Waves 1 Flashcards
What is a wave?
propagation of energy through a medium
- medium itself barely moves
Ocean waves are generated by ___ or ___
- wind
- geological events (earthquakes, landslides, etc)
The net water motion of ocean waves is
zero, water particles rotate in place
What is wave height (H)
the vertical distance from crest to trough
How can we calculate Steepness?
H/L
(wave height / wave length)
- no units!
For a given wave height, shorter wavelengths lead to ___ steepness
higher
Period vs Frequency
period is time between successive cycles (sec)
frequency is number of cycles per second (Hz)
What is celerity?
speed of a wave in m/s
L/T
(wavelength / period)
the generating force creates
a disturbance in the fluid level
the restoring force tries to ___ but ___
- remove/flatten out the disturbance
- it overshoots and creates a trough
the ___ overshooting the ___ creates an oscillation
- restoring force
- initial generating force
Typical wavelength of waves generated by winds
60 - 150m
Typical wavelength of waves generated by geological events
200km
tsunamis!
Typical wavelength of tides
20 000km
that’s half the circumference of earth!
Waves generated by the gravitational pull of the moon and sun are called…
tides
What is the restoring force for small waves? (wavelength < ___cm)
< 1.7cm
Surface tension or cohesion of water created by H-bonds btwn molecules
What are the two kinds of restoring forces?
- surface tension
- gravity
If the generating force was a small object that dropped into the water, the restoring force would be…
surface tension
What is the restoring force for larger waves? (wavelength > ___cm)
> 1.7cm
Gravity: the weight of water in the CREST pulls the crest back down
Classification of waves can be based on (2 things)
- Generating force
- Restoring force
What is a generating force?
force that adds energy to the water
Classify: Wind Wave
Generating force:
- wind
Wavelength: up to 150m
Classify: Seiche
Generating force:
- changes in atmospheric pressure (MOST important)
- storm surge
- tsunami
Wavelength: 1km
What is the most important generating force for seiches?
changes in atmospheric pressure
Classify: Tsunami
Generating force:
- faulting of sea floor
- volcanic eruption
- landslide
Wavelength: 200km
Classify: Tide
Generating force:
- gravitational attraction
- rotation of earth
Wavelength: 20,000km
Classify: Capillary Wave
Restoring force:
- surface tension
Wavelength: < 1.7cm
Classify: Surface Gravity Waves
Restoring force:
- gravity
Wavelength: > 1.7cm
Wind waves, seiches, tsunamis, and tides are types of
Surface gravity waves
How do water particles move as a wave passes?
Forward circular motion / orbits
where energy passes thru water but net movement of water particles is negligible
Wave base marks the ___ of the wave and can be calculated by ___
bottom (where orbitals disappear)
wavelength ÷ 2
In open water, water particles have an orbital motion that ___ with depth
decreases
The orbital diameter of wave motion is equal to the
wave height (H)
Is there orbital movement below the wave base?
nope
What are deep water waves?
d >= L / 2
(water depth >= wave base)
- waves don’t feel the sea floor
What are shallow water waves?
d <= L / 20
- waves interact with sea floor
- friction flattens orbits to oval shape
What is an intermediate wave?
L / 20 <= depth <= L / 2
The speed of deep water waves depend on ___ only
Wavelength
Equation to calculate the speed of a deep water wave
c = 1.25 √ L
OR
c= 2.56 x T
in m/s!
Storms produce winds travelling in various directions, which generates winds with different ____
wavelengths
Due to a storm, the chaotic sea has ___ and the swell in front of it has ___ wavelengths
chaotic: many wavelengths
swell: long wavelengths
How do deep water waves tend to travel?
in groups that have similar wavelengths
In the sea, complex swells result from what?
waves of diff wavelengths
describe the dispersion of deep water waves
- waves with longer wavelengths travel faster and outruns the others
- results in waves travelling in groups w/ similar wavelengths
- long wavelength waves are more ahead of the storm
waves are sorted by wavelength by the time they reach the shore, due to ____
dispersion
The speed of shallow & intermediate waves depends on ____ only
depth
c = √gd
c = 3.1 √ d
True or false: the speed of shallow & intermediate waves are affected by wavelength
false!
they’re affected only by depth
Shallow waves travel faster in ___ water and slower in ___
faster in deeper, slower in shallower cuz floor creates friction
What happens as waves approach the shore (move into shallower water)?
- wave height (H) and steepness INCREASE
- speed and wavelength decreases
- period stays the same
- orbitals flatten
- waves break
- some kinetic energy is lost due to friction
How does friction affect water moving past the ocean floor?
- decreases wave speed (c)
- increases wave height (H)
- some kinetic energy is lost
Waves break when
speed of water in crest > wave velocity
(outrace bottom and falls forward)
- when H/d ~ 3/4
How to calculate when a wave will break
H/L > 1/7 (0.14)
- at 1/7, the wave does NOT break yet
T or F: waves are sorted by wavelength by the time they reach the shore
True
Cuz dispersion
