due to subduction zoning. One plate moving under another causing verticle motion. Sea floor rupture/breakage. No vertical motion makes tsunami unlikely.
Also generated by volcanos either from landslides and small quakes or from underwater eruption of mass and water displaced that way. Mass dumped into the ocean. i.e. Krakatau (indonesia) flooded many areas due to its underwater landslide.
